颗粒聚团结构对曳力特性影响的数值模拟

黏性颗粒多以聚团形式存在于气固两相系统中，流体施加于聚团的曳力对两相流动及传热传质起着至关重要的作用，而聚团的不规则、分形结构增加了曳力特性的复杂性。基于黏性离散单元方法生成不同分形结构的聚团，利用计算流体力学方法（CFD）直接求解分形聚团内部多孔结构的气流流动，得到了气体流过聚团时的周围与内部流场，研究了低Reynolds数（Re=0.1～10）条件下聚团结构特征对曳力的影响。结果表明：聚团的疏密程度显著影响聚团整体流场分布，多孔疏松结构增强了聚团的渗透性，使其与流体接触面积增加，所受曳力增加。分析不同结构聚团的曳力系数发现：除了聚团孔隙率、分形维数等结构参数的影响，气体流经聚团的方向也影响聚团曳力系数。在此基础上，综合考虑聚团分形维数、聚团与气流的夹角方向、Reynolds数拟合得到聚团曳力系数关联式。     

基于格子Boltzmann方法的单颗粒绕流数值模拟

采用格子Boltzmann方法（LBM）研究了单颗粒绕流流动过程。通过使用LBM中的LBGK（lattice Bhatnagar-Gross-Krook）模型和二阶精度的曲线边界条件处理方法,实现了对单颗粒绕流问题的定常及非定常流动过程中涡结构的模拟。采用动量交换法分别计算了Reynolds数在0.1～200范围内27个不同Reynolds数时的曳力系数,并将计算结果拟合得到基于LBM数值模拟的曳力曲线。计算结果表明,LBM在气固两相流的模拟计算中具有精确、可靠的优点,使用LBM模拟计算曳力曲线的方法经济、易行,并且可以克服由传统实验方法获得曳力曲线的局限性。     

低雷诺数下气固两相中浓稀相界面对微尺度曳力的影响及建模

采用具有二阶精度的浸没边界-格子玻尔兹曼法对低雷诺数下流体流经不同颗粒聚团结构的过程进行了解析到颗粒表面的直接数值模拟(PR-DNS)。结果显示在颗粒聚团表面的浓稀相界面处,传统的微观均匀BVK曳力模型[AIChE Journal, 2007, 53(2): 489-501]的预测结果与PR-DNS结果有明显的差别;同时文献中所构建的考虑界面影响的微观曳力模型(Int. J. Multiphase Flow, 2020, 128: 103266)也无法准确预测稀相固含率不为0的情况。因此,本工作提出了一种将界面附近网格分解求取曳力的方法。通过与不同颗粒聚团结构的PR-DNS结果及其他曳力模型预测结果对比发现,新模型不但在稀相固含率趋近于0时与文献中模型具有相近的预测能力,且在稀相固含率不为0时,具有明显优于文献模型的相关系数及拟合优度。     

循环流化床中颗粒聚团特性的模拟

考虑到循环流化床中分散颗粒和颗粒聚团同时存在的多尺度结构,确定了密相和稀相加速度与计算网格局部参数之间的关系,建立了多尺度曳力消耗能量最小的稳定性条件,基于双变量极值理论,构建了考虑颗粒团聚效应的多尺度气固相间曳力模型。结合双流体模型,对循环流化床内气固流动特性以及颗粒聚团特性进行了模拟研究。通过与实验值比较,考虑颗粒聚团影响的计算模型可以更好地贴近实验结果,颗粒聚团直径随颗粒浓度增大呈现先增大后减小的分布趋势,气体和颗粒的加速度在模拟中与重力加速度同处一个数量级,求解过程中不能被忽略。     

