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In this work, different CFD models to compute flows inside a steam ejector were investigated. The results were compared to the analytical models as well as the experimental results from the literature. All the simulations gave realistic results from the hydrodynamic perspective with a relative error of the entrainment ratio between 25% and 40% compared to reference experimental data. However, an analysis of the temperature profiles showed that only realistic results from the thermodynamic perspective were given by multiphase calculations. The first multiphase model tested was the so-called Wet-Steam model from ANSYS Fluent. This model gave inconsistent results for the steam ejector CFD simulation due to the physical boundaries of this model. The second model tested was the Eulerian mixture model, which gave the most realistic results in terms of the physical conditions of the liquid and vapor phases inside the ejector. It also showed that the phase change could have a significant impact on the value of the critical output pressure as a way to improve the performance of the ejector. [ABSTRACT FROM AUTHOR]

The present work provides a detailed numerical investigation of a turbopump for waterjet applications in cavitating conditions. In particular, the study focuses on the complexities of cavitation modelling, serving as a pivotal reference for future computational research, especially in off-design hydro-jet scenarios, and it aims to extend current model assessments of the existing methods, by disputing their standard formulations. Thus, a computational domain of a single rotor-stator blade passage is solved using steady-state Reynolds-Averaged Navier–Stokes equations, coupled with one-, two-, and four-equation turbulence models, and compared with available measurements, encompassing both nominal and thrust breakdown conditions. Through grid dependency analysis, a medium refinement with the Shear Stress Transport turbulence model is chosen as the optimal configuration, reducing either computational time and relative error in breakdown efficiency to 1%. This arrangement is coupled with a systematic study of the Zwart cavitation model parameters through multipliers ranging from 10 − 2 to 10 2 . Results reveal that properly tuning these values allows for a more accurate reconstruction of the initial phases of cavitation up to breakdown. Notably, increasing the nucleation radius reduces the difference between the estimated head rise and experimental values near breakdown, reducing the maximum error by 4%. This variation constrains vapour concentration, promoting cavitation volume extension in the passage. A similar observation occurs when modifying the condensation coefficient, whereas altering the vaporization coefficient yields opposite effects. [ABSTRACT FROM AUTHOR]

This study reports the development of a numerical method to simulate two-phase flows of Newtonian fluids that are incompressible, immiscible, and isothermal. The interface in the simulation is located and reconstructed using the geometric volume of fluid (VOF) method. The implementation of the piecewise-linear interface calculation (PLIC) scheme of the VOF method is performed to solve the three-dimensional (3D) interface transport during the dynamics of two-phase flows. In this method, the interface is approximated by a line segment in each interfacial cell. The balance of forces at the interface is accounted for using the continuum interfacial force (CSF) model. To solve the Navier–Stokes equations, meshless finite difference schemes from the HiG-Flow computational fluid dynamics software are employed. The 3D PLIC-VOF HiG-Flow algorithm is used to simulate several benchmark two-phase flows for the purpose of validating the numerical implementation. First, the performance of the PLIC implementation is evaluated by conducting two standard advection numerical tests: the 3D shearing flow test and the 3D deforming field test. Good agreement is obtained for the 3D interface shape using both the 3D PLIC-VOF HiG-Flow algorithm and those found in the scientific literature, specifically, the piecewise-constant flux surface calculation, the volume of fluid method implemented in OpenFOAM, and the high-order finite-element software FEEL. In addition, the absolute error of the volume tracking advection calculation obtained by our 3D PLIC-VOF HiG-Flow algorithm is found to be smaller than the one found in the scientific literature for both the 3D shearing and 3D deforming flow tests. The volume fraction conservation absolute errors obtained using our algorithm are 4.48 × 10 − 5 and 9.41 × 10 − 6 for both shearing and deforming flow tests, respectively, being two orders lower than the results presented in the scientific literature at the same level of mesh refinement. Lastly, the 3D bubble rising problem is simulated for different fluid densities ( ρ 1 / ρ 2 = 10 and ρ 1 / ρ 2 = 1000 ) and viscosity ratios ( μ 1 / μ 2 = 10 and μ 1 / μ 2 = 100 ). Again, good agreement is obtained for the 3D interface shape using both the newly implemented algorithm and OpenFOAM, DROPS, and NaSt3D software. The 3D PLIC-VOF HiG-Flow algorithm predicted a stable ellipsoidal droplet shape for ρ 1 / ρ 2 = 10 and μ 1 / μ 2 = 10 , and a stable cap shape for ρ 1 / ρ 2 = 1000 and μ 1 / μ 2 = 100 . The bubble's rise velocity and evolution of the bubble's center of mass are also computed with the 3D PLIC-VOF HiG-Flow algorithm and found to be in agreement with those software. The rise velocity of the droplet for both the ellipsoidal and cap flow regime's is found, in the initial stages of the simulation, to gradually increase from its initial value of zero to a maximum magnitude; then, the steady-state velocity of the droplet decreases, being more accentuated for the cap regime. [ABSTRACT FROM AUTHOR]

