Application of CFD for simulation of a baffled tubular membrane

Computational fluid dynamics (CFD) investigation of a tubular membrane channel containing a set of baffles was conducted for predicting turbulent flow. Simulation was performed using an array of baffles oriented either in the flow or in the reverse direction. A range of local parameters such as stream function, velocity, static pressure, wall shear stress, turbulent kinetic energy, and turbulent dissipation energy on the membrane surface was computed using CFD code FLUENT. The simulation results indicate that the presence of baffle can improve the local shear stress on the membrane surface and produces eddy activities which enhance the filtration performance. The observed flux enhancement can be attributed to the intense fluctuations of wall velocity and shear stress which can disrupt the growth of boundary layer on the membrane surface. The experimental evaluation was performed through cross flow microfiltration of titanium dioxide suspension which showed an acceptable agreement with the CFD predictions.     

竖直管外气液逆流环状降膜速度与温度分布

建立了竖直管外环状降膜气液逆流传热传质条件下稳态层流降膜一维速度分布和二维温度分布模型,以及膜厚和降膜表面热通量的数值计算方法。表面热通量的模型计算值与实验值在气体Reynolds数Re_g<1200的范围内吻合较好,表明基于界面摩擦因子求解模型的方法在两相均为层流条件下是可靠的。模型显示了降膜速度分布和温度分布的非线性特征,降膜表面附近陡降的温度梯度表明,减小膜厚是强化降膜传热传质过程的有效途径。     

垂直及倾斜上升管内气液两相弹状流壁面切应力的模拟

用VOF模型对垂直及倾斜上升管内弹状流壁面切应力进行数值模拟。计算结果表明,垂直上升流动时,液膜厚度始终小于对应位置倾斜上升弹状流的液膜厚度,壁面切应力从气弹头部至尾部逐渐增大至恒定不变,在尾流区呈杂乱无章状态。倾斜上升流动时,气泡头部顶点偏向管中心线上方,倾角越小,相同轴向位置处测得的液膜厚度越大。当FrTB较小时,倾斜管内弹状流上管壁面的切应力曲线在液膜区有明显波动,而下管壁面在对应区域的切应力分布则比较光滑。随着FrTB的增大,上下壁面切应力分布曲线越来越靠近。     

气升式陶瓷膜过滤过程的气液两相流模拟

采用VOF双流体模型对19通道气升式陶瓷膜过滤装置进行气液两相流的流体动力学模拟,研究了曝气孔直径和曝气量对气升式陶瓷膜过滤装置的气含率、环流液速、膜面剪切力及膜管内湍流强度的影响,模拟结果与实验结果的误差在5%~10%之间。结果表明,气升管与降液管的气含率都随曝气量增大而增大,随曝气孔直径减小而增大;环流液速、膜面剪切力及膜管内的湍流强度都随曝气量增大先增大,当曝气量达到400L·h^-1时其增大趋势变缓。通过实验和模拟比较了3种不同孔径的曝气头,环流液速与曝气孔的直径关系不大,仅与曝气量相关,但曝气孔直径越小,其膜面剪切力越大,越有利于过滤过程的进行。     

稀疏气固两相湍流边界层拟序结构变动特性

应用PIV两相同时测量方法,对壁面Reynolds数为430的水平槽道稀疏气固两相湍流边界层拟序结构变动特性进行了研究。选取质量载荷为10^-4~10^-3的110 μm聚乙烯颗粒作为离散相。结果表明,低载荷颗粒仍能显著改变湍流拟序结构,进而影响宏观湍流属性。颗粒重力沉降形成的粗糙壁面增强了壁面附近湍流猝发行为,导致黏性底层中的气相法向脉动速度和雷诺剪切应力显著增大。颗粒与壁面的碰撞加强了低速流体上抛、削弱了高速流体下扫,同时增强了轨道交叉效应,从而抑制了湍流拟序结构发展,显著减小了黏性底层以上区域的法向脉动速度和雷诺剪切应力。此外,颗粒惯性还减小了黏性底层厚度、增大了流向速度梯度,导致气相流向脉动速度峰值增大,且其对应位置也更加靠近壁面。     

