稠密气固两相流各向异性颗粒相矩方法

基于气体分子动力学和颗粒动理学方法,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.采用Koch等计算方法模拟气固脉动速度关联矩.考虑气体-颗粒间相互作用,建立稠密气体-颗粒流动模型.数值模拟提升管内气固两相流动特性,模拟结果表明提升管内颗粒相湍流脉动具有明显的各向异性.预测颗粒速度、浓度和颗粒脉动速度二阶矩与Tartan等实测结果相吻合.模拟结果表明轴向颗粒速度脉动强度约为平均颗粒相脉动强度的1.5倍,轴向颗粒脉动能大约是径向颗粒脉动能3.0倍.     

强吸气旋转圆筒壁面湍流边界层建模及计算

提出了湍流边界层的一种简单、快速计算方法,用以求解强吸气作用下旋转圆筒表面边界层流动.首先,理论分析了同心圆筒间的旋转流体运动,外筒静止、内筒旋转且为多孔吸气条件.强吸气情况下旋转流动主要表现为内筒壁面附近的边界层流动,基于这一事实得到了周向速度分布的解析表达式.其次,通过引入新参数扩展Cebeci-Smith代数湍流模型,使其能考虑流线曲率、壁面吸气、低Reynolds数效应等因素.针对这些因素的综合影响,采用解析修正和经验参数对模型进行调整.同时,基于Reynolds应力湍流模型的仿真结果,校准代数湍流模型中的经验参数.最后,给出基于广义Cebeci-Smith湍流模型的旋转壁面边界层流动的迭代算法,该算法适用于需要特殊迭代过程的轴向及周向流动均匀情况.计算了不同旋转速度和吸气强度组合工况下的边界层流动,其周向速度和湍流强度分布与基于Reynolds应力湍流模型的计算结果非常接近.并且表明,当Reynolds应力湍流模型数值模拟预测内筒边界层为稳定层流时,该方法也再现了相同初始条件下的层流边界层.     

MODELLING AND CALCULATION OF THE TURBULENT BOUNDARY LAYER ON A ROTATING CYLINDER SURFACE WITH STRONG SUCTION 1)

提出了湍流边界层的一种简单、快速计算方法, 用以求解强吸气作用下旋转圆筒表面边界层流动. 首先, 理论分析了同心圆筒间的旋转流体运动, 外筒静止、内筒旋转且为多孔吸气条件. 强吸气情况下旋转流动主要表现为内筒壁面附近的边界层流动, 基于这一事实得到了周向速度分布的解析表达式. 其次, 通过引入新参数扩展Cebeci-Smith代数湍流模型, 使其能考虑流线曲率、壁面吸气、低Reynolds数效应等因素. 针对这些因素的综合影响, 采用解析修正和经验参数对模型进行调整. 同时, 基于Reynolds应力湍流模型的仿真结果, 校准代数湍流模型中的经验参数. 最后, 给出基于广义Cebeci-Smith湍流模型的旋转壁面边界层流动的迭代算法, 该算法适用于需要特殊迭代过程的轴向及周向流动均匀情况. 计算了不同旋转速度和吸气强度组合工况下的边界层流动, 其周向速度和湍流强度分布与基于Reynolds应力湍流模型的计算结果非常接近. 并且表明, 当Reynolds应力湍流模型数值模拟预测内筒边界层为稳定层流时, 该方法也再现了相同初始条件下的层流边界层.     

