Numerical Simulation of Solids Suspension in a Stirred Tank

Abstract

Large-eddy simulations of the turbulent flow driven by a Rushton turbine have been coupled to a Lagrangian description of spherical, solid particles immersed in the flow. The working fluid was water, whereas the solid particles had the properties of glass beads. Simulations were restricted to a lab-scale tank (volume 10-2 m3), and relatively low solids volume fractions (up to 3.6%). Two sets of particles were considered with particle dia. of 0.30 mm and 0.47 mm, respectively. It has been investigated to what level of detail the particle motion needs to be modeled in order to meet Zwietering's just suspended criterion. It appeared to be essential to take particle-particle collisions into account, mainly because of their exclusion effect that prevents unrealistic buildup of particle concentrations closely above the bottom. The simulations give detailed insight in the behavior of the particles, and in the way that the liquid flow is altered by the presence of the particles. The frequency and intensity of particle-particle collisions, and particle-impeller collisions, have been investigated. Furthermore, it will be demonstrated that the rotational Reynolds numbers of the big (0.47 mm) particles were of the same order of magnitude as their translational counterparts.