The present work provides a detailed numerical investigation of a turbopump for waterjet applications in cavitating conditions. In particular, the study focuses on the complexities of cavitation modelling, serving as a pivotal reference for future computational research, especially in off-design hydro-jet scenarios, and it aims to extend current model assessments of the existing methods, by disputing their standard formulations. Thus, a computational domain of a single rotor-stator blade passage is solved using steady-state Reynolds-Averaged Navier–Stokes equations, coupled with one-, two-, and four-equation turbulence models, and compared with available measurements, encompassing both nominal and thrust breakdown conditions. Through grid dependency analysis, a medium refinement with the Shear Stress Transport turbulence model is chosen as the optimal configuration, reducing either computational time and relative error in breakdown efficiency to 1%. This arrangement is coupled with a systematic study of the Zwart cavitation model parameters through multipliers ranging from 10−2 to 102. Results reveal that properly tuning these values allows for a more accurate reconstruction of the initial phases of cavitation up to breakdown. Notably, increasing the nucleation radius reduces the difference between the estimated head rise and experimental values near breakdown, reducing the maximum error by 4%. This variation constrains vapour concentration, promoting cavitation volume extension in the passage. A similar observation occurs when modifying the condensation coefficient, whereas altering the vaporization coefficient yields opposite effects.

This study delves into applying Distributed Learning Machines (DLM), a subset of Physics-Informed Neural Networks (PINN), in tackling benchmark challenges within two-phase flows. Specifically, two variants of DLM, namely Distributed Physics-Informed Neural Networks (DPINN) and Transfer Physics-Informed Neural Networks (TPINN), are studied. The DLM architecture strategically divides the global domain into distinct non-overlapping sub-domains, with interconnected solutions facilitated by interface conditions embedded in the loss function. Forward and inverse benchmark problems in two-phase flows are explored: (a) bubble in a reversing vortex and (b) Bubble Rising under Buoyancy. The Volume of Fluid (VOF) method handles interface transport in both scenarios, with the inverse problem incorporating interface-position data during the training phase. The forward problem highlights the effectiveness of DPINN in capturing the interface using a simple transport equation. The distinctive contribution of this work lies in its exploration of the inverse problem, offering insights into the scalability of distributed architectures when dealing with a system of governing equations. Following the validation of an initial PINN model against Computational Fluid Dynamics (CFD) data, the study extends to DPINN and TPINN. A parametric study optimizes network hyperparameters, emphasizing the regularization of loss terms within the DPINN loss function. A self-adaptive weighting strategy based on a Gaussian probabilistic model dynamically adjusts loss weights during training to overcome challenges associated with manual parameter tuning. The evaluation of accuracy against CFD data and published results underscore the efficacy of DLMs in addressing two-phase flow problems. Additionally, the computational efficiency of distributed networks is explored compared to traditional PINNs. [Display omitted] • Introduced Distributed Learning Machines for two-phase flow problems. • Demonstrated DPINN, TPINN effectiveness in complex flow modeling. • Highlighted computational efficiency of distributed neural networks. • Investigated self-adaptive strategy for network loss function balance. [ABSTRACT FROM AUTHOR]

