ALBERT

Description: 〈p〉Publication date: October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 141〈/p〉 〈p〉Author(s): D.Y. Yeo, H.C. NO〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this paper, two-phase drag models for a packed bed of uniform-size particles were suggested, and they were applied to the calculation of pressure drop and dryout heat flux. We provided physical basis for the two-phase flow regime model through the analysis of the interfacial friction (〈em〉F〈sub〉i〈/sub〉〈/em〉). The suggested model provides flow patterns representing bubbly, slug, and channel flow and considering three criteria including d〈sup〉2〈/sup〉〈em〉F〈sub〉i〈/sub〉〈/em〉/d〈em〉α〈/em〉〈sup〉2〈/sup〉 = 0, 〈em〉F〈sub〉i〈/sub〉〈/em〉 = maximum, and 〈em〉F〈sub〉i〈/sub〉〈/em〉 = 0. The results obtained from the three criteria were drawn with several observation-based experimental ones to generate the flow regime map (void fraction vs. particle diameter). Through the current flow regime map, we clearly saw the existence of channel flow in a packed bed with particles smaller than around 3.5 mm. Then, mechanistic interfacial friction models were developed on basis of the current two-phase flow map of bubbly flow, slug flow, channel flow and annular flow. The suggested interfacial friction models were validated with top- and bottom-flooding air-water experiments and boiling experiments. We found out that the capability of pressure drop estimation by the current model were significantly improved for a bed with small particles. Finally, a zero-dimensional dryout heat flux (DHF) model was derived using the suggested interfacial friction models, and validated against DHF experimental data for beds with 1-D configuration. The root-mean-square error (RMSE) of the suggested DHF model was 35%, which was the smallest among the RMSEs of the previous DHF models.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 141〈/p〉 〈p〉Author(s): Tianyu Ma, Lei Feng, Hu Wang, Haifeng Liu, Mingfa Yao〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Spray impingement has aroused more and more interests in recent years with the increase of injection pressure in internal combustion engines. In this paper, the near wall combustion after fuel spray impingement is studied based on an updated film development model and experiment in constant volume vessel. Relationship between spray injection and flame development under different ambient temperature is analyzed as well as the near wall distribution of combustion products. In the first part, a mathematical model that considered the effects of surface tension on spray impingement is built for better prediction of film development. Reasonable results are obtained at room temperature spray impingement case, and the split distribution of film depth is also well captured. In the second part, flame development in the near wall region is investigated based on the proposed numerical model and experiments. The result shows that the flame becomes circumferentially nonuniformed at lower ambient temperature (723 K) and the evolution of downstream flame become sensitive to the wall temperature. At lower ambient temperature (723 K), increasing wall temperature could enlarge the high temperature zone, which is helpful to accelerate the film evaporation and soot oxidation. At higher ambient temperature condition, the spray impact angle should be reduced to create more concentrated combustion (stronger stratification), which could improve the combustion efficiency in the near wall region.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 141〈/p〉 〈p〉Author(s): Youqiang Liao, Xiaohui Sun, Baojiang Sun, Yonghai Gao, Zhiyuan Wang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The three–phase gas–liquid–solid flow, caused by hydrate decomposition in cuttings is a main concern during drilling through gas–hydrate reservoir. In this study, a transient gas–liquid–solid flow model is developed considering the coupling interactions between hydrate dynamic decomposition, cuttings transport and heat transfer in multiphase flow. Using this model, the transient gas–liquid–solid flow behaviors are investigated. Numerical simulations show that the decomposition rate of hydrate in formation is only 1/140 of that in annular cuttings for a unit depth, therefore, the influences of hydrate decomposition in hydrate layers can be neglected. Hydrate particles undergo three processes from bottom hole to wellhead in annulus: non–decomposition, slow decomposition and rapid decomposition. In annulus where the depth is more than 400 m, hydrates decompose slowly and the decomposed gas hardly expands due to the high pressure. While, if the hydrates and decomposed gas return upwards to the position where the depth less than 400 m, the gas void fraction increases significantly, not only due to the faster decomposition rate of hydrates but also due to the more intense expansion of decomposed gas. After the hydrate particles return upwards to the wellhead, the behaviors of gas–liquid–solid flow tend to be a quasi–stable state. If there is no backpressure device at the wellhead, that is, the wellhead backpressure is 0 MPa, the gas void fraction at the wellhead can reach 0.68, which is enough to cause blowout accident. Increasing wellhead backpressure to 2 MPa through managed pressure devices and lowering the inlet temperature of drilling fluid to 17.5 °C except adjusting drilling fluid density can manage the gas void fraction within 10%.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 141〈/p〉 〈p〉Author(s): Ronghui Qi, Chuanshuai Dong, Li-Zhi Zhang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The focus of this work is on the liquid-air mass transfer mechanism that is critical for liquid desiccant dehumidification and many other absorption processes. Most existing mass transfer correlations heavily rely on specific experiments and show poor universality. Therefore, we proposed a new set of mass transfer correlations theoretically based on the film instability during falling film dehumidification. The flow dynamic, Marangoni effect and liquid/air contact conditions that affecting the interface characteristics and wetting factors are considered. The correlations were verified by comparing with experimental data from several widely-cited literatures. The tests in these literatures were conducted under a wide range of operating conditions and dehumidifier types. The newly-developed correlations provide an acceptable prediction for liquid-air mass transfer, showing close trends to all previous experimental results. The overall error of the new predictions, 20–30%, is close to those of empirical equations built in the specific literature. The factors that affect the interphase mass transfer by changing the film instability and the wetting factor are also analyzed. The increase in liquid Reynolds number shows the most significant effect as it could effectively increase the film instability and liquid-air contact area. The liquid contact angle on solid surfaces, regarding the wettability, also affects the mass transfer considerably. By reducing the contact angle from 90° to 10°, although the increase in Sherwood number is slight due to the suppression of film instability, the wetting factor is almost doubled, resulting in a significant growth in mass transfer performance. This new correlation examines the falling film mass transfer process in more detail, and is based on fewer simplifying assumptions and attempts to take more realistic situations into account. Findings presented herein contribute to a more fully understanding on the falling film behaviors during liquid/gas contact such as liquid desiccant dehumidification.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 144〈/p〉 〈p〉Author(s): Wahiba Yaïci, Evgueniy Entchev〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉In recent years, considerable interest has been given to the application of solar-powered cooling technology for use in buildings. Solar cooling systems look like to be a suitable substitution to the traditional vapour-compression electrical-driven machines. Solar systems have the advantage of using harmless working fluids, especially water. They also have the capacity to decrease the peak loads for electricity utilities and can contribute to a substantial reduction of the harmful CO〈sub〉2〈/sub〉 emissions, which produce the notorious greenhouse effect that in turn is responsible for global warming and its devastating consequences. Amongst cooling technologies, low-temperature, solar-powered adsorption chillers/heat pumps are arising as a sustainable alternative to electrical vapour-compression systems.〈/p〉 〈p〉This study aims at examining the impact of design and operating factors on an adsorption cooling system’s performance in a residential application. An unsteady Computational Fluid Dynamics (CFD) combined with a heat and mass transfer model of the adsorption cooling system using adsorbent/water pair, was created in order to predict the following: (1) Flow behaviour; (2) Pressure; (3) Temperature; and (4) Water adsorption distributions. For possible adsorbents, both silica gel and zeolite 13X were considered; however, it is worth mentioning that silica gel was used at a lower working temperature range, as required by the operation. This makes silica gel an efficient option for solar/heat driven residential cooling applications. For the CFD model implemented equations, two geometries found in literature were employed for validation. Validation of the unsteady simulation results with experimental data found in literature showed favourable agreements. In a parametric study, various computation cases underwent simulation over the duration of the adsorption mode, which considered the following set of factors: heat transfer fluid (HTF) velocity (〈em〉v〈/em〉); adsorbent bed thickness (〈em〉l〈sub〉bed〈/sub〉〈/em〉); heat exchanger tube thickness (〈em〉b〈/em〉); and adsorbent particle diameter (〈em〉d〈sub〉p〈/sub〉〈/em〉) in order to perform a detailed investigation for main geometrical and operating parameters’ influence upon system performance. Results obtained from CFD disclosed the significance of 〈em〉v〈/em〉, 〈em〉l〈sub〉bed〈/sub〉〈/em〉 and 〈em〉d〈sub〉p〈/sub〉〈/em〉 whereas 〈em〉b〈/em〉 was found having relatively minor modifications within the system performance. Additionally, the development of CFD combined with heat and mass transfer model serves as an effective tool for both simulation and optimisation of adsorption cooling systems as well as for performance predicting purposes.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 144〈/p〉 〈p〉Author(s): Wenzhong Gao, Qiaye Qi, Jiahao Zhang, Guangming Chen, Dawei Wu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Spray flash evaporation is an effective desalination method, which increases specific surface area of salty water by liquid atomization, thereby improving desalination performance and maximising low-grade heat source utilization. During evaporation, explosive boiling phenomenon occurs inside superheated droplets on a heated surface. In order to understand the mechanism of explosive boiling, spray flash evaporation of distilled water and 3.5 wt% salty water in a high vacuum vessel was observed visually. Meanwhile, a parametric study was carried out to scrutinize the impacts of the variation of ambient pressure, heat flux, and surface superheat degree. The experiment data indicates that nucleate site is located in the upper layer of a droplet due to internal superheated liquid and Marangoni convection. In different operating conditions, bubble fragmentation process or crown fragmentation process happens at nucleate site. The fragmentation time of pure water, which is mainly influenced by heat flux and surface superheat degree, shrinks with higher heat flux and higher surface superheat degree. The fragmentation time of 3.5 wt% salty water decreases with ambient pressure drops and superheat degree increments.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 144〈/p〉 〈p〉Author(s): Miaomiao Chen, Zhigang Yang, Zheyan Jin〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In the present study, we experimentally investigated the melting process of an ice bead on the smooth and micro-grooved surfaces under a hot shear flow. One smooth silicon surface and three micro-grooved silicon surfaces were fabricated and tested. A parameter study of the substrate surface temperature and the air flow speed was conducted. During the experiment, an ice bead was first formed from a freezing water droplet on a cold substrate surface. Then, the ice bead was exposed to a hot shear flow and its melting process was recorded by a CCD camera and an infrared camera simultaneously. As for the micro-grooved surfaces, the direction of the hot shear flow was parallel to the micro-grooves. The results showed that the melting process of the ice bead on the smooth and micro-grooved surfaces under a hot shear flow could be divided into three categories. The air flow speed, the surface temperature, and the type of the surface had a significant influence on which category the melting process of the ice bead belonged to. Besides, the presence of the micro-grooves was found to apparently affect the wetting length, the removable time, and the temperature along the centerline of the ice bead. In general, the micro-grooved surfaces were found to be more favorable for the ice bead melting process than the smooth surface.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 144〈/p〉 〈p〉Author(s): Kai Yang, Geng-Hui Jiang, Hai-Feng Peng, Xiao-Wei Gao〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this paper, combination of radial integration boundary element method (RIBEM) with complex variable and Levenberg-Marquardt algorithm (LMA) is firstly proposed to identify temperature-dependent conductivity in the inverse heat conduction problem. To obtain the simulative temperature, radial integration boundary element method is used to solve the transient nonlinear heat conduction problem with temperature-dependent conductivity. What’s more, RIBEM with complex variable, which transforms the real variables into complex ones in boundary element method, makes it possible to perform complex variable derivative method (CVDM) in LMA. Furthermore, because of the introduction of CVDM, the sensitivity coefficient matrix can be calculated accurately and efficiently, and then the identification of unknown variable can be achieved admirably in the inverse heat problem. Finally, different initial guess value and measurement errors are considered, respectively, and various numerical examples are presented to fully demonstrate the accuracy and feasibility of the proposed method in identifying temperature-dependent conductivity.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 144〈/p〉 〈p〉Author(s): Wonkeun Baik, Rin Yun〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The condensation heat-transfer coefficient and pressure drop for pure CO〈sub〉2〈/sub〉 and CO〈sub〉2〈/sub〉 + N〈sub〉2〈/sub〉 mixtures were investigated under the near-critical condition, which simulates the CO〈sub〉2〈/sub〉 transporting conditions under Carbon Capture, Transportation, and Storage (CCS). The experimental apparatus consists of a test section, heat exchangers, mass flow meters, temperature sensors, a magnetic gear pump, and a differential pressure transducer. The test section made with a copper tube was assembled with Polyvinyl Chloride (PVC) pipe to form a double tube. The condensation temperature and mole fraction of N〈sub〉2〈/sub〉 of the CO〈sub〉2〈/sub〉 mixtures ranged from 20 °C to 30 °C, and 1 to 5%, respectively. The mass flux was changed from 500, 600 and 700 kg·m〈sup〉−2〈/sup〉s〈sup〉−1〈/sup〉. The average heat transfer coefficient of the CO〈sub〉2〈/sub〉 + N〈sub〉2〈/sub〉 mixtures decreased by 2.23 to 15.9% based on the average heat transfer coefficient of pure CO〈sub〉2〈/sub〉, and the pressure drop decreased by 53.4 to 77.3% at the condensation temperature of 25 °C with increase of the mole fraction of N〈sub〉2〈/sub〉.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 144〈/p〉 〈p〉Author(s): Yong Jin, Mohammed Albaity, Yusuf Shi, Noreddine Ghaffour, Peng Wang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Water condensation is an important phase change phenomenon whose applications range from power generation to water desalination. In the present study, we compared condensation occurring on two different substrates (namely square and strip) and demonstrated the effect of substrate geometry on water condensation. It is found that condensation on different regions of the same substrate is dramatically different due to different local vapor flux. In general, the condensation rate is linearly proportional to vapor flux while average vapor flux can be improved by creating geometrical discontinuity (strip substrate) within rigid substrates. Experimental result of water collection confirms that the condensation rate is increased by around 40% on the strip substrate compared to the square substrate. This study demonstrates that water condensation can be enhanced by rationally tuning the geometry of the condensation substrate. Performance of water condensation of a specific substrate can be predicated by simulating the vapor flux over the substrate.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 144〈/p〉 〈p〉Author(s): Hao Zhang, Ningfei Wang, Zhiwen Wu, Wanzhi Han, Rui Du〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A new theoretical model has been developed to calculate the regression rate of solid fuel scramjet (SF-Scramjet). This model obtains the local regression rate by calculating the local heat flux on the fuel surface, which is obtained by a standard wall function. A two-dimensional, axisymmetric, turbulent, one-reaction model which used the new method was developed to study the solid fuel regression rate for SF-Scramjet numerically with a flight environment of 25 km and a Mach number of 6. The quasi-steady state numerical method has been adopted to calculate the regression rate at different times. Experiments were carried out on the ground dedicated connected-pipe static test facility under the same boundary conditions to verify the correctness of the model. The results demonstrate that the numerical results agree well with the experimental results in the first few seconds. In the next few seconds of combustion, the numerical results of the regression rate gradually deviate from the experimental results. This may be due to the gradual accumulation of errors in the quasi-steady method.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 144〈/p〉 〈p〉Author(s): Fan Wu, Liang Li, Jiefeng Wang, Xiaojun Fan, Changhe Du〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this paper, four swirl and impingement composite cooling structures are established to deeply study the flow and heat transfer characteristics, where the swirl nozzles and impingement nozzles are reasonably arranged. Numerical simulation is conducted by solving the Reynolds Averaged Navier-Stokes (RANS) equations with the standard 〈em〉k〈/em〉-〈em〉ω〈/em〉 model. Meanwhile, numerical results are compared with the cooling behaviors of swirl cooling and impingement cooling under the same condition. Results revealed that the pressure distribution of four composite cooling structures is quite different from that of swirl cooling and impingement cooling. Hence, the nozzle mass flow ratio distribution of composite cooling structures displays a large fluctuation with the variation of the nozzle location, which has an influence on the flow and heat transfer characteristics. Moreover, the heat transfer characteristics of swirl and impingement composite cooling combine the advantages of impingement cooling and swirl cooling, where there both exists extremely high local heat transfer regions and uniform heat transfer regions. As for composite cooling 3 and composite cooling 4, the alternate locations of impingement nozzles and swirl nozzles could effectively increase the band-shaped high heat transfer area. Meanwhile, the low heat transfer area caused by the continuous arrangement of impingement nozzles is reduced. Among four composite cooling structures, the composite cooling 4 has the highest average heat transfer coefficient and the minimum pressure loss. The globally average heat transfer of composite cooling 4 is 3.49% lower than swirl cooling but is 19.12% higher than impingement cooling. Its total pressure loss is 4.29% lower than swirl cooling and is slightly lower compared with impingement cooling.