Three‐Dimensional Numerical Simulation of Dense Pneumatic Conveying of Pulverized Coal in a Vertical Pipe at High Pressure

A two‐fluid model based on the kinetic theory of granular flow was used to study three‐dimensional steady state flow behavior of dense phase pneumatic conveying of pulverized coal in a vertical pipe, where the average solid concentration ranges from 11 % to 30 %, and the transport pressure ranges from 2.6 Mpa to 3.3 Mpa. Since the solid concentration is rather high, a k–?–k_p–?_p model which considers the turbulence interaction between the gas and particle phase, was incorporated into the two‐fluid model. The simulation results including profiles of gas and particle phase axial velocity, profiles of solid concentration, profiles of the turbulence intensity of the particle phase, as well as the value of the pressure gradient were reported. Then, the influences of solid concentration and transport pressure on the flow behaviors were discussed. The experiment was also carried out to validate the accuracy of the simulation results which showed that the predictions of pressure gradient were in good agreement with the experimental data. Simulation results indicate that the location of maximal solid concentration deviates from the pipe center and the deviation becomes more obvious with the solid concentration increasing, which is analogous to the phenomenon in the liquid/solid flow. Besides, pressure gradient declines as the transport pressure decreases, which is validated by experiment described in the paper. Moreover, the analysis indicates that it is necessary to consider the turbulence of particles for the simulation of dense phase pneumatic conveying at high pressure.     

SCX超细分级机进料管内气固两相流数值模拟

以两相流理论为基础,对SCX超细分级机进料管改进后内部的气固两相流动特性进行了数值模拟。分析了气相压力、速度、湍动能和固相颗粒浓度的分布情况,得到了进料管内气固两相的分布规律。结果表明,加入导流片之后,进料管内气相压力、速度、湍动能径向梯度减小,偏流现象减弱;导流片阻碍了固相颗粒的横向运动,改变了颗粒的运动轨迹,使得颗粒浓度分布更加均匀,为后续的分级提供了良好的基础。     

The turbulent characteristics of the gas-solid suspension in a square cyclone separator

Three-dimensional particle dynamics analyzer was employed to study the gas-solid flow in a square-shaped cyclone separator which was designed for large CFB application. Distribution of flow vector, fluctuating velocity, turbulent kinetic energy, turbulent intensity and particle concentration were discussed. The swirling flow inside the cyclone showed the Rankine vortex characteristics, i.e., strong swirling vortex at the central region of the cross-section and weak swirling quasi-free vortex near the wall. The quasi-laminar motion of particles enhanced the turbulent movement at the corners due to particle-particle/wall collision, which led to the local peak value of the turbulent kinetic energy and turbulent intensity. The corner is one of the major region to cause pressure drop because the suspension at the corners consumed more energy of the flow. The corners were found to be beneficial to particle separation mainly because the strong fluctuating flow consumed much of the kinetic energy of both the particle and the gas.     

Numerical investigation on the hydrodynamics of an LSCFB riser

Analysis of fluid flow in a liquid-solid circulation fluidized bed (LSCFB) is necessary to understand its behavior under different operating parameters. In this work, ample parametric studies have been carried out numerically, which provides a view how an LSCFB operates under different operating parameters, and the numerical model has been validated using the experimental data. This study aims to get an insight of the behavior of LSCFB under different operating parameters, which include solids circulation rate, primary and auxiliary liquid velocity. In addition to this task, numerical modeling has also been carried out to predict the behavior of different particles with different densities upon fluidization in an LSCFB, which resolves the problem of experimentation with a wide spectrum of new particles that might have a wide variety of applications in an LSCFB. LSCFBs always involve high Reynolds number flow and dense solids concentration, which demands for proper modeling of the turbulent flow, liquid-solid interactions and particle-particle interactions. Kinetic theory based on Eulerian-Eulerian two-phase model is used to account for particle interactions and is applied to model the solids viscosity and solids pressure, which takes into account the particle-particle collisions.     

