A modified DSMC method for simulating gas–particle two-phase impinging streams

A modified direct simulation Monte Carlo (DSMC) method is proposed in the present paper for obtaining the particle motion behavior which is of essential importance for research of the transfer processes in a gas–particle two-phase impinging streams (GPIS). In GPIS, the particle concentration distribution is extremely nonuniform. The particle collisions play an important role in the particle motion behavior inside the impingement zone with high particle concentration, nevertheless, the interactions between gas phase and particle phase play an important role outside the impingement zone. And the volume of the impingement zone is relatively small, and the time required for a particle to pass through this zone is very short. According to these characteristics of multiple spatial scale and multiple time scale, the DSMC method is modified to be more suitable to the numerical study of GPIS. This modified DSMC method is not restricted to flow field cells in the calculation of particle motion and collisions, and the particle time step in this method is adaptive and calculated using local particle parameters. The algorithm and numerical solution method of this method are also given in this paper. The predicted results of the particle motion behavior in GPIS are in reasonable agreement with the experimental data, which indicates the validity of the present method.     

Numerical Analysis of Gas-Particle Flow in Vertical Impinging Stream Chamber

For impingement stream drying, it is important to understand the complex gas-particle flow field in order to control the quality of dried products accurately. Hence a numerical analysis of three-dimensional gas-particle flow field was carried out including momentum, heat and mass transfers between the two phases. Simulation results for millet drying in a vertical impingement chamber were obtained and discussed. There is a great difference between gas phase and particle phase flow fields. The impingement planes of gas phase and particle phase are not coincident along the axial direction. Not all the millet particles can be taken away by the gas flow leaving some wet particles in impingement chamber for a prolonged drying. The temperature of millet rises up quickly approaching the value of drying medium in some zones. Consequently, it is important to regulate the inlet gas temperature for drying of thermal sensitive materials.     

Numerical Analysis of Gas-Particle Flow in Vertical Impinging Stream Chamber

Abstract

For impingement stream drying, it is important to understand the complex gas-particle flow field in order to control the quality of dried products accurately. Hence a numerical analysis of three-dimensional gas-particle flow field was carried out including momentum, heat and mass transfers between the two phases. Simulation results for millet drying in a vertical impingement chamber were obtained and discussed. There is a great difference between gas phase and particle phase flow fields. The impingement planes of gas phase and particle phase are not coincident along the axial direction. Not all the millet particles can be taken away by the gas flow leaving some wet particles in impingement chamber for a prolonged drying. The temperature of millet rises up quickly approaching the value of drying medium in some zones. Consequently, it is important to regulate the inlet gas temperature for drying of thermal sensitive materials.     

IMPINGEMENT STREAM DRYERS FOR PARTICLES AND PASTES

This paper reports on the available experimental and analytical studies on a relatively novel flash drying configuration involving use of jets impinging against each other. The impingement zone provides ideal conditions for highly enhanced heat and mass transfer from particles or droplets to the gas stream. Repeated particle penetration into the opposite jet stream allows longer residence times in the impingement zone to accomplish desired drying. The collisions of the particles also permit breakage of lumps and better drying even for poly disperse materials. Advantages and limitations of this concept are discussed along with a discussion of some industrial applications     

IMPINGEMENT STREAM DRYERS FOR PARTICLES AND PASTES

This paper reports on the available experimental and analytical studies on a relatively novel flash drying configuration involving use of jets impinging against each other. The impingement zone provides ideal conditions for highly enhanced heat and mass transfer from particles or droplets to the gas stream. Repeated particle penetration into the opposite jet stream allows longer residence times in the impingement zone to accomplish desired drying. The collisions of the particles also permit breakage of lumps and better drying even for poly disperse materials. Advantages and limitations of this concept are discussed along with a discussion of some industrial applications     

DEM-LES of coal combustion in a bubbling fluidized bed. Part I: gas-particle turbulent flow structure

The gas and particle motions in a bubbling fluidized bed both with and without chemical reactions are numerically simulated. The solid phase is modelled as Discrete Element Method (DEM) and the gas phase is modelled as 2-D Navier-Stokes equations for 2-phase flow with fluid turbulence calculated by large Eddy simulation (LES), in which the effect of particles on subgrid scale gas flow is taken into account. The gas/particle flow structure, the mean velocities and turbulent intensities can be predicted as a function of several operating parameters (particle size, bed temperature, and inlet gas velocity). The lower the inlet gas velocity, the higher the ratio of particle collision. The distributions of the particle anisotropic velocity show that the particles have no local equilibrium, and the distribution of gas kinetic energy corresponds to the distribution of gas-particle coupling moment in the fluidized bed. An intensive particle turbulent region exists near the wall, and the gas Reynolds stress is always much higher than the particle stress. The presence of the large reactive particles in the fluidized bed may affect significantly the gas and particle velocities and turbulent intensities. The effects of the bed temperature and inlet gas velocity on the gas particle flow structure, velocity, and turbulent intensity are also studied.     