[Display omitted] • The VOF simulations of air entrainment phenomena in a water column are conducted. • Jet velocity and column dimensions affect the air entrainment. • Three main patterns are identified: dispersed bubbles, medium air pockets and large air pockets. • Low jet Reynolds and Froude numbers ensure the fluid does not overflow. This study focuses on computational fluid dynamics (CFD) modelling of the air entrainment phenomena in water columns which are commonly used in processing plants. The CFD model is first validated against experimental data. Then, a comprehensive set of CFD simulations are conducted to understand the effect of inlet jet velocity, nozzle diameter, tank diameter, and tank height on the bubble formation. Three main patterns are identified: dispersed bubbles, medium air pockets (MAP) and large air pockets (LAP). For the dispersed bubble case, the initially formed air pockets break into smaller bubbles and hence the water level rises linearly. For the MAP case, the size of the air pockets remains larger than for the dispersed bubble case, though, with time they burst at the surface. Finally, for the LAP case, the inkling jet interacts violently with the wall and hence large air pockets are trapped within the column, causing the overflow of the fluid. The flows are further analysed using non-dimensionless Reynolds and Froude numbers, providing an acceptable range of operating parameters that ensure the fluid does not overflow. [ABSTRACT FROM AUTHOR]

The authors performed 3D numerical simulation of the unsteady outflow of a two-phase liquid-vapor flow from a tube with spherical monodisperse filling. The numerical model was developed by the smoothed particle method, and its verification was carried out using experimental data on the motion of a bubble flow in a tube. The minimum size of smoothed particles and their required number to build a correct computational model have been determined. Structural and hydraulic characteristics of the flow in the presence of phase transitions have been obtained. The calculated data on the three-dimensional spatial distribution of the vapor phase and pressure losses both over the cross section of the unit cell formed by the dispersed backfill and along the entire length of the channel are given. The analysis of the dynamics of changes in pressure losses along the length of the tube has been carried out; two characteristic sections with different dynamics of the growth of pressure losses have been found. Data were obtained on the distribution of the volumetric steam content in the tube at its various positions: horizontal, vertical, and with an inclination of 45°. The influence of the angle of inclination of the tube on the character of the distribution of steam over its cross section is shown. In this case, the greatest difference in the structure is observed between the vertical and horizontal positions. It has also been demonstrated that its structure tends to be homogeneous as the flow moves. The calculation model used made it possible to determine pressure loss in the working section from the inlet of the vapor-liquid flow into the tube to the outlet under conditions corresponding to the onset of the critical flow regime. Comparison of the results of numerical calculations, including the critical mass flow rates at the outflow of a vapor-liquid flow from the tube, with the available experimental data and previously obtained results of analytical studies, noted their good match. [ABSTRACT FROM AUTHOR]

In this work, the numerical code DualPhysics, based on the Lagrangian particle and mesh free method Smoothed Particle Hydrodynamics, has been employed to solve the slightly compressible isothermal two-phase water-oil flow. The continuity and momentum equations are solved, and we used the modified Tait equation of state to determine the pressure. To validate the numerical code, we solved the modified Couette flow of two fluids. As a practical case, we solved the isothermal and two-dimensional two-phase water-oil flow. The mixing of the fluids occurs after passing through a 45 degree Y junction placed at the entrance of the horizontal pipeline. Results showed the potential for using the numerical code, although some modifications and alterations are still necessary to solve practical problems.