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 144〈/p〉 〈p〉Author(s): Fang Li, Wenhui Zhu, Hu He〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Microchannel cooling technology using nanofluid is one of the effective ways to solve the rapid heat dissipation in a limited space for high power density device. In this work, the field synergy and heat transfer performance of nanofluid in the microchannel with non-uniform internal spoiler cavities configuration based on conjugate heat transfer and homogenous model were investigated. The heat transfer enhancement mechanism of nanofluid in this complex microchannel was discussed through the analysis of field synergy angle, field synergy factor, and flow and heat transfer characteristics. It was found that the flow and heat transfer performance of the nanofluid in the microchannel can be well explained by the field synergy principle. The nanofluid heat transfer enhancement phenomenon could be attributed to the improvement of the field synergy of the thermal boundary layer under the axial thermal conductive effect. In addition, the field synergy analysis of nanofluid in complex microchannel revealed that the heat transfer enhancement was affected by the combination of perturbation effect, the axial thermal conduction effect and the thermal boundary redevelopment.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 144〈/p〉 〈p〉Author(s): Yang Xu, Chanhee Moon, Jin-Jun Wang, Oleg G. Penyazkov, Kyung Chun Kim〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This study experimentally investigated the combined effects of the wall temperature (〈em〉T〈sub〉w〈/sub〉〈/em〉〈sub〉0〈/sub〉) and the orifice-to-wall distance (〈em〉H〈/em〉/〈em〉D〈/em〉) on the flow and heat transfer characteristics of an impinging synthetic jet. Thermographic phosphor thermometry (TPT) was used to measure the wall surface temperature, and the quantitative flow velocity was obtained using time-resolved particle image velocimetry (PIV). A cavity-diaphragm actuator was employed to generate a round synthetic jet to impinge onto a heated wall that was coated with Mg〈sub〉4〈/sub〉FGeO〈sub〉6〈/sub〉:Mn (MFG) for use as a temperature sensor. Three orifice-to-wall distances (〈em〉H〈/em〉/〈em〉D〈/em〉 = 10, 15, and 20) and three wall temperatures (〈em〉T〈sub〉w〈/sub〉〈/em〉〈sub〉0〈/sub〉 = 60 °C, 90 °C, and 120 °C) were tested for comparison, whereas the the operating conditions of the incident synthetic jet were kept constant. It was found that the maximum temperature drop at the stagnation point could reach approximately 50°C although the orifice-to-wall distance was relatively large in this study, which indicated a good cooling performance of the synthetic jet. The penetration of the wall shear layer played an important role on the cooling performance of the impinging synthetic jet. For a heated wall with high 〈em〉T〈sub〉w〈/sub〉〈/em〉〈sub〉0〈/sub〉, the enhanced buoyancy and thermal boundary layer resulted in the formation of a strong wall shear layer, which was more difficult for the impinging synthetic jet to affect or penetrate. For a large 〈em〉H〈/em〉/〈em〉D〈/em〉, the vortex rings of the synthetic jet lose coherence completely before impacting the wall. Thus, they have no ability to penetrate the wall’s shear layer to interact with it directly. As a result, the cooling performance of the impinging synthetic jet gradually decreased as both 〈em〉T〈sub〉w〈/sub〉〈/em〉〈sub〉0〈/sub〉 and 〈em〉H〈/em〉/〈em〉D〈/em〉 increased. In particular, the maximum cooling effect by the synthetic jet impingement can achieve 64% of the theoretical maximum.〈/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-S0017931019325189-ga1.jpg" width="419" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 144〈/p〉 〈p〉Author(s): Panding Wang, Hongshuai Lei, Xiaolei Zhu, Haosen Chen, Daining Fang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Powder-based metallic additive manufacturing (AM) technology is opening new avenues to fabricate highly complex components from metallic powders. However, most of the powder-scale modeling methods are limited to single track process and ideal particle microstructure. Nevertheless, the presence of hollow particles significantly influences the heat conduction during AM processing and experimental quantification of the heat conduction between hollow particles is extremely challenging. Herein, we have used X-ray micro-computed tomography (μCT) to reconstruct 3D structures of AlSi10Mg particles. The morphology, location and distribution of intact and hollow particles are studied to analyze their role in AM processing. Based on X-ray tomography images, two 3D image-based finite element models of statistically representative particles with imperfect geometry are reconstructed and compared to simulate the thermal conduction in the powder bed. Simulation results shows that thermal conduction is governed not only by cell topology but also by cavities in particles induced by powder production. The calculation results are consistent with the Serial-Parallel Model, which is based on the reconstruction geometry model and statistical results. The results reveal that the presence of cavities in particles significantly influences the thermal conduction and, consequently, reduces the sintered density during selective laser sintering (SLS).〈/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-S0017931019301486-ga1.jpg" width="319" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 144〈/p〉 〈p〉Author(s): Ukmin Han, Heeseung Kang, Hongyoung Lim, Jeongwan Han, Hoseong Lee〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Recently, there has been great attention to find new materials of a heat exchanger to replace aluminum which has been typically used for a long time as a basic material of heat exchangers. In this context, polymeric materials are considered as candidates due to their characteristics of lightweight and excellent corrosion resistance. Despite substantial benefits of polymeric materials, there is a critical issue as used for the heat exchanger, which is the low heat transfer performance due to their extremely low thermal conductivity. In this study, to overcome this limitation, a novel polymer heat exchanger is proposed with the new heat flow design. The finless teardrop-shaped tube bundle polymer heat exchanger is newly designed and its thermal-hydraulic performances are investigated with experiments and simulations. Then, the geometries of the novel polymer heat exchanger are optimized to maximize the thermal and hydraulic performance by using the online approximation-assisted optimization technique.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 144〈/p〉 〈p〉Author(s): Rui Hou, Fengbo Wen, Yuxi Luo, Xiaolei Tang, Songtao Wang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The thermal and flow fields of round and trenched holes in the flat-plate model are investigated using large eddy simulation (LES) after validated against the experimental results. The focus is on understanding the influence of the trenched hole on downstream vortex structures at blowing ratios M = 0.5 and 1.0, which may benefit its effective application in cooling design. At M = 1.0, the transverse trench increases turbulent fluctuations and augments the complexity of vortex structures. Dynamic mode decomposition analysis is employed to extract the primary vortex structures. The dominant vortices of the trenched hole include K-H vortices and hairpin-like vortices near the centerline. This series of hairpin-like vortices present a larger spatial size than the hairpin vortex downstream of the round hole. Besides, they correspond to the downstream counter-rotating vortex pair in the mean flow field, which is detrimental to the local film cooling effectiveness. At lower blowing ratio M = 0.5, the previous spatially large hairpin-like vortices are replaced by a smaller one which alternatively appears on both sides of the centerline. In the mean flow field, each branch of the CVP is caught between two anti-CVPs. The suppression of adjacent vortices guarantees the attachment of coolant to the surface.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Wenyao Zhang, Qiuwang Wang, Min Zeng, Cunlu Zhao〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A systematic theoretical study of thermoelectric effect and temperature-gradient-driven electrokinetic flow of electrolyte solutions in charged nanocapillaries is presented. The study is based on a semianalytical model developed by simultaneously solving the nonisothermal Poisson-Nernst-Planck-Navier-Stokes equations with the lubrication theory. Particularly, this paper clarifies the interplay and relative importance of the thermoelectric mechanisms due to (a) the convective transport of ions caused by the fluid flow, (b) the dependence of ion electrophoretic mobility on temperature, (c) the difference in the intrinsic Soret coefficients of cation and anion. Additionally, synergy conditions for the three thermoelectric mechanisms to fully cooperate are proposed for thermo-phobic/philic electrolytes. The temperature-gradient-driven electrokinetic flow is shown to be a nearly unidirectional flow whose axial velocity profiles vary with the axial location. Also, the flow can be regarded as a consequence of the counteraction or cooperation between a thermoelectric-field-driven electroosmotic flow and a thermo-osmotic flow driven by the osmotic pressure gradient and dielectric body force. Moreover, the Seebeck coefficient and the fluid average velocity are demonstrated to be affected by electrolyte-related parameters. The results are beneficial for understanding the temperature-gradient-driven electrokinetic transport in nanocapillaries and also serve as theoretical foundation for the design of low-grade waste heat recovery devices and thermoosmotic pumps.〈/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-S0017931019322598-ga1.jpg" width="268" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Marco Gandolfi, Giulio Benetti, Christ Glorieux, Claudio Giannetti, Francesco Banfi〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In order to account for non-Fourier heat transport, occurring on short time and length scales, the often-praised Dual-Phase-Lag (DPL) model was conceived, introducing a causality relation between the onset of heat flux and the temperature gradient. The most prominent aspect of the first-order DPL model is the prediction of wave-like temperature propagation, the detection of which still remains elusive. Among the challenges to make further progress is the capability to disentangle the intertwining of the parameters affecting wave-like behaviour. This work contributes to the quest, providing a straightforward, easy-to-adopt, analytical mean to inspect the optimal conditions to observe temperature wave oscillations. The complex-valued dispersion relation for the temperature scalar field is investigated for the case of a localised temperature pulse in space, and for the case of a forced temperature oscillation in time. A modal quality factor is introduced showing that, for the case of the temperature gradient preceding the heat flux, the material acts as a bandpass filter for the temperature wave. The bandpass filter characteristics are accessed in terms of the relevant delay times entering the DPL model. The optimal region in parameters space is discussed in a variety of systems, covering nine and twelve decades in space and time-scale respectively. The here presented approach is of interest for the design of nanoscale thermal devices operating on ultra-fast and ultra-short time scales, a scenario here addressed for the case of quantum materials and graphite.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Xiaoling Yu, Zhao Lu, Liyu Zhang, Lichuan Wei, Xin Cui, Liwen Jin〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The thermal characteristics of lithium-ion battery affect significantly charging/discharging performance, cycle life and safety of electric vehicles (EVs) battery packs. In this study, a stagger-arranged battery pack consisting of three battery modules was developed to explore its transient thermal characteristics in charging/discharging process under the two cooling strategies, i.e., natural cooling and forced air cooling. The investigation of heat generation behavior of the battery with Li(Ni〈sub〉x〈/sub〉Co〈sub〉y〈/sub〉Al〈sub〉z〈/sub〉)O〈sub〉2〈/sub〉 cathode showed that the heat generation rate of the battery remains almost unchanged along the main discharging process, while a rapid increase in heat production is detected at the end of discharging. It was found that the maximum temperature and temperature difference in the battery pack subject to a moderate charging/discharging rate, e.g., 0.5 C, can be maintained within the desirable ranges by natural cooling. The forced air cooling strategy employing longitudinal airflow remarkably improves the battery’s transient thermal characteristics with achieving the depth of discharge (DOD) up to 84.2%, which is capable to prolong the battery pack’s cycle life to a large extent. Lastly, the appropriate air supply velocity of 0.8 m⋅s〈sup〉−1〈/sup〉 is recommended for the proposed battery pack subject to a higher discharging rate, e.g., 1 C, from the viewpoint of cooling effectiveness.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Yonghui Jia, Hang Wang, Qichi Le〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A transient 2D axisymmetric mathematical model that couples the pulse electromagnetic field with fluid flow and solidification was established by using the COMSOL Multiphysics software. Based on the measured pulse currents under different electromagnetic parameters, the model was firstly validated, and then the solidification processes of direct-chill (DC) casting in the absence and presence of pulse magnetic field (PMF) were simulated and discussed, including the variations in fluid flow, heat transfer, and solidification characteristics at different locations of the melt. Finally, effects of pulse electromagnetic parameters (current intensity, electromagnetic frequency, and duty cycle) on Lorentz force, flow field, temperature field, and solidification during DC casting of AZ80 magnesium alloy were studied systematically. The forced convection induced by PMF can significantly accelerate the melt flow and heat extraction. As the increase of current intensity, Lorentz force, melt convection, and heat extraction are strengthened considerably, and the increase of frequency has the opposite effect on them. The effect of duty cycle on solidification process is extremely limited. For billets with different magnesium alloy systems and sizes, a fined and uniform solidified structures can be obtained by adjusting the current intensity and electromagnetic frequency in pulse electromagnetic DC casting.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Yicun Tang, Yuan Zhang, Jingchun Min〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Experimental studies have been conducted to investigate the wet porous material convective air-drying characteristics. An experimental setup is designed and constructed to examine the air-drying process of a wet sand layer, and it allows dynamic measurements of both the sand layer weight and temperature. Hot air flows down towards the sand layer upper surface to realize a uniform drying of it and multiple thermocouples are embedded in the sand layer to evaluate its temperature distribution. Experiments are carried out for three sand layer thicknesses of 2, 4 and 8 mm and for three hot air temperatures of 45, 60 and 75 °C. Water film evaporation experiments are also performed to obtain the convective heat and mass transfer coefficients, which are then used for the lumped parameter analysis of the sand layer heat and mass transfer. The results show that in the water film evaporation experiment, the water temperature initially increases and then remains constant at the air wet-bulb temperature for a long period of time until the water all evaporates. In the sand layer drying experiment, however, the sand layer temperature continues to increase throughout the drying process, which can be divided into three stages, i.e. the initial rapid, intermediate slow, and final rapid increase stages. The lumped parameter method is employed to analyze the sand layer heat transfer because of the small sand layer thickness. Such analysis uses the heat transfer coefficient obtained in the water film evaporation experiment and the sand layer temperature increasing rate to calculate the sand layer moisture content and drying rate. The obtained results agree well with the experimental data. The evolution of water film cover fraction on the sand layer upper surface during drying is also discussed by assuming that the water evaporation occurs mainly at the sand layer upper surface at the drying stage at which the sand layer upper surface is fully wet or partly wet.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Xiaohui Bai, Zihao Zheng, Akira Nakayama〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A systematic 3D numerical experiment was conducted for forced convection in a series of isothermally heated sandwich panel structures filled with metal foam, rectangular corrugated cellular structures and various lattice core structures, such as vertical lattices, slanted lattices, Kagome lattices, tetrahedral lattices and pyramidal lattices. An in-house computer code based on a finite volume method with SIMPLE algorithm was used to solve the 3D set of the governing equations, namely, the continuity equation, Navier-Stokes equation, fluid phase energy equation and solid phase heat conduction equation, simultaneously. The values of the Nusselt number under equal pumping power were numerically determined to make a fair heat transfer performance evaluation on these various structures. This performance evaluation subsequently led to a proposal for a novel lattice core structure, “windward bend structure”, in which the lattices are bended in the windward direction and arranged in a staggered fashion. This novel structure exhibited an excellent heat transfer performance due to its enhanced macroscopic thermal dispersion closely associated with the flow pathline deflection towards the heated end-walls. This study provides numerical evidence to prove that the windward bend structure is a strong candidate for the new generation of compact heat exchanger systems.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Felipe Huerta, Velisa Vesovic〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉New analytical solutions have been derived for the isobaric evaporation of a pure liquid cryogen. In particular, expressions have been provided for the liquid volume, evaporation rate, Boil-off-Gas (BOG) rate, vapour temperature and vapour to liquid heat transfer rate as a function of time. Both equilibrium and non-equilibrium scenarios have been considered. In the former, the vapour and liquid cryogen are assumed to be in thermal equilibrium, while in the latter the vapour is treated as superheated with respect to the liquid and acts as an additional heat source.