DIRECT NUMERICAL SIMULATION OF VORTEX STRUCTURES AND PARTICLE DISPERSION IN TURBULENT FREE SHEAR FLOWS

In order to provide references for the study of jets from combustor and associated industrial applications, direct numerical simulation was employed to study a three-dimensional temporally evolving plane mixing layer laden with particles in the upper region initially. The coherent structures in the mixing layer between two parallel streams were simulated using a pseudo-spectral method. Particles with different Stokes numbers were traced using the Lagrangian approach based on one-way coupling between the continuous and the dispersed phases. Both the large-scale vortex structures and the particle dispersion patterns with different Stokes numbers were investigated. The results clearly showed that particle dispersion is closely related to the large-scale organized structures and the three-dimensionality. Particles with Stokes number of the order of unity were found to have the largest concentration on the outer edges of the large-scale vortex structures, and the variation of particle concentration along the spanwise direction increased with the development of the three-dimensionality, which was mainly due to the presence of the streamwise large-scale structures. When the counter-rotating “rib” large-scale vortices paired, part of the particles were thrown out from the high concentration area in the upper region to the lower region of the mixing layer and finally developed into a “mushroom” pattern of the particle distribution along the spanwise direction.     

三维混合层中大涡结构与扩散颗粒的相互作用

In order to understand the interaction between large-scale vortex structure and particles, a two-way coupling temporal mixing layer laden with particles at a Stokes number of 5 with different mass loading planted initially in the upper half region is numerically studied. The pseudospectral method is used for the flow fluid and the Lagrangian approach is employed to trace particles. The momentum coupling effect introduced by a particle is approximated to a point force. The simulation results show that the coherent structures are still dominant in the mixing layer, but the large-scale vortex structure and particle dispersion are modulated. The length of large-scale vortex structure is shortened and the pairing is delayed. At the same time, the particles are distributed more evenly in the whole flow field as the mass loading is increased, but the particle dispersion along the transverse direction differs from that along the spanwise direction, which indicates that the effect by the addition of particle on the suanwise large-scale vortex structure is different from the streamwise counterpart.     

Transient, three-dimensional simulation of particle dispersion in flows around a circular cylinder Re = 140-260)

To understand three-dimensional dispersion of particles in the flow around a bluff body, direct numerical simulations of particle-laden wakes of a circular cylinder with Reynolds number ranging from 140 to 260 were performed. The domain decomposition method and high-order finite-difference schemes were applied to solve the fluid flow. A Lagrangian tracking solver was developed to trace the trajectories of each particle in the non-uniform grid system. It is observed that the predicted coherent structures and vortex dislocation frequency agree well with those of previous experimental studies. Particles at low Stokes numbers follow the vortex motion and can disperse into the core regions of the vortex-street and the recirculating region. Particles at larger Stokes numbers try to maintain their own motion, but take on a non-uniform distribution in the wake due to the strong folding effect of shed vortices. But for particles at intermediate Stokes numbers, higher concentration occurs in the outer boundary regions of the vortices as well as the thin band regions connecting adjacent vortex structures. Besides the spanwise vortex structures, the streamwise vortex tubes also have a remarkable effect on particle dispersion, which results in a ‘mushroom’ shape particle distribution in the wake. Furthermore, non-uniform forcing at inlet has additional effect on particle dispersion and mixing.     

Numerical Simulation of Particle Dispersion in the Wake of a Circular Cylinder

In this article, numerical simulation of the Navier-Stokes equations was performed for the large-scale structures of a two-dimensional temporally developing cylinder flow and the associated dispersion patterns of particles were simulated. The time-dependent Navier-Stokes equations were integrated in time using a mixed explicit-implicit operator splitting rules. The spatial discretization was processed using spectral-element method. Non-reflecting conditions were employed at the outflow boundary. Particles with different Stokes numbers were traced by the Lagrangian approach based on one-way coupling between the continuous and the dispersed phases.

The simulation results of the flow field agree well with experimental data. Due to the effects of the coherent structures, the particles demonstrate a more organized dispersion process in the space and a periodic dispersion characteristic in the time. Particle dispersion increases with the flow Reynolds number and so does for particle concentration, which is independent of particle size. However, for particles at different Stokes numbers, the dispersion patterns are different. The particles at smaller Stokes number congregate mainly in the vortex core regions and the particles at larger Stokes number disperse much less along the lateral direction with the even distribution. The higher density distribution at the outer boundary of large-scale vortex structure characterizes the dispersion pattern of particles at the Stokes numbers of order of unity. Furthermore, these particles disperse largely along the lateral direction and show the nonuniform distribution of concentration.     

