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。     

垂直并流向上气固两相流中流动结构的分析及曳力系数的计算

This study investigates the heterogeneous structure and its influence on drag coefficient for concurrent up gas-solid flow.The energy-minimization multi-scale (EMMS) model is modified to simulate the variation of structure parameters with solids concentration,showing the tendency for particles to aggregated to form clusters and for fluid to pass around clusters.The global drag coefficient is resolved into that for the dense phase,for the dilutephase and for the so-called inter-phase,all of which can be obtained from their respective phase-specific structure parameters.The computational results show that the drag coefficients of the different phases are quite different,and the global drag coefficient calculated from the EMMS approach is much lower than that from the correlation of Wen and Yu.The simulation results demonstrate that the EMMS approach can well describe the heterogeneous flow structure,and is very promising for incorporation into the two-fluid model or the discrete particle model as the closure law for drag coefficient.     

双流体模型中曳力及恢复系数对气固流动的影响

应用双流体模型CFD模拟的方法,从恢复系数和曳力两方面,研究了气固密相流化床中颗粒之间和气固相之间的相互作用对床内非均匀流动结构形成与变化的影响.计算结果表明颗粒间非弹性碰撞和气固间曳力的增大均使气固两相流动的非均匀性增大.通过比较二者对非均匀流动结构的影响,发现气固间曳力是形成非均匀流动结构的决定因素.从碰撞耗散、颗粒动能和颗粒势能的角度分析了二者的作用机理,发现恢复系数和曳力对流动结构的作用主要区别在于对颗粒团聚和床层膨胀的影响程度不同.     

Numerical simulation of flow behavior of particles and clusters in riser using two granular temperatures

Cluster in CFB riser significantly affects performance of circulating fluidized beds. To model hydrodynamic behavior in CFB risers, three phase flows were assumed in the riser, the gas phase, the dispersed particle phase, and the clusters phase. The gas-solid multi-fluid model is extended to give the macroscopic averaged equations with constitutive equations for both particle phases from kinetic theory of granular flow. The clusters and the dispersed particles have their own fluctuating energy or two individual granular temperatures. Interactions between the cluster and its surrounding dispersed particles were obtained from kinetic theory of granular flow. Drag force for gas to dispersed particles and the clusters are empirically determined. The momentum exchange between dispersed particles and clusters is modeled using the concept of molecular dynamics. Cluster properties are predicted with the cluster-based approach. The distributions of volume fractions and velocities of gas, dispersed particles and clusters are predicted. Computed solid mass fluxes and volume fractions agree with Manyele et al. [S.V. Manyele, J.H. Parssinen, J.X. Zhu, Characterizing particle aggregates in a high-density and high-flux CFB riser, Chemical Engineering Journal, 88 (2002) 151-161.] and Knowlton [T.M. Knowlton, Modelling benchmark exercise. Workshop at the Eighth Engineering Foundation Conference on Fluidization, Tours, France, 1995.] experimental data.     

拟泡-乳曳力模型在大型高温费托流化床反应器中的CFD模拟研究

以带冷却盘管的大型高温费托流化床反应器为研究对象,开展三维计算流体力学模拟研究。传统双流体模型基于局部平均的假设,认为单位控制体内气固两相均匀分布,网格尺寸必须足够小才能正确揭示局部非均匀结构的所有细节。采用双流体模型模拟大型工业化流化床装置时,将导致网格数量过于庞大,远超现有计算能力。为提高计算效率的同时不损失模拟精度,提出了基于局部非均匀假设、适用于粗网格的拟泡-乳三相非均匀曳力(PBTD)模型。该模型将流化床分为乳化相气体、乳化相颗粒以及气泡三相,分别建立守恒方程,体现气泡的非均匀特性对气固曳力的影响。乳化相内气固曳力以及气泡相与乳化相内颗粒的曳力分开考虑。采用PBTD模型耦合传质和反应模型,建立基于局部非均匀假设的高温费托合成反应器三维流动-传递-反应模型,包括各相守恒控制方程、气泡尺寸模型、相间物质和动量交换模型、高温费托合成反应动力学模型以及初始和边界条件,预测反应器内的流场和组分浓度分布。研究结果表明:在粗网格条件下,非均匀曳力模型可以预测床层内相含率的分布情况,预测的床层膨胀高度与经验公式计算值接近,偏差为1.2%。反应器出口气体组分的质量分数与试验测量值相近,偏差在1.5%~16.0%。模拟结果证实,基于非均匀假设的PBTD模型适用于模拟工业规模的鼓泡流化床反应器,对其设计开发和工业运行具有指导价值。     

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.     