Experimental investigation for near-wall lift of coarser particles in slurry pipeline using γ-ray densitometer

Experimental data are collected in 54.9 mm diameter horizontal pipe for concentration profiles using γ-ray densitometer. Two sizes of glass beads with mean diameter and geometric standard deviation of 440 μm and 1.2 and 125 μm and 1.15, respectively, are used. These data are collected for flow velocity up to 5 m/s and overall concentration up to 50% by volume for each velocity. Experimental data measured by sampling probe and pressure gradients in the previous study [D.R. Kaushal, K. Sato, T. Toyota, K. Funatsu, Y. Tomita, Effect of particle size distribution on pressure drop and concentration profile in pipeline flow of highly concentrated slurry, International Journal of Multiphase Flow, 31(7) (2005) 809-823.] are used to compare with γ-ray densitometer measurements and to study slip velocity and near-wall lift of particles in the pipeline. In overall, γ-ray densitometer and sampling probe give quite similar concentration profiles except for few cases of coarser (440 μm) particles at lower flow velocities. Measurements show that, for finer particles, point of maximum concentrations are near the pipe bottom (y/D = 0.1) and for coarser particles, maximum points are relatively away from the pipe bottom with decrease in shift as flow velocity increases. Pressure gradient profiles of equivalent fluid for finer particles are found to resemble with water data except for 50% concentration, however, more skewed pressure gradient profiles of equivalent fluid are found for coarser particles. Experimental results indicate absence of near-wall lift for finer particles due to submergence of particles in the lowest layer into the viscous sublayer and presence of considerable near-wall lift for coarser particles due to impact of viscous-turbulent interface on the bottom most layer of particles and increased particle-particle interactions. It is observed that near-wall lift decreases with increase in flow velocity; however, the effect of slip velocity on pressure drop is greater at lower flow velocities and less at higher flow velocities than near-wall lift of coarser particles in slurry pipeline. For finer particles, the departure from equivalent fluid pressure gradient profile at 50% concentration is attributed to the sudden increase of viscous sublayer thickness.     

Multilayer Resuspension of Small Identical Particles by Turbulent Flow

ABSTRACT

A model for the resuspension of a multilayer deposit by turbulent flow is developed. The resuspension rate is obtained by solving a set of coupled, first-order kinetic equations. The multilayer resuspension rate depends explicitly on single-particle resuspension rates that are determined from a modified energy-transfer model. The surface-particle and particle-particle interaction potentials are calculated by a microscopic approach based on the integration of the Lennard-Jones intermolecular interaction potential. The effect of the surface roughness, which leads to a distribution of the adhesive forces, is considered, as well as the energy transfer from the fluctuating part of the turbulent flow to the particle. It is shown that for a geometrical arrangement of deposited particles with a co-ordination number of two (particles stacked on top of each other) particles from the top layers resuspend at lower friction velocities than particles adjacent to the surface. The predicted long-term resuspension rate decays algebraically with exposure time. Calculations are presented for a two-layer deposit of either SnO₂ and Al₂O₃ particles on a stainless steel surface.     

Effects of the variation of mass loading and particle density in gas-solid particle flow in pipes

The method of two dimensional Reynolds Averaged Navier-Stokes (RANS) equations has been employed for the simulation of turbulent particulate flow. This approach was fitted with appropriate closure equations that take into account all the pertinent forces and effects on the solid particles, such as: particle-turbulence interactions; turbulence modulation; particle-particle interactions; particle-wall interactions; gravitation, viscous drag and lift forces. The flow domain in all cases was a cylindrical pipe and the computations were carried for upward pipe flow. The finite volume technique was used for the numerical solution of the governing and closure equations. The results show the effect of loading and particle density on the profiles of the velocity, the turbulence intensity and the solids concentration.     

