In this study, the gas–liquid flow through an interdigitated anode flow field of a PEM water electrolysis cell (PEMEC) is analysed using a three-dimensional, transient, computational fluid dynamics ...(CFD) model. To account for two-phase flow, the volume of fluid (VOF) method in ANSYS Fluent 17.2 is used. The modelled geometry consists of the anode channels and the anode transport layer (ATL). To reduce the complexity of the phenomena governing PEMEC operation, the dependence upon electro-chemistry is disregarded. Instead, a fixed source of the gas is applied at the interface between the ATL and the catalyst layer. An important phenomenon that the model is able to capture is the gas–liquid contact angle on both the channel wall and ATL-channel interface. Particularly, the latter interface is crucial in capturing bubble entrainment into the channel. To validate the numerical simulation, photos taken of the gas–liquid flow in a transparent micro-channel, are qualitative compared against the simulation results. The experimental observations confirm the models prediction of long Taylor bubbles with small bubbles in between. From the simulation results, further intriguing details of the flow are revealed. From the bottom to the top of the outgoing channel, the film thickness gradually increases from zero to 200 μm. This increase in the film thickness is due to the particular superficial velocity field that develops in an interdigitated flow. Here both the superficial velocities change along the length of the channel. The model is capable of revealing effect of different bubble shapes/lengths in the outgoing channel. Shape and the sequence of the bubbles affect the water flow distribution in the ATL. The model presented in this work is the first step in the development of a comprehensive CFD model that comprises multiphase flow in porous media and micro-channel, electro-chemistry in catalyst layers, ion transport in membrane, hydrogen evolution, etc. The model can aid in the study of gas–liquid flow and its impact on the performance of a PEMEC.
•VOF method is used for detailed CFD modelling of gas–liquid flow in micro-channels and transport layer was presented.•Gas-liquid contact angles on both micro-channel walls and porous-channel interface were considered in the simulation.•A layer of fine cells used in the micro-channel close to walls to capture water wall film.•Wall water film thickness changes from bottom to the top of the micro-channel.•Long Taylor bubbles with small bubbles in between were seen in both simulation and experiment.
•Head-on ternary droplet collision is investigated numerically.•The shape evolution and energy change of head-on ternary droplet collision are described.•The axial and radial geometrical ...characteristic of head-on ternary droplet collision are analyzed.•The effect of the Weber number, central droplet size and distance offset on the head-on ternary droplet collision are investigated.
In recent years, ternary droplet collision has been proposed in some new application prospects, such as three-dimensional (3D) reactive inkjet printer. To better apply ternary droplet collision, head-on ternary droplet collision of same liquid in the gaseous environment is investigated numerically. The investigation is based on the finite volume numerical solution of the Navier–Stokes equations in axisymmetric form. Volume of Fluid (VOF) methodology and adaptive grid technique are used to capture the liquid-gas interface and the reliability of the methodology is verified by published experimental data. Head-on ternary droplet collision for a wide range of Weber number is studied. The shape and velocity field evolution of three colliding droplets (coalescence and separation) is described in detail. Moreover, the time evolution of the kinetic energy and the surface energy of three colliding droplets, the viscous dissipation and the maximum deformation are evaluated. Furthermore, based on the dimensionless flow parameters, relevant correlations determining the length of ligament, the maximum diameter of ligament, the length of bridge and the maximum radial length are also presented quantitatively. Finally, the effect of central droplet size and distance offset on the head-on ternary droplet collision are investigated, the results are also compared and analyzed. It is believed that the present work is meaningful for better applying ternary droplet collision.
•Developing the incompressible two-phase solver for modeling the natural convection by Boussinesq approximation.•Investigating the effect of contact angle (ϕ), wall slope (θ), and the Bond number ...(Bo).•Examine the bubble regimes near the inclined wall’s effect.•Studying the role of contact area by comparing the single and pair bubble cases with the same bubble volume.•Providing a correlation for the average Nusselt number based on the Bo and θ.
