颗粒旋转对鼓泡流化床内气固两相流动特性影响的数值模拟

运用考虑颗粒自旋转流动对颗粒碰撞能量交换和耗散影响的颗粒动理学方法,建立鼓泡流化床气固两相Euler-Euler双流体模型,数值模拟流化床内气体颗粒两相流动特性。分析表明,颗粒平动温度与旋转温度之比是法向和切向颗粒弹性恢复系数和摩擦系数的函数。与不考虑颗粒旋转效应计算结果相比,考虑颗粒旋转效应后床内较容易形成气泡,颗粒自旋转运动将导致床内非均匀结构更明显。并且床层平均空隙率和床层膨胀高度增加,床中心区域颗粒轴向速度提高,床内颗粒平动温度下降。考虑颗粒旋转效应后预测的颗粒轴向速度和颗粒脉动速度与文献实验结果基本吻合。考虑颗粒旋转效应后获得的气泡直径更接近于前人经验关联式。     

稠密气固两相流动的颗粒二阶矩方法及鼓泡床流化特性的模拟

采用颗粒动理学方法,考虑颗粒速度脉动各向异性,建立颗粒相二阶矩模型。应用初等输运理论,对三阶关联项进行模化和封闭。考虑颗粒与壁面之间的能量传递和交换,建立颗粒相边界条件模型。数值模拟鼓泡流化床内气固两相流动特性,模拟结果表明鼓泡流化床内颗粒相湍流脉动具有明显的各向异性。预测颗粒速度与Muller等和Yuu等实测结果相吻合。预测颗粒脉动速度二阶矩与Muller等实验结果变化趋势相同。统计得到的固相雷诺应力型二阶矩与Muller等实测颗粒脉动速度二阶矩和Yuu等实测颗粒脉动速度相吻合。     

Hydrodynamics of binary fluidization in a riser: CFD simulation using two granular temperatures

A new computational fluid-dynamic (CFD) model with a separate granular temperature (2/3 random particle kinetic energy per unit of mass) equation for each phase or particle size was developed using constitutive equations derived earlier by Huilin, Gidaspow and Manger. In agreement with the experiment and model of Mathiesen, Solberg and Hjertager the new model computes the observed core-annular flow regime. It predicts the trends of the observed radial and axial particle diameter distributions. For elastic particles the computed particle velocity distributions are parabolic. They are close to the laminar type approximate analytical solution for flow in a pipe, where the mean velocity equals the inlet flux divided by the particle density and volume fraction. The computed turbulent intensity is lower for large particles than for small particles, as measured. This is in agreement with an approximate analytical solution for the granular temperature in the developed flow region of a riser for elastic particles. Computations show that for sufficiently inelastic particles the granular temperature in the center can be lower than near the wall resembling the measured particle fluctuating velocity distribution.     

Flow behaviors of a large spout-fluid bed at high pressure and temperature by 3D simulation with kinetic theory of granular flow

Flow behaviors of a large spout-fluid bed (I.D. 1.0 m) at high pressure and temperature were investigated by Eulerian simulation. The gas phase was modeled with k − ε turbulent model and the particle phase was modeled with kinetic theory of granular flow. The development of an internal jet, gas-solid flow patterns, particle concentrations, particle velocities and jet penetration depths at high pressure and temperature at different operating conditions were simulated. The results show that the bed operated at an initial bed height larger than the maximum spoutable bed height resembles the flow patterns of jetting fluidized beds. The radial profiles of particle velocities and concentrations at high temperature and pressure have the similar characteristic shapes to those at ambient pressure and temperature. The particle concentrations and velocities appear to depend on the bed heights when increasing pressure while keeping the gas velocities and temperature constant. The particle velocities in the lower region of the bed increase with increasing pressure, while they tend to decrease in the middle and upper regions of the bed. The particle concentrations have an opposite dependency with increasing pressure. They decrease in the lower region of the bed but increase in the middle and upper regions of the bed. Besides, the jet penetration depths are found to increase with increasing pressure.     

