Porous metal foam holds substantial promise as a material for bipolar plates in high-performance proton exchange membrane fuel cells (PEMFC) due to its exceptional properties in reactant ...redistribution. However, the intricate structure of metal foam flow field (MFF) presents challenges for mass transfer and water management. In this study, a three-dimensional two-phase flow model for metal foam is developed to analyze and capture flow patterns. MFFs are reconstructed based on detailed structural analysis and various pore sizes are discussed. Employing the Volume of Fluid (VOF) method, we delve into multiphase flow dynamics, specifically focusing on the removal of liquid droplets within the MFF. It reveals the MFF with moderate pore sizes exhibit a trade-off performance, striking a balance between optimal mass transfer and minimal parasitic loss. The porous structure's pivotal role is highlighted, contributing significantly to both through-plane and in-plane mass transfer and convection. Furthermore, we classify three flow patterns of liquid droplet, with the “split-up” pattern emerging as the most effective for water management and mass transfer. This study illuminates water behavior in porous metal foam bipolar plates, introduces a systematic methodology for assessing mass transfer and water management capabilities in MFFs for PEMFC.
•CFD model for gas-liquid flows with different interface length scales is presented.•The model is based on VOF for large interfaces and Lagrangian approach for dispersed bubbles.•Slug flow is ...studied, incorporating the effect of dispersed bubbles on the flow around Taylor bubbles.•New insights are presented about influence of dispersed bubbles on rising velocity of Taylor bubbles.
This work presents a multiscale three-dimensional CFD model for the simulation of two-phase gas-liquid flows with different interface length scales, with focus on slug flow pattern. The model is based on the coupling of the Volume-of-Fluid (VOF) method, used to model the large-scale interface dynamics, and the Discrete Bubble Model (DBM) for modeling the small-scale bubbles, which allows to simulate two-phase flows with different interface length scales. A validation study is conducted independently for the VOF and DBM methods, by comparing the numerical results with experimental data from the literature, showing a very good agreement. A model for the collision between the VOF interface and Lagrangian bubbles is proposed and also independently validated. The coupled VOF-DBM model is used to study a liquid-gas two-phase flow with different interface length scales, where large Taylor bubbles and small dispersed bubbles are present. The results demonstrate that the presence of the small dispersed bubbles alters the flow structure around the Taylor bubble, increases the terminal velocity of the Taylor bubble and affects the flow structure in the wake region, which is directly related to heat and mass transfer rates in slug flow.
•Compressible multiphase model based on a geometrical PLIC-VOF based simulation method.•Fully conservative hyperbolic system is solved based on a Riemann solver.•Reconstruction of the Riemann solver ...for oscillation-free behavior near contact discontinuity.•Dynamic behavior of bubble collapses, high-speed jets, and pressure loads.•3D bubble collapse near a free surface and an oblique wall.
The dynamic behavior of bubble collapses, water jets, and pressure loads during the collapse of the bubble near walls and a free surface were numerically investigated via a geometrical volume of fluid (VOF)-based simulation method. The numerical method is based on the compressible Navier–Stokes equations in a conservative form that describe the flow of compressible viscous fluids. The equations are discretized on a general curvilinear grid using an associated Godunov-type numerical scheme, and a reconstruction of a computational finite-volume Riemann solver is introduced for suppression of oscillation near the interface between fluids. The interface was tracked using the VOF reconstruction method. The VOF method is based on a geometrical tool and Lagrangian propagation of the interface reconstructed by a piecewise linear interface calculation (PLIC), resulting in strictly mass-conserving and sharp interface solutions. The numerical procedure was validated for capturing sharp interface and strong shock waves. For the simulation of bubble collapse, grid dependence studies of a spark-generated bubble were studies of both spherical and non-spherical bubbles were conducted. The results showed good agreement between the simulation and experiment of the bubble dynamics during the collapse process. Subsequently, an investigation of a single bubble near a wall with different standoff distances was performed. The pressure loads induced by the jets impacting the walls were calculated and analyzed. Furthermore, a more complex case of bubble collapse near an oblique wall and free surface was simulated. The resulting bubble dynamics with the jets and free surface shape were compared via photographs of the experiments.
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Pinch-off of a compound jet in 3D glass capillary microfluidic device, which combines co-flowing and countercurrent flow focusing geometries, was investigated using an incompressible ...three-phase axisymmetric Volume of Fluid–Continuum Surface Force (VOF–CSF) numerical model. The model showed good agreement with the experimental drop generation and was capable of predicting formation of core/shell droplets in dripping, narrowing jetting and widening jetting regimes. In dripping and widening jetting regimes, the presence of a vortex flow around the upstream end of the necking thread facilitates the jet break-up. No vortex flow was observed in narrowing jetting regime and pinch-off occurred due to higher velocity at the downstream end of the coaxial thread compared to that at the upstream end. In all regimes, the inner jet ruptured before the outer jet, preventing a leakage of the inner drop into the outer fluid. The necking region moves at the maximum speed in the narrowing jetting regime, due to the highest level of shear at the outer surface of the thread. However, in widening jetting regime, the neck travels the longest distance downstream before it breaks.
