Multiphase Computational Fluid Dynamics Model Research Articles

Optimization study of a PEM fuel cell performance using 3D multi-phase computational fluid dynamics model

An optimization study using a comprehensive 3D, multi-phase, non-isothermal model of a PEM (proton exchange membrane) fuel cell that incorporates significant physical processes and key parameters affecting fuel cell performance is presented and discussed in detail. The model accounts for both gas and liquid phase in the same computational domain, and thus allows for the implementation of phase change inside the gas diffusion layers. The model includes the transport of gaseous species, liquid water, protons, energy, and water dissolved in the ion-conducting polymer. Water is assumed to be exchanged among three phases: liquid, vapour, and dissolved, with equilibrium among these phases being assumed. This model also takes into account convection and diffusion of different species in the channels as well as in the porous gas diffusion layer, heat transfer in the solids as well as in the gases, and electrochemical reactions. The results showed that the present multi-phase model is capable of identifying important parameters for the wetting behaviour of the gas diffusion layers and can be used to identify conditions that might lead to the onset of pore plugging, which has a detrimental effect on the fuel cell performance. This model is used to study the effects of several operating, design, and material parameters on fuel cell performance. Detailed analyses of the fuel cell performance under various operating conditions have been conducted and examined.

Prediction of hygro—thermal stress distribution in proton exchange membranes using a three-dimensional multi-phase computational fluid dynamics model

A three-dimensional, multi-phase, non-isothermal computational fluid dynamics model of a proton exchange membrane fuel cell has been developed to simulate the hygro and thermal stresses in polymer membrane, which developed during the cell operation. The behaviour of the membrane during the operation of a unit cell has been studied and investigated. The model accounts for both gas and liquid phase in the same computational domain, and thus allows for the implementation of phase change inside the gas diffusion layers. The model includes the transport of gaseous species, liquid water, protons, energy, and water dissolved in the ion-conducting polymer. The new feature of the present model is to incorporate the effect of hygro and thermal stresses into actual three-dimensional, multi-phase, non-isothermal fuel cell model. In addition to hygro—thermal stresses, the model features an algorithm that allows for a more realistic representation of the local activation overpotentials, which leads to improved prediction of the local current density distribution in high accuracy, and therefore, high accuracy prediction of temperature distribution in the cell and then thermal stresses. This model also takes into account convection and diffusion of different species in the channels as well as in the porous gas diffusion layer, heat transfer in the solids as well as in the gases, and electrochemical reactions.

Predictions of non-equilibrium phase transition in a model low-pressure steam turbine

An Eulerian-Eulerian multi-phase computational fluid dynamics model for predicting phase transition under non-equilibrium conditions in the low-pressure stages of a steam turbine is presented. In a novel way the model introduces dispersed liquid phases into the solution according to phase transition type and location in the turbine. Such an approach provides added information on the location and extent of phase transition regions, including associated droplet size, number, and wetness distributions. The model provides a mass weighted size distribution throughout the turbine with a resolution depending on the number of phases chosen. The study presents examples of the models usefulness in isolating phase transition activity and growth at design and off-design conditions. Comparison against results based on equilibrium phase transition in the turbine is provided for verification and discussion. Validation against measured droplet and flow data from the model turbine is also discussed.

Large eddy simulation of hydrocyclone—prediction of air-core diameter and shape

Hydrocyclones are widely used in the mining and chemical industries. An attempt has been made in this study, to develop a CFD (computational fluid dynamics) model, which is capable of predicting the flow patterns inside the hydrocyclone, including accurate prediction of flow split as well as the size of the air-core. The flow velocities and air-core diameters are predicted by DRSM (differential Reynolds stress model) and LES (large eddy simulations) models were compared to experimental results. The predicted water splits and air-core diameter with LES and RSM turbulence models along with VOF (volume of fluid) model for the air phase, through the outlets for various inlet pressures were also analyzed. The LES turbulence model led to an improved turbulence field prediction and thereby to more accurate prediction of pressure and velocity fields. This improvement was distinctive for the axial profile of pressure, indicating that air-core development is principally a transport effect rather than a pressure effect. An increase in feed flow rate will increase the air-core diameter by increasing the centrifugal force on fluid elements, which in turn increases the tangential velocity component, thereby lowering the pressure at the hydrocyclone axis near the apex. It has been observed that when the spigot diameter increased, the air-core diameter increased, since the tangential velocity increases when underflow diameter increases. As expected, a rise in the viscosity of liquid reduces the air-core diameter at constant feed velocity, by lowering the pressure drop over the cyclone. This LES turbulent multi-phase CFD model, once extended to particulate phase flow in hydrocyclone, would be a useful tool for studying design dimensions. More importantly, alternative geometries may be examined swiftly.