大涡模拟方法模拟鼓泡床内气固两相流动

采用大涡模拟（LES）方法模拟气相湍流,颗粒动理学方法考虑颗粒相碰撞产生的动量和能量传递和耗散,采用颗粒相大涡模拟方法（LESp）模拟颗粒脉动导致的能量耗散,同时考虑介观尺度对颗粒相压力的影响,建立了气体-颗粒LES-θ-LESp双流体模型,研究鼓泡流化床内气固两相流动的特性。数值模拟与文献实测颗粒速度和实测颗粒浓度结果具有相同的变化趋势。     

Prediction of particle motion in a two-dimensional bubbling fluidized bed using discrete hard-sphere model

The solids motion in a gas-solid fluidized bed was investigated using a discrete hard-sphere model. Detailed collision between particles and a nearest list method are presented. The turbulent viscosity of gas phase was predicted by subgrid scale (SGS) model. The interaction between gas and particles phases was governed by Newton's third law. The distributions of concentration, velocity and granular temperature of particles are obtained. The radial distribution function is calculated from the simulated spatio-temporal particle distribution. The normal and shear stresses of particles are predicted from the simulated instantaneous particle velocity. The pressure and viscosity of particles are obtained from both the kinetic theory of granular flow and the calculated stresses of particles. For elastic particles the individual lateral and vertical particle velocity distribution functions are isotropic and Maxwellian. The observed anisotropy becomes more pronounced with increasing degree of inelasticity of the particles.     

Hydrodynamic modelling of dense gas-fluidised beds: Comparison of the kinetic theory of granular flow with 3D hard-sphere discrete particle simulations

A novel technique to sample particle velocity distributions and collision characteristics from dynamic discrete particle simulations of intrinsically unsteady, non-homogeneous systems, such as those encountered in dense gas-fluidised beds, is presented. The results are compared to the isotropic Maxwellian particle velocity distribution and the impact velocity distribution that constitute the zeroth-order Enskog approximation for the kinetic theory of granular flow. Excellent agreement with the kinetic theory is obtained for elastic particles. The individual particle velocity distribution function is isotropic and Maxwellian. A good fit of the collision velocity distribution and frequency is obtained, using the radial distribution function proposed by Carnahan and Starling (J. Chem. Phys. 51 (1969) 635). However, for inelastic and rough particles an anisotropic Maxwellian velocity distribution is obtained. It is concluded that the formation of dense particle clusters disturbs spatial homogeneity and results in collisional anisotropy. Analysis of the impact velocity shows that, in dense gas-fluidised beds, not all impact angles are of equal likelihood. The observed anisotropy becomes more pronounced with increasing degree of inelasticity of the particles.     

Fluidization characteristics of rice husk in a bubbling fluidized bed

An investigation was carried out in a circulating fluidized bed to explore if rice husk could be fluidized without mixing it with any foreign solids. Experimental results indicate that bed cross‐section has an important influence on the fluidization characteristics of rice husk. Larger the cross‐section, easier it is to fluidize the husk without slugging. Particle size, sphericity, bulk density, and transport velocity of husk were measured. The riser was operated under bubbling bed regimes. Efforts were made to measure the minimum fluidization velocity by varying the bed depth and cross‐section.     

The effect of distributor design on gas dispersion in a bubbling fluidized bed

In this study, the behavior of gas dispersion in a bubbling fluidized bed was investigated. Carbon dioxide was used as the tracer gas. Most of the gas jets from tuyeres are towards the same direction, parallel with the longitudinal axis. The movement of particles in the lateral direction was enhanced by the momentum of horizontal gas jets within the bed.The experimental results show that the effect of superficial gas velocity on the gas mixing depends on the distributor type. Comparing with perforated distributor, a better performance of gas mixing was observed while the bed was equipped with horizontal nozzle distributor.     

