Computational Fluid Dynamics Modeling of Gas‐Liquid Two‐Phase Flow around a Spherical Particle

Microscale studies, which can provide basic information for meso‐ and macroscale studies, are essential for the realization of flow characteristics of a packed bed. In the present study, the effects of gas velocity, liquid velocity, liquid‐solid contact angle, and liquid viscosity on the flow behavior were parametrically investigated for gas‐liquid two‐phase flow around a spherical particle, using computational fluid dynamics (CFD) methodology in combination with the volume‐of‐fluid (VOF) model. The VOF model was first validated and proved to be in good agreement with the experimental data. The simulation results show that the film thickness decreases with increasing gas velocity. This trend is more obvious with increasing operating pressure. With increasing liquid velocity, the film thickness tends to be uniform on the particle surface. The flow regime can change from film flow to transition flow to bubble flow with increasing contact angle. In addition, only at relatively high values does the liquid viscosity affect the residence time of the liquid on the particle surface.     

Validation of a Discrete Particle Model in a 2D Spout‐Fluid Bed Using Non‐Intrusive Optical Measuring Techniques

To gain insight into the hydrodynamics of spout‐fluid beds, an experimental and numerical study was carried out. Particle image velocimetry was successfully developed and applied to determine particle velocity profiles, whereas voidage profiles were determined by digital image analysis. A 3D hard‐sphere discrete particle model was used to simulate the flow in a spout‐fluid bed. The simulations and experiments showed a similar influence of the background fluidization velocity on the spout behaviour.     

Full‐Loop Simulation of Gas‐Solids Flow in a Pilot‐Scale Circulating Fluidized Bed

In order to study the system hydrodynamics in a circulating fluidized bed (CFB), a 3D full‐loop simulation was conducted for a pilot‐scale CFB. The Eulerian‐Eulerian two‐fluid model with the kinetic theory of granular theory helped to simulate the gas‐solids flow in the CFB. The system hydrodynamics including pressure balance, vectors of gas and solids, distribution of solids holdup, and instantaneous circulating rates were obtained to get a comprehensive understanding of the system. It was predicted that the main driving force was the pressure drop of the storage tank. The storage height and valve opening were critical operating factors to control the riser operation. The effects of operating conditions including solids circulating rates and superficial gas velocity on the hydrodynamics were investigated to provide guidance for the stable operation of the CFB system.     

Catalytic Ozone Decomposition in a Gas‐Solids Circulating Fluidized‐Bed Riser

Computational fluid dynamics (CFD) modeling of the catalytic ozone decomposition reaction in a circulating fluidized‐bed (CFB) riser, using iron‐impregnated fluid catalytic cracking particles as catalyst, is carried out. The catalytic reaction is defined as a one‐step reaction, and the reaction equation is modified by with respect to the particle surface area, A_p, and an empirical coefficient. The Eularian‐Eularian method with the kinetic theory of granular flow is used to solve the gas‐solids two‐phase flow in the CFB riser. The simulation results are compared with experimental data, and the reaction rate is modified by using an empirical coefficient, to provide better simulation results than the original reaction rate. Moreover, the particle size has great effects on the reaction rate. The generality of the CFD model is further validated under different operating conditions of the riser.     

Computational Fluid Dynamics Study of Heat Transfer in Turbulent Fluidized Beds

The fluidization and heat transfer behaviors of a turbulent fluidized bed were investigated using computational fluid dynamics (CFD). The effects of inlet superficial velocity on heat transfer behaviors in a turbulent fluidized bed were analyzed and compared with those operated in other fluidization regimes. The effects of using particles belonging to different Geldart groups in a turbulent fluidized bed on fluidization and heat transfer behaviors were evaluated. For both fluidization regimes investigated, the solids temperature distribution during the heat transfer process became less uniform when the particle size was reduced.     

Simulation and Experimental Validation of Two‐Phase Flow in an Aerosol Counter‐Flow Reactor using Computational Fluid Dynamics

A simulation of the hydrodynamic behavior of an aerosol‐counter flow reactor was conducted using an Euler‐Lagrange method. The simulation results were then verified with experiments. The process simulated was a separation process required during the production of biodiesel (fatty acid methyl ester). In this process, the liquid ester/glycerol phases are continuously injected through a hollow cone nozzle with an overpressure of 10⁶ Pa into the reactor, operated at 15000 Pa. The liquid is atomized because of the pressure drop and a liquid particle spray is generated with an inlet velocity of 44.72 m/s. Water vapor of temperature 333 K is injected tangentially through two side, gas inlets with an inlet velocity of 1.2 m/s. Excess methanol is subjected to a mass transfer from the liquid phase into the gas phase, which is withdrawn through the head of the reactor and condensed in an external condenser unit. The stripping of the methanol off the liquid leads to a sharp interface between the glycerol and the ester phase, which can then be easily separated by gravity or pumping. The gas velocity field, pressure field and the liquid particle trajectories were calculated successfully. Simulated dwell time distribution curves were derived and analyzed with the open‐open vessel dispersion model. Experimental dwell time distribution curves were measured, analyzed with the open‐open vessel dispersion model, and compared with the simulated curves. A good consistency between simulated and measured Bodenstein numbers was achieved, but 25 % of the simulated particles exited at the reactor's head, contrary to experimental observations. The difference between simulated and measured dwell times was within one order of magnitude.     

