颗粒相壁面条件对提升管内气固流动模拟的影响

引言提升管是非均匀结构显著的气固两相流动体系,其流动特性主要表现为轴向空隙率的"S"形分布、径向的"环-核"结构以及团聚物的生成和破碎等。近年来,国内外学者致力于数值计算方法的研究增多,其中双流体模型的应用最为广泛;颗     

壁面粗糙度对气粒两相流动中颗粒行为影响的实验研究

The effect of wall roughness on particle behavior in two-phase flows in a horizontal backward-facing step is studied using a phase-Doppler particle anemometer. The results show that the wall roughness widens the particle velocity probability density distribution, enhances the redistribution of particle velocity into different directions,reduces the particle longitudinal mean velocity and increases the longitudinal and transverse fluctuation velocities and Reynolds shear stress. The effect of roughness on particle motion in the recirculation zone is weaker than that in the fully developed flow region. The effect of roughness for small particles is restricted only in the near-wall region, while that for large particle diffuses to the whole flow field.     

颗粒相壁面条件对非球形颗粒流动影响的数值模拟

采用双流体模型对设置竖直隔板的气固密相流化床中非球形颗粒的运动进行了模拟,颗粒形状的影响由相间曳力模型考虑,重点考察壁面处颗粒边界条件的影响。同时进行了实验室规模三维流化床的流化实验,以验证模型的有效性。通过压降轴向分布、颗粒浓度径向分布以及物料出口处颗粒质量流率功率谱估计等定量分析,结果表明:对不设置内构件的自由床,壁面反射系数对系统宏观流动特性影响较小,而对壁面处局部颗粒运动影响较大;对壁面面积大幅增加的内构件床,壁面反射系数可显著改变气体和颗粒的运动特征,取值需控制在适当范围内。     

壁面条件对FCC颗粒湍动床气固流动影响的模拟

采用基于颗粒动力学的双流体模型对FCC颗粒湍动床和挡板床内气固流动进行模拟研究,考察颗粒相壁面边界条件对其气固流动的影响。研究结果表明,采用Johnson-Jackson壁面边界条件描述壁面与颗粒的相互作用时,壁面反射系数对气固流动行为影响较大,而颗粒-壁面碰撞恢复系数影响甚微。通过将模拟结果与实验数据对比,确定壁面反射系数取0.001时能较好地描述壁面与颗粒间的作用力。对于挡板床,挡板的加入增大了壁面面积,壁面对颗粒的作用力更加显著,挡板壁面采用部分滑移条件时,穿过导向挡板向上和向下运动的颗粒数量均较大于采用无滑移条件时的结果。挡板壁面的光滑程度对其在床层中的作用有一定影响。     

基于粗糙颗粒动理学流化床内颗粒与幂律流体两相流动特性的数值模拟研究

在颗粒动理学理论（KTGF）的基础上,通过引入表征粗糙颗粒摩擦和切向非弹性的切向弹性恢复系数β,以及综合反映颗粒平动和旋转运动脉动强度的颗粒拟总温e₀,结合输运理论建立了考虑颗粒旋转作用的颗粒相质量、动量和颗粒拟总温守恒方程。并在求解了同时具有平动和旋转运动的能量耗散和颗粒相应力等参数的前提下提出了颗粒相压力、剪切黏度和能量耗散等本构关系式以及边界条件,最终得出了粗糙颗粒动理学理论（KTRS）。通过改变液相的流变特性,分析了幂律流变模型中流动指数n和稠度系数K_l对流化床内流固两相流动特性的影响,模拟结果表明：随着流动指数和稠度系数的增大,液相湍动能耗散率逐渐增大,而颗粒相压力逐渐减小,颗粒旋转先增大后减小。     

颗粒特性对撞击分离器性能影响的实验与数值研究

颗粒-壁面撞击行为和气固相间作用对撞击分离器的性能具有重要影响。基于玻璃珠及煤粉的单颗粒撞击实验数据建立平均撞击恢复系数模型,采用非球形颗粒曳力模型对平板式撞击分离器的分离性能和气固流动开展数值研究。结果表明,采用基于实验的平均恢复系数模型以及考虑颗粒形状的曳力模型,能够准确地预测撞击式分离器的总分离效率和分级分离效率。颗粒分离过程中,Stokes数较大的颗粒对颗粒-壁面撞击模型比较敏感,Stokes数较小的颗粒对气固曳力模型比较敏感。     

非球形混合颗粒的沉降特征长度

提出以Stokes径和Newton径作为计算非球形颗粒终端速度的特征长度,推导出联用双径计算终端速度的方程,并将混合颗粒的粒径分布计入双径值中.理论推导作了实验验证.     