Extension of the kinetic theory of granular flow to include dense quasi-static stresses

Fluidized bed technology has diverse industrial applications ranging from the gasification of coal in the power industry to chemical reactions for the plastic industry. Due to their complex chaotic non-linear behaviour understanding the hydrodynamic behaviour in fluidized beds is often limited to pressure drop measurements and a mass balance of the system. Computational fluid dynamics has the capability to model multiphase flows and can assist in understanding gas-solid fluidized beds by modeling their hydrodynamics. The multiphase Eulerian-Eulerian gas-solid model, extended and validated here improves on the kinetic theory of granular flow by including a closure term for the quasi-static stress associated with the long term particle contact at high solid concentrations. Similar quasi-static models have been widely applied to slow granular flow such as chute flow, flow down an incline plane and geophysical flow. However combining the kinetic theory of granular flow and the quasi-static stress model for the application of fluidized beds is limited. The objective of the present paper is to compare two quasi-static stress models to the experimental fluidized bed data of Bouillard et al. [4]. A quasi-static granular flow model (QSGF) initially developed by Gray and Stiles [18] is compared to the commonly used Srivastava and Sundaresan [37]. Both models show good agreement with the experimental bubble diameter and averaged porosity profiles. However only the QSGF model shows a distinct asymmetry in the bubble shape which was documented by Bouillard et al. [4].     

基于EMMS介尺度模型的双分散鼓泡流化床的模拟

双分散气固鼓泡流化床中颗粒通常具有不同粒径或密度,导致产生颗粒偏析等现象,影响传递和反应行为。颗粒分离和混合与气泡运动密不可分,其中相间曳力起关键作用。最近Ahmad等提出了一种基于气泡结构的双分散介尺度曳力模型,能成功预测双分散鼓泡流化床的床层膨胀系数。本研究耦合该曳力模型与连续介质方法,模拟了两种不同的双分散鼓泡流化床,通过分析不同流化状态下的气泡运动、颗粒浓度比的轴向分布等参数,进一步检验模型的适用性。研究表明,当双分散颗粒处于完全流化状态时,耦合双分散介尺度曳力模型可合理预测不同颗粒的分离现象;而其处于过渡流化状态时,新曳力模型和传统模型均无法获得合理结果,此时调节固固曳力可改进模拟结果。     

Transient multi gas—solid reactions in a bubbling fluidized bed

A “multimodel” for gas‐solid reactions in a reacting particle has been applied to a bubbling fluidized bed reactor. The particle is tracked and bed and particle variables are determined continuously. The conservation equations of mass and heat with auxiliary relations are solved in an accelerating particle, which may rise or fall. The effects of bulk pressure, velocity and temperature, and particle diameter are studied. Heat and mass transfer coefficients may fluctuate up to 75% and 148% respectively. Doubling the pressure changes h_c by 75% and k_c by ?45%. Increase in pellet diameter reduces both h_c and k_c.     

塔式鼓泡流化床内的涌渗流动特性

为强化气固接触,提出了一种新型塔式鼓泡循环床反应器,采用多个带有风帽的中间分布板、沿床层高度方向将反应器分隔成若干腔室,流化过程中形成一种特殊的涌渗（gushing）现象;依据连续拍照所得图像,基于快速傅里叶变换和小波包变换的方法,对床内压力脉动信号进行分析,研究涌渗产生和消亡条件,频率和能量大小,以期掌握涌渗的形成规律。结果表明,此反应器内流化数为3.47时涌渗产生明显,周期性寿命为1~2 s,频率分布于0.1~0.5 Hz区域;保持流化风速不变,改变反应器结构,加入一层中间分布板构成双腔室、调整下腔室高径比为3：1以及适当增大风帽开孔率时,涌渗主频明显,能量适中,涌渗效果得到优化。因此,塔式鼓泡流化床内的涌渗流动特性取决于流化风速以及塔式鼓泡床的几何结构。     

Numerical study of biomass gasification combined with CO2 absorption in a bubbling fluidized bed