CFD predictions for flow‐regime transitions in bubble columns

AbstractThis work evaluates the ability of multiphase computational fluid dynamics (CFD) models to predict known flow regimes in air–water bubble columns. An initial grid‐resolution study shows that grid spacing of 0.25 cm or smaller must be used for adequate resolution. The ability of the two‐fluid model to predict homogeneous‐ and transitional‐flow behavior is analyzed next, and the flow predictions are found to be highly dependent on the model formulation (that is, bubble‐induced turbulence, drag, virtual mass, lift, rotation, and strain). At low gas velocities, homogeneous flow is observed for only a particular set of force models. At higher gas velocities, the same set of models yields reasonable predictions of transitional flow for small columns. Bubble size and liquid coflow also affect flow structures and flow stability at high gas flow rates. Scale‐up to larger column diameters is studied for both the homogeneous‐ and transitional‐flow regimes. In the homogeneous regime, the flow behavior is found to be independent of column diameter. However, because of neglect of coalescence, transition to churn‐turbulent flow is not observed at high gas velocities for large column diameters. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Measurement and Prediction of Pressure Drop in Pneumatic Conveying: Effect of Particle Characteristics, Mass Loading, and Reynolds Number

This paper reports the effect of Reynolds number, mass loading, and particle shape and size on pressure drop in a vertical gas−solids pneumatic conveying line. We isolate the effect of one variable while holding all others constant. A commonly used pressure drop correlation and a state-of-the-art multiphase computational fluid dynamics (CFD) models are then assessed by comparing their predictions to experimental data. Deficiencies in the models and the correlation are identified, and possible modifications are proposed. The most notable deficiency is the inability of both the experimental correlation and the CFD model to accurately predict the pressure drop for gas−solids flow with highly aspherical particles.

CFD Simulations of bubbling/collapsing fluidized beds for three Geldart Groups

AbstractA multiphase computational fluid dynamics (CFD) model was applied to a commonly used industrial experiment known as the collapsing fluidized‐bed experiment. The experiment involves several hydrodynamic regimes including the bed expansion, bubbling, sedimentation, and consolidation of the fluidized bed. The CFD model is capable of predicting all four of these regimes. The three Geldart Groups, C, A, and B were simulated in a bubbling and subsequently collapsing fluidized bed. Results show that hydrodynamic Models A and B, the use of the modified Ergun equation or the MFIX code drag models, and solids rheology have limited impact on the bubbling‐ and collapsing‐bed simulations. The major finding of this study is that the solids modulus, G, is the controlling parameter during the consolidation regime. The simulated bubble sizes and bed collapse rates for all three Geldart groups were found to be within reported experimental error. The computed high turbulent intensity of Geldart Group A particles also agrees with data in the literature measured using an acoustic shot noise probe. It is also demonstrated that the traditional interpretation of the collapsing bed as consisting of separate bubble escape and sedimentation regimes is incorrect and that, in fact, they occur simultaneously.

Erosion prediction in slurry pipeline tee-junctions

A significant erosion problem is found to occur on steel blanks located inside tee-junctions in a slurry pipeline system. A 3-D computational fluid dynamics (CFD) model is developed to predict the motion of caustic liquor and bauxite particles through a tee-junction using an Eulerian–Eulerian continuum approach in conjunction with the k– ε turbulence model. Initial simulations assuming a uniform inlet flow are unable to demonstrate the cause of the observed erosion. However, subsequent modelling with a swirling inlet flow, based on a more thorough assessment of the upstream vessels and piping, results in the prediction of an accumulation of particles on the steel blank at the centre of a slow-moving vortex, the location of which is in excellent agreement with the observed wear on the plant. Furthermore, the predicted wear location is found to be insensitive to the assumed level of inlet swirl and the numerical scheme employed. The multi-phase CFD model is also used to assess several potential solutions to the erosion problem, resulting in the replacement of the tee-junctions with a pivoting elbow design which is found to exhibit significantly reduced erosion rates on the plant. These results demonstrate the effectiveness with which CFD techniques can be used in the solution of industrial erosion problems. They also highlight the potential sensitivity of modelling results to inlet boundary condition assumptions and emphasise the need to adequately account for upstream influences when applying CFD techniques to the simulation of industrial flows.

Evaluation of a Three-Dimensional Numerical Model of a Scaled Circulating Fluidized Bed

This work concerns the evaluation of a numerical model of a scaled circulating fluidized bed (CFB) with a square cross section. The evaluation consists of a comparison of the numerical results with measurements achieved using laser Doppler anemometry (LDA) and pressure taps. The CFB is fluidized using metal particles, which enable the laboratory-scale CFB to be operated under scaled conditions of a full-scale CFB. The numerical part is based on a three-dimensional multiphase computational fluid dynamics (CFD) model that uses an Eulerian description for the gas and solid phases. The conservation equations for the solid phases are based on the kinetic theory for granular flow, which provides adequate modeling of the solid phase when the Eulerian approach is used. Three-dimensional simulations are found to be superior to two-dimensional simulations in comparisons to the experimental findings.