微尺度泊肃叶流的高阶连续模型分析

分别从分子运动论及连续流理论出发,对体积力驱动的微尺度平面泊肃叶(Poiseuille)流的横向分布特征进行了分析. 分子水平模拟采用直接模拟蒙特卡罗(direct simulation Monte Carlo, DSMC)方法;连续流理论则主要考察了伯内特(Burnett)及超伯内特(Super-Burnett)等高阶连续模型,在平行流假设下,获得一组高阶非线性常微分方程,补充完整的边界条件,并应用龙格-库塔(Runge-Kutta)方法求解. 结果表明,即使对于过渡领域流动,高阶连续模型可以给出与DSMC 结果完全相符的压力分布,而速度分布当努森(Knudsen)数约为0.2时即在壁面开始出现偏差;对于温度的横向分布,伯内特模型回复到纳维-斯托克斯(Navier-Stokes)水平,不能得到与DSMC一致的双峰结构,而超伯内特模型在滑移流动领域与DSMC定性相符,在过渡领域却仅能正确预测主流区温度分布,壁面附近差异明显;横向热流与纳维-斯托克斯模型预测接近,但机理上存在本质区别. 本文结果提示选用连续模型时,不仅要根据流动参数来判断,还可以根据所关注的物理量来进行调整,适度扩大连续模型的适用范围. 但即使采用高阶本构关系,连续模型仍然不能完全描述壁面附近区域的非平衡效应(如努森层效应),这是试图扩大连续模型适用范围时必然会遇到的困难.     

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.      

尼龙粉末在SLS预热温度下的离散元模型参数确定及其流动特性分析

尼龙粉末是增材制造中常用的粉体材料,温度对其流动性有重要影响. 探索尼龙粉末增材制造预热温度下的流动性是研究选择性激光烧结(selective laser sintering, SLS)工艺中粉体铺展成形的基础. 选取SLS技术中的尼龙粉末为原材料,采用离散元数值方法,研究尼龙粉末的流动行为,是增材制造工艺数值模拟和铺粉工艺优化的研究热点. 以Hertz-Mindlin模型为基础,基于Hamaker理论模型和库伦定律,在尼龙粉末的接触动力学模型中引入范德华力和静电力,建立预热温度下尼龙粉末流动的离散元模型(discrete element method, DEM),通过对比相应实验结果,标定了该模型的参数. 对加热旋转圆筒中尼龙粉末流动过程进行了DEM数值模拟,校核了所建模型的正确性,并研究了粉体粒径分布对尼龙粉末流动特性的影响规律. 研究表明,尼龙粉末黏附力是静电力与范德华力的共同作用结果;随着粉体粒径的增大,尼龙粉末崩塌角增大,流动性增强;相对于高斯粒径分布,粒径均匀分布的尼龙粉末颗粒流动性更强. 研究结果可指导SLS中铺粉工艺的优化.      

大涡模拟的壁模型及其应用

大涡模拟是研究湍流的非定常特性的重要方法. 但解析壁面层的大涡模拟所需的计算量与直接数值模拟相当,是大涡模拟在高雷诺数壁湍流数值模拟中所面临的主要困难. 解析壁面层所需的网格尺度与壁面黏性长度同量级,是引起壁湍流大涡模拟计算量增加的主要原因. 壁模型通过模化近壁流动避免了完全解析壁面层,可以显著地降低壁湍流大涡模拟的计算量,是克服上述困难的有效方法. 本文介绍了大涡模拟壁模型的主要类型;详细讨论了常用的壁面应力模型,特别是平衡层模型和双层模型的构建思路和特点;基于近壁流动的特征讨论了应力边界条件的必要性和适用性;指出了壁面应力模型的局限性以及考虑非平衡效应修正的各种方法;讨论了壁面应力模型的研究历史、最新进展和发展趋势,给出了常用的壁面应力模型的分支与发展关系图;并基于Werner-Wengle模型实现了周期山状流的大涡模拟.      