Comparison of Two‐Fluid and Discrete Particle Modeling of Dense Gas‐Particle Flows in Gas‐Fluidized Beds

Both two‐fluid models embedding the kinetic theory of granular flow for particulate phase stress (TFM) and discrete particle models (DPM) are widely used for the numerical simulation of gas fluidization. In this study, a detailed comparison between results obtained from both TFM and DPM is reported, including axial and radial solid concentration profiles, solids circulation patterns, pressure drop and its standard deviation and granular temperature. It was shown that good agreement can be obtained even in cases of low restitution coefficient, which suggests the possible applicability of kinetic theory of granular flow beyond its nominal range of validity and clearly indicates that the continuum treatment of the solids phase in TFM provides a good approximation of its discrete nature.     

A Comprehensive Computational Fluid Dynamics Study of Droplet‐Film Impact and Heat Transfer

The Eulerian multiphase model and continuum surface force (CSF) are employed to simulate the liquid droplet impinging onto a solid wall with a pre‐existing thin film of the same liquid. The numerical results are compared with the experimental data reported in the literature, indicating a reasonable matching. The flow field and splashing behavior of a droplet impinging onto a liquid film are analyzed. The reason for the edge of the crown to eject into secondary drops is found. The splashing behavior can be influenced by the impacting velocity and fluid properties. The effects of impact velocity, droplet diameter, depth of film, liquid property, and droplet and wall temperature on the heat removal are investigated. Numerical results demonstrate that an increase in impact velocity, droplet diameter, film depth, cooling droplet, and wall temperature enhances the dissipated heat. These results can provide a reference for designing spray‐cooling systems.     

Computational Fluid Dynamics Simulation of Pilot‐Plant‐Scale Two‐Phase Separators

Two computational fluid dynamics (CFD) modeling approaches, the discrete phase model (DPM) and the combination of volume of fluid (VOF) and DPM, are developed to simulate the phase separation phenomenon in four pilot‐plant‐scale separators. The incipient vapor phase velocity, at which liquid droplet carryover occurs, and separation efficiency plots are used as criteria for evaluating the developed CFD models. The simulation results indicate that the VOF‐DPM approach is a substantial modification to the DPM approach in terms of the predicted separation efficiency data and diagrams. CFD simulation profiles demonstrate that all the separators are essentially operating at a constant pressure. The CFD results also show that mist eliminators may operate more efficiently in horizontal separators than in vertical separators.     

Numerical and Experimental Study of Catalyst Loading and Body Effects on a Gas‐Liquid Trickle‐Flow Bed

The influence of tortuosity and fluid volume fractions on trickle‐flow bed performance was analyzed. Hydrodynamics of the gas‐liquid downward flow through trickle beds, filled with industrial trilobe catalysts, were investigated experimentally and numerically. The pressure drop and liquid holdup were measured at different gas and liquid velocities and in two different loading methods, namely, sock and dense catalyst loading. The effect of sharp corners on hydrodynamic parameters was considered in a bed with rectangular cross section. The reactor was simulated, considering a three‐phase model, appropriate porosity function, and interfacial forces based on the Eulerian‐Eulerian approach. Computational fluid dynamics (CFD) simulation results for pressure drop and liquid holdup agreed well with experimental data. Finally, the velocity distribution in two types of loading and the effect of bed geometry in CFD results demonstrated that pressure drop and liquid holdup were reduced compared to a cylindrical one due to high voidage at sharp corners.     

Drying of Liquid Feedstocks in a Spout‐Fluid‐Bed with Draft‐Tube Submerged in Inert Solids: Hydrodynamics and Drying Performance

A spout‐fluid bed with draft tube submerged in a bed of polypropylene beads was used for drying maltodextrin solutions. The hydrodynamics of the dryer were studied by determining the annular air flow vertical profile at different spouting velocities, using an additional air flow rate through the annulus equivalent to 0.5 U_mf. The drying performance of the dryer was studied through the determination of several dryer response parameters (product moisture, evaporative capacity and volumetric evaporative capacity). These parameters were compared with those obtained in a conventional spouted bed with inert solids and a spray dryer.     

