旋流气固分离器内气粒两相运动特性及分离效率

基于旋转气固多相流复杂的运动特征,结合旋流气固分离器的特点,同时考虑颗粒间的碰撞与并聚和喷水加湿对颗粒间并聚及碰撞、颗粒运动特性的影响,并考虑颗粒对气相的作用构建了描述旋流气固分离器内湍流气固多相流的三维时均方程组.模型封闭采用κ-ε/ RNG模型,数值模拟了旋流气固分离器内气固两相流场、固相运动特性及分离效率,提出了合理的旋流气固分离器分离效率计算关联式.     

考虑颗粒旋转的颗粒动力学模拟提升管气固两相流动特性

流化床内颗粒旋转会影响颗粒相的流动特性,目前在流化床数值模拟中普遍采用的颗粒动力学模型却没有考虑颗粒的旋转效应。今运用基于颗粒动力学理论的欧拉-欧拉气固多相流模型,考虑颗粒旋转流动对颗粒碰撞能量交换和耗散的影响,提出了考虑颗粒旋转效应的颗粒动力学模型以及颗粒相守恒方程,数值模拟提升管内气体颗粒两相流动特性。计算结果表明提升管内中心区域为低浓度-高速的颗粒上升流动、壁面区域为高浓度-低速的颗粒下降流动。分析了颗粒粗糙度系数对颗粒相能量耗散、颗粒平动温度和黏度的影响。随着颗粒粗糙度系数的增加,颗粒碰撞能量耗散先逐渐增加后减小。颗粒平动温度和黏度的变化趋势是相反的,表明颗粒旋转产生摩擦将导致颗粒旋转脉动能量的改变,影响提升管内气体-颗粒两相宏观流动特性。     

旋转床超重力环境下多相流传递过程研究进展

旋转填充床作为一种高效的传质、分离与反应设备,在化工、环境保护、纳米材料制备、能源、制药等工业过程得到广泛应用。本文对旋转填充床超重力环境下,流体力学特性、传质性能、微观混合、多尺度传递特性等方面的研究进行了总结归纳。近年来,随着计算机科学与多相流传递过程的研究进展,对传递过程的研究也由实验手段为主转变为实验与数值模拟相结合的手段,对有关的数值模拟研究以及相应的多相流模型也予以总结描述。在此基础上,对旋转填充床超重力环境下多相流研究的未来发展提出了有关设想。     

搅拌釜内流场实验研究与数值模拟的进展

概述了搅拌釜内流场实验研究与数值模拟的进展,介绍了速度场、混合时间、循环流量和温度场的各种测量方法,同时就搅拌釜内流场数值模拟中涉及的各种湍流模型以及旋转桨叶的处理方法进行了对比分析。最后指出传热特性、多相流以及多参数耦合数学模型是搅拌釜研究的重点。     

DSMC方法在大规模气固两相撞击流中的应用

将直接模拟Monte Carlo（DSMC）方法应用于颗粒数目庞大的大规模气固两相撞击流的数值模拟研究,旨在解决基于拉格朗日法模型难以模拟含大量颗粒碰撞的多相流问题,建立了气固两相撞击流的数理模型。应用所建立模型计算分析了撞击流中的气相流动、颗粒运动、颗粒及颗粒碰撞位置分布;并对模型中考虑颗粒碰撞和不考虑颗粒碰撞时,计算获得的颗粒运动行为、停留时间以及对气相的影响等结果进行了对比分析。结果表明：经发展,DSMC方法能够有效地应用于大规模气固两相撞击流的数值研究;颗粒运动区域可分为颗粒碰撞区、颗粒射流区和颗粒发散区;颗粒碰撞主要发生在颗粒碰撞区内,使得颗粒在该区域富集,且明显缩短颗粒在撞击区的停留时间;在所研究的较小固气比条件下,颗粒的存在对气相流动的影响不显著。     

颗粒群研究：多相流多尺度数值模拟的基础

用两流体模型及颗粒轨道模型数值模拟过程工程设备中的多相流,都需要表达浓度较高的分散相和连续相间的作用力的本构方程. 本工作综述了多相流中颗粒受力研究的现状和尚待解决的问题. 对单个颗粒(包括流体颗粒)在静止和运动流体中的受力情况的研究结果比较丰富,但对颗粒群的研究则很不充分. 对颗粒受的曳力研究较多,但还缺乏可信的计算升力、非稳态力等的公式. 因此颗粒群的研究成为多相流准确多尺度数值模拟的关键. 开发能处理整体和局部均存在非均匀性的多相流的高效算法也是有待克服的困难.     