滑移壁面蛇形微通道两相流数值模拟

基于Fluent平台,采用CLSVOF方法对滑移壁面蛇形微通道气液两相流动进行了数值计算。计算选用的方法与理论结果具有较好的一致性,同时可以表明疏水壁面会产生滑移现象,且在高度较小的微通道内滑移效果更显著,从而减小通道内流体流动阻力,实现减阻;不同壁面性质通道内流体流动情况的计算结果表明,滑移壁面对截面速度分布趋势几乎没有影响,但上下壁面疏水性不同会影响通道截面最大速度分布。此外接触角及相对粗糙度对滑移特性影响较大,合理设计壁面润湿性及微粗糙元结构可以最大限度发挥滑移现象引起的减阻效果;与无滑移壁面相比,滑移壁面微通道内传热效果更好,且随滑移速度的增大,通道换热增强。     

水力学聚焦微通道中气液两相流动的数值模拟

采用计算流体力学方法,考察了水力学聚焦微通道中气、液两相的流动状态。气、液两相在微通道中呈膜状流动,成膜过程可划分为3个阶段,液相表面压力、黏性力和表面张力在该过程中不断变化。考察了两相入口压强对两相流量和膜厚的影响,模拟结果显示:保持两相压强比值不变,同时改变两相压强,影响不显著;改变两相压强比值,影响显著。     

新型气固环流反应器内颗粒流动的CFD模拟

采用基于结构的EMMS曳力模型,对一种新型气固环流反应器中的颗粒流动特性进行数值模拟。模拟的固含率与颗粒速率预测值与实验数据具有一致性,验证了模型的适用性。模拟结果表明：导流筒表观气速增加,导流筒中的床层固含率减小,向上的颗粒速率增加;反应器中存在多个颗粒逆流和错流混合区,促进了颗粒沿径向的混合;槽孔处,导流筒中的固含率以及颗粒速率分布更加均匀,而环隙中存在颗粒浓集区;进料区在0≤L≤0.058 m,0< r/R< 0.3的范围内固含率增加并且颗粒存在明显的径向流动。     

液固流态化动态过程中相间作用力的数值模拟及实验验证

选择恰当的相间作用力模型是液固流态化动态特性CFD建模的关键。首先采用Richardson-Zaki关联式验证了稳态操作条件下整体固含率的实验结果,然后在基于颗粒动理学理论的欧拉-欧拉双流体模型中比较了Wen-Yu、Gidaspow、Syamlal-O’Brien、Dallavalle和TGS 5个曳力计算公式对液固流化床收缩和膨胀特性的数值模拟结果,进而探讨了Moraga等提出的升力模型影响行为及主要相间作用力影响机制。与实验测量数据比较结果表明:收缩过程中Syamlal-O’Brien和TGS曳力模型对响应时间预测较为准确,TGS曳力模型对整体固含率的预测精度较高;膨胀过程中TGS曳力模型对响应时间和整体固含率的预测优于其他模型。整体而言,基于静止颗粒群绕流直接模拟得到的TGS曳力模型忽略了颗粒-颗粒相互作用,与液固散式体系中颗粒动力学特性相符合。升力模型对动态特性模拟结果影响较小,CFD模拟时根据选择体系可予以适当忽略。     

A numerical study exploring the effect of particle properties on the fluidization of adhesive particles

The effects of varying the elastic modulus, coefficient of restitution, and coefficient of friction of adhesive particles on fluidized bed dynamics have been investigated via numerical simulations. It is found that lower values of the elastic modulus and coefficient of restitution lead to a greater degree of particle clustering, and the formation of smaller bubbles. Coordination numbers are found to initially increase, and then fall, with increasing coefficient of friction, while bubble velocities follow the opposite trend. It is concluded that artificially reducing the elastic modulus of adhesive particles has a significant impact on the fluidization behavior. The change in dynamics of the fluidized bed due to varying the coefficient of friction is more complex: particle clustering increases up to a point, beyond which clusters become increasingly rigid. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1467–1477, 2016     