Slip flow of coal water slurries in pipelines

Experiments were carried out on a pilot scale slurry transport apparatus to investigate slip flow behavior of coal water slurries (CWSs) in pipes with various diameters (25, 32, 40 and 50 mm). The effects of volume concentration, particle size and slurry temperature on wall slip behavior (wall slip velocity, critical wall shear stress and slippage contribution) were investigated. A numerical technique based on Tikhonov regularization was applied to determine the wall slip behavior. As the slurry temperature or the particle size increased, the critical wall shear stress was observed to decrease and the wall slip velocity was observed to increase significantly, while when the solid concentration was very close to the maximum packing fraction, a slight increase in volume concentration would lead to a rapid decrease in wall slip velocity and a sharp increase in critical wall shear stress. The temperature influence on critical wall shear stress and yield stress increased as volume concentration increased. At low wall shear stress, the slippage contribution was mainly dependent on the difference between yield stress and critical shear stress. While at high wall shear stress, it was dependent on both of the shear viscosity of the bulk slurries and the wall slip velocity.     

温度和固相粒径与浓度对水煤浆管内流动壁面滑移的影响

通过改变管径、温度、煤粉粒度和浓度,在中试规模的输送装置上研究水煤浆直管内的滑移流动规律,联合采用Mooney滑移修正方法和Tikhonov正则化方法确定浆体壁面滑移特性。结果表明,随温度和固相粒径的增大,临界剪切应力降低,壁面滑移速度显著增加;浓度越高,临界剪切应力及产生相同滑移速度所需的壁面剪切应力越大,温度升高对临界剪切应力和屈服应力的降低越显著;低壁面剪切应力下的滑移贡献率主要取决于临界剪切应力及屈服应力的相对大小,高壁面应力下主要取决于壁面滑移速度和浆体真实流变特性。     

Analysis of pulsatile flow and its role on particle removal from surfaces

The use of pulsatile flow for energy efficient particle removal from surfaces is evaluated through modeling calculations. The governing equation for pulsatile flow in a channel between parallel plates with an oscillatory pressure input is solved and wall shear stress, identified as a measure for particle removal, calculated for fixed power input. It is observed that as the frequency of oscillation is increased the average wall shear stress with an oscillatory pressure input is higher than the corresponding steady state value only above a critical frequency. Similar results are obtained for pulsatile flow in a pipe. Explanation for this observation is presented based on how velocity profile changes as a function of frequency and consequently its effect on wall shear stress versus power dissipated. Based on these observations we propose that there is a critical frequency above which an oscillatory pressure input will be energy efficient for particle removal.     

Scherfließen und Wandgleiten von Massen für den Pulverspritzguss

This contribution presents coupling of laws for shear flow and wall slipping by the shear stress at the slipping interface. It includes the special case of Coulombian friction postulated by Uhland as well as the assumption of a constant sliding velocity along the flow channel according to Mooney. As an example, Ostwald and de Waele's law of shear thinning flow is combined with a shear stress of sliding depending on internal pressure by a power law. Examined feedstocks for metal injection molding showed a rheological behavior according to the model presented.     

Hydrodynamics of an external-loop gas-lift system with restrictions located in the downcomer

Experiments were conducted to study the fluid dynamics in the case that slug flow occurs in the riser of an external-loop gas-lift system with a restriction section located in the downcomer. Complex fluctuation behaviors of the liquid circulation velocity and the wall shear stress in the riser were observed and discussed. Based on the slug flow hydrodynamic behaviors and the balance of momentum and pressure drop over the circulating loop, a model was developed to predict the main parameters of the system: the liquid circulation velocity, the void fraction, the length and velocities of Taylor bubbles and liquid slugs. The predicted results of these parameters were compared with the experimental data and a good agreement was obtained.     