• Physics-Informed neural networks (PINNs) are used for single and two-phase flows. • The study investigates the rising bubble problem with and without heat transfer. • The pertinent physics of wake interaction and thermal boundary layer are discussed. • Physics-Informed neural networks are agnostic to geometry and fluid properties. • The maximum error in the position of the centre of mass by PINN is less than 2.8 %. This paper presents data-driven simulations of two-phase fluid processes with heat transfer. A Physics-Informed Neural Network (PINN) was applied to capture the behaviour of phase interfaces in two-phase flows and model the hydrodynamics and heat transfer of flow configurations representative of established numerical test cases. The developed PINN approach was trained on simulation data derived from physically based Computational Fluid Dynamics (CFD) simulations with interface capturing. The present study considers fundamental problems, including tracking the rise of a single gas bubble in a denser fluid and exploring the heat transfer in the wake of a bubble rising close to a heated wall. Tracking of a rising bubble phase interface of fluids with disparate properties was performed, revealing a maximum error of only 5.2% at the interface edge and a maximum error of 2.8% at the position of the centre of mass. Inferred (hidden variable) flows are studied in addition to a purely extrapolative inverse isothermal bubble case. When no velocity data was supplied, velocity field predictions remained accurate. Rise of an inferred isothermal bubble with unseen fluid properties was found to produce a maximum mean-squared error of 0.28 and centre of mass error of 1.25%. For the case of the rising bubble with a hot wall, the maximum error in the temperature domain using specified boundary conditions was 6.8%, while the bubble position analysis reveals a maximum positional error of 3.6%. These results demonstrate that PINN is agnostic to geometry and fluid properties when studying the combined effects of convection and buoyancy on two-phase flows for the first time. This work serves as a starting point for PINN in multiphase cases involving heat transfer over a range of geometries. Eventually, PINN will be used in such cases to provide solutions for forward, inverse, and extrapolative cases. Each of which represent a dramatic saving in computational cost compared to traditional CFD. Contours of volume fraction (α) and temperature (T*) predicted by the CFD (Top) and PINN (Bottom) for the evolution of flow showing how the rising bubble influences the thermal boundary layer on the wall. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • A computational fluid dynamics (CFD) model for liquid–solid circulating fluidized bed (LSCFB) • Hydrodynamics in LSCFB under different operating conditions. • Radial non-uniformity and axial uniformity in LSCFB. A computational fluid dynamics (CFD) model was employed to investigate the hydrodynamics of the liquid–solid circulating fluidized bed (LSCFB). The numerical simulations of the flow in the LSCFB under different operating conditions, including different superficial liquid velocities, superficial solid velocities, particle densities and shapes, are carried out using the CFD model developed in the previous work. The numerical predictions show correct trends and good agreements with the experimental data. It is demonstrated that the radial non-uniformity and axial uniformity exist in the flow structures under different operating conditions. By increasing the superficial solid velocity, the average solids holdup and radial non-uniformity increase, while the opposite trends are observed by increasing the superficial liquid velocity. Besides, the solids holdup decreases with the decrease in the particle density. It is also observed that all the flow distributions in the radial and axial directions in LSCFBs are more uniform than those in GSCFBs. [ABSTRACT FROM AUTHOR]

This study employs DNS of two-phase flows to enhance primary atomization understanding and modeling to be used in numerical simulation in RANS or LES framework. In particular, the work has been aimed at improving the information on the liquid-gas interface evolution for modeling approaches, such as the Eulerian–Lagrangian Spray Atomization (ELSA) framework. Even though this approach has been already successfully employed to describe the complete liquid atomization process from the primary region to the dilute spray, improvements are still expected on the derivation of the drop size distribution (DSD). The main aim of the present work is the introduction of a new framework to achieve a continuous description of the DSD formation during the atomization process. The attention is here focused on the extraction from DNS data of the behavior of geometrical variable of the liquid-gas interface, such as the mean (H) and Gauss (G) surface curvatures. The use of a Surface Curvature Distribution is also proposed and studied. A Rayleigh–Plateau instability along a column of liquid and a droplet collision case are first of all considered to analyze and to verify the capabilities of the code to correctly predicting the curvature distributions. A statistical analysis based on the curvatures data, in terms of probability density function, is presented in order to determine the physical parameters that control the curvatures on this test case. Then, the same formulation is applied in the analysis of the two phase Homogeneous Isotropic Turbulence (HIT) configuration to study how the curvatures evolve all along the atomization process. Joint PDFs are used to illustrate the topological changes of the interface when increasing the liquid volume fraction. [ABSTRACT FROM AUTHOR]

In this paper, we propose a lattice Boltzmann model for simulation of two-phase flows pertinent to thermal nonideal fluids in two dimensions. This LBM has a modified pseudo-potential so that it recovers a full set of hydrodynamic equations for two-phase flows through the Chapman-Enskog expansion procedure. Numerical measurements of thermal conductivity and of surface tension agree well with theoretical predictions. Simulations of bubble rising and of pool boiling with heat transfer are carried out. They demonstrate that the model is applicable to two-phase flows with thermal effects. Using finite difference Lattice Boltzmann method ensures numerical stability of the scheme.

