多孔介质电渗流动计算流体力学模拟与实验研究（Ⅱ）电渗流动与传质特性

采用流体动力学方法模拟多孔介质内的电渗流动行为，探讨了电场强度、多孔介质的孔隙率等对填充床层中电渗流场、宏观电渗流动及传质特性的影响，并以羟基磷灰石电色谱、采用DEAE Sepharose Fast Flow为介质的离子交换电色谱及Blue Sepharose Fast Flow为介质的亲和电色谱分离过程为例将理论计算结果与实验结果进行了对比，二者吻合较好，证实了上述理论研究的正确性并显示出此方法在发展新型固-液电动分离技术中的应用前景.     

CFD simulation of solid–liquid flow in a two impinging streams cyclone reactor: Prediction of mean residence time and holdup of solid particles

The hydrodynamic behavior of a two impinging streams cyclone reactor (TISCR) was simulated using the computational fluid dynamics (CFD) technique. An Eulerian multiphase model has been used to compute the unsteady flow of a solid–liquid suspension in a 3D geometric configuration. The mean residence time (t_m) and holdup of solid particles were calculated from a number of simulated results obtained at different solid and liquid flow rates. The CFD simulation results were compared with the experimental data available in the literature and a fairly well agreement was observed. Such a correlation was gradually improved with increasing solid flow rate.     

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.     

气液两相流摆动特性实验的数值模拟

为了给鼓泡塔反应器设计提供依据,运用计算流体力学(CFD)软件模拟了鼓泡塔气液两相流动态行为。采用双欧拉法对鼓泡塔矩形反应器内不同曝气量下气液两相流的摆动特性进行了模拟考察,液相采用标准κ-ε紊流模型,气相采用分散相零方程模型,分析了网格尺寸、时间步长以及相间作用力对模拟结果的影响,模拟的曝气量为42.5~237 m L/s。结果表明,当相间作用力仅考虑阻力时,气液两相流呈现周期性摆动规律;随着气流量的增加,气泡羽流的摆动幅度和频率增大,同时液体的气含率也在增加;模拟的气液两相流摆动频率数据与实验值吻合较好,两者的相对误差为7.2%~12.9%。     

Multiphase CFD Simulation of a Solid Bowl Centrifuge

This study presents some results from the numerical simulation of the flow in an industrial solid bowl centrifuge used for particle separation in industrial fluid processing. The computational fluid dynamics (CFD) software Fluent was used to simulate this multiphase flow. Simplified two‐dimensional and three‐dimensional geometries were built and meshed from the real centrifuge geometry. The CFD results show a boundary layer of axially fast moving fluid at the gas‐liquid interface. Below this layer there is a thin recirculation. The obtained tangential velocity values are lower than the ones for the rigid‐body motion. Also, the trajectories of the solid particles are evaluated.     

规整填料内单相流的LDV实验研究

To date, many models have been developed to calculate the flow field in the structured packing by the computational fluid dynamics （CFD） technique, but little experimental work has been carried out to serve the vali-dation of flow simulation. In this work, the velocity profiles of single-phase flow in structured packing are measured at the Reynolds numbers of 20.0, 55.7 and 520.1, using the laser Doppler velocimetry （LDV）. The time-averaged and instantaneous velocities of three components are obtained simultaneously. The CFD simulation is also carried out to numerically predict the velocity distribution within the structured packing. Comparison shows that the flow pattern, velocity distribution and turbulent kinetic energy （for turbulent flow） on the horizontal plane predicted by CFD simulation are in good agreement with the LDV measured data. The values of the x-and z-velocity components are quantitatively well predicted over the plane in the center of the packing, but the predicted y-component is sig-nificantly smaller than the experimental data. It can be concluded that experimental measurement is important for further improvement of CFD model.     

STUDY OF THE TURBULENT FLOW IN AN UNBAFFLED STIRRED TANK BY DETACHED EDDY SIMULATION

Detached eddy simulation (DES) of the liquid-phase turbulent flow in an unbaffled stirred tank agitated by a six-blade, 45°-pitched blade turbine was performed in this study. The tank wall is cylindrical with no baffle and the fluid flow problem was solved in a single reference frame (SRF) rotating with the impeller. For the purpose of comparison, computation based on large eddy simulation (LES) was also carried out. The commercial code Fluent was used for all simulations. Predictions of the phase-averaged turbulent flow quantities and power consumption were conducted. Results obtained by DES were compared with experimental laser Doppler velocimetry (LDV) data from the literature and with the predictions obtained by LES. It was found that numerical results of mean velocity and turbulent kinetic energy profiles as well as the power consumption are in good agreement with the LDV data. When performed on the same computational grid, which is under-resolved in the sense of LES, DES allows better accuracy than LES in that it works better in the boundary layers on the surface of the impeller and the stirred tank walls. It can be concluded that DES has the potential to predict accurately the turbulent flow in stirred tanks and can be used as an effective tool to study the hydrodynamics in stirred tanks.     

