LBM-DEM四向耦合喷动床内介尺度模拟与分析

研究喷动床内颗粒的流动特性对于喷动床的设计和优化具有重要意义。基于气固两相流流动的LBMDEM四向耦合模型,对单孔射流喷动床中颗粒的流动进行数值模拟。其中,气相采用修正的格子玻尔兹曼方法,颗粒相采用离散单元法,流固之间受力的双向耦合基于牛顿第三定律,颗粒与颗粒及颗粒与壁面的受力双向耦合采用软球模型。模拟得到了流化过程、颗粒与气体的速度分布、床层膨胀高度变化以及床宽对流化过程的影响。结果表明,喷动床内存在强烈的内循环,床宽增加导致颗粒运动能力减弱,射流速度增加使颗粒运动更加剧烈,床层膨胀高度增加。     

二维喷动床CFD-DEM模拟中曳力模型的影响

CFD-DEM已经广泛应用到喷动床的研究中,其模拟的准确性与用于处理颗粒-流体相互作用的曳力模型密切相关。为了探究不同曳力模型对喷动床CFD-DEM模拟结果的影响,基于非结构化网格的喷动床仿真,使用7个曳力模型分别对锥底喷动床内气固两相运动进行了数值模拟。综合床层压降、喷动高度和颗粒速度特性三个方面,Wen-Yu模型和Gibilaro模型预测的气固两相运动最剧烈,其次是Di Felice模型、Syamlal-O’Brien模型、Gidaspow模型和Huilin-Gidaspow模型,BVK模型预测的气固两相运动最平缓。由于模拟的气固两相体系属于密相体系,Huilin-Gidaspow模型的光滑过渡函数没有产生效果,所以Gidaspow模型和Huilin-Gidaspow模型在各个方面的预测结果基本一致。     

基于格子Boltzmann方法的电流变液动力学建模及数值模拟

电流变悬浮液内部结构对外电场的快速响应发生在指定的控制空间中,在这一特指的时间和空间尺度上电流变悬浮液的物理行为特征主要为剪切速率低和流动阻尼大,即Mach和Reynolds数一般不大,可以视为微尺度流动来加以研究。针对这一流动特征,基于介观动理论的格子Boltzmann方法,建立了电流变悬浮液两相流动的离散颗粒运动模型,通过该模型进行了动力学模拟,结果表明,该模型解决了分子动力学模型难以描述的因颗粒运动造成局部流场流变特性改变的难题,以及该流场双向耦合过程中对颗粒运动的影响。     

粗糙颗粒动理学及稠密气固两相流动的数值模拟

考虑颗粒碰撞过程中摩擦作用,给出了粗糙颗粒碰撞动力学.引入颗粒相拟总温来表征颗粒平动和转动脉动能量的特征.基于气体分子运动论,建立颗粒碰撞中平动和旋转共同作用的粗糙颗粒动理学,给出了颗粒相压力和黏度等输运参数计算模型.运用基于颗粒动理学的欧拉-欧拉气固两相流模型,数值模拟了流化床内气体颗粒两相流动特性,分析了颗粒旋转流动对颗粒碰撞能量交换和耗散的影响.模拟得到的流化床内径向颗粒浓度和提升管内颗粒轴向速度与他人实验结果相吻合.模拟结果表明随着颗粒浓度的增加,颗粒相压力和能量耗散逐渐增加,而颗粒拟总温先增加后下降.随着颗粒粗糙度系数的增加,床内平均颗粒相拟总温和能量耗散增加,表明颗粒旋转产生的摩擦将导致颗粒旋转脉动能量的改变,影响床内气体-颗粒两相宏观流动特性.      

基于离散单元法和人工神经网络的近壁颗粒动力学特征研究

颗粒与壁面的相互作用往往对颗粒流动具有显著影响. 为研究颗粒与壁面作用机理, 对滚筒内颗粒流动过程进行离散单元法(DEM)数值模拟. 基于模拟结果统计分析靠近壁面处颗粒的运动特征, 结果表明, 小摩擦系数时颗粒平动和旋转速度均近似满足正态分布, 但由于壁面影响, 摩擦系数增大时颗粒沿滚筒轴向的旋转速度偏离正态分布, 颗粒动力学理论推导壁面边界条件时应考虑速度正态分布的修正及速度脉动的各向异性. 采用人工神经网络(ANN)构建了颗粒无因次旋转温度、滑移速度和平动温度之间的函数模型, 进而可以在常规双流模型壁面边界条件中考虑颗粒旋转的影响. 基于DEM模拟及结果分析可以为壁面边界条件的理论构造和半经验修正提供基础数据和封闭模型.      