旋转盘反应器流场特性的PIV测试和CFD模拟

通过PIV实验和CFD数值模拟研究了旋转盘表面的流动特性和变化规律。基于VOF多相流模型和Realizablek-ε湍流模型,CFD模拟结果与实验结果形成对比,考察了不同转速和流量条件下旋转盘表面的液膜厚度分布、停留时间和分布情况。结果表明:盘的中心区域受流速控制,流动方向更接近径向流动,边缘区域受转速控制,流动方向更接近圆周流动。液膜厚度随径向半径的增大先增大后减小,转速的增加导致边缘区膜厚减小。流速的增加导致中心区域的膜厚度减小和边缘区域的膜厚度增加。平均停留时间随转速和流速的增加而降低。停留时间分布表明,在实验条件下,液体流动更接近活塞流动。     

旋风分离器气相旋转流流场动态特性的研究进展

旋风分离器的气相旋转流场对颗粒的分离过程有重要影响。本文指出实验测量和数值模拟表明这种流场具有很强的动态特性,表现为速度和压力随时间的低频高幅脉动变化。但以往的研究主要关注流场的稳态时均特性,缺少对流场动态特性的研究。旋风分离器内流场动态特性主要产生于旋转流的旋转中心围绕着几何中心作随机摆动,由此造成了流动参数的脉动和湍流强度的急剧增大,也导致了对时均流场中一些现象的分析不清晰。此外,由于各种气固分离模型没有考虑流场的动态效应造成了计算结果不够准确。文章指出目前这种流场动态特性主要是实验测量分析,数值模拟方法尚难以准确描述,还需在计算模型上改进。开展流场动态特性的研究对开发高效低阻旋风分离器和改进其分离性能是非常必要的。     

撞击流中颗粒旋转特性

搭建了研究撞击流中颗粒旋转特性的气固两相撞击流实验台,使用高速摄像机拍摄一个截面为0.15 m×0.08 m的撞击区域内固体颗粒的运动。利用所搭建的实验台设置了单喷口和双喷口两种实验方式来研究颗粒旋转影响因素,得出撞击流内颗粒的旋转特性。结果表明:固体颗粒在气相中运动过程一直伴随着其自身的旋转;气相场对颗粒转速的影响较小,可忽略不计;相同实验条件下,颗粒直径越小其转速越大;颗粒以及气相速度越大,则固体颗粒在碰撞后的转速越大,当加速气相速度为25 m·s^-1,氧化铝陶瓷直径为0.003 m时,颗粒碰撞前后转速差平均值可达280 r·s^-1;颗粒间碰撞过程中,颗粒相对运动偏置角度对转速变化影响很小。     

撞击流中颗粒旋转特性

搭建了研究撞击流中颗粒旋转特性的气固两相撞击流实验台,使用高速摄像机拍摄一个截面为0.15 m×0.08m的撞击区域内固体颗粒的运动。利用所搭建的实验台设置了单喷口和双喷口两种实验方式来研究颗粒旋转影响因素,得出撞击流内颗粒的旋转特性。结果表明:固体颗粒在气相中运动过程一直伴随着其自身的旋转;气相场对颗粒转速的影响较小,可忽略不计;相同实验条件下,颗粒直径越小其转速越大;颗粒以及气相速度越大,则固体颗粒在碰撞后的转速越大,当加速气相速度为25 m·s-1,氧化铝陶瓷直径为0.003 m时,颗粒碰撞前后转速差平均值可达280 r·s-1;颗粒间碰撞过程中,颗粒相对运动偏置角度对转速变化影响很小。     

搅拌釜内固-液悬浮体系的CFD-DEM模拟(英文)

Computational fluid dynamics-discrete element method （CFD-DEM） coupled approach was employed to simulate the solid suspension behavior in a Rushton stirred tank with consideration of transitional and rotational motions of millions of particles with complex interactions with liquid and the rotating impeller. The simulations were satisfactorily validated with experimental data in literature in terms of measured particle velocities in the tank. Influences of operating conditions and physical properties of particles （i.e., particle diameter and density） on the two-phase flow field in the stirred tank involving particle distribution, particle velocity and vortex were studied. The wide distribution of particle angular velocity ranging from 0 to 105 r·min-1 is revealed. The Magnus force is comparable to the drag force during the particle movement in the tank. The strong particle rotation will generate extra shear force on the particles so that the particle morphology may be affected, especially in the bio-/polymer-product related processes. It can be concluded that the CFD-DEM coupled approach provides a theoretical way to under-stand the physics of particle movement in micro-to macro-scales in the solid suspension of a stirred tank.     

Analysis of gas-particle flow characteristics in impinging streams

A three dimensional Euler–Lagrange model for the gas-particle two-phase impinging streams (GPIS) is developed based on the direct simulation Monte Carlo (DSMC) method with consideration of particle rotation and collision. The gas-particle flow characteristics involved in GPIS as well as the effects of inlet gas velocity and particle rotation are analyzed. The results indicate that two pairs of counter-rotating gas vortices are developed at two sides of the opposite jet flows, which is able to entrain the particles and thus greatly weaken the deposition of particles. Interparticle collisions in the impingement zone produce two effects on the particle behaviors: the direct escaping of particles from impingement zone and the progressive accumulation of particles in impingement zone. Under the same inlet particle mass flow rate, the particle concentration in the impingement zone decreases with increasing inlet velocity of gas due to the increasing impinging reaction of interparticle collisions and growing entrainment of gas vortices. In addition, the rotation of particle provides an additional driving force to push the particles away from the impingement zone, leading to the higher speed of escaping particles and smaller maximum particle concentration at the center of impingement zone than those without particle rotation.     