Modelling of heavy and buoyant particle dispersion in a two-dimensional turbulent mixing layer

Heavy and buoyant particle dispersion in the turbulent mixing layer was investigated numerically using a two-phase flow discrete vortex modelling. It was revealed from the modelling that inclusion of two-way momentum coupling is essential for properly modelling heavy particle dispersive transport in turbulent free shear flows. For heavy particles with small Stokes numbers, the dispersion is predominated by the large-scale vortex structures and they exert small influence on the carrier fluid flow. Heavy particles with large St directionally align along the braid region between the neighbouring vortices. However, the lateral dispersion of particles of large St is smaller than that of particles of small St.For buoyant particles with the density being slightly greater than that of the carrier fluid, numerical simulation revealed that the buoyant particles scatter over the whole vortex core rather than collect along the fringes of the vortex. The Lagrangian statistics calculation of buoyant particle dispersion showed that both the inertial and crossing-trajectory effects affect the particle dispersion behaviour and particle eddy diffusivity. The dispersion behaviour of buoyant particles is highly associated with the particle Stokes number. Large St buoyant particles exhibit a larger dispersion. It was also indicated from the numerical simulation that buoyant particles might disperse larger than the fluid tracers. The correlation between the buoyant particle and fluid tracer velocities was affected by including the coupling effect.     

气固可压缩湍射流中颗粒扩散的直接数值模拟

A numerical method was developed to directly simulate the compressible, particle-laden turbulent jets.The fourth order compact finite difference schemes were used to discretize the space derivatives. The Lagrangian method was adopted to simulate the particle motion based on one-way coupling. It is found that the turbulent intensity profiles attain self-similar status in the jet downstream regions. At the Stokes number of 1, particles are concentrated largely in the outer boundaries of the large-scale vortex structures with the most uneven distribution and the widest dispersion in the lateral direction. Particles at the much smaller Stokes numbers are distributed evenly in the flow field, and the lateral dispersion is also considerable. Distribution of particles at much larger Stokes numbers is more uniform and the lateral dispersion becomes small. In addition, the inflow conditions have different effects on the particle dispersion. The direct numerical simulation (DNS) results accord with the previous experiments and numerical studies.     

Direct numerical simulation of a three-dimensional particle laden plane mixing layer considering inter-particle collisions

To investigate the behavior of inter-particle collision and its effects on multiphase flow, the direct numerical simulation of a three-dimensional gas–solid two-phase plane mixing layer is conducted. The flow is assumed to be temporally evolving and incompressible. The particle trajectories are traced by the one-way or two-way coupled Lagrangian method separately. The deterministic hard-sphere model is used to describe the inter-particle collision. Calculations are performed for a particle Stokes numbers of 3. The results show that the preferential concentration phenomenon of particles is found after the beginning of the rolling up of the large-scale vortex structures due to the influence of the vortex. It is also found that the inter-particle collision occurs frequently in the local regions with higher particle concentration of the flow field. The evolution of inter-particle collision can be divided into 3 stages under the influence of the growth of the vortex and the particle dispersion. The results under the two-way coupling show that the particle distribution is more uniform. The modifications of the mixed fluid thickness, the Reynolds stresses, and the mean stream-wise velocity of two phases due to inter-particle collision are quantitatively investigated.     

受限空间内空心锥形喷雾-横流掺混规律

在自建的冷态横流-旋流喷雾两相掺混系统实验台上,采用PIV测量了掺混通道内气液两相掺混过程中液滴群的运动特性,获得了掺混流场中不同位置的液滴分布图像与流场结构特性。实验段结构为方腔（横截面尺寸为95 mm×95 mm）,喷嘴采用空心锥形雾化喷嘴。对影响掺混效果的主要参数（横流速度、喷嘴雾化压力、喷嘴雾化粒径）进行了详细研究,绘出了最佳掺混效果下各参数关系曲线。掺混过程主要受不同尺度的旋涡结构影响,液滴多富集于旋涡边缘,稳定的大尺度涡不利于掺混。提高掺混效果的途径即是避免流场中出现稳定的大尺度旋涡结构,采用喷嘴前倾布置、增加喷嘴个数、确定合适的横流速度均是提高掺混效果的有效途径。分析方法与研究结果为工程实际应用中掺混室结构的设计及掺混性能的改进提供了依据和参考。     

Simultaneous numerical simulation of nano and fine particle coagulation and dispersion in a round jet