〈/p〉 〈p〉The derived solutions for two scenarios were validated against the numerical results for the evaporation of liquid methane and of liquid nitrogen in small, medium sized and large storage tanks that are used in industry. For the equilibrium model, the analytical solutions are exact. For the non-equilibrium model, the analytical solutions are valid for the whole duration of evaporation, except for a short transient period at the beginning of the evaporation. For physical quantities of industrial interest, they provide accurate estimates of liquid volume, BOG rate and BOG temperature, with the maximum deviations not exceeding 1%, 2% and 4.5%, respectively. The vapour to liquid heat transfer rate is also well predicted to within a maximum deviation of 5%.〈/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-S0017931019333812-ga1.jpg" width="483" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Farzad Pourfattah, Ali Akbar Abbasian Arani, Mohammad Reza Babaie, Hoang M. Nguyen, Amin Asadi〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The main objective of the present study is to numerically simulate the heat transfer and fluid flow characteristics of water/CuO nanofluid under laminar flow regime in a manifold microchannel. Ensuring the accuracy of the numerical procedure, the obtained results have been compared with the experimental data available in the literature. The Euler multi-phase method considering temperature-dependent thermophysical properties have been employed to simulate the fluid flow and heat transfer. The effects of in/out ratio, Reynolds number, and solid concentration of nanoparticles as independent parameters on the heat transfer and flow field characteristics have been investigated. The obtained results revealed that the heat transfer and friction coefficient had been enhanced by increasing the in/out ratio at a constant Reynolds number. Furthermore, it is observed that by increasing the Reynolds number, the effects of in/out ratio becomes more considerable. According to obtained results, by increasing the solid concentration of the nanoparticles, the friction coefficient increases due to the enhancement of the effective viscosity. Based on the present investigation, in/out ratio equal to 0.25 provides the maximum amount of performance evaluation criterion at the Reynolds number of 100 and solid concentration 2 vol%. Thus, under the studied conditions, the heat transfer enhancement, due to adding the nanoparticles, is higher than that of the pressure drop.〈/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-S0017931019333307-ga1.jpg" width="213" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Runkeng Liu, Zhenyu Liu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The cooling process on superheated surface at nanoscale is significantly distinct from that at macroscale or microscale. In this work, the boiling phenomena of liquid argon thin film on concave hemispherical nanostructure surfaces have been investigated with molecular dynamics (MD) simulation. For each surface, the substrate temperature was progressively increased and the non-equilibrium MD simulation was carried out to record the variation of atomic motion trajectories, number of vapor and liquid atoms, kinetic energy and internal energy with time. By comparing the obtained predictions of boiling heat transfer on different nanostructure configurations (concave hemispherical, flat and convex hemispherical), it shows that the nanostructure on solid wall can enhance the heat transfer process and concave hemispherical nanostructure presents a better thermal energy transfer capacity at the low substrate temperature. Moreover, the surfaces with different wettable concave nanostructure were considered to study the wettability effect on boiling heat transfer characteristics. The hydrophilic concave hemispherical nanostructure surface has a better heat transfer performance and the hydrophobic concave hemispherical surface has a better tolerance for heat transfer deterioration. The findings in this work prove the capability of enhancement technique for boiling heat transfer processes using concave nanostructure surface, which can be applied in future advanced cooling solutions.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Tianxi Xie, Yoshio Utaka, Zhihao Chen, Shoji Mori〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this study, the effect of heating surface size (squares with 10–40 mm edges) on critical heat flux (CHF) when applying a novel method of different-mode-interacting boiling in narrow gaps in a water pool was investigated. Nonuniform heating plates with alternately arranged materials with high and low thermal conductances were applied under various material widths operated with different gaps. Significant enhancements in CHF were observed for the nonuniform heating surface compared to a uniform surface, along with a clear effect of the heating surface size on CHF. Furthermore, the effects of surface size, gap size, and material width on CHF were classified into two trends; decreasing the gap and increasing the surface size tended to monotonically decrease CHF, while the material width had an optimum value for maximizing CHF for each surface size and gap. Therefore, it has been shown that different-mode-interacting boiling was effective in improving CHF even when the surface size was increased.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Wenbin Fei, Guillermo A. Narsilio, Joost H. van der Linden, Mahdi M. Disfani〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Coordination number can be used to quantify the particle connectivity and deformability of a granular material. However, it is a local feature of particles at the microscale, and the use of an average coordination number does not allow for full characterization of the microstructural variation in the granular material. Mesoscale structures can be used to overcome this limitation: triangular-like structures at the mesoscale tend to be rigid, whereas square-like structures tend to be deformable. However, the effect of these structures on heat transfer has not been studied in deforming granular materials. A better understating of how microstructure variation affects effective thermal conductivity is necessary. This work constructs contact networks representing the granular materials with particles as nodes and linking neighbouring nodes with edges that represent particle contacts. Then, ‘3-cycles’ (i.e., a triangular structure) and ‘clustering coefficients’ are extracted from the contact network. As contact thermal conductance is vital to heat transfer and affected by particle shape, microscale three-dimensional particle shape descriptors are also calculated. To compute the effective thermal conductivity of the granular assembly, a thermal network model is established by adding ‘near-contact’ edges to the contact network and assigning a thermal conductance to each edge. The results show that mesoscale local clustering coefficients can indicate the rigidity of granular materials and, together with particle shape descriptors, can be used to predict well the effective thermal conductivity of granular materials under deformation.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Anamika Maurya, Naveen Tiwari, R.P. Chhabra〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The present numerical work investigates the momentum and heat transfer characteristics of a Bingham plastic fluid in a rectangular T-channel. A rotating cylinder which is placed in a T-junction mimics the behaviour of a rotating valve to regulate the fluid flow and enthalpy in the two branches of the channel. Numerical simulations are carried out over a wide range of conditions (based on the cylinder diameter): Reynolds number 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si148.svg"〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈msup〉〈mrow〉〈mn〉10〈/mn〉〈/mrow〉〈mrow〉〈mo〉-〈/mo〉〈mn〉2〈/mn〉〈/mrow〉〈/msup〉〈mo〉⩽〈/mo〉〈mi mathvariant="italic"〉Re〈/mi〉〈mi〉≤〈/mi〉〈mn〉40〈/mn〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈/math〉, Bingham number 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si149.svg"〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈msup〉〈mrow〉〈mn〉10〈/mn〉〈/mrow〉〈mrow〉〈mo〉-〈/mo〉〈mn〉2〈/mn〉〈/mrow〉〈/msup〉〈mo〉⩽〈/mo〉〈mi mathvariant="italic"〉Bn〈/mi〉〈mi〉≤〈/mi〉〈mn〉20〈/mn〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈/math〉, Prandtl number 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si150.svg"〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈mn〉10〈/mn〉〈mo〉⩽〈/mo〉〈mi mathvariant="italic"〉Pr〈/mi〉〈mi〉≤〈/mi〉〈mn〉100〈/mn〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈/math〉 and rotational velocity of the cylinder 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si151.svg"〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈mo linebreak="badbreak"〉-〈/mo〉〈mn〉5〈/mn〉〈mo〉⩽〈/mo〉〈mi〉α〈/mi〉〈mi〉≤〈/mi〉〈mn〉5〈/mn〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈/math〉 where 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si152.svg"〉〈mrow〉〈mi〉α〈/mi〉〈/mrow〉〈/math〉 is the circumferential velocity normalized by the inlet velocity. Extensive results are presented in the form of streamline patterns, isotherm contours and yielded/unyielded regions in the vicinity of the T-junction. In particular, the major thrust of this study is to predict the hydrodynamic forces and torque exerted on the cylinder, flow split ratio (i.e., flow rates in the two outlet branches), critical Bingham number (beyond which no flow separation occurs). Also, the outlet temperatures, enthalpy gain, distribution of Nusselt number over the cylinder surface and its average value as a function of the governing dimensionless parameters have been investigated. The present results show that the rotation of the cylinder can aid or suppress the formation of the recirculation zones effectively which appear on the left wall of the main branch and lower wall of the side branch depending upon the Reynolds number while it always suppresses the cylinder wake. As expected, the Bingham number stabilizes the flow by suppressing the recirculation zone while the Reynolds number tends to promote it. The flow split ratio is found to be significantly affected by the direction of the cylinder rotation. The cylinder rotation also shows a strong impact on the mean values of the hydrodynamic forces and torque. The temperature of the exiting relative streams is seen to be higher for the lower Reynolds and Prandtl numbers, and higher Bingham numbers while cylinder rotation shows a weak effect. The enthalpy gain by the main branch is found to be significantly affected by all the parameters. It has also been observed that the rate of heat transfer is higher for a clockwise rotating cylinder than in the case of an anticlockwise rotating cylinder.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Hong Liu, Han Chen, Chang Cai, Ming Jia, Hongchao Yin〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In order to enhance the intensity and other mechanical properties of the steel, fast cooling technique is generally employed in the metallurgical industry. In the present paper the spray cooling characteristics during steel making process were numerically studied by CFD. A two-phase flow model was established based on the Euler-Lagrangian method. The Lagrangian method was applied to account for the behavior of the discrete droplets while the Eulerian scheme was employed to describe the continuous phase. The wall-jet and Lagrangian wall-film models were applied according to the surface temperature. The accuracy of the model was verified by comparing the temperature with available experimental data. The heat transfer characteristics and metal cooling characteristics during spray cooling were studied and the heat transfer mechanisms during film boiling, transition boiling, nucleate boiling and forced convection regimes were analyzed. The surface temperature exhibits spatially non-uniformity due to the cooling characteristics relating to the flow field of the spray. The influence of the droplet velocity was investigated and the results show that the cooling speed is accelerated with the increase of the droplet velocity and the peak of the heat flux is also corresponding with higher surface temperature.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 7 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer〈/p〉 〈p〉Author(s): 〈/p〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Xunfeng Lu, Weihong Li, Xueying Li, Jing Ren, Hongde Jiang, Phil Ligrani〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Within the present investigation, the combination of impingement jet arrays and arrays of target surface micro pin-fins is investigated. The use of arrays of micro pin fins for installation in different application environments is made possible by recent developments in additive manufacturing. Considered are effects of impingement jet Reynolds number (with values from 2000 to 20,000), jet-to-target plate distance (with Z/D values of 0.75 and 3.0), micro pin-fin shape (rectangle and pentahedron), and micro rectangle pin-fin height (with h values of 0.05D, 0.2D, and 0.4D). Presented are variations of discharge coefficient, area-averaged Nusselt number, and area-averaged Nusselt number ratio. Area-averaged Nusselt number variations for the different data sets are a result of the effects of increased wetted surface area, turbulent transport and turbulent mixing, thermal conduction resistance from pin-fin material, and local flow separations and recirculation regions. When compared at a particular streamwise location, Z/D, and 〈em〉Re〈/em〉 value, area-averaged Nusselt number ratios generally increase with height for the rectangle micro pin-fins. When Z/D = 3.0, Nusselt number ratios show heat transfer augmentations of 30–130 percent, which are approximately constant as streamwise row location changes, provided comparisons are made for constant 〈em〉Re〈/em〉, Z/D, and h/D. Different behavior is present Z/D = 0.75, since associated Nusselt number ratios vary significantly with streamwise row location, with augmentations which range from 0 to 160 percent, because of the influences of narrow jet-to-target plate distance and impingement passage flow confinement.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Yuan Tang, Shao-long Zhu, Li-min Qiu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Mass transfer calculation through vapor-liquid interface is the most important key issue for condensation simulation. However, it is a challenge to determine the coefficient of mass transfer model and the uncertain coefficient leads to unclear physics of the calculated results. Up to now, the coefficient is generally chosen based on numerical trial and errors or empirical parameters. A new mass transfer model for condensation is proposed. And a calculation method for the coefficient in this model is derived, which correlates the coefficients with the measurable interfacial temperature difference. The simulation results of the proposed model agree well with the theoretical results of Nusselt theory for laminar film condensation. The convergence analysis of the proposed model illustrates the mechanism that smaller coefficient causes serious deviations between the calculated interfacial temperature and the saturated temperature, while overlarge coefficient causes convergence difficulties. The convergence analysis is also applicable to the Lee model and the Schrage model.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Suna Yan, Fengwen Wang, Jun Hong, Ole Sigmund〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This paper investigates the topology optimization of microchannel heat sinks. A two-layer heat sink model is developed allowing to do topology optimizations at close to two-dimensional computational cost. In the model, reduced two-dimensional fluid dynamics equations proposed in the literature based on a plane flow assumption are adopted. By assuming a fourth-order polynomial temperature profile of the heat sink thermal-fluid layer and a linear temperature profile in the substrate, two-dimensional heat transfer governing equations of the two layers are obtained which are thermally coupled through an out-of-plane heat flux term. Topology optimizations of a square heat sink are carried out using the two-layer model. Comparison with a three-dimensional conjugate heat transfer analysis of optimized designs in COMSOL Multiphysics validates the accuracy of the two-layer model. The re-evaluation of an optimized design by a one-layer model commonly seen in the literature shows the inadequacy of the one-layer model in predicting physical fields properly. In addition, the influence of physical and optimization parameters on the layout complexity of optimized designs is studied and related to the Peclet number. Optimizations under diffusion-dominated conditions are performed and typical optimized topologies for heat conduction structures are seen.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Martin Thebault, Stéphanie Giroux-Julien, Victoria Timchenko, Christophe Ménézo, John Reizes〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A spatially developing transitional flow in a vertical channel with one side heated uniformly and subjected to random velocity fluctuations at the inlet is investigated in this experimental and numerical study. Two Rayleigh numbers are studied. Experimentally, Particle Image Velocimetry is used to obtain a complete two-dimensional velocity field of the streamwise flow development. A three-dimensional Large-Eddy-Simulation investigation is also performed to obtain a complete streamwise evolution of the flow. The results allow the definition of three transition indicators on the basis of the time-averaged velocities and temperatures fields as well as on the turbulent statistics of the flow. The detailed experimental and numerical spatial development of the transitional natural convective flow is then presented and the ability of the indicators to capture the early and the advanced stages of the transition is assessed. The numerical simulations were performed on the assumptions of a thermally stratified environment, submitted to an environmental noise, and for which wall to wall radiations were considered. These conditions aim at representing realistic experimental conditions.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Pongjet Promvonge, Sompol Skullong〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉An experimental work has been carried out to investigate the influence of combined turbulence promoters (or turbulators) on forced convection and fluid flow resistance behaviors in a solar air heater duct. Two turbulators included V-ribs with punched holes and chamfered V-grooves were introduced. The V-rib and the V-groove having the attack angle of 45° were mounted repeatedly on the absorber plate with their arrangements for V-tip pointing upstream and pointing downstream. Air as the test fluid flowed into the duct with Reynolds number (Re) ranging from 5300 to 23,000. The rib parameters were three relative rib-pitches (R〈sub〉P〈/sub〉 = 1.0, 1.5 and 2.0), three inclination angles (〈em〉β〈/em〉 = 45°, 0° and −45°) of rib-punched holes having a single relative rib height or blockage ratio, R〈sub〉B〈/sub〉 = 0.5. The groove parameters included three relative groove-pitch lengths (R〈sub〉P〈/sub〉 = 1.0, 1.5 and 2.0) similar to the V-rib case. Influences of this newly designed absorber plate on Nusselt number (Nu) and friction factor (〈em〉f〈/em〉) have been examined and compared with similar results of the smooth duct alone. The experimental results demonstrated that the combined turbulators at 〈em〉β〈/em〉 = 45° and R〈sub〉P〈/sub〉 = 1.0 provide the maximum Nu and 〈em〉f〈/em〉, especially for the V-up case due to stronger vortex flows and the impinging flows from the punched holes over the absorber plate. Further, a new thermal enhancement factor (TEF) at similar pumping power has been proposed and it indicates that the combined V-up rib-groove with 〈em〉β〈/em〉 = 45° and R〈sub〉P〈/sub〉 = 1.5 has the highest TEF of about 2.47.