单颗粒和颗粒群研究的挑战

Numerical simulation of multiphase flows in processing equipment in industry with two-fluid models and Eulerian-Lagrangian approaches requires the constitutive equations describing the interactions between the dispersed phase of high concentration and the continuous phase. The status of research on the forces on dispersed solid and fluid particles is reviewed in this article. As compared with the knowledge on drag of single solid particles, study on particle swarms and on other forces is not sufficient to meet the demand of reliable and efficient numerical simulation of multiphase flows. Thus, thorough study on the particle swarms becomes the key to accurate multi-scale simulation of multiphase flows. Besides, the development of efficient algorithm dealing with the non-uniformity on both equipment and mesoscopic scales is recognized as an important issue to be resolved. The research topics in the near future are suggested.     

气固并流向上流动中颗粒聚集的机理

1 INTRODUCTION Cocurrent upward gas-solid flows are widely used in industrial situations such as circulating fluid- ized bed, fluid catalytic cracking and pneumatic transport. Due to its industrial relevance, many quan- titative experimental and numerical investigations have been carried out[1―4]. Investigations show that upward gas-solid flow always appears heterogenously structured and the distribution of particles is always non-uniform unless the solid phase is very dense or very dilut…     

颗粒尾涡增强湍流的大涡模拟以及气固两相流中湍流变动的数值模拟

The turbulence enhancement by particle wake effect is studied by large eddy simulation （LES） of turbulent gas flows passing a single particle. The predicted time-averaged and root-mean-square fluctuation velocities behind the particle are in agreement with the Reynolds-averaged Navier-Stokes modeling results and experimental results. A semi-empirical turbulence enhancement model is proposed by the present-authors based on the LES resuits. This model is incorporated into the second-order moment two-phase turbulence model for simulating vertical gas-particle pipe flows and horizontal gas-particle channel flows. The simulation results show that compared with the model not accounting for the particle wake effect, the present model gives simulation results for the gas turbulence modulation in much better agreement with the experimental results.     

垂直管中零净液流量气液两相流的持液率研究

Zero net-liquid flow (ZNLF) is a special case of upward gas-liquid two-phase flow. It is a phenomenon observed as a gas-liquid mixture flows in a conduit but the net liquid flow rate is zero. Investigation on the liquid holdup of ZNLF is conducted in a vertical ten-meter tube with diameter of 76 mm, both for Newtonian and nonNewtonian fluids. The gas phase is air. The Newtonian fluid is water and the non-Newtonian fluids are water-based guar gel solutions. The correlations developed for predicting liquid holdup on the basis of Lockhart-Martinelli parameter are not suitable to ZNLF. A constitutive correlation for the liquid holdup of vertical ZNLF was put forward by using the mass balance. It is found that the liquid holdup in ZNLF is dependent on both the gas flow rate and the flow distribution coefficient.     

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.     

以新型渐缩式流道增进质子交换膜燃料电池性能

Based on use of multi-dimensional models, this investigation simulates the performance of a proton exchange membrane fuel cell by varying the channel pattern. In the one-dimensional model, the porosity of the gas diffusion layer is 0.3. The model reveals the water vapor distribution of the fuel cell and demonstrates that the amount of water vapor increases linearly with the reduction reaction adjacent to the gas channel and the gas diffusion layer. Secondly, four novel tapered gas channels are simulated by a two-dimensional model. The model considers the distributions of oxygen, the pressure drop, the amount of water vapor distribution and the polarization curves. The results indicate that as the channel depth decreases, the oxygen in the tapered gas channel can be accelerated and forced into the gas diffusion layer to improve the cell performance. The three-dimensional model is employed to simulate the phenomenon associated with four novel tapered gas channels. The results also show that the best performance is realized in the least tapered gas channel. Finally, an experimentally determined mechanism is found to be consistent with the results of the simulation.     

考虑颗粒凝并情况下充分发展圆管湍流场中的纳米颗粒的迁移(英文)

Numerical simulations of nanoparticle migration in a fully developed turbulent pipe flow are performed.The evolution of particle number concentration,total particle mass,polydispersity,particle diameter and geometric standard deviation is obtained by using a moment method to approximate the particle general dynamic equation.The effects of Schmidt number and Damkhler number on the evolution of the particle parameters are analyzed.The results show that nanoparticles move to the pipe center.The particle number concentration and total particle mass are distributed non-uniformly along the radial direction.In an initially monodisperse particle field,the particle clusters with various sizes will be produced because of coagulation.As time progresses,the particle cluster diameter grows from an initial value at different rates depending on the radial position.The largest particle clusters are found in the pipe center.The particle cluster number concentration and total particle mass decrease with the increase of Schmidt number in the region near the pipe center,and the particles with lower Schmidt number are of many dif-ferent sizes,i.e.more polydispersity.The particle cluster diameter and geometric standard deviation increase with the increase of Damkhler number at the same radial position.The migration properties for nano-sized particles are different from that for micro-sized particles.     

