ALBERT

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Shengxiang Deng, Changda Nie, Haojie Jiang, Wei-Biao Ye〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Embedded fin in phase change materials (PCMs) is one of the most efficient methods to enhance the heat transfer between the PCM and heat transfer fluid (HTF). An appropriate arrangement of the fins plays significant role to design a highly efficient latent heat thermal energy storage (LHTES) unit. The aim of this study is to find the most efficient arrangement of fins to accelerate the charging rate. A two-dimensional numerical model based on finite volume method (FVM) was developed with considering natural convection and the calculation results were validated with experimental data. The heat transfer characteristics of LHTES unit with different fins arrangements were firstly explored. These include no fins, straight fins, angled fins, lower fins and upper fins. Then, the effects of fins number (〈em〉N〈/em〉), dimensionless fins length (〈em〉l〈/em〉), heat transfer fluid temperature (〈em〉T〈/em〉〈sub〉w〈/sub〉) and outer tube material on melting performance for four arrangements were studied. In addition, the best type of arrangements to increase the efficiency of heat exchanger was suggested. The performance enhancement of LHTES through fins configuration were quantitatively described based on complete melting time and heat storage capacity, and the conclusions are arrived as follows: when 〈em〉N〈/em〉 ≤ 6, the optimum arrangement is the lower fins, while it is the angled fins when 〈em〉N〈/em〉 〉 6. For 〈em〉N〈/em〉 = 6, only 〈em〉l〈/em〉 could change the optimum arrangement of fins. At 〈em〉l〈/em〉 equals 0.5 and 0.95, the optimum arrangement is angled case. While at 〈em〉l〈/em〉 = 0.75, the optimum arrangement is lower case. It is also found that the heat storage capacity of lower fins configuration is minimal compared to other three configurations.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Lily, B. Munshi, K. Barik, S.S. Mohapatra〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The dissolved surfactant in water droplet alters the surface tension, contact angle and viscosity and thereby heat transfer mechanism changes accordingly. The sliding characteristic of the water and vapor film resulting from the agglomeration of partially evaporated droplets on the hot plate are controlled by the aforesaid properties of the fluid. In addition to the above, the sliding characteristic further depends on the orientation of the plate. For the identification of appropriate conditions to get high heat transfer rates, the effect of viscosity, surface tension and orientation of the plate in the favorable direction of heat transfer needs to be investigated. The literature does not reveal the combined effect of the aforesaid process parameters on heat transfer rate. Therefore, in the current work an attempt has been made to reveal the effect of viscosity, surface tension and the orientation of the plate on heat transfer. To determine the effect of viscosity and surface tension in the presence of the foaming characteristics, three types of surfactants such as Sodiumdodecyl sulfate (SDS), Cetyltrimethylammonium bromide (CTAB) and Polyoxyethylene (20) sorbitan monolaurate (Tween 20) below critical micelle concentration have been used. The result reveals that the reduction of surface tension and viscosity up to 48% and 45% enhances the heat transfer rate (CHF from 1.39 MW/m〈sup〉2〈/sup〉 to 1.58 MW/m〈sup〉2〈/sup〉) at 30° plate inclination due to reduction in the residence time from 40 ms to 20 ms and enhancement in wettability characteristic (contact angle from 72.34° to 30.96°). The aforesaid result has been achieved at an optimum concentration of SDS (600 ppm), CTAB (180 ppm) and Tween 20 (42 ppm). In addition to the above, to identify the effect of viscosity and surface tension in the absence of foaming, ethanol has been selected as an additive. In comparison with Tween 20, the ethanol added water spray at an inclination of θ = 30° shows insignificant enhancement due to very small reduction and increment in surface tension and viscosity, respectively. The accuracy and enhancement are identified by comparing the obtained results with the information reported in the literature.〈/p〉〈/div〉 〈/div〉 〈div xml:lang="en"〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S0017931018301200-fx1.jpg" width="500" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Yuxiang Hong, Juan Du, Shuangfeng Wang, Wei-Biao Ye, Si-Min Huang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Thermal-hydraulic behaviors of air turbulent flow in a traverse corrugated tube (TCT) inserted with twin and triple wire coils (WCs) were investigated experimentally. The effects of two different arrangements (twin WCs and triple WCs) and four space ratios (〈em〉S〈/em〉/〈em〉D〈/em〉 = 0, 3.88, 8.62 and 18.1) on Nusselt number (〈em〉Nu〈/em〉), friction factor (〈em〉f〈/em〉), performance evaluation criterion (〈em〉PEC〈/em〉), Bejan number (〈em〉Be〈/em〉) and augmented entropy generation number (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mi〉ϕ〈/mi〉〈/mrow〉〈mrow〉〈mtext〉s〈/mtext〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉) were investigated at Reynolds number (〈em〉Re〈/em〉) ranging from 6000 to 18,000. The gained results point out that the compound use of TCT with WCs gives considerably enhanced heat transfer and also increased pressure loss than the plain tube with improved 〈em〉Nu〈/em〉 ratios in the range of 1.74–2.61 and augmented 〈em〉f〈/em〉 ratios in the range of 4.57–21.34. By analyzing different parameters, it is found that 〈em〉Nu〈/em〉 and 〈em〉f〈/em〉 in the TCT mounted with WCs increases with WC number increasing and space ratio decreasing. Although the co-use of TCT with WCs exhibits superior 〈em〉Nu〈/em〉 than the alone use of the TCT, the former performs higher flow resistance and lower 〈em〉PEC〈/em〉 than the latter. The increased heat transfer of about 22.3–84.2% and augmented pressure loss of around 2.91–12.90 times over that of the TCT are obtained for the combined use of TCT with WCs. The largest 〈em〉PEC〈/em〉 of about 1.09 is achieved in the TCT with twin WCs at 〈em〉S〈/em〉/〈em〉D〈/em〉 = 18.1 while that is about 1.26 for the TCT alone. In terms of thermodynamic characteristics, both the TCT alone and the co-use of TCT with multiple WCs can reduce the contribution of heat transfer entropy generation to the overall entropy generation, and the 〈em〉Be〈/em〉 could be decreased to values of about 0.91 and 0.32 by the former and the latter, respectively. In addition, the obtained results show that the lowest 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mi〉ϕ〈/mi〉〈/mrow〉〈mrow〉〈mtext〉s〈/mtext〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 of about 0.46 is gained by the TCT with triple WCs at 〈em〉S〈/em〉/〈em〉D〈/em〉 = 0 and 〈em〉Re〈/em〉 = 6428 in this study scope. Finally, the 〈em〉Nu〈/em〉 and 〈em〉f〈/em〉 empirical correlations within respectively ±9.0% and ±11.0% from experimental data are installed.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Sunggu Kang, Jonghwan Cha, Kyeongbeom Seo, Sejun Kim, Youngsun Cha, Howon Lee, Jinsung Park, Wonjoon Choi〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Integrated circuits or miniaturized portable electronics require adaptive thermal control under certain temperatures. Thermal metamaterials (TMs), which artificially manipulate the heat passing through mediums have shown innovative thermal functions at a continuum scale. However, they cannot implement tunable thermal functions at local spots depending on the operating temperatures. Herein, we introduce temperature-responsive TMs enabled by modular design of thermally tunable unit cells. As ambient temperature changes, tunable thermal shifters can dynamically turn on/off their intrinsic functions to guide anisotropic heat transfer through the transition of thermal conductivities from the inner phase change nanocomposites (PCNCs), and their modular design realizes temperature-responsive thermal shields having switchable functions. The layered structures of stainless steel and the PCNC of 〈em〉n〈/em〉-octadecane embedding carbon nanotubes and copper powder are fabricated as tunable thermal shifters. Their 4 × 4 modular structure confirms the feasibility of temperature-responsive TMs, verified by the disappearance and appearance of thermally shielded regimes at low- and high-temperature ranges. The potential use of the developed concept was demonstrated as tunable interfaces between thermal dissipation and insulation for protecting temperature-sensitive components. This work can offer new capabilities for conventional passive TMs, such as local thermal adaptation, active thermal control interface, and thermal disturbance mitigation.〈/p〉〈/div〉 〈/div〉 〈div xml:lang="en"〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S0017931018337992-ga1.jpg" width="433" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Nathan Colgan, Joseph L. Bottini, Raúl Martínez-Cuenca, Caleb S. Brooks〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Critical Heat Flux (CHF) is the maximal limit of heat flux in two-phase nucleate boiling heat transfer; therefore, an understanding of CHF under a wide range of conditions is important for safe system operation. In this work, CHF experiments are conducted over a range of subatmospheric system pressures in a vertical square channel that is heated on one side. The experimental conditions cover a pressure range of 20 kPa to 108 kPa, a mass flux range of 45–190 kg/m〈sup〉2〈/sup〉-s, and an inlet subcooling range of 0–14 K. Heat flux is gradually increased until an excursion of the wall temperature occurs, indicating CHF. For the experimental conditions considered, CHF increases with rising system pressure, mass flux, and inlet subcooling, although the effects of mass flux and inlet subcooling are weak. A new correlation for CHF is developed and found to predict the data with an average error of ±15.6%.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Guobin Zhang, Xu Xie, Biao Xie, Qing Du, Kui Jiao〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Limited by the computational efficiency and stability, traditional 3D (three-dimensional) CFD (computational fluid dynamics) simulations of PEMFC (proton exchange membrane fuel cell) are always in single-channel scale, which neglect the realistic flow field structures in commercial PEMFC. In this study, a large-scale PEMFC (109.93 cm〈sup〉2〈/sup〉), which is a repeated unit in commercial stacks and includes realistic anode and cathode flow fields, is investigated in detail utilizing a comprehensive 3D multi-phase model. In particular, the Eulerian-Eulerian model is chosen for the solution of gas and liquid two-phase flow in flow fields and the surface tension, wall adhesion, drag force and gravity are all taken into consideration. The gas concentration and liquid water amount in each channel of flow field are studied to test the influence of flow field. Moreover, it is proved that increasing operating pressure is helpful to improve PEMFC performance by increasing the reactant gas concentration and membrane water content significantly. Besides, counter-flow arrangement of hydrogen and air facilitates uniform distribution of membrane content and electrochemical reaction. And in this case, the coolant flow direction designed to be the same with that of air is beneficial to PEMFC performance.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Shumpei Hara, Andrew J. Maxson, Yasuo Kawaguchi〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This paper presents an exergy transfer analysis for turbulent drag reducing surfactant solution flow with and without heat transfer enhancement devices which employ static or dynamic mixers. Heat transfer and pressure drop measurements were performed at an inlet temperature of 297.7–298.2 K in pipe flow with a concentric tube heat exchanger. The environmental temperature in the analysis was set to be 288.2 K in assumption of a cooling system. The experimental range of the Reynolds number was 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si11.gif" overflow="scroll"〉〈mrow〉〈mn〉1.0〈/mn〉〈mo〉×〈/mo〉〈msup〉〈mrow〉〈mn〉10〈/mn〉〈/mrow〉〈mrow〉〈mn〉4〈/mn〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉 to 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si12.gif" overflow="scroll"〉〈mrow〉〈mn〉4.2〈/mn〉〈mo〉×〈/mo〉〈msup〉〈mrow〉〈mn〉10〈/mn〉〈/mrow〉〈mrow〉〈mn〉4〈/mn〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉. Adding surfactant to a water flow decreases the exergy transfer Nusselt number, 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si13.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mi mathvariant="italic"〉Nu〈/mi〉〈/mrow〉〈mrow〉〈mi mathvariant="normal"〉e〈/mi〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉, owing to the heat transfer reduction and increases the exergy transfer efficiency, 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si14.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mi〉η〈/mi〉〈/mrow〉〈mrow〉〈mi mathvariant="italic"〉eff〈/mi〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉, by a maximum of 9.3% at high Reynolds numbers at moderate fluid transport distance owing to the drag reduction. Also the secondary flow caused by the enhancement devices increases 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si13.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mi mathvariant="italic"〉Nu〈/mi〉〈/mrow〉〈mrow〉〈mi mathvariant="normal"〉e〈/mi〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉. The proposed flow performance curve which provides 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si14.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mi〉η〈/mi〉〈/mrow〉〈mrow〉〈mi mathvariant="italic"〉eff〈/mi〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 for an arbitrary 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si13.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mi mathvariant="italic"〉Nu〈/mi〉〈/mrow〉〈mrow〉〈mi mathvariant="normal"〉e〈/mi〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 shows that 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si14.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mi〉η〈/mi〉〈/mrow〉〈mrow〉〈mi mathvariant="italic"〉eff〈/mi〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 in viscoelastic fluid flow compared to Newtonian fluid flow is small independently of the fluid transport distance, and that the installation of heat transfer enhancement devices has a positive effect on energy-saving in certain ranges of Reynolds numbers and fluid transport distances.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): A.V. Reshetnikov, K.A. Busov, N.A. Mazheiko, V.N. Skokov〈/p〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Lingen Chen, Huijun Feng, Zhihui Xie, Fengrui Sun〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Constructal theory has been widely used in performance optimizations of various problems. Among these problems, engineering problem is a hot topic for constructal theory. The developments of constructal theory about engineering problem in China over the past decade are reviewed in this paper. Multi-disciplinary, multi-objective and multi-scale constructal optimizations of various transfer processes in engineering, such as heat conduction and thermal insulation processes, fluid flow processes, convective heat transfer processes of fins, heat sources, cavities, cooling channels, vascular networks and heat exchangers, mass transfer processes of porous mediums, heat and mass transfer processes of solid-gas reactors and solid oxide fuel cells, iron and steel production processes and generalized transfer processes, have been conducted since 2006. Performance improvements are realized, new design requirements are satisfied, and more practical and generalized results are obtained after new research objects, model improvements, optimization objectives, boundary conditions, constraint conditions and transfer extensions are considered. It shows that the developments of constructal theory are still being made in China, which will provide more design guidelines not only for engineering problem, but also for natural and social problems.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Mohammad Zargartalebi, Jalel Azaiez〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Heat generation in small devices hinders the optimum performance of such equipment. Microchannel heat sinks (MCHS) with embedded pins are a common solution to this problem. The performance of these heat sinks is, however, highly dependent on the pin configuration and the coolant properties. Despite known nanoparticle stability problems, nanofluids have been extensively used to improve coolant conductivity. This study is dedicated to the analysis of nanoparticle-based coolants in randomly distributed pin-based MCHS with special attention to nanoparticle adsorption on pin surface. Statistical analysis showed that the behavior of different random realizations is different, and an averaged behavior can be found depending on the number of embedded pins. The averaged behavior is used to analyze the effects of pin configuration and coolant properties. It is shown that the performance of random distributions outweighs that of the inline one, and that as the number of pins increases, the effects of randomness decrease. The impact of nanoparticles on heat transfer is more highlighted as the number of pins decreases. Nanoparticle adsorption on the pin surface is found to have double-edged effects and can be beneficial or detrimental to heat removal and these effects depend on the pins' configuration. It is shown that nanoparticle adsorption on pin surface has generally negative effects on heat removal performance but it can be outweighed by the positive effects of nanoparticle-driven surface area stimulation.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Elnaz Norouzi, Chanwoo Park, Gisuk Hwang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Efficient thermal management of electronics is crucial for their reliable performance and lifetime. Direct cooling of nanoscale heat source components may offer the most efficient thermal management solution. Heat pipe, a phase-change-based cooling device, has been widely used for electronic cooling at system level because of high heat flux cooling capability, but it is challenging to realize in nanoscale, primarily due to the limited choices of fluid circulation mechanisms. This paper presents a nanoscale heat pipe (NHP) which uses a surface diffusion to return the condensate liquid via a nano post connecting the condenser with the evaporator. The NHP is made of a Pt nanogap with the connecting post, which is filled with Ar atoms as the working fluid. The heat transfer modes in the NHP include phase change (evaporation and condensation), convection/conduction heat transfer of Ar atoms, and conduction heat transfer of Pt atoms in the post. This study examines the coupled effects of the average surface temperature, surface temperature difference, Ar number density, condensate mobility, and system size on the heat and mass transfer of the NHP using nonequilibrium molecular dynamics simulation. It was found that the heat transfer by the gaseous Ar atoms in the NHP using the surface-diffusion-driven fluid circulation increases by approximately 44% in the reference NHP design as compared to the heat transfer through the gaseous Ar atoms in a nanogap without a post. For a surface temperature difference Δ〈em〉T〈/em〉 = 60 K, the heat flux of the NHP reaches as high as 240.2 MW/m〈sup〉2〈/sup〉 and the thermal resistance is 2.5 × 10〈sup〉−7〈/sup〉 m〈sup〉2〈/sup〉 K/W. The maximum heat transfer through the adsorbed Ar atoms on the Pt post occurs at an intermediate Ar-Pt surface force 〈em〉ε〈/em〉 = 1.5 kcal/mol. It is also found that the combined heat transfer through both the gaseous and adsorbed Ar atoms outpaces the conduction heat transfer through the Pt post at the length larger than 1000 Å. The effects of the thermal conductivity and cross-sectional area of the post on the NHP performance are also discussed. The in-depth discussions on the nanoscale heat and mass transfer of the NHP will provide an insight into the development of next-generation nanoscale thermal management systems.