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1

Digitale Medien

Transport and solidification phenomena in molten microdroplet pileup (2002)

Haferl, S. ; Poulikakos, D.

[S.l.] : American Institute of Physics (AIP)

Journal of Applied Physics 92 (2002), S. 1675-1689

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ISSN: 1089-7550

Quelle: AIP Digital Archive

Thema: Physik

Notizen: This article presents a predominantly numerical investigation of the transient transport phenomena occurring during the pileup (deposition one upon another) of molten, picoliter-size liquid metal droplets relevant to a host of novel micromanufacturing processes. The investigated phenomena last fractions of a millisecond in severely deforming domains of typical size of a small fraction of a millimeter. The prevailing physical mechanisms of the pileup process (occurring simultaneously) are identified and quantified numerically. These are the fluid mechanics of the bulk liquid, capillarity effects at the liquid–solid interface, heat transfer, solidification, and thermal contact resistance effects at all interfaces. In terms of values of the Reynolds, Weber, and Stefan number the following ranges are covered: Re=281–453, We=2.39–5.99, and Ste=0.187–0.895. This corresponds to molten solder droplets impinging at velocities ranging between 1.12 and 1.74 m/s having an average diameter of (approximate)78 μm. The initial substrate temperature ranges between 25 and 150 °C. The initial droplet temperature is 210 °C. The numerical model presented is based on a Lagrangian formulation of the Navier–Stokes equations accounting for surface tension, thermal contact resistance, solidification, and a Navier slip condition at the dynamic contact line. Results of simulations are presented showing the effect of thermal contact resistance and slip at the dynamic contact line on the transients and the outcome of a pileup. Comparisons of the simulated pileup with experimental visualizations are shown, demonstrating good agreement in cases where inertia dominates over capillary effects. For decreasing Stefan number (i.e., higher substrate temperatures) an increasing importance of wetting is observed. For these cases the limitations of the employed popular boundary condition at the dynamic contact line is demonstrated and the need for experimental data (currently nonexistent in the literature) that would yield an improved condition at the contact line accounting for the temperature dependence of wetting phenomena is underpinned. © 2002 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.1492019

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2

Digitale Medien

Modeling of the deformation of a liquid droplet impinging upon a flat surface (1993)

Fukai, J. ; Zhao, Z. ; Poulikakos, D. ; [weitere]

New York, NY : American Institute of Physics (AIP)

Physics of Fluids 5 (1993), S. 2588-2599

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ISSN: 1089-7666

Quelle: AIP Digital Archive

Thema: Physik

Notizen: This article presents a theoretical study of the deformation of a spherical liquid droplet impinging upon a flat surface. The study accounts for the presence of surface tension during the spreading process. The theoretical model is solved numerically utilizing deforming finite elements and grid generation to simulate accurately the large deformations, as well as the domain nonuniformities characteristic of the spreading process. The results document the effects of impact velocity, droplet diameter, surface tension, and material properties on the fluid dynamics of the deforming droplet. Two liquids with markedly different thermophysical properties, water and liquid tin, are utilized in the numerical simulations because of their relevance in the industrial processes of spray cooling and spray deposition, respectively. The occurrence of droplet recoiling and mass accumulation around the splat periphery are standout features of the numerical simulations and yield a nonmonotonic dependence of the maximum splat radius on time.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.858724

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3

Digitale Medien

Wetting effects on the spreading of a liquid droplet colliding with a flat surface: Experiment and modeling (1995)

Fukai, J. ; Shiiba, Y. ; Yamamoto, T. ; [weitere]

[S.l.] : American Institute of Physics (AIP)

Physics of Fluids 7 (1995), S. 236-247

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ISSN: 1089-7666

Quelle: AIP Digital Archive

Thema: Physik

Notizen: In this paper an experimental and theoretical study of the deformation of a spherical liquid droplet colliding with a flat surface is presented. The theoretical model accounts for the presence of inertia, viscous, gravitation, surface tension, and wetting effects, including the phenomenon of contact-angle hysteresis. Experiments with impingement surfaces of different wettability were performed. The study showed that the maximum splat radius decreased as the value of the advancing contact angle increased. The effect of impact velocity on droplet spreading was more pronounced when the wetting was limited. The experimental results were compared to the numerical predictions in terms of droplet deformation, splat radius, and splat height. The theoretical model predicted well the deformation of the impacting droplet, not only in the spreading phase, but also during recoiling and oscillation. The wettability of the substrate upon which the droplet impinges was found to affect significantly all phases of the spreading process, including the formation and development of a ring structure around the splat. © 1995 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.868622

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4

Digitale Medien

Buoyancy-driven convection in a horizontal fluid layer extending over a porous substrate (1986)

Poulikakos, D.