循环流化床中颗粒聚团特性的模拟

考虑到循环流化床中分散颗粒和颗粒聚团同时存在的多尺度结构,确定了密相和稀相加速度与计算网格局部参数之间的关系,建立了多尺度曳力消耗能量最小的稳定性条件,基于双变量极值理论,构建了考虑颗粒团聚效应的多尺度气固相间曳力模型。结合双流体模型,对循环流化床内气固流动特性以及颗粒聚团特性进行了模拟研究。通过与实验值比较,考虑颗粒聚团影响的计算模型可以更好地贴近实验结果,颗粒聚团直径随颗粒浓度增大呈现先增大后减小的分布趋势,气体和颗粒的加速度在模拟中与重力加速度同处一个数量级,求解过程中不能被忽略。     

纳米颗粒聚团流化特性的数值模拟

基于气固两相流体动力学和流态化理论,建立气体纳米颗粒气固两相双流体模型.模型中采用了Jung&Gidaspow[1]测量的固相应力模量,并应用了王垚,金涌,魏飞[2]提出的聚团曳力系数计算模型.对纳米颗粒聚团的流化过程进行了数值模拟,得到纳米颗粒的流化特性.模拟得出的床层膨胀比与文献中实验的结果较为接近.     

颗粒振动影响动量传递过程的格子Boltzmann方法模拟

流化床内部颗粒振动对传递过程有着重要影响。格子Boltzmann方法耦合改进的浸入运动边界法模拟了不同振幅比A/D和频率比k=f_e/f₀下的单颗粒振动情况,并研究了不同排布和间距的双颗粒振动对传递过程中升阻力系数以及涡脱落频率的影响。结果表明,颗粒Reynolds数Re=100,单颗粒横向振动时,振幅增大导致锁定区间变大,颗粒锁定区间内的曳力系数大于锁定区间外,有利于传递。单颗粒流向振动,A/D=1.50时,随振动频率增大,流体流动模式为：2S模式→2P模式→2P+2S模式→混沌。相同振幅下k <1.25时,颗粒横向振动的曳力系数大于流向振动的曳力系数;k>1.25时则与之相反。因此,当k <1.25时,横向振动更有利于传递;k>1.25时,流向振动更有利于传递。串联双颗粒相互抑制涡的形成使曳力系数减小,不利于传递;相反,并联双颗粒促进传递作用,且在间距H=3D时效果最佳。以上数值结果为强化传递过程提供了一种思路。     

气固两相流动中非球形颗粒所受曳力的数值研究

采用数值计算的方法研究了非球形颗粒在气固两相流动中所受的曳力,在Re＜100的条件下对流经3种具有典型几何特征的非球形颗粒（正方体、圆柱体和圆台体）的流体流动进行了详细的数值计算.通过比较计算结果与文献中所推荐的3种曳力系数修正关系发现：Ganser方法与本文计算结果符合最好.通过对两种不同颗粒摆放条件的流动进行计算发现：对于不同的颗粒与来流角度,非球形颗粒所受到的曳力具有较大的差别.     

颗粒流体两相流基本方程中压差力和曳力系数的计算

本文总结了颗粒流体两相流基本方程的压差力表达方式和推导颗粒流体系统曳力系数计算公式的力平衡原理，提出了一种分析压差力的简单方法，阐明了各种压差力表达方式间的关系及对应的两相流基本方程的曳力系数的计算方法     

浆态床内固含率轴向分布的数值模拟

采用DBS曳力模型计算气液相间作用,分别采用Gidaspow曳力模型、经Brucato修正的Gidaspow曳力模型和Schiller?Naumann曳力模型计算液固相间作用,忽略气固间的直接作用,对比了浆态床内不同颗粒粒径体系轴向固含率的模拟和实验结果. 结果表明,不同液固相间曳力模型对气含率的预测影响不大;在颗粒粒径较大(140 ?m)的体系中,较低表观气速下气液DBS与液固Schiller?Naumann曳力模型组合模拟的固含率随床高度增加而减小,与实验结果吻合,而其它曳力模型组合的模拟结果较差,轴向分布较均匀;在颗粒粒径较小(35 ?m)的体系中,几种曳力模型组合的模拟结果均与实验结果吻合较好,轴向分布较均匀.     

Drag force in discrete particle models—Continuum scale or single particle scale?

