Experimental Study On Single Particle Suspension In Transitional Flow In Stirred Tank

Solid-liquid suspension are widely found in nature and used in industrial processes.Mixing is a common chemical engineering unit operation,and it is also widely used in bioreactors,heterogeneous catalysis,and wastewater treatment.Studying the characteristics of solid-liquid suspension in a stirred tank can help us understand the law of particle motion intuitively and lay a foundation for exploring the mechanism of particle suspension under different conditions.However,the current research on particle suspension mostly focuses on the macroscopic dynamic behavior of a large number of particles.There are few studies on the solid-liquid suspension details of single particle or a small number of particles in a stirred tank.The single particle motion characteristics in stirred tank have been summarized by several researchers,but their work is mostly concentrated on the laminar flow with lower Reynolds number.The study on the suspension characteristics of single particle helps to improve the suspension mechanism.In this paper,the suspension characteristics of single spherical particles in the transitional flow of Re=3 00?1200 in a square stirred tank were studied.Two impeller types,three off-bottom clearances,four particle densities,and different Reynolds number affecting single particle suspension process were investigated.The particle motion trajectory was captured by high-speed camera and processed in Matlab,and the flow field characteristics were studied by Two Dimensional Particle Image Velocimetry(2D-PIV).The results show that:(1)There are two cases of critical suspension,once and multiple critical suspensions of the single particle under disk turbine.As the off-bottom clearances decreases,the two kind critical suspension speed will both decrease significantly.The critical suspension speed change of particles whose dimensionless density difference less than 1 is similar as that in disk turbine tank,but it will be reduced by about 60%.The critical suspension speed of particles whose dimensionless density difference greater than 1 decreases as the off-bottom clearance increase.(2)The density of particle and off-bottom clearance have significant impact on the trajectories particle rolling over the bottom in disk stirred tank;particle will move upward spirally,downward spirally or rolling under the rotating disk continuously after it lifts off vertically.The impeller Reynolds number and off-bottom clearance have significant impact on the trajectories particle rolling over the bottom in Rushton stirred tank,the suspension trajectories of particles whose dimensionless density difference less than 1 are basically same as which in disk stirred tank.The particles whose dimensionless density difference greater than 1 make an irregular circle motion over the bottom and jump as suspension;(3)The diversion of the fluid near the axis below the rotating disk in stirred tank is the cause of the change in particle motion.The suspension of the particles is mainly caused by the mean flow.The vortex moving near the bottom of the tank under the Rushton turbine maybe the cause of particles moving irregularly.