CFD simulations of turbulent fluid flow and dust dispersion in the 20 liter explosion vessel

A CFD model was developed with the aim at simulating the turbulent flow field induced by dust feeding and dispersion within the 20 L bomb, and the associated effects on the distribution of dust concentration. The model was validated considering a set of data (pressure time histories and root mean square velocity) available in the literature. The time sequences of velocity vector and kinetic energy maps have shown that multiple turbulent vortex structures are established within the sphere. These vortices generate dead volumes for the dust which is pushed toward the walls of the sphere. The obtained results are relevant to the practice of dust explosion testing and the interpretation of test results and, then, they should be taken as reference to improve the conditions for standard tests. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2485–2496, 2013     

磷石膏颗粒湍动流化特性实验及模拟

针对磷石膏颗粒湍动流化体系曳力变化的问题,在实验的基础上,考虑非均匀结构对曳力的影响,引入修正因子φ修正Gidaspow曳力模型,对2D流化床进行了数值模拟研究。通过将Gidaspow模型在不同φ值下的模拟结果与实验结果进行对比,研究φ值的改变对模拟结果的影响规律及一定气速范围内磷石膏颗粒湍动流化体系曳力变化特性。结果表明,Gidaspow模型高估了实验体系曳力,对体系流化特性的预测效果较差;适当φ值的引入能明显提高Gidaspow模型对床层膨胀、压降及体系非均匀度的模拟精度。模拟结果反映出φ值越小,床层膨胀高度越低,床内颗粒浓度分布越不均匀,床层压降波动性越大。随着气速的升高(0.144~0.240 m/s),颗粒沿水平方向上的聚集程度加剧,φ值呈非线性减小(0.31~0.24)。流化体系的非均匀度随着气速增加而增大,颗粒浓度沿径向存在较大梯度,两侧边壁处附近出现环-核结构且流场分布对称性较差。     

Modeling of Gas‐Particle Turbulent Flow in Spout‐Fluid Bed by Computational Fluid Dynamics with Discrete Element Method

Gas and solid turbulent flow in a cylindrical spout‐fluid bed with conical base were investigated by incorporating various gas‐particle interaction models for two‐way coupling simulation of discrete particle dynamics. The gas flow field was computed by a k‐ϵ two‐equation turbulent model, the motion of solid particles was modeled by the discrete element method. Drag force, contact force, Saffman lift force, Magnus lift force and gravitational force acting on individual particles were considered in the mathematical models. Calculations on the cylindrical spout‐fluid bed with an inside diameter of 152 mm, a height of 700 mm, a conical base of 60° and the ratio of void area of 3.2 % were carried out. Based on the simulation, the gas‐solid flow patterns at various spouting gas velocities are presented. Besides, the changes in particle velocity, particle concentration, collision energy, particle and gas turbulent intensities at different proportions of fluidizing gas to total gas flow are discussed.     

Calculation of the movement of polydisperse mixtures of solid particles in the flow of a Newtonian fluid in a horizontal pipe

The influence of pressure force; weight; the Archimedes, Magnus, and Safman forces; turbophoresis; and hydrodynamic resistance on solid particles, as well as impact interactions between particles and the lack of space of liquid flux in the interparticle region was taken into account during the motion of polymodal two-phase flow. The distribution of the volume concentration of solid particles was defined by their diffusion process and the motion velocity in transverse direction under the influence of the listed forces. The considered computation method showed satisfactory agreement with experimental data on the distribution of solid phase concentration and average velocity of suspension carrying flux.     