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Fulong Cui, Fangjun Hong, Chen Li, Ping Cheng〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The two-phase flow, heat transfer and pressure drop of distributed jet array impingement boiling of HFE-7000 on pin-fin surfaces are investigated with particular emphasis on the characteristics, triggering mechanism and affecting factors on two-phase flow instability. It is found that the two-phase flow in a jet chamber becomes unstable at certain high heat fluxes when large bubbles intermittently choke effusion holes. However, the two-phase flow returns to a stable condition as heat flux is further increased after stable vapor columns are formed between fin tops and effusion holes. Although this kind of instability only causes slight decrease in the heat transfer coefficient, it leads to significant fluctuations of inlet and outlet pressures as well as an abrupt increase in pressure drop. The transitional heat flux at which pressure oscillations occur corresponds to that of two-phase flow instabilities, but the frequency of pressure oscillation is much lower than that of two-phase flow patterns. Two-phase instabilities can be delayed by increasing flow rates, but the degree of instability cannot be reduced once it occurs. These instabilities can also be attenuated or even eliminated by decreasing jet-to-target distance or enlarging effusion ports.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Jiawen Yu, Yiqiang Jiang, Weihua Cai, Xiaojun Li, Zuchao Zhu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉An experimental and simulation investigation of condensation flow pattern and heat transfer characteristics of zeotropic hydrocarbon mixtures methane/propane and ethane/propane in a helical tube was presented. Flow visualization was conducted to capture flow pattern during flow condensation. The flow mechanisms were categorized into six different flow patterns: slug flow, stratified flow, transition flow, wavy flow, half-annular flow and annular flow. In addition, several existing two-phase flow pattern maps in horizontal tube were compared with the flow pattern data. Then a new flow pattern transition line between annular and non-annular flow pattern was proposed. Based on the flow pattern transition line, the heat transfer data under different flow pattern were compared with the existing heat transfer corrections. Finally, an improved heat transfer correlation was proposed and well coincided with the data with a mean absolute relative deviation (MARD) of 13.8%.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): J. Orosco, C.F.M. Coimbra〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We propose a spectral thermophysical model for the infrared optical and radiative properties of metals. The model is suitable for metals possessing nontrivial valency and interband dynamics activated at infrared wavelengths, and consists of an anomalous intraband component and a Gaussian-Lorentzian interband component. The utility of the model is demonstrated by application to platinum, a material of technical importance that also possesses the relevant intra- and interband characteristics. The model yields accurate estimates of the temperature-dependent spectral directional radiative properties over the wavelength range from 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si61.svg"〉〈mrow〉〈mn〉1.5〈/mn〉〈/mrow〉〈/math〉 to 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si62.svg"〉〈mrow〉〈mn〉16〈/mn〉〈mspace width="0.25em"〉〈/mspace〉〈mi mathvariant="normal"〉μ〈/mi〉〈/mrow〉〈/math〉m, and the temperature range from 0 to 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si63.svg"〉〈mrow〉〈mn〉1400〈/mn〉〈/mrow〉〈/math〉 K. Results indicate that when computing the total normal, directional, or hemispherical properties, the model can be used for accurate extrapolation over the entire Planck-weighted spectrum. High-fidelity reproduction of the directional properties validates the inverse extrapolated estimates of the complex-refractive index. This indicates that the model can be interfaced with alternative Fresnel frameworks, such as those used to characterize surfaces that have been systematically or randomly roughened. A M〈span〉atlab〈/span〉 code is provided as supplemental material for reproducibility and convenient implementation of the model.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Aoran Fan, Yudong Hu, Haidong Wang, Weigang Ma, Xing Zhang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This paper presents a “dual-wavelength flash Raman (DFR) mapping method” for in-situ measuring the thermal diffusivity of suspended 2D nanomaterials. Using a periodical heating laser pulse to heat the sample, and using another different wavelength laser pulse as a probe to measure the temperature rises of the sample by its Raman band shifts, the temperature variations of the sample during the heating and cooling period can be determined by changing the time deviation between the heating pulse and the probing pulse. Through changing the position of the probing laser spot, the temperature variation curves at various positions can be measured. The sensitivity and uncertainty analysis shows the measurement accuracy of the mapping method can be significantly improved compared with it of the concentric DFR method. Furthermore, instead of normalization analysis in the DFR method, a specific definition of temperature phase is proposed to analyze the above-mentioned temperature variation curves for characterizing the thermal diffusivity. The uncertainty analysis with a hundred virtual experiments verified that phase analysis is more accurate than the normalization analysis of the same experimental data. When the uncertainty of the temperature measurement is ±2% and each temperature measurement repeated 4 times, with multi-position phase analysis, the uncertainty of the measured thermal diffusivity can be within ±5%.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Amy E. Mensch, Thomas G. Cleary〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A thin laminar flow channel with a transverse temperature gradient was used to examine thermophoretic deposition of soot aerosol particles in experiments and modeled in Fire Dynamics Simulator (FDS) simulations. Conditions investigated included three flowrates, with nominal Reynolds number based on the hydraulic diameter of 55, 115 and 230, and two applied temperature gradients, nominally 10 °C/mm and 20 °C/mm, with repeats. Soot was generated from a propene diffusion flame. The burner exhaust was mixed with dilution air, and most large agglomerates greater than 1 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si26.svg"〉〈mrow〉〈mi mathvariant="normal"〉μ〈/mi〉〈/mrow〉〈/math〉m aerodynamic diameter were removed prior to the channel inlet. The expected thermophoretic velocity of the aerosol was calculated from the applied temperature gradient. A calculated deposition velocity was determined from the mass of deposition, the channel inlet soot concentration, and the exposure time. Uniform soot deposition allowed targets to be used to measure the mass of deposition on the cold side of the channel. The mass of deposition was also determined by subtracting the mass of soot exiting the channel from the mass of soot entering the channel during the exposure time. The deposition velocities from these two methods generally agreed with the thermophoretic velocity and with each other. The deposition mass predicted by the FDS model also compared well with the experiments in most cases. The disagreements for the lowest flow rate cases are attributed to buoyant flow effects adding uncertainty to the actual temperature gradients present in the channel. (The opinions, findings, and conclusions expressed in this paper are the authors’ and do not represent the views or policies of NIST or the United States Government.)〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Rydge B. Mulford, Matthew R. Jones, Brian D. Iverson〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Efficient heat transfer is critical in the design and optimization of thermal control systems. Static radiative heat exchangers are often simple and reliable systems but typically cannot be adapted to environmental changes. Adaptable radiative heat exchangers can be adjusted in response to variations in the thermal environment or operating conditions and have the potential for increased efficiency and reduced cost. Dynamic control of a radiative heat exchanger is possible through geometric manipulation of a segmented, self-irradiating fin, consisting of rigid panels that are linked by thermal hinges in an accordion arrangement. In this paper, a numerical model is described to predict the temperature profile and efficiency of a radiative heat exchanger, accounting for conduction and self-irradiation. Governing equations are cast in terms of the conduction-radiation interaction parameter, surface emissivity, actuation angle, and the thermal conductance of the hinges linking the panels. Results indicate that a turn-down ratio (largest possible heat rate divided by smallest possible heat rate) of greater than three is possible for realistic panel geometries and materials. Self-irradiation decreases the turn-down ratio, and there is evidence that an optimal number of rigid panels exists for any combination of panel geometry and device temperature. The maximum efficiency occurs when the plates are in the collapsed position, but the heat rate is at a minimum in this configuration. Finally, the properties and geometry of the plates are shown to have a more significant effect on the turn-down ratio than the properties of the thermal hinges.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Yong Zhao, Lei Wang, Zhenhua Chai, Baochang Shi〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this paper, the effects of heating direction on the natural convection melting in a cubic cavity are comparatively studied by an improved two-relaxation-time (TRT) lattice Boltzmann model, which can not only recover the correct macroscopic equations but also dramatically reduce the numerical diffusion across the phase interface. In order to reveal the effects of heating direction on the melting efficiency at different Rayleigh number (〈em〉Ra〈/em〉) and Prandtl number (〈em〉Pr〈/em〉) more clearly, both one-side heating and two-side heating are considered. The present numerical results show that in either case, the melting efficiency is increased with the increase of 〈em〉Ra〈/em〉, and the differences of them mainly depend on the value of 〈em〉Pr〈/em〉 and the heating direction. Further, the distributions of streamlines and temperature at various times for different cases are also studied.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Huizhu Yang, Yongyao Li, Yue Yang, Yonggang Zhu, Jian Wen〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉To better apply the correlations established from passages single stack format to evaluate the performance of plate-fin heat exchanger with double stack arrangement, a new calculation method of surface efficiency is proposed in the paper. The variations of local Colburn factor and local pressure drop are studied. Besides, the distribution of dimensionless temperature and local fin efficiency are investigated. Comparison of Colburn factor between single stack and double stack modes is discussed. Seven different fin geometries with 38 sets of simulation data are used to verify the effectiveness of the proposed new method. The results show that the deviations of the surface efficiency between simulated results and formula values are the smallest for the proposed new method. Using usual, Li & Shen, Kraus and proposed new methods to calculate the surface efficiency of the double stack format, the mean absolute deviation of Colburn factor between single stack and double stack modes is 3.7% with air as the medium, while it is 28.1%, 16.6%, 12.9% and 3.2%, respectively for water as the medium. Therefore, regardless of medium used, the proposed new method could give the data of the single stack and double stack modes in a uniform format for guiding the thermal design of plate-fin heat exchanger with serrated fin.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Shuwen Huang, Bo Tao, Jindang Li, Zhouping Yin〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This paper presents a method for on-line estimating a time-varying surface heat flux of a nonlinear heat conduction system with complex geometry from transient temperature measurements. The study consisted of two problems: the forward problem and the inverse problem. For the forward problem, a fast and accurate method based on artificial neural network (ANN) was developed to nonlinearly map any known heat flux to the corresponding temperatures. The training data was obtained by off-line finite element simulations. For the inverse problem, an adaptive sequential Tikhonov regularization (ASTR) method was proposed to estimate the boundary heat flux, which shows superiority in on-line applications due to its independence of future measurement. At each time step of the on-line process, the trained ANN was called by the ASTR procedure to calculate the temperature and sensitivity coefficient. The coupled method, referred to as ASTR-ANN, was tested numerically in a three-dimensional solid system with nonlinear thermal properties and complex geometry. Comparisons with several existing nonlinear inverse methods were also conducted, and the results demonstrated the validity of the proposed ASTR-ANN method.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): S.Y. Chen, Z.M. Liu, X.C. Zhao, Z.Y. Lv, X.G. Fan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A three-dimensional numerical model was established to investigate the thermal coupling mechanism in the K-TIG (Keyhole Tungsten Inert Gas) keyhole and weld pool. Especially, to reflect the energy distribution in the new-developed high-current free burning arc, a novel, suitable combined heat source model was proposed. The dynamic process of heat source and the corresponding temperature field were studied to reflect the effect of arc on the workpiece. Keyhole evolution with temperature distribution and fluid flow in the weld pool was investigated to further explore the keyhole influence on the heat transfer and mechanical energy of the weld pool. Some new features of K-TIG were discovered. The higher temperature molten metal is distributed mainly in the upper part instead of the bottom of the weld pool, and the weld pool volume of K-TIG is large, up to 50 mm〈sup〉3〈/sup〉. Finally, the first emerging time of a fully penetrated keyhole, keyhole size, and fusion line size were measured in stationary welding experiments. The simulation results agreed well with the measured data. The results lay a foundation for understanding the coupling behavior in energy source-keyhole-weld pool system, and they could promote the engineering application of K-TIG process.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Truong V. Vu, Quan H. Luu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this paper, we present the front-tracking-based results of containerless solidification of a liquid droplet with laminar forced convection. The droplet starts solidifying from a nucleus of solid phase assumed to be located at its bottom, and the phase change interface propagates through the droplet as time progresses. The problem is governed by various parameters such as the Reynolds number 〈em〉Re〈/em〉, the Prandtl number 〈em〉Pr〈/em〉, the Stefan number 〈em〉St〈/em〉, the Weber number 〈em〉We〈/em〉, the temperature of the surrounding fluid 〈em〉θ〈sub〉in〈/sub〉〈/em〉, volume change in terms of the solid-to-liquid density ratio 〈em〉ρ〈sub〉sl〈/sub〉〈/em〉, the growth angle 〈em〉ϕ〈sub〉gr〈/sub〉〈/em〉 and the solidification nucleus size 〈em〉r〈sub〉0〈/sub〉〈/em〉. We focus on small-sized droplets with low 〈em〉Re〈/em〉, and thus an axisymmetric configuration is used. Numerical results show that the presence of the forcing flow whose temperature below the solidifying point of the droplet liquid enhances the solidification process with a decrease in the solidification time, i.e. time for completion of solidification, with respect to an increase in 〈em〉Re〈/em〉 from 20 to 200. However, variation of 〈em〉Re〈/em〉 in this range does not affect much the shape of the solidified droplet. Similarly, the droplet shape after complete solidification is minor affected by 〈em〉Pr〈/em〉 varied from 0.01 to 0.32, 〈em〉We〈/em〉 varied from 0.05 to 6.4 or 〈em〉θ〈sub〉in〈/sub〉〈/em〉 varied from −2.0 to 0. In contrast, the solidified droplet shape is strongly affected by 〈em〉St〈/em〉 varied in the range of 0.025–1.6, 〈em〉ϕ〈sub〉gr〈/sub〉〈/em〉 varied in the range of 0–28°, 〈em〉ρ〈sub〉sl〈/sub〉〈/em〉 varied in the range of 0.8–1.2 or 〈em〉k〈sub〉sl〈/sub〉〈/em〉 varied in the range of 0.125–8.0 with an increasing in the droplet aspect ratio (or with a decrease in the top angle) with respect to a decrease in any of 〈em〉St〈/em〉, 〈em〉ρ〈sub〉sl〈/sub〉〈/em〉, 〈em〉k〈sub〉sl〈/sub〉〈/em〉 or with respect to an increase in 〈em〉ϕ〈sub〉gr〈/sub〉〈/em〉. However, the size of the nucleus normalized by the initial liquid droplet diameter, 〈em〉r〈/em〉〈sub〉0〈/sub〉/〈em〉D〈/em〉, changed from 0.08 to 0.15 has no effect on the solidified droplet shape. Concerning the solidification time, it increases with an increase in 〈em〉Pr〈/em〉, 〈em〉ϕ〈sub〉gr〈/sub〉〈/em〉, 〈em〉θ〈sub〉in〈/sub〉〈/em〉 or with a decrease in 〈em〉St〈/em〉, 〈em〉k〈sub〉sl〈/sub〉〈/em〉.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 142〈/p〉 〈p〉Author(s): Elizaveta Ya. Gatapova, Oleg A. Kabov, Vladimir S. Ajaev〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Modeling of evaporation in the microregion separating an adsorbed film and a macroscopic meniscus is important for a number of applications such as pool boiling and micro heat pipes. We develop a model of a moving microregion incorporating the effects of evaporation, viscous flow, surface tension, and two-component disjoining pressure and apply it to study motion of microregions over heated surfaces with wettability defects or patterns. Substantial heat transfer enhancement is found when a receding microregion passes over the portion of the substrate with higher wettability than the surrounding areas. The effect is explained in physical terms by widening of the part of the microregion with low thermal resistance. To achieve sustained heat transfer enhancement, we then consider a configuration in which the substrate is patterned by an array of high-wettability stripes and investigate the evaporative flux as a function of the geometry of the pattern. The effects of Marangoni stress at the interface are also studied and found to result in slight reduction of the average evaporative flux, as the tangential stress at the interface reduces the liquid supply into the microregion.