MOLECULAR DYNAMICS SIMULATIONS OF FILLED AND EMPTY CAGE-LIKE WATER CLUSTERS IN LIQUID WATER AND THEIR SIGNIFICANCE TO GAS HYDRATE FORMATION MECHANISMS

Molecular dynamics simulations are performed to observe the evolutions of 5~(12) and 5~(12)6~2 cage-like water clusters filled with or without a methane molecule immersed in bulk liquid water at 250 K and 230 K. The lifetimes of these clusters are calculated according to their Lindemann index δ (t) using the criteria of δ≥0.07. For both the filled and empty clusters, we find the dynamics of bulk water determines the lifetimes of cage-like water clusters, and that the lifetime of 5~(12) 6~2 cage-like cluster is the same as that of 5~(12) cage-like cluster. Although the methane molecule indeed makes the filled cage-like cluster more stable than the empty one, the empty cage-like cluster still has chance to be long-lived compared with the filled clusters. These observations support the labile cluster hypothesis on the formation mechanisms of gas hydrates.     

垂直稀相气力输送实验研究

An experimental study of vertical gas conveying of Geldart-D powder as a dilute phase is performed in a pipe of length 22m and internal diameter 0.05m using a fluidized blow tank at gas velocity ranging from 5m.S-1 to 13m.s-1 and loading ratio up to about 30. The characteristics of gas conveying, such as pressure drop, the choking velocity and the minimum primary velocity of the fluidized blow tank, are discussed in detail.     

三维混合层中大涡结构与扩散颗粒的相互作用

In order to understand the interaction between large-scale vortex structure and particles, a two-way coupling temporal mixing layer laden with particles at a Stokes number of 5 with different mass loading planted initially in the upper half region is numerically studied. The pseudospectral method is used for the flow fluid and the Lagrangian approach is employed to trace particles. The momentum coupling effect introduced by a particle is approximated to a point force. The simulation results show that the coherent structures are still dominant in the mixing layer, but the large-scale vortex structure and particle dispersion are modulated. The length of large-scale vortex structure is shortened and the pairing is delayed. At the same time, the particles are distributed more evenly in the whole flow field as the mass loading is increased, but the particle dispersion along the transverse direction differs from that along the spanwise direction, which indicates that the effect by the addition of particle on the suanwise large-scale vortex structure is different from the streamwise counterpart.     

下行床气粒流动行为的Eulerian-Lagrangian模拟

采用计算流体力学和离散单元方法耦合模型（CFD-DEM）对二维下行床内的气粒流动行为进行了全床数值模拟。模拟结果展示了下行床典型操作条件下特有的气固动态流动结构：沿流动方向存在明显的入口控制区、过渡区和（完全）发展区;颗粒聚团并不是出现在浓度相对较高的入口区,而是在过渡区之后的发展过程中逐渐形成较多的、松散的动态聚团结构。下行床发展段呈现典型的近壁浓环结构,这与实验结果基本一致。考察了颗粒之间以及颗粒与壁面之间的碰撞参数对下行床内气固流动结构的影响,发现完全弹性碰撞颗粒体系在入口区呈现最快速的颗粒分散;而对本文研究的操作条件,颗粒碰撞参数对发展段时均流体力学行为只产生轻微的影响。     

Modeling of cluster structure-dependent drag with Eulerian approach for circulating fluidized beds

Flow behavior of gas and solids is simulated in combination the gas-solid two-fluid model with a cluster structure-dependent (CSD) drag coefficient model. The dispersed phase is modeled by a Eulerian approach based upon the kinetic theory of granular flow (KTGF) including models for describing the dispersed phase interactions with the continuous phase. The drag forces of gas-solid phases are predicted from the local structure parameters of the dense and dilute phases based on the minimization of the energy consumed by heterogeneous drag. The cluster structure-dependent (CSD) drag coefficients are incorporated into the two-fluid model to simulate flow behavior of gas and particles in a riser. Simulation results indicate that the dynamic formation and dissolution of clusters can be captured with the cluster structure-dependent drag coefficient model. Simulated solid velocity and concentration of particles profiles are in reasonable agreement with experimental results.     

