On equation of discrete solid particles' motion in arbitrary flow field and its properties

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亚微米颗粒在汇作用下运动机理的实验研究

当前,城市空气质量的不断恶化,引起了公众的普遍性关注.空气中的悬浮颗粒物,是城市大气环境重要污染源之一,其分布、运动及扩散规律已成为科学领域的研究热点.与连续流体不同,大气中的悬浮颗粒物是离散的,确定颗粒运动的模型是研究大气细微颗粒污染问题的关键.本文拟研究小空间静稳空气中亚微米级颗粒在汇作用下的运动规律,并构建其运动模型.在密闭实验空间中通过燃烧生成亚微米颗粒,利用静电吸附装置模拟颗粒汇,并通过粒子图像测速(particle image velocimetry,PIV)实验和激光多普勒测速仪(lasser Doppler velocimeter,LDV)实验技术测量分析不同空间内亚微米颗粒在大气中的热运动速度和在汇作用下的运动规律,并推导出颗粒物的速度分布经验公式.结果显示:粒子在汇作用下的运动与连续流体汇运动规律类似,但在小空间内颗粒的运动不满足流体连续方程;说明在无气流夹带输运情况下,利用汇作用及颗粒的扩散而发展的颗粒净化技术是可行的.     

Effect of particle size on a two-phase turbulent jet

The effect of particle size on two-phase turbulent jet flow structure is studied in the present experimental investigation. Polystyrene solid particles of 210, 460, and 780 μm were used. The particles' mass loading ratios ranged from 0 to 3.6. The flow Reynolds number was 2 ‘ 10⁴, which was based on the pipe nozzle diameter and the fluid-phase centerline velocity at the nozzle exit. A two-color laser-Doppler anemometer (LDA), combined with the amplitude discrimination method and the velocity filter method, was employed for measurement. The measurement range of the jet flow was from the initial pipe exit to 90D downstream. Results are presented for the mean velocities of particle and fluid phases, the flow's turbulent intensities and the flow's Reynolds stresses. The energy spectra and the correlation functions of the two-phase jet flow were also obtained by using another one-component He-Ne LDA system.     

Numerical simulation of the accumulation of heavy particles in a circular bounded vortex flow

In present work, an Eulerian–Lagrangian CFD model based on the discrete element method (DEM) and immersed boundary method (IBM) has been developed, validated and used to investigate the accumulation of heavy particles in a circular bounded viscous vortex flow. The inter-particle and particle-wall collisions are resolved by a hard-sphere model. Effects of one-way and two-way coupling, Reynolds number, and particle diameter are systematically explored. Results show that, in case of one-way coupling, the majority of particles will spiral into an accumulation point located near the stagnation point of the flow field. The accumulation point represents a stable equilibrium point as the drag created by the flow field balances the destabilizing centrifugal force on the particle. However, in case of two-way coupling, there does not exist a stable accumulation point due to the strong interaction between the particles and fluid dynamics. Instead most particles are expelled from the circular domain and accumulate on the confining wall. The percentage of accumulated particles on the wall increases with increasing Reynolds number and particle diameter. Moreover, influence of three well-known drag models is also studied and they give consistent results on the particle accumulation behavior, although small quantitative differences can still be discerned.     

气固两相混合层二维涡配对的数值研究

采用双向耦合模型中的速度耦合模型,数值模拟了气固两相混合层中涡的配对、合并过程,文中采用拟谱方法对流场进行了直接数值模拟,用Lagrange模型跟踪固粒,结果发现,在双向耦合过程中Stokes数仍然是重要的参数,但不是唯一影响流场的参数,流场的发展还与固粒的体积浓度、固粒的相对密度以及固粒大小等因素有关。固粒抑制流场的发展,阻碍涡的配对、合并,加快了涡量的扩散;小St数的固粒仍然跟随流体运动,大St数的固粒趋向于低涡量区的趋势减弱。     