Numerical Modeling of Particle Dynamics in a Cylindrical Chamber Containing a Rotating Disk

Numerical modeling was performed to study the submicron particle dynamics in a confined flow field containing a rotating disk, temperature gradient, and various inlet gas flow rates. The Lagrangian model was employed to compute particle trajectories under the temperature gradient, disk rotation speed, and inlet gas flow rate effects. The trajectories of particles with diameters of 1 μm, 0.1 μm, and 0.01 μm were examined in this study. When the inlet gas temperature was lower than that of the disk, particle-free zones were created due to upward thermophoretic force for 1 μm and 0.1 μm particles. Disk rotation was found to depress the size of the particle-free zone. Particle deposition onto the disk for 0.01 μm particles was possible because of the Brownian motion effect. A detailed evaluation of the particle-free zone size as a function of the temperature gradient, disk rotation speed, and inlet gas flow rate was performed. When the inlet gas temperature was higher than the disk temperature, particle deposition onto the disk was enhanced due to the downward thermophoretic force for 1 μm and 0.1 μm particles. Disk rotation was found to increase the deposition rate. For 0.01 μm particles, Brownian motion was more important than thermophoretic force in controlling particle behavior. The particle deposition rates as a function of the temperature gradient, disk rotation speed, and inlet gas flow rate were performed.     

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。     

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.     

循环流化床内颗粒运动的预测与测量

本文提出了一种二维气固流动的数值计算模型，它考虑了循环流化床中颗粒的脉动和颗粒间磁撞等因素。该模型将颗粒运动的脉动考虑成一种基于气流脉动频谱的随机Fourier级数，称为脉动频谱随机轨道模型（FSRT）。这一模型可用于预测循环床中颗粒相的速度、颗粒轨迹和颗粒相浓度。对循环床中颗粒相速度和浓度进行了测量，并与FSRT模型的计算结果进行对比，同时对循环床中颗粒运动的一些行为也进行了预测。     

Rushton涡轮搅拌槽内流场特性及颗粒运动行为数值模拟

基于计算流体动力学（CFD）方法,采用大涡模拟（LES）和拉格朗日颗粒追踪技术计算了Rushton涡轮搅拌槽内流场特性及三种St颗粒的运动行为。平均流场（切向速度、轴向速度和径向速度）、颗粒速度及浓度分布方面与实验值的吻合度较好,验证了数值模拟的可靠性。结果表明,搅拌流场及颗粒运动均呈现循环流特性,当转速N=313r/min不变时,St=0.24的小颗粒几乎实现了均匀分布;而St=37.3的大颗粒与流体的跟随性较差,底部沉积率较高,容器顶部会出现一定的颗粒空白区。叶轮附近产生一系列的湍流涡结构,并且由于剧烈的颗粒-壁面碰撞,该位置颗粒拟温度最高;小颗粒（St=0.24）的运移主要受叶片后方尾涡的控制,均匀分布在低涡量区;而大颗粒（St=37.3）由于具有较大的惯性,运动不再由涡主导,很快被叶轮甩向边壁,穿过了尾涡所形成的高涡量区,故而叶轮对附近大颗粒的搅拌效果较差。     

基于阶跃射流的撞击流反应器流场动态特性分析

利用实验与数值模拟方法对动态阶跃型撞击流反应器流场特性进行研究,分析不同入口速度条件下流体流动规律、湍流特性以及能量水平。结果表明,动态阶跃型入口条件下,撞击面在两喷嘴之间周期性移动,流动参数也会发生周期性变化。随着入口平均速率的增大,驻点速度逐渐增大;随着两喷嘴入口速率差的增加,撞击面移动速度加快,撞击区流体湍流强度逐渐增加;随着入口平均速率与入口速率差的增大,XOZ平面在一个周期内的平均湍动能逐渐减小。对比动态撞击流反应器与稳态撞击流反应器内流场特性,探究动态入口条件对撞击流反应器流场特性的影响。结果表明,动态阶跃撞击流反应器湍流黏度、湍流强度和湍动能等参数均明显高于稳态撞击流反应器,撞击轴线上的湍动能梯度分布大于稳态撞击流反应器。动态入口条件下撞击流反应器流体湍动更剧烈,能量水平更高,有利于增加流场内流体扰动与促进混合。     

A Model for the Relative Velocity of a Particle in an Impingement Dryer: Application to Heat Transfer

A simplified mathematical model for the relative gas-particle motion in a confined jet impingement dryer is developed. Model predictions based on an unsteady momentum balance are in good agreement with the observed cycling motion of a spherical particle. The model is applied to coriander seeds submerged in a flow field of superheated steam. It is found that relative motion occurs in unsteady turbulent regime, and that steady settling velocity of particles is never achieved. Model results are applied to correlate experimental heat transfer data of an impingement dryer. Experimental Nu numbers compare fairly well with correlations for fluidized systems.     

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。     

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。     

T型撞击流混合器内流动特性的PIV研究

采用粒子图像测速技术对入射管直径为3 mm、混合腔直径为16 mm的T型撞击流混合器内的流动特性进行了研究,考察了不同流速比和撞击轴线上方空间条件下混合腔内的速度和湍流动能分布. 结果表明,在相同入射管直径和流速下,撞击驻点位于混合腔中心处,无因次化的速度和湍流动能分布趋势基本一致. 高湍流动能区主要集中在撞击点附近区域,其无因次化数值是传统Rushton涡轮搅拌槽叶端处的3倍. 流速比对撞击驻点位置影响显著;减小撞击轴线上方空间可增加高湍流动能分布区域,利于物料混合.     