确定性颗粒轨道模型在流化床模拟中的研究进展

随着计算机硬件的发展,基于颗粒尺度模拟稠密气固两相流的颗粒轨道模型发展很快.本文回顾了目前流化床模拟中确定性颗粒轨道模型的研究状况,概述了处理颗粒碰撞的硬球模型、软球模型以及DSMC方法在国内外流化床模拟中的研究成果,评述了颗粒间相互作用处理方法的优缺点以及适用的系统等,指出了确定性颗粒轨道模型发展亟待解决的问题,展望了确定性颗粒轨道模型未来的发展趋势.     

一个气固两相流动阻力的新模型

为了解决现有经验气固阻力模型的普适性问题,合理描述颗粒团聚现象对气固阻力的严重影响,从理论分析入手,将传统的CFD方法与系统能量分析方法相结合,建立了计及颗粒团聚效应的气固阻力分析模型.与现有模型相比,新模型不仅合理地描述了气固两相相互作用的物理过程,而且避免了以往采用经验系数所导致的误差和局限性.经循环流化床数值模拟的检验证明,新模型的计算结果与实验数据吻合,从而在稠密气固两相流动的数值模拟中具有相当的优越性.     

Numerical predictions of flow behavior and cluster size of particles in riser with particle rotation model and cluster-based approach

Flow behavior of particles in a circulating fluidized bed (CFB) riser is numerically simulated using a two-fluid model incorporating with the kinetic theory for particle rotation and friction stress models. The particle rotations resulting from slightly friction particle-particle collisions was considered by introducing an effective coefficient of restitution based on the kinetic theory for granular flow derived by Jenkins and Zhang [2002. Kinetic theory for identical, frictional, nearly elastic spheres. Physics of Fluids 14, 1228-1235]. The normal friction stress model proposed by Johnson et al. [1990. Frictional-collisional equations of motion for particles flows and their application to chutes. Journal of Fluid Mechanics 210, 501-535] and a modified frictional shear viscosity model proposed by Syamlal et al. [1993. MFIX Documentation and Theory Guide, DOE/METC94/1004, NTIS/DE94000087] were used as the particle frictional stress model. The drag force between gas and particle phases was modified with cluster-based approach (CBA). The flow behavior of particles and the cluster size in a riser of Wei et al. [1998. Profiles of particle velocity and solids fraction in a high-density riser. Powder Technology 100, 183-189] and Issangya et al. [2000. Further measurements of flow dynamics in a high-density circulating fluidized bed riser. Powder Technology 111, 104-113] experiments are predicted. Effects of the rotation and friction stress models on the computed results are analyzed. It is concluded that particle rotations reduce the cluster size and alter the particle flows and distributions through more particle fluctuation energy dissipations. Effects of frictional stress on flow behavior and cluster size are not significant because the particle phase in the CFB riser is not dense enough to take into account for the particle-particle contact interactions.     

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。     

Numerical and experimental study on the effect of solid particle sphericity on cyclone pressure drop

The continuous flow inside cyclone separator is usually simulated by solving the Reynolds averaged Navier–Stokes equations in Eulerian reference frame whereas the dispersed phase is modeled using Lagrangian approach. Although these methods have had a remarkable success, more advanced ideas are needed to model particulate phase in cyclones, especially the non-spherical shaped particles. Numerical simulation is verified with experimental results for the gas-solid flow in a cyclone separator. Reynolds Averaged Navier–Stokes equations (RANS) employing the RNG-based k–ε turbulence model are used to simulate the gas phase. 3-D particle tracking procedure is used for the solid phase. Three different equations for the drag coefficient are utilized in the numerical modeling to acquire more understanding of the behavior of non-spherical particles in cyclones. Computations resulted in the difference of pressure between the inlet and exit of the cyclone, and results are compared with experimental data. Experiments included measuring the separation efficiency of different shapes and sizes of particles. The results indicate that the CFD simulation can effectively reveal the pressure drop behavior as well as separation efficiency of gas-non-spherical particle flow in cyclone.     