The discrete vortex method coupled with the particle-tracking and moment methods have been used to calculate the trajectories of individual nano and fine particles undergoing the coagulation and dispersion in a round jet. The results show that the Stokes number plays an important role in the dispersion of fine particles. The radial dispersion of fine particles for the wavenumber of 5 is the largest when an azimuthal perturbation is introduced to its flow. However, the width of particle dispersion is not always proportional to the amplitude of the azimuthal perturbation. As the round jet flow develops, the mass concentration of nanoparticles within the jet core decreases continuously. The number concentration of nanoparticle decreases rapidly near the jet exit, then decreases slowly in the far field until an asymptotic state is attained. Both the polydispersity and diameter of nanoparticle increase, the rate of increase for the former is the largest near the jet exit, and the largest particles are found near the jet core. The particle diameter is relatively constant across the width of the jet within and outside the jet core except the region near the interface of the jet and the outside. The standard deviation of nanoparticle size distribution increases at the jet exit, approaching slowly at an asymptotic value in the jet core. The minimum and maximum of standard deviation appear between the jet core and edge, and at the jet centerline, respectively. The region of mixing of nanoparticles becomes narrower, however, the particle number concentration grows with increasing Schmidt number. The particle size increases along the radial direction with increasing Damkohler number, and the standard deviation does not change when the Damkohler number is monotonous.     

Detached eddy simulation of particle dispersion in a gas-solid two-phase fuel rich/lean burner flow

To predict the particle dispersion in the burner jet is of great importance in industrial application and in the designing of coal burner with good performance. The objective of this study was to numerically investigate the particle dispersion mechanisms in the gas-solid two-phase jet from a fuel rich/lean burner. The detached-eddy-simulation (DES) approach was employed to study the turbulent flow in the fuel rich/lean separator and the gas-solid multiphase jet from the exit of a fuel rich/lean burner. The vortex shedding process was simulated and its effect on the fuel rich/lean separating performance was evaluated. Combined with the Lagrangian tracking procedure for the particle phase, the coal particles with various Stokes numbers equal to 0.000434, 0.043, 1.08, 4.34 (corresponding to particle diameter 1, 10, 50, 100 μm, respectively) in the gas-solid fuel rich/lean jet were studied, which shows that although there are coherent vortex structures behind the central partition plate, these vorticities are small, the fuel-rich stream and the fuel lean stream will not mix quickly downstream of the exit of the nozzle. The large vortex structures at the jet outer boundary are the main factors that make the small particles to mix together. The coal particles with large Stokes number (St>1) disperse very slowly in the jet flow, which realizes the fuel rich/lean combustion in a rather long distance downstream the exit of the nozzle, resulting in low NO_x emissions.     

不同油气参数下SVQS系统内颗粒浓度场的模拟

为研究提升管出口旋流快分(SVQS)系统中催化剂颗粒的运动规律与浓度分布特征,对不同油气参数下SVQS系统的气固分离进行了数值流动模拟,从粒径、浓度分布和颗粒返混三个方面进行了分析。结果表明,在入口浓度一定的情况下,油气密度越大或黏度越小,封闭罩壁面处颗粒浓度越大,呈螺旋状分布的细带带宽和螺距不同;在隔流筒盖板处有颗粒大量聚集,并形成具有周期脱落特点的颗粒环带;隔流筒底部出现的“短路流”现象,削弱了SVQS系统的分离性能;不同粒径的颗粒下行时始终处于螺旋分层排列状态,且SVQS系统对直径为0.013 mm以下的细小颗粒的分离性能较弱。油气密度越大或黏度越小,颗粒间的跟随性更佳,浓度分布更均匀,SVQS系统对中、细颗粒的捕捉能力变强。SVQS系统中存在颗粒返混现象,且随油气密度增大或黏度减小而削弱。削弱颗粒返混、抑制颗粒高峰是提高该系统分离性能的重要方式。     

Numerical study of particle dispersion in the wake of gas-particle flows past a circular cylinder using discrete vortex method

A numerical investigation on the particle dispersion in the wake of particle-laden gas flows past a circular cylinder at Reynolds number of 10⁵ is presented. In the numerical method, the Discrete Vortex Method with the diffusion velocity model is employed to calculate the unsteady gas flow fields and a Lagrangian approach is applied to track individual particles. A dispersion function is defined to represent the dispersion scale of the particle. The distributions of gas velocities and vortex blobs, the trajectories and dispersion functions as well as distributions for particles with various Stokes numbers ranging from 0.01 to 1000 are obtained. The numerical results show that: (1) very small sized particles with St = 0.01 can distribute both in the vortex core and around the vortex periphery, whereas intermediate sized particles with St = 1.0, 10 are distributed around the vortex periphery, and very large sized particles with St = 1000 do not feel the gas flow; (2) only at small Stokes number (St = 0.01, 0.1) the particles do not impact with the cylinder; (3) the particle's dispersion intensity decreases precipitously as St is increased from 0.01 to 10.     