多射流锥形对撞流气流床流场特性实验及模拟研究

为了考察多射流锥形对撞煤加氢气流床内的冷态流场情况,以3 t/d的热态煤加氢气化炉为依据建立了气流床冷模装置。使用三维动态颗粒分析仪(3D-PDA)对气流床内的速度场进行了测量,同时使用Fluent软件,采用κ-ε模型对气流床内的流动情况进行了模拟研究,模拟结果与实验结果能较好地吻合。结果显示:多射流锥形对撞气流床内的流体流动分为对撞区、射流区、回流区和管流区,在考察范围内,随着进气速度的增加,回流区的体积占比先增大后减小;随中心喷嘴携带气速度的增加,射流区速度增大,且较进气速度的影响更为敏感;适当增加进气角度,则会降低射流区速度,增大回流区体积。     

Principal characteristics of turbulent gas-particulate flow in the vicinity of single tube and tube bundle structure

In this paper the particle rebounding characteristics of a gas-particle flow over a cylindrical body and an in-line tube bundle arrangement is investigated. With the aid of both experimental and numerical approaches, the mean particulate flow patterns, comprising both incident and rebound particles resulting from the impact of particles on solid walls, are examined. In the experimental investigation, a two-dimensional laser-Doppler anemometry (LDA) technique is used in the immediate vicinity of the body surface to measure the instantaneous incident and rebound particle velocities. The Reynolds-averaging Navier-Stokes equations are solved for the continuum gas phase and the results are used in conjunction with a Lagrangian trajectory model to predict the particle-rebound characteristics. For the single tube model, the experimental observations, also confirmed through computations, reveal a particle rebound zone where the mean particulate flow pattern is significantly modified due to the contribution of the rebound particles during the process of particle-wall impact interaction. This particle rebound zone is found to be a function of mainly the Stokes number (particle inertia), and to a lesser extent on the fluid Reynolds number (gas flow condition) except for high gas flow velocities. For the in-line tube bundle model, particles being rebounded from the first row of tubes at upstream migrated downstream and impinged the other tubes in an extremely complex and random disposition. Detailed measurements on the flow and turbulent characteristics within the subset containing two cylindrical tubes representing the flow over the first and second row tubes in the tube bundle configuration revealed that the heavier particles possessed higher axial and transverse velocity fluctuations than the gas and lighter particles. A means of quantifying the erosion rate using a semi-empirical relationship and CFD approach is presented. The erosion distributions were found to be significantly different between the lighter and heavier particles. Analysis of the effect of the above-mentioned parameters on the rebounding particle flow characteristics and their interrelationship has provided a better understanding on the behaviour of particulate flow impinging on a solid wall body or series of solid bodies. The usefulness of employing the experimental and computational approaches to quantify the particle-wall impact interaction phenomena in this study provides the basis for additional investigations to be undertaken to better comprehend the particulate behaviour in tube bundle structure, for example staggered tube arrangement commonly found in many commercial heat exchangers.     

Numerical modeling of gas-particle flow using a comprehensive kinetic theory with turbulence modulation

In most fluidized beds, both solids flux and gas phase Reynolds number are high and the flow are usually turbulent. It is therefore necessary to consider both the effects of particle-particle collisions and particle phase turbulence in any mathematical model for simulating gas-particle flows. A comprehensive model is developed in the present work in which a two-equation (k-?) turbulence model is used for calculating the gas phase. In addition, a transport equation of particle phase turbulent kinetic energy is proposed and used for modeling the particle phase turbulence (k_p model). Similar to that of the single gas phase, effective viscosity of the particle phase is the sum of the laminar viscosity caused by particle-particle collisions described by kinetic theory and the turbulent viscosity caused by collections of particles described by the k_p model. The proposed model is used to predict gas-particle flows in a vertical pipe. Results obtained using this model compare well with experimental data.     

Dynamic characteristics of the volatile cloudy zone in a gas–solid fluidized bed with hydrocarbon liquid spray

In a gas–solid fluidized bed with continuous hydrocarbon liquid spray, a volatile “cloudy zone” could be formed, defined as a dynamically steady liquid-affected zone, including droplets, wet particles, and the gas which passes through the zone. A new flow pattern with the dynamic coexistence of cloudy zone and non-cloudy zone (gas–solid zone), is accordingly established. The temperature, particle concentration, and particle velocity fields are measured in real-time via infrared thermography and particle imaging velocimetry, respectively. Results show that the area and range of central position of the cloudy zone illustrate a heavier fluctuant trend with the increasing velocity of liquid spray, and the main frequency of area fluctuation is close to that of the bubble rising. Moreover, the particle concentration and particle velocity in the cloudy zone are lower than those in the non-cloudy zone, breaking the conventional symmetrical distributions of hydrodynamic parameters of particles in a gas–solid fluidized bed.