In this numerical study, the bubble injection effect on the heat transfer rate is investigated next to an inclined heated wall. The volume of fluid (VoF) method solver in the OpenFOAM package is extended with an energy equation and Boussinesq approximation to consider the natural convection flow. The solver is validated with several single and multiphase benchmarks, then the effect of various parameters such as wall slope angle (θ), contact angle (ϕ), bubble pair, Bond number (Bo), and bubble regimes are studied.
The results show the maximum Nusselt number occurs at θ=60° where a bubble has a higher velocity than θ=30°, 45°, 90° and the average Nusselt number boost up 26%. While the Nusselt number is elevated in the higher Bond number (larger bubble), the studies show for the identical gas volume, the bubble pair increases the heat transfer rate more than one single large bubble. The heat transfer reduces about 9% when the contact angle increases from 0° to 120°. The intermittent contact regime has better heat transfer performance than the sliding contact regime and non-contact regime. The Nusselt number enhanced by more than 92% in this regime. The correlation for the Nusselt number based on the present study revealed that the two main influential parameters are wall slope and Bond number.
An in-house Navier–Stokes multiphase VOF (Volume-Of-Fluid) solver is used to study sloshing phenomena in a partially filled rectangular tank subjected to vertical sinusoidal excitation, with the main ...goal of characterizing the effects of vertical acceleration and excitation frequency on the flow dynamics. The solver is validated through comparison with analytical solutions and with available experiments of vertical sloshing. We find that the computed forces acting on the tank walls are well predicted, both in two- and three-dimensional numerical simulations. The flow dynamics is found to be significantly affected by the forcing parameters, and to exhibit more chaotic and three-dimensional nature in cases with strong acceleration and low forcing frequency. As a consequence, certain properties such as the energy dissipation and the mixing efficiency of the system are poorly predicted by two-dimensional simulations in that range of parameters, necessitating the use of more expensive three-dimensional simulations. The time history of the sloshing force and instantaneous flow visualizations are used to analyze the effects of liquid impacting on the walls on energy exchanges between the fluid and the tank. Finally, the evolution of mixing efficiency and its influence on the energy losses are discussed.
•A multiphase Navier–Stokes solver is used to predict vertical sloshing.•Excellent agreement of computed forces with recent experiments.•The flow dynamics is strongly affected from tank acceleration and forcing frequency.•Initial filling level of liquid and mixing efficiency have an impact on energy losses.
•Film drainage and bubble coalescence is investigated with high-resolution numerical methods.•Effect of particles and their properties on thin liquid film drainage and rupture is studied in ...detail.•The influence of particle concentration on the coalescence time in co-axial cases is multmodal.•Particles may block coalescence, change drainage axisymmetry and affect film instability.•Wake entrainment is a key factor influencing coalescence in bubble columns.
Bubble coalescence and breakup is still a complex challenging topic. How far it is understood affects directly the analysis and design optimization of multiphase reactors. Despite years of active research, bubble coalescence in three-phase systems is far from being understood. Contradictory results on the effect of particles are often reported. Although it still lacks a unique explanation, a general conjecture is that the presence of solid particles affects the film drainage process, and hence the bubble coalescence time and behaviour. This paper presents insights into bubble-pair coalescence in slurry by coupling the multiphase particle in cell (MP-PIC) method with the volume of fluid (VOF) method. The mesh resolution for VOF fields is down to micrometers, which allows for analysis of the film drainage and rupture mechanism in detail. The accuracy of MP-PIC fields during the refinement of CFD grids is guaranteed by a chimera approach (Caliskan and Miskovic, Chemical Engineering Journal Advances 5 (2021) 100054), which allows two overlapping meshes in the Lagrangian–Eulerian framework, namely, a fine mesh for the CFD fields and a coarser mesh for the MP-PIC ones.