射流流化床中锥形分布板对流动的影响

给出了具有锥形分布板的射流流化床中浓密气固两相流动的多相流体力学基本方程组. 采用二维正交曲线坐标并生成了数值网格,用改进的IPSA方法求解二维正交曲线坐标中的多相流基本方程组,并编制了大型通用程序,流场可视化使用Tecplot软件. 对于给定的模拟计算,计算结果与实验值吻合. 模拟计算中改变了锥形筛板的角度、射流管的直径、床层高度、分布板开孔率的分布、射流气速、床层表观气速等,通过模拟得到床内的流动图像,考察了射流高度及颗粒循环的影响.     

Numerical simulation of a pitched‐blade turbine stirred tank with mirror fluid method

Mirror fluid method [Yang and Mao, Phys. Rev. 2005; E 71:036704] combined with local grid refinement is proposed to deal with the numerical simulation of turbulent flow in a pitched‐blade turbine stirred tank. By such a novel method, the domain occupied by the impeller is assigned suitable flow parameters explicitly by the mirror relation, so that the correct shear and normal forces on the fluid side of an interface segment is eventually guaranteed. Satisfactory agreement between our predictions and the reported experimental data is achieved both in single‐phase baffled or unbaffled stirred tanks and solid–liquid two‐phase systems. © 2012 Canadian Society for Chemical Engineering     

Arbitrary shaped ice particle melting under the influence of natural convection

This work is devoted to numerical simulations of an arbitrary shaped ice particle melting inside water under the influence of natural convection. Specifically, four different shapes of the ice particle have been studied: sphere, cylinder, cross shaped cylinder, and irregular sphere with radial bumps on its surface. A 2D axisymmetric particle‐resolved numerical model has been employed on a fixed grid to study the detailed melting dynamics of an ice particle. The solid‐liquid interface is treated as a porous medium characterized by the permeability coefficient which is used to damp the velocity values inside the interface. The model results have been compared with an existing experimental results produced by A. Shukla et al. (Metal Mater Trans B. 2011; 42(1):224–235). Very good agreement between our predictions and experimental data have been achieved. Based on the analysis of numerical simulation results, melting process is found to advance through two distinct regimes, namely, establishment of the natural convection and active melting of ice particle exhibiting substantial amount of fluid‐particle interactions. A set of dimensionless parameters have been identified to distinguish between regimes. Finally, we developed a semi‐empirical to predict the melting of any arbitrary shaped ice particle and validated it against the particle‐resolved numerical simulation and experimental results. The comparison showed good agreement. Finally, the presented semi‐empirical model can be used as sub‐grid model in Euler‐Lagrange based numerical models to study the phase change phenomena in particulate flow systems. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3158–3176, 2017     

Fluid dynamics and mixing of single-phase flow in a stirred vessel with a grid disc impeller: Experimental and numerical investigations

We report experimental and numerical investigations of a novel grid disc impeller for mixing of single-phase flow in stirred vessels. We performed detailed mean velocity measurements using LDA to understand the flow generated by the grid disc impeller and we also measured the mixing time and power consumption of the grid disc impeller. Measurements showed that the performance of the grid disc impeller, which has radial flow characteristics, is equivalent to a standard propeller in terms of the degree of mixing achieved per unit power consumption. We also performed numerical simulations to predict the flow generated by the grid disc impeller and its mixing performance. It was shown that the present computational model predicts the mean velocities, power consumption and mixing time in good agreement with the measurements. The experimentally validated computational model was further used to understand the effects of impeller rotational speed and grid disc configuration on the fluid dynamics and mixing performance of the grid disc impeller.     

CFD Prediction of Flow and Homogenization in a Stirred Vessel: Part I Vessel with One and Two Impellers

A simulation of flow field and tracer homogenization was performed using the commercial CFD software FLUENT 6.1. The aim is to investigate the potential of CFD software to predict concentration distribution of added tracer in cylindrical vessels. The calculated results – dimensionless velocity profiles, power and pumping numbers, dimensionless concentration curves, and mixing times – were compared with experiments in stirred vessels. In Part I, the study was performed for vessels agitated by one or two impellers on a centric shaft. Two different impellers were used – a 6‐bladed 45° pitched blade turbine and a standard Rushton turbine. The standard k‐? turbulence model and multiple reference frames method were used for the simulations. The influence of the grid type was also investigated; three types of grid – a structured, unstructured and a special user‐defined grid – were studied.     