A study of bubble nucleation in a mixture of molten polymer and volatile liquid in a shear flow field

Bubble nucleation in a mixture of volatile liquid and polymer melt under shear flow conditions was investigated, using a light scattering technique. In the study, a mixture of polystyrene and trichlorofluoromethane was extruded through a slit die having glass windows and bubble nucleation in the flow channel was observed optically. A He-Ne laser was used to illuminate the nucleating and growing bubbles. The light flux scattered by the growing bubbles at a fixed angle was detected by a photomultiplier with the aid of a high-voltage power supply. The bubble nucleating site in the flow channel was located using a computer controlled tracking system, which was designed to move the entire optical system automatically in the three dimensional space, and also had the ability to follow the software control command and cooperate with the data acquisition system. When the site of bubble nucleation was located, the coordinates of this site in the flow channel and the experimental conditions were automatically recorded on a floppy diskette by entering a software command. The pressure profile along the flow channel was measured by pressure transducers, with the aid of a microprocessor-based pressure reading system. It has been found that the site of bubble nucleation varies with the position in the direction perpendicular to the flow direction, which is attributed to the nonuniform velocity and stress distributions in the slit flow channel. The present investigation suggests that bubble nucleation can be induced either by flow and/or shear stress; specifically, flow-induced bubble nucleation is the dominant mechanism at positions near the center of the die opening, and shear-induced bubble nucleation is the dominant mechanism at positions near the die wall. It should be mentioned that the bubble near the die wall may also be generated by cavitation brought about by the surface roughness of the wall and also by thermal fluctuations due to the heat transfer between the metal (die wall) and the mixture of polymer and volatile component. The present study indicates that bubble nucleation in a shear flow field can occur at an unsaturated condition. This is in contrast to bubble nucleation under static conditions, where supersaturation is necessary.     

基于有限元的流道出口胶料速率均匀性分析

胶料熔体在流道出口沿宽度方向上速率的均匀性是保证挤出制品品质的重要因素,在已优化的流道结构基础上研究胶料在流道出口速率均匀性的影响因素。利用Fluent流体动力学软件进行模拟,改变工艺参数,分析速度场、应力场、压力场的分布。结果表明,胎面胶料黏度减小或入口压力增大均会引起流道出口中间的速率偏大;而温度的改变对流道出口速率均匀性影响不明显。     

The open-channel flow of fluidized solids

Observations of the flow properties of fluidized solids through a horizontal open channel with a porous tile distributor are reported. The shear stress at the vertical wall of the experimental section was measured directly and that across the distributor estimated from the difference between the total stress represented by the pressure drop along the experimental section and the wall shear stress. Experiments were carried out with a bed of catalyst of mean diameter 77 μm and ash of wide size distribution with a mean diameter of 380 μm over a range of fluidizing conditions between 1.75 and 3 U_mf. Earlier results with a 196 μm sand have been reinterpreted on the same basis. Comparison is also made with flow property measurements using a modified Stormer viscometer.The freely fluidized and flowing fluidized beds showed pseudoplastic flow characteristics. The flowing sand and ash beds also displayed dilatant characteristics at higher fluidizing velocities. The apparent viscosity reduced with initial increase in gas flow rate, passed through a minimum and then increased. However, the minimum was not reached over the range of test with either the flowing catalyst or ash bed experiments. Whereas the distributor shear stress was consistently lower than that at the wall in the catalyst experiments, it exceeded that at the wall in both the sand and ash experiments at low shear rates and its relative value then decreased to less than that at the wall as the shear rate increased. Bed depth was a significant factor and flow behaviour was influenced by the width of the experimental channel.While there is reasonable correspondence between variation in shear stress measurements made in the modified Stormer viscometer and directly at the channel wall, the viscometer reading is sensitive to the position of the rotor within the bed. This and the varying slip at the distributor in the flowing bed system precludes direct application of small scale test data in the design of open-channel flow systems.     

CFD studies coupling hydrodynamics and solid‐liquid mass transfer in slug flow for matter removal from tube walls

Organic matter deposition on internal surfaces constitutes a drawback that impairs the efficiency of several industrial processes. To overcome this problem, sparging a train of bubbles could be useful since its presence strongly increases the wall shear stress. A detailed numerical mass‐transfer study between a finite soluble wall and the liquid around a rising Taylor bubble was performed, simultaneously solving velocity and concentration fields. The bubble passage throws solute backward and is responsible for radial dispersion. There is also an increase in the transfer rate with enhancements between 10 and 20% (depending on liquid average velocity and bubble length) compared to single‐phase flow. Mass‐transfer coefficients along the different hydrodynamic regions around the bubble nose, liquid film, and wake were characterized and their values compared with those from literature. The results suggest a promising potential of bubble train flow to enhance organic matter removal from walls in biological systems. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2420–2439, 2017     

