Numerical Simulation Of Flow Reaction Characteristics In Fluidized Beds With Meso-scale Model Of Rough Sphere

In dense gas-solid two-phase flow,due to the combined effect of gas flow disturbance and particle collision,the particle system will exhibit non-uniform structures such as clusters or bubbles,which will cause the interphase drag to decrease sharply and affect the stable operation of fluidized bed.Therefore,the study of mesoscale effects in gas-solid two-phase flow plays a very important role in the development of theoretical models and their application in engineering.One important factor in the formation of mesoscale structures is the energy dissipation caused by nonelastic particle collisions.The roughness of real particles leads to the rotation during the collision process,which introduces additional rotational dissipation.Therefore,it is necessary to consider the physics behind mesoscale structures under the effect of rough particle rotation.By introducing the effect of rough particle collision and rotation on the mesoscale structure,a bubble structure-dependent(BSD)model for rough particle is developed.In the model construction,the roughness and rotation characteristics of particles are embodied in the form of particle pressure and collision dissipation through kinetic theory of rough sphere(KTRS).Furthermore,when the particle concentration is high,the mesoscale effect of friction stress is considered by introducing random fluctuations of particle concentration in the frictional zone.Regarding the mesoscale structural of gas-solid two-phase flow under fast fluidization,the mesoscale cluster stress is introduced in the particle stress to consider the influence of cluster-scale fluctuation on the collision process.Combined with the effect of collision and rotation of particles,a cluster structure-dependent model of rough particles(KTRS-CSD)is constructed.Using the proposed bubble-based mesoscale model(KTRS-BSD),a numerical simulation of a three-dimensional bubbling fluidized bed is carried out to analyze the collision law under mesoscale structure and the effect of particle roughness on the evolution of mesoscale structure.The results show that with the recession of mesoscale bubbles,additional fluctuations are introduced into the system,which leads to an increase in particle collision dissipation.Due to the disturbance of bubbles,the collision dissipation in the central area is more significant than that near the wall.The frictional stress considering the mesoscale effect fluctuates more intensively than that without the mesoscale effect.The study also shows that the particle rotation effect plays an important role in low particle concentration area,which makes the bubble fraction and emulsion phase concentration decrease with the increase of tangential restitution coefficient in low particle concentration area.The developed model is used to simulate the fast fluidized bed and explore the gas-solid distribution under fast fluidization.The cluster mesoscale informations of rough particles are obtained.The influence of cluster-scale fluctuation on collision and dissipation processes is analyzed.The simulation results show that the mesoscale effects play an important role in solid-phase stress,and the contribution from the stress induced by cluster-scale fluctuation is more significant than that from the micro-scale particle stress.The magnitudes of particle pressure and particle viscosity increase significantly near the wall.The relationship between mesoscale energy dissipation and particle roughness satisfies a non-monotonic relationship.The energy dissipation caused by collision reaches its peak when the tangential restitution coefficient is-0.5.Owing to the influence of particle roughness on collisional process,both cluster diameter and cluster concentration will decrease with increasing particle roughness.For the binary mixture of distinct particle size,a KTRS-TFM-DEM hybrid model is constructed by considering the fluctuation transfer between continuous and discrete particle phases based on Euler-Euler-Lagrange framework.A simulation of the fluidization of binary mixture of distinct particle size is carried out using TFM-DEM hybrid model with the KTRS and verified by experiments.The results show that the developed TFM-DEM hybrid model with the KTRS can well describe the fluidization mixing process of binary mixture of distinct particle size.After considering the fluctuation transfer between solid-solid phases,the sinking speed of fuel particles decreases,and the predicted fuel particle distribution in the bed layer is more consistent with the experiment.Moreover,ignoring fluctuation transfer will affect the energy transport from bed material particles to fuel particles,resulting in a decrease in fluctuation energy of fuel particles and an increase in fluctuation energy of bed material particles.The KTRS-TFM-DEM model is extended to the reaction process and used to simulate oxygen-enriched biomass gasification reaction.The characteristics of fuel particle size distribution during reaction process and gas product distribution in bed layer are obtained.The results show that unreacted biomass is difficult to pass through stacked particles due to its large particle size and is therefore concentrated on the surface of the bed layer.The larger the particle size of bed material particles,the more concentrated the distribution of fuel particles along the bed height.The distribution of combustible gas in the bed is affected by the mixing of the two particle phases.Smaller bed material particles lead to better mixing and thus uniform distribution of combustible gas.