The process of thermocapillary bubbles rising in a rotating 3D cylinder in zero gravity was analysed and presented numerically with the aid of computational fluid dynamics (CFD) by means of the volume of fluid (VOF) method. Calculations were carried out to investigate in detail the effect of the rotational speed of the hosted liquid on the trajectory of both single and group bubbles driven by the Marangoni force in zero-gravity conditions. For rotational speeds from 0.25 to 2 rad/s, bubble displacement with angular motion was found to be directed between the hotter surface and the rotational axis. This is contrary to the conventional bubble flow from areas of high pressure to low pressure, radial direction, or from cold to hot regions, axial direction. The results demonstrate that for the ratio of rotational speeds to the thermocapillary bubble velocity larger than unity, the surface tension gradient is the dominant force and the bubble motion towards the hotter. On the other hand, for ratio less than 1, the bubble motion is dominated and is significantly affected by centrifugal force. As rotation speed increases, the amount of deflection increases and the Marangoni effect vanishes. The current study is novel in the sense that single- and multi-bubble motion incorporating thermocapillary forces in a rotating liquid in a zero-gravity environment has never been numerically investigated. [ABSTRACT FROM AUTHOR]

A three-dimensional steady-state mathematical model for an oil-in-water dispersed flow was developed to study the effects of lift force on the dispersed phase behaviour in a vertical pipe. The Euler/Lagrange multiphase scheme implemented in CFD code ANSYS Fluent was used to investigate the interaction between the oil droplets (dispersed phase) and water (continuous phase). The flow field of the continuous phase was resolved by solving the Reynoldsaveraged Navier-Stokes conservation equations with the fe-e turbulence model and standard wall function. The motion of oil droplets was obtained from the resolved flow field of the continuous phase as such that a two-way coupling was enforced to account for the influence of oil droplets on the water flow. Simulations were performed to model published experimental observation. The computational domain was a vertical pipe (3.2 m in length and 38 mm in diameter). Inter-phase forces (gravitational, drag, buoyancy and shear-lift) were taken into consideration where the shear-lift force is of our primary focus. Three different mixture velocities (1.5,2.0, 2.5 m/s) were simulated with oil fraction ranging from 0 to 0.5. Uniform droplet diameters of 0.5-6 mm were tested. Here, droplet breakup and coalescence were not modelled. Comparison with experimental results was given in terms of the ratio of pressure gradient. The simulations show that the shear-lift force plays an important role in the overall force balance acting on the droplets and influences the radial distribution of the oil droplets (i.e. droplets tend to concentrate at the core of the pipe rather than near the wall). Oil droplets diameters are found to depend on the mixture velocity for the case under consideration. [ABSTRACT FROM AUTHOR]

The accurate and efficient modelling of two-phase flows is at present mostly limited to structured, unskewed meshes, due to the additional topological and numerical complexity of arbitrary, unstructured meshes. Compressive VOF methods which discretize the interface advection with algebraic differencing schemes are computationally efficient and inherently applicable to arbitrary meshes. However, compressive VOF methods evidently suffer severely from numerical diffusion on meshes with topological skewness. In this paper we present a compressive VOF method using a state-of-the-art donor–acceptor advection scheme which includes novel modifications to substantially reduce numerical diffusion on arbitrary meshes without adding computational complexity. The new methodology accurately captures evolving interfaces on any arbitrary, non-overlapping mesh and conserves mass within the limits of the applied solver tolerance. A thorough validation of the presented methods is conducted, examining the pure advection of the interface indicator function as well as the application to evolving interfaces with surface tension. Crucially, the results on equidistant Cartesian and arbitrary tetrahedral meshes are shown to be comparable and accurate. [ABSTRACT FROM AUTHOR]