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Yuxiang Hong, Wei-Biao Ye, Juan Du, Si-Min Huang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The work aims to address the effects of mushy zone constant (1 × 10〈sup〉4〈/sup〉 ≤ 〈em〉C〈/em〉 ≤ 1 × 10〈sup〉8〈/sup〉 kg m〈sup〉−3〈/sup〉 s〈sup〉−1〈/sup〉) and gravitational acceleration (1.635 ≤ 〈em〉g〈/em〉 ≤ 9.810 m s〈sup〉−2〈/sup〉) on the thermal storage/release behaviors of paraffin wax (phase change material, PCM), by performing numerical investigations in a rectangular cavity with partially thermal active walls. The enthalpy-porosity method is employed to simulate the solid–liquid phase-change process and flow evolution at the PCM interface. The numerical model is validated by the published literature data. It is confirmed that the thermal storage behaviors are significantly influenced by the mushy zone constant and gravitational acceleration. PCM liquid fraction is essentially increased as decreased in mushy zone constant and increased in gravitational acceleration, with maximum discrepancies are reached 88%. However, very limited impacts are conducted on the thermal release process with maximum PCM liquid fraction discrepancy only 2%. Furthermore, the proposed equations regarding how to calculate the value of mushy zone constant are also outlined and commented. Therefore, proper examination and verification for mushy zone constant is very important and necessary before solid–liquid phase-change thermal storage process is accurately simulated by enthalpy-porosity model.〈/p〉〈/div〉 〈/div〉 〈div xml:lang="en"〉 〈h5〉Graphical abstract〈/h5〉 〈div〉 〈p〉“Porosity function”, 〈em〉A〈/em〉(〈em〉ϕ〈/em〉), which makes the momentum equation in the mushy zone “mimic” the Carman-Kozeny equation for fluid flow in a porous media, is determined by a mushy zone constant, 〈em〉C〈/em〉, and PCM liquid fraction, 〈em〉ϕ〈/em〉. On the other hand, the thermal storage/release processes may be occurred in low-gravitational situations with gravity acceleration, 〈em〉g〈/em〉 〈 9.81 m s〈sup〉−2〈/sup〉, on the moon (〈em〉g〈/em〉/6) for example. Therefore, the work deals with the validated numerical modeling of the thermal storage/release behaviors of paraffin wax in a partially thermal active rectangular cavity, and directs at the influences of mushy zone constant (shown in figures a1, a2) as well as gravitational acceleration (shown in figures b1, b2). It is found that the mushy zone constant and gravitational acceleration significantly influence the thermal storage behaviors, while they conduct very limited impacts on the thermal release process.〈/p〉 〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S0017931018340158-ga1.jpg" width="260" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Hong Liang, Yang Li, Jiangxing Chen, Jiangrong Xu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this paper, a novel lattice Boltzmann (LB) model based on the Allen-Cahn phase-field theory is proposed for simulating axisymmetric multiphase flows. The most striking feature of the model is that it enables to handle multiphase flows with large density ratio, which are unavailable in all previous axisymmetric LB models. The present model utilizes two LB evolution equations, one of which is used to solve fluid interface, and another is adopted to solve hydrodynamic properties. To simulate axisymmetric multiphase flows effectively, the appropriate source term and equilibrium distribution function are introduced into the LB equation for interface tracking, and simultaneously, a simple and efficient forcing distribution function is also delicately designed in the LB equation for hydrodynamic properties. Unlike many existing LB models, the source and forcing terms of the model arising from the axisymmetric effect include no additional gradients, and consequently, the present model contains only one non-local phase field variable, which in this regard is much simpler. In addition, to enhance the model’s numerical stability, an advanced multiple-relaxation-time (MRT) model is also applied for the collision operator. We further conducted the Chapman-Enskog analysis to demonstrate the consistencies of our present MRT-LB model with the axisymmetric Allen-Cahn equation and hydrodynamic equations. A series of numerical examples, including static droplet, oscillation of a viscous droplet, breakup of a liquid thread, and bubble rising in a continuous phase, are used to test the performance of the proposed model. It is found that the present model can generate relatively small spurious velocities and can capture interfacial dynamics with higher accuracy than the previously improved axisymmetric LB model. Besides, it is also found that our present numerical results show excellent agreement with analytical solutions or available experimental data for a wide range of density ratios, which highlights the strengths of the proposed model.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Dalei Jing, Jian Song〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The present work investigates and compares the hydraulic and thermal performances of two tree-like networks with constant surface area constraint and constant volume constraint. The results find that the optimal channel diameter ratio to reach a minimum hydraulic resistance satisfies 〈em〉β〈sub〉m〈/sub〉〈/em〉 = 〈em〉N〈/em〉〈sup〉−2/5〈/sup〉 for the tree-like network with surface area constraint, but 〈em〉β〈sub〉m〈/sub〉〈/em〉 = 〈em〉N〈/em〉〈sup〉−1/3〈/sup〉 for the tree-like network with volume constraint. The convective heat transfer in both of the two tree-like networks with different size constraints is independent on the channel diameter and diameter ratio, but increases with the increasing channel length, channel length ratio, branching level and branching number. For the two tree-like networks with exactly the same channel length, length ratio, branching number and branching level but different channel diameter, the present work provides a segmenting model to choose the network with better hydraulic and thermal performances.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): N.W. Awang, D. Ramasamy, K. Kadirgama, M. Samykano, G. Najafi, Nor Azwadi Che Sidik〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Nano-lubricant is a new kind of engineering lubricant composed of nanometer-sized particle dispersed in a base lubricant. Recently, nanoparticles have been explored as lubricant additives for improving the stability and lubricity properties of a technological fluid. Cellulose nanocrystals (CNC), a unique and natural material extracted from native cellulose, has gained much attention in many field application due to its remarkable physical properties, special surface chemistry, and excellent biological properties, making them attractive as a green lubricant additive. The purpose of this study is to investigate the characterization of the CNC nanoparticles and to evaluate the influence of CNC nanoparticles on the lubricating properties added to the base oil. In this study, CNC nanoparticles were prepared and suspended in five different volume concentrations in the engine oil (0.1, 0.3, 0.5, 0.7 and 0.9%). The kinematic viscosity and viscosity index of the resulting nano lubricant was determined while varying both the nanoparticle volume fraction and the temperature. The size, morphology, and structure of CNC nanoparticles were characterized using Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive X-ray (EDX) and X-ray diffractions (XRD). The dispersion analysis of CNC nanoparticles in lubricating oil using UV spectrometer confirms that CNC nanoparticles possess good stability and solubility in the lubricant and improve the lubricating properties of the engine oil. The overall results of this experiment reveal that the addition of CNC nanoparticle with base 0il SAE40 lubricant shows the highest value of VI and most suitable concentration for improving properties of the base oil.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Ram Pravesh, Amit Dhiman, R.P. Bharti〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Aiding buoyancy mixed convection features of Newtonian fluids across a periodic array of heated cylinders have been studied numerically using a commercial CFD solver ANSYS FLUENT. The governing equations have been solved for the following ranges of physical parameters: Reynolds (1  ≤  Re  ≤  40), Prandtl (0.70  ≤  Pr  ≤  50) and Richardson (0  ≤  Ri  ≤  2) numbers and fluid volume fractions of 0.70–0.99. Qualitatively, the dense and curved streamlines and isotherms were seen with the increasing inertial (Re), viscous diffusion (Pr) and buoyancy parameter (Ri) across all the fluid volume fractions. The drag coefficients were observed to be diminished with an upturn in Re and fluid volume fractions, whereas an opposite behavior was noticed with rising in Pr and buoyancy parameter. The Nusselt numbers were found to be enhanced with Re and Pr numbers and moreover with fluid volume fractions also as in contrast to forced convection (Ri = 0) cases. Aiding buoyancy enhances flow as well as heat transfer features and yields unsteady behavior also at the higher fluid volume fractions and Re for all the values of Pr and Ri numbers. Moreover, statistical correlations have been developed for the total drag coefficient and average Nusselt number to gain the more physical insight of the results. Lastly, the findings have been compared with the literature which displayed the good agreement within the ranges of parameters studied herein.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Ran Yao, Jianhua Wang, Ming Wang, Lei Wang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In most investigations on the film cooling performance of turbine endwalls, real annular endwalls were usually simplified into flat endwalls. However, up to now, the influence of flat endwall simplification on cooling performance is seldom considered. In present work, a film cooling experiment was conducted in the hot gas wind tunnel at the University of Science and Technology of China (USTC), in order to validate the conjugate heat transfer algorithm and SST k-ω turbulence model used in the following numerical simulations. Then a series of numerical simulations was conducted by two endwall models: one is a real annular endwall, and the other is a flat endwall simplified from the first. The cooling performances of the two endwalls were analyzed and compared, and the numerical results reveal the following interesting and important phenomena: (1) Under real conditions, the adiabatic film effectiveness of the annular endwall is generally higher than that of the flat endwall, but at our experimental environment, the overall cooling effectiveness of the annular endwall is lower than that of the flat endwall. (2) Under the real conditions of gas turbine operation, the annular endwall exhibits a “Matthew effect”, i.e., better overall cooling performance appear in good film-covered region, but worse overall cooling performance appear in poorly film-covered region. (3) Under the real conditions, the temperature differences between the two endwalls can be larger than 25 K, and the temperature gradient in annular endwall is much higher. With other words, using flat endwall simplification may cause a high risk of endwall lifespan. This is very important for the designers of turbine endwall and users of the experimental data obtained by flat endwalls.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): A. Nikulin, A.S. Moita, A.L.N. Moreira, S.M.S. Murshed, A. Huminic, Y. Grosu, A. Faik, J. Nieto-Maestre, O. Khliyeva〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The laminar, transient and turbulent heat transfer and hydrodynamic of a new nanofluid isopropanol/Al〈sub〉2〈/sub〉O〈sub〉3〈/sub〉 is investigated in a closed flow loop with a horizontal mini-channel test section (3.5 mm inner diameter) under uniform heat flux conditions. The experiments performed at various inlet temperatures (15, 25, 35 °C), mass flow rates (from 0.00076 to 0.041 kg/s) and nanoparticle concentrations (0.387, 0.992, 3.12, 4.71 mass%). We found that despite the pressure drop increases with Reynolds number and nanoparticles mass fraction the dependence of friction factor for the isopropanol/Al〈sub〉2〈/sub〉O〈sub〉3〈/sub〉 nanofluid remains the same as for the base fluid. The heat transfer performance of isopropanol/Al〈sub〉2〈/sub〉O〈sub〉3〈/sub〉 nanofluid was evaluated in two ways 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si13.gif" overflow="scroll"〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈mi〉i〈/mi〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈/math〉 depending on the Reynolds number and 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si14.gif" overflow="scroll"〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈mi mathvariant="italic"〉ii〈/mi〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈/math〉 product of the mass flow rate and specific heat capacity. The first approach indicates to significant enhancement of the heat transfer coefficient with addition of nanoparticles in all range of experimental parameters. The second approach shows no effect of nanoparticles on the heat transfer coefficient in laminar flow and its deterioration in transient and turbulent flows. Both effects of nanoparticles on the heat transfer are attributed to change in intensity of the turbulence in nanofluids compared to the base fluids. Finally, an influence of nanoparticles on the start of laminar-turbulent transition was examined.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Dongxu Wu, Congliang Huang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this work, an effective thermal conductivity (ETC) model of open cell foam composite is proposed based on regular pentagonal dodecahedron geometrical model, considering the realistic shape of ligaments, hollowness in ligaments and ligament orientations. Compared with other ETC models, this model exhibits higher accurate predictions with respect to the available experimental data in a wide span of porosities spanning from about 0.6 up to 1. Results show that the ETC of foam composites will increase with increase of the hollowness of ligaments at the same foam porosity. For a high porosity, the influence of the hollowness of ligaments on the ETC is negligible when the foam material has a high TC. The ETC of copper foam-paraffin composite predicted by our model agrees well with that obtained by experimental methods carried out in this work, both suggest that the ETC almost linearly depend on the melting degree of paraffin. According to the linear dependence of ETC on the melting degree of paraffin, the melting degree of the paraffin in copper foam-paraffin composite can be roughly obtained by fitting our model to the experimental ETC. The model developed in this work is expected to give an accurate prediction of open cell foam composites, and also could be applied to evaluate the melting degree of phase change materials in open cell foam composites.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Zujun Peng, Ying Chen, Xue Feng〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This paper proposes a novel approach to estimate the temperature-dependent thermal contact conductance (TCC) during the isothermal cooling phase based on the induction heating. The realizability of the ideal isothermal cooling phase has been validated through the finite-element model, even for specimens with low thermal conductivity. During the cooling process, the temperature difference of the specimen can be less than 0.5%. Moreover, based on the temperature curve of isothermal cooling, nonlinear TCC can be estimated using the proposed method over a broad temperature range, e.g., 500–800 K, and the relative numerical errors will be less than 5% with virtual temperature data. Considering the real measurement condition, we introduce the Savitzky-Golay (SG) filter to reduce the noise’s effect on computation accuracy. Finally, the TCC experiments have been carried out to further verify the isothermal cooling hypothesis and the effectiveness of the method.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Wei Zhang, Xiaoping Chen, Hui Yang, Hong Liang, Yikun Wei〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This work presents the first numerical investigation on the forced convection of flow across two tandem cylinders with rounded corners in a channel at 〈em〉Re〈/em〉 = 100. Both cylinders have the geometry of a square rounded at all corners with a radius of curvature 〈em〉R〈/em〉, which is non-dimensionalized as 〈em〉R〈/em〉〈sup〉+〈/sup〉 = 〈em〉R〈/em〉/〈em〉D〈/em〉 where 〈em〉D〈/em〉 is the cylinder diameter, thus the cylinder geometry can be square (〈em〉R〈/em〉〈sup〉+〈/sup〉 = 0.0), partially rounded (〈em〉R〈/em〉〈sup〉+〈/sup〉 = 0.1–0.4) or circular (〈em〉R〈/em〉〈sup〉+〈/sup〉 = 0.5). The two cylinders are separated at a distance in the streamwise direction as characterized by the parameter of gap ratio (〈em〉GR〈/em〉) chosen at 〈em〉GR〈/em〉 = 1(1)8. The objective of this work is to explore the effects of two significant parameters, i.e., gap ratio and corner radius, on the flow unsteadiness and heat transfer characteristics of the tandem arrangement that has not been studied before. The effects of the two parameters are exhibited and analyzed by the instantaneous temperature and vorticity fields, variation of representative aerodynamic and heat transfer quantities, spatial distributions of local heat transfer rate, flow behaviors in the gap and the near-wake regions, and temperature distribution and variation on the channel wall. The results are presented by time-averaged and fluctuating quantities to reflect both mean and pulsating behaviors. We observed that the cylinder geometry determines the unsteadiness of the near-wake flow after the downstream cylinder; the flow is always unsteady for square-like cylinders where the corner radius is small, while the flow can be stabilized by the circular-like cylinders with larger corner radii that the flow fluctuation is greatly weakened or even fully suppressed at small 〈em〉GR〈/em〉s. Numerical results also reveal that the gap flow is steady at small 〈em〉GR〈/em〉s and unsteady at large 〈em〉GR〈/em〉s, as categorized as 〈em〉steady gap flow regime〈/em〉 and 〈em〉unsteady gap flow regime〈/em〉. There are drastic variations for the representative characteristic quantities at the critical 〈em〉GR〈/em〉 where the gap flow transits from steady to unsteady. The different flow regimes categorized by 〈em〉GR〈/em〉 and 〈em〉R〈/em〉〈sup〉+〈/sup〉 also substantially determine the flow patterns in the gap and near-wake regions, the mean and fluctuating of heat transfer rate on the cylinder surface and the temperature variation on the channel wall.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Masoud Ghasemian, Marko Princevac, Yong W. Kim, Hans D. Hamm〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Both steady and unsteady Reynolds Averaged Navier-Stokes (RANS) techniques coupled with the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈mi〉k〈/mi〉〈mo〉-〈/mo〉〈mi〉ε〈/mi〉〈/mrow〉〈/math〉 and 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.gif" overflow="scroll"〉〈mrow〉〈mi〉k〈/mi〉〈mo〉-〈/mo〉〈mi〉ω〈/mi〉〈mspace width="0.35em"〉〈/mspace〉〈mo stretchy="false"〉(〈/mo〉〈mi mathvariant="italic"〉SST〈/mi〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈/math〉 turbulence models are utilized to study the flow characteristics and hot gas ingestion through rim seal in a subscale 1.5-stage axial gas turbine. A scalar transport equation is solved for a tracer gas to represent the coolant flow interaction with the main stream flow. To validate the numerical methodology, radial pressure and sealing effectiveness distributions are compared with the experimental data. The 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.gif" overflow="scroll"〉〈mrow〉〈mi〉k〈/mi〉〈mo〉-〈/mo〉〈mi〉ω〈/mi〉〈mspace width="0.35em"〉〈/mspace〉〈mo stretchy="false"〉(〈/mo〉〈mi mathvariant="italic"〉SST〈/mi〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈/math〉 turbulence model has the capability to predict secondary flow characteristics, reattachment and separation, thus leading to better agreement with the experimental data. Both radial and circumferential pressure distributions are analyzed to get deeper insight into rotationally and externally induced ingress mechanisms. The circumferential pressure peak-to-trough amplitude is significantly attenuated in the cavity region compared to annulus region. Finally, different purge flow rates and rotational speeds are examined. Results indicate that as purge flow rate increases, the static pressure in the disk cavity region raises remarkably and consequently the sealing effectiveness improves. Averaged sealing effectiveness in the rim cavity decreases linearly with the rotational speed. To visualize different mechanisms of ingestion, streamline and flow field are shown.