Simulation of particles and gas flow behavior in a riser using a filtered two-fluid model

Flow behavior of gas and particles is predicted by a filtered two-fluid model by taking into the effect of particle clustering on the interphase momentum-transfer account. The filtered gas–solid two-fluid model is proposed on the basis of the kinetic theory of granular flow. The subgrid closures for the solid pressure and drag coefficient (Andrews et al., 2005) and the solid viscosity (Riber et al., 2009) are used in the filtered two-fluid model. The model predicts the heterogeneous particle flow structure, and the distributions of gas and particle velocities and turbulent intensities. Simulated solids concentration and mass fluxes are in agreement with experimental data. Predicted effective solid phase viscosity and pressure increase with the increase of model constant c_g and c_s. At the low concentration of particles, simulations indicate that the anisotropy is obvious in the riser. Simulations show the subgrid closures for viscosity of gas phase and solid phase led to a qualitative change in the simulation results.     

Practical validation of the two-fluid model applied to dense gas-solid flows in fluidized beds

This paper discusses the simulation of bubbling gas-solid flows by using the Eulerian two-fluid approach. Predictions of particle motion, bed expansion, bubble size and bubble velocity in bubbling beds containing Geldart B particles are compared with experimental results and correlations found in the literature. In addition, gas mixing in a bed of Geldart A particles is investigated.An in-house code has been developed based on the finite-volume method and the time-splitting approach using a staggered grid arrangement. The velocities in both phases are obtained by solving the 2D Reynolds-averaged Navier/Stokes equations using a partial elimination algorithm (PEA) and a coupled solver. The k-ε turbulence model is used to describe the turbulent quantities in the continuous phase.In general, the model predictions are in good agreement with experimental data found in the literature. Most important observations are: the level of the restitution coefficient was found to be crucial in order to obtain successful results from 2D axisymmetric simulations of a system containing Geldart B particles. Bubble size and bubble rise velocities are not as sensitive to the restitution coefficient. The turbulence model is of outmost importance concerning gas mixing in a fluidized bed of Geldart A particles.From these numerical analyzes an optimized granular flow two-fluid model can be designed for the purpose of simulating reactive systems in fluidized bed reactors.     

Global modelling of a gas-liquid-solid airlift reactor

This paper presents a global model of three phase flow (gas-liquid-solid) in an internal airlift reactor. The airlift is composed of four zones: a riser (on the aerated side on the internal wall), a downcomer (on the opposite side) and two turning zones above and below the internal wall. Tap water is the liquid continuous phase and the dispersed phases are air bubbles and polyethylene particles. The global modelling of the airlift involves mass and momentum equations for the three phases. The model enables phase velocities and phase volume fractions to be estimated, which can be compared to experimental data. Closure relations for the gas and solid drift velocities are based on the model proposed by Zuber and Findlay. The drift flux coefficients are derived from CFD numerical simulations of the airlift. Gas bubble and solid particle averaged slip velocities are deduced from momentum balances, including drag coefficient correlations. The link between Zuber and Findlay model and the two-fluid model is established. In the experiment as well as in the model, the gas flow rate is fixed. However, the liquid and solid flow rates are unknown. Two closure relations are needed to predict these flow rates: the first closure relation expresses that the volume of solid injected into the airlift remains constant; the second closure relation expresses a global balance between the difference of column height in the riser and the downcomer and the total pressure drop in the airlift. The main parameters of a three phase airlift reactor, like gas and solid volume fractions, are well predicted by the global model. With increasing solid filling rate (40%), the model starts to depart from the experimental values as soon as coalescence of bubbles appears.     