[S.l.] : American Institute of Physics (AIP)

Physics of Fluids 29 (1986), S. 3949-3957

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ISSN: 1089-7666

Quelle: AIP Digital Archive

Thema: Physik

Notizen: In this study a series of numerical simulations is reported that aims to document the phenomenon of buoyancy-driven flow instability in a fluid layer extending over a porous substrate. The numerical simulations focus primarily on the parametric domain in which the flow in the system is well established, i.e., the value of the Rayleigh number is larger than critical. A general flow model is used to describe the flow inside the porous bed. This flow model accounts for friction caused by macroscopic shear [Brinkman extension of the Darcy model; Appl. Sci. Res. Sect. A 1, 27 (1947)], as well as for the phenomenon of flow inertia [Forchheimer's extension of the Darcy model; Dtsch. Ingenieure 45, 1782 (1901)]. Several important characteristics of the flow and temperature fields inside the composite layer (porous/fluid) are reported and the dependence of these characteristics on the problem dimensionless groups is documented.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.865734

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5

Digitale Medien

Transient double-diffusive convection experiments in a horizontal fluid layer extending over a bed of spheres (1989)

Kazmierczak, M. ; Poulikakos, D.

New York, NY : American Institute of Physics (AIP)

Physics of Fluids 1 (1989), S. 480-489

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ISSN: 1089-7666

Quelle: AIP Digital Archive

Thema: Physik

Notizen: The problem of double-diffusive convection for a thermohaline system consisting of a horizontal fluid layer situated above a saturated porous bed is studied experimentally. The porous bed is constructed from 5 mm spherical glass beads. The system initially contains a linear stabilizing salt distribution and is suddenly heated uniformly from below. The evolving temperature and concentration fields were determined at various times and were used to piece together a reliable picture of the behavior of the composite layer. In the majority of the experiments, convective flow was initiated and sustained in the fluid portion of the system. The flow was characterized by thorough mixing, resulting in uniform temperature and concentration distributions. The presence of the porous layer was responsible for a large delay in the flow initiation near the bottom wall, resulting in a considerable temperature increase. In some cases, the fluid saturating the porous matrix remained stagnant throughout the experiment, despite the fact that a vivid flow region existed directly above the porous region. The effect of the thermal Rayleigh number, the salinity Rayleigh number, and the height of the porous substrate are also investigated in the course of the present study.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.857418

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6

Digitale Medien

Experiments on double-diffusion in a composite system comprised of a packed layer of spheres and an underlying fluid layer (1997)

Rastogi, S. K. ; Poulikakos, D.

Springer

Heat and mass transfer 32 (1997), S. 181-191

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ISSN: 1432-1181

Quelle: Springer Online Journal Archives 1860-2000

Thema: Maschinenbau , Physik

Beschreibung / Inhaltsverzeichnis: Zusammenfassung In dieser Arbeit wird das Doppeldiffusionsproblem bezüglich eines Kompositsystems experimentell untersucht, welches aus einer flüssigkeitsgetränkten Kugelpackungschicht und einer darunter liegenden, reinen Flüssigkeitsschicht besteht. Als Flüssigkeit wird Wasser mit gelöstem Ammoniumchlorid verwendet. Die anfängliche Stoffkonzentration in der porösen Schicht ist linear und stabil, die in der reinen Flüssigkeit ist gleichförmig. Das anfänglich isotherme System wird plötzlich von oben gekühlt. Über unmittelbare Temperaturmessungen und Visualization des Dichtefeldes mittels holographischer Interferometrie werden die sich entwickelnden Temperaturund Strömungsfelder registriert. Der Einfluß der thermischen Rayleigh-Zahl, der konzentrationsbezogenen Rayleigh-Zahl, der Wärmeleitfähigkeit, der die poröse Matrix bildenden Kugeln und der Höhe der porösen Matrix auf die Ausbildung der Temperaturund Strömungsfelder wird aufgezeigt. Aus dem Vergleich der experimentellen Befunde mit den Vorausberechnungen mittels eines vorhandenen theoretischen Modells werden die Grenzen dieses Modells bestimmt, so wie der Zeitabschnitt, innerhalb dessen es zufriedenstellende Aussagen liefert. Die Ergebnisse dieser Studie sind für Doppeldiffusionsphänomene in Mischphasen und Flüssigkeitsgebieten erstarrender Binärmischungen von Bedeutung.