Fluid–particle correlated motion and turbulent energy transfer in a two-dimensional particle-laden shear flow

Discrete vortex simulations of a dilute two-dimensional particle-laden shear layer with one-way coupling were performed to study fluid–particle correlated motion and the transfer of turbulent kinetic energy between the phases. The resulting modification of carrier phase turbulence, estimated according to current computational models, was evaluated. Particle Stokes numbers were between 1.0 and 4.5, so that the particles showed considerable temporal concentration fluctuations due to centrifuging by the fluid flow structures, and the mass loading was 12% corresponding to a volume fraction of 6.0×10^?5.Fluid velocities and particle concentration and velocities and their covariances, which appear in a commonly used model equation for carrier phase turbulence modification, were evaluated. Additionally, the probability density functions of fluid velocity fluctuations viewed by the particles are presented and compared with their Eulerian counterparts. It was found that particles view reduced velocity fluctuations due to preferential clustering. The model for carrier-phase turbulence modification predicted turbulence reduction, depending on the particle Stokes number. The mechanism responsible for turbulence reduction was the correlated velocity fluctuations of fluid and particles and this reduction could reach values up to one third of the fluid flow dissipation. Preferential particle concentration together with a relative velocity between the phases could generate turbulent kinetic energy of the gas phase, however this production was nearly an order of magnitude smaller compared to reduction of turbulence due to the correlated motion. The findings were compared with experiments available in the literature and help to clarify the view when turbulence reduction or augmentation occurs.     

Three-Dimensional Simulation of Exhaust Particle Dispersion and Concentration Fields in the Near-Wake Region of the Studied Ground Vehicle

In the present study, the interaction effects of different vehicle speeds and exhaust tailpipe exit velocity and temperature conditions on the three-dimensional flow structure, exhaust particle dynamic behavior, formation and evolution processes (i.e., nucleation, coagulation, condensation, and dispersion), number and volume concentration, and nucleation rate fields in the near-wake region behind the studied ground vehicle in urban road microenvironment were comprehensively simulated using large-eddy simulation (LES) with the aerosol dynamics and dispersion model based on our recently established direct quadrature method of moments (DQMOM) approach. The numerical results show that the particles are drawn up into the recirculation region and then moved toward the upper vortex by its lower vortex accordingly. The particle nucleation rate is much more associated with the distribution of the root mean square (RMS) temperature than with that of the temperature itself. Some of the sulfuric acid concentration emitted from the vehicular exhaust tailpipe are trapped by the vortex and then recirculated back to the rear end of the studied ground vehicle. As a result, the exhaust particle concentration in this region is much higher than a farther distance downstream and the nucleation rate is also enhanced in this region. The exhaust particle volume concentration is the highest in the recirculation region behind the studied ground vehicle and is diluted gradually beyond the recirculation region in the downstream.     

MECHANISM OF PLASTICITY FROM COLLOID STANDPOINT1

Introduction .—Brief statement of present conceptions of the colloid chemists. Theory .—Plasticity is caused by the, enveloping film of colloidal material which surrounds the clay grains, this film being of opposite polarity to the grain proper. Addition of electrolyte acting as deflocculant causes outer layer of film to be attracted to ion of same polarity as clay particle, thus reducing effective diameter and increasing polarity of plasticity inducing particles. Particles which were held by surface forces of “particles in contact” are no longer in contact and are actually repelling each other due to like and increased polarity. A flocculating ion drives action in opposite direction causing an increase in depth of colloid film thereby increasing surface in actual contact and decreasing repulsive force of plasticity inducing particles. Transport numbers as well as polarity are governing factors in ions of electrolytes. Measurements .—Touch method is proven to be unreliable. Desirability of a unit of measurement. Plea for a unit of colloidality and standard method for measurement.     

圆湍射流中的拟序结构和颗粒弥散

引言 气固两相圆射流广泛存在于各种工程应用当中.仅以能源工程领域来说,煤粉的输送、分离、燃烧效率及污染物的生成控制等过程都跟煤粉颗粒在湍流气流中的弥散方式有关.预测并控制颗粒在气流中的弥散对于实现稳定燃烧、强化传热、降低噪声等有十分重要的意义.为此,需要深入理解气固两相射流中拟序结构的演化特性和不同颗粒的弥散规律.     