湍流边界层中固体颗粒运动的数值模拟

根据Ｌａｇｒａｎｇｅ颗粒运动微分方程及不可压缩湍流边界层中流体的壁面速度分布规律，数值求解了颗粒在湍流边界层中的运动，考虑了Ｓａｆｆｍａｎ升为对颗粒运动的影响，壁面对运动阻力的影响，给出了固体颗粒沉积边壁，在边界层外缘上所需的最小速度和最小入射角，计算结果还表明边界层对固体颗粒撞击边壁的速度和入射角有较大影响，从数值结果可可以发现一个重要现象。     

空间模式下可压缩气固两相混合层流动特性研究

采用Euler-Lagrange颗粒-轨道双向耦合模型对空间模式下含有固粒的二维可压缩混合层流场进行了研究。气相流场采用具有空间三阶精度的WNND格式进行数值模拟,固相方程采用单边三点差分离散。在考虑流场对固粒作用的同时,也计及了固粒对流场的反作用。在对流马赫数为0.5时,研究了颗粒相对密度、颗粒尺寸、Stokes数等因素对粒子运动和流场结构的影响。研究结果表明:在可压缩空间模式混合层中,固粒的Stokes数仍然是主要影响参数之一;相同Stokes数下不同密度的固粒对流场的干扰不同,轻固粒对流场的干扰明显要小。     

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LBM-DEM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。     

A hybrid Eulerian–Lagrangian method to simulate the dispersed phase in turbulent gas-particle flows

This paper presents a methodology to combine stochastic Lagrangian approach and continuum model to simulate the dispersed phase in gas-particle turbulent flows using that both approaches are based on the same Boltzmann-like kinetic equation governing the joint fluid-particle probability density function (pdf). The proposed hybrid method is based on the separate application of each approach in two adjacent domains and their coupling at the interface via flux boundary conditions. Validation of the method is carried out for non-colliding solid particles suspended in homogeneous turbulent shear flow without two-way coupling.     

基于流固耦合作用的抽水引起地面沉降计算模型

过量开采地下承压水会引起地基土体的变形，进一步导致地面沉降和地裂缝等地质灾害，抽水引起地层的变形过程是土层中孔隙水和土颗粒相互作用的复杂过程．两步法和流固耦合法是计算地面沉降常用的两种思路，它们的适用性各有不同．本文基于理想的地层模型，分别建立两步法计算模型和流固耦合计算模型，利用FLAC3D6.0有限差分软件对模型进行求解，对比分析两类方法计算结果的差异．结果表明流固耦合法符合力学机制，能够更好地反映土体变形的发展过程，但计算时间更长．而两步法适用于稳定流，在渗流达到稳定流时两步法计算的孔压分布和地面沉降能够满足工程精度要求．本文的研究成果为合理选取地面沉降计算方法提供依据．     

等离子熔射粉末颗粒飞行过程格子Boltzmann法仿真

为考察等离子熔射过程中粉末的飞行过程,本文在已开发的正六边形7-b it格子Bo ltzm ann(LB)方法等离子射流的温度场和速度场的计算模型基础上,采用单个颗粒加速方程,建立了一个随机算法,实现了对粉末颗粒在射流场中运动过程的仿真;计算结果通过动画演示了粉末飞行的全过程,表明初始位置越靠近射流场出口中心的粉末颗粒加速越充分,并且在射流场一定的情况下,减小粉末颗粒直径可以提高粉末速度,但会降低粉末利用率。     

考虑热流固耦合作用的多孔介质孔隙尺度两相流动模拟

为了研究深层油气资源在岩石多孔介质内的运移过程, 使用一种基于Darcy-Brinkman-Biot的流固耦合数值方法, 结合传热模型, 完成了Duhamel-Neumann热弹性应力的计算, 实现了在孔隙模拟多孔介质内的考虑热流固耦合作用的两相流动过程. 模型通过求解Navier-Stokes方程完成对孔隙空间内多相流体的计算, 通过求解Darcy方程完成流体在岩石固体颗粒内的计算, 二者通过以动能方式耦合的形式, 计算出岩石固体颗粒质点的位移, 从而实现了流固耦合计算. 在此基础上, 加入传热模型考虑温度场对两相渗流过程的影响. 温度场通过以产生热弹性应力的形式作用于岩石固体颗粒, 总体上实现热流固耦合过程. 基于数值模型, 模拟油水两相流体在二维多孔介质模型内受热流固耦合作用的流动过程. 研究结果表明: 热应力与流固耦合作用产生的应力方向相反, 使得总应力比单独考虑流固耦合作用下的应力小; 温度的增加使得模型孔隙度增加, 但当注入温差达到150 K后, 孔隙度不再有明显增加; 温度的增加使得水相的相对渗流能力增加, 等渗点左移.      