The aim of this work is to investigate the relationship between evaporation from heated water surfaces and mean aero thermal properties of a forced airflow. Flows with Reynolds numbers varying between 2475 and 49,503 were considered. Both wind tunnel measurements and numerical simulations were used. The experimental results were obtained in an installation consisting of a low speed wind tunnel and an evaporation tank. The numerical study was performed through a 3D-CFD code. To validate the numerical formulation the predicted results were compared with the experimental measurements for the mass transfer at the free surface of the water tank and with correlations available in the literature. A good agreement was achieved which indicates an interesting capacity of the CFD program to predict the phenomena engaged. The dependence of the rate of evaporation with air velocity, water–air temperature difference and relative humidity is also addressed. The results obtained with both methodologies clearly show that the rate of evaporation is mainly dependent of the airflow velocity. The water–air temperature difference and the relative humidity also have an important effect, but much less than the airflow velocity. For small airflow velocities the rate of water evaporation is also small and presents only a slightly dependence from the temperature difference and the relative humidity. [ABSTRACT FROM AUTHOR]

Highlights: [•] A compressible two-phase flow CFD model is developed. [•] We use this model to simulate high-pressure cavitating liquid jets. [•] Ambient air is assumed to be non-condensable and ideally mixed with the vapor. [•] A hybrid HLLC–Rusanov scheme is used to improve numerical robustness. [•] Practical implementation of a phase equilibrium solver is discussed. [ABSTRACT FROM AUTHOR]

Abstract: The present work deals with the numerical investigations of vaned demisters using a computational fluid dynamics tool to validate the presently adopted computational methodology. A vane demister profile identified from literature, on which experimental data is available, is chosen and water particles at the rated flow rate and distribution are injected. Discrete phase modeling approach with Rosin–Rammler size distribution is chosen for specifying the water droplet injection with a combination of break-up and/or coalescence option, along with few turbulence model recommended in the literature are tested for suitability. The walls of the demister are set to capture the water droplets impinging on them and will be removed from the numerical calculations. Various options within the solver are numerically tested to recommend an appropriate combination of solver setting. The results indicate that the Wave model is the best option among several other droplet breakup models available in the solver. It was observed that inclusion of Saffman lift force does not influence the results much. Spalart–Almaras model or Realizable k–ε model can be considered as a suitable turbulence closure for numerical prediction of vane separation efficiency in curved vane demisters. [Copyright &y& Elsevier]

In this paper, we review and compare the level set, phase-field, and immersed boundary methods for incompressible two-phase flows. The models are based on modified Navier-Stokes and interface evolution equations. We present the basic concepts behind these approaches and discuss the advantages and disadvantages of each method. We also present numerical solutions of the three methods and perform characteristic numerical experiments for two-phase fluid flows. [ABSTRACT FROM AUTHOR]

Recently, machine learning has been used to substitute parts of conventional computational fluid dynamics (CFD) solvers, e.g., the cell face reconstruction in the finite-volume method or the curvature computation in the Volume-of-Fluid (VOF) method. The latter showed improvements in terms of accuracy for coarsely resolved interfaces, however at the expense of convergence and symmetry. In this work, a hybrid data-driven approach is proposed, addressing the aforementioned shortcomings. We focus on interface reconstruction (IR) in the level-set method, i.e., the computation of the volume fraction and apertures. In the hybrid data-driven IR, a classification neural network decides based on the local interface resolution whether to use conventional linear IR or neural network IR. The proposed approach improves accuracy for coarsely resolved interfaces and recovers the conventional IR for high resolutions, yielding first order overall convergence. Symmetry is preserved by mirroring and rotating the input level-set grid and subsequently averaging the predictions. The hybrid model is implemented into a CFD solver and demonstrated for two-phase flows. Furthermore, we provide details of floating-point symmetric implementation and computational efficiency. • Hybrid scheme for interface reconstruction in the level-set method. • Neural networks to predict volume fraction and apertures from level-set field. • The scheme preserves symmetry by mirroring and rotating input level-set grid. • The model shows first order overall convergence. • Implementation into CFD solver to demonstrate improvements on two-phase flows. [ABSTRACT FROM AUTHOR]