Unresolved discrete particle (DP) models, where a Lagrangian method is used for the solid phase, and a Eulerian method for the fluid phase, have become increasingly popular. The fluctuations of the drag force on individual particles in a homogeneous random array obtained from fully resolved simulations using a lattice Boltzmann method were analysed; such fluctuations are by construction ignored in the unresolved DP model. The drag on individual particles in the array can differ up to 40% with the drag that would be used in DP type simulations was found. A detailed analysis shows that the drag on an individual particle depends strongly on all its surrounding neighbors within a distance of at least two particle diameters. As in DP models, the local flow field is unresolved, the conclusion is that this root mean square (RMS) deviation in the drag force is inherent to the model. © 2012 American Institute of Chemical Engineers AIChE J, 59: 316–324, 2013     

Numerical study of cluster and particle rebound effects in a circulating fluidised bed

Gas-particle flows in a vertical two-dimensional configuration appropriate for circulating fluidised bed applications were investigated numerically. In the computational study presented herein the motion of particles was calculated based on a Lagrangian approach and particles were assumed to interact through binary, instantaneous, non-frontal, inelastic collisions including friction. The model for the interstitial gas phase is based on the Navier-Stokes equations for two-phase flows. The numerical study of cluster structures has been validated with experimental results from literature in a previous investigation. Numerical experiments were performed in order to study the effects of different cluster and particle rebound characteristics on the gas-particle flow behaviour.Firstly, we investigated the hard sphere collision model and its effect on gas-particle flow behaviour. The coefficient of restitution in an impact depends not only on the material properties of the colliding objects, but also on their relative impact velocity. We compared the effect of a variable restitution coefficient, dependent on the relative impact velocity, with the classical approach, which supposes the coefficient of restitution to be constant and independent of the relative impact velocity.Secondly, we studied the effects of different cluster properties on the gas-particle flow behaviour. Opposing clustering effects have been observed for different particle concentrations: within a range of low concentrations, groups of particles fall faster than individual particles due to cluster formation, and within a well-defined higher concentration range, return flow predominates and hindered settling characterises the suspension. We propose herein a drag law, which takes into account both opposing effects and have compared the resulting flow behaviour with that predicted by a classical drag law, which takes into account only the hindered settling effect.     

Cluster-based drag coefficient model for simulating gas-solid flow in a fast-fluidized bed

Drag coefficient is of essential importance for simulation of heterogeneous gas-solid flows in fast-fluidized beds, which is greatly affected by their clustering nature. In this paper, a cluster-based drag coefficient model is developed using a hydrodynamic equivalent cluster diameter for calculating Reynolds number of the particle phase. Numerical simulation is carried out in a gas-solid fast-fluidized bed with an Eulerian-Lagrangian approach and the gaseous turbulent flow is simulated using large eddy simulation (LES). A Lagrange approach is used to predict the properties of particle phase from the equation of motion. The collisions between particles are taken into account by means of direct simulation Monte Carlo (DSMC) method. Compared with the drag coefficient model proposed by Wen and Yu, results predicted by the cluster-based drag coefficient model are in good agreement with experimental results, indicating that the cluster-based drag coefficient model is suitable to describe various statuses in fast-fluidized beds.     

基于LBM-DEM的鼓泡床内气泡-颗粒动力学数值模拟

将修正后的格子Boltzmann方法（LBM）与离散单元法（DEM）相结合,建立LBM-DEM四向耦合模型对单口射流鼓泡床中气泡运动进行模拟。其中,流体相采用格子Boltzmann方法中经典的D2Q9模型,颗粒相求解采用离散单元软球模型,颗粒曳力求解采用Gidaspow模型,流固耦合基于牛顿第三定律。应用Fortran语言编程对上述模型进行求解,模拟得到了鼓泡床内气泡演化过程,并与相关实验进行对比,有效验证了当前模型的准确性。同时,分析了床层内颗粒速度、颗粒体积分数以及能量分布。结果表明：颗粒时均速度分布不仅能体现颗粒运动强弱,也可以反映气泡运动过程;床内空隙率与颗粒体积分数分布在预测床层膨胀高度上具有高度的一致性;初始堆积效应使得床内颗粒势能始终大于颗粒动能;随颗粒密度增加,势能增大,动能逐渐减小。     