Interior Ballistics Two‐Phase Reactive Flow Model Applied to Small Caliber Projectile‐Gun System

Multi‐dimensional multi‐component two‐phase flow modeling of solid propellant combustion in weapons is the new trend of the interior ballistics codes. Most of these codes are designated to large caliber guns and rockets simulation. Only a small number of investigations on small‐caliber gun have been recently reported, where the need of high‐performance and reliable small‐caliber guns stimulated significant interest in developing techniques to understand the phenomenology of small‐caliber ballistics and predict the behavior and the performance of this type of weapons. In this paper, a numerical model describing the combustion of solid propellant in small‐caliber gun is presented. The governing equations with customize parameters were derived in the form of coupled, non‐linear axisymmetric partial differential equations. They were further implemented into the CFD code Fluent. A numerical test showed that Fluent is able to handle correctly the interaction between the moving projectile and the combustion gases in the chamber. The interior ballistics curves along with the performance of small‐caliber gun 5.56 mm were adequately predicted. The numerical results were in agreement with the experimental results.     

Particle‐scale simulation of the flow and heat transfer behaviors in fluidized bed with immersed tube

A kind of new modified computational fluid dynamics‐discrete element method (CFD‐DEM) method was founded by combining CFD based on unstructured mesh and DEM. The turbulent dense gas–solid two phase flow and the heat transfer in the equipment with complex geometry can be simulated by the programs based on the new method when the k‐ε turbulence model and the multiway coupling heat transfer model among particles, walls and gas were employed. The new CFD‐DEM coupling method that combining k‐ε turbulence model and heat transfer model, was employed to simulate the flow and the heat transfer behaviors in the fluidized bed with an immersed tube. The microscale mechanism of heat transfer in the fluidized bed was explored by the simulation results and the critical factors that influence the heat transfer between the tube and the bed were discussed. The profiles of average solids fraction and heat transfer coefficient between gas‐tube and particle‐tube around the tube were obtained and the influences of fluidization parameters such as gas velocity and particle diameter on the transfer coefficient were explored by simulations. The computational results agree well with the experiment, which shows that the new CFD‐DEM method is feasible and accurate for the simulation of complex gas–solid flow with heat transfer. And this will improve the farther simulation study of the gas–solid two phase flow with chemical reactions in the fluidized bed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Particle‐resolved direct numerical simulation of gas–solid dynamics in experimental fluidized beds

Particle‐resolved direct numerical simulations (PR‐DNS) of a simplified experimental shallow fluidized bed and a laboratory bubbling fluidized bed are performed by using immersed boundary method coupled with a soft‐sphere model. Detailed information on gas flow and individual particles’ motion are obtained and analyzed to study the gas–solid dynamics. For the shallow bed, the successful predictions of particle coherent oscillation and bed expansion and contraction indicate all scales of motion in the flow are well captured by the PD‐DNS. For the bubbling bed, the PR‐DNS predicted time averaged particle velocities show a better agreement with experimental measurements than those of the computational fluid dynamics coupled with discrete element models (CFD‐DEM), which further validates the predictive capability of the developed PR‐DNS. Analysis of the PR‐DNS drag force shows that the prevailing CFD‐DEM drag correlations underestimate the particle drag force in fluidized beds. The particle mobility effect on drag correlation needs further investigation. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1917–1932, 2016     

Turbulence modelling of multiphase flow in high-pressure trickle-bed reactors

Computational fluid dynamics (CFD) has been used as a successful tool for single-phase reactors. However, fixed-bed reactors design depends overly in empirical correlations for the prediction of heat and mass transfer phenomena. Therefore, the aim of this work is to present the application of CFD to the simulation of three-dimensional interstitial flow in a multiphase reactor. A case study comprising a high-pressure trickle-bed reactor (30 bar) was modelled by means of an Euler-Euler CFD model. The numerical simulations were evaluated quantitatively by experimental data from the literature. During grid optimization and validation, the effects of mesh size, time step and convergence criteria were evaluated plotting the hydrodynamic predictions as a function of liquid flow rate. Among the discretization methods for the momentum equation, a monotonic upwind scheme for conservation laws was found to give better computed results for either liquid holdup or two-phase pressure drop since it reduces effectively the numerical dispersion in convective terms of transport equation.After the parametric optimization of numerical solution parameters, four RANS multiphase turbulence models were investigated in the whole range of simulated gas and liquid flow rates. During RANS turbulence modelling, standard k-ε dispersed turbulence model gave the better compromise between computer expense and numerical accuracy in comparison with both realizable, renormalization group and Reynolds stress based models. Finally, several computational runs were performed at different temperatures for the evaluation of either axial averaged velocity and turbulent kinetic energy profiles for gas and liquid phases. Flow disequilibrium and strong heterogeneities detected along the packed bed demonstrated liquid distribution issues with slighter impact at high temperatures.     