Particle‐resolved PIV experiments of solid‐liquid mixing in a turbulent stirred tank

Particle Image Velocimetry (PIV) experiments on turbulent solid‐liquid stirred tank flow with careful refractive index matching of the two phases have been performed. The spatial resolution of the PIV data is finer than the size of the spherical, uniformly sized solid particles, thereby providing insight in the flow around individual particles. The impeller is a down‐pumping pitch‐blade turbine. The impeller‐based Reynolds number has been fixed to Re = 10⁴. Overall solids volume fractions up to 8% have been investigated. The PIV experiments are impeller‐angle resolved, that is, conditioned on the angular position of the impeller. The two‐phase systems are in partially suspended states with an inhomogeneous distribution of solids: high solids loadings near the bottom and near the outer walls of the tank, much less solids in the bulk of the tank. The liquid velocity fields show very strong phase coupling effects with the particles increasingly attenuating the overall circulation patterns as well as the liquid velocity fluctuation levels when the solids volume fraction is increased. © 2017 American Institute of Chemical Engineers AIChE J, 63: 389–402, 2018     

Design and Analysis of a Submerged Membrane Reactor by CFD Simulation

CFD was applied to demonstrate the effect of reactor configurations on the fluid flow pattern in submerged membrane reactors. A mixture model, a realizable k‐? model, and the multiple reference frame (MRF) technique were employed to simulate the solid‐liquid turbulent flow. Influences of the introduction of a ceramic membrane, the relative position between ceramic membrane and impeller, and the types of impeller on velocity profiles and concentration distributions were systematically discussed. These simulation results were validated qualitatively with experimental data for various reactor configurations.     

Dilute turbulent gas-solid flow in risers with particle-particle interactions

Earlier work of Sinclair and Jackson that treats the laminar flow of gas-solid suspensions is extended to model dilute turbulent flow. The random particle motion, often exceeding the turbulent flucutations in the gas, is obtained using a model based on the kinetic theory of granular materials. A two-equation low Reynolds number turbulence model is modified to account for the presence of the dilute particle phase. Comparisons of the model predictions with available experimental data for the mean and fluctuating velocity profiles for both phases indicate that the resulting theory captures many of the flow features observed in the pneumatic transport of large particles. THe model predictions did not manifest an exterme sensitivity to the degree of inelasticity in the particle-particle collisions for the range of solid loading ratios investigated.     

Effects of Electrohydrodynamic Flow and Turbulent Diffusion on Collection Efficiency of an Electrostatic Precipitator with Cavity Walls

The effects of electrohydrodynamic (EHD) flow and turbulent diffusion on the collection efficiency of particles in a model ESP composed of the plates with a cavity were studied through numerical computation. Electric field and ion space charge density in the ESP were calculated by the Poisson equation of electric potential and the current continuity equation of ion space charge. The EHD flow field was solved by the continuity and momentum equations of gas phase, including the electrical body force induced by the movement of ions under the electric field. RNG k - l model was utilized to analyze turbulent flow. Particle concentration distribution was calculated from the convective diffusion equation of particle phase. As the ion space charge increased, the collection efficiency of charged particles increased because the electric potential increased over the entire domain in the ESP. The collection efficiency decreased as the EHD flow became stronger when the electrical migration velocity of charged particles was high. However, the collection efficiency could increase for the stronger EHD flow when the electrical migration velocity of charged particles was relatively lower. Also, the collection efficiency decreased as the turbulent diffusion of particles increased when the electrical migration velocity of particles was high. However, the collection efficiency could increase with the turbulent diffusion when the electrical migration velocity of particles was relatively lower.     

Simulation of particles and gas flow behavior in a riser using a filtered two-fluid model

Flow behavior of gas and particles is predicted by a filtered two-fluid model by taking into the effect of particle clustering on the interphase momentum-transfer account. The filtered gas–solid two-fluid model is proposed on the basis of the kinetic theory of granular flow. The subgrid closures for the solid pressure and drag coefficient (Andrews et al., 2005) and the solid viscosity (Riber et al., 2009) are used in the filtered two-fluid model. The model predicts the heterogeneous particle flow structure, and the distributions of gas and particle velocities and turbulent intensities. Simulated solids concentration and mass fluxes are in agreement with experimental data. Predicted effective solid phase viscosity and pressure increase with the increase of model constant c_g and c_s. At the low concentration of particles, simulations indicate that the anisotropy is obvious in the riser. Simulations show the subgrid closures for viscosity of gas phase and solid phase led to a qualitative change in the simulation results.     

