Study On Suspension Characteristics Of Spherical Sinking Particles In A Stirred Tank With Transitional Flow

Solid-liquid stirred tank is widely used in biological fermentation,industrial crystallization process and sludge treatment of wastewater.The study of solid-liquid stirred tank includes particle suspension characteristics and two-phase flow characteristics.Most of the existing studies emphasized the critical suspension characteristics of a pile of particles in a stirred tank and ignored the suspension mechanism of particles.In this work,the suspension motion characteristics of multi-particles are first explored by using image capturing.Then the flow field characteristics of solid-liquid two-phase during blade startup are measured by using PIV and refractive index matching technology.The experiment used high-speed imaging to study the suspension characteristics of multiple spherical particles in a stirred tank with a disk impeller.The images were processed in the MATLAB environment and the trajectory and velocity of the particles were obtained.The results show that:the multi-particle suspension movement of transition flow has certain randomness.The particle suspension probability is greatly affected by its own density.The particle group gathers at the bottom of the tank to form a stable configuration.The existence of this configuration seriously affects the suspension motion of particles.The single particle has a stable vertical upward movement and stagnation point during the suspension process,while the multi-particles only do spiral motion.The refractive index matching technology was used to match the PMMA particles with the mixture of castor oil and cinnamon oil,and the solid volume fraction is 10 %.Combined with refractive index matching and PIV,the effects of rotational speed and pumping configurations on particle bed change,liquid flow field and solid particle distribution during blade startup were studied.The results show that: during the impeller start-up process,the particle bed is continuously eroded by the fluid,and the particles accumulate towards the middle of the bed.When the impeller speed increases,the number of suspended particles increases and the height of suspension increases.The average solid phase concentration at the bottom of the impeller decreases,while the area above the impeller is opposite.The impeller lifting operation is more conducive to particle suspension.The existence of particles hinders the formation of fluid flow circulation and reduces the liquid phase velocity to varying degrees.