Numerical simulation of liquid-solid flow in an unbaffled stirred tank with a pitched-blade turbine downflow

Computational fluid dynamics (CFD) provides an efficient method for investigating highly complex fluid flows in mechanically stirred tanks, but less attention was given to unbaffled stirred tanks generated with axial impeller, which are frequently used in process industries. The present study is intended to evaluate the CFD predictions of key properties related to the mixing against measurements and to provide a detailed insight into the process. A three-blade 70 degrees PBTD (Pitched Blade Turbine Downflow) was used to generate axial flows in a stirred tank, of which the local solid concentration profiles were numerically simulated. The liquid-solid system consisted of spherical silica beads in water. A method using "vector distance" in a Eulerian reference frame was proposed to resolve the impeller and the bottom of the tank of complex geometry. The predicted results of two-phase flows at various impeller speeds and solid holdups are presented. In addition, various Schmidt numbers and laminar viscosity coefficients of dispersion phase were tested to improve the predictions. In the experiments, an optical fiber probe was used to measure solid particle concentration, and the effects of impeller speed on solid holdup were examined. The solid particle distribution was affected with the impeller speed in the range of 113-173 rpm in the stirred tank. The radical concentration of solids particle presents different profiles at different axial cross sections. Good agreement between the experiments and simulations was observed.