Simulation of Orifice Flow Influenced by Lateral Flow in a Trough‐Type Liquid Distributor

A computational fluid dynamics (CFD) simulation is performed to investigate the influence of lateral flow on the orifice flow in a trough‐type liquid distributor. The discharge coefficients from the simulation are in good agreement with the experimental values, indicating that the CFD simulation is accurate in describing the outflow through orifices. The lateral flow near an orifice can change the velocity and pressure distributions of flow regions in front of this orifice, causing a decrease in the discharge coefficient. This phenomenon is supported by the theory of flow past a blunt body. An important implication derived from this finding is that the influence of lateral flow should be minimized in the design of a trough‐type liquid distributor, because the decrease in the discharge coefficient leads to non‐uniform outflow. The structure of a trough‐type liquid distributor is optimized to improve the liquid distribution performance by reducing the influence of lateral flow.     

CFD simulations and particle image velocimetry measurements in an industrial scale rotating disc contactor

Fluid dynamics of the single‐phase and two‐phase flow in a segment of a rotating disc contactor (RDC) liquid–liquid extraction column with 450 mm inner diameter were studied by performing computational fluid dynamics (CFD) simulations and particle image velocimetry (PIV) measurements. The fluid dynamics were investigated to test the predictivity of CFD at industrial scale. Different turbulence models in conjunction with the Eulerian approach were applied in the single‐phase and two‐phase simulations. The turbulent flow characteristics were analyzed by PIV measurements to validate the CFD simulations. An iso‐optical system composed of CaCl₂/water–butylacetate allows for the two‐phase PIV measurements. Local turbulent energy dissipation was derived from velocity gradients in PIV data. In this connection, the influence of the PIV spatial resolution on the measured energy dissipation was also analyzed, and different fit functions were tested to scale the measured energy dissipation. Simulated velocity fields as well as the energy dissipation were compared with the experimental PIV data. The results from the simulations and experiments are in good agreement. The work shows that CFD can predict hydrodynamic characteristics even at bigger scales but is still subject to some minor restrictions. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Numerical simulations of the reactive mixing in a commercially operated stirred ethoxylation reactor

The computational fluid dynamics (CFD) technique was applied to describe the mixing and the chemical reactions in commercially operated stirred ethoxylation reactors. Two reactor sizes in the existing ethoxylation operations were studied in this work: a laboratory-scale autoclave with a single-Rushton turbine and an industrial-scale reactor with a dual-Rushton turbine. The ethoxylation reactor contents were described as an incompressible, turbulent single-phase liquid mixing regime with chemical species undergoing heat and mass transfer. Since the accurate experimental ethoxylation flow data could not be extracted from the industrial operations, the development of the CFD model for the ethoxylation process was undertaken in two stages. The first stage simulated a single-phase liquid agitation system based on the literature with experimental data on velocities, such as Wu and Patterson [1989. Laser-Doppler measurements of turbulent-flow parameters in a stirred mixer. Chemical Engineering Science 44, 2207–2221], for a Rushton stirred reactor of standard configuration. Once validated, the numerical model was applied to compute the flow field in ethoxylation reactors. The second stage integrated the ethoxylation kinetics into the numerical model and simulated the ethoxylation process. In the simulation of the mean flow field, the qualitative features of the literature data were well reproduced. The computed results of both the ethylene oxide consumption and the temperature calculation compared very well with the measurements in the laboratory-autoclave operations. Reasonably good agreement was also reached between the simulated and experimental data on the time-dependent changes of ethylene oxide mass fraction in the bulk liquid in the industrial ethoxylation operations. These demonstrate that the CFD process model was capable of predicting the reaction behaviour and would be useful for exploration of any opportunity for increasing the ethoxylation capacity in the industrial operations.     

Comparative Evaluation of OpenFOAM® and ANSYS® Fluent for the Modeling of Annular Reactors

A three‐dimensional numerical model was solved for a turbulent incompressible steady flow and the discrepancies and similarities between the software OpenFOAM® and ANSYS® Fluent on the flow modeling were analyzed. The evaluation was performed for the meshing, setup, and simulation step of the modeling process. Outcomes were contrasted with experimental data to validate the fluid flow obtained with both softwares. With this aim, an experimental pulse‐response technique was applied to obtain the residence time distribution (RTD) curve to be compared with the RTD curves achieved from the modeling. Additionally, images of the tracer transport in the experimental reactor were taken, showing good agreement with the computational fluid dynamics (CFD) predictions.