•The micro droplet impact on a structured superhydrophobic surface is simulated with VOF model.•Characteristics of both Cassie and Wenzel impact regimes are discussed.•The Laplace pressure of the ...droplet promotes the impalement transition in the Cassie impact regime.•The surface adhesion of the Wenzel impact regime decreases with the intrinsic contact angle.•The height of the micro pillar also significantly influences the impact result.
In this work, the volume-of-fluid (VOF) model is applied to numerically study the micro droplet impact on the structured superhydrophobic surface with a large range of impact velocity. The droplet impact processes of both Cassie and Wenzel regimes are obtained and discussed. The influences of the intrinsic contact angle and pillar height on the impact are also studied in the simulation. The simulation results show that, due to the small diameter of the micro droplet, the effect of the Laplace pressure on the droplet impinging cannot be neglected, which could facilitate the impalement transition in the Cassie impact regime. The surface with a larger intrinsic contact angle can not only reduce the penetrate depth of the Cassie impact, but also significantly reduce the adhesion force of the surface in the Wenzel impact regime. When the intrinsic contact angle is large, the droplet can rebound even in the Wenzel impact regime. The height of the pillar also influences the bouncing ability of the droplet in the Wenzel regime. A shorter pillar is beneficial for droplet bouncing in the Wenzel impact regime while it is bad for maintaining the Cassie regime. These results can be used as a reference in understanding the mechanism of high-speed supercool droplet impact on anti-icing superhydrophobic surface and designing highly efficient anti-icing surfaces.
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Super-hydrophobicity is one of the significant natural phenomena, which has inspired researchers to fabricate artificial smart materials using advanced manufacturing techniques. In ...this study, a super-hydrophobic aluminum surface was prepared by nanosecond laser texturing and FAS modification in sequence. The surface wettability turned from original hydrophilicity to super-hydrophilicity immediately after laser treatment. Then it changed to super-hydrophobicity showing a WCA of 157.6 ± 1.2° with a SA of 1.7 ± 0.7° when the laser-induced rough surface being coated with a layer of FAS molecules. The transforming mechanism was further explored from physical and chemical aspects based on the analyses of surface morphology and surface chemistry. Besides, the motion process of droplet impacting super-hydrophobic surface was systematically analyzed via the optimization of simulation calculation grid and the simulation method of volume of fluid (VOF). Based on this simulation method, the morphological changes, the inside pressure distribution and velocity of the droplet were further investigated. And the motion mechanism of the droplet on super-hydrophobic surface was clearly revealed in this paper. The simulation results and the images captured by high-speed camera were highly consistent, which indicated that the computational fluid dynamics (CFD) is an effective method to predict the droplet motion on super- hydrophobic surfaces. This paper can provide an explicit guidance for the selection of suitable methods for functional surfaces with different requirements in the industry.
Knowing hydraulic parameters is very important in the design of spillways, which are the safety structures of dams. While obtaining these parameters, traditionally theoretical and empirical ...approaches are used and scale model experiments are performed. Today, with the increase in computer processing capacity, numerical modeling techniques are used as an alternative to long and costly experimental studies. In this study, the pressure parameter changing along the longitudinal section in the chute spillway was examined using computational fluid dynamics (CFD). In the 3D CFD model of the spillway, the VOF method and the standard k-e turbulence model, which can solve two-phase flows, were used for the design discharge. The obtained pressure parameter results were examined and interpreted along the spillway section. According to the pressure results obtained as a result of numerical analysis, points along the spillway where cavitation may occur have been determined.
•Regular dripping, irregular dripping, synchronized jetting and unsynchronized jetting models were classified.•Velocity fields, pressure distribution and sizes of double emulsions were investigated ...numerically.•Deformation behaviors of core and shell droplets were explored.
Double emulsions are attracting much attention from academic community and industry fields. Emerging microfluidics technology offered new ways to prepare unprecedentedly controllable double emulsions. Here a numerical simulation method, volume of fluid (VOF) technique, was adopted to study the generation and evolution of double emulsions in a circular microchannel, which is helpful for understanding the underlying physics of double emulsion preparation and optimizing the operation conditions. The flow pattern regions where double emulsions existed were found and classified according to dripping and jetting models of double emulsion generation. Velocity fields inside and outside double emulsions were investigated, and several distinct velocity vortexes were observed. Several characteristic parameters involving pressure distribution of fluids and deformation index of double emulsions were studied. At last, preparation of double emulsions with desired configurations, including same inner droplets engulfed by different outer droplets and same outer droplets encapsulating different inner droplets, were explored in depth.
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