Simulation Study Of Particle Flow,Heat Transfer And Coating Uniformity In Spray Fluidized Bed Based On CFD-DEM Model

Spray fluidized beds have a wide range of applications in food,pharmaceutical,chemical and energy fields.It can coat and agglomerate particles in batches to achieve precise transformation of particle properties.The spray fluidized bed includes fluidization,atomization,drying and other processes.Several experiments are required to obtain the ideal operating parameters of the spray fluidized bed.Through numerical simulation research,it is possible to deeply understand the internal relationship between particle circulation,drying and granulation,and to quickly optimize the operation and structure of the fluidized bed.Based on computational fluid dynamics-discrete particle model(CFD-DEM model),we develops a comprehensive model of spray fluidized bed,optimizes the structure and proposes a directional spray fluidized bed.The main research contents and results are as follows:(1)In terms of modeling,a simplified spray model is established based on CFDDEM model,spray zones are set up in a source term manner,uniform spray volume is obtained based on particle residence time in the spray zone.The water evaporation model and heat transfer model are coupled to describe the drying process of wet particles.A cohesive particle contact model is coupled to describe wet particle collisions.A comprehensive model of the multi-physics process within the spray fluidized bed is established.(2)In terms of Wurster fluidized bed,studies on heating,drying and wet particle fluidization were carried out.The results show that the particles rise in the center tube and descend in the annular region.The time-averaged overall flow field is symmetrical,and a vortex is formed near the inclined wall.The particle cycle time is inconsistent,resulting in two peaks in the temperature distribution of the particles during heating and drying.The particles containing moisture are heated in the draft tube and descend to the annular zone,where the particle temperature decreases due to the evaporation absorb heat from particles.Considering the viscosity of wet particles,agglomerates are formed in the spray zone,and the stability of particle fluidization circulation decreases.When the spray volume and the cohesive force are large,the draft tube is blocked and fluidization fails.When the particles can fluidize and cycle,the average particle temperature and moisture content remain stable.If the fluidization fails,moisture accumulates in the fluidized bed.(3)Combining the Wuster type fluidized bed and the deflector,a directional spray fluidized bed is designed.Make the movement direction and path of the particles in the descending zone as consistent as possible to ensure the uniformity of the particle coating.By CFD-DEM numerical simulation,the particle cycle time,particle size distribution and particle size variation coefficient under different structures were compared.The ideal directional spray fluidized bed structure is obtained,which can realize the orderly circulation movement of particles.Coating uniformity is characterized using particle size probability density distribution and particle size variation coefficient.The effects of fluidization air velocity,spray rate and fluidization air temperature on coating uniformity are obtained.The particle size variation coefficient fluctuates periodically due to the cyclic motion of the particles.The amplitude decreases with time,and the coefficient of variation increases overall with time.Increasing the fluidization air velocity properly,increasing the fluidization temperature and reducing the spray amount can improve the coating uniformity.The optimized directional spray fluidized bed has excellent coating uniformity.