The rigid motions of powders widely exist in powder-based laser additive manufacturing, like powder deposition, entrainment and spattering. These phenomena simultaneously happen as the laser melts ...these powders, and can have significant impacts on the quality of the printed products. However, existing models can hardly efficiently reproduce the co-existence of melting and rigid motion which greatly influences the molten pool and molten track evolution. The bottleneck therein lies in the demand that unresolved Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) has on the CFD grid size, that is, larger than 3 times of the particle diameter. However, this large size ratio (CFD grid cell vs particle diameter) handicaps the description of the particle deformation during melting. Hence in this paper, a kernel approximation-based semi-resolved CFD-DEM numerical model that allows the refinement of CFD grid is employed, and then coupled with the Volume of Fluid (VOF) method to reproduce the molten metal-gas interface. On this basis, the rigid motions of the powders, like entrainment and spattering, are finally simultaneously realized as the molten pool evolves at high efficiency. Powders remained in the unclosed molten pool trail defect are well explained with the proposed model as well. This model can also be applied to simulate the particle-fluid interactions in Directed Energy Deposition. It is believed that this semi-resolved VOF-DEM Additive Manufacturing model will serve as a good tool for future investigations into the particle behaviors in powder-based additive manufacturing.
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•Semi-resolved VOF-DEM Additive Manufacturing model is developed.•Particle rigid motion is simultaneously modeled with molten pool evolution.•Spattered powders left in the defect are well explained.•Modeling of complex particulate flows in Directed Laser Deposition is realized.
•Holdup and interfacial area were numerically evaluated in a structured packing.•Visualisation of flow morphology in a packing periodic element.•An effective contact angle for the investigated ...packing was determined.•Interfacial area and holdup are lower than the values from literature.
Systematic development of column internals with high separation efficiency can be realised only if the flow pattern within the internals is captured. In this work, numerical simulations of the gas-liquid flow in a Montz B1-250 packing, a typical representative of commercially available corrugated sheet structured packings, were carried out to investigate the wetting behaviour. The complexity of the studied geometry was increased step by step, starting with a single inclined plate, over a single packing layer, and finally, covering two adjacent packing layers in contact. For each complexity level, the holdup and interfacial area were evaluated. For all studied geometries, the static contact angle and the liquid load were varied to investigate their influence on the wetting. It was found that the contact points had a significant influence on the flow distribution. They act as mixing and redistribution points for the liquid flow and induce local accumulation of the liquid phase, resulting in an increased holdup around them. The simulated values of the interfacial area and holdup are lower than the values found in the literature.
•Simulation of pool boiling heat transfer on the microchannels with mixed wettability is carried out.•Bubble dynamics on the microchannels with mixed wettability is investigated.•Integrated effect of ...microchannel and mixed wettability on HTC of pool boiling is investigated.•Significant enhancement in pool boiling heat transfer is found on the hydrophilic microchannel with hydrophobic bottom.
Comparing with the abundant experimental investigations on the pool boiling heat transfer, this study aims to explore the integrated effect of engineered microchannels and the mixed wettability on the enhancement in pool boiling heat transfer in a wide range of geometric and physical conditions. A two-dimensional transient volume of fluid (VOF) model was adopted to simulate the bubble behavior and thermal performance on different surfaces including the hydrophilic plain, hydrophilic microchannel, hydrophilic microchannel with hydrophobic top, and hydrophilic microchannel with hydrophobic bottom, respectively. The vapor volume fraction, velocity vector field, temperature of solid/liquid interface, and average heat transfer coefficients (HTCs) on different geometric surfaces were obtained based on the numerical model. It was found that hydrophobic areas are necessary to be added in the hydrophilic channels to facilitate the bubble nucleation and detachment due to the merit of the mixed wettability. However, it is crucial to optimize the width and location of the hydrophobic areas and the height of the hydrophilic microchannel for the prevention of early formation of vapor blanket and promotion of liquid replenishment. With the hydrophobic area on the top of the microchannel, the maximum solid/liquid interface temperature was 392.8 K, which decreased by 0.91 % comparing with that on the hydrophilic microchannel. While with a hydrophobic bottom (W1 = 2 and 4 μm), the max average HTCs were 508.2 kW/(m2K) and 503.5 kW/(m2K), which were 16.4 % and 11.4 % higher than that in hydrophilic microchannel (W1 = 2 and 4 μm), respectively. The simulation results indicate that the microchannel with optimized mixing of the wettability can efficiently promote the bubble to move upward and liquid to rewet along with the hydrophilic wall, forming the separation of vapor–liquid pathways to further enhance pool boiling heat transfer.