尼龙粉末在SLS预热温度下的离散元模型参数确定及其流动特性分析

尼龙粉末是增材制造中常用的粉体材料,温度对其流动性有重要影响.探索尼龙粉末增材制造预热温度下的流动性是研究选择性激光烧结(selective laser sintering, SLS)工艺中粉体铺展成形的基础.选取SLS技术中的尼龙粉末为原材料,采用离散元数值方法,研究尼龙粉末的流动行为,是增材制造工艺数值模拟和铺粉工艺优化的研究热点.以Hertz-Mindlin模型为基础,基于Hamaker理论模型和库伦定律,在尼龙粉末的接触动力学模型中引入范德华力和静电力,建立预热温度下尼龙粉末流动的离散元模型(discrete element method,DEM),通过对比相应实验结果,标定了该模型的参数.对加热旋转圆筒中尼龙粉末流动过程进行了DEM数值模拟,校核了所建模型的正确性,并研究了粉体粒径分布对尼龙粉末流动特性的影响规律.研究表明,尼龙粉末黏附力是静电力与范德华力的共同作用结果;随着粉体粒径的增大,尼龙粉末崩塌角增大,流动性增强;相对于高斯粒径分布,粒径均匀分布的尼龙粉末颗粒流动性更强.研究结果可指导SLS中铺粉工艺的优化.     

微纳米结构壁面对流场及动压润滑的影响研究

固体边界具有的微纳米结构将影响流体在近壁面处的流动行为,进而由于尺度效应改变流体在整个微间隙的流动或润滑规律.将壁面可渗透微纳米结构等效为多孔介质薄膜,采用Brinkman方程来描述流体在近壁面边界渗透层内的流动,并将其与自由流动区域的不可压缩流体Navier-Stokes控制方程耦合,在界面处的连续边界条件下求解和分析了速度分布规律和压力变化规律.针对恒定法向承载力的油膜润滑条件,进一步讨论了静止表面或运动表面的微纳米结构对近壁面流动行为的影响;并揭示了考虑壁面微纳米结构的流体动压润滑的油膜厚度和摩擦系数的变化规律.论文结果为具有可渗透微结构表面的微间隙流动与润滑提供了理论参考.     

大涡模拟的壁模型及其应用

大涡模拟是研究湍流的非定常特性的重要方法.但解析壁面层的大涡模拟所需的计算量与直接数值模拟相当,是大涡模拟在高雷诺数壁湍流数值模拟中所面临的主要困难.解析壁面层所需的网格尺度与壁面黏性长度同量级,是引起壁湍流大涡模拟计算量增加的主要原因.壁模型通过模化近壁流动避免了完全解析壁面层,可以显著地降低壁湍流大涡模拟的计算量,是克服上述困难的有效方法.本文介绍了大涡模拟壁模型的主要类型;详细讨论了常用的壁面应力模型,特别是平衡层模型和双层模型的构建思路和特点;基于近壁流动的特征讨论了应力边界条件的必要性和适用性;指出了壁面应力模型的局限性以及考虑非平衡效应修正的各种方法;讨论了壁面应力模型的研究历史、最新进展和发展趋势,给出了常用的壁面应力模型的分支与发展关系图;并基于Werner-Wengle模型实现了周期山状流的大涡模拟.     

Effect of Shape on Inertial Particle Dynamics in a Channel Flow

Particle dynamics in a channel flow are investigated using large eddy simulation and a Lagrangian particle tracking technique. Following validation of single-phase flow predictions against DNS results, fluid velocities are subsequently used to study the behaviour of particles of differing shape assuming one-way coupling between the fluid and the particles. The influence of shape- and orientation-dependent drag and lift forces on both the translational and rotational motion of the particles is accounted for to ensure accurate representation of the flow dynamics of non-spherical particles. The size of the particles studied was obtained based on an equivalent-volume sphere, and differing shapes were modelled using super-quadratic ellipsoid forms by varying their aspect ratio, with their orientation predicted using the incidence angle between the particle relative velocity and the particle principal axis. Results are presented for spherical, needle- and platelet-like particles at a number of different boundary layer locations along the wall-normal direction within the channel. The time evolution and probability density function of selected particle translational and rotational properties show a clear distinction between the behaviour of the various particles types, and indicate the significance of particle shape when modelling many practically relevant flows.     