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 142〈/p〉 〈p〉Author(s): Pengfei Wang, Jin Jiang, Shunyang Li, Xiangyu Luo, Shaojie Wang, Wensheng Zhao〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A three-dimensional (3D) computational fluid dynamics (CFD) study is conducted to investigate the influence of tube ellipticity ratio 〈em〉e〈/em〉, tube rotation angle 〈em〉θ〈/em〉 and fin spacing 〈em〉H〈/em〉 on heat transfer capability. The tube ellipticity ratios range from 0.4 to 1.0 with increments of 0.1. The Reynolds number in the fluid domain varies from 1300 to 2100. The tube rotation angle ranges between 0° and 90°. The performance of heat exchanger is evaluated by non-dimensional parameters Colburn factor 〈em〉j〈/em〉, friction factor 〈em〉f〈/em〉, and Area goodness factor 〈em〉j〈/em〉/〈em〉f〈/em〉. It can be concluded from this study that when Reynolds number and elliptical ratio are the same, a maximum of Colburn factor 〈em〉j〈/em〉 occurs as 〈em〉θ〈/em〉 increases, while the friction factor 〈em〉f〈/em〉 keeps increasing. Furthermore, the maximum value decreases as the ellipticity ratio increases. The area goodness factor decreases sharply when 〈em〉θ〈/em〉 〉 45° for heat exchangers with different ellipticity ratios. The study also suggests that the inclined elliptical fin-tube heat exchanger with the tube ellipticity ratio 〈em〉e〈/em〉 = 0.6 and rotation angle 〈em〉θ〈/em〉 = 30° has the best heat exchange performance and airflow characteristics.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 142〈/p〉 〈p〉Author(s): X. Bai, A. Nakayama〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉An integral solution procedure has been developed to describe the entire evolution of the thermal boundary layer in a channel filled with a fluid-saturated porous medium. The upper and lower walls are heated under a constant heat flux condition, and local thermal nonequilibrium is assumed to apply. The development of the thermal boundary layer in this channel is divided into three distinctive regions, namely, the entrance, transition and the nearly fully-developed regions, in which separate fluid and solid phase thermal boundary layers develop near heated walls with different growth rates. In this integral analysis, each region is considered in terms of the interactions between the fluid and solid thermal boundary layers; this eventually yields a set of algebraic equations for the easy and accurate estimation of the local Nusselt number. The solutions thus obtained for the three regions are combined to reveal the entire development of the local Nusselt number from the entrance to fully-developed stage. This analytic procedure, for the first time, reveals a complete region map showing the locations of the transition from one region to another, and these depend on the Biot number, the thermal conductivity ratio and the Graetz number in a complex manner.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 142〈/p〉 〈p〉Author(s): David Martínez-Maradiaga, Alberto Damonte, Alessandro Manzo, Jan H.K. Haertel, Kurt Engelbrecht〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Thermal management is fundamental to ensure that electronics components operate at their design temperatures for improved performance and lifetime. As current electronic devices become more compact and more power dense, the amount of heat to be dissipated per area also increases. Therefore, it is necessary to design heat sinks capable of maintaining a low operating temperature and a small packaging envelope. Topology optimization, due to its geometric freedom, can be a useful tool to develop passive heat sinks capable of rejecting as much heat as possible in a limited space. This paper presents the design, modeling, and testing of topology optimized heat sinks for a commercial tablet. Firstly, a numerical model of the tablet’s thermal behavior is developed. Secondly, the topology optimization problem is formulated and implemented. Two topology optimization approaches are used: the non-robust approach and the robust approach. COMSOL’s optimization module is used to conduct the optimization and the Globally Convergent version of the Method of Moving Asymptotes is used as the optimization algorithm. Finally, three heat sinks were fabricated in aluminum: the two resulting topology optimized designs (robust and non-robust), and one baseline L-shaped heat sink. The latter heat sink is used to compare the performance of topology optimized and traditionally designed heat sinks. It was shown that topology optimized heat sinks can reduce the temperature of the heat dissipating components of a consumer tablet.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 145〈/p〉 〈p〉Author(s): Simao Guo, Yuchuan Guo, Xiangmiao Mi, Guanbo Wang, Dazhi Qian, Bo Hu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Energy systems based on inertial confinement fusion or fusion-fission hybrid reactor generate energy in the form of pulse heating, which may lead to new flow instability phenomenon in the cooling system. In order to make thermal hydraulic design of pulse heating energy system safe and reliable, it is necessary to deeply understand the oscillation characteristics of flow and heat transfer under the pulse heating condition. As for the first stage of this complicated research, the numerical simulation of coolant flow in a circular tube under pulse heating condition is executed in this paper. The oscillation characteristics of flow and heat transfer under different pulse heating conditions are discussed in detail, and the effect of heating power, pulse heating period and duty ratio are studied. Four flow development stages (stage A – stage D) are identified, and three kind of flow oscillation phenomenon are found according to the numerical results with increasing the pulse heating power. The analysis shows that oscillation is mainly caused by pulse heating itself or the combination of pulse heating and density wave oscillation (DWO). Increasing the pulse heating period is beneficial to decrease flow oscillation, and decreasing the duty ratio, the flow oscillation is more obvious. The flow development stage maps are also given in the plane of average power and inlet subcooled temperature. With the increase of period, the stage D appears more easily at lower average power. When the duty ratio decreases, the region of stage C in instability map becomes larger.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 145〈/p〉 〈p〉Author(s): Ang Zhang, Fengyuan Liu, Jinglian Du, Zhipeng Guo, Qigui Wang, Shoumei Xiong〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The thermosolutal interaction influences eutectic evolution and thus properties of solidified materials. Limited by numerical capability, however, very few studies are performed on the eutectic growth with coupled heat and solute diffusion. Combining the phase-field lattice-Boltzmann approach and a parallel-adaptive mesh refinement algorithm, a novel numerical scheme is developed to efficiently simulate the thermosolutal multiphase eutectic evolution. The contact angles at the triple point agree well with the analytical solution, and the temperature always obtains the local extreme at the solid/liquid interface due to the release of latent heat. The effects of the Lewis number and imposed heat sink on the eutectic evolution are discussed in detail, which includes the change of the lamellar growth velocity and the form of lamellae creation. The dimension is further extended to 3D to investigate the evolution of plate- and rod-like eutectics. How to simulate eutectic evolution with a larger Lewis number (e.g., 10〈sup〉6〈/sup〉) and how to develop a more general eutectic model are also discussed. As the first attempt to solve the thermosolutal eutectic evolution, our investigation paves a way for further quantitative analysis and direct comparison with experiments.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 145〈/p〉 〈p〉Author(s): Min Chai, Kun Luo, Changxiao Shao, Haiou Wang, Jianren Fan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this paper, our previous finite difference discretization method for Robin boundary conditions is implemented into an interface-resolved detailed numerical simulation framework to solve evaporation problems on an irregular interface. The framework utilizes a conservative level set method for describing the interface, a ghost fluid method for addressing the discontinuities on the interface, and a revised heat flux based model for predicting the evaporation rate to improve the accuracy, stability and applicability of the simulations. Specific care has been devoted to the accurate treatments of the involved complex boundary conditions on the interface. Numerical results are in excellent agreements with the analytical solutions, indicating the success of the methods for the above-mentioned sub-processes coupled with the fluid solver. The numerical framework is robust and achieves first-order overall convergence accuracy. Then, the non-uniform evaporation of single droplets is simulated to demonstrate the fascination of such an interface-resolved framework on in-depth understanding of physical mechanisms and therefore on modification of traditional evaporation models. Finally, rigorous validations are conducted in a three-dimensional configuration of droplet evaporation. In summary, the framework proposed in this paper represents a promising tool for non-uniform evaporation modelling, which will be applied to more practical problems in future work.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Long Jiao, Rong Chen, Dingding Ye, Wei Li, Dongliang Li〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The droplets sorting in the well-defined microdroplets array is significantly important in fields including biomedicine, pharmacy, primary diagnosis and chemosynthesis due to its pivotal role in the follow-up analysis, collection and other applications. Recently, the light control of droplets using the interactions between light and fluids allows for the contactless droplets sorting with prominent features. Among the interactions, the photothermal effect has shown significant potential. In this work, we investigated the photothermal effect-induced interface dynamics of neighboring droplets in the microdroplets array on the superhydrophobic surface with hydrophilic patterns. It was found that the irradiated droplet was rapidly collapsed by the photothermal effect-induced evaporation-condensation-extension while the adjacent droplet’s outboard triple-phase contact line was gradually extended and showed more significant extension due to the limited vapor condensation in the space between the droplets. The light-induced interface behaviors of neighboring droplets under different droplet gaps and laser powers were also analyzed. Based on the findings, a proof of concept of the droplets sorting platform manipulated by the light was demonstrated. With the assistance of the hydrophilic patterns on the superhydrophobic surface and the laser heating, the target droplet reached the hydrophilic patterns could spontaneously move to the designated position via the imposed asymmetrical surface tension, realizing the droplets sorting. The results indicated that the light-induced interface migration of the droplets in the microdroplets array could sort the target droplets with high yields and remarkable simplicity and show promising potential in many biochemical analysis applications but not limited.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Lei Bai, Hongwu Deng, Zhi Tao, Shuqing Tian, Lu Qiu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The convective heat transfer characteristics in a rotating pin-fin arrayed rectangular channel with multiple inlets and outlets were investigated. The first inlet was located at the bottom of the channel whereas the secondary coolant was injected into the channel through the side-wall holes. On the opposite sidewall, the flow was discharged through eight slots. The major-to-total mass flow ratio (〈em〉MR〈/em〉) varied from 0 to 1 in the experiments. The effective Reynolds number (defined with the total mass flow rate), the rotation number and the density ratio varied from 12,000 to 28,000, from 0 to 1, and from 0.08 to 0.16, respectively. The overall buoyancy number ranged from 0 to 0.85 whereas the local buoyancy number changed from 0 to 56.2. In the non-rotating cases, the results showed that the heat transfer in the inner channel was stronger than the mid-span and outer region counterparts when the flow rate ratio exceeded 0.59. In the mid-span region, there was a critical 〈em〉MR〈/em〉, after which the decreasing trend of the heat transfer was reversed. For the rotating channels, the rotational effects were less apparent as 〈em〉MR〈/em〉 increased. In the case of 〈em〉MR〈/em〉 = 1, the rotation promoted the heat transfer in the entire channel except the corners of the channel. Moreover, the buoyancy played an important role in influencing the heat transfer when the local buoyancy number is larger than a threshold.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Hao Deng, Yuze Hou, Wenmiao Chen, Fengwen Pan, Kui Jiao〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉An in-depth understanding of pore-scale transport process and electrode reaction kinetics is critical to ease the sluggish oxygen reduction reaction of proton exchange membrane fuel cells. In this study, the effects of catalyst layer microstructure on the oxygen diffusion and electrochemical reaction consumption are analyzed, based on the developed 3D single-phase lattice Boltzmann model and optimized stochastic reconstruction algorithm. The results suggest that increasing the ionomer content and platinum loading are beneficial to increasing the active sites, while the oxygen transport along the thickness direction becomes more difficult due to the narrowed pores, so the platinum particles in the back region have less participation in the reaction, which causes the significant catalyst waste and limits the further cost reduction and improvement of power density. Improving the carbon particle diameter ensures the enlarged pores and more effective Pt particles covered by ionomer, thus enhances the oxygen supply and electrochemical reaction rate.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Min Chan Kim, Kwang Ho Song〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉To understand the effect of cross diffusion on the onset and the growth of the gravitational instabilities in a ternary solution more rigorously, theoretical and numerical analyses are conducted by considering all cross diffusion coefficients. We clearly showed that the stable concentration field is possible even for the complex-eigenvalues systems, such as acetone(1)-benzene(2)-CCl〈sub〉4〈/sub〉(common solvent) ternary mixture. By employing Faddeeva functions, we extended the previous asymptotic analysis into the complex eigenvalue systems. In addition, by considering all possible cross diffusion coefficients, i.e., complex conjugate, and real and distinct eigenvalues system, we derive the linear stability equations in the uncoupled form, solve them, and conduct nonlinear numerical simulations employing the linear stability result as an initial condition. Through the present asymptotic, linear and nonlinear analyses, the instability motions in the double-diffusive (DD), diffusive-layer convection (DLC) and extended double diffusive (EDD) regimes are clearly identified. The present asymptotic, linear and nonlinear analyses support each other and are in good agreement with the previous theoretical, numerical and experimental work.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Tong-Miin Liou, Chun-Sheng Wang, Jui-Cheng Tseng, I-Ming Hsieh, Chieh-Chu Chen〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This study reports an innovative design of 3-D louver-type turbulator based on previous ribs, baffles, and 2-D louver-type turbulators. Its effects on turbulent flow field, detailed temperature distribution, and pressure loss in a two-pass channel with square cross-section are investigated by Particle Image Velocimetry (PIV), Infrared Thermography (IRT), and pressure transducer. Four pitch ratios (〈em〉Pi〈/em〉/〈em〉D〈sub〉H〈/sub〉〈/em〉), namely 1, 2, 3, and ∞, are explored, and the slat attack angle (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si4.svg"〉〈mrow〉〈mi〉α〈/mi〉〈/mrow〉〈/math〉) of the louver-type turbulator ranges from −40° to 30°. The Reynolds number (〈em〉Re〈/em〉) for PIV and IRT measurement is fixed at 10,000 and in the range of 5000–20,000, respectively. It is found that the present turbulators with 〈em〉Pi〈/em〉/〈em〉D〈sub〉H〈/sub〉〈/em〉 = ∞ not only disturb the core flow and degenerate wake zone behind the turbulator but also induce four pairs of Dean-like counter-rotating vortices which result in strong impingement on the bottom and top walls. The main contributions of this study are twofold: (1) the present design fills the blank at moderate friction factor ratio (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈mrow〉〈mover accent="true"〉〈mi〉f〈/mi〉〈mo stretchy="true"〉‾〈/mo〉〈/mover〉〈mo stretchy="false"〉/〈/mo〉〈msub〉〈mi〉f〈/mi〉〈mi〉∞〈/mi〉〈/msub〉〈/mrow〉〈/math〉 ≤100) compared with previous ribs and baffles; (2) it attains much higher Nusselt number ratio (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈mover accent="true"〉〈mrow〉〈mi mathvariant="italic"〉Nu〈/mi〉〈/mrow〉〈mo stretchy="true"〉‾〈/mo〉〈/mover〉〈mo stretchy="false"〉/〈/mo〉〈msub〉〈mrow〉〈mi mathvariant="italic"〉Nu〈/mi〉〈/mrow〉〈mi〉∞〈/mi〉〈/msub〉〈/mrow〉〈/math〉 up to 4.4) at that 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈mrow〉〈mover accent="true"〉〈mi〉f〈/mi〉〈mo stretchy="true"〉‾〈/mo〉〈/mover〉〈mo stretchy="false"〉/〈/mo〉〈msub〉〈mi〉f〈/mi〉〈mi〉∞〈/mi〉〈/msub〉〈/mrow〉〈/math〉 region or for the same value of 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈mover accent="true"〉〈mrow〉〈mi mathvariant="italic"〉Nu〈/mi〉〈/mrow〉〈mo stretchy="true"〉‾〈/mo〉〈/mover〉〈mo stretchy="false"〉/〈/mo〉〈msub〉〈mrow〉〈mi mathvariant="italic"〉Nu〈/mi〉〈/mrow〉〈mi〉∞〈/mi〉〈/msub〉〈/mrow〉〈/math〉 the value of 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈mrow〉〈mover accent="true"〉〈mi〉f〈/mi〉〈mo stretchy="true"〉‾〈/mo〉〈/mover〉〈mo stretchy="false"〉/〈/mo〉〈msub〉〈mi〉f〈/mi〉〈mi〉∞〈/mi〉〈/msub〉〈/mrow〉〈/math〉 is 44.1% lower.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Kittipass Wasinarom, Jarruwat Charoensuk, Visarn Lilavivat〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A numerical model for lean premixed combustion of LPG (70% propane and 30% butane) within a porous inert medium was developed. Experiments were conducted at three different firing rates at the equivalent ratios of 0.4 and 0.6. The model was developed with the thermal non-equilibrium concept between phases and validated with three cases of experimental results. The discussion of model calibration was undertaken by focusing on the effects of the extinction coefficient and convection heat transfer effective area. Comparisons were made of the temperature profile, as well as the peak temperature, with the calculated adiabatic temperature. The model agreed well with experimental results and was robust throughout three firing rates. Moreover, it was found that the two aforementioned thermal parameters had different roles in temperature distribution, which provided insight on flame front location and heat transfer between phases within the porous domain.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Qi Wang, Ruiqiang Guo, Cheng Chi, Kai Zhang, Baoling Huang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Fundamental understanding of thermal transport properties in ultrathin Si-based films is essential for the thermal management of nanoelectronic and nanophotonic devices. Using Density-Functional-based tight-binding method and direct iterative solution of linearized Boltzmann transport equation without empirical assumptions, we systematically investigated mode-wise in-plane phonon transport in ultrathin Si films of a few nanometer thickness (0.77–1.90 nm) and the effects of surface morphology. The dimensionality reduction of these films leads to quantum confinement and softening of silicon bonds and in turn changes the dispersion and phonon-phonon interactions. The ultrathin Si films with naturally reconstructed surfaces show a counterintuitively high in-plane thermal conductivity (∼30 W/m-K at 300 K) with relatively weak size dependence and large acoustic phonon contribution, demonstrating that dimensionality reduction alone cannot suppress phonon transport in ultrathin films efficiently. The in-plane thermal conductivities of ultrathin films are very sensitive to surface defects and even atomic-level surface defects can induce up to 10-fold reduction in thermal conductivity due to much enhanced phonon scatterings in low frequency regime. Our direct first-principle-based calculations also show that the conventional modeling of thin films with bulk properties and suppression function may lead to large uncertainty when applied to ultrathin films.