Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session G02: Multiphase Flows: Modeling and Theory I |
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Chair: Roberto Mauri, University of Pisa Room: 2B |
Sunday, November 24, 2019 3:48PM - 4:01PM |
G02.00001: Dynamic transition of dendrite orientation in the spinodal decomposition of viscous binary mixtures under a thermal gradient. Roberto Mauri, Antonio Bertei In this study, spinodal decomposition of a very viscous regular binary mixture bounded within two walls cooled at different temperatures is simulated by using the diffuse interface model. Under a temperature gradient, phase separation starts from the cooler wall forming dendritic structures growing anisotropically with time. Two remarkably different dynamics are identified depending on whether heat propagates slower or faster than mass. For small thermal conductivity (i.e., small Lewis number), dendrites grow parallelly to the temperature gradient, keeping such an alignment until the steady-state. On the other hand, for large Lewis number, during the early stages phase separation proceeds within stripes oriented along iso-temperature lines, i.e., with dendrites aligned perpendicularly to the temperature gradient, which, however, gradually shift their orientation parallel to the temperature gradient as the steady-state is approached. Such a dynamic transition of dendrite orientation upon a temperature gradient when heat propagates faster than mass is found to hold also for non-equimolar mixtures and for different species thermal conductivities. These results shed light on the dynamics of phase separation in constrained systems and anisotropic conditions. [Preview Abstract] |
Sunday, November 24, 2019 4:01PM - 4:14PM |
G02.00002: Modeling of Marangoni corner flow with phase change in the constrained vapor bubble system~ James Barrett, Vladimir Ajaev In the constrained vapor bubble system, designed by the group of J. Plawsky, liquid-vapor mixture inside an elongated cuvette of rectangular cross-section is subject to axial temperature gradients resulting in phase change and complex flow patterns.~Experimental studies of this system have been conducted on the ground and onboard the International Space Station. A number of unexpected phenomena, most notably flooding of the hot end of the cuvette, were discovered in the microgravity experiment. In the present study, we focus on mathematical models designed to~explain some of these phenomena via a study of corner flows driven by both Marangoni and an opposing capillary flow due to changes in both temperature and curvature of the menisci respectively along the wedge. Also, phase change takes place at the liquid vapor interface and, depending on the location, can be either evaporation or condensation. A model is presented for a steady state corner wedge flow and compared with experimental observations. An imposed temperature gradient gives rise to Marangoni flow in the direction of decreasing temperature. The change in total flow Q along the wedge is used to determine the mass flux~from the liquid meniscus into the vapor due to phase change. [Preview Abstract] |
Sunday, November 24, 2019 4:14PM - 4:27PM |
G02.00003: Exploring the effect of temperature discontinuity at interfaces in transient liquid-vapor phase change processes Anirban Chandra, Zhi Liang, Assad Oberai, Onkar Sahni, Pawel Keblinski Quantitative prediction of the rate of phase change in micro/meso-scale systems requires proper boundary conditions at the interface between phases. It was recently shown that the temperature jump at phase interfaces does not alter the mass fluxes significantly in a quasi-steady state condition; however, a dissimilar temperature profile was observed with the assumption of temperature continuity/discontinuity. In this study, we explore the effect of temperature discontinuities at liquid-vapor interfaces before the system reaches a quasi-steady state. We use a locally discontinuous finite element method to solve the 3D compressible Navier-Stokes equations while correctly accounting for jump-conditions across phase interfaces using explicit interface tracking and discontinuous interpolation only at the interface. Expressions for temperature jumps and phase change rates are obtained from theoretical considerations and augmented by MD simulations. [Preview Abstract] |
Sunday, November 24, 2019 4:27PM - 4:40PM |