规整填料塔内两相流动的三维计算流体力学建模(英文)

Characterizing the complex two-phase hydrodynamics in structured packed columns requires a power- ful modeling tool. The traditional two-dimensional model exhibits limitations when one attempts to model the de- tailed two-phase flow inside the columns. The present paper presents a three-dimensional computational fluid dy- namics （CFD） model to simulate the two-phase flow in a representative unit of the column. The unit consists of an CFD calculations on column packed with Flexipak 1Y were implemented within the volume of fluid （VOF） mathe- matical framework. The CFD model was validated by comparing the calculated thickness of liquid film with the available experimental data. Special attention was given to quantitative analysis of the effects of gravity on the hy- drodynamics. Fluctuations in the liquid mass flow rate and the calculated pressure drop loss were found to be quali- tatively in agreement with the experimental observations.     

Three-dimensional Computational Fluid Dynamics Modeling of Two-phase Flow in a Structured Packing Column

Characterizing the complex two-phase hydrodynamics in structured packed columns requires a power-ful modeling tool. The traditional two-dimensional model exhibits limitations when one attempts to model...     

Three‐Dimensional Computational Fluid Dynamics Modeling of a Planar Solid Oxide Fuel Cell

A three‐dimensional computational fluid dynamics model was developed to study the performance of a planar solid oxide fuel cell (SOFC). The governing equations were solved with the finite volume method. The model was validated by comparing the simulation results with data from literature. Parametric simulations were performed to investigate the coupled heat/mass transfer and electrochemical reactions in a planar SOFC. Different from previous two‐dimensional studies the present three‐dimensional analyses revealed that the current density was higher at the center along the flow channel while lower under the interconnect ribs, due to slower diffusion of gas species under the ribs. The effects of inlet gas flow rate and electrode porosity on SOFC performance were examined as well. The analyses provide a better understanding of the working mechanisms of SOFCs. The model can serve as a useful tool for SOFC design optimization.     

Effect of a Draft Tube on the Fluid Dynamics of a Spouted Bed: Experimental and CFD Studies

Knowledge about the gas and particle dynamics in spouted beds is important in the evaluation of particle circulation rates and the efficiency of gas-solid contacts. In this work, the mechanism of transition from a static bed to a spouted bed was numerically simulated using a Eulerian multiphase model. This model was applied to two distinct spouted bed geometries: a conventional device and a spouted bed with draft tube. The radial voidage and particle velocity profiles along the longitudinal position in the annular and spout regions were simulated for the geometries under study. The characteristic simulated curves were congruous with the experimental data.     

Effect of a Draft Tube on the Fluid Dynamics of a Spouted Bed: Experimental and CFD Studies

Knowledge about the gas and particle dynamics in spouted beds is important in the evaluation of particle circulation rates and the efficiency of gas-solid contacts. In this work, the mechanism of transition from a static bed to a spouted bed was numerically simulated using a Eulerian multiphase model. This model was applied to two distinct spouted bed geometries: a conventional device and a spouted bed with draft tube. The radial voidage and particle velocity profiles along the longitudinal position in the annular and spout regions were simulated for the geometries under study. The characteristic simulated curves were congruous with the experimental data.     

Design Criteria for Oilfield Separators Improved by Computational Fluid Dynamics

Oilfield separator data ranging from light‐oil conditions to heavy‐oil conditions were incorporated into suitable two‐phase and three‐phase computational fluid dynamics (CFD) models to provide improved design criteria for separator design methods. The CFD simulation results revealed that the most important affecting parameters are vapor density and oil viscosity. In contrast with the classic design methods, noticeable residence times were required for liquid droplets to penetrate through the fluid interfaces. Moreover, it was indicated that the Abraham equation should be used instead of the Stokes' law in the liquid‐liquid separation calculations. The velocity constraints caused by re‐entrainment in horizontal separators were also studied and led to novel correlations.     

旋流板内两相流场的CFD模拟与分析

The flow field of gas and liquid in a φ150mm rotating-stream-tray (RST) scrubber is simulated by using computational fluid dynamic (CFD) method. The simulation is based on the two-equation RNG κ-ε turbulence model, Eulerian multiphase model, mad a real-shape 3D model with a huge number of meshes. The simulation results include detailed information about velocity, pressure, volume fraction and so on. Some features of the flow field are obtained: liquid is atomized in a thin annular zone; a high velocity air zone prevents water drops at the bottom from flying towards the wall;the pressure varies sharply at the end of blades and so on. The results will be helpful for structure optimization and engineering design.