Biomass gasification combined with CO₂ absorption-enhanced reforming (AER) in a bubbling fluidized bed (BFB) reactor is numerically studied via the multiphase particle-in-cell (MP-PIC) method featuring thermochemical and polydispersity sub-models. A novel bubble detection algorithm is proposed for efficiently characterizing bubble morphology. The effects of several crucial operating parameters on the microscale particle behaviors, mesoscale bubble dynamics, and macroscale reactor performance of the AER gasification process are analyzed. Compared with conventional gasification, AER gasification reduces the CO₂ concentration by 33.58% but elevates the H₂ concentration by 32.13%. Higher operating temperature and steam-to-biomass (S/B) ratio promote H₂ generation but deteriorate gasification performance. A lower operating pressure improves gas–solid contact efficiency and gasification performance as the increased operating pressure inhibits bubble dynamics and particle kinematics. Compared with pure sand as bed material, the mixed bed material (CaO:sand = 1:1) significantly improves gasification performance by enhancing H₂ generation and CO₂ removal.     

Numerical simulations of flow behavior of gas and particles in spouted beds using frictional-kinetic stresses model

Flow behavior of gas and particles is simulated in the spouted beds using a Eulerian-Eulerian two-fluid model on the basis of kinetic theory of granular flow. The kinetic-frictional constitutive model for dense assemblies of solids is incorporated. The kinetic stress is modeled using the kinetic theory of granular flow, while the friction stress is from the combination of the normal frictional stress model proposed by Johnson and Jackson (1987) and the frictional shear viscosity model proposed by Schaeffer (1987) to account for strain rate fluctuations and slow relaxation of the assembly to the yield surface. An inverse tangent function is used to provide a smooth transitioning from the plastic and viscous regimes. The distributions of concentration, velocity and granular temperature of particles are obtained in the spouted bed. Calculated particle velocities and concentrations in spouted beds are in agreement with the experimental data obtained by He et al. (1994a, b). Simulated results indicate that flow behavior of particles is affected by the concentration of the transition point in spouted beds.     

Numerical simulation of gas-particle flow with a second-order moment method in bubbling fluidized beds

Flow behavior of gas and particles is performed by means of gas-solid two-fluid model with the second-order moment model of particles in the bubbling fluidized bed. The distributions of velocity and moments of particles are predicted in the bubbling fluidized beds. Predictions are compared with experimental data measured by Jung et al. (2005) in a bubbling fluidized bed and Patil et al. (2005) experiments in a bubbling fluidized bed with a jet. The simulated second-order moment in the vertical direction is on average 1.5-2.3 times larger than that in the lateral direction in the bubbling fluidized bed (Jung et al., 2005). For a bubbling fluidized bed with a jet, the ratio of normal second-order moment in the vertical direction to in the lateral direction is in the range of 0.5-2.5 (Patil et al., 2005). The bubblelike Reynolds normal stresses per unit bulk density used by Gidaspow et al. (2002) are computed from the simulated hydrodynamic velocities. The simulated bubblelike Reynolds normal stresses in the vertical direction is on average 4.5-6.0 times larger than that in the lateral direction in the bubbling fluidized bed (Jung et al., 2005). The predictions are in agreement with experimental second-order moments measured by Jung et al. (2005) and porosity measured by Patil et al. (2005).     