大速差射流燃烧室中烟煤与贫煤燃烧的数值模拟

本文基于颗粒相的轨道模型,对大速差射流燃烧室中烟煤粉与贫煤粉的二维流动,混合及燃烧进行了数值模拟,模拟结果从两相耦合的角度,阐明了煤粉颗粒在燃烧室中运动的规律,煤粉与大速差射流诱导的中心气体逆流之间的混合及其对煤粉火焰稳定的影响,指出此种燃烧室中煤粉火焰稳定的回流区燃烧机理,气相流场及回流区的预报结果与实验符合良好。     

水平井及大斜度井砾石充填α波砂床平衡高度随机概率模型及实验研究

α波砂床平衡高度预测是水平井砾石充填过程数值模拟及施工参数优化设计的核心问题之一。根据水流紊动理论,将砂床表面流体瞬时流速视为正态分布的随机变量,将颗粒运动状态转换视为随机事件,分析状态转换事件概率及最终状态概率的数字特征。根据最终状态概率代表颗粒运动趋势的原理,砂床高度的变化是床层表面颗粒运动状态转换的结果。由平衡砂床高度和摩阻流速及平均流速的关系,建立了同时适用于水平井和大斜度井砾石充填的α波砂床平衡高度预测的随机概率模型。利用研制的大斜度井砾石充填全尺寸实验模拟装置,进行井筒倾角、筛管偏置度、流量、砂浓度对α波砂床平衡高度影响的实验研究,对随机概率模型的计算结果进行了对比验证。随机概率模型与实验结果比较吻合,平均误差10.5%,进行必要的修正可以用于大斜度井砾石充填α波砂床平衡高度预测。     

注塑成型喷射现象数值模拟及机理分析

喷射是熔体充填过程中一种特殊流动形态,也是注塑成型的不良现象,为了预测射流长度、形态演化,揭示喷射的产生机理,建立了粘性、非等温的熔体三维流动模型,构造了熔体流动前沿演化的输运方程。用有限体积法离散控制方程,提出了压强与速度及速度与温度的两重解耦合方案,开发了模拟程序,分析了模拟成型过程中粘性力、惯性力、速度、温度、压力等物理量的大小及变化规律。基于模拟结果分析了粘性力、惯性力的大小对喷射的影响,发现:当惯性力大于粘性力时产生喷射现象,随着粘性力增大,射流变为蛇形流;射流长度依赖于注射速度,速度越大,射流越长,熔体温度对射流长度基本没有影响。此外,实验结果表明,注塑成型喷射现象数值模拟及机理分析方法可以较好地预测射流长度、蛇形流的形态以及蛇形流中、后端摆动幅度。     

具有壁面射流激励的圆管内气相流动大涡模拟

采用大涡模拟方法和单方程亚格子模式对小尺度量进行模拟。研究了不同强度壁面射流激励对圆管内气相流动的影响,模拟结果给出了射流对瞬态拟序结构发展、时平均流向速度分布的影响。随着射流强度的增加,射流入口附近流体的回流现象增强。射流强度足够大时可以减小管壁处的切应力值,同时会减小壁面附近流动速度,这种速度分布会导致气体夹带颗粒的能力下降,从而在实际两相流动中容易造成壁面附近的气粒返混现象。     

气固两相流介尺度LBM-DEM模型

LBM-DEM耦合方法通常是指一种颗粒流体系统直接数值模拟算法,即是一种不引入经验曳力模型的计算方法,颗粒尺寸通常比计算网格的长度大一个量级,颗粒的受力通过表面的粘性力与压力积分获得,其优点是能描述每个颗粒周围的详细流场,产生详细的颗粒-流体相互作用的动力学信息,可以探索颗粒流体界面的流动、传递和反应的详细信息及两相相互作用的本构关系,但其缺点是计算量巨大,无法应用于真实流化床过程模拟。本文针对气固流化床中的流体以及固体颗粒间的多相流体力学行为,建立了一种稠密气固两相流的介尺度LBMDEM模型,即LBM-DEM耦合的离散颗粒模型,实现在颗粒尺度上流化床的快速离散模拟。该耦合模型采用格子玻尔兹曼方法(LBM)描述气相的流动和传递行为,离散单元法(DEM)用于描述颗粒相的运动,并利用能量最小多尺度(EMMS)曳力解决气固耦合不成熟问题,以提高其模拟精度。通过经典快速流态化的模拟,验证了介尺度LBM-DEM耦合模型的有效性。模拟结果表明介尺度LBM-DEM模型是一种探索实验室规模气固系统的有力手段。     