A study on periodic boundary condition in direct numerical simulation for gas–solid flow

Direct numerical simulation(DNS) of gas–solid flow at high resolution has been carried out by coupling the lattice Boltzmann method(LBM) for gas flow and the discrete element method(DEM) for solid particles. However,the body force periodic boundary condition(FPBC) commonly used to cut down the huge computational cost of such simulation has faced accuracy concerns. In this study, a novel two-region periodic boundary condition(TPBC) is presented to remedy this problem, with the flow driven in the region with body force and freely evolving in the other region. With simulation cases for simple circulating fluidized bed risers, the validity and advantages of TPBC are demonstrated with more reasonable heterogeneity of the particle distribution as compared to the corresponding case with FPBC.     

Influence of particle-specific gravity and particle shape on the averaged axial velocity of nearly neutrally buoyant particles in horizontal pipes

This paper presents an experimental study on the velocities of single, nearly neutrally buoyant particles in water in horizontal flows. Particle specific gravity was found to have a strong effect on the particle velocity. The modified particle Froude number was found to be an effective parameter to reflect the effect of density. A force-balance model has been developed that includes the effects of particle specific gravity and particle shape.     

Numerical simulation on fluidization characteristics of tobacco particles in fluidized bed dryers

Three-dimensional simulations have been carried out to examine the gas–particle flow behavior of tobacco material in a fluidized bed dryer. The Euler–Euler model has been used to study the distribution of particles in the fluidized bed dryer. The simulation results indicate that tobacco particles usually concentrate in the near-wall region, and there exists a maximum particle concentration in the feed pipe. The predictions on the regions with high concentration of particles in the fluidized bed dryer agree well with the experimental findings. Moreover, this kind of dynamic particle aggregation might lead to particle clusters, and investigations of the particle motion and mixing behavior in the simulated systems indicate that there are particle clusters during fluidization. The diverse nature of clusters enriches the flow behaviors of particles and consequently leads to the macro-scale heterogeneity featuring fast fluidization: dilute at the top and dense at the bottom in the axial direction as well as the core–annulus structure in the radial direction. Therefore, the particle clusters is one of the key problems in drying processes, which must be known for understanding the material distribution inside the dryer, as well as for the system design of fluidized bed dryers. According to the results, some improvements on the fluidized bed dryer have been brought out and the relative numerical experiments have been performed. The numerical experiments show that the improvements can realize better uniformity and lead to a decrease in the particle concentration, which provides useful ways to solve the clustering problem.     

提升管内细长颗粒流化运动的气固多向耦合模型的构建

细长颗粒气固两相流研究已成为气固两相流研究的重点问题之一。而各种细长颗粒在流场中的受力、运动模型构建以及细长颗粒/流场间的耦合关系的构建是细长颗粒两相流数值模拟研究的难点之一。在已经基于刚体动力学原理构建了细长粒子气固两相流单向耦合模型的基础上,结合基于κ-ε模型的球形粒子-流场气固间耦合关联式,并改进细长颗粒间碰撞模型,从而构建了细长颗粒气固两相流动多向耦合数值模拟平台。并且采用此平台对某一实际流化床内的细长粒子气固两相流场进行了数值模拟研究。研究表明,不同时间细长颗粒在流场内的停留位置具有较强的随机性;细长颗粒在流化运动过程中伴随有较明显的取向选择性;细长颗粒的存在会导致当地流场的速度、压强均有较为明显的下降。     

Study of wall boundary condition in numerical simulations of bubbling fluidized beds

The influence of the solid-phase wall boundary condition was investigated in Eulerian-Eulerian numerical simulations of a bubbling fluidized bed. Parametric studies of the particle-wall restitution coefficient and specularity coefficient were performed to evaluate their impact on the predicted flow hydrodynamics in terms of bed expansion, local voidage, and solid velocity. Both two- and three-dimensional simulations were conducted and compared with available experimental data on solid velocity and bubble properties. It is found that the wall effect plays an important role in CFD models. Such factors as the voidage at the bubble boundary, averaging method, and minimum bubble size also influence the mean bubble diameter.