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Meijie Chen, Yurong He, Qin Ye, Zhenduo Zhang, Yanwei Hu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A nanofluid phase change material (PCM) is prepared by adding a small amount of CuO nanopowders to paraffin, for use in solar thermal conversion, thermal storage, and thermoelectric applications. Results show that adding CuO nanopowders to paraffin can greatly improve the solar thermal conversion capacity by enhancing the light absorption ability of PCMs. The steady temperature increased with increasing mass fraction of CuO NPs at low mass concentrations (0–0.1%). The largest increase was about 2.3 times that of the pure paraffin with comparable latent heat. In the solar thermoelectricity experiments, the open-circuit voltage improved (1.35 V) with the CuO/Paraffin composite (〈em〉f〈/em〉〈sub〉m〈/sub〉 = 0.1%) under the same conditions, and was almost 1.8 times that of the pure paraffin. More importantly, a unique feature of the CuO/Paraffin composite PCM is the extension of thermal release, which enables the continuation of the voltage output when the solar simulator is switched off. The introduction of a very low mass concentration of CuO nanopowders can endow the composite PCMs with strong solar absorption ability, and contribute to realizing efficient solar thermal and solar thermoelectric energy conversion and storage. This provides a new prospect for solar radiation usage efficiency and direct solar energy conversion and utilization.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Ki Wook Jung, Chirag R. Kharangate, Hyoungsoon Lee, James Palko, Feng Zhou, Mehdi Asheghi, Ercan M. Dede, Kenneth E. Goodson〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Single-phase thermal-fluidic performance of an embedded silicon microchannel cold-plate (25 parallel channels: 75 μm × 150 μm) with a 3-D liquid distribution manifold (6 inlets: 700 μm × 150 μm) and vapor extraction conduits, is investigated using water as working fluid. A 3D manifold is fabricated from silicon and bonded to a silicon microchannel substrate to form a monolithic microcooler (μ-cooler). A metal serpentine bridge (5〈sup〉2〈/sup〉 mm〈sup〉2〈/sup〉 of footprint) and multiple resistance temperature detectors (RTDs) are used for electrical Joule-heating and thermometry, respectively. The experimental results for maximum and average temperatures of the chip, pressure drop, thermal resistance (as low as 0.68 K/W), average heat transfer coefficient (∼30,000–50,000 W/m〈sup〉2〈/sup〉 K) for flow rates of 0.03, 0.06 and 0.1 l/min and heat fluxes of 60, 100 and 250 W/cm〈sup〉2〈/sup〉 are reported. The embedded microchannel-3D manifold μ-cooler device is capable of removing 250 W/cm〈sup〉2〈/sup〉 at a maximum temperature of 90 °C with less than 3 kPa pressure drop for a flow rate of 0.1 l/min. The results from conjugate thermal-fluidic numerical simulations agree well with the experimental data over the wide range of heat fluxes and flow conditions. The numerical simulation results also hint at the possibility of removing up to ∼850 W/cm〈sup〉2〈/sup〉 using single-phase water at a maximum temperature of 166 °C at the same pressure drop and flow rate. This offers a very attractive strategy/option for cooling of high heat flux power electronics using single-phase water.〈/p〉〈/div〉 〈/div〉 〈div xml:lang="en"〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S0017931018326206-ga1.jpg" width="159" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Qi Peng, Li Jia, Chao Dang, Zhoujian An, Yongxin Zhang, Liaofei Yin〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Condensation is a ubiquitous phase-change phenomenon in nature and has been widely adopted in various energy-intensive industrial application. Many efforts have been focused on regulating droplet dynamics to enhance the condensation heat transfer by developing micro/nanostructured surface. In this work, a microgrooved surface with CuO nanostructures was fabricated by combination of simple machining and scalable self-limiting chemical oxidation method for regulating droplet dynamic behavior. The wettability, droplet dynamics and heat transfer characteristics on such surface were compared with that on plain and microgrooved hydrophobic surfaces. The anisotropic wettability was observed on microgrooved surface and enhanced by creating nanostructures. 15–43% higher heat flux was reached on microgrooved hydrophobic surface compared to plain hydrophobic surface due to an increase of effective heat transfer area, the sweeping effect of liquid columns for droplets on adjacent plateaus and cooperation of liquid columns flowing and liquid bridges sliding. Several novel droplet dynamic behaviors that the suspension of large droplets, suction of spindle-shaped droplets and strong self-oscillation of falling droplets were observed on the microgrooved surface with nanostructures and enhanced the condensation heat transfer. Compared with plain hydrophobic surface, the heat flux was enhanced up to 55–102%.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): J.Y. Ho, K.C. Leong, T.N. Wong〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉An experimental investigation on the use of sinusoidal pin fins to enhance filmwise condensation of steam on vertical plates was carried out. Nine surfaces with pin fin arrays were fabricated by Selective Laser Melting (SLM) and tested in a condensation chamber. The pin fin arrays have the same fin base diameter (〈em〉d〈sub〉b〈/sub〉〈/em〉) but are of different fin heights (〈em〉l〈/em〉) and fin pitches (〈em〉p〈/em〉). At the same fin pitch (〈em〉p〈/em〉 = 1.25 mm and 1.67 mm), the heat flux (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈msup〉〈mrow〉〈mi〉q〈/mi〉〈/mrow〉〈mrow〉〈mo〉″〈/mo〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉) and condensation heat transfer coefficient (〈em〉h〈/em〉) increase as 〈em〉l〈/em〉 increases from 1.25 mm to 1.66 mm. However, with further increment in 〈em〉l〈/em〉 from 1.66 mm to 2.49 mm, reductions in 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈msup〉〈mrow〉〈mi〉q〈/mi〉〈/mrow〉〈mrow〉〈mo〉″〈/mo〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉 and 〈em〉h〈/em〉 were observed. At the same fin heights of 〈em〉l〈/em〉 = 1.25 mm and 1.66 mm, an increase in 〈em〉p〈/em〉 from 1.25 mm to 1.67 mm has negligible effects on 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈msup〉〈mrow〉〈mi〉q〈/mi〉〈/mrow〉〈mrow〉〈mo〉″〈/mo〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉 and 〈em〉h〈/em〉. However, with further increment in 〈em〉p〈/em〉 from 1.67 mm to 2.50 mm, significant reductions in 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈msup〉〈mrow〉〈mi〉q〈/mi〉〈/mrow〉〈mrow〉〈mo〉″〈/mo〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉 and 〈em〉h〈/em〉 were observed. Visualization studies of the static condensate retention height (〈em〉H〈sub〉ave〈/sub〉〈/em〉) show that two distinct flooding regions can be identified for sinusoidal pin fin surfaces. The static condensate retention height of both regions decreases with increasing 〈em〉p〈/em〉 but remains unchanged with varying 〈em〉l〈/em〉. A fin analysis was performed to determine the average heat transfer coefficients (〈em〉h〈sub〉t〈/sub〉〈/em〉) which considers the total heat transfer areas (〈em〉A〈sub〉t〈/sub〉〈/em〉) of the enhanced surfaces. Our results show that the highest thermal enhancement factor (η) of 1.86 is achieved with Specimens 〈em〉S〈/em〉4 and 〈em〉S〈/em〉5 whereas Specimens 〈em〉S〈/em〉3 has the highest average heat transfer coefficient (〈em〉h〈sub〉t〈/sub〉〈/em〉). In comparison with cylindrical pin fin surfaces, the sinusoidal pin fin surfaces show better heat transfer performances at the same 〈em〉p〈/em〉/〈em〉l〈/em〉 ratio.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Deepshikha Bhargava, Nopbhorn Leeprechanon, Phadungsak Rattanadecho, Teerapot Wessapan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Accidental overexposure to non-standard mobile phone radiation can occur in many situations. The overpower limit of mobile phone radiation interacts with the human body which could result in an adverse effect on human health. It is envisaged that the severity of the physiological effect can take place with small temperature increase in the delicate organs or tissues such as eyes, brain, skin, etc. However, the resulting thermo-physiological response of the body tissues to overpower limit of mobile phone radiation is still not well implemented. The aim of this study is to analyze the effect of overexposure of mobile phone radiation on the specific absorption rate (SAR) and temperature increase in three-dimensional heterogeneous human head models. The study focuses attention on the differences in the electromagnetic (EM) absorption characteristics with higher power level among different usage pattern. The effect of three different usage patterns - voice calling, video calling, and texting- on SAR and temperature distributions in different types of head tissues is systematically investigated. This paper also investigates the effects of different user ages, radiated powers, and gap distances between mobile phone and human heads, on SAR and temperature distributions. Results obtained from this analysis considering the safety guidelines show a high impact of mobile phone radiation in the voice calling position. Hence, comparisons of the absorption of mobile phone radiation are calculated between an adult and a 7-year-old child head model, for the voice calling position at different gap distances. In addition, the results indicate that child head always has a higher absorption rate of mobile phone radiation than the adult head. The rate of absorption in tissue increases as the distance between mobile phone and head decreases and the radiated power increases, depending on their dielectric and thermal properties. The obtained results can be helpful in determining exposure limits for the power output of the mobile phone, and the distance a user should maintain from the mobile phone in thermo-physiological aspects.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 130〈/p〉 〈p〉Author(s): Syed Muhammad Ammar, Naseem Abbas, Saleem Abbas, Hafiz Muhammad Ali, Iftikhar Hussain, Muhammad Mansoor Janjua〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Condensation frictional pressure drop was obtained in multiport micro channel smooth and grooved flat tubes using R134a. The study consists of two smooth and one grooved tube. Refrigerant R134a is studied over a mass flux 490–1600 kg/m〈sup〉2〈/sup〉s, heat flux 5.5–19 kW/m〈sup〉2〈/sup〉 and saturation temperature 51–68 °C. Results reveal that frictional pressure drop increases with an increment of mass flux, vapor quality and decreases with the increase of saturation temperature. The effect of heat flux on frictional pressure drop was negligible. Grooved tube shows the lowest pressure drop and surprisingly highest heat transfer coefficient. The frictional pressure drop data was compared with famous correlations. Zhang and Webb predicted 86% of the data with in ±30%.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): V.B. Zametaev, A.R. Gorbushin, I.I. Lipatov〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Turbulent mixing layer between the moving 2D viscous incompressible fluid and fluid in rest is studied. The characteristic Reynolds number of the flow is assumed to be large and the layer thickness being small. To analyze the problem, the method of multiple scales was applied, which allowed to find and investigate the steady secondary flow inside the turbulent mixing layer. Self-induced entrainment of fluid from the external stream is the main flow in this case, which ensures the supply of kinetic energy from the maximum speed zone to the turbulence generation zone. Secondary steady solutions were found analytically for the longitudinal velocity component. The found solutions were compared with the available experimental data. Understanding of turbulence nature has a principal value for mass and heat transfer in different flows.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Xueli Wang, Zan Wu, Jinjia Wei, Bengt Sundén〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Based on a modified force balance model, new correlations were proposed for the prediction of vapor bubble departure radius in saturated and subcooled pool boiling under atmospheric pressure. To predict the departure radius, the wall temperature and contact angle are two important input parameters. Instead of the static contact angle, the present correlations use the dynamic advancing contact angle at root of the bubble base at the moment before bubble detachment (i.e., the maximum dynamic advancing contact angle) to calculate the bubble departure radius. The results show that for the bubble departure radius obtained in this study, the developed correlation can predict all the data points within a maximum error of 3.8% in both normal earth gravity and 0.01g〈sub〉e〈/sub〉 reduced gravity. Moreover, for data sets in the literature including 1g saturated boiling, 1g subcooled boiling, saturated boiling in reduced gravity, and subcooled boiling in reduced gravity, it is also demonstrated that compared with the thirteen existing correlations, the proposed correlations exhibit a big improvement in predicting bubble departure radius.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Quanbin Zhao, Weixiong Chen, Lutao Wang, Daotong Chong, Junjie Yan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The pressure oscillation characteristic of steam jet condensation in water was really important for the safety operation of related devices. In present study, the pressure oscillation characteristic of steam condensation through two and three holes would be investigated, and the character of dominant oscillation frequency with operation conditions (steam mass flux and water temperature) would be investigated. Meanwhile, the impact of geometrical factors (hole pitch and number) would also be analyzed. The experimental results indicated that the dominant oscillation frequency would decrease with water temperature increasing. Then as steam mass flux increased, the dominant frequency had a peak value for three holes nozzle, which always appeared when the steam mass flux varied from 250 to 300 kg·m〈sup〉−2〈/sup〉·s〈sup〉−1〈/sup〉. The dominant frequency increased as the hole pitch increased, and the dominant frequency difference between hole pitches gradually grew smaller as the water temperature raised. Meanwhile, the dominant frequency would decline as hole number increased. Besides, a experimental correlation considering the effect of hole number and pitch was applied to predict dominant oscillation frequency, while the maximum deviation was less than 15%.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Juan C. Godinez, Dani Fadda, Jungho Lee, Seung M. You〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The effects of an aluminum high temperature conductive microporous coating (Al-HTCMC) on the nucleate boiling heat transfer (NBHT) coefficient and critical heat flux (CHF) are studied in saturated distilled water at 1 atm. Aluminum powders with three different mean particle diameters (d〈sub〉m〈/sub〉 = 11, 24, and 66 µm) are used in the fabrication of the Al-HTCMC. For each mean particle diameter, an optimal coating thickness to yield the highest NBHT coefficient is determined. The optimized Al-HTCMC thickness is found to result in comparable NBHT coefficients regardless of the particle diameter. Pool boiling tests with a plain aluminum surface are used for comparison. The coated and plain aluminum surfaces are treated equally before the pool boiling tests to establish a Boehmite oxidation nano layer on the aluminum surfaces. Following the Boehmite treatment, the contact angle is unmeasurable (∼0°) with the Al-HTCMC surface and 12° with a plain aluminum surface. Then, pool boiling tests are performed and reveal comparable CHF (1725–1850 kW/m〈sup〉2〈/sup〉) values with or without the Al-HTCMC. However, the Al-HTCMC is shown experimentally to improve the NBHT coefficient by a factor of five as the wall superheat is reduced by from 31 K to 6 K just before CHF. The results obtained are also compared to similar work using an HTCMC layer on a copper surface to demonstrate the performance of the Al-HTCMC.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Xinting Wang, Yunmin Liang, Yue Sun, Zhichun Liu, Wei Liu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this paper, the heat transfer and flow performance of a double shell-pass rod baffle heat exchanger (DS-RBHX) is investigated experimentally. Likewise, a single shell-pass rod baffle heat exchanger (SS-RBHX) is set as the control. Water serves as the working fluid both in the shell side and tube side. Experimental results indicate that the overall heat transfer coefficient of the DS-RBHX is higher than that of the SS-RBHX for all measurements. As the shell-side volume flow rate varies from 2.8 to 15.2 m〈sup〉3〈/sup〉/h, the shell-side heat transfer coefficient and pressure drop of the DS-RBHX increase by 33.5–54.0% and 34.0–74.3%, respectively. From the perspective of the comprehensive performance, the shell-side heat transfer coefficient of the DS-RBHX is 14.4–24.3% higher than that of the SS-RBHX under the same shell-side pressure drop. Consequently, it is proved that the DS-RBHX has better comprehensive performance compared with the SS-RBHX. On the basis of experimental results, numerical studies are conducted to analyze the shell-side behaviors of the DS-RBHX further. According to numerical results, three kinds of guide shells, arranged at the end of the sleeve, are proposed to reduce the flow dead zone in the shell-side outlet zone. The behaviors of DS-RBHXs with the guide shell (DS-RBHX-GSs) are obtained numerically. The results show that all three guide shells improve the heat transfer performance of the shell-side outlet zone, particularly in the outer side. Moreover, the guide shell of the DS-RBHX-GS2 has more significant effects than the others.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Yeong Woong Oh, Yoon Suk Choi, Man Yeong Ha, June Kee Min〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A numerical study on the natural convection heat transfer of slanted-pin fins mounted on a vertical wall has been conducted for the laminar flow regime. Assuming the air is the ideal gas, three-dimensional governing equations of flow and heat transfer was solved using the periodic boundary condition in the horizontal lateral direction with the SIMPLE algorithm. The effect of radiation heat transfer was considered using the discrete ordinate method based on the evidence of the code validation through a comparison with experimental data. The effects of the fin-inclination angle ranging from −45° to + 45° and the aspect ratio of the rectangular-fin pins for 0.25–4.0 were examined for a modified Rayleigh number range of 5.2 × 10〈sup〉10〈/sup〉–1.3 × 10〈sup〉10〈/sup〉 under constant heat flux conditions. For the positively-inclined fins, the enhancement of the heat transfer performance on the heated plate and fin-side surface was captured, which is similar to previous forced convection studies. For negative inclination angles, however, it was observed that the penetration of cold air from the quiescent region affects the heat transfer coefficient distribution on the top side of the fins in the vertical fin interspacing. As a result, in the present calculation, the negatively-inclined fins showed the best heat-transfer performance under a natural convection condition. Details on the buoyant-flow and heat transfer characteristics, such as the distributions of the local- and average-heat transfer coefficient, are quantitatively summarized.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): M.C.F. Silva, J.B.L.M. Campos, J.D.P. Araújo〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The mechanism of wall-liquid mass transfer of a solute in micro-scale systems has a huge relevance in many practical scenarios with particular interest for medical devices. A possible enhancement on this kind of phenomenon through the application of slug flow regime was studied with CFD techniques. Different flow conditions were considered to enable the inspection on the distinct hydrodynamics that may occur on the Taylor bubble surroundings in micro-scale. The VOF methodology was used to track the gas–liquid interface and the mass and hydrodynamic fields were simultaneously solved.〈/p〉 〈p〉The effects of the bubble passage on the mass transfer from a finite soluble wall to the flowing fluid were analyzed for each flow condition, and the corresponding mass transfer coefficients were quantified.〈/p〉 〈p〉Overall, this numerical work indicates that the flow due to the presence of one Taylor bubble leads to a moderate increase of the wall-liquid mass transfer coefficients. This increase can be enhanced if, instead of one, a continuous flow of bubbles is considered. The abrupt variation on the wall shear stress induced by the bubble movement is important to promote the referred increase.