In this paper, a numerical study has been carried out to model and simulate the air-liquid two-phase flows both inside and around a rotating disk atomizer, which is fitted with multiple spray nozzles. The physical problem was simulated with an Eulerian multiphase model in a single computational domain. The realizable two-equation turbulence model was used for the turbulent flow. The governing equations were solved with a finite-volume-based numerical method. The rotary frame method was used for the spinning disk. Numerical simulation was conducted in a rotational disk speed range of 1000 to 10000 rpm and a liquid feed flow rate range of 100 to 175 gpm for the liquid feeder. Both uniform and non-uniform liquid distribution conditions were considered. Detailed results about flow velocity and volume fraction fields inside and outside of the atomizer are presented and discussed. It was found that when liquid is nonuniformly distributed through the distributors, some of the nozzles could reach flooding conditions at lower rotational disk speeds and liquid feed volume flow rates, as compared to uniform distribution cases. [ABSTRACT FROM AUTHOR]

Abstract: Liquid water transport in gas channels may influence and even prevent gas supply in a proton exchange membrane (PEM) fuel cell, so significantly affect cell operation. A two-phase two-dimensional lattice Boltzmann model is employed in this paper to simulate the development and interaction of two liquid droplets in a gas channel of a PEM fuel cell, focusing mainly on parametric effects, including the gas flow velocity, initial droplet distance, different micropore combinations, and the gas diffusion layer (GDL) surface wetting properties, on liquid droplet transport behaviors. Results confirm that an increased gas stream velocity and liquid pore distance may prevent liquid droplet interaction and enhance liquid water removal. Numerical simulations further indicate that different pore size combinations may promote droplet interaction, particularly with a large pore in front of a small one. On the contrary, a more hydrophobic GDL surface can decrease liquid droplet interaction, benefit liquid water removal, and consequently improve PEM fuel cell water management. [Copyright &y& Elsevier]

Abstract: Side weirs are extensively used in the hydraulic and environmental engineering applications. The modeling of free surface flow over a labyrinth side weir is a sophisticated problem in the hydraulic engineering. The water surface profiles over the triangular labyrinth side weirs were investigated by many of the researchers experimentally and theoretically. In this study, the free surface flow over the triangular labyrinth side weir was modeled by using Volume of Fluids (VOF) method to describe the flow characteristics in subcritical flow conditions. A valid method, Grid Convergence Index (GCI) was used to determine the numerical uncertainty of the simulation results. The simulation results were compared with experimental observations, and good agreements were obtained between the both results. [Copyright &y& Elsevier]

Abstract: Counter current flow limitation CCFL is an important phenomenon for numerous engineering applications and safety of light water reactors. In particular, the possible occurrence of CCFL in the hot-leg of a PWR during SBLOCA or LOCA accidents is of special interest for nuclear safety research. A review of the related literature has made in order to present the most important studies about the phenomenon and to reach common general understanding of the different factors that govern CCFL. Eventually this will allow explaining contradictions among different explanations provided by different authors. Most important factors were geometrical characteristics, liquid superficial velocity, and physical properties. The review shows that despite numerous experimental works, many scaling and geometrical effects are still not fully understood. For Instance there exist no consistent explanation of the channel diameter and inclined riser length effect upon results. The same can be stated-though to a minimum extent – for the inclination angle while channel length (or channel to diameter ratio) effect was clear and consistent. Since most experimental work was done in down-scaled hot-leg simulators, it becomes interesting to build a coherent knowledge about these effects and to explain arising contradictions in order to safely extrapolate results to full-scale hot-leg. The review has shown that many differences were simply due to geometrical effects, this leads to the need to “standardize” experimental data according to geometrical parameters. This should results in a better understanding of the phenomenon and corresponding scaling effects. Additionally, important variables such as: pressure drop, void fraction and shear stress were also investigated and discussed. A compilation of CCFL data was built and analyzed. Since the new simulation trends tend to use CFD codes where geometrical and spatial deviations are excluded by using 3D modeling, emphasis was placed upon introducing correlations for onset of CCFL out of collected data. Existing correlations for interfacial shear stress friction factor and the void fraction as a function of gas superficial velocity were also gathered and briefly discussed. The effect of condensation, physical parameters, and hysteresis upon CCFL was also introduced. [Copyright &y& Elsevier]