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Hui He, Liang-ming Pan, Quan-yao Ren, Ting-pu Ye, Ding-fei Zhang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Local liquid film behavior of annular flow on rod bundle geometry is vital for accurate prediction of dryout. This paper is the Part II of a two-part study devoting to modelling and prediction of local liquid film behavior of annular flow on rod bundle geometry based on the unique feature of experimental phenomenon in Part I of this work. In this paper, a mechanism model is developed to predict the local liquid film thickness of annular two-phase flow on rod bundle geometry, in which the spatial distribution of gas flow in the cross-section of rod bundle is specified, and the local velocity of liquid film is obtained by employing the triangular relationship that taking into account properly the momentum and mass balances for the liquid film and the gas core. Ordinary differential equation (ODE) for the liquid film thickness is developed according to the Navier-Stokes equations for momentum balance of film flow on a rod to analyze the spatial characteristic of film thickness, in which the droplet entrainment-deposition process and the effect of circumferential shear stress are considered as boundary conditions during mass transfer at the gas-liquid interface. The present model predicts the experimental local liquid film thickness with the maximum relative error of ±10%. Circumferential coherent wave structure is formed by the combination effect of transversal stress gradient in the wave, droplet redeposition and film flow by circumferential shear stress, and the circumferential shear stress reduces the relative importance of the droplet redeposition with respect to the transversal stress gradient making the peripheral distribution of film thickness more uniform.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): S.P. Aktershev, E.A. Chinnov, E.N. Shatskiy〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The formation of a three-dimensional rivulet structure in a heated falling liquid film is investigated theoretically. For a description of the dynamics of a non-isothermal film, the theoretical model is developed taking into account the thermocapillary force. Within the framework of the spatial approach, a linear analysis of the stability of the heated film relative to perturbations in the spanwise direction to the flow is performed for the first time. A spatial growth rate of these perturbations down-stream is obtained. Analysis of experimental data for different liquids and different Reynolds numbers has shown that the distance between the rivulets corresponds well to the wavelength of most amplified perturbation. The stationary rivulet structure in the film is simulated by numerical method for two types of heating conditions implemented in our previous experiments: with constant wall temperature and heat flux on the wall. Calculations show, that on a heater with constant temperature of a wall the developed rivulet structure has a quasistationary character and very slightly changes downstream. The development of rivulets on a heater with a constant heat flux on the wall has been calculated for the first time. The good agreement between calculations and our experimental data is found for both types of heating conditions implemented in the experiments.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Liang Jun Zheng, Dong Hee Kang, Na Kyong Kim, Young Jik Youn, Hyun Wook Kang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Theoretical model of thermoelectric generator (TEG) cooled by direct evaporative cooling technology is developed. In the theoretical model, thermoelectric material properties depending on temperature change and effects of heat loss by radiation, conduction, and Thomson phenomenon are considered. To investigate the effect of evaporative cooling to the efficiency of TEG, the simulation conditions are set as hot-side temperature of 40–120 °C and 1 mm thickness of thin water film covering for cold-side. The ambient temperature is 25 °C and the relative humidity of the air is set from 25% to 75%. Based on the developed theoretical model, the output power of TEG is increased as increasing hot side temperature under fixed relative humidity or as decreasing relative humidity under fixed hot-side temperature. Consequently, the maximum output power of 130.69 W m〈sup〉−2〈/sup〉 and an efficiency of 1.63% are obtained with 25% relative humidity and 120 °C for the hot-side temperature condition. The results show that the output power and efficiency are increased 100.53 and 10.53 times higher than without evaporative cooling (natural convection case, output power of 1.30 W m〈sup〉−2〈/sup〉 and efficiency of 0.1548%), respectively.〈/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-S0017931019304971-ga1.jpg" width="500" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Bo Wang, Ruifeng Tian〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Corrugated plate dryer is a very important steam-water separation equipment for nuclear engineering. And heat and mass transfer phenomenon is commonly observed in the separation process. It is meaningful to study the flow, fluctuation and breakdown characteristics of the water film on corrugated plate wall. Water film thickness of steady flow is measured based on plane laser induced fluorescence (PLIF) technique and time series of water film thickness are obtained. Besides, amplitude and frequency domain information of water film fluctuations are analyzed by the power spectral density (PSD) method. A two-dimensional model of water film breakdown is established. A model of the relative position of the liquid film rupture is established. Critical airflow velocity when the water film breakdown is measured experimentally. Based on theoretical results, equations for calculating critical airflow velocity of the corrugated plate under different water film thicknesses are fitted in the form of nike function. Fitting equations for calculating critical airflow velocity in the form of nike function agree well with experimental results. The relative position of water film breakdown is merely affected by the water film Reynolds number and the structure parameters of the corrugated plate.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 142〈/p〉 〈p〉Author(s): Vahid Madadi Avargani, Reza Karimi, Touraj Tavakoli Gheinani〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this work, an integrated system is introduced to regenerate the desiccant materials in solar air conditioning systems. A solar parabolic dish collector and a helically baffled cylindrical cavity receiver were coupled to two series finned-tube heat exchangers, and a fixed bed filled with silica gel. A silicone oil as heat transfer fluid absorbs the solar energy and heats the air to regenerate the silica gels in the bed. The system was studied both experimentally and theoretically. A comprehensive mathematical model was developed for the entire system, and the proposed model was validated with experimental data. The regeneration rate of the desiccant materials and the average daily thermal regeneration efficiency of the system were obtained up to 0.4 kg water/h m〈sup〉2〈/sup〉 and 30% respectively, which compared to the other solar systems, the present system has a good performance and is more efficient. Based on the weather conditions of the installed location of the system, the operational or even geometrical parameters of the system can be designed in such a way that for specified required cooling capacity, the system can provide the required energy for regeneration of desiccant materials. The system performance was studied during a sample day based on real solar irradiation intensity and ambient conditions, and the influence of the effective parameters was investigated on the regeneration rate of the silica gels.〈/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-S0017931019320769-ga1.jpg" width="284" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): J.E. Li, B.L. Wang, C. Zhang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This paper studies the problem of a thermal and electrical electrode/punch on a thermoelectric material layer within the framework of fully coupled, nonlinear thermoelectricity model. The resultant thermal flux and energy current on the electrode/punch are prescribed. Explicit expressions for the thermal/electrical fields at the electrode/punch tip are obtained and are seen to be singular. For infinite layer thickness the thermoelectrical fields are obtained in closed-form. For finite layer thickness, the relationship between the applied flux/current and the voltage/temperature underneath the electrode/punch are obtained numerically by a singular integral equation technique. Effect of layer thickness is discussed. The results are helpful in measuring the thermoelectric coupling properties of thermoelectric materials by electrode/punch experiments.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Z.X. Yao, J.X. Li, K. Wang, Y.N. Song, X. Li〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Subsea pipeline of waxy crude oil may face a high risk of the wax plug during shut down or pigging. Direct Electrical Heating (DEH) technology is expected to be an efficient and convenient way to melt the wax plug in the subsea pipeline. An industrial-scale experimental facility with a test tube of 10-in. diameter and a power supply of 0–1350 A is set up, and then several sets of experiments are carried out. Simultaneously, the melting process of the wax plug is simulated by FLUENT software. The numerical model is validated from the aspects of liquid fraction and solid patterns. The melting rate and temperature of the wax plug with different wax content and current are analyzed, and the temperature of the pipe wall is considered for the sake of pipeline integrity. Furthermore, to avoid secondary solidification, the recooling process of the wax plug and pipe wall is also discussed. Finally, the effects of the “Maximum Allowable Temperature”, “Maximum Safe Power”, “Effective Liquid Fraction” and “Applicable Heating Time” on economic evaluation are investigated, and the correlations are developed between the above factors and common pipe diameters (6–32-in.), which shows heating with maximum safe power is also the most economical method in engineering.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Omar S. Al-Yahia, Ho Joon Yoon, Daeseong Jo〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Bubble behaviors and their interactions with each other affect the two-phase flow characteristics. In this study, experiments are conducted to investigate the bubble characteristics during subcooled flow boiling under uniform and non-uniform transverse heat flux distribution. The non-uniform heat flux distribution creates non-uniform bubble characteristics on the heated surface as opposed to under uniform heating. The working fluid is demineralized water that flows through a narrow rectangular channel heated from one side. The experimental loop used in the study operates at low pressure. A wide range of experimental operating conditions, such as inlet temperature (35–65 °C), thermal power (500–6250 W), and mass flow rates (0.03–0.13 kg/s), are applied to the upward flow channel. The bubble behaviors are visualized using a high-speed camera (2200 fps) at a resolution of 512 × 512 pixels. The results indicate that the bubbles exhibit different departure diameters, nucleation site density distributions, and bubble departure frequencies. In the uniform case, bubbles are generated uniformly across the whole transverse direction of the heated surface. In the non-uniform case, more bubbles are generated where the heat flux is concentrated, which disturbs the flow velocity profile in the transverse direction. The differences in bubble generation in the transverse direction result in differences in the two-phase flow instability through the heated channel. As a result, new empirical correlations are proposed based on the experimental results to estimate the bubble departure diameter, nucleation site density, and bubble departure frequency. The correlations are applicable for both heated surface conditions, uniform and non-uniform, under low-pressure conditions. CFD analysis using ANSYS FLUENT incorporates the RPI wall boiling model is conducted to validate the empirical correlations. Comparison of the CFD calculations with the experimental data for void fraction, wall temperature, and bulk temperature show good agreement. The simulation results show an accurate prediction of the ONB (Onset of Nucleate Boiling) and OFI (Onset of Flow Instability).〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): P. Wang, B.L. Wang, J.E. Li〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Temperature and performance modeling of thermoelectric generators (TEGs) have long been discussed. However, due to the high nonlinearity of heat conduction in thermoelectric (TE) materials, the analytical model of TEGs is difficult to develop and there is no such model exists at this time. In this paper, we revisit the problem and develop an analytical model to evaluate the performances of a TEG. The model considers contact resistance between TE leg and electrode, and temperature dependent TE material properties. Through the analytical model, we give simplified expressions to calculate the leg temperature profile, energy conversion efficiency, and output power for a single TE leg pair. These expressions are validated by either experiment or finite element (FE) simulation. The results show that an optimal cross-section area exists for maximum efficiency. The optimal leg length decreases as the thermal contact conductance increases while the optimal leg length has no relation with electrical contact resistance. Based on the results of this paper, we provide some useful suggestions for the design of high-performance thermoelectric generators.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): A. Amiri Delouei, A. Emamian, S. Karimnejad, H. Sajjadi, A. Tarokh〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The paper has dealt with an exact analytical solution of steady-state heat conduction for the special case of a functionally graded (FG) cylindrical sector. In this regard, Fourier theory is utilized to develop the steady-state temperature field. The material properties according to the power-law function are considered to vary in radial and circumferential directions and in both directions, the most general thermal boundary conditions are applied. Adequate verification of the solution is demonstrated. The correctness of the exact analytical solution in terms of industrial examples is examined by solving a heat conduction problem for a cylindrical segment subjected to a combination of boundary conditions. Furthermore, the effects of various parameters such as material constant, geometry, and thermal conductivity ratio on the temperature distribution are explored. The findings are advantageous to comprehend the flexibility of the two-dimensional FG materials for designing process and optimizing configurations under multi-functional requirements such as intelligent control applications.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 142〈/p〉 〈p〉Author(s): D. Sarker, W. Ding, C. Schneider, U. Hampel〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The present study reports the mutual effect of heater surface wettability, roughness and bulk liquid velocity on the bubble dynamics and departure in nucleate boiling. Boiling experiments were conducted at atmospheric pressure with degassed-deionized water at low subcooling (1.9 ± 0.25 K) for vertically oriented stainless steel heaters. Self-assembled monolayer (SAM) coating and wet-etching technique were used to alter the heater surface wettability and roughness. Liquid contact angle hysteresis (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si25.svg"〉〈mrow〉〈msub〉〈mi〉θ〈/mi〉〈mtext〉hys〈/mtext〉〈/msub〉〈/mrow〉〈/math〉) and root mean square roughness (〈em〉Sq〈/em〉) of the heater surfaces were adjusted between 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si26.svg"〉〈mrow〉〈mtext〉42.32〈/mtext〉〈mtext〉°〈/mtext〉〈mo〉≤〈/mo〉〈msub〉〈mi〉θ〈/mi〉〈mtext〉hys〈/mtext〉〈/msub〉〈mo〉≤〈/mo〉〈mtext〉68.56〈/mtext〉〈mtext〉°〈/mtext〉〈/mrow〉〈/math〉 and roughness 0.01 μm ≤ 〈em〉Sq〈/em〉 ≤ 0.549 μm. High resolution optical shadowgraphy has been used to record the bubble life cycle. Experimental results show that higher bulk liquid velocity yields smaller bubble departure diameters for all heater surface characteristics. Bubble departure diameters are greater for low wetting surfaces. The bubble growth rate and departure diameter were found maximum for an intermediate surface roughness 〈em〉Sq〈/em〉 between 0.108 and 0.218 μm. The corresponding roughness height is referred to as the ‘optimal roughness height’ in this study. Eventually, a bubble departure criterion was derived from the expressions of forces which act on a nucleating bubble throughout its growth cycle. 90% of the departing bubbles satisfy the bubble departure criterion with ±25% deviation.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Piotr Duda〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Most IHCP-solving methods available in literature are formulated for simple-shaped bodies and cannot be applied to complex geometries. In this work, the 3-D problem is simplified to an axisymmetric analysis. If the nozzle diameter is not much smaller than the diameter of the cylindrical component, a corrected axisymmetric model is proposed. Next, a method is presented for solving the axisymmetric IHCP in a complex domain. It is based on the control volume finite element method. Based on temperature transients measured on the outer surface, the temperature distribution is reconstructed by marching from the known to the unknown boundary. The developed method is applied for temperature identification in a cylindrical component with a nozzle. The presented algorithm is tested using measured temperatures generated from a direct solution. The transient temperature distribution obtained from the method presented in the paper is compared with the values obtained from the direct solution. The proposed method is also used to estimate the unknown boundary condition. The information about the heat transfer coefficient value makes it possible to describe the heat transfer phenomena occurring inside the component.〈/p〉 〈p〉The presented method makes it possible to optimize the power unit start-up and shutdown, contributes to a reduction in heat losses arising during the operations and enables extension of the power unit life. The method can be used in monitoring systems of both conventional and nuclear power plants.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 1 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer〈/p〉 〈p〉Author(s): U. Madanan, R.J. Goldstein〈/p〉

Description: 〈p〉Publication date: Available online 1 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer〈/p〉 〈p〉Author(s): Filippo de Monte, A. Haji-Sheikh〈/p〉

Description: 〈p〉Publication date: Available online 1 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer〈/p〉 〈p〉Author(s): Dirk Bertsche, Paul Knipper, Thomas Wetzel〈/p〉