Superquadric DEM-SPH coupling method for interaction between non-spherical granular materials and fluids

The research on the coupling method of non-spherical granular materials and fluids aims to predict the particle–fluid interaction in this study. A coupling method based on superquadric elements is developed to describe the interaction between non-spherical solid particles and fluids. The discrete element method (DEM) and the smoothed particle hydrodynamics (SPH) are adopted to simulate granular materials and fluids. The repulsive force model is adopted to calculate the coupling force and then a contact detection method is established for the interaction between the superquadric element and the fluid particle. The contact detection method captures the shape of superquadric element and calculates the distance from the fluid particle to the surface of superquadric element. Simulation cases focusing on the coupling force model, energy transfer, and large-scale calculations have been implemented to verify the validity of the proposed coupling method. The coupling force model accurately represents the water entry process of a spherical solid particle, and reasonably reflects the difference of solid particles with different shapes. In the water entry process of multiple solid particles, the total energy of the water entry process of multiple solid particles tends to be stable. The collapse process of the partially submerged granular column is simulated and analyzed under different parameters. Therefore, this coupling method is suitable to simulate fluid–particle systems containing solid particles with multiple shapes.     

Effect of inertial particles with different specific heat capacities on heat transfer in particle-laden turbulent flow

The effect of inertial particles with different specific heat on heat transfer in particle-laden turbulent channel flows is studied using the direct numerical simulation(DNS) and the Lagrangian particle tracking method. The simulation uses a two-way coupling model to consider the momentum and thermal interactions between the particles and turbulence. The study shows that the temperature fields display differences between the particle-laden flow with different specific heat particles and the particle-free flow,indicating that the particle specific heat is an important factor that affects the heat transfer process in a particle-laden flow. It is found that the heat transfer capacity of the particle-laden flow gradually increases with the increase of the particle specific heat. This is due to the positive contribution of the particle increase to the heat transfer. In addition,the Nusselt number of a particle-laden flow is compared with that of a particle-free flow.It is found that particles with a large specific heat strengthen heat transfer of turbulent flow, while those with small specific heat weaken heat transfer of turbulent flow.     

CFD–DEM simulation of particle deposition characteristics of pleated air filter media based on porous media model

In this study, the three-dimensional physical model of pleated air filtration media was simplified to porous media model, and the calculation parameters of porous media were obtained based on experimental data. The model of V-shaped pleated air filter media is constructed, the height of the media pleat is 50 mm and the pleat thickness is 4 mm, the pleat angle is 3.7°. The Hertz-Mindlin contact model was modified by Johnson Kendall Roberts (JKR) adhesion contact model. The deposition process of particles in media was simulated based on computational fluid dynamics (CFD) theory and discrete element method (DEM). Results show that the CFD–DEM coupling method can be effectively applied to the macro research of pleated air filter media. The particles will form dust layer and dendrite structure on the fiber surface, and the dust layer will affect the subsequent air flow organization, and the dendrite structure will eventually form a “particle wall”. The formation of the “particle wall” will prevent the particles from moving further in the fluid domain, which makes area of pleated angle become the “low efficiency” part about the particle deposition. Compared with area of pleated angle, the particles are concentrated in the opening area and the middle area of the pleated to agglomerate and deposit.     