We present and discuss the results of a comprehensive study addressing the non-aerated region of the skimming flow in steep stepped spillways. Although flows in stepped spillways are usually characterized by high air concentrations concomitant with high rates of energy dissipation, the non-aerated region becomes important in small dams and/or spillways with high specific discharges. A relatively large physical model of such spillway was used to acquire data on flow velocities and water levels and, then, well-resolved numerical simulations were performed with a commercial code to reproduce those experimental conditions. The numerical runs benefited from the ability of using multi-block grids in a Cartesian coordinate system, from capturing the free surface with the TruVOF method embedded in the code, and from the use of two turbulence models: the $${k{-}\varepsilon}$$ and the RNG $${k{-}\varepsilon}$$ models. Numerical results are in good agreement with the experimental data corresponding to three volumetric flow rates in terms of the time-averaged velocities measured at diverse steps in the spillway, and they are in very satisfactory agreement for water levels along the spillway. In addition, the numerical results provide information on the turbulence statistics of the flow. This work also discusses important aspects of the flow, such as the values of the exponents of the power-law velocity profiles, and the characteristics of the development of the boundary layer in the spillway. [ABSTRACT FROM AUTHOR]

The use of graphics hardware for general purpose computations allows scientists to enormously speed up their numerical codes. We presently investigate the impact of this technology on our computational fluid dynamics solver for the three-dimensional two-phase incompressible Navier-Stokes equations, which is based on the level set technique and applies Chorin’s projection approach. To our knowledge, this is the first time, that a two-phase solver for the Navier-Stokes equations profits from the computation power of modern graphics hardware. As part of our project, a Jacobi preconditioned conjugate gradient solver for the pressure Poisson equation and the reinitialization of the level set function of our CPU based code were ported to the graphics processing unit (GPU). They are implemented in double precision and parallelized by the Message Passing Interface (MPI). We obtain speedups of 16.2 and 8.6 for the Poisson solver and the reinitialization on one GPU in contrast to a single CPU. Our implementation scales close to perfect on multiple GPUs of a distributed memory cluster. This results in excellent speedups of 115.8 and 53.7 on eight GPUs of our cluster. Furthermore our whole multi-GPU accelerated solver achieves an impressive speedup of 69.6 on eight GPUs/CPUs. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a dual-scale model is developed to simulate the radial spreading of liquids in thin fibrous sheets. Using 3-D microscale simulations, the required constitutive equations, capillary pressure and relative permeability, have been determined at each saturation level and used in a macroscale model developed based on the Richards’ equation of two-phase flow in porous media. The dual-scale approach allowed us to include the partially-saturated region of a porous medium in calculations. Simulating different fibrous sheets with identical parameters but different in-plane fiber orientations, it is revealed that the rate of fluid spread increases with increasing the in-plane alignment of the fibers. Our simulations are discussed with respect to existing studies in the literature. [Copyright &y& Elsevier]

A computational fluid dynamics (CFD) model of the two-phase flow within a flow distribution chamber is presented in this paper. Several sets of results have been validated against experimental data obtained from a scale model of the chamber. The interaction between air and water within the chamber, the shape and location of the free surface are modelled by the volume-of-fluid (VOF) model. The k–ℇ and renormalization group (RNG) k–ℇ models are the two turbulence models used in the simulations. It is found that both the two-dimensional and the three-dimensional models using the RNG k–ℇ model give a good qualitative description of the flow features within the chamber. The three-dimensional results also show very good quantitative agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Within the framework of averaged two-phase mixture flow simulations of PWR Steam Generators (SG), this paper provides a geometric version of a pseudo-FMG FAS preconditioning of the balance equations used in the CEA Genepi code. The 3D steady-state flow is reached by a transient computation using a fractional step algorithm and a projection method. Our application is based on the PVM package. The difficulties of applying geometric FAS multigrid methods to the balance equations solver are addressed. The effects on the convergence behaviour of the numerical parameters are investigated. An original parallel red-black pseudo-FMG FAS multigrid algorithm is also presented. The use of dynamic multigrid cycles leads to perceptible improvements in the computation convergences. Numerical tests (academic and industrial simulations) underline a noticeable computation speed-up, essentially for a large number of freedom degrees: the speed-up reached for 2 or 3 grids ranges between 2 and 3. [ABSTRACT FROM AUTHOR]