G02.00004: Liquid-Vapor Interface Configuration for Capillary Flow in a Heat Pipe Muhammad Rizwanur Rahman, Prashant Waghmare, Morris R. Flynn The shape of the liquid-vapor interface within a heat pipe wick is responsive to the wetting characteristics of the working fluid, wick geometry and other operational variables. This study critically investigates the underlying physics that dictate the interface shape and the manner in which this shape changes owing to a change of operating conditions. Far from being of academic interest, these details are essential to the effective functioning of the heat pipe, the interface shape dictating the degree of capillary pumping that occurs. In formulating our mathematical model, we exploit ideas otherwise applied to plastron respiration by aquatic insects. We thereby reject the popular simplification that the three-phase contact point is somehow anchored at a fixed elevation within the wick. Although multiple solutions are predicted, corresponding to different interface shapes, only one is mechanically stable and therefore physically-acceptable. When coupled with a complementary thermodynamic model, the unique interface shape can be predicted for prescribed operating conditions. Such information offers insights on the recess depth for entrainment minimization as well as fill ratio and optimized geometric dimensions for maximizing axial heat flow. [Preview Abstract] |
Sunday, November 24, 2019 4:40PM - 4:53PM |
G02.00005: Core Annular Flow Theory as Applied to the Adiabatic Section of Heat Pipes Aishwarya Rath, Morris R. Flynn Core annular flow theory is used to model the parallel flow of fluids of different phases and has been applied to industrial applications from bitumen hydrotransport to sub-aqueous drag reduction. Here we consider the extension of core annular flow theory to the study of the adiabatic section of heat pipes. Our aim is to develop a first-principles estimate of the conditions necessary to maximize the (counter) flow of liquid and vapor and, by extension, the axial flow of heat. Both planar and axisymmetric geometries are examined. Moreover, we consider heat pipes either containing or devoid of a wick. In these two respective cases, the peripheral return flow of liquid is driven by capillarity and by gravity. Our model is used to predict velocity profiles and the appropriate pressure gradient ratio (vapor-to-liquid). We further obtain estimates for the optimum thickness of the liquid layer. Note finally that when the liquid flow occurs via capillary pumping, there is a minimum surface tension below which the wick cannot supply a sufficient flow of liquid. We characterize this critical point in terms of e.g. the viscosity ratio, the density ratio and the wick depth, porosity and permeability. [Preview Abstract] |
Sunday, November 24, 2019 4:53PM - 5:06PM |
G02.00006: A non-Reynolds lubrication model and application to droplet levitation Shintaro Takeuchi, Jingchen Gu, Amaury Barral, Yoshiyuki Tagawa An attempt for extending the applicability of the Reynolds lubrication theory is presented. By considering a larger surface-to-surface distance than that for the Reynolds lubrication theory, the effect of the non-negligible pressure gradient in the surface-normal direction is incorporated into the lubrication model. The analysis shows that the local pressure is separated into (i) a base component satisfying the Reynolds lubrication theory and (ii) an adjusting component varying in the surface-normal direction, and the second component is found to be related with the velocity of the local Couette-Poiseuille flow. The lubrication model is verified in a non-Reynolds regime of a flow between a moving curved object and stationary object, and good agreement in the pressure distributions by analytical and numerical methods is observed in both wall-normal and longitudinal directions. The lubrication model is also applied to a droplet levitation problem over a moving wall (Sawaguchi 2019) involving a $\mu$m-thin air film, and the levitation lift reproduced from the full drop profile is found to provide a more accurate and complete view of the levitation problem. [Preview Abstract] |
Sunday, November 24, 2019 5:06PM - 5:19PM |
G02.00007: Population Balance Modelling and CFD analysis in a Multi-scale Approach for Flash Nano-precipitation Alessio Lavino, Paola Carbone, Daniele Marchisio, Omar Matar Polymer nanoparticles (NP) formation is investigated by a multi-scale modelling approach focusing on poly-$\varepsilon$-caprolactone (PCL) self-assembly in acetone-water mixtures via flash nano-precipitation (FNP). The control of the final NP size at the outlet of the mixer and the evolution of the particle size distribution, cluster mass distribution (CMD), are guaranteed by a suitable population balance model (PBM) closed with the quadrature method of moments (QMOM). The CMD moments are transported and coupled with computational fluid dynamics (CFD). NP size is characterised in terms of mean radius of gyration, expressed through Flory theory. Molecules are treated as “building blocks” undergoing aggregation once the solubility limit is exceeded. The rate at which two NP collide and aggregate is expressed by the aggregation kernels, built upon molecular dynamics simulations. Turbulent fluctuations on NP formation are taken into account thanks to the direct quadrature method of moments coupled with the interaction-and-exchange-with-the-mean (DQMOM-IEM) method. Favre-averaged continuity and Navier-Stokes equations are implemented, in order to consider the density fluctuations of the system. The model is validated against experiments, showing an excellent agreement. [Preview Abstract] |
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