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Chen Zeng, Sichao Tan, Shouxu Qiao, Fulong Zhao, Tao Meng〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉To calculate the thermal characteristics of the rectangle liquid droplet radiator more accurately, a novel method, which estimated the characteristics of each droplet, was introduced. The shape factors of two and three droplets were theoretically deduced and verified by the Monte Carlo method. It was found that the shape factor of two droplets depends only on the ratio of the distance between the two droplets to the radii of droplets. According to this characteristic, the shape factor of two droplets was calculated and fitted to be a formula, which makes it convenient to calculate the shape factor in the following analysis. Based on the superposition rule of the shape factor, a shape factor matrix was introduced to calculate the effect of surrounding droplets when analyzing the heat transfer of the droplet sheet. The temperature distribution of the droplet sheet was calculated based on the shape factor matrix, which obtained using the combination of theoretical method and Monte Carlo method. The results showed that the temperature difference among the droplets on a same width-thickness plane was negligible. Therefore, a simplified model was developed based on this characteristic. The discrepancies of the temperature distributions obtained by the original model and the simplified model were found to be less than 0.35%, which demonstrates the feasibility of the simplified model to analyze the thermal characteristics of the rectangle droplet radiator. By further analyzing the heat transfer characteristics of the droplet sheet, proposals were made for design of the rectangle liquid droplet radiators.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Zhongyan Mu, Xin Chen, Zengchao Zheng, Anguo Huang, Shengyong Pang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Current laser-plasma interaction theory supports that the plasma energy e.g. electron temperature would increase by the effect of inverse bremsstrahlung (IB) absorption, when a laser beam passed through the plasma. However, in this paper, we found an interesting laser cooling arc plasma effect (LCAPE) during kilo-Watt fiber laser-TIG hybrid welding. Based on theoretical modelling and experiments, we observed that a temperature decrease of more than 5000 K at the tail of the argon plasma occurred under different process parameters during hybrid welding of 316L stainless steel. We proposed the LCAPE is caused by the laser-induced metal vapor. The mechanism mainly includes the convection cooling and enhanced radiation of the arc plasma by the metal vapor. Our findings could broaden the theory of laser-plasma interaction and provide a theoretical reference to the modulation and control of plasma in industries.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): J.M. Gorman, E.M. Sparrow, J. Ahn〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This investigation presents numerical results for both laminar and turbulent flow and heat transfer for an in-line tube bank ranging in size from 1 to 20 tube rows over a Reynolds number range of 100–1000. Both the longitudinal and transverse pitches were fixed at 1.5D. It was demonstrated that the most useful heat transfer results were expressible as a total-tube-bank-averaged Nusselt number value, which is in contradiction to other investigations. For sufficiently lengthy tube banks, the existence of a fully developed regime characterized by an axially unchanging array-average Nusselt number was identified. It was found that the highest array-average heat transfer coefficients occurred in the initial portion of the tube bank, also in contradiction with information conveyed in some of the literature. A special case in which only a single tube in the array was thermally active was investigated in deference to experiments conducted under that condition. The present results obtained by numerical simulation compared favorably to existing experimental data.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Changkun Li, Dewen Zhao, Jialin Wen, Xinchun Lu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Marangoni drying induced by organic vapor is an important process in integrated circuit manufacturing to obtain an ultra-clean surface, but its application is limited by the utilization of flammable vapor. Thermal Marangoni drying induced by a temperature gradient is a safe and environmentally friendly technology. However, the regulation of thermal Marangoni-driven flow is complicated, and current understanding of the thermal Marangoni drying mechanism is inadequate. In this work, we present a coupled model of thermal Marangoni drying that combines the two-phase flow, heat transfer, and water vapor transfer in air. The evolution of entrained water film thickness, dynamics of Marangoni-driven flow, and evaporative cooling are numerically simulated. The results show that the achieved minimum residual thickness of the entrained water film is thinned more than tenfold compared with that of the wafer withdrawn without the thermal Marangoni effect. The temperature rise and Marangoni stress grow dramatically in the film and meniscus at the initial time, and then they remain as almost invariant after achieving the dynamic equilibrium between the heating and evaporative cooling. The convective water vapor transfer in air and the reduction of entrained water film thickness improve the evaporative flux, which in turn suppresses the increase in the thermal Marangoni effect until the dynamic equilibrium is attained. Furthermore, the higher heat source, lower wafer thermal conductivity and smaller wafer thickness can enhance drying performance. The investigation of drying dynamics will contribute to a comprehensive understanding of the thermal Marangoni drying process and provide guidance to its industrial applications.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Sergei V. Ryzhkov, Victor V. Kuzenov〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉An approximate mathematical model of the heat transfer processes in the laminar and turbulent boundary layers, which occur near the surface of an aircraft moving at the hypersonic speed in the Earth’s atmosphere, is derived. This mathematical model makes it possible to calculate the convective heat transfer on the surface of typical structural elements of modern perspective aircrafts. 2D versions of the calculations of convective heat fluxes for bodies of simple geometric shapes are performed.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Xuehong Chen, Fengzhong Sun, Youliang Chen, Ming Gao〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The heat transfer deterioration that occurs in the inner rain zone weakens the thermal performance of natural draft wet cooling towers (NDWCTs). Existing NDWCT retrofit methods including the air deflectors and the cross wall have limited effects on this deterioration. In this paper, we propose a new retrofit method in which air ducts are installed in the rain zone and air deflectors are installed around the air inlet to improve the total tower thermal performance. To clarify the effect and mechanism of our retrofit method, a hot test for a NDWCT model is performed under various crosswind velocities, and a 3D numerical model for a NDWCT with air deflectors and air ducts is established and validated. Using the proposed method, the thermal performance of a NDWCT is substantially improved with less crosswind sensitivity. It is found that the flow diversion efficiency of the air deflectors weakens the adverse impact of the ambient crosswind on air inflow of the tower, and the additional ambient air introduced through the air ducts enhances the heat transfer in the central rain zone. Compared with the single effect of the air deflectors or the cross wall, the combined effect of the air ducts and air deflectors is more efficient in improving the thermal performance of NDWCTs.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Chun Wei Zhang, Hai Zhou, Yong Zeng, Lei Zheng, Yue Lin Zhan, Ke Dong Bi〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The thermal conductivity (TC) of non-periodic Si/Ge superlattice nanowire (SLNW) is investigated by non-equilibrium molecular dynamics simulation (NEMD). It is found that the TC of non-periodic Si/Ge SLNW can be significantly reduced at room temperature. Compared to the minimum TC of periodic Si/Ge SLNWand pure Si nanowire, the TC of non-periodic Si/Ge SLNW is further decreased to be around 47.4%, 4.4% of that of periodic Si/Ge SLNW and pure Si nanowire respectively. By introducing 20% Ge atom doping, the TC of non-periodic Si/Ge SLNW with 10-unit cell (UC) thickness is reduced by 38%. The reduction of TC of non-period Si/Ge SLNW is first due to the destruction of phonon coherent transport. Additionally, the change of periodic length can cause shift of density of state (DOS), which makes the interface of Si/Ge with different lengths play roles to scatter the phonon with different wavelengths. Therefore, the distribution of thickness from atomic to nano scale can decrease the TC by scattering phonon with more wavelengths. The results provide an efficient way for future thermoelectric applications.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Seong Jin Chang, Seunghwan Wi, Jongki Lee, Sumin Kim〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Various industries require production equipment for uniform mass-production short periods of time, such as in the construction industry. This study presents vacuum impregnation equipment that is capable of mass-producing uniform, shape-stabilized PCM (MUSPCM). The vacuum impregnation equipment can manufacture MUSPCM of various diameters without additional filtering and crushing process because the mixing ratio of the SSPCM was analyzed beforehand through a TGA analysis. As the result, MUSPCM using vacuum impregnation equipment can save 32.5 times based on production of 15.0 kg compared to conventional vacuum impregnation. In addition, the analysis of the thermo-physical properties and long-term stability of the MUSPCM were characterized via DSC and TGA analysis. The results indicate that a large amount of uniform MUSPCM were produced, and the long-term phase stability was excellent. Therefore, the developed vacuum impregnation equipment is expected to be useful in various industries, including construction.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Bin Zhao, Yi Ren, Diankui Gao, Lizhi Xu, Yuanyuan Zhang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In order to study the heat transfer rule of microreactor in depth, the Bandelet finite element model is established using Bandelet basis function as interpolating function. Firstly, the mathematical model and property of Bandelet transformation are analyzed. Secondly, the heat transfer Bandelet finite element model of microreactor is constructed. Finally, the effectiveness of Bandelet finite element method is verified through comparing analysis between simulation and experimental results, the computing precision and accuracy of heat transfer of microreactor can be improved based on the proposed method. In addition, the relationship between Nusselt number and Reynolds number is analyzed. The inlet velocity has obvious effect on the heat transfer of microreactor. The influence of main geometric sizes concluding height and diameter of micro-pin-fin on heat transfer performance of microreactor is obtained. The analysis results can offer favorable basis for optimization of microreactor and enrich the theoretical system of heat transfer of microreactor.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Manav Vohra, Sankaran Mahadevan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This paper develops an efficient methodology for both forward and inverse problems in uncertainty quantification with respect to molecular dynamics simulation. Specifically, our objectives are to investigate the impact of uncertainty in the Stillinger-Weber (SW) potential parameters on NEMD-based predictions of bulk thermal conductivity of silicon (forward problem), and perform a Bayesian calibration of these parameters using experimental data (inverse problem). However, both analyses typically require tens of thousands of model evaluations and therefore, relying purely on atomistic simulations would be impractical. The common strategy of building a surrogate model in the space of the uncertain parameters is also unaffordable due to the need for many training evaluations using atomistic simulations. Therefore, computational effort is minimized in this paper by reducing the dimensionality of the input space of the surrogate model by first computing the so-called active subspace. The active subspace is found to be 1-dimensional, indicating enormous scope for dimension reduction and computational savings. A surrogate model is then built in the 1-dimensional subspace to help quantify the variability of the bulk thermal conductivity, and is shown to have reasonable accuracy. The active subspace is also used to perform efficient global sensitivity analysis (GSA) of the SW parameters. Finally, we use the active subspace-based surrogate model for fast calibration of SW parameters in a Bayesian setting.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Yanchu Liu, Shuangfeng Wang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉An experimental study was conducted in order to investigate the phase distribution of gas-liquid slug flow in six parallel micro-channels in presence of different branch spacing of 0.8 mm(2〈em〉d〈/em〉), 4 mm(10〈em〉d〈/em〉) and 12 mm(30〈em〉d〈/em〉), respectively. The parallel micro-channels was composed of a header with hydraulic diameter of 0.48 mm and six branch channels with hydraulic diameter of 0.40 mm, all with rectangle cross sections. The entire test section was machined by PMMA to facilitate flow visualization. Nitrogen and 0.03 wt% sodium dodecyl sulfate (SDS) solution at ambient pressure and room temperature were used as the test fluids. A flow-regime map of bubbly, slug, slug-annular and annular was generated, covering the range of gas and liquid inlet superficial velocities of 0.28 ≤ 〈em〉J〈sub〉G〈/sub〉〈/em〉 ≤ 33.3 m/s and 0.008 ≤ 〈em〉J〈sub〉L〈/sub〉〈/em〉 ≤ 2.52 m/s, respectively. The phase distribution experiments of slug flow were conducted. The fluid dynamics of two-phase flow splitting in parallel micro-channels was captured by high speed recording technique. It was found that the phase distribution characteristics of two-phase flow in parallel channels highly depend on the inlet flow conditions and the distance between channels. Besides, the effect of branch spacing took on distinct characteristics under different inlet flow conditions. At low mass flux and quality, the increase of branch spacing can facilitate the liquid phase to flow into channels at the rear part of the header, while the first three channels are more supplied with liquid as the branch spacing increasing at high mass flux and quality. Specially, an improvement of gas distribution was also observed with the increase of the branch spacing. Finally, a correlation capable of predicting the liquid phase distribution of slug flow in parallel micro-channels was developed.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Edwin Martin Cárdenas Contreras, Guilherme Azevedo Oliveira, Enio Pedone Bandarra Filho〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This paper presents an experimental investigation of the thermohydraulic performance of nanofluids, composed of graphene and silver nanoparticles with a binary mixture of equal parts of water and ethylene glycol (50:50 vol%) as a base fluid, in automotive radiators. The nanofluids were prepared by high pressure homogenization method with volumetric concentrations of 0.01%, 0.05% and 0.1%. The thermophysical properties were measured experimentally and compared with correlations and others results of similar research found in the literature. The nanofluids were tested in an automotive radiator installed in a wind tunnel, simulating the operation of an automotive cooling system. The experiments were conducted at mass flow rates between 0.08 and 0.11 kg/s, with coolant inlet temperatures between 55 and 85 °C. The air velocity on the radiator was kept constant at 2.1 m/s. The heat transfer rate and the pumping power of the fluids tested were determined under the test conditions stipulated. With regard to the pumping power at high temperatures and mass flow rates, the nanofluids showed increases up to 4.1%. The silver nanofluids produced an increase up to 4.4% in the heat transfer rate, while the graphene samples demonstrated a decrease in thermohydraulic performance when compared with the base fluid.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Hao Jia, Bin Chen, Dong Li〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Port wine stains (PWSs) are congenital dermal vessel proliferations mainly treated with laser therapy. The complete removal of the vessel lesions is rarely achieved because of a lack of discriminatory analysis of the two competitive laser damages to blood vessels, namely, pressure damage and thermal damage. Unlike complete vessel constriction, which is caused by thermal damage that can be measured by temperature-related integral Ω, vessel rupture results from pressure damage, which has been seldom studied. In this study, the rupture potential index based on wall pressure (RPIP) was calculated as the ratio of locally acting pressure to the pressure threshold. RPIP 〉 1 and Ω 〉 10〈sup〉3〈/sup〉 were adopted as benchmarks to judge pressure damage (vessel rupture) and thermal damage (complete vessel constriction), respectively. A computational fluid dynamics simulation was carried out to provide the temperature and pressure field in the PWS vessel model during irradiation by 595 nm pulsed dye laser (PDL) or 1064 nm Nd:YAG laser. Numerical results showed that for the 595 nm laser, vessels constantly underwent rupture. The area of high RPIP determined the degree of rupture by predicting the large and multiple rupture locations of the vessel. By contrast, for the 1064 nm laser, complete constriction was the main damage type. To a single vessel of 100 μm diameter, the optimized laser parameters were 〈em〉E〈/em〉 = 10 J/cm〈sup〉2〈/sup〉 with 〈em〉t〈sub〉p〈/sub〉〈/em〉 = 6 ms for 595 nm PDL and 〈em〉E〈/em〉 = 180 J/cm〈sup〉2〈/sup〉 with 〈em〉t〈sub〉p〈/sub〉〈/em〉 = 6 ms 1064 nm for Nd:YAG laser.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): T.P. Lyubimova, R.V. Skuridin〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The effect of acoustic wave propagating in the direction of temperature gradient on a stability of stationary plane-parallel flow of binary fluid, generated by horizontal temperature and concentration gradients in a horizontal layer is investigated for two types of thermal boundary conditions: perfectly conductive and adiabatic boundaries. The study is performed analytically for the longwave perturbations and numerically for finite-wavelength perturbations. Stability maps for fixed Prandtl number value 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈mi mathvariant="italic"〉Pr〈/mi〉〈mo〉=〈/mo〉〈mn〉0.01〈/mn〉〈/mrow〉〈/math〉 and different values of Schmidt number and acoustic Reynolds number are obtained. The most dangerous instability modes and critical perturbation structure are discussed. Comparison with the case where acoustic wave is absent is performed. It is shown, that longwave perturbations are always stabilized by acoustic wave. Numerical investigation demonstrates, that acoustic wave also stabilizes the flow with respect to finite-wavelength perturbations when negative concentrational Grashof number is higher in modulus than certain value depending on acoustic wave intensity.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Jin Wang, Guolong Li, Hengxuan Zhu, Jing Luo, Bengt Sundén〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Some experimental tests were conducted to reveal the enhancement of the ferrofluid heat transfer under a permanent magnetic field. This research aims to investigate the effect of various external magnetic fields on convective heat transfer characteristics of the ferrofluid (magnetic nanofluid). Comparison of theoretical predictions and experimental data were conducted to validate the rationality of the test results, and a good agreement with less than 10% deviations was found. The deviations from experimental data decrease with an increase of the Reynolds number (〈em〉Re〈/em〉) from 391 to 805. Results from the case with 5 cannulas indicate that a continuous increase in the magnetic flux density (by increasing the quantity of the magnets) can improve the heat transfer enhancement significantly. The ferrofluids with a magnetic cannula shows heat transfer enhancements of 26.5% and 54.5% at 〈em〉Re〈/em〉 = 391 and 805, respectively.