CFD simulation of complete drawdown of floating solids in a stirred tank.

Computational fluid dynamics (CFD) simulations were carried out to find a CFD methodology to predict the critical impeller speed ( N _CS) for the complete drawdown of floating solids in a stirred tank with an up- and down-pumping pitched blade turbine (PBTU and PBTD). CFD models along with six mixing parameters predicted that N _CS was 7.5 rps and 5.8 rps, respectively, for PBTU and PBTD, when impeller submergence ( S) was equal to half of the liquid height ( H/2). Reasonable agreement was observed between CFD simulations and published empirical correlation. N _CS was smaller for PBTD relative to PBTU at S = H/2, which agreed with results in the literature. The drawdown mechanisms based on the mean drag, turbulent fluctuations or large scale eddies failed to explain the difference in N _CS between PBTU and PBTD. The flow patterns based on a simulated velocity field were further investigated and successfully accounted for this difference. [ABSTRACT FROM AUTHOR]