Investigation On Flow,Heat And Mass Transfer Characteristics In Wet Particle Fluidized Beds

Fluidized beds are widely applied in energy,chemical engineering and food fields due to sufficient momentum and energy exchange space for particle systems.In fact,in industrial processes,liquid existence is inevitable,which results in different flow characteristics compared with those of dry particle systems.Therefore,the wet particle system is chosen as the research object.Flow and heat mass transfer characteristics of wet particles in different types of fluidized bed are explored by numerical simulation and experiment to better guide industrial production and reactor design in fields of energy,chemical engineering and food.Based on the soft-sphere model,a numerical model of wet particle flow and heat mass transfer is established.The Height Function surface tension model is extended from the flat surface contact angle boundary condition to the spherical surface contact angle boundary condition.Thus,the numerical model of collision between a single particle and liquid was established.Based on the Discrete Element Method(DEM),the effect of cohesive liquid on particle flow characteristics was considered by introducing the liquid bridge force model.The rolling friction model was also added.By adding heat and mass transfer module,considering the heat conduction between particles,heat convection between gas and solid phases,and liquid evaporation,a numerical model describing the wet particle drying process was established.The processes,including particle sedimentation,particle bouncing and a droplet spreading on a particle surface,were investigated by the Direct Numerical Simulation model.Simulation results were compared with the experimental results for model validation.The effects of liquid properties on collisions between a particle and liquid layer were analyzed.It was found that the structure of liquid bridge formed by particles colliding with liquid film with different surface tension and thickness were different.Particle Image Velocimetry Technology was applied for measuring and observing wet particle flow behaviors in a spouted fluidized bed.The DEM built for the wet particle system was applied to simulate flow characteristics of wet particles corresponding to experimental conditions.The numerical model was validated.Moreover,interaction effects of cohesive liquid and rolling friction on particle flow behaviors were obtained.The results showed that simulation results were in good agreement with experimental results.It was found that with an increase of liquid content,cohesive liquid played a more important role on wet particle flow behaviors.A gas channel was observed and became curved gradually.Particles were more difficult to be fluidized with more liquid introduced.Moreover,it was found that the interaction effect of rolling friction and cohesive liquid was different in different regions of spouted beds.An experimental platform for measuring heat and mass transfer behaviors of wet particle was built based on wet particle flow characteristics.Meanwhile,the DEM model coupling flow,heat and mass transfer was used to simulate the heat and mass transfer characteristics of wet particles in different types of fluidized beds.Effects of pulse gas amplitudes and frequencies on heat and mass transfer characteristics were analyzed in a bubbling fluidized bed.Moreover,the effect of initial liquid content was explored in a spouted fluidized bed.It was found that reducing the fraction of emulsion phase can improve the heat and mass transfer characteristics effectively,and also avoid defluidization leading to unstable operation.The major heat transfer means was convection between gas and solid phases in spouted fluidized beds.Liquid evaporation only accounted for a small proportion of the total heat transfer amount.The heat transfer rate decreases gradually due to more uniform temperature distribution.