An extended unresolved CFD-DEM coupling method for simulation of fluid and non-spherical particles

This study develops an extended unresolved CFD-DEM coupling method for simulation of the fluid–solid flow with non-spherical particles. The limitation of fluid grid size is discussed, by simulating the settling of a cylinder in a Newtonian fluid based on the resolved and unresolved CFD-DEM coupling method. Then, the calculation of porosity and the fluid–particle relative velocity based on the particle shape enlargement method for simulation of non-spherical particles is proposed. The availability of the particle shape enlargement method for the simulation of non-spherical particles with different sphericity is discussed in this work, by comparing it with the results from the equivalent diameter enlargement method. The limitation of the equivalent diameter enlargement method for non-spherical particles is revealed from the simulation results. Several typical cases are employed to elaborate and verify the extended unresolved CFD-DEM method based on particle shape enlargement method, by presenting a good consistency with the experimental results. It proves that the extended unresolved CFD-DEM method is suitable for different CFD grid size ratios, and consolidates that it is a universal calculation method for CFD-DEM coupling simulation.     

Numerical investigation of fluid–particle interactions for embolic stroke

Roughly one-third of all strokes are caused by an embolus traveling to a cerebral artery and blocking blood flow in the brain. The objective of this study is to gain a detailed understanding of the dynamics of embolic particles within arteries. Patient computed tomography image is used to construct a three-dimensional model of the carotid bifurcation. An idealized carotid bifurcation model of same vessel diameters was also constructed for comparison. Blood flow velocities and embolic particle trajectories are resolved using a coupled Euler–Lagrange approach. Blood is modeled as a Newtonian fluid, discretized using the finite volume method, with physiologically appropriate inflow and outflow boundary conditions. The embolus trajectory is modeled using Lagrangian particle equations accounting for embolus interaction with blood as well as vessel wall. Both one- and two-way fluid–particle coupling are considered, the latter being implemented using momentum sources augmented to the discretized flow equations. It was observed that for small-to-moderate particle sizes (relative to vessel diameters), the estimated particle distribution ratio—with and without the inclusion of two-way fluid–particle momentum exchange—were found to be similar. The maximum observed differences in distribution ratio with and without the coupling were found to be higher for the idealized bifurcation model. Additionally, the distribution was found to be reasonably matching the volumetric flow distribution for the idealized model, while a notable deviation from volumetric flow was observed in the anatomical model. It was also observed from an analysis of particle path lines that particle interaction with helical flow, characteristic of anatomical vasculature models, could play a prominent role in transport of embolic particle. The results indicate therefore that flow helicity could be an important hemodynamic indicator for analysis of embolus particle transport. Additionally, in the presence of helical flow, and vessel curvature, inclusion of two-way momentum exchange was found to have a secondary effect for transporting small to moderate embolus particles—and one-way coupling could be used as a reasonable approximation, thereby causing substantial savings in computational resources.     

稠密颗粒床中点火传火过程的两维两相流数值模拟

本文结合火炮工程背景,建立伴随化学反应的稠密颗粒群气固两相非定常两维轴对称流和点火器内的非定常一维两相流数学模型,并考虑点火器对主装药床的耦合作用。采用MacCormack两步差分格式,模拟点火传火过程中火焰传播及其物理量沿轴向和径向变化规律。部分计算结果与实验结果相吻合。发现点火开始阶段存在明显的径向效应,当火焰波传到固壁后,轴向流占主要优势。     

Direct numerical simulation of particle dispersion in a turbulent jet considering inter-particle collisions

To investigate the behaviour of inter-particle collision and its effects on particle dispersion, direct numerical simulation of a three-dimensional two-phase turbulent jet was conducted. The finite volume method and the fractional-step projection algorithm were used to solve the governing equations of the gas phase fluid and the Lagrangian method was applied to trace the particles. The deterministic hard-sphere model was used to describe the inter-particle collision. In order to allow an analysis of inter-particle collisions independent of the effect of particles on the flow, two-way coupling was neglected. The inter-particle collision occurs frequently in the local regions with higher particle concentration of the flow field. Under the influence of the local accumulation and the turbulent transport effects, the variation of the average inter-particle collision number with the Stokes number takes on a complex non-linear relationship. The particle distribution is more uniform as a result of inter-particle collisions, and the lateral and the spanwise dispersion of the particles considering inter-particle collision also increase. Furthermore, for the case of particles with the Rosin–Rammler distribution (the medial particle size is set d₅₀ = 36.7 μm), the collision number is significantly larger than that of the particles at the Stokes number of 10, and their effects on calculated results are also more significant.