Numerical simulation of hydrodynamics in downers using a CFD-DEM coupled approach

The gas-solid flows in a two-dimensional downer of 10 m in height and 0.10 m in width were simulated using a CFD-DEM method, where the motion of particles was modeled by discrete element method (DEM) and the gas flow was described by Navier-Stokes equations. The simulations revealed a rich variety of developing flow structures in the downer under different operating conditions. The two-phase flow development can be clearly characterized by the micro-scale particle distributions in the downer. Near the inlet, the particle distribution is dominated by the distributor design. Then, the particles disperse in the column, forming a homogeneous transit region. After that clusters start to form and modulate the gas-solid flow field till the fully-developed state. The particle-scale simulation disclosed that the clusters are composed of loosely collected particles, and these particles have the same flow direction as the bulk flow so that no particle backmixing can be observed. As the particles in the downer have the tendency to maintain the inertia, the capability of lateral transfer of particles is relatively weak, which was illustrated by tracking the movement of the single particles and clusters. The simulations of the inlet effect on the hydrodynamics in the downer showed that the gas-solid flow structure and the mixing behavior are sensitive to the inlet design. An inappropriate design or operation would probably cause the undesired flow phenomena such as the wide distribution of residence times. The time-averaged hydrodynamics based on the transient simulations showed good agreement with the experimental findings in the literature. The simulation based on the CFD-DEM coupled approach provides a theoretical way to comprehensively understand the physics at micro- to macro-scales in the co-currently downward gas-solid flows.     

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.     

鼓泡流化床内高灰煤气化过程的颗粒尺度特性研究

基于多相流体质点网格方法(MP-PIC)对高灰煤在三维鼓泡流化床气化过程进行了数值模拟研究。在欧拉-拉格朗日框架下将气相和固相分别视作连续介质和离散相处理。首先,将模拟得到的出口处气体组分结果与实验数据进行对比,实验数据与模拟结果具有良好的一致性。其次,研究了煤颗粒在气化炉内的温度、传热系数、速度和停留时间,从颗粒尺度揭示了鼓泡流化床气化炉内的颗粒分布特性和气固流动特征。结果表明:在气化炉入口附近煤颗粒与床层温差最大,传热系数最大;由于流化床内强非线性的气固流动,床中煤温度和传热系数的空间分布不均匀;煤颗粒和床料的瞬时速度具有稳定的波动幅度,其中垂直方向速度波动最明显,且煤颗粒的瞬时速度比床料的瞬时速度略大;由于颗粒间的剧烈碰撞,延长了煤颗粒停留时间。此外,对鼓泡流化床中煤气化过程颗粒尺度的研究,有助于深入了解固体颗粒的流动行为以及气固相相互作用特性,对鼓泡流化床反应器的设计优化具有重要意义。     

A model for cluster size in risers

Particles in gas-solid flows aggregate to move as “clusters” and simultaneously develop gas “voids” free of particles. A model for gas-solid flow in risers considering the presence of clusters and voids is presented. Equations are developed to estimate the size of clusters considering “ratio of cluster volume to void volume to be equal to the ratio of cluster volumetric fraction to void volumetric fraction in the bed”. Estimates of cluster size compare well with the experimental observations reported in the literature. Equations developed for slip ratios and gas to particle effective drag coefficients are inline with the trends reported in the literature.     

基于重置温度方法的双参数介尺度气固传热模型构建

颗粒聚团等介尺度结构在气固两相流中普遍存在,这些介尺度非均匀结构直接影响气固流动特性及气固接触效率,进而影响气固相间传热、传质及化学反应过程。在更适合工业大尺度气固传热模拟的粗网格方法中缺乏准确度高、简单易用且可以考虑介尺度非均匀结构影响的传热模型。采用计算流体力学-离散单元法（CFD-DEM）研究了气固两相流相间传热,为了保证气固相间持续传热,采用了两种维持气固相间传热温差的方法,并讨论了两种方法的优缺点。方法一：给气相能量方程添加热源项;方法二：每间隔一段时间重置气相温度,重置温度后气固两相自由传热,两种方法中均保持固相温度不变。结果表明聚团界面位置的局部单位体积气固传热量最大,重置温度方法在稀相和界面位置的局部单位体积传热量与总单位体积传热量之比大于热源项方法,而在浓相位置的局部单位体积传热量与总单位体积传热量之比小于热源项方法。通过过滤CFD-DEM计算数据,为重置温度方法构建了双参数（过滤固含率、过滤尺度）传热系数修正因子模型,通过先验分析评价了模型的表现,研究表明所构建模型在过滤网格尺度为5~40倍颗粒直径范围内优于文献中已有的双参数模型。     

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.     

FCC提升管反应器中颗粒聚团对裂化反应的影响

在对气固流动体系颗粒聚团实验现象分析的基础上,以减压馏分油裂化反应为例,对反应器中最常见的球形和椭球形聚团上的流动、传热和反应过程进行了模拟计算,得到了聚团内外油气和催化剂颗粒的速度分布、温度分布、浓度分布以及反应速率分布。研究结果表明,催化剂颗粒聚团的存在阻碍了油气与催化剂颗粒的充分接触,进而造成系统内速度和温度的不均匀分布,影响了裂化反应的发生,使得有颗粒聚团时的一次反应速率明显低于无聚团时的反应速率,颗粒聚团显著影响了油气在颗粒上的反应时间,最终导致气体和焦炭产率升高,对裂化反应产品收率分布十分不利。