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): M.K. Khasanov, M.V. Stolpovsky, I.K. Gimaltdinov〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Based on the equations of mechanics of multiphase media, a mathematical model of the injection of liquid carbon dioxide into a natural reservoir saturated with methane and its gas hydrate is constructed. Numerical solutions for two different flow regimes are obtained and investigated. In the first regime, methane is replaced by carbon dioxide in the gas hydrate without the release of free water. In the second mode, gas hydrate decomposes into methane and water, and further formation of CO〈sub〉2〈/sub〉 gas hydrate from water and carbon dioxide occurs. Numerical calculations show that the first mode is realized at low values of permeability and injection pressure, as well as high values of the temperatures of injected carbon dioxide, initial temperature of the reservoir and pressure at the right boundary of the reservoir. For each regime, the conditions under which the pressure in the liquid carbon dioxide filtration region can drop below the equilibrium boiling point of CO〈sub〉2〈/sub〉 are investigated. Numerical calculations show that this is possible at low injection pressures and pressures at the right boundary of the reservoir. Critical diagrams were constructed that determine the “injection pressure-pressure on the right-hand boundary of the reservoir”, “injection pressure-permeability”, “injection pressure-injection temperature” and “injection pressure-initial temperature of the reservoir” in the region of existence of four qualitatively different flow regimes.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Aniket M. Rishi, Satish G. Kandlikar, Anju Gupta〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Graphene nanoplatelets (GNP) are known for their excellent thermal and mechanical properties making them suitable candidates for a variety of engineering applications. In this work, a novel GNP/Cu porous coating obtained via a multistep electrodeposition technique is presented and tested for their efficacy in improved critical heat flux (CHF) and heat transfer coefficient (HTC). An array of hierarchical porous coatings was obtained by systematically increasing the GNP concentration in the electrodeposition bath and were found to be superhydrophilic with very high wicking rates. Our pool boiling tests indicate that 2% GNP/Cu (wt/vol.) surfaces yielded a CHF of 286 W/cm〈sup〉2〈/sup〉 and a heat transfer coefficient of 204 kW/m〈sup〉2〈/sup〉-°C, representing an improvement of 130% in CHF and 290% in HTC compared to pristine copper surfaces. The reported CHF and HTC represent the highest values reported in the literature till date for pool boiling on a plain surface. This enhancement in heat transfer properties is attributed to the hierarchical pores that serve as the nucleation sites and influence the overall bubble dynamics that is responsible carry the heat between liquid and vapor phases. The porous surfaces also improved the surface wickability and wettability that further promoted nucleation and microlayer evaporation.〈/p〉〈/div〉 〈/div〉 〈div xml:lang="en"〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S0017931018345769-ga1.jpg" width="500" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Tao Wen, Yimo Luo, Weifeng He, Wenjie Gang, Liyuan Sheng〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The present study proposed a novel quasi-3D model for simultaneous simulation of heat and mass transfer process in a falling film dehumidifier with penetration mass transfer theory and CFD technology. Different from the existing 2D model, the new model took the shrinkage of falling film into consideration by introducing variable film wettability. Besides, an experimental system with a single channel falling film dehumidifier was designed and built up to validate the developed quasi-3D model. Results indicated that the shrinkage model could predict the flow pattern accurately, and the relative different between calculated and experimental wetting ratio was less than 4%. Combined with the shrinkage model, the CFD model was proved to be reliable to simulate the dehumidification process with the relative deviation of absolute moisture removal less than 10%. It was found that the non-wetting of falling film and mass transfer resistance in the air side hindered the dehumidification performance. Consequently, super-hydrophilic coating and curved plate were proposed to enlarge the wetting ratio and decrease the mass transfer resistance in the air side. By adopting the nano-TiO〈sub〉2〈/sub〉 coating, the wetting ratio of dehumidifier increased from 81% to 97.8%, which led to a relative improvement of 15.2% for absolute moisture removal. In addition, the employment of curved dehumidifier could enhance the dehumidification performance by 70%.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 131〈/p〉 〈p〉Author(s): Young Min Seo, Man Yeong Ha, Yong Gap Park〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This study conducted three-dimensional numerical simulations of the natural convection in a cold, long, rectangular enclosure containing a hot elliptical cylinder. The immersed boundary method (IBM) was used to capture the virtual wall boundary of the inner cylinder based on the finite volume method (FVM). The effect of inclination angles of 0° ≤ 〈em〉ψ〈/em〉 ≤ 90° were analyzed by a visualization technique with various radii of the cylinder at a relatively high Rayleigh number of 〈em〉Ra〈/em〉 = 10〈sup〉6〈/sup〉. The variation in the flow and thermal structures and the corresponding heat transfer characteristics were investigated in regard to the transition of the flow regime from the steady state to the unsteady state. The heat transfer characteristics from the cylinder surface and enclosure walls were also addressed. As a result, the heat transfer characteristics were improved by the variation in the inclination angle in accordance with the mean radius of the elliptical cylinder.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 131〈/p〉 〈p〉Author(s): Jia-Hui Huang, Shuang-fei Li, Zhen-Hua Liu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉An experimental study on thermosyphon boiling in 3-D micro-channels with inclined angle changed from 90° to 0° was carried out, to develop a new passive cooling technology for 3-D chip cooling. The specific micro-channel structure of actual 3-D chip was simulated as evaporating section of thermosyphon, and the critical heat flux and heat transfer coefficient of 3-D micro-channel thermosyphon boiling in different inclined angles were studied in detail. Experiments were carried out using two kinds of working liquids of deionized water and R113. The length and gap (the distance between two heated walls) of test channels with a rectangular section were in the range of 30–100 mm and 30–50 μm, respectively. Stainless steel wires with different numbers were evenly laid in the channel along the flow direction for forming change in width of micro-channel. The width of rectangular section changed from 0.4 mm to 4 mm. The study results show that the boiling characteristics in 3-D micro-channels thermosyphon can be predicted through the extension of the correlations used for 2-D micro-channels. The present 3-D micro-channel thermosyphon boiling is a promising passive technology for 3-D chip cooling.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 131〈/p〉 〈p〉Author(s): Mohammad Reza Shaeri, Richard W. Bonner〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉An analytical model is proposed to predict the average Nusselt numbers of laterally perforated-finned heat sinks (LA-PFHSs) with high aspect ratios in forced convection laminar flows. The model is developed based on the experimental data acquired from testing air-cooled heat sinks including square cross-sectional perforations distributed equidistantly along the length of the fins. The experiments were conducted using three different perforation sizes and five different porosities at each perforation size. The accuracy of the experiments was validated by comparing the experimental pressure drops and heat transfer coefficients of the heat sink without perforation with those obtained from the widely accepted correlations in the literature. The developed model in this study predicts the Nusselt number as a function of Reynolds number, Prandtl number, fin and perforation geometrical parameters, porosity, and the distances between perforations. The model showed excellent predictions for the Nusselt numbers of all LA-PFHSs tested in this study to be within ±12% of the experimental data and a mean absolute error of 4.90%. This study is the first attempt in the literature to develop an analytical model based on experimental data for investigating heat transfer in LA-PFHSs.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 131〈/p〉 〈p〉Author(s): Peng Peng, Xinzhong Li, Yanqing Su, Jingjie Guo, Hengzhi Fu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The Sn-40at.%Mn peritectic alloy were directionally solidified in a temperature gradient at different growth velocities. It was demonstrated that the diffusion-controlled remelting/resolidification process occurred on the secondary dendrite arms in the mushy zone of Sn-40at.%Mn peritectic alloy. Theoretical analysis has shown that the diffusion-controlled solute transport which occurred between secondary dendrite arms was produced by both the temperature gradient zone melting (TGZM) and the Gibbs-Thomson effects (capillary effect). An analytical model was established to describe the diffusion-controlled solute transport during the remelting/resolidification process. The coupling influences of the TGZM and Gibbs-Thomson effects on the remelting/resolidification process during peritectic solidification was analyzed in terms of the specific surface area (〈em〉S〈sub〉V〈/sub〉〈/em〉) of dendrites. It was found that the remelting/resolidification process by the Gibbs-Thomson effect was retarded by peritectic reaction, while that by the TGZM effect was accelerated by peritectic reaction. Since the diffusion-controlled solute transport by the TGZM effect is dominant as compared with that by the Gibbs-Thomson effect in this work, the diffusion-controlled remelting/resolidification process of dendrites is accelerated during peritectic solidification in a temperature gradient.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 131〈/p〉 〈p〉Author(s): Eren Sevinchan, Ibrahim Dincer, Haoxiang Lang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Theoretical and experimental heat transfer studies are conducted for performance assessment and evaluation of various thermal insulation materials for robotic operations in this study. Nineteen different thermal insulation materials are theoretically analysed, and three of these materials (namely stone wool, fiberglass and extruded polyurethane) are tested at both low and high temperatures to obtain their thermal performance under the extreme ambient conditions. According to the experimental results obtained at 40 °C, the heat transfer rates between the inside and outside of the robot are obtained 120 W for stone wool, 126 W for fiberglass, and 173.9 W for extruded polyurethane. On the other hand, at −25 °C, the heat transfer rates are 97 W for stone wool, 107.8 W for fiberglass, and 127.8 W for extruded polyurethane, respectively.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 131〈/p〉 〈p〉Author(s): Shong-Leih Lee, Jeng-Bin Chiou, Guo-Sian Cyue〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Mixed convection in a square enclosure arising from thermal buoyancy force and a rotating flat plate is investigated in the paper. The moving boundary problem is solved with the implicit virtual boundary method on a fixed non-staggered Cartesian grid system. Computations are performed for Prandtl number 0.71 at various Rayleigh numbers 〈em〉Ra〈/em〉 and rotational Reynolds numbers 〈em〉Re〈/em〉. For a rotor of length 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈mi〉d〈/mi〉〈mo〉=〈/mo〉〈mn〉0.6〈/mn〉〈/mrow〉〈/math〉 rotating at 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.gif" overflow="scroll"〉〈mrow〉〈mi mathvariant="italic"〉Re〈/mi〉〈mo〉=〈/mo〉〈mn〉430〈/mn〉〈/mrow〉〈/math〉, the numerical results show that thermal oscillation occurs when the Rayleigh number exceeds 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.gif" overflow="scroll"〉〈mrow〉〈mn〉0.55〈/mn〉〈mo〉×〈/mo〉〈msup〉〈mrow〉〈mn〉10〈/mn〉〈/mrow〉〈mrow〉〈mn〉6〈/mn〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉. There is a critical Rayleigh number (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si4.gif" overflow="scroll"〉〈mrow〉〈mn〉0.13〈/mn〉〈mo〉×〈/mo〉〈msup〉〈mrow〉〈mn〉10〈/mn〉〈/mrow〉〈mrow〉〈mn〉6〈/mn〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉). Below that the rotor enhances the heat transfer. By contrast, the rotor would suppress the heat transfer beyond the critical Rayleigh number. In the case of 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si5.gif" overflow="scroll"〉〈mrow〉〈mi mathvariant="italic"〉Ra〈/mi〉〈mo〉=〈/mo〉〈msup〉〈mrow〉〈mn〉10〈/mn〉〈/mrow〉〈mrow〉〈mn〉6〈/mn〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉, the Nusselt number oscillates 23 cycles when the rotor makes 29 revolutions. For a longer rotor 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si6.gif" overflow="scroll"〉〈mrow〉〈mi〉d〈/mi〉〈mo〉=〈/mo〉〈mn〉0.9〈/mn〉〈/mrow〉〈/math〉 rotating at 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.gif" overflow="scroll"〉〈mrow〉〈mi mathvariant="italic"〉Re〈/mi〉〈mo〉=〈/mo〉〈mn〉430〈/mn〉〈/mrow〉〈/math〉, thermal oscillation exists for all Rayleigh numbers, and the critical Rayleigh number increases to 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si8.gif" overflow="scroll"〉〈mrow〉〈mn〉0.38〈/mn〉〈mo〉×〈/mo〉〈msup〉〈mrow〉〈mn〉10〈/mn〉〈/mrow〉〈mrow〉〈mn〉6〈/mn〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉. In the case of 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si5.gif" overflow="scroll"〉〈mrow〉〈mi mathvariant="italic"〉Ra〈/mi〉〈mo〉=〈/mo〉〈msup〉〈mrow〉〈mn〉10〈/mn〉〈/mrow〉〈mrow〉〈mn〉6〈/mn〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉, the Nusselt number oscillates twice when the rotor makes one revolution.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 131〈/p〉 〈p〉Author(s): Chuan-Yong Zhu, Zeng-Yao Li, Hao-Qiang Pang, Ning Pan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Due to the complex microstructure in aerogels and the intricate heat transfer mechanism of solid-gas coupling heat conduction, modeling of the gas-contributed thermal conductivity of this type of material is quite difficult. The present work introduces a novel numerical methodology for computing the gas-contributed thermal conductivity of aerogels by analyzing their microstructural characteristics and heat transfer mechanism of the thermal coupling between the gas phase and the solid backbone of the system. Specifically, structures of aerogels are reconstructed by an improved three-dimensional diffusion-limited cluster-cluster aggregation (DLCA) method, and the contribution of the solid-gas coupling heat transfer to the gas-contributed thermal conductivity of aerogels is quantified. The present numerical model is fully validated by the available experimental data for different aerogels with porosity ranging from 78% to 97.7%. The proposed numerical method is flexible and versatile because it is capable to account for both the geometrical and topological details of the aerogel structure.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 131〈/p〉 〈p〉Author(s): Yu-Shu Shi, Di Liu, Yu Wang, Fu-Yun Zhao, Yu-Xing Li〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this paper forced flow structure and mixed convection in a ventilated porous enclosure with a local contaminant source centrally positioned on the floor have been investigated numerically. The physical model for the momentum conservation equation makes use of the Darcy-Brinkman equation, which allows the no-slip boundary condition on a solid wall to be satisfied. The set of coupled equations is solved via the SIMPLE algorithm. Comparisons with previously published work are performed and found to be in excellent agreement. An extensive series of numerical simulations is conducted in the range of, 200 ≤ 〈em〉Re〈/em〉 ≤ 10,000, 0.02 ≤ 〈em〉Re〈/em〉 · 〈em〉Da〈/em〉 ≤ 20 and, 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈mn〉0.01〈/mn〉〈mo〉⩽〈/mo〉〈mi〉f〈/mi〉〈mo〉=〈/mo〉〈mo stretchy="true"〉(〈/mo〉〈msub〉〈mrow〉〈mi mathvariant="italic"〉Gr〈/mi〉〈/mrow〉〈mrow〉〈mtext〉s〈/mtext〉〈/mrow〉〈/msub〉〈mo〉·〈/mo〉〈msup〉〈mrow〉〈mi mathvariant="italic"〉Da〈/mi〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msup〉〈mo stretchy="true"〉)〈/mo〉〈mo〉/〈/mo〉〈mo stretchy="true"〉(〈/mo〉〈mi mathvariant="italic"〉Re〈/mi〉〈mo〉·〈/mo〉〈mi mathvariant="italic"〉Da〈/mi〉〈mo stretchy="true"〉)〈/mo〉〈mo〉⩽〈/mo〉〈mn〉1000〈/mn〉〈/mrow〉〈/math〉 where 〈em〉Re〈/em〉 is the Reynolds number, 〈em〉Da〈/em〉 is the Darcy number, 〈em〉Gr〈/em〉〈sub〉s〈/sub〉 is the solutal Grashof number and 〈em〉f〈/em〉 is derived from the discretized 〈em〉V〈/em〉 equation and represents the relative magnitude between the effective solutal buoyancy and the effective external force. Streamlines are produced to illustrate the forced flow structure transition from eddy-free motion to multi-eddy pattern and 〈em〉Re〈/em〉 · 〈em〉Da〈/em〉 ≥ 0.2 is the critical condition of eddy emergence. The analysis of overall Sherwood number and iso-concentrations indicate that the parameter 〈em〉f〈/em〉 is appropriate to be regarded as the criterion of mixed convection in porous medium.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 131〈/p〉 〈p〉Author(s): Amin Shahsavar, Shoaib Khanmohammadi, Arash Karimipour, Marjan Goodarzi〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉This research aims to understand the impacts of volume concentration of Fe〈sub〉3〈/sub〉O〈sub〉4〈/sub〉 nanoparticles and temperature on the viscosity & thermal conductivity of liquid paraffin based nanofluid. Several experiments are conducted in the Fe〈sub〉3〈/sub〉O〈sub〉4〈/sub〉 concentration range of 0.5–3% and temperature range of 20–90 °C. Oleic acid is utilized as a surfactant for the improved dispersibility and stability of nanofluids. It was found that the nanofluid behaves as a shear thinning fluid. Additionally, it was revealed that both the thermal conductivity and viscosity boost with increasing the nanoparticle concentration, whereas when the temperature increases the viscosity reduces and the thermal conductivity rises.〈/p〉 〈p〉Moreover, the Artificial Neural Network (ANN) was utilized to model the thermal conductivity and viscosity of the nanofluid using experimental data. The accuracy of the models was assessed based on four known statistical indices including root meant square (〈em〉RMS〈/em〉), root mean square error (〈em〉RMSE〈/em〉), mean absolute deviation (〈em〉MAE〈/em〉), and coefficient of determination (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈msup〉〈mrow〉〈mi〉R〈/mi〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉). Results showed that the proposed model of thermal conductivity could estimate outputs with 〈em〉RMS〈/em〉, 〈em〉RMSE〈/em〉, 〈em〉MAE〈/em〉 & 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈msup〉〈mrow〉〈mi〉R〈/mi〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉 values of 0.0678, 0.0179, 0.0041 and 0.96, respectively.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 131〈/p〉 〈p〉Author(s): X. Zhang, J. Kang, Z. Guo, Q. Han〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A phase-field-lattice-Boltzmann method, coupled with parallel computing and adaptive mesh refinement (Para-AMR), was employed to study the effect of forced flow on permeability of the mushy zone during solidification. Accuracy of the method was testified by comparing with predictions of conventional models, as well as other numerical investigations in literature. Results showed that the permeability of the mushy zone changed as the forced flow was imposed. For columnar dendritic networks, the permeability decreased exponentially either parallel or normal to the dendritic growth direction due to impinging and interlocking of secondary arms. The permeability of equiaxed dendritic networks decreased significantly when the flow was present. The permeability changed as a result of morphological transformation of the dendritic network, which became more columnar-like under a higher incoming velocity. Based on the numerical results and theoretical analysis, a new model was proposed for predicting permeability under the forced flow. Results showed that the model was suitable for describing the permeability of both columnar and equiaxed dendritic structures under a moderate forced flow, and could be used as a viable alternative to determine the permeability in a wide range of solid fraction during solidification.