非均匀气固流态化系统中颗粒流体相间作用的计算

曳力系数是双流体模型模拟气固两相流动的关键参数.文献中应用的关联式都基于平均方法,不再适用于模拟快速流态化系统的非均匀流动结构.本文试图阐明非均匀结构对曳力系数的影响,应用改进的能量最小多尺度模型提出一种计算微元体曳力系数的新方法.计算结果表明应用该模型计算出的曳力系数远小于基于平均方法关联式的计算结果,符合实验得出的结论.     

粗糙颗粒碰撞参数对鼓泡流化床内气固流动模拟的影响

结合粗糙颗粒动力学理论和双流体方法,数值模拟了碰撞参数对鼓泡流化床内稠密气固两相流动特性的影响. 结果表明,增大摩擦系数或减小法向弹性恢复系数会使床内颗粒分布更为不均,并增强床层膨胀及压力降脉动. 合理选取摩擦系数模拟得到时均气固流场分布,与实验吻合,罂粟籽颗粒的摩擦系数取0.3~0.6较合适. 法向弹性恢复系数改变不影响时均气固流场分布的基本形态,其取值敏感性不如摩擦系数;切向弹性恢复系数对鼓泡流化床动态特性及时均气固流场的影响相对较弱.     

Modelling of dispersed bubble and droplet flow at high phase fractions

The present paper describes an Eulerian two-fluid model for the prediction of dispersed two-phase (gas/liquid and liquid/liquid) flow at high volume fractions of the dispersed phase. The model is based on the standard Eulerian approaches for modelling two-phase flow that have hitherto been limited in validity to dilute systems. An extension to high phase fractions is made here and this involves two aspects. First, the closure models for inter-phase forces (namely drag and lift) are modified to account for the high concentration of the dispersed phase. Second, a turbulence model based on the k-ε equations for the mixture of the two phases is formulated. This turbulence model is suitable for computations at all phase fraction values and reduces to the equivalent single phase model in the extremes when only one or other of the phases is present. The model uses a response function to link the turbulent fluctuations of the continuous and dispersed phases. The variation of this response function with phase fraction is determined from experimental evidence made available recently. The overall model is applied to the prediction of air/water bubble flow in a pipe with a sudden enlargement where phase fractions can reach 25% and for which experimental data exist. The calculations show that marked improvement in the quality of the predictions, as compared to measurements, is obtained over the available model for dilute systems.     

The important role of local dispersed phase hold-ups for the calculation of three-phase bubble columns

A computational fluid dynamics model is used to calculate a three-phase (air-water-solid particles) flow in a bubble column. The calculation of multi-phase flows is significantly influenced by the formulation of the inter-phase drag and the modelling of the turbulence. Both are influenced by the dispersed phases. The k-ε turbulence model extended with terms accounting for the bubble-induced turbulence is used to calculate the eddy viscosity of the liquid phase. Bubble-bubble and particle-particle interactions are considered as well as a direct momentum transfer between the two dispersed phases bubbles and solid particles. The local volume fractions of the dispersed phases are considered for the calculation of the drag coefficients between the dispersed phases and the continuous phase. Measured local gas and solid hold-ups as well as measured liquid velocities are compared with the corresponding calculated results. The measured and the calculated results show good agreement.     

不同曳力模型对鼓泡床内气固两相流的模拟研究

基于双流体模型,应用Fluent商业软件包对带有单喷嘴的二维鼓泡床中气固两相流进行模拟研究。采用三种基于不同机理的曳力模型;半经验的Gidaspow模型、由格子波尔兹曼方法导出的Koch-Hill模型与修正的半经验的McKeen模型,通过模拟气泡的形成、上升及破裂过程,气泡形状和颗粒运动特征,计算气体泄漏率、气泡直径及气泡上升速度,对不同曳力模型进行比较研究,其中编程实现了Koch-Hill和McKeen曳力模型模块。通过与文献中的实验结果进行对比发现,Gidaspow模型对气泡形状的模拟效果较好,与实验数据的误差介于其他两种模型之间;Koch-Hill模型捕捉到的气泡特征最逼真,且床层膨胀效果明显,但定量计算的误差最大;而McKeen模型与实验数据的误差最小,但对气泡形状的模拟效果较差。