Influences of the fluidizing and spouting pulsation on particle motion in spout-fluid beds

Effects of variable airflow on particle motion in spout-fluid beds are studied. Computational fluid dynamics using Navier–Stokes equations for the gas phase coupled with the discrete element method using Newton’s laws for the solid phase have been employed. Results indicate that increasing the fluidizing velocity diminishes dead zones and increases both the total height of the bed and the traversed distance by particles in the steady spout-fluid bed. In pulsed airflows, two configurations are investigated, namely, the spouted pulsed-fluidized bed with pulsed flow of the fluidizing velocity, and the pulsed-spouted fluidized bed with pulsed flow of the spouting velocity. The positive effect of pulsation on particle motion is shown and the effects of parameters, such as amplitude and frequency, on the dynamics of the bed are investigated in each configuration. An increase of up to 19% in traversed distance is found for the range studied, which suggests flow pulsation as a promising technique for increasing particle mixing in spout-fluid beds.     

CFD-DEM simulation of spouting of corn-shaped particles

Three dimensionally coupled computational fluid dynamics (CFD) and discrete element method (DEM) were used to investigate the flow of corn-shaped particles in a cylindrical spouted bed with a conical base. The particle motion was modeled by the DEM, and the gas motion by the k-? two-equation turbulent model. A two-way coupling numerical iterative scheme was used to incorporate the effects of gas–particle interactions in terms of momentum exchange. The corn-shaped particles were constructed by a multi-sphere method. Drag force, contact force, Saffman lift force, Magnus lift force, and gravitational force acting on each individual particle were considered in establishing the mathematical modeling. Calculations were carried out in a cylindrical spouted bed with an inside diameter of 200 mm, a height of 700 mm, and a conical base of 60°. Comparison of simulations with experiments showed the availability of the multi-sphere method in simulating spouting action with corn-shaped particles, but it depended strongly on the number and the arrangement of the spherical elements. Gas–solid flow patterns, pressure drop, particle velocity and particle concentration at various spouting gas velocity were discussed. The results showed that particle velocity reaches a maximum at the axis and then decreases gradually along the radial direction in the whole bed. Particle concentration increases along the radial direction in the spout region but decreases in the fountain region, while it is nearly constant in the annulus region. Increasing spouting gas velocity leads to larger pressure drop, remarkably increased speed of particle moving upward or downward, but decreased particle concentration.     

Numerical simulation of uranium tetrafluoride fluorination in a multistage spouted bed using the improved CFD-DEM chemical reaction model

A CFD-DEM reaction coupling model was established to simulate UF₄ fluorination process, in which heat and mass transfer, heterogeneous chemical reaction, and particle shrinkage model were considered. The gas behavior was described by the conservation laws of mass, momentum, and energy. The solid phase is modeled with the discrete element method, considering the gas–solid interphase force, contact force, heat transfer, and chemical reaction models based on the discretized surface. Each particle can be individually tracked and associated with specific physical properties. The proposed CFD-DEM reaction coupling model based on particle shrinking reaction model with discretized surface was validated by the experimental and literature results at first. Then a multistage conical spouted bed was proposed and the process of UF₄ fluoridation reaction in it was investigated. The fluidization characteristics and the concentration distribution of gaseous products in the spouted bed with an extended gas velocity range were obtained and analyzed. In addition, the effects of different parameters, such as superficial gas velocity, temperature, fluorine concentration, on fluoridation rate and the fluorine conversion rate were investigated based on the proposed CFD-DEM reaction coupling model. The results obtained in this work are beneficial for method development of the chemical reaction simulation research in particle scale using the CFD-DEM model, and useful for operation and equipment parameters design of the uranium tetrafluoride fluorinate industrial process in the future.     

