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 (NCS) 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 NCS 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. NCS 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 NCS between PBTU and PBTD. The flow patterns based on a simulated velocity field were further investigated and successfully accounted for this difference.