Minimum Energy Dissipation Rate Drag Coefficient Model And Numerical Simulation On The Active And Passive Regulation Of Cluster In Riser

Due to the gas-solid interactions and the collisional interactions between particles,the particles in the gas-solid fluidized bed reactor aggregate to form the heterogeneous structure,resulting in the formation of the mesoscale flow structure in which dispersed particles and cluster coexist in the flow.The existence of mesoscale structure plays a major role for momentum,heat and mass transfer in the fluidized bed.Therefore,judging the occurrence of cluster flow is the key to accurate prediction of gas-solid two phase flow by numerical simulation.The inelastic collision between particles is the main reason for inducing the formation of the heterogeneous structure.Therefore,accurately predicting the inelastic collision granular restitution coefficient is an important factor to judge the particle cluster flow.In addition,if the phenomenon of particle agglomeration is not effectively controlled in the commercial application of coal combustion and catalytic cracking fluidized bed reactors,it will result in the bed defluidization.Therefore,the mechanism of particle agglomeration is used to suppress the adverse effects of agglomeration and achieve the regulation of particle agglomeration process is one of the core research issues in industry and academia.In this study,the criterion of cluster existence identifying the heterogeneous flow and homogenous flow of gas-particles suspension and a varied granular restitution coefficient model are proposed,and the numerical simulation and analysis of active and passive control of particle cluster in the fluidized bed riser are carried out.Based on the flow structure of coexistence of dispersed particles and particle cluster in the computational cell,a criterion for distinguishing homogeneous flow and heterogeneous flow is proposed according to the energy dissipation rate generated bu different control mechanisims.It is clarified that the particle cluster is jointly controlled by three mechanisms:gas-solid interaction,particle collision and gas viscosity.The particle cluster is controlled by the coordinated control of the energy dissipation rate of the gas-solid phase drag force and the energy dissipation rate of the particle collision,ignoring the gas viscouse energy dissipation rate.Based on the two-fluid model and the kinetic theory of granular flow,considering the momentum transfer between dispersed particle and particle cluster,a cluster minimum energy dissipation rate drag model is established.A dynamic generalized granular restitution coefficient correlation is proposed.The coupled Euler gas phase-Euler solid phase-Lagrangian discrete particle model is used to simulate the gas-solid two phase flow in the fluidized bed by considering the multiple collisions of particles.The average granular restitution coefficient in the computational cell is obtained by statistical calculation of the relative velocity of discrete particle collision,and this average value is used in the calculation of the particle collision process of the Euler solid phase.The distribution of dynamic generalized granular restitution coefficients with granular volume fraction at low and high granular volume fraction is obtained by simulating the gas-solid flow in the fluidized bed.The dynamic generalized granular coefficient of restitution-based kinetic theory of granular flow is obtained by introducing this coefficient into the kinetic theory of granular flow.The gas-solid flow behaviors in the fluidized bed is simulated by the proposed dynamic generalized granular coefficient of restitution-based kinetic theory of granular flow.The calculated granular volume fraction and standard deviation of gas volume fraction are agree with the experimental data.The minimum average granular volume fraction in the entire riser is calculated using the constant granular restitution coefficient corresponding to the granular volume fractionα_s=(α_s,m+σ).By comparisons of the predicted granular volume fraction,solid velocity and solid reynolds stress distribution using dynamic generalized granular coefficient of restitution and several constant granular coefficient of restitution model,there are obvious differences between the calculation results of dynamic generalized granular coefficient of restitution and constant granular coefficient of restitution.Compared with the prediction results under the constant granular coefficient of restitution,the radial distribution of granular volume fraction predicted by the dynamic generalized granular coefficient of restitution-based kinetic theory of granular flow is the closest to the experimental data.The hydrodynamics characteristics in the circulating fluidized bed riser are simulated by means of the cluster minimum energy dissipation rate drag model.The solid volume fraction and cluster solid volume fraction obtained by cluster minimum energy dissipation rate drag model are in good agreement with the experimental data.The simulation describes the dynamic process of evolution of particle cluster in the flow.The intermittent factor of clusters is defined.The mean intermittent factor of clusters in the riser is obtained,which varies in the range of 0.233-0.266,indicating that particle cluster occurs intermittently in the computational cell.The effects of drag energy dissipation rate,collision energy dissipation rate and gas viscous energy dissipation rate on the formation of particle cluster are studied.The gas viscous energy dissipation rate is low about two to four order of magnitudes in comparison to hydrodynamic and collision energy dissipation rates.By analyzing dominant mechanisms of clusters,it is concluded that the collision-hydrodynamic dominant mechanism for particle clustering is a central mechanism.That is,most cluster are formed in the competition and compromise between gas-solid interation and collision of particles.Numerical simulations of gas and particle cluster flow in inlet gas pulsation-assisted riser and gas-nozzle assisted fluidized bed risers controlled by active method.The effects of inlet gas velocity pulsation frequency and amplitude on the cluster characteristics are analyzed.The cluster diameter,the cluster solid volume fractions and the number fraction of clusters in the riser first decrease and then increase with the increase of pulsation frequency and amplitude,and then reach the smallest at the inlet gas pulsating amplitude of 1.0 m/s and frequency of 4.0 Hz using inlet gas pulsing velocity.The effects of upward,horizontal and downward gas injection directions of nozzle on the distribution of particle cluster are analyzed.A non-uniform index reflecting the radial flow difference of particles is proposed,and the flow is divided into an inlet,transition and mixing zone in terms of the variation of non-uniform index.In the transition and mixing regions,the cluster diameters and velocities are lowest for the downward gas jets,and highest for the upward gas jets.The cluster solid volume fractions and existence time fractions are larger for the upward gas jets than that for the horizontal and downward gas jets in the transition and mixing regions.Numerical simulations of gas and solid flow in two-stage fluidized bed risers controlled by passive method.The distribution of particle cluster parameters in the axial and radial directions under the expanding-diameter riser and contracting-diameter risers are obtained.Compared to hydrodynamics of gas and particles in a conventional constant-diameter riser,the solid volume fraction and diameter of clusters in contracting-diameter riser decreases.The solid volume fraction,number and diameter of clusters decrease with the decrease of the diameter ratio in the second chamber of contracting-diameter riser.In the second chamber of expanding-diameter riser,the cluster solid volume fraction is decreased and the diameter of cluster is increased with the increase of diameter ratio.