Hydrodynamic modeling of a circulating fluidized bed

Hydrodynamics plays a crucial role in defining the performance of circulating fluidized beds (CFB). The numerical simulation of CFBs is very important in the prediction of its flow behavior. From this point of view, in the present study a dynamic two dimensional model is developed considering the hydrodynamic behavior of CFB. In the modeling, the CFB riser is analyzed in two regions: The bottom zone in turbulent fluidization regime is modeled in detail as two-phase flow which is subdivided into a solid-free bubble phase and a solid-laden emulsion phase. In the upper zone core-annulus solids flow structure is established. Simulation model takes into account the axial and radial distribution of voidage, velocity and pressure drop for gas and solid phase, and solids volume fraction and particle size distribution for solid phase. The model results are compared with and validated against atmospheric cold bed CFB units' experimental data given in the literature for axial and radial distribution of void fraction, solids volume fraction and particle velocity, total pressure drop along the bed height and radial solids flux. Ranges of experimental data used in comparisons are as follows: bed diameter from 0.05-0.418 m, bed height from 5-18 m, mean particle diameter from 67-520 μm, particle density from 1398 to 2620 kg/m³, mass fluxes from 21.3 to 300 kg/m²s and gas superficial velocities from 2.52-9.1 m/s.As a result of sensitivity analysis, the variation in mean particle diameter and superficial velocity, does affect the pressure especially in the core region and it does not affect considerably the pressure in the annulus region. Radial pressure profile is getting flatter in the core region as the mean particle diameter increases. Similar results can be obtained for lower superficial velocities. It has also been found that the contribution to the total pressure drop by gas and solids friction components is negligibly small when compared to the acceleration and solids hydrodynamic head components. At the bottom of the riser, in the core region the acceleration component of the pressure drop in total pressure drop changes from 0.65% to 0.28% from the riser center to the core-annulus interface, respectively; within the annulus region the acceleration component in total pressure drop changes from 0.22% to 0.11% radially from the core-annulus interface to the riser wall. On the other hand, the acceleration component weakens as it moves upwards in the riser decreasing to 1% in both regions at the top of the riser which is an important indicator of the fact that hydrodynamic head of solids is the most important factor in the total pressure drop.     

CFD simulation of the high shear mixing process using kinetic theory of granular flow and frictional stress models

In order to enhance process understanding and to develop predictive process models in high shear granulation, there is an ongoing search for simulation tools and experimental methods to model and measure the velocity and shear fields in the mixer. In this study, the Eulerian-Eulerian approach to model multiphase flows has been used to simulate the mixer flow. Experimental velocity profiles for the solid phase at the wall in the mixer have been obtained using a high speed camera following the experimental procedure as described by Darelius et al. [2007a. Measurement of the velocity field and frictional properties of wet masses in a high shear mixer. Chemical Engineering Science, 62, 2366-2374]. The governing equations for modelling the dense mixer flow have been closed by using closure relations from the kinetic theory of granular flow (KTGF) combined with frictional stress models. The free slip and partial slip boundary conditions for the solid phase velocity at the vessel wall have been utilized. The partial slip model originally developed for dilute flows by Tu and Fletcher [1995. Numerical computation of turbulent gas-solid particle flow in a 90^° bend. A.I.Ch.E. Journal, 41, 2187-2197] has been employed. It was found that the bed height could be well predicted by implementing the partial slip model, whereas the free slip model could not capture the experimentally found bed height satisfactorily. In the simulation, the swirling motion of the rotating torus formed was over-predicted and the tangential wall velocity was under-predicted, probably due to the fact that the frictional stress model needs to be further developed, e.g. to tackle cohesive particles in dense flow. The advantage of using the Eulerian-Eulerian approach compared to discrete element methods is that there is no computational limitation on the number of particles being modelled, and thus manufacturing scale granulators can be modelled as well.     

流化床技术制备粒状多晶硅的研究进展

概述流化床技术制备粒状多晶硅的研究进展,简介粒状多晶硅的生长机制和物理性能,阐述流化床技术在多晶硅生产领域的发展方向。     

Bubble dynamics in bubbling fluidized beds of ellipsoidal particles

This article presents a CFD-DEM study on the effect of particle shape on bubble dynamics in bubbling fluidized beds. The particles used are ellipsoids, covering from disk-type to cylinder-type. The phenomena such as bubble coalescence and splitting are successfully generated, and the results are compared with literature, showing a good agreement. The results demonstrate that the bubble forming/rising regions and patterns are influenced significantly by particle shape. Ellipsoidal particles have asymmetrical bubble patterns with two or more circulation vortices while the bubbles for spherical particles form at the bed centerline and rise through the center of the bed. Hence, the vertical mass flux at the bed centerline for spheres is always positive, and ellipsoids have negative or positive vertical mass fluxes. The solid mixing estimated based on the dispersion coefficient revealed poor mixing for ellipsoids. Spherical particles have a larger bubble size and higher bubble rising velocity than ellipsoids.