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 131〈/p〉 〈p〉Author(s): M.A. Chernysheva, Y.F. Maydanik〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The paper presents a model of heat and mass transfer in a cylindrical evaporator of a loop heat pipe (LHP) with allowance for the peculiarities of heat exchange in the evaporation zone formed by vapor-removal grooves. The results of numerical simulation of temperature and pressure distribution in a cylindrical evaporator 8 mm in diameter are presented. Data have been obtained for four working fluids, namely, ammonia, acetone, Freon R152a and Freon R141b. The results of calculation of the pressure drop in a wick obtained on the basis of the present model and that with a simplified wick geometry in the form of a porous cylindrical wall have been compared. Analysis of the results showed that detailed simulation of the evaporator is required. The necessity of taking into account the geometry of the evaporation zone in problems on the determination of the pressure drop in a wick has been substantiated. It is shown that such an approach is particularly topical for fine-pored wicks and such working fluids as Freon R152a and Freon R141b.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 131〈/p〉 〈p〉Author(s): M. Azam, A. Shakoor, H.F. Rasool, M. Khan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A numerical analysis for unsteady magnetohydrodynamic (MHD) flow of Cross nanofluid subject to non-linear thermal radiation is carried out. The Buongiorno’s nanofluid model involving Brownian motion and thermophoresis is adopted. Two more realistic conditions namely convective condition and zero nanoparticles mass flux condition are implemented on the boundary. Mathematical problem is modelled with the aid of momentum, temperature as well as nanoparticles concentration equations adopting suitable transforming variables. The resulting highly nonlinear differential systems are solved numerically with the help of shooting Runge-Kutta-Fehlberg method. Numerical computations for Nusselt number as well as skin friction coefficient are performed. Variations of velocity, temperature as well as nanoparticles concentration profiles are examined by varying the involved parameters. A comparative analysis is conducted between existing study and present investigation in limiting case and found to be in excellent agreement. It is interesting to note that thermal as well as nanoparticles concentration boundary layer thicknesses are the upgrading functions of unsteadiness parameter. Additionally, rate of heat transfer is depreciated by upgrading the values of radiation parameter as well as thermophoresis parameter. Furthermore, the magnitude of wall shear stress is an enhancing function of the magnetic parameter. It is also noted that rate of heat transfer enhance with the enhancement of temperature ratio parameter as well as Biot number.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 131〈/p〉 〈p〉Author(s): Tie Wei〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This paper investigates heat transfer regimes in fully developed plane channel flows without considering the buoyancy effect. Analyzing the governing thermal energy equation, aided by direct numerical simulation (DNS) data, six heat transfer regimes are identified including (i) laminar flow and laminar heat transfer (LamF-LamH), (ii) transitional flow and laminar-like heat transfer (TraF-LamH), (iii) transitional flow and transitional heat transfer (TraF-TraH), (iv) turbulent flow but laminar-like heat transfer (TurF-LamH), (v) turbulent flow and transitional heat transfer (TurF-TraH), and (vi) turbulent flow and turbulent heat transfer (TurF-TurH). One key result is the clarification of a TurF-LamH regime which exists only for low Prandtl number fluid (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si42.gif" overflow="scroll"〉〈mrow〉〈mi mathvariant="italic"〉Pr〈/mi〉〈mo〉≪〈/mo〉〈mn〉1〈/mn〉〈/mrow〉〈/math〉). A critical non-dimensional number is determined as 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si44.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mi mathvariant="italic"〉Re〈/mi〉〈/mrow〉〈mrow〉〈mi〉τ〈/mi〉〈/mrow〉〈/msub〉〈msup〉〈mrow〉〈mi mathvariant="italic"〉Pr〈/mi〉〈/mrow〉〈mrow〉〈mn〉1〈/mn〉〈mo〉/〈/mo〉〈mn〉2〈/mn〉〈/mrow〉〈/msup〉〈mi〉≲〈/mi〉〈mn〉50〈/mn〉〈/mrow〉〈/math〉 where 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si7.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mi mathvariant="italic"〉Re〈/mi〉〈/mrow〉〈mrow〉〈mi〉τ〈/mi〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 is the Reynolds number defined by the channel half-height 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si51.gif" overflow="scroll"〉〈mrow〉〈mi〉δ〈/mi〉〈/mrow〉〈/math〉 and the frictional velocity 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si52.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mi〉u〈/mi〉〈/mrow〉〈mrow〉〈mi〉τ〈/mi〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉. In the TurF-LamH regime, the simplified thermal energy equation yields a prediction of Nusselt number as 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si45.gif" overflow="scroll"〉〈mrow〉〈mi mathvariant="italic"〉Nu〈/mi〉〈mo〉≈〈/mo〉〈mn〉6.0〈/mn〉〈/mrow〉〈/math〉. The clarification of the TurF-LamH regime provides valuable insight into the understanding of the Nusselt number data for liquid metals, which have very low Prandtl number. Another major finding is that, in the TurF-TurH regime, the Kader-Yaglom-style equation is shown to be better than the traditional power law correlations at predicting Nusselt number, for either low or high Prandtl numbers. No separate correlations are needed for the prediction of Nusselt number at low Prandtl numbers and high Prandtl numbers. Another advantage of the Kader-Yaglom-style equation is that the equation can be theoretically connected to the log-law for the mean velocity and the mean temperature. More importantly, in the TurF-TurH regime the prediction of Kader-Yaglom-style equation can be reliably extended to ultra high Reynolds numbers, for either low or high Prandtl numbers.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 131〈/p〉 〈p〉Author(s): Ali Farghadan, Amirhossein Arzani〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Cardiovascular disease typically initiates at the vessel wall where near-wall transport of certain biochemicals influences disease initiation and progression. Wall shear stress (WSS) influences these transport processes in a complex manner. WSS magnitude and the shear force exerted on the endothelial cells determine the biochemical flux at the vessel wall. In addition, it has been recently shown that Lagrangian WSS structures (topological features) influence near-wall transport in high Schmidt and Peclet numbers. In this study, the influence of WSS topology and magnitude on surface concentration patterns in shear-dependent biochemical transport problems was explored in a coronary artery stenosis and a carotid artery model. Shear-enhanced and shear-reduced biochemical flux boundary conditions were defined at the wall and surface concentration patterns were obtained by solving the advection-diffusion-reaction equation using the finite element method. Surface concentration patterns were demonstrated to depend on a competition between WSS topology and magnitude. A threshold point was identified where WSS topology determined surface concentration patterns for flux equations closer to homogeneous flux, whereas WSS magnitude dominated surface concentration once the wall flux was sufficiently heterogeneous and strongly dependent on shear stress. Finally, nitric oxide (NO) transport was investigated as an example of an important biochemical with shear-enhanced flux at the vessel wall. It was shown that NO transport was close to the identified threshold where WSS topology and magnitude both influenced surface concentration. This study shows that WSS could potentially be used as a powerful parameter to predict qualitative surface concentration patterns without the need to solve numerically challenging cardiovascular mass transport problems.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 131〈/p〉 〈p〉Author(s): Yunran Min, Yi Chen, Hongxing Yang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Indirect evaporative cooling is recognized as an alternative air-cooling solution with low carbon potential and considerable energy efficiency. An indirect evaporative cooler (IEC) can handle both of the sensible and latent cooling loads because of possible condensation when it is used as a precooling unit in an air-conditioning system in hot and humid regions. Cross flow and counter flow, as two basic flow configurations of an IEC, differ in condensation behavior that affects their cooling performance. In this paper, a novel 2-D model of cross flow IEC considering condensation is established and validated. The performance of the cross flow and counter flow IEC is thoroughly compared under the same configuration. The channel gap and height to length ratio (H/L) are optimized to provide references for the design and operation of the IEC under condensation conditions. Results show that under the same operating conditions, the condensation ratio of counter flow IEC is 2–15% higher than that of the cross flow IEC, leading to 2–7% decrease of wet-bulb effectiveness. The difference in the total heat transfer rate between the two configurations is less than 5% when the number of transfer units (NTU〈sub〉p〈/sub〉) is lower than 3.1. For cross flow IEC, there is an optimal value in H/L among 0.4–0.8 considering the cooling capacity and energy consumption.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 131〈/p〉 〈p〉Author(s): Bimal Subedi, Sung Hyoun Kim, Seok Pil Jang, M.A. Kedzierski〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This paper presents a theoretical investigation of the thermal characteristics of flat-micro heat pipes (FMHPs) with multi-heat sources and sinks. Analytical solutions of the pressure and the temperature distributions of FMHPs with multi-heat sources and sinks were obtained based on the modified liquid pressure drop. The solutions were used to identify the key engineering parameters of a mesh wick with microscale length that affect the maximum heat transfer rate of the FMHPs with multi-heat sources and sinks. The effects of the key engineering parameters on the maximum heat transfer rate of the FMHPs were presented for two limits. The first limit is the capillary limit and the other is the allowable maximum temperature limit which is used to ensure that the maximum surface temperature of the FMHP with the maximum heat transfer rate calculated at the capillary limit does not exceed the allowable maximum temperature of the electronic components. Finally, the theoretically results for the optimized wick structure for the corresponding maximum heat transfer rate and the surface temperature distribution of the FMHP were compared for the capillary limit only and for the maximum temperature limit cases, respectively.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 133〈/p〉 〈p〉Author(s): Carlos Zing, Shadi Mahjoob, Kambiz Vafai〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉An innovative porous filled heat exchanger is modeled to investigate the cooling effectiveness and temperature distribution at its base subject to a high heat flux. The effects of different nanofluid coolants (5% titanium dioxide (TiO〈sub〉2〈/sub〉) in water, 1% alumina in water, 0.03% multi walled carbon nanotubes (MWCNT) in water, and 1% diamond in 40:60 ethylene glycol/water), different porous materials (copper and annealed pyrolytic graphite (APG)), and porosity values are investigated. The coolant enters from an inlet channel normal to the base, moves through the porous medium, and leaves the heat exchanger through two opposite exit channels parallel to the base. The effects of the inclination angle of the foam filled channel, inlet velocity value, and heat flux value are also studied. In addition, the effect of the inlet cross section is investigated by studying two different designs. One of the designs has a rectangular cross sectional inlet channel (extended all along the transverse direction) and the other design has a square one. The results indicate the importance of the utilization of a high conductive porous material. Utilization of APG porous matrix improves the cooling effectiveness at the base of the heat exchanger, for all studied coolants of pure water and water based nanofluids. The results also show that utilizing titanium dioxide nanofluids (TiO〈sub〉2〈/sub〉) as coolant for both copper and APG porous matrices at low and high porosity structures, and for both square and rectangular inlet cross sections improves the cooling efficiency and temperature uniformity over the base. Investigation of the effect of inlet channel geometry, i.e., square and rectangular, indicates that employing a square cross section inlet channel would result in lower temperature values along the streamwise direction while higher temperature values are observed far from the center in transverse direction.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 133〈/p〉 〈p〉Author(s): Abdulwahab A. Alnaqi, Saeed Aghakhani, Ahmad Hajatzadeh Pordanjani, Reza Bakhtiari, Amin Asadi, Minh-Duc Tran〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this paper, the effects of radiation and magnetic field on the convection heat transfer rate and the nanofluid entropy generation in a diagonal square cavity with a conductor fin have been numerically investigated. A fin with a thermal conductivity coefficient of k〈sup〉*〈/sup〉 = 100 is located on one of the walls of the cavity. A volumetric heat source is considered in the fluid that is producing heat in the form of radiation. The effect of this source is added as a source to the energy equation and its value is shown by the Rd radiation parameter. Mass, momentum, and energy conservation equations in two-dimensional mode are discretization with finite difference method based on the control volume and solved using the simple algorithm. The model used for the thermal conductivity coefficient is the phenomenon model, taking into account the Brownian motion of the particles. In this paper, the effects of Rayleigh numbers, Hartmann numbers, radiation parameter and volume percentages of nanoparticles on the entropy generation and heat transfer have been investigated. The results show that increasing the Rayleigh number and reducing the Hartmann number increases the Nusselt number. Alternatively, adding 6% of the nanoparticles to the base fluid in the absence of radiation increases the heat transfer rate and entropy generation by 5.9% and 16.6%, respectively. By adding the radiation parameter, Rd = 3, and the volume percentage of nanoparticles of 6%, the heat transfer rate and total entropy generation increased by 3.4% and 11.2%, respectively. It was also observed that increasing the radiation parameter at high Rayleigh numbers increases the Nusselt number and entropy generation and decreases the Bejan number. Increasing the heat transfer rate is more significant by increasing the radiation parameter at higher Rayleigh numbers.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): René Spencer Chatwell, Matthias Heinen, Jadran Vrabec〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉An analytical solution of a model fluid’s time behavior, known as the Stefan problem, is presented. A scenario is investigated in which a planar two-component liquid film is continuously evaporating into a thermodynamically non-ideal vapor phase. Evaporation is initiated and maintained by a spatial chemical potential gradient, while its rate is limited by the components’ diffusion fluxes across the vapor-liquid interface. Local thermodynamic equilibrium is found to be present throughout the process. In contrast to the classical approach relying on equations of state, all required non-idealities are formulated in relation to the Gibbs energy and are determined by molecular simulations. Initially, the liquid is an equimolar mixture of two components of different volatility, whereas the adjacent vapor phase is dominated by a dense inert gas. To validate the analytical model and verify all exploited assumptions, the results are contrasted to large scale molecular dynamics simulations.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 133〈/p〉 〈p〉Author(s): Elvira F. Tanjung, Daeseong Jo〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A visualization study on boiling phenomena at various surface orientations was conducted to investigate the bubble behaviors and the critical heat flux (CHF) mechanism. The vapor behavior observed in this study indicates that the pool boiling can be categorized into three regions: upward facing (0° ≤ θ 〈 90°), vertical and near-downward facing (90° ≤ θ ≤ 165°), and downward facing (165° 〈 θ ≤ 180°). In the upward-facing region, the vapor was generated and detached easily in the vertical direction of the heated surface. In the vertical and near-downward facing region, the vapor grew and drifted upward the surface in a wavy shape owing to the vapor flow. When the heater faced downward (165° 〈 θ ≤ 180°), the vapor was trapped and blanketed the entire heated surface. According to the vapor behaviors at various inclination angles, the CHF decreased with the increase of the surface orientation from horizontal facing upward to downward. The vapor film thickness decreased as the surface orientation increased from 0° to 180°. The vapor velocity affected the distance between the two wavelength peaks of the vapor. Faster vapor flow yielded a shorter wavelength. Additionally, the CHF location was affected by the surface orientation owing to the vapor behavior. These results indicate that the surface orientation significantly affects the vapor behavior, CHF, CHF location, vapor velocity, wavelength, and maximum and minimum vapor thicknesses on a printed circuit board in a saturated water pool.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 133〈/p〉 〈p〉Author(s): Thue S. Bording, Søren B. Nielsen, Niels Balling〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Needle probe measurements are used in a wide variety of fields for measuring the thermal conductivity of materials. It is most common to extract the thermal conductivity from the temperature rise data using the asymptotic solution to the heat equation. With special reference to, but not limited to Earth materials, this study presents an inversion procedure using pulsed needle probe data to determine both thermal conductivity and thermal diffusivity. The measured temperature response data are interpreted using a finite element forward model and a Markov Chain Monte Carlo inversion algorithm. The thermal properties of the needle probe are part of the forward model and determined by measurements on calibration standards. We examine several factors by synthetic modelling and quantify their effects by Monte Carlo inversions. This include (1) the heat production rate of the probe in order to produce similar temperature increases in materials of different thermal properties, (2) the contact resistance between needle probe and sample, (3) the limitations imposed by sample diameter and the thermal properties of the surrounding medium, and (4) the duration of heating period required to obtain good results. Laboratory test measurements on selected materials (water, glycerol, ceramic standard, clay) demonstrate good agreement with expected values obtained from literature. We demonstrate that the combination of numerical forward modelling and Monte Carlo inversion, applied to the pulsed needle, is a flexible and powerful methodology for accurate laboratory measurements of material thermal properties and with well-defined estimates of uncertainty. With the given equipment and laboratory setup, thermal conductivity is measured to within few percent, while thermal diffusivity generally has a somewhat higher degree of uncertainty, up to about five percent.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Weifang Han, Chunhua Ge, Rui Zhang, Xiangdong Zhang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Hexagonal boron nitride (h-BN) has triggered great interest in thermal management system because of its superb thermal conductivity, excellent chemical inertness and outstanding tribological behavior. However, the application of BN in water suffers from the poor dispersion stability. Herein, we develop a spray drying and pyrolysis route to directly fabricate porous BN microrods with fish-scale-like structures. BN microrods exhibit long-term dispersion stability in aqueous solution due to their high hydroxylation degrees. Although the viscosity of water was increased after adding BN microrods, the friction coefficient and wear scar diameter were reduced obviously. Moreover, thermal conductivity of BN/water fluid was improved from 0.589 to 1.117 W/m K when the content of BN was 0.1 vol%, which was higher than that of previous results. These results demonstrate that the fish-scale-like structured BN microrods have great potential and irreplaceable advantage in thermal and tribological management.