Description: 〈p〉Publication date: October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 142〈/p〉 〈p〉Author(s): Dae Yeon Kim, Omid Nematollahi, Kyung Chun Kim〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This paper reports a visualization study on the flow boiling of R245fa in a rectangular channel filled with an open-cell random porous structure made of copper. The boiling heat transfer of the two-phase vertical upward flow inside the channel was measured while the refrigerant was asymmetrically heated by a hot-water-source channel. An experiment was conducted with a constant saturation pressure of 5.9 bar, inlet vapor quality of 0.05–0.99, channel heights of 3 and 5 mm, and mass flux of 133–300 kg/m〈sup〉2〈/sup〉 s. The results are compared with those of a plain channel in the same operating conditions. The two-phase flow characteristics are discussed along with the trend of the boiling heat transfer coefficient and mean vapor quality according to the flow patterns obtained with and without the metallic porous media. A non-dimensional analysis was also conducted for comparison.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Shi Lin, Xi Liu, Xuelai Li〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The visualization method in 3D simulation is established to investigate the spatial distribution of film thickness and local heat transfer coefficient to avoid the dryout phenomenon and improve the heat transfer performance. The simulated results are in good agreement with the experimental data in the literature. The results indicate that the spatial distribution of liquid film is peak-valley-peak, and there exists a minimum thickness. The change path of the hazardous area is a space curve rather than a plane curve. The film thickness with dimensionless axial length of 0.375 is closest to the average film thickness. The hazardous area approaches the inlet column as the tube diameter increases in the axial direction. The tube diameter has a great influence on the hazardous area, and the large tube diameter promotes it to move to the inlet column. The spatial distribution rule of the local heat transfer coefficient is the same as that of film thickness. The place with the worst heat transfer performance is approximately in the same position as the place with the thinnest liquid film. The results provide a good guidance for further study on falling film process.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): A. Urbano, S. Tanguy, C. Colin〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Understanding and controlling nucleate pool boiling phenomena in zero gravity conditions is fundamental for space applications. An analytical model for the equilibrium radius reached by a bubble nucleated in sub-cooled conditions is established in this work and verified numerically. Indeed, direct numerical simulations of two phase flows conjugated with the heat conduction in the solid wall are carried out in order to verify and correct the analytical model. Fine grids, with cells size of the order of the micron, are mandatory in order to capture the subtle equilibrium between condensation and evaporation that characterises stationary conditions. This has been possible thanks to the house made solver 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si53.svg"〉〈mrow〉〈mi mathvariant="bold-script"〉DIVA〈/mi〉〈/mrow〉〈/math〉, validated for nucleate pool boiling simulations, and that permits to carry out parallel numerical simulations. Results show that the equilibrium radius of the bubble is a function of the thermal gradient, of the Jakob numbers associated with condensation and evaporation and of the apparent contact angle. The analysis of the thermal field is carried out and an interpretation of the physical processes that characterise the equilibrium is given. In addition, useful information on the heat transfer behaviour, reported in terms of Nu numbers, completes the work.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Yubo Wang, Zhigang Liu, Yingjie Chang, Xiangyuan Zhao, Liejin Guo〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The regimes of gas-liquid two-phase flow in horizontal pipe at high pressure were experimentally investigated, and characteristics of wave stratified flow, such as wave length, wave height and propagation speed were calculated. The pressure drop was analyzed and friction factors, such as gas-wall, liquid-wall and interfacial friction factors, were calculated and new correlations were deduced. Influence of interface wave on interfacial friction factor was studied, and characteristics of interface wave were included to deduce new correlation for it, and good accuracy was proved. For the gas-water two-phase flow at 2 MPa, transition from wave stratified to annular flow appears with smaller gas superficial velocity compared with that at atmosphere, and intermittent flow appears with greater liquid superficial velocity. Regime transition models are tested, for the gas-liquid two-phase flow at atmosphere the sheltering coefficient with value of 0.01 is appropriate. However, it is promoted to be 0.06 at 2 MPa, and good performance of regime transition model was reached.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): M. Streza, O. Grad, D. Lazar, M. Depriester, S. Longuemart, A.H. Sahraoui, G. Blanita, D. Lupu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The development of effective methods for hydrogen storage is of paramount importance in using hydrogen as a transportation fuel for on-board applications. The rate at which the hydrogen is adsorbed/desorbed on porous materials in compressed pellets is directly related to the thermal conductivity of the adsorbent. This work aims to increase the hydrogen adsorption rate in MIL-101(Cr) and MIL-100(Fe) compressed pellets by using reduced graphene oxide (rGO) as an additive, in order to get an increased thermal conductivity and thus a more efficient heat transport through the pellets. To achieve this goal, a complex study was undertaken using different techniques, namely photothermal radiometry (PTR) for thermal conductivity investigation, a volumetric home-made device for kinetic measurements and other techniques (XRD, SEM, TEM, BET, TG-DTA) for structural and morphological characterization of the samples. It has been found that the thermal conductivity of the pellets increases with the graphene addition. A significant enhancement in thermal conductivity (by factors of 4 compared to pellets without additives) is obtained and reaches a maximum of 0.58 W/mK for MIL-100(Fe) pellet (ρ = 0.65 g/cm〈sup〉3〈/sup〉). The hydrogen adsorption equilibrium time in neat samples is reached in about 180 s. The presence of 10 wt% rGO in both MIL-100 and MIL-101 pellets improves the hydrogen adsorption kinetics and favors the equilibrium in shorter times, respectively 20 and 40 s, than in neat samples. The experimental data are in very good agreement with the Linear Driving Force Model (LDF) for gas adsorption kinetics.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Zaid B. Jildeh, Patrick Kirchner, Klaus Baltes, Patrick H. Wagner, Michael J. Schöning〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Hydrogen peroxide (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si61.svg"〉〈mrow〉〈msub〉〈mrow〉〈mtext〉H〈/mtext〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈msub〉〈mrow〉〈mtext〉O〈/mtext〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉) is a typical surface sterilization agent for packaging materials used in the pharmaceutical, food and beverage industries. We use the finite-elements method to analyze the conceptual design of an in-line thermal evaporation unit to produce a heated gas mixture of air and evaporated 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si62.svg"〉〈mrow〉〈msub〉〈mrow〉〈mtext〉H〈/mtext〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈msub〉〈mrow〉〈mtext〉O〈/mtext〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 solution. For the numerical model, the required phase-transition variables of pure 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si63.svg"〉〈mrow〉〈msub〉〈mrow〉〈mtext〉H〈/mtext〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈msub〉〈mrow〉〈mtext〉O〈/mtext〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 solution and of the aerosol mixture are acquired from vapor-liquid equilibrium (VLE) diagrams derived from vapor-pressure formulations. This work combines homogeneous single-phase turbulent flow with heat-transfer physics to describe the operation of the evaporation unit. We introduce the apparent heat-capacity concept to approximate the non-isothermal phase-transition process of the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si64.svg"〉〈mrow〉〈msub〉〈mrow〉〈mtext〉H〈/mtext〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈msub〉〈mrow〉〈mtext〉O〈/mtext〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉-containing aerosol. Empirical and analytical functions are defined to represent the temperature- and pressure-dependent material properties of the aqueous 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si65.svg"〉〈mrow〉〈msub〉〈mrow〉〈mtext〉H〈/mtext〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈msub〉〈mrow〉〈mtext〉O〈/mtext〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 solution, the aerosol and the gas mixture. To validate the numerical model, the simulation results are compared to experimental data on the heating power required to produce the gas mixture. This shows good agreement with the deviations below 10%. Experimental observations on the formation of deposits due to the evaporation of stabilized 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si66.svg"〉〈mrow〉〈msub〉〈mrow〉〈mtext〉H〈/mtext〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈msub〉〈mrow〉〈mtext〉O〈/mtext〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 solution fits the prediction made from simulation results.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Nikita Razuvanov, Peter Frick, Ivan Belyaev, Valentin Sviridov〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Liquid metal flows under a high thermal load and strong magnetic field are of great interest in the context of the development of fusion reactor cooling systems. In this study, a downward flow of mercury in a heated duct, affected by a coplanar magnetic field (directed transverse to the main flow and along the long side of the duct), is studied experimentally for symmetrical (two-sided) and asymmetrical (one-sided) duct heating, showing different flow behaviors in the magnetic field. We show that in spite of the qualitatively different structure of the downward flow in the duct with different heating modes, the applied coplanar magnetic field provides a rather general reaction, which includes two scenarios determined by the Richardson number. At high Richardson numbers (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si71.svg"〉〈mrow〉〈mtext〉Ri〈/mtext〉〈mspace width="0.12em"〉〈/mspace〉〈mi〉≳〈/mi〉〈mspace width="0.12em"〉〈/mspace〉〈mn〉1〈/mn〉〈/mrow〉〈/math〉), the Nusselt number tends toward the turbulent limit, as expected in the channel without a magnetic field, and at low Richardson numbers (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si72.svg"〉〈mrow〉〈mtext〉Ri〈/mtext〉〈mspace width="0.12em"〉〈/mspace〉〈mi〉≲〈/mi〉〈mspace width="0.12em"〉〈/mspace〉〈mn〉1〈/mn〉〈/mrow〉〈/math〉), the Nusselt number tends to its laminar limit.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Weijia Qian, Yuzhen Lin, Xin Hui〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A temporal linear instability analysis has been conducted to investigate the effect of heat and mass transfer on the instability of an annular liquid sheet axially moving in the gas medium. The dispersion relation is obtained and solved numerically. Energy budget is calculated to provide physical insights of various instability driving mechanisms. The results show that heat and mass transfer promotes the wave growth rate mainly at small wave numbers. In contrast to the case without heat and mass transfer, the wave growth rate with strong heat and mass transfer peaks at zero wave number. Liquid viscosity is found to have a minimal stabilizing effect on the sheet instability. Regardless of the heat and mass transfer, increasing liquid Weber number suppresses the sheet instability below a crossover point of wave number at 1.15, and promotes the sheet instability above the crossover point. In the presence of strong heat and mass transfer, increasing gas-to-liquid density ratio reduces the wave growth rate at small wave numbers below 0.28, and beyond which increases the wave growth rate. Reducing sheet thickness promotes the sheet instability for all wave numbers. The energy budgets show that the inner gas disturbance is most responsible for promoting the wave growth rate at large liquid Weber numbers; and the inner interface dominates the outer one in destabilizing the annular liquid sheet.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Sining Li, Hongna Zhang, Jianping Cheng, Xiaobin Li, Weihua Cai, Zengyao Li, Fengchen Li〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Continuous improvement of energy-efficient systems has led to various technical features such as miniaturization, integration, and portability. This development became the leading trend of the contemporary industry, with the evolvement of various new and miniaturized instruments such as micro-electro-mechanical systems and micro-satellite. Highly miniaturized and integrated electronic devices often release a large amount of heat within their micro scale inter-components areas, which under certain circumstances, may lead to the micro-device’s functional failure. This calls for techno-scientific efforts for efficient heat removal from the mentioned devices. Numerous innovatively scenarios have been so far explored to satisfy the greatly increasing heat removal demand. This paper explores two major types of heat removal technics for micro-scale single-phase flow, namely passive and active type. The passive type includes the geometry modification and working media change, where corresponding effects on heat transfer performance and fluid flow are addressed by Nusselt number and friction factor, respectively. Their most important advantages are stability and robustness in operation or integration with complex systems. For the active type, only the pulsating inlet flow and acoustic wave are considered in the present paper, which are less reviewed recently. Generally, the heat transfer enhancement is attributed to the effect of disturbed thermal boundary layers on mixing of fluids. The present review article is organized in a way that provides a historical perspective of the recent developments in this area, and advances comprehensive comments from overall situations; which may serve a substantial guidance for researchers in different background who intend to step into this area.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Fritjof Nilsson, Ali Moyassari, Ángela Bautista, Abraham Castro, Ignacio Arbeloa, Mikael Järn, Urban Lundgren, Jan Welinder, Kenth Johansson〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Aggregation of ice on electrical cables and apparatus can cause severe equipment malfunction and is thus considered as a serious problem, especially in arctic climate zones. In particular, cable damage caused by ice accumulation on railway catenary wires is in wintertime a common origin for delayed trains in the northern parts of Europe. This study examines how resistive heating can be used for preventing formation of ice on metallic, non-insulated electrical cables. The heat equation and the Navier Stokes equations were solved simultaneously with FEM in 3D in order to predict the cable temperature as function of external temperature, applied voltage, wind speed, wind direction, and heating time. An analytical expression for the heat transfer coefficient was derived from the FEM simulations and it was concluded that the influence of wind direction can typically be neglected. Experimental validation measurements were performed on Kanthal cables in a climate chamber, giving temperature increase results in good agreement with the simulation predictions. The resistive heating efficiency, i.e. the ratio between applied electrical energy and resulting thermal energy, was found to be approximately 68% in this particular study.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Yosheph Yang, Ikhyun Kim, Gisu Park〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A combined experimental and numerical approach has been proposed to study the oxygen catalytic recombination coefficient of SiC-coated material for two different types of surface condition: roughened and pre-heated. For the experimental approach, the shock tube facility, which test gas consists of 21% oxygen and 79% argon by volume, is utilized to measure the oxygen catalytic recombination coefficient at near-room surface temperature. The coefficient is deduced from the local heat transfer measurement at the end-wall model of the shock tube via catalytic boundary layer theory. The experimental data indicated that the coefficient for the SiC-coated material varied between 0.0024 and 0.01, depending on the surface condition. The results suggested that even at low surface temperature, both the level of surface roughness and the oxidation due to the pre-heating should be considered carefully for the atomic recombination process. For the numerical approach, the finite rate catalytic modeling is elaborated in detail. The modeling is conducted to better understand the physical interaction for the recombination mechanism. Given the surface reactions considered in the modeling, the catalytic recombination coefficient was not significantly affected by the pressure at low wall temperature. Conversely, the variation in the coefficient was observed to be dependent on the activation energy and the number of active site concentrations at the surface.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Kasper Gram Bilde, Kim Sørensen, Thomas Condra〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A natural circulation biomass boiler is a highly dynamic system, especially during start-up where the entire system is initially cold and at rest. During the start-up procedure, swelling inside the steam drum is often experienced due to the evaporation and rapid volumetric expansion of water. A mathematical model describing the dynamics of a natural circulation evaporator is presented. The boiler system is divided into subsystems where the mass, momentum and energy equations are formulated and applied. The complex geometry of the boiler is simplified and discretized in one dimension. The model captures the transient evaporation of water and the dynamic instabilities affiliated with the phase transition. During the phase transition to the two-phase regime, the steam produced displaces water. The rapid volumetric expansion of water causes mass to be transferred to the steam drum, resulting in high water levels. It is concluded that in order to model the dynamic behaviour of a natural circulation boiler with a steam drum, the subcooled region and the phase transition needs to be considered, since the phase transitions in the evaporator tubes result in rapid swelling inside the steam drum.