•Drop breakup for various We and Re numbers at isothermal as well as evaporating conditions.•VOF model coupled with a local evaporation model and adaptive grid refinement.•Quantification of the ...effect of heating and evaporation on droplet breakup.•Breakup is affected by heating mainly at low We numbers.•An enhanced 0-D model is proposed to predict droplet heating and evaporation of deformed droplets.
The Navier–Stokes equations, energy and vapor transport equations coupled with the VOF methodology and a vaporization rate model are numerically solved to predict aerodynamic droplet breakup in a high temperature gas environment. The numerical model accounts for variable properties and uses an adaptive local grid refinement technique on the gas–liquid interface to enhance the accuracy of the computations. The parameters examined include Weber (We) numbers in the range 15–90 and gas phase temperatures in the range 400–1000K for a volatile n-heptane droplet. Initially isothermal flow conditions are examined in order to assess the effect of Weber (We) and Reynolds (Re) number. The latter was altered by varying the gas phase properties in the aforementioned temperature range. It is verified that the We number is the controlling parameter, while the Re number affects the droplet breakup at low We number conditions. The inclusion of droplet heating and evaporation mechanisms has revealed that heating effects have generally a small impact on the phenomenon due to its short duration except for low We number cases. Droplet deformation enhances heat transfer and droplet evaporation. An improved 0-D model is proposed, able to predict the droplet heating and vaporization of highly deformed droplets.
•A temperature gradient can induce surface flow on a falling drop.•Surface flow can act against the buoyancy flow, reducing the settling velocity.•Water drop can get elongated due to the thermally ...driven surface flows.•Surface flow can retard the film drainage beneath the water drop and a solid surface.•Retarded film drainage slows down the wetting dynamics and convective heat transfer.
The influence of the thermally-induced Marangoni stresses on the falling and spreading dynamics of a droplet surrounded by another liquid phase is numerically studied. A water droplet is released in a three-dimensional (3D) domain filled with oil. The droplet descends, comes at an apparent resting position above an oil film close to the bottom surface, and eventually wets the solid surface when the underneath film ruptures. A linear vertical temperature gradient is applied in the solution domain (with the bottom surface as the cold side) which imposes a surface tension gradient across the oil-water interface. The Marangoni source term is added to the momentum equation coupling the momentum and the energy equations. It is assumed that the dynamic contact angle changes according to the Kistler relation during the spreading. The Volume of Fluid (VOF) method is used to capture the interface between the phases. The solver is validated for both the falling and spreading phases. During the buoyancy-driven falling regime, the droplet retains its spherical shape at the low Reynolds number O(1) and finite Bond number O(0.001). It is revealed that the spreading rate of the droplet is a decreasing function of Marangoni number (Ma). Unlike the isothermal systems, where the bottom side of the droplet becomes slightly flattened at the resting stage, the Marangoni stress imposes an upward force on the droplet which elongates the shape of droplet in the temperature gradient direction. Consequently, the ultimate spreading radius at the equilibrium state is smaller at larger Ma. The slow rate of spreading and also the small wetted area at large temperature gradients adversely affect the heat transfer rate from the liquid to the cold plate where the local convective heat transfer coefficient and the average Nusselt number (Nu) are decreasing functions of the Marangoni stress.
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