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 133〈/p〉 〈p〉Author(s): Yigang Luan, Lianfeng Yang, Shi Bu, Tao Sun, Haiou Sun, Pietro Zunino〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this work, connecting holes are used between the two legs of a two-pass channel with and without ribs. The flow and heat transfer are obtained by numerical simulation at the Reynolds number of 10,000, 30,000 and 50,000. Different rib configurations and positions of the connecting holes are considered. The result shows positive effect of the holes on the performance especially in ribbed channels. Connecting holes can largely reduce the pressure drop across the channel but the decrease in heat transfer is very small, thus the overall thermal performance can be improved significantly. For the smooth channel, pressure loss can be reduced by 14.66% at the heat transfer loss of 1.68%, for the ribbed channel pressure loss can be reduced by 47.99% with merely 8.24% decrease in heat transfer if the ribs and connecting holes are carefully arranged. Connecting holes can affect the boundary layer separation and reattachment between ribs. Separation bubble behind the rib can be suppressed or moved by connecting holes and acceleration occurs within the low velocity region. Therefore the weak heat transfer related to the separation can be improved. Based on this interaction between ribs and holes, further improvement in the performance of two-pass channel can be expected after better design of ribs and connecting holes.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): François Allard, Martin Désilets, Marc LeBreux, Alexandre Blais〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A thermal-electric model is developed using finite elements and thoroughly validated against thermal measurements performed in industrial aluminum electrolysis cells (AEC). Knowledge about the geometries of anode cover material (ACM) and anode crust is improved by measuring their profiles in industrial cells. Moreover, the shape of the cavity formed by the melting of the anode crust is predicted with the numerical model, using a radiosity module combined to an iterative method. The thermal conductivity and the emissivity of both ACM and anode crust are also evaluated based on experimental measurements. The thermal-electric model accurately predicts the measurements obtained from heat flux sensors and thermocouples installed on industrial anodes. Modeling results show that increasing the ACM thickness reduces the top heat losses and increases the heat dissipation from the side, while the bottom losses remain constant. With thicker top insulation, the side ledge shrinks and the anode crust melts. The impact of a film of sludge under the liquid aluminum is quantified with the model. Accordingly, the sludge increases the cathode voltage drop (CVD), enlarges the cavity, reduces the side ledge thickness and amplifies the side heat dissipation. The thermal-electric modeling provides insights to improve the design and operation of AEC in order to reach higher efficiency.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): H. Sajjadi, A. Amiri Delouei, M. Izadi, R. Mohebbi〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this paper, a new double multi relaxation time (MRT) Lattice Boltzmann method (LBM) has been used to simulate magnetohydrodynamics (MHD) natural convection in a porous media. A new nanofluid named; multi-walled carbon nanotubes–iron oxide/water nanofluid (MWCNT–Fe〈sub〉3〈/sub〉O〈sub〉4〈/sub〉/water hybrid nanofluid) has been utilized to investigate the effect of nanoparticle on heat transfer. The thermo-physical properties of the nanofluid have been extracted from experimental results. D2Q9 and D2Q5 lattices are used to solve the flow and temperature fields respectively, and the effect of various parameters such as; Darcy number (Da) (10〈sup〉−2〈/sup〉–10〈sup〉−1〈/sup〉), Rayleigh number (10〈sup〉3〈/sup〉 ≤ Ra ≤ 10〈sup〉5〈/sup〉), porosity (0.4 ≤ ε ≤ 0.9), volume fraction of nanoparticles (0≤ Ø ≤ 0.003) and Hartmann number (0 ≤ Ha ≤ 50) have been investigated. Based on the present results, the new double MRT LBM is a proper method to solve the complex flows such as MHD natural convection in porous media. The results show that augmentation of Rayleigh number increases the heat transfer rate for all cases however by increasing the Hartmann number the effect of Rayleigh number decreases. Also, adding the nanoparticles enhances the average Nusselt number as increases 4.9% for Ra = 10〈sup〉5〈/sup〉, Da = 10〈sup〉−1〈/sup〉, Ha = 50, and ε = 0.9 when the volume fraction of nanoparticles rises from 0 to 0.003. Results indicate that by enhancing Darcy number heat transfer rate increases and average Nusselt number improves by porosity.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): S.A. Hosseini, N. Darabiha, D. Thévenin〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Many practical flow configurations involve energy transfer in fluids, or in solids and fluids with different thermo-physical properties. The classical advection-diffusion lattice Boltzmann (LB) solver admits some errors when dealing with such configurations. Given that the macroscopic equation recovered by this model is only valid in the limit of incompressible flows with constant heat capacities, one would, for example, observe inconsistent fluxes at the interface of a fluid and solid with different densities or specific heat capacities. This inconsistency being second-order in space, it will have non-negligible effects on the final results. In this work, a modified equilibrium distribution function (EDF) is proposed to overcome these issues. The proposed scheme recovers the correct partial differential equation (PDE) describing energy transfer, as shown by a multi-scale Chapman–Enskog analysis. The performance of the model is checked through a variety of test-cases, involving conjugate heat transfer and variable specific heat capacities in both steady and unsteady configurations. In all cases the obtained results are in excellent agreement with reference data.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Daechan Jeon, Se Hyun Kim, Wonjoon Choi, Chan Byon〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this study, an innovative thermal interface material (TIM) paper based on a composite of cellulose and graphene is investigated experimentally. Six types of commercially-available papers: a wool paper; an aqua satin; a merit paper; a new craft board; and two oriental traditional papers (Bulgyeong and Daerye) are used to fabricate the paper-graphene composites via bar coating and a slot die coating. The fabricated TIM papers are lightweight, flexible and robust against tensile strength. The in-plane and through-plane thermal conductivities of the TIM papers are measured using a laser-flash-method (LFM). The measured in-plane thermal conductivities are of the order of 5 W/m-K, whereas the through-plane thermal conductivities are of the order of 0.1 W/m-K. These results suggest that the addition of graphene significantly enhance the in-plane thermal conductivity of papers, while the through-plane thermal conductivities are not significantly improved. The mechanical properties of the TIM papers are also tested. This work provides a new possibility for development of next-generation thermal interface materials with good thermal and mechanical properties.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Evangelos Tsotsas〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Simulation of transient heat transfer from the wall of contact equipment (e.g. rotary drum) to a mechanically agitated bed of particles in gas atmosphere by the discrete element method (DEM) requires knowledge of the particle-particle heat transfer coefficient. A simple analytical equation is proposed in order to calculate this quantity from the effective thermal conductivity of the respective packed bed for the described class of problems. Comparison with other calculation methods that assign particle-particle heat transfer exclusively to either the solid or the gas phase shows that large differences can occur. It stresses the need for further work on the fundamentals of thermal DEM for such applications, and for a critical attitude until those have been clarified.〈/p〉〈/div〉 〈/div〉 〈div xml:lang="en"〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S0017931018340857-ga1.jpg" width="444" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): S.M. Abolarin, M. Everts, J.P. Meyer〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The purpose of this study was to experimentally investigate the heat transfer and pressure drop enhancement characteristics in a smooth circular copper tube with peripheral u-cut twisted tape (PUCTT) inserts without and with ring (PUCTTR) inserts. The PUCTT inserts were fabricated from 1 mm thick and 18 mm wide copper strips. The twisted tapes contained peripheral cuts with depth ratios of 0.105 and 0.216 respectively and were twisted to obtain a twist ratio of 5. Ring inserts were soldered on the PUCTT inserts to form PUCTTR inserts with ring space ratios of 1.25, 2.5 and 5 respectively. A total of five 900 mm long PUCTT inserts and one 770 mm insert were connected longitudinally to form a single PUCTT insert with an overall length of 5.27 m. The PUCTT and PUCTTR inserts were placed in a smooth copper tube with an inner diameter of 19 mm. Water was used as the test fluid and experiments were conducted between Reynolds numbers of 315 and 11,404 at constant heat flux boundary condition. This Reynolds number range covered the transitional flow regime, as well as sufficient parts of the laminar and turbulent flow regimes. This study focussed on the identification of the transitional flow regime with the PUCTT and PUCTTR inserts, as well as to investigate the influence of the depth ratio of the peripheral cuts and the ring space ratio on the transitional flow regime. It was found that the start and end of the transitional flow regime were affected by both the depth and ring space ratios. An increase in depth ratio caused the transitional flow regime to occur earlier. Furthermore, the transitional flow regime occurred earlier with PUCTTR inserts than with PUCTT inserts and transition occurred even earlier as the ring space ratio was reduced. It was concluded that an increase in depth ratio and reduction in ring space ratio significantly enhanced heat transfer in the transitional flow regime. Heat transfer and pressure drop correlations were therefore developed to predict the experimental data in the laminar, transitional and turbulent flow regimes as a function of Reynolds number, depth ratio and ring space ratio.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): I. Dueramae, M. Yoneyama, N. Shinyashiki, S. Yagihara, R. Kita〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The effect of hydrophobicity on thermal diffusion of acetylated dextran in aqueous solution was studied in the temperature range of 15–50 °C by optical beam deflection (OBD) technique. The hydrophobicity was controlled by substitution of hydroxyl groups on dextran chain with acetylated groups. The Soret coefficient 〈em〉S〈/em〉〈sub〉T〈/sub〉 and thermal diffusion coefficient 〈em〉D〈/em〉〈sub〉T〈/sub〉 significantly increased with an increase of temperature. The hydrophobicity plays an important role in the thermal diffusion behavior, i.e. 〈em〉S〈/em〉〈sub〉T〈/sub〉 and 〈em〉D〈/em〉〈sub〉T〈/sub〉 switch the sign from negative to positive magnitude with an increase of hydrophobic contents. We observed a correlation between the parameters obtained from Iacopini and Piazza equation (Europhy. Lett. 63 (2003) 247–253.) and the hydrophobic content of the binary aqueous solutions. A master curve can be used to describe the temperature dependence of 〈em〉S〈/em〉〈sub〉T〈/sub〉 and 〈em〉D〈/em〉〈sub〉T〈/sub〉 for acetylated samples. The 〈em〉S〈/em〉〈sub〉T〈/sub〉 linearly decreases with an increase of particle size for dextran whereas it scales linearly with the volume of particles for acetylated samples.〈/p〉〈/div〉 〈/div〉 〈div xml:lang="en"〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S001793101835141X-ga1.jpg" width="376" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Michele Celli, Antonio Barletta〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The linear stability analysis of a fluid saturated porous layer is carried out. The porous layer is inclined to the horizontal and is infinitely wide. One boundary of the layer is permeable while the other one is impermeable. The two boundaries are subject to different temperatures, so that convective instability may arise when such a temperature difference exceeds a threshold value. The basic state whose stability is studied consists of a single cell with no net mass flow rate. The critical values of the governing parameters are computed numerically and are presented as functions of the inclination angle. The threshold relative to the horizontal layer recovers the results already present in the literature. The inclination angle comes out to be a stabilising parameter such that the vertical layer cannot become unstable.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 133〈/p〉 〈p〉Author(s): Chao Bai, Hongzhen Cao, Guanmin Zhang, Maocheng Tian〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Small channel heat transfer with condensation or boiling is utilized to handle the more and more intense heat dissipation load nowadays. To further enhance the annular condensation heat transfer inside these channels, diverging-shaped small channel is considered in this work analytically. It is testified that diverging channels do improve condensation heat transfer significantly compared to small channels with constant cross-sectional area. With refrigerant mass flux density increasing or channel size decreasing, diverging channels become more and more efficient in enhancing condensation heat transfer.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 133〈/p〉 〈p〉Author(s): Yung-Wei Chen〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Based on the idea of the Lie-group shooting method and the backward group preserving scheme, a novel backward-forward algorithm is developed to solve non-linear, nonhomogeneous, multi-dimensional backward heat conduction problems under long time spans. For a nonhomogeneous, multi-dimensional, backward heat conduction problem, it is very difficult to integrate towards the time direction, even when using a high-order numerical scheme. To avoid time integration and increase the computational efficiency, a novel backward-forward Lie-group scheme is proposed. According to the quadratic equation of the Lie-group shooting method, a solution is applied to obtain the initial condition and to examine the final condition. Using the reciprocal relationship between the solutions of forward and backward schemes, the proposed algorithm can avoid the time integration of the numerical scheme and numerical divergence. To illustrate the effectiveness and accuracy of the proposed algorithm, several benchmarks in multi-dimensions are tested. The numerical results indicate that the proposed algorithm can achieve an efficient and accurate solution, even with noisy measurement data by comparing the estimation results with the existing literatures.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Juan Xiao, Simin Wang, Shupei Ye, Jiarui Wang, Jian Wen, Jiyuan Tu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Preheating coal water slurry (CWS) before entering the gasifier can improve gasification efficiency and reduce oxygen consumption. Shell-and-tube heat exchangers (STHXs) with ladder-type fold baffles were applied to preheating of CWS. And experiments were performed when CWS was in shell side and conductive oil flowed in tube side. The different concentration (52.13 wt%, 55.56 wt% and 57.67 wt%) of CWS fitted Bingham model had a significant effect on thermal-hydraulic performance. The experimental results show that, both shell-side temperature difference and overall heat transfer coefficient are the highest when the concentration of CWS is 52.13 wt%, and the shell-side pressure drop is the lowest. The temperature difference per unit pressure drop of CWS with 52.13 wt% and 55.56 wt% increases by 119.3%–290.2% and 55.2%–96.9% compared with that of CWS with 57.67 wt%, respectively, and overall heat transfer coefficient per unit pressure drop also decreases with the increasing of concentration of CWS. Based on experimental data, empirical correlations of 〈em〉Nu〈/em〉 and 〈em〉f〈/em〉 are obtained and the adjusted coefficient of determination (Adj.〈em〉R〈/em〉〈sup〉2〈/sup〉) is between 0.980 and 0.997. And compared with STHXs with fold baffles, selecting STHXs with ladder-type fold baffles is priority to preheat CWS.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Hiroyuki Abe, Robert Anthony Antonia〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Direct numerical simulations (DNSs) in a turbulent channel flow are used to examine the behavior of the turbulent Prandtl number 〈em〉Pr〈sub〉t〈/sub〉〈/em〉 for air (〈em〉Pr〈/em〉 = 0.71) and mercury (〈em〉Pr〈/em〉 = 0.025), with a view to calculating the mean temperature. Constant time-averaged (surface) heat flux (CHF) is used as a thermal boundary condition. For each 〈em〉Pr〈/em〉, four values of the Kármán number (〈em〉h〈sup〉+〈/sup〉〈/em〉 = 180, 395, 640, 1020) are used. Datasets for the constant heating source (CHS) are also examined. For 〈em〉Pr〈/em〉 = 0.71, 〈em〉Pr〈sub〉t〈/sub〉〈/em〉 is approximately 1.1 at the wall, varies between 0.9 and 1.1 in the region 〈em〉y〈sup〉+〈/sup〉〈/em〉 〈100, and is approximated by 0.9–0.3(〈em〉y/h〈/em〉)〈sup〉2〈/sup〉 for 〈em〉y〈/em〉/〈em〉h〈/em〉 〉 0.2. The latter relation, with a low 〈em〉Re〈/em〉 correction term (i.e. 25/〈em〉h〈/em〉〈sup〉+〈/sup〉), yields an excellent prediction for the mean temperature up to 〈em〉h〈sup〉+〈/sup〉〈/em〉 = 2000, whereas a calculation based on 〈em〉Pr〈sub〉t〈/sub〉〈/em〉 = 0.85 underestimates the mean temperature. The calculated maximum wall-normal turbulent heat flux and Nusselt number also agree well with the empirical relations over a wide range of 〈em〉h〈/em〉〈sup〉+〈/sup〉. For 〈em〉Pr〈/em〉 = 0.025, 〈em〉Pr〈sub〉t〈/sub〉〈/em〉 departs significantly from unity inside the inner region (〈em〉y/h〈/em〉 〈 0.2) owing to the strong conductive effect, whilst the magnitude in the outer region (〈em〉y/h〈/em〉 〉 0.2) tends to approach that corresponding to 〈em〉Pr〈/em〉 = 0.71 as 〈em〉h〈/em〉〈sup〉+〈/sup〉 increases due to the increase in the Peclet number. The 〈em〉h〈/em〉〈sup〉+〈/sup〉 dependence of 〈em〉Pr〈sub〉t〈/sub〉〈/em〉 in the logarithmic and outer regions is represented adequately by the turbulent Peclet number, i.e. 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mi mathvariant="italic"〉Pe〈/mi〉〈/mrow〉〈mrow〉〈mi〉t〈/mi〉〈/mrow〉〈/msub〉〈mo〉=〈/mo〉〈mi mathvariant="italic"〉Pr〈/mi〉〈mrow〉〈mfenced open="(" close=")"〉〈mrow〉〈msub〉〈mrow〉〈mi〉ν〈/mi〉〈/mrow〉〈mrow〉〈mi〉t〈/mi〉〈/mrow〉〈/msub〉〈mo〉/〈/mo〉〈mi〉ν〈/mi〉〈/mrow〉〈/mfenced〉〈/mrow〉〈/mrow〉〈/math〉. The resulting 〈em〉Pr〈sub〉t〈/sub〉〈/em〉 relation, which is an extension of the expression established by Kays (1994), leads to a correct calculation of the mean temperature not only for mercury (〈em〉Pr〈/em〉 = 0.025) but also for liquid sodium (〈em〉Pr〈/em〉 = 0.01). The mean temperature defect profile exhibits an outer-layer similarity when 〈em〉Pe〈/em〉(≡〈em〉Re〈sub〉b〈/sub〉Pr〈/em〉) ≥ 2000; the Nusselt number is represented by 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.gif" overflow="scroll"〉〈mrow〉〈mi mathvariant="italic"〉Nu〈/mi〉〈mo〉=〈/mo〉〈mn〉4.8〈/mn〉〈mo〉+〈/mo〉〈mn〉0.0147〈/mn〉〈msup〉〈mrow〉〈mi mathvariant="italic"〉Pe〈/mi〉〈/mrow〉〈mrow〉〈mn〉0.84〈/mn〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉 reasonably well.