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.     

DEM simulation of gas-solid flow behaviors in spout-fluid bed

Three-dimensional gas and particle turbulent motions in a rectangular spout-fluid bed were simulated. The particle motion was modeled by discrete element method and the gas motion was modeled by k-ε two-equation turbulent model. Shear induced Saffman lift force, rotation induced Magnus lift force as well as drag force, contract force and gravitational force acting on individual particles were considered when establishing the mathematics models. A two-way coupling numerical iterative scheme was used to incorporate the effects of gas-particle interactions in volume fraction, momentum and kinetic energy. The gas-solid flow patterns, forces acting on particles, the particles mean velocities, jet penetration depths, gas turbulent intensities and particle turbulent intensities were discussed. Selected stimulation results were compared to some published experimental and simulation results.     

Theoretical and experimental investigations on particle rotation speed in CFB riser

Particle rotation is a common phenomenon in gas-solid two-phase flows. The paper presents theoretical and experimental investigations on particle rotation speed in the gas-solid flow inside a cold CFB riser. The possible particle rotation speed, caused by non-homogeneous flow field and particle collision, and its variation with time were investigated. The average particle rotation speed was predicted with considerations of particle size, average particle collision velocity, particle collision rate and particle number density. It is found that particle collision is the most significant reason for particle rotation. The maximal and average rotation speed for particles with an average size of several hundred micrometers in the CFB riser under typical working condition may be several thousand revolutions per second and several hundred revolutions per second, respectively. The rotation speed of glass beads with an average size of in the upper dilute zone of a cold CFB riser was measured by using a high speed digital imaging system. The variation of particle rotation speed with time was observed, which is in accordance with the theoretical result. The average rotation speed for glass beads was statistically analyzed based on a large number of particle examples. With several factors taken into account, the experimental result is considered to agree with the theoretical one.     

Visualization of the shear region in a cohesive powder by scanning with a polarized neutron beam

The flow of cohesive powder occurs by the formation of shear planes or zones. How these form and how particles microscopically behave in a shear zone is fundamental for understanding powder flow. In this work Neutron Depolarization has been used to study in-line particles in a powder sample. The Neutron Depolarization technique gives a unique insight in the particle rotations and width of the shear zone. It has been shown that rotation of particles during a normal consolidation becomes less when the sample is more compacted. Shear displacement experiments showed that particles rotate in a preferred direction. The width of the region in which the preferred rotation takes place is found to be at least 1000 to 2000 particle diameters.     

Experimental study of particle rotation characteristics with high-speed digital imaging system

Particle rotation plays an important role on several aspects in gas-solid two-phase flow. However, it has not been paid much attention due to a lack of appropriate measurement methods. An attempt has been made in the present paper on the experimental study of particle rotation characteristics in a cold pilot-scale Circulating Fluidized Bed (CFB) riser, by using a high-speed digital imaging measurement system. It is found that one can measure rotation speeds manually for particles with special speckles on their surfaces or irregular shapes by observing particle image sequences. A dual-frequency imaging method was presented to enlarge the maximal measurable rotation speed at finite frame frequency and the measured rotation speeds are validated theoretically. Furthermore, particle rotation characteristics in a cross-section in upper dilute-phase zone were analyzed statistically. The results show that the average particle rotation speed is about 300 rev/s with the top speed of 2000 rev/s, when the superficial gas velocity U_g, external solids mass flux G_s and average particle diameter are 5 m/s, 1.5 kg/(m² s) and 0.5 mm, separately. The average particle rotation speed near the wall area is higher than that in the center area at the testing cross-section. Those particles, with either smaller size or higher radial component of translational speed, may have higher average rotation speed. The average rotation speed of irregular particles is apparently higher than that of the spherical ones.     

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。     

Hydrodynamic modeling of particle rotation for segregation in bubbling gas-fluidized beds

A multi-fluid Eulerian model has been improved by incorporating particle rotation using kinetic theory for rapid granular flow of slightly frictional spheres. A simplified model was implemented without changing the current kinetic theory framework by introducing an effective coefficient of restitution to account for additional energy dissipation due to frictional collisions. Simulations without and with particle rotation were performed to study the bubble dynamics and bed expansion in a monodispersed bubbling gas-fluidized bed and the segregation phenomena in a bidispersed bubbling gas-fluidized bed. Results were compared between simulations without and with particle rotation and with corresponding experimental results. It was found that the multi-fluid model with particle rotation better captures the bubble dynamics and time-averaged bed behavior. The model predictions of segregation percentages agreed with experimental data in the fluidization regime where kinetic theory is valid to describe segregation and mixing.