Liquid-wall shear stress in stratified liquid/gas flow

Stratified liquid/gas flow was experimentally investigated in a horizontal or slightly inclined circular pipe. The polarographic method was used to determine the liquid-wall shear stress. Both the liquid fraction and pressure gradient were also measured. Sixteen wall electrodes were positioned around the tube perimeter. Local liquid-wall shear stress profiles are presented for horizontal and near-horizontal flows. The averaged values are compared with the results obtained from a stratified flow model and models using the experimental data of liquid fraction and pressure drop. This paper was presented at the International Workshop on Electrodiffusion Diagnostics of Flows held in Dourdan, France, May 1993.     

层叠流道中天然高分子挤压流动过程的数值模拟

为探究层叠流道中天然高分子材料挤压流动过程与混合特性，建立层叠流道的三维物理模型和有限元网格模型，利用POLYFLOW软件计算了食品熔体在不同的入口流量，挤压过程中的压力、速度、剪切速率分布。结果表明，压力沿着挤出方向呈梯度递减，随着入口流量增大中间位置压力差增大；剪切速率在流道中变化明显，剪切速率从流道中间位置向四周递减，从壁面到流道内部递减；速度在流道内沿着挤出方向先增大后减小，靠近壁面速度小；层叠扭转流道的结构和尺寸设计，有助于提高天然高分子物料的混合效果，并且第1、4层流道混合效果优于第2、3层流道，提高入口流量也有利于提高物料的混合效果。     

湿天然气在上倾管道的气液两相流动特性研究

利用VOF (Volume of Fluid)模型模拟了湿天然气在上倾管道的气液两相流动特性。讨论了管道内气体入口流速对气液两相流动特性的影响。结果表明,当管道内气体入口流速较低时,由于气液界面的切应力小于液相在上倾段的重力分力,因此气体无法将液体携带至管道上倾段。随着气速的增加,气液界面的切应力逐渐增大,气液界面开始出现波纹,气体逐渐将液体携带并完全平铺于管道上倾段。     

微小通道内液体扰流微孔逸出气泡串的迁移行为

对微小矩形通道内,液体扰流下微孔孔口逸出气泡串的迁移行为进行了可视化实验。基于图像处理方法,分别研究了液体流量和通道倾斜角度对气泡串尺寸、运动速度以及运动轨迹的影响。发现了不同工况下通道内的微气泡堆积现象,并对其进行了分析。结果表明：增大液体流量或者通道倾角,使得形成的气泡尺寸减小,气泡运动轨迹与通道壁面夹角减小,气泡运动速度增大。液体流量与通道倾角影响了通道内微气泡堆积的位置及数量。     

Numerical study of flow at a liquid-porous medium interface

The problem of the two-dimensional creeping flow of a viscous incompressible liquid in a flat channel partially filled with a model fibrous porous medium that is represented by a regular system of square cylinders (prisms) located across the flow is solved. Two types of flows are considered: a shear flow due to the motion of the upper wall of the channel and a gradient flow due to the presence of a pressure drop along the channel. The hydrodynamic microscopic fields of velocity are numerically found. The macroparameters such as the rate of filtration, the permeability of a system of cylinders, the flow rate of the liquid through the channel, tangential stresses on the upper wall of the channel and the porous boundary, and a slip coefficient, the use of which in the Saffman slip boundary condition makes it possible to considerably simplify the solution of the original problem, are obtained as a result of averaging.     

中等Reynolds数线性剪切泡状流的液相雷诺应力

Analytical investigation of liquid Reynolds stress in shear bubbly flow with intermediate Reynolds numbers is absent. In this paper, the velocity field around a sphere bubble in linear shear liquid is assumed to be the linear superposition of the velocity field of uniform liquid passing a sphere bubble and the linear shear velocity field. The formula of shear liquid Reynolds stress was derived by averaging the velocity field in the cell-ensemble averaging method, and the formula was corrected under conditions of intermediate Reynolds number. The formula was compared with that of Sato, and the predicted results of local liquid velocity of the fully developed upward bubbly flow in pipes were compared with the experimental data. The results show that the formula is valid and accurate in prediction.