基于离散相模型的旋转填充床内的流场分析

利用CFD软件Fluent初步模拟了旋转填充床(Rotating Packed Bed,RPB)内的流体流动.计算中气相采用RNGk-ε湍流模型,液相采用拉格朗日离散相模型(DPM).通过一定简化后建立了旋转填充床二维模型,考察了液体颗粒在填充床内的速度分布,以及转速对液体颗粒速度的影响.结果显示液滴速度随转速的增加而增加,转速对液滴径向速度的影响不大.此外还针对转速、气体进口速度对干床压降的影响做了一定的研究,发现压降随转速的增加而增加,但其影响没有气体进口速度对压降的影响明显.填料的内缘处存在剧烈的端效应,模拟结果表明,端效应区的湍动强度明显大于其他区域.     

Experimental and numerical investigation of the hydroerosive grinding

This paper presents an Euler-Euler approach for the numerical simulation of the hydroerosive grinding (HE) process. It describes a two-phase slurry flow consisting of a liquid and a dispersed solid phase which causes wear at walls of devices. The continuous fluid phase is solved using a finite volume scheme in which the Large Eddy Simulation (LES) [1] model is applied to resolve large-scale turbulent structures. The solid phase is dispersed and treated as a second continuum in which drag and lift forces as well as added mass, pressure and history force are taken into account. Considering particle-particle interactions, the granular model from Gidaspow [2] is used for particle volume concentrations over 1%. Investigations of erosion processes proofed that non-spherically shaped particles as well as harder particles increase the wear on devices significantly. Consequently, non-spherical particles are utilised for the hydroerosive grinding. Their steady drag, unsteady drag and lift coefficients, depending on the particle Reynolds number, are determined by a direct numerical simulation via an in-house LES Lattice-Boltzmann solver. This Lattice-Boltzmann method was presented for laminar flows by Hölzer and Sommerfeld [3]. In this work, interpolating functions of these coefficients are implemented in the Euler-Euler approach which enables the simulation of non-spherical particle transport. Hydroerosive grinding experiments in 3D throttles and 3D planar geometries are carried out to determine an erosion model depending on particle impact velocity, particle size, particle concentration and wall hardness. Implementation of a mesh-morphing algorithm combined with the Euler-Euler scheme of the commercial solver ANSYS CFX11 [4] enables an online simulation of the hydroerosive grinding process. Additionally, the online simulation is used to validate the applied numerical methods. Very good agreements are achieved and will be presented in this paper.     

基于颗粒动力学理论的搅拌器中固液流动的数值模拟

在欧拉双流体模型基础上引入颗粒动力学理论(KTGF),对带挡板圆盘涡桨式搅拌器内的固液两相流动进行数值模拟。结果表明,搅拌器底部颗粒温度分布与固相浓度分布趋势吻合,转速低于600 r/min时,槽底会形成明显的颗粒沉积,转速从600 r/min增至1500 r/min,堆积区向轴中心收缩,基于颗粒动力学理论可以合理解释挡板及叶轮转速对固相浓度分布的影响。随叶轮转速增大,搅拌器内固液两相湍流运动加剧,颗粒温度、湍动能及轴向速度增加,颗粒分布更均匀,但达到完全悬浮状态后颗粒温度趋于稳定。搅拌器底部和挡板处颗粒堆积导致了局部颗粒浓度增加及颗粒平均自由行程减少,颗粒温度反而降低;同时挡板布置使搅拌器内形成了双循环回路,加强了流体的湍流程度,增强了湍动能,但导致颗粒在挡板处积聚,不利于固相在挡板处均匀分布。