A numerical investigation into the solids phase chromatography using a combined continuous and discrete approach

Solids phase chromatography for particle classification is based on different retention times of particles with different properties when they are elutriated through a confined geometry.This work aims at a fundamental understanding of such a technology by using the combined continuous and discrete method.A packed bed is employed as the model confined geometry.The numerical method is compared first with experimental observations,followed by a parametric analysis of the effects on the flow hydrodynamics and solids behaviour of various parameters including the number of injected particles,the superficial gas velocity,the contact stiffness and the diameter ratio of the packed column to the packed particles.The results show that the modelling captures some important features of the flow of an injected pulse of fine particles in a packed bed. An increase in the number of injected particles or the superficial gas velocity reduces the retention time,whereas the contact stiffness does not show much effect over the range of 5×102 to5×104 N/m.It is also found that the effect on the retention time of the diameter ratio of the packed column to the packed particles seems complex showing a non-monotonous dependence.     

Mixing and segregation of solid particles in a conical spouted bed: Effect of particle size and density

In this work, the mixing and segregation of binary mixtures of particles with different sizes and densities in a pseudo-2D spouted bed were studied experimentally. A binary mixture of solid particles including sand, gypsum, and polyurethane was used. To determine the particles mass fraction, and their mixing and segregation in the bed, an image-processing technique was developed and used. Important hydrodynamic parameters, such as the axial and radial segregation profiles of the solid particles, were measured. The effects of air velocity, particle size, and particle mass fraction were also evaluated. The flow regime in the spouted bed and the time required for reaching the equilibrium state of the solid particles were discussed. The results showed that the segregation of solid particles and the time to equilibrium both decreased when the air velocity increased to much larger than the minimum spouting velocity. The axial segregation increased with the diameter ratio of the particles. Upon completion of the test, coarse particles were concentrated mainly in the spout region, while fine particles were aggregated in the annulus region. Examination of the flow pattern in the spouted bed showed that the particles near the wall had longer flow paths, while those near the spout region had shorter flow paths.     

A numerical investigation into the solids phase chromatography using a combined continuous and discrete approach

Solids phase chromatography for particle classification is based on different retention times of particles with different properties when they are elutriated through a confined geometry. This work aims at a fundamental understanding of such a technology by using the combined continuous and discrete method. A packed bed is employed as the model confined geometry. The numerical method is compared first with experimental observations, followed by a parametric analysis of the effects on the flow hydrodynamics and solids behaviour of various parameters including the number of injected particles, the superficial gas velocity, the contact stiffness and the diameter ratio of the packed column to the packed particles. The results show that the modelling captures some important features of the flow of an injected pulse of fine particles in a packed bed. An increase in the number of injected particles or the superficial gas velocity reduces the retention time, whereas the contact stiffness does not show much effect over the range of 5 × 10^2 to 5× 10^4 N/m. It is also found that the effect on the retention time of the diameter ratio of the packed column to the packed particles seems complex showing a non-monotonous dependence.     

Some hydrodynamic aspects of 3-phase inverse fluidized bed

Hydrodynamics of 3-phase inverse fluidized bed is studied experimentally using low density particles for different liquid and gas velocities. The hydrodynamic characteristics studied include pressure drop, minimum liquid and gas fluidization velocities and phase holdups. The minimum liquid fluidization velocity determined using the bed pressure gradient, decreases with increase in gas velocity. The axial profiles of phase holdups shows that the liquid holdup increases along the bed height, whereas the solid holdup decreases down the bed. However, the gas holdup is almost uniform in the bed.     

Lagrangian–Eulerian simulation of slugging fluidized bed

This work studies gas–solid slugging fluidized beds with Type-D particles, using two-dimensional simulations based on discrete element model (DEM). DEM performance is quantitatively validated by two commonly accepted correlations for determining slugging behavior. The voidage profiles simulated with bed height corresponding to Baeyens and Geldart (1974) correlation for onset of slugging demonstrate a transitional flow pattern from free bubbling to slugging. The present calculated values for the maximum slugging bed height are in good agreement with the correlation from Matsen et al. (1969). Simulations show that fluidized beds with Type-D particles can operate in the round-nosed slugging regime and also shows that wall slugs and square-nosed slugs tend to be formed with increase in superficial gas velocity and in bed height, respectively.     

Sand attrition in conical spouted beds

A study was carried out on the attrition in conical spouted beds using two sands with different properties for several bed heights and gas flow rates. Furthermore, the influence of a draft tube was studied at ambient and high temperatures. The main objective was to acquire knowledge on the attrition of sand beds for biomass pyrolysis in a pilot plant provided with a conical spouted bed reactor. A first-order kinetic equation is proposed for sand attrition in a conical spouted bed at room temperature. The predicted attrition rate constant depends exponentially on excess air velocity over that for minimum spouting. Both the draft tube and temperature increase contribute to reduction of attrition.