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Juan Li, Zhangyu Zhu, Liang Zhao, Hao Peng〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The single-phase heat transfer and flow characteristics of microchannels with microribs were investigated experimentally in this paper. Based on a smooth rectangular microchannel (MC-S), rectangular microchannels with rectangular ribs on one side (MC-OSR) and both sides (MC-BSR) were proposed. The effects of the volumetric flow rate and inlet temperature on the Nusselt number and friction factor were analyzed. Five positions were assigned evenly in the flow direction at the wall to measure the temperature and determine the wall temperature distribution. The results showed that the Nusselt number increased as the volumetric flow rate and inlet temperature increase. The friction factor decreased with increasing inlet temperature. The thermal performance index of the MC-OSR was higher than those of the MC-BSR and MC-S, which reflected the superior heat transfer performance of the MC-OSR. The wall temperature was nonlinearly distributed in the flow direction. The amplitude of the temperature difference gradually decreased. Compared to the MC-S, the entrance effects of the MC-OSR and MC-BSR were more obvious mainly due to the presence of ribs.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): H. Maral, E. Alpman, L. Kavurmacıoğlu, Cengiz Camci〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In turbomachines, a properly dimensioned gap between the rotating blade tip and the stationary casing is required in order to avoid mechanical failures due to blade rubbing. Maintaining a tip gap allows the relative motion of the blade, however a leakage flow almost always exists due to the strong pressure differentials existing near the tip airfoil boundaries. Tip leakage flow which is a 3-dimensional and highly complex flow system is responsible from a considerable amount of total pressure loss in a turbine stage. Besides, tip leakage flows induce adverse thermal effects near the blade tip, eventually causing an increase in cooling demand. Various passive control methods exist to weaken the adverse effects of tip leakage flows, in an effort to increase turbine stage efficiency. In this paper, a novel tip carving approach is applied to mitigate the undesired aerothermal effects of the tip leakage flow. A numerical investigation is carried out to obtain the optimum shape of the carved blade tip with an objective function to minimize both heat transfer and leakage loss. A genetic algorithm is used for the optimization, integrated with a meta model which predicts the objective functions quickly. Various meta-models such as Artificial Neural Network (ANN), Extreme Learning Machine (ELM) and Support Vector Machine (SVM) are tested for this purpose. An initial database consisting of 55 blade tip geometries is created for meta-model training using “Sobol design of experiments” methodology. This database is then successively enlarged using a coarse-to-fine approach in order to improve the prediction capabilities of the meta-models. Once a sufficient level of prediction error and a proper consistency is achieved, the optimization process is terminated. Current results indicate that carved blade tip designs are likely to achieve a considerable improvement in aero-thermal performance of axial turbine stages. Multi-objective optimization of the blade tip surface of the carved type is a promising approach in gas turbines since it paves the way for undiscovered blade tip designs for further performance improvements.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Abgail P. Pinheiro, João Marcelo Vedovoto, Aristeu da Silveira Neto, Berend G.M. van Wachem〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Ethanol is considered a promising alternative fuel due to its advantages compared to conventional fossil fuels. Since fuel evaporation plays an important role in many applications, especially in energy systems involving spray combustion, this work aims to investigate the effects of ambient temperature, pressure and vapor concentration on the evaporation of a single ethanol droplet. In order to validate the theoretical model, numerical simulations of an ethanol droplet evaporation are performed and the results are validated with experimental data. The ambient temperature, pressure and vapor concentration were varied in the ranges of 400–1000 K, 0.1–2.0 MPa and 0.0–0.75, respectively. The results reveal that an increase in the ambient pressure causes an augmentation in the ratio of initial heat-up time to the whole evaporation lifetime, which enhances the unsteady effects of the droplet evaporation. This same ratio is almost independent of the gas temperature at low ambient pressure; however, as the ambient pressure is increased, the tendency of this ratio to increase with the gas temperature becomes significant. Additionally, the results show that there is a threshold ambient temperature, which determines whether the average area reduction rate will increase or decrease as the ambient pressure is increased. Finally, the results show that condensation effects are observed for non-zero ambient vapor concentration, and there is also a threshold ambient temperature which determines whether the average area reduction rate will increase or decrease as the ambient vapor concentration is increased.〈/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-S0017931019309214-ga1.jpg" width="300" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): George Dogkas, Panagiotis Bitsikas, Dimitrios Tertipis, Emmanouil Rogdakis〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉A one-dimensional analytical model and a three-dimensional CFD numerical model were used for the investigation of the effect that the rotational speed has on several thermodynamic quantities and the performance of a Vuilleumier machine. The machine, was designed as a combination of two opposing Stirling engines. The one-dimensional program was based on the energy balance at each control volume in order to calculate the energy flow inside the machine and then losses were added. On the other hand, the CFD model utilized fundamental conservation equations at each of the numerous computational cells which resulted to accurate calculations at every dimension. It was able to provide the heat transfer coefficients inside the heat exchangers that were in turn utilized by the analytical model.〈/p〉 〈p〉The pressure drop was computed directly at each space by the numerical model and with equations for flow losses from the bibliography by the analytical model. Pressure drop increased significantly with the speed. The effectiveness of the regenerators was evaluated by an existing analytical model and resulted to reduce drastically with the drop of speed. The effectiveness plays very important role on the efficiency of the machine. Furthermore, there appears to be a discrepancy between the heat flow in the heat exchangers and the wall-gas temperature difference at high speeds which has to be examined according to the oscillatory nature of the gas flow. Heat transfer coefficients were generated in relationship with the Reynolds number for each speed investigated, yielding less thermal resistance when the speed is high. Finally, the change of heat amounts through the four heat exchangers and change of the efficiency of the Vuilleumier machine with the speed, resulted to be similar with experimental data.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Manjunath C. Rajagopal, Ho Chan Chang, Timothy Man, Gowtham Kuntumalla, Yuquan Meng, Sreenath Sundar, Hanyang Zhao, Srinivasa Salapaka, Chenhui Shao, Placid Ferreira, Nenad Miljkovic, Sanjiv Sinha〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Heat exchangers for low temperature (〈150 °C) waste heat recovery (WHR) are challenging due to severe constraints on materials costs, manufacturing methods, and maintenance opportunities after installation. Here, the use of polymers instead of metals, offers an avenue for cost reduction in WHR applications, provided the typical low thermal conductivity of polymers (∼0.2 W m〈sup〉−1〈/sup〉 K〈sup〉−1〈/sup〉) that results in poor overall heat transfer coefficient can be overcome. Previous approaches to enhance the thermal conductivity of polymers remain problematic in terms of cost, thermal, and thermomechanical considerations. In this work, we propose a novel hybrid metal-polymer composite assembled from strips of polymer and copper. Polymer strips with metal-clad edges are wound helically and the interfaces between the strips joined to obtain a tube for a cross-flow heat exchanger. Interfaces are designed to enhance effective heat conduction across the material and increase the overall heat transfer coefficient. Through numerical simulations, we obtain metal-polymer layouts that achieve wall effective thermal conductivity ∼1 W m〈sup〉−1〈/sup〉 K〈sup〉−1〈/sup〉 at ∼23% or ∼35% volume fraction of copper or aluminum, respectively. The conductivity enhancement is sufficient for typical low temperature WHR, resulting in a ∼20% enhancement in the overall heat transfer coefficient compared to a similar all-polymer heat exchanger. Thermomechanical simulations revealed that the composite pipe designs can withstand 〉1.4 MPa internal pressure, under the normal operating conditions of the heat exchanger. Our work provides guidelines for designing macroscopic metal-polymer composites that can be adapted for specific requirements such as reduced materials costs, ease of manufacturing, and heat transfer enhancement. The optimized design of hybrid metal-polymer heat exchanger tubes potentially provides a scalable and cost-effective route toward harvesting waste heat from low temperature sources.〈/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-S0017931019325748-ga1.jpg" width="482" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): V. Yasnou, A. Mialdun, D. Melnikov, V. Shevtsova〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This paper is addressing the challenge related to understanding transient mass transfer associated with a thermal gradient through a liquid mixture containing a layer of a porous medium saturated with liquid. Digital optical interferometry in combination with tomography was used to unveil the difference in the mechanism of mass transfer caused by the Soret effect in a free liquid and a porous medium and, especially, at their border. With continuous monitoring of mass transfer throughout the system, it was found that at the beginning of the experiment (the first 24 h) the temporal evolution of the concentration difference between the interfaces of the porous layer looked as if the inverse and strong component separation occurred in the porous medium. It was evidenced that different characteristic times of the thermodiffusion process in free liquids and a porous medium were essential for the formation of the apparent inverse separation. This study not only provides physical insights into the mass transfer dynamics, but also opens up the possibility to measure several transport coefficients from a single experiment. We have obtained the Soret, diffusion and thermodiffusion coefficients of the THN–〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si66.svg"〉〈mrow〉〈mi〉n〈/mi〉〈msub〉〈mrow〉〈mi mathvariant="normal"〉C〈/mi〉〈/mrow〉〈mrow〉〈mn〉12〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 binary mixture as well as tortuosity and thermal conductivity of a saturated porous medium. The Soret coefficients, determined in the porous layer and in the compound system using the mean concentration in free volumes are similar, and about 10% higher than the benchmark value (Platten et al., 2003), but coincide with the value reported in literature for measurements in porous media (Platten and Costeseque, 2004).〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 143〈/p〉 〈p〉Author(s): Byunggyun Kim, Suhan Park〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This study visualizes and analyzes the changes in cavitation flow and spray characteristics that depend on the needle position and length-to-width ratios of the nozzle. We tested five needle positions that simulated the eccentricity of the needle. The length-to-width ratios were set at 2.0 and 2.67. The width of the nozzle was taken as 3 mm and 4 mm, and the length of the nozzle was fixed at 8 mm. The cross section of the test nozzles was rectangular. The injection pressure was set at 1–4 bar in 0.1 bar increments, and the inside of the nozzle was visualized using shadow graphic visualization. The flow rate of the fluid, the injection pressure applied to the inside of the nozzle, and the material properties of the fluid were measured in terms of the Reynolds number, the cavitation number, and the discharge coefficient, respectively. We analyzed the spray angle, cavitation length, and cavitation width inside the nozzle from the captured images by using image processing, as well as the correlations between the spray angle and cavitation length and width.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 137〈/p〉 〈p〉Author(s): Ahmadreza Ayoobi, Ahmadreza Faghih Khorasani, Mohammad Reza Tavakoli, Mohammad Reza Salimpour〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Pool boiling heat transfer and critical heat flux (CHF) in saturated water were experimentally studied under transient power conditions. A chrome-aluminum-iron alloy wire supported horizontally in a pool of water was used as the heating element. The heating rate in the test section was increased linearly depending on time by applying voltage control for 1 s to 5000 s. The heat flux was obtained by a second-order function of time. In this study, the increase in heat flux starts from the free convection regime to the film boiling regime. The transient boiling heat transfer coefficient (TBHTC), transient wire superheat temperature, transient heat flux and transient CHF were also obtained. The results showed that with increasing the time period, the TBHTC in the nucleate boiling increased, decreased in transition from the nucleate boiling to the film boiling, and again increased in the film boiling and the incipient boiling point decreased which may be due to a decreased number of nucleation sites and the explosive nature of the initial stage of nucleate boiling. The TBHTC decreased in the second part of the film boiling as both the heat flux and the vapor film thickness around the wire had increased. In addition, increasing the time period decreased the CHF and superheating temperatures in the CHF by up to a period of 100 s. After that, its trend approached the steady state boiling. The transient CHF reduction in the time period of 100 s is 40.5% relative to steady state condition which is the minimum transient CHF. The maximum increase in the transient CHF is in the time period of 1 s and equals to 46.7%. A high-speed camera was used to show bubble and vapor film behavior around the heating wire.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 137〈/p〉 〈p〉Author(s): Kristina Navickaitė, Andrea Mocerino, Luca Cattani, Fabio Bozzoli, Christian Bahl, Klaus Liltrop, Xiaodan Zhang, Kurt Engelbrecht〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A double corrugated tube geometry for improved heat transfer performance inspired by vascular heat exchangers found in fish such as the tuna and the opah is presented. The geometry features a cross section that varies continuously in the flow direction while maintaining a constant hydraulic diameter, which gives enhanced heat transfer at a relatively low increase in pressure drop. Five ellipse-based tubes with varying corrugation severity and period that emulate blood vessels of fish were produced in an aluminium alloy using additive manufacturing technology. Thermal performance of the novel tube design was experimentally investigated in a counter flow tube-in-shell heat exchanger in a range of Reynolds numbers from 1000 to 2500. Correlations for the Nusselt number and friction factor for each tube are proposed and the experimental results show that the Nusselt number increases up to 500% in corrugated tubes compared to a straight tube. The global thermo-hydraulic performance, evaluated for the same pumping power, of the double corrugated tubes is up to 160% higher than that of a straight tube.〈/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-S0017931018346301-ga1.jpg" width="275" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 137〈/p〉 〈p〉Author(s): Liaofei Yin, Peixue Jiang, Ruina Xu, Haowei Hu, Li Jia〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The open microchannel configuration can mitigate several drawbacks of closed microchannel for flow boiling heat transfer, but the studies related to flow boiling in open microchannels are limited up to now. In this study, experiments were conducted to investigate the heat transfer and pressure drop characteristics during flow boiling of deionized water (DI water) in open microchannels and to analyze the effects of operating conditions and channel dimensions. The experiments were carried out with the inlet subcooling Δ〈em〉T〈/em〉〈sub〉sub〈/sub〉 = 20, 35, 50 °C at different mass fluxes ranging from 174 to 374 kg/m〈sup〉2〈/sup〉 s for varied applied heat flux 214.9–1355.1 W/cm〈sup〉2〈/sup〉. Two open microchannel samples with different geometry dimensions were tested. High speed flow visualizations were realized to illustrate the flow pattern transitions during flow boiling in open microchannels. Experimental results showed that the onset of stratified flow (OSF) denoted an alternation of dominant heat transfer mechanism at which the heat transfer coefficient (HTC) was largest during the flow boiling. The effects of operating conditions and channel dimensions on the HTC and two-phase pressure drop in open microchannels were mainly observed in the stratified flow regimes, during which nucleate boiling and convective evaporation jointly controlled the flow boiling process. In addition, the open microchannel with smaller size but a great number of channels had a better heat dissipation capability along with a higher pressure drop in stratified flow regimes.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 137〈/p〉 〈p〉Author(s): Lucky V. Tran, Carson D. Slabaugh〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This work demonstrates that it is possible, and with relative ease, to determine unique characteristic temperatures for the red & green light intensity signals of a narrowband thermochromic liquid crystal mixture. It is proposed to utilize additional indicators in future transient thermochromic liquid crystals measurements to increase data throughput, improve robustness, and reduce uncertainty in the calculated heat transfer coefficient. Potential algorithmic implementations for a multi-color technique are discussed. Optimum conditions for minimizing the uncertainty in the calculated heat transfer coefficient are discussed and compared to that of the traditional single-color technique. The multi-color technique is also extendable to multi-band techniques.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 International Journal of Heat and Mass Transfer, Volume 137〈/p〉 〈p〉Author(s): Kyun Ho Lee〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The major objective of the present study is to demonstrate the performance and efficiency of the Repulsive Particle Swarm Optimization (RPSO) method as an inverse analysis solver by extending its application to the inverse heat conduction problem. As the first outcome, an estimation of unknown parameters of a time-varying plane heat source in a one-dimensional inverse heat conduction problem was considered. The overall performance of the RPSO method was examined based on the effects of the forms of unknown heat source, the number of parameters, the measurement errors and the population sizes on the estimation accuracy. In addition, the final results were compared with those of the Levenberg-Marquardt Method (LMM), which is widely used for the inverse heat conduction problem, to investigate the advantages and disadvantages of the RPSO method.〈/p〉 〈p〉The present results prove that the RPSO method has more robust characteristics and yields rather confidential inverse estimations than the LMM for the inverse heat conduction problem although it requires more computational cost.〈/p〉 〈/div〉 〈/div〉