Lattice-boltzmann Simulation Of Fluid-particle Flow Heterogeneous Structure And Phase Interaction

The two-phase flow process involving solid particles plays an important role and influence on energy,environment and industrial production.However,the study of particle dynamics of fluid-particle two-phase flow is very complicated.Even for the simplest gas-solid flow system,there may be a heterogeneous structure,usually represented by agglomerated structures of particles.Therefore,the study of heterogeneous structures in fluid-particle two-phase flow systems is crucial and has great practical value.Because of the widespread presence of heterogeneous structures of particles,numerical methods for simulating particle flow require accurate description of this instability.The Lattice-Boltzmann Method(LBM)is a mesoscopic scale numerical simulation method further developed on the basis of Lattice Gas Automata(LGA)theory.LBM can replace complex real motion with simple particle motion,which has great advantages for studying complex fluid-particle two-phase flow.In this study,the numerical simulation of three-dimensional fluid-particle two-phase sedimentation system is carried out by the LBM.The numerical simulation spans a rather wide parameter space: mean-flow Reynolds numbers on the order of 1 and 10,mean solid volume fractions from 0.1 to 0.4,solid-to-fluid density ratios from 2 to 1000.It can be observed from the results that under the condition of the same Archimedes number and average particle volume fraction,with the increase of density ratio,the particle distribution structure in the system will change from homogeneous state to heterogeneous state,mainly plug-like particle cluster.And systems with different Archimedes numbers and average particle volume fractions exhibit different heterogeneous structures as density ratios change.In this study,the fluid-particle two-phase system mainly presents four structures:(1)steady and nearly homogeneous,(2)unsteady and chaotic,(3)steady and dense plug structure,(4)steady and dilute plug structure.Under different Archimedes numbers,Richardson-Zaki's prediction of particle sedimentation velocity is different for the lower two average volume fractions;For the higher two average volume fractions,when the system is in nearly homogeneous state,the Richardson-Zaki prediction is approximately accurate.However,when the system has a heterogeneous structure,the predicted value has a large deviation from the LBM value.After the particles enter the cluster,the sedimentation velocity of the particles decreases.At the same time,the particle rotation velocity fluctuation in the horizontal direction is larger than the value in the sedimentation direction,and the rotation velocity fluctuation of the particles in the clusters is lowered.In systems with different structures but the rest of the conditions are the same,the radial distribution functions also differ.Under the condition that the Archimedes number and the average particle volume fraction are the same,the force from the fluid received by the particle will increase due to the appearance of the particle cluster,and the denser the particle cluster,the greater the force from the fluid.When the density of the two phases is becomes larger,the virtual mass effect and Magnus lift of the particles are negligible.