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Yan-Hong Wang, Shao-Yu Wang, Gui Lu, Xiao-Dong Wang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Two main reasons including practical and scientific significance continuously motivate the studies of explosive boiling of nanoscale liquid films. Whether the nanoscale explosive boiling agrees with classical nucleation theory is still an open issue. In this work, we study the effects of surface wettability on the explosive boiling of nanoscale liquid films using molecular dynamics simulations. A critical film thickness is proposed to address the debate of whether the classical nucleation theory fails in nanoscale boiling. The explosive boiling modes and dynamics are summarized with the phase diagram based on various simulation cases, considering the effects of surface wettability, film thickness, and surface superheat. When the films thickness exceeds the critical thickness, hydrophobic surfaces are more favorable for explosive boiling, which still agrees with the classical nucleation theory. However, a much higher superheat is required to trigger the explosive boiling of a thin liquid film when its thickness is less than the critical thickness on hydrophobic surfaces, which consists with previous molecular dynamics simulations. Furthermore, we also find that the explosive boiling is trigged faster for films on hydrophilic surfaces than those on hydrophobic surfaces with the same superheat owing to the lower Kapitza thermal resistance. With such heat transfer superiority, hydrophilic surfaces can heat liquid films faster to explosion. The deliveries of this work and the concept of the critical thickness might help to understand the still fledgling field of nanoscale boiling phase change and its relevant mechanisms.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Takuma Kogawa, Eita Shoji, Junnosuke Okajima, Atsuki Komiya, Shigenao Maruyama〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this study, the radiation effects on the temperature field and perturbation of the thermal boundary layer of natural convection for gas were evaluated by an interferometer. The natural convection investigated in this study was that of a cavity, which had differentially heated vertical walls, surrounded by adiabatic walls. The target Rayleigh number was of the order of 10〈sup〉8〈/sup〉–10〈sup〉9〈/sup〉. By using the interferometer adopted in a phase-shifting technique, surface and gas radiation effects were evaluated. To compare the experimental results, a numerical calculation was also conducted using a large-eddy simulation coupled with radiative heat transfer. By varying the emissivity of the adiabatic walls while maintaining the emissivity of the isothermal walls, the surface radiation effect on the natural convection was analyzed. To neglect the gas radiation effect, air was used as the working fluid. When the adiabatic walls were black-body surfaces, the temperature difference around the adiabatic walls was higher than that of the reflective surface as a result of the optical path difference. From this result, the high temperature difference due to the surface radiation effect was confirmed. To investigate the gas radiation effect on natural convection, trifluoromethane gas, which has significant radiative absorption, was used as a working fluid. By evaluating the interference fringe by the interferometer, the wave motion of the thermal boundary layer around the heated and adiabatic walls was visualized. Comparing the numerical calculation results (when varying the calculation conditions) revealed that the wave motion and turbulence boundary layer around the heated and adiabatic walls were caused by the gas radiation effect.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Zhaoqing Ke, Chung-Lung Chen, Kuojiang Li, Sheng Wang, Chien-Hua Chen〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This paper reports a numerical investigation of fluid flow and heat transfer in a rectangular channel with delta-shaped winglet longitudinal vortex generators (LVGs) under different configurations. In addition to the conventional common-flow-down configuration and the common-flow-up configuration, a unique mixed configuration is suggested. The “method of images” is successfully adopted to analyze the dynamics of the longitudinal vortices due to wall interference. The Nusselt number, friction factor and overall performance coefficient for the three configurations are compared at various Reynolds numbers (all less than 2200), LVG row numbers, channel heights, and LVG aspect ratios. It is found that the channel height and LVG aspect ratio are the two most critical factors influencing the effectiveness of the different LVG configurations, which may explain why inconsistent conclusions have been presented by previous studies comparing the performance of the common-flow-down and common-flow-up configurations. When the channel height is relatively small, the unique mixed configuration is found to be the most effective at enhancing fluid mixing (and therefore improving heat transfer) at streamwise cross sections. When the LVG aspect ratio becomes large, the common-flow-down configuration outperforms the common-flow-up configuration. This paper sheds some insight on how to design the optimal LVG configuration for enhancing heat transfer performance.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): M. Ciofalo, M. Di Liberto, L. Gurreri, M. La Cerva, L. Scelsi, G. Micale〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The problem of mass transfer in ducts with transpiring walls is analysed: the concepts of “solvent” and “solute” fluxes are introduced, all possible sign combinations for these fluxes are considered, and relevant examples from membrane processes such as electrodialysis, reverse osmosis and filtration are identified. Besides the dimensionless numbers commonly defined in studying flow and mass transfer problems, new dimensionless quantities appropriate to transpiration problems are introduced, and their limiting values, associated with “drying”, “desalting” and “saturation” conditions, are identified. A simple model predicting the Sherwood number Sh under all possible flux sign combinations is derived from the single simplifying assumption that concentration profiles remain self-similar (so that the Sherwood number based on diffusion only remains unchanged) also under transpiration conditions. The simple model provides not only local values of Sh, but also its axial profiles. Predictions are validated against fully predictive CFD results, not based on the above simplifying assumption, and a good agreement is demonstrated provided the transpiration rate complies with certain limitations, depending on the Schmidt number.〈/p〉〈/div〉 〈/div〉 〈div xml:lang="en"〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S0017931018337025-ga1.jpg" width="500" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Austin Palya, Omid A. Ranjbar, Zhibin Lin, Alexey N. Volkov〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Expansion of vapor plume induced by short-pulse laser irradiation of a bottom of a cylindrical cavity or planar trench in a copper target into helium, argon, and xenon background gases at pressure varying from zero to 1 bar is studied numerically. The simulations are performed based on a hybrid computational model that includes an unsteady heat conduction equation, Hertz-Knudsen model of evaporation, and a kinetic model of two-component gas flow implemented in the form of the Direct Simulation Monte Carlo method. It is found that increasing background pressure results in the formation of a complex structure of shock waves, which drastically changes the flow compared to the case of expansion into a vacuum. The moving shocks strongly affect the deposition of the ablated material inside the cavity. In particular, they induce the formation of a near-surface layer with a reduced fraction of vapor, which protects the cavity wall from vapor redeposition. The overall effect of the increasing background gas pressure on the flow structure and redeposition of vapor inside the cavity can be described as a trade-off between the confinement effect, i.e. a reduction in the propagation speed of the primary shock and overall rate of the plume expansion, and the focusing effect of moving shock waves that induce the radial flow towards the cavity axis, increase vapor density around the axis of symmetry, and reduce vapor density at the cavity wall. The efficiency of vapor removal out of the cavity can increase or decrease with increasing molar mass of the background gas depending on whether the flow is dominated by the focusing or confinement effect. These findings suggest that tuning the background gas parameters and geometrical parameters of spatial confinement can be used to effectively control the laser-induced plume expansion process in applications of laser ablation.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): S. Nangle-Smith, J.S. Cotton〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉EHD is an active heat transfer enhancement technique for convective boiling or condensing dielectric fluids whose performance varies widely in the literature for even the same geometry and fluids. As such, the performance of EHD convective boiling devices remains largely unpredictable. Two empirical EHD convective boiling heat transfer coefficient correlations exist in the literature with good performance when compared to their exact test dataset and geometry, however they have been shown to have poor correlations in predicting the performance for external test datasets.〈/p〉 〈p〉In this paper, a phenomenological approach is taken in the development of a new EHD convective boiling heat transfer coefficient correlation and a novel EHD convective boiling pressure drop correlation. The performance correlations are based on widely-used free-field convective boiling correlations for heat transfer coefficient and pressure drop with phenomenological enhancement factors based on the two phase flow pattern as predicted using the EHD two phase flow pattern map which accounts for the effect of electric field strength on flow pattern redistribution, as described in a previously published paper (Nangle-Smith and Cotton, 2018).〈/p〉 〈p〉A review of the EHD convective boiling experimental literature was conducted to determine confounding factors that can impact the large variance in the reported performance. Flow pattern and applied heat flux were identified as common parameters not maintained constant in the experimental data. Two datasets, including data from the present study, in which flow pattern and applied heat flux are maintained constant were used in the development of the performance correlations.〈/p〉 〈p〉Considerable improvement over previous EHD convective boiling performance correlations was found both in error and the physical significance of coefficients. The correlations were developed for thermodynamic qualities in the range of 20–60% below the onset of dryout or mist flow patterns and for applied heat fluxes 〈 30 kW/m〈sup〉2〈/sup〉. Furthermore, test conditions in this study were chosen to focus on the dielectrophoretic effect and therefore studies with significant electrophoresis were not included. Thus, the correlation is limited to positive applied voltages, saturated flow boiling conditions, and field strengths below the onset of charge injection. The authors recommend this approach for EHD two phase performance correlation development as it is more mechanistic, is analogous to the state-of-the-art approaches for free-field two phase performance, and is likely to yield more accurate models as more experimental data becomes available.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Jue Wang, Yi Cheng, Xiao-Bin Li, Feng-Chen Li〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Surfactant additives can prevent bubbles merging and change the flow mode in flow boiling. This work presented the effect of bubbles merging on flow boiling heat transfer performance of water. Flow boiling experiment and lattice Boltzmann method (LBM) were carried out in a rectangular channel. With a high speed camera, the flow boiling process has been recorded. D2Q9 model and the Peng-Robinson (P-R) equation of two phase state (EOS) were used to simulate flow boiling process. The present experimental results showed that surfactant addition could enhance the flow boiling heat transfer and increased the critical heat flux (CHF). And there were more nucleation sites, larger bubbles density and smaller bubble size in surfactant solution. The present simulation results showed that the flow boiling without bubbles merging has higher and more stable average 〈em〉Nu〈/em〉 number. The bubbles merging made the middle nucleation sites lose effectiveness. It implies that preventing bubbles merging is another factor of surfactant addition enhancing boiling heat transfer.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Jiateng Zhao, Wei Jiang, Zhonghao Rao〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Aiming to combining the strength of flexibility and adaptation of oscillating heat pipe (OHP) with specific application in the field of waste heat recovery and storage, an OHP with external expansion structure was designed and fabricated. The start-up characteristic, working status, overall thermal resistance, local thermal resistance and dispersion ratio for the local thermal resistances under different filling ratios (FR), working fluids and heat loads were revealed and compared experimentally. The results showed that for the OHP with water, it exhibits well start-up performance under wide range of FRs and is able to work effectively under wide range of FR at proper heat load. Meanwhile, it also shows well adaptability to heat load under proper FR. The OHP has not obvious difference for each branch in the heat transfer performance under wide working conditions. When filled with self-rewetting fluid (SRWF), it has smaller overall thermal resistance than the water case at specific range of heat load. Comparing to the case with water, the OHP with SRWF can show better uniformity in the heat transfer capacity for each branch under appropriate condition. However, the uniformity of branch does not always contribute to increasing the heat transfer performance.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Robin Campet, Manqi Zhu, Eleonore Riber, Bénédicte Cuenot, Marouan Nemri〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This work presents a study of the turbulent flow in a single-started helically ribbed tube with low blockage ratio. The Large Eddy Simulation (LES) approach is used in a wall-resolved periodic configuration. Both an adiabatic and a wall-heated simulations are performed and validated against experiment. Velocity profiles and wall temperatures were measured at the Von Karman Institute (VKI) using Stereoscopic Particle image Velocimetry (S-PIV) and Liquid Crystal Thermography (LCT) by Mayo et al. (2018). Comparisons show that the numerical methodology gives accurate results in terms of mean and fluctuating velocity fields as well as the correct friction drag. The wall temperature profile is also in good agreement with the experiment. The rib induces a large recirculation zone immediately downstream, with a reattachment point occurring a few rib heights farther downstream. The helical shape of the rib also induces a strong swirling motion close to the wall. The pressure drop is found equal to 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si20.gif" overflow="scroll"〉〈mrow〉〈mn〉3.37〈/mn〉〈/mrow〉〈/math〉 Pa/m and is mostly due to the pressure drag. Maximum heat transfer is found just upstream of the reattachment point and on top of the ribs, which is in good agreement with experimentally obtained values. The mean Nusselt number in the ribbed tube is found 2.3 times higher than in a smooth tube confirming the positive impact of such geometry on heat transfer.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Eduardo Aktinol, Vijay K. Dhir, Talbot Jaeger, Walt Mirczak〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Pool boiling has been extensively investigated for the last eight decades experimentally and for almost two decades numerically. As a parameter of interest, the system pressure is generally kept constant in numerical simulations of a given set of experimental conditions. Alternatively, the system pressure may be an input parameter that may depend on time. However, in the absence of active cooling of hot liquid especially in microgravity, bubbles can grow to take up a significant portion of a boiling chamber and thus pressure can vary significantly over time in a fixed-volume chamber. In this work, numerical simulations of the boiling process are performed in which the increase in thermodynamic pressure inside a fixed-volume chamber and its feedback on the boiling process is included. The test liquid used in the simulations is R-134a. The results show that the effect of pressure on volume changes due to phase change is strong. The pressure response provides a feedback for the volume changes by altering heat transfer rates due to corresponding changes in saturation temperature. The pressure response to volume change is stronger for a chamber that has comparatively more liquid than vapor due to reduced compressibility. In microgravity bubble dynamics vary greatly depending on chamber fill level and heat transfer rates also depend on chamber fill level during boiling and natural convection.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Houpei Li, Pega Hrnjak〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This paper presents visualization of the two-phase flow of R134a in a 24-port microchannel tube with the average hydraulic diameter of 0.643 mm. The experiment is conducted on the same facility in the previous work for R32 (Li and Hrnjak, 2019). Mass flux covers the range from 50 to 250 kg-m〈sup〉−2〈/sup〉s〈sup〉−1〈/sup〉. The two-phase flow is generated by adding heat in several steps to originally subcooled refrigerant at the very entrance. When mass flux is 50 kg-m〈sup〉−2〈/sup〉s〈sup〉−1〈/sup〉, no annular flow is observed in the tube. The annular flow starts at x = 0.9 when mass flux is 100 kg-m〈sup〉−2〈/sup〉s〈sup〉−1〈/sup〉 and x = 0.6 when 250 kg-m〈sup〉−2〈/sup〉s〈sup〉−1〈/sup〉. The transitional flow starts at x = 0.25 (G = 50 kg-m〈sup〉−2〈/sup〉s〈sup〉−1〈/sup〉) and x = 0.8 (250 kg-m〈sup〉−2〈/sup〉s〈sup〉−1〈/sup〉). Revellin and Thome (2007), Ong and Thome (2011), and Zhao and Hu (2000) correlations are plotted against the measurements. None of them fit our data since the experiment is in lower mass flux than the database of their correlations’. The interface velocity and vapor plug length fraction (close to void fraction) are measured from the high speed videos. The velocity and vapor fraction agree to results based on the homogeneous assumption. In plug/slug flow, both the vapor plug and liquid slug lengths are uneven. The length distribution of plug and slug seems to follow Beta distribution. When the quality is low, the quantity of short vapor plugs is larger than long plugs. At fixed mass flux, when quality increases, length of vapor plugs increases and length of liquid slugs decreases.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 132〈/p〉 〈p〉Author(s): Yan Hou, Pengfei Jie, Huanguang Wang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The experimental study of heat transfer and film dynamics of droplet impacting onto flowing film using Novec-7100 as the cooling liquid was presented in this article. The droplet velocity and the liquid film velocity could be adjusted in this experimental study. The Reynolds number of the flowing liquid film varied from 1257 to 6290. In this study, the substrate of heater was Zinc Selenide (ZnSe), and the heating element was a thin layer of Indium Tin Oxide (ITO). The overflow plate was used to make the liquid film thickness fixed at 1 mm. The heat transfer mechanisms of flowing liquid film (flowing film only), droplet impinging on the dry surface (droplet-dry surface) and droplet impinging on the flowing liquid film (droplet-flowing film) were discussed in this paper respectively. The results showed that when the averaged temperature of the heated surface does not reach the liquid saturation temperature, bubbles appear in the liquid film, and the bubbles are more dense downstream. When the averaged heat flux of the heated surface ranges from 6.2 × 10〈sup〉3〈/sup〉 W/m〈sup〉2〈/sup〉 to 3.4 × 10〈sup〉4〈/sup〉 W/m〈sup〉2〈/sup〉, the averaged heat transfer coefficient of the droplet-flowing film can be calculated by the superposition of the averaged heat transfer coefficient of the flowing film only and the averaged heat transfer coefficient of the droplet-dry surface, the maximum error is 14%. When the averaged heat flux of the heated surface ranges from 3.4 × 10〈sup〉4〈/sup〉 W/m〈sup〉2〈/sup〉 to 6.8 × 10〈sup〉4〈/sup〉 W/m〈sup〉2〈/sup〉, the Reynolds number of the flowing liquid film and the effect of the coupling between droplet impingement and flow boiling should be considered in the calculation of the averaged heat transfer coefficient of the droplet-flowing film. In addition, boiling heat transfer is the main heat transfer mechanism of droplet impinging on thin liquid film when the averaged heat flux of the heated surface ranges from 2 × 10〈sup〉4〈/sup〉 W/m〈sup〉2〈/sup〉 to 6.8 × 10〈sup〉4〈/sup〉 W/m〈sup〉2〈/sup〉.〈/p〉〈/div〉 〈/div〉