Notizen: Abstract In this paper an experimental study is reported on the problem of double-diffusion in a composite system comprised of a liquid-saturated packed layer of spheres and an underlying clear (of solid matrix) fluid layer. The liquid is a mixture of water and ammonium chloride. The initial species concentration of the porous layer is linear and stable and of the clear liquid layer uniform. The system is initially isothermal and it is suddenly cooled from above. The study investigates the evolving temperature and flow fields in the system by utilizing direct temperature measurements as well as holographic interferometry visualization of the density field. The effect of the thermal Rayleigh number, the species Rayleigh number, the thermal conductivity of the beads constituting the porous matrix, and the height of the porous matrix on the evolving temperature and flow fields are determined. Comparisons of the experimental results to the predictions of an existing theoretical model define the limitations of this model and the time domain in which the model performs acceptably well. The findings of this study are relevant to double-diffusion phenomena occuring in the mixed phase and liquid regions of solidifying binary mixtures.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/s002310050110

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7

Digitale Medien

Splat-quench solidification: estimating the maximum spreading of a droplet impacting a solid surface (1993)

Bennett, T. ; Poulikakos, D.

Springer

Journal of materials science 28 (1993), S. 963-970

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ISSN: 1573-4803

Quelle: Springer Online Journal Archives 1860-2000

Thema: Maschinenbau

Notizen: Abstract This study investigates the mechanisms that contribute to determining the maximum spreading of a liquid droplet impacting a solid surface in connection with splat-quench solidification. This paper defines two domains, the viscous dissipation domain and the surface tension domain, which are characterized by the Weber and the Reynolds numbers, and that are discriminated by the principal mechanism responsible for arresting the splat spreading. This paper illustrates the importance of correctly determining the equilibrium contact angle (a surface tension characteristic that quantifies the wetting of the substrate) for predicting the maximum spreading of the splat. Conditions under which solidification of the splat would or would not be expected to contribute to terminating the spreading of the splat are considered. However, our a priori assumption is that the effect of solidification on the spreading of a droplet, superheated at impact, is secondary compared to the effects of viscous dissipation and surface tension.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/BF00400880

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8

Digitale Medien

Heat transfer aspects of splat-quench solidification: modelling and experiment (1994)

Bennett, T. ; Poulikakos, D.

Springer

Journal of materials science 29 (1994), S. 2025-2039

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ISSN: 1573-4803

Quelle: Springer Online Journal Archives 1860-2000

Thema: Maschinenbau

Notizen: Abstract In this paper a combined theoretical and experimental study is reported on the process of solidification of a liquid metal droplet by impaction on a cold substrate (splat-quenching). The study is focused on the heat transfer aspects of this process and on the identification of parameters affecting the heat transfer mechanism. To this end, the effect of the droplet impact velocity and temperature, the effect of the substrate material and its initial temperature, and the effect of the thermal contact resistance between the splat and the substrate are investigated. A two-dimensional conduction model accounting for the freezing process in the splat and for the solidification kinetics has predicted reasonably well the trends observed in the experimental part of the study.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/BF01154677

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9

Digitale Medien

Remelting phenomena in the process of splat solidification (1995)

Kang, B. ; Waldvogel, J. ; Poulikakos, D.

Springer

Journal of materials science 30 (1995), S. 4912-4925

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ISSN: 1573-4803

Quelle: Springer Online Journal Archives 1860-2000

Thema: Maschinenbau

Notizen: Abstract A combined theoretical and experimental study is reported which investigates remelting phenomena during the splat cooling of two liquid-metal droplets impacting sequentially on a substrate. Under conditions of sufficiently high superheat it was proposed theoretically and demonstrated experimentally that an initial deposit is remelted by the subsequent impact of molten material. It is shown that the amount of superheat as well as the variation of thermophysical properties, particularly the latent heat and the melting temperature, influence the degree of remelting. Experimental findings supported to a certain extent the theoretical model assumptions that the splats could be represented by thin discs and that the heat transfer and solidification within the splat propagates in the axial direction only. However, the experiments showed that these assumptions are better suited for the central region of the splat. The occurrence of remelting often depended on the radial location for a given amount of superheat. For most part, the splat exhibited globular microstructure. Lamellar structures were observed near the top and the periphery of the splat, indicating slower cooling rates at these locations. The theoretical model constituted a good compromise between accuracy and simplicity and predicted the correct trends of the remelting phenomenon.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/BF01154504

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