Collision Characteristics Of Wet Particles For Sprayed Fluidized Beds

Spray fluidized bed has been widely used in various granulation fields as it can efficiently achieve particle coating or agglomeration.However,the particle structure and morphology are influenced by numerous factors such as liquid and particle properties and operating parameters of fluidized beds.The complexity of the physical process and the many influencing factors make the development of spray fluidized bed technology face a major challenge.Therefore,at the microscopic level,it is of great importance to study the complex droplet-particle and particle-particle interactions,which will provide clues for determining the operating parameters of spray fluidized beds and regulating the particle structure and morphology.In this paper,wet particle collisions are carried out,and the main conclusions are as follows:In this study,we perform experiments on droplets impacting heated particles,combining a high-speed camera and infrared camera to observe the collision behaviors of droplets and particles.The particle temperature is in the range of 35-150℃.The solution used to generate droplet is a 30% sodium benzoate solution,whose viscosity is adjusted by adding hydroxypropyl methylcellulose(HPMC).The results show that the liquid viscosity has a significant effect on the spreading and bounce characteristics of droplets.The collisions between droplets from a 30% sodium benzoate solution and heated particles have four regimes: deposition,deposition-bubble,breakup rebound,and rebound regimes,while the break-bubble regime observed for water droplets is not observed for sodium benzoate solution.For the solution with the addition of 1% HPMC or 2% HPMC,only two regimes are observed:deposition and deposition-bubble regimes.The transition temperature between deposition and deposition-bubble regimes is between 90℃ and 110℃,independent of the Weber number.The collision dynamics are analyzed by plotting the evolution of the film thickness and spread factor with time.This reveals that a collision comprises three phases(initial deformation,inertial dominated,and viscous dominated).In the first two phases,the liquid film spreading characteristics are consistent for solutions with different temperatures and viscosities.However,in the third phase,when the liquid viscosity is small,the difference of liquid film at different temperatures is larger,and this difference decreases significantly with the increase of solution viscosity.By coupling the VOF(Volume of Fluid)model and overset mesh model,we simulate the normal collision between a particle and a liquid film,called the “particle-plane” collision,and the normal collision between a particle and a particle with a liquid droplet deposited,called the “particle-particle” collision.The liquid bridge evolution and particle motion during the collision are analyzed.The results show that there are significant differences between the two collision cases.For the “particle-particle” collision,the entire process is divided into three phases: approach,collision,and separation.With the approach of two particles,the liquid squeezed in the gap.At the moment of particle collision,the velocity of moving particles reverses and presents a significant decrease.In the separation phase,the liquid bridge stretches and finally ruptures.Under the same conditions,the wet restitution coefficients for the “particle-plane” collisions are significantly lower than those for the “particle-particle”collisions.With increasing droplet diameter and liquid viscosity,the dissipation of kinetic energy increases,and the wet restitution coefficients for the “particle-plane” and“particle-particle” collisions decrease but the decreasing trend of the wet restitution coefficient of "particle-plane" collision is more obvious.Furthermore,the proportion of energy dissipation caused by differential pressure,viscous,and surface tension is obtained by extracting the “force-distance” curve and compare with the kinetic energy dissipation of direct solid collision.The results indicate that differential pressure and solid collision are the dominant sources of kinetic energy dissipation.By coupling the VOF model and overset mesh model,we simulate the "particle-particle" oblique collision and analyze the evolution of the liquid bridge and particle motion during the collision.Unlike the normal collision,the normal direction between two particles of the particle constantly changes in the oblique collision,and the liquid bridge has obvious deflection and distortion,which affects the particle motion.The effects of liquid viscosity,collision velocity,and collision angle on the restitution coefficient,particle motion,and liquid bridge rupture behavior are further analyzed.Due to the partial conversion of tangential kinetic energy to normal kinetic energy,the normal restitution coefficient is higher than the tangential restitution coefficient,while the total restitution coefficient is in between.With the increase of liquid viscosity,the tangential restitution coefficient slightly increases,while the normal restitution coefficient and total restitution coefficient decrease.With the increase of collision velocity,the tangential restitution coefficient increases,and the normal restitution coefficient decreases,while the total restitution coefficient remains almost unchanged.With the increase of collision angle,the total restitution coefficient has an increasing trend.