Translation and rotation of particles in different flow pattern areas of a silo

The present paper reports the results obtained for translational and rotational velocity profiles of spherical particles for the mixed flow in a conical silo. The discrete element method (DEM) based on Hertz-Mindlin (no slip) with RVD rolling friction contact model is used for simulations. Opposite correlations are found between translational and rotational velocities in different flow areas of the silo. In particular, the abrasion caused by rotation is dominant in the funnel flow area. In addition, increase of the mass flow rate of silo can effectively reduce the abrasion induced by rotation. This highlights that understanding of dynamic characteristics of particles is helpful for optimization of silos and reduction of granular material abrasion.     

Simulations of magnetorheological suspensions in Poiseuille flow

Particle-level simulations are conducted to study magnetorheological fluids in plane Poiseuille flow. The importance of the boundary conditions for the particles at the channel walls is examined by considering two extreme cases: no friction and infinite coefficient of friction. The inclusion of friction produces Bingham fluid behavior, as commonly observed experimentally for MR suspensions. Lamellar structures, similar to those reported for electrorheological fluids in shear flow, are observed in the post-yield region for both particle boundary conditions. The formation of these lamellae is accompanied by an increase in the bulk fluid velocity. The slip boundary condition produces higher fluid velocities and thicker lamellar structures.     

Simulation of proppant transport at intersection of hydraulic fracture and natural fracture of wellbores using CFD-DEM

Proppants transport is an advanced technique to improve the hydraulic fracture phenomenon, in order to promote the versatility of gas/oil reservoirs. A numerical simulation of proppants transport at both hydraulic fracture (HF) and natural fracture (NF) intersection is performed to provide a better understanding of key factors which cause, or contribute to proppants transport in HF–NF intersection. Computational fluid dynamics (CFD) in association with discrete element method (DEM) is used to model the complex interactions between proppant particles, host fluid medium and fractured walls. The effect of non-spherical geometry of particles is considered in this model, using the multi-sphere method. All interaction forces between fluid flow and particles are considered in the computational model. Moreover, the interactions of particle–particle and particle–wall are taken into account via Hertz–Mindlin model. The results of the CFD-DEM simulations are compared to the experimental data. It is found that the CFD-DEM simulation is capable of predicting proppant transport and deposition quality at intersections which are in agreement with experimental data. The results indicate that the HF–NF intersection type, fluid velocity and NF aperture affect the quality of blockage occurrence, presenting a new index, called the blockage coefficient which indicates the severity of the blockage.     

A new model for dissipative particle dynamics boundary condition of walls with different wettabilities

The implementation of solid-fluid boundary condition has been a major challenge for dissipative particle dynamics(DPD) method. Current implementations of boundary conditions usually try to approach a uniform density distribution and a velocity profile close to analytical solution. The density oscillations and slip velocity are intentionally eliminated, and different wall properties disappear in the same analytical solution. This paper develops a new wall model that combines image and frozen part...     

Particle dynamics calculations of wall stresses and slip velocities for couette flow of smooth inelastic spheres

The objective of this work is to characterize the effects of boundary geometry on the flow of dry granular materials composed of smooth, inelastic spheres between parallel, bumpy walls in the absence of gravity. Particle dynamic simulations are done in which wall stresses and slip velocities are computed over a wide range of parameters, including shear gap height, geometry of the wall particles, wall to free particle diameter ratio, and normal restitution coefficient. Calculated wall stresses and slip velocities are found to be highly sensitive to boundary roughness, which is characterized in terms of the mean spacing value for the wall particles. Most noticeable is a pronounced stress drop for dense flows with an associated large slip velocity in a system having a narrow shear gap height.     