绕流具有光合生化反应管束的格子Boltzmann模拟

采用格子Boltzmann方法模拟了有机废水溶液绕流表面附着有光合生物膜管束的流动、传质及光合生化反应过程,并分析了管束排列,管间距及Reynolds数对流场、浓度场、平均阻力系数及底物的Sherwood数的影响。结果表明：溶液绕流小管间距时具有较高的Sherwood数,同时阻力系数也较高;与顺排管束相比,绕流叉排管束的平均Sherwood数增大了一倍左右,其增大幅度随Re的增大而增大,而阻力系数仅增大了约10%。一定条件下,绕流叉排管束时,出口处具有较低的底物浓度和较高的产物浓度。结果表明,很小的流场扰动可对浓度场产生较大影响,从而,叉排管束更有利于传输传质及生物降解。     

Investigation of the accuracy of the extrapolation method for the lattice Boltzmann simulation of viscous fluid flow in a Maxblend impeller system

This paper offers a thorough assessment on the performance of the extrapolation method for the lattice Boltzmann simulation of viscous mixing flows. This method appears to be well-suited for the treatment of the complex boundary conditions found in various mixing systems. Here, the ability to simulate accurate power consumption and pumping capacity is evaluated on several configurations of the Maxblend mixing system, which has proven efficient in a wide range of applications. First, the impact of the boundary conditions on the spatial convergence of the lattice Boltzmann method (LBM) is determined on the 3D Couette flow, clearly showing that small modifications of the boundary conditions may reduce the accuracy of the predicted shear rate and power. Second, a parallel LBM scheme was used to simulate fluid flow within a Maxblend mixing system. For the unbaffled configuration, the simulated power consumption and the pumping capacity are observed to be in good agreement with experimental data and finite element simulation results. The effect of the bottom clearance is also successfully evaluated, suggesting that the standard bottom clearance is not optimum in the transitional regime. Lastly, results for the most geometrically complex case (baffled configuration) indicate that the power consumption is affected by numerical perturbations appearing around the moving impeller. Overall, these results show that, when combined with the extrapolation method for the treatment of boundary conditions, the LBM is an efficient tool for the investigation of viscous flow in mixers of industrial relevance.     

基于格子Boltzmann方法模拟纳米流体强化传质过程

使用Boltzmann方法对纳米流体中的传质过程进行了模拟,给出了修改后的Boltzmann方程和计算纳米流体中扩散系数的方程,通过两种方法,基于有限体积颗粒的LBE方法和基于点源颗粒的LBE方法进行了模拟,并与宣益民的实验进行了对比。最后计算了CO2在纳米流体中的扩散系数,计算结果表明,纳米流体因为其纳米颗粒的微扰动对传质有着很大的强化效果,为CO2吸收提供了一种新思路,并且证明了纳米流体的强化传质主要是靠对流传质。     

A modified pseudopotential for a lattice Boltzmann simulation of bubbly flow

The pseudopotential in Shan and Chen-type multiphase models was investigated and modified based on a virial equation of state with newly proposed parameters. This modified pseudopotential was used in a lattice Boltzmann model and shown to be suitable for simulating sufficiently large gas–liquid density ratios with good numerical stability and only small spurious velocities. The spurious velocity was reduced by reducing the pseudo-sound speed by the use of suitable parameters. The multicomponent multiphase model based on this modified pseudopotential can be used in bubbly flow simulations. Bubble rise behavior was simulated using a 3D multicomponent and multiphase model with a high density ratio. The predicted terminal velocity and drag coefficient of a single bubble agreed well with those calculated from empirical correlations. The drag coefficient of bubbles in the homogenous regime decreased with increased gas holdup. A new relationship between the bubble drag coefficient and gas holdup in the homogenous regime was proposed.