Time-dependent flow structures and Lagrangian mixing in Rushton-impeller baffled-tank reactor

The object of this work is to investigate the role of large-scale convective structures in promoting mixing in a stirred tank. We focus on a standard geometry (flat bottom, four-baffle reactor stirred by a six-blade Rusthon impeller) and we use an Eulerian-Lagrangian approach to investigate numerically the dispersion of fluid particles. The three-dimensional, time-dependent, fully developed flow field is calculated with a computationally efficient procedure using a RANS solver with k-ε turbulence modeling and the flow field is assessed precisely against experimental data. Then, fluid parcels are tracked in the calculated flow field. Analyzing the trajectory of fluid parcels, the segregated regions within the flow are identified and mixing indicators are calculated (mixing time, circulation length and sojour time distribution). A physical explanation is thus proposed to establish a link between large-scale mixing and complex fluid dynamics generated by the interactions of radial-discharge jet, ring vortices, and upper counter rotating vortex.     

Turbulent particle mass flux in a two-phase shear flow

This paper presents measurements of mean and rms of fluctuations of concentration, particle turbulent velocities, shear stress and covariance of the fluctuations of particle number density and particle velocities in a horizontal plane shear layer. Particle Image Velocimetry (PIV) was used to obtain simultaneously particle velocities and number densities to evaluate models for the prediction of particle dispersion in Reynolds-Averaged Navier-Stokes calculation approaches. The flow was horizontal with the low speed side on top and laden with nearly mono-dispersed 55 and 90 µm glass beads, which were injected at the upper, low speed side of the flow. The Stokes number of the particles was in the range of 0.41 to 4.3 and the drift parameter due to gravity was in the range 0.18 to 1.5. The experimental results quantified how particle ‘centrifuging’ by the large fluid vortices influenced the measured quantities. The turbulent particle mass flux was compared with models based on the gradient of mean particle concentration. Different dispersion coefficients were evaluated by introducing the measured quantities into the model equation and it was found that dispersion coefficients based on the fluid eddy diffusivity performed poorly leading to an order of magnitude errors. A dispersion coefficient in tensor form, based on the product of particle shear stress and particle integral time scale, led to good agreement with measured turbulent particle mass fluxes with errors between 0 and 50%.     

Particle dispersion in a turbulent natural convection channel flow

Direct numerical simulations of particle dispersion in the turbulent natural convection flow between two vertical walls kept at constant but different temperatures are reported. It is assumed that the particles do not affect the flow (i.e. the dilute phase approximation is adopted). Particles with different levels of inertia, or Stokes numbers (0.843≤St≤17.45), are tracked according to the drag force imposed by the fluid. The gravity force is included for two cases, St=0.843 and St=17.45. The different levels of turbulence near the wall and near the center of the channel produce, as in isothermal turbulent channel or pipe flow, a larger concentration of particles near the wall. This effect becomes more important, and the deposition velocity of particles on the wall increases, as the particle inertia is increased. The simulations at St=8.38 and St=17.45 predict similar concentration profiles and deposition velocities according to the large inertia of these particles. The deposition velocities, obtained when the gravity force is ignored in the particle equations, follow the trend observed and measured for isothermal turbulent channel flows in the diffusion impaction regime. For the conditions considered, the gravity vector imposes a strong descending motion on the particles and this produces the increase of the particle concentration near the wall and a reduction of the deposition velocities in comparison with the results without the gravity force.     

下行床气粒流动行为的Eulerian-Lagrangian模拟

采用计算流体力学和离散单元方法耦合模型（CFD-DEM）对二维下行床内的气粒流动行为进行了全床数值模拟。模拟结果展示了下行床典型操作条件下特有的气固动态流动结构：沿流动方向存在明显的入口控制区、过渡区和（完全）发展区;颗粒聚团并不是出现在浓度相对较高的入口区,而是在过渡区之后的发展过程中逐渐形成较多的、松散的动态聚团结构。下行床发展段呈现典型的近壁浓环结构,这与实验结果基本一致。考察了颗粒之间以及颗粒与壁面之间的碰撞参数对下行床内气固流动结构的影响,发现完全弹性碰撞颗粒体系在入口区呈现最快速的颗粒分散;而对本文研究的操作条件,颗粒碰撞参数对发展段时均流体力学行为只产生轻微的影响。