Application of DEM modified with enlarged particle model to simulation of bead motion in a bead mill

We applied the discrete element method （DEM） of simulation modified by an enlarged particle model to simulate bead motion in a large bead mill. The stainless-steel bead mill has inner diameter of 102 mm and mill length of 198 mm. The bead diameter and filling ratio were fixed respectively at 0.5 mm and 85%. The agitator rotational speed was changed from 1863 to 3261 rpm. The bead motion was monitored experimentally using a high-speed video camera through a transparent mill body. For the simulation, enlarged particle sizes were set as 3-6 mm in diameter. With the DEM modified by the enlarged particle model, the motion of enlarged particles in a mill was simulated.The velocity data of the simulated enlarged particles were compared with those obtained in the experiment. The simulated velocity of the enlarged particles depends on the virtual frictional coefficient in the DEM model. The optimized value of the virtual frictional coefficient can be determined by considering the accumulated mean value. Results show that the velocity of the enlarged particles simulated increases with an increase in the optimum virtual frictional coefficient, but the simulated velocity agrees well with that determined experimentally by optimizing the virtual frictional coefficient in the simulation. The computing time in the simulation decreases with increased particle size.     

Numerical simulation of the hydrodynamic interaction between a sedimenting particle and a neutrally buoyant particle

A new method for the simulation of the translational and rotational motions of a system containing a sedimenting particle interacting with a neutrally buoyant particle has been developed. The method is based on coupling the quasi-static Stokes equations for the fluid with the rigid body equations of motion for the particles. The Stokes equations are solved at each time step with the boundary element method. The stresses are then integrated over the surface of each particle to determine the resultant forces and moments. These forces and moments are inserted into the rigid body equations of motion to determine the translational and rotational motions of the particles. Unlike many other simulation techniques, no restrictions are placed on the shape of the particles. Superparametric boundary elements are employed to achieve accurate geometric representations of the particles. The simulation method is able to predict the local fluid velocity, resolve the forces and moments exerted on the particles, and track the particle trajectories and orientations.     

Circulation intensity and axial dispersion of non-cohesive solid particles in a V-blender via DEM simulation

In this study, discrete element method （DEM） was employed to simulate the movement of non-cohesive mono-dispersed particles in a V-blender along with particle-particle and particle-boundary interactions. To validate the model, DEM results were successfully compared to positron emission particle tracking （PEPT） data reported in literature. The validated model was then utilized to explore the effects of rotational speed and fill level on circulation intensity and axial dispersion coefficient of non-cohesive particles in the V-blender. The results showed that the circulation intensity increased with an increase in the rotational speed from 15 to 60 rpm. As the fill level increased from 20% to 46%, the circulation intensity decreased, reached its minimum value at a fill level of 34% for all rotational speeds, and did not change significantly at fill levels greater than 34%. The DEM results also revealed that the axial dispersion coefficient of particles in the V-blender was a linear function of the rotational speed. These trends were in good agreement with the experimentallv determined values reported bv previous researchers.     

近壁泊肃叶流中活性粒子迁移规律研究

研究活性粒子在剪切流中的迁移规律对实现颗粒分离和过程强化均具有重要意义.基于耗散粒子动力学理论,建立了描述微通道内近壁泊肃叶流中活性粒子迁移运动的数学模型,考察了活性粒子圆周运动角速度、手性诱导角速度、直行运动速度和转向扩散系数对大肠杆菌和常规活性粒子横向迁移速度和受迫转向频率的影响规律,并确定近壁剪切流中活性粒子横向迁移的形成机制.结果表明,近壁剪切流场中大肠杆菌的横向迁移速度随剪切速率增大先快速增加继而趋于稳定;大肠杆菌横向迁移速度随圆周运动角速度增大而减小,随手性诱导角速度、直行运动速度和转向扩散系数的增大而增大;大肠杆菌的受迫转向频率受圆周运动角速度、直行运动速度和转向扩散系数的影响小,而随手性诱导角速度的增大而加快;相比大肠杆菌,常规活性粒子横向迁移速度显著减小、受迫转向频率明显变慢,二者受直行运动速度和转向扩散系数的影响规律与大肠杆菌类似.直行运动是活性粒子形成横向迁移运动的前提,其他运动参数和结构参数均可一定程度促进或抑制活性粒子在近壁剪切流场中的横向迁移.