Large eddy simulations of flow instabilities in a stirred tank generate by a Rushton turbine

The aim of this paper is to investigate the flow instabilities in a baffled, stirred tank generated by a single Rushton turbine by means of large eddy simulation (LES) and simulation using the k-ε turbulent model. A sliding mesh method was used for the coupling between the rotating and the stationary frame of reference. The calculations were conducted on the “Shengcao-21C” supercomputer using CFD code CFX5. The flow fields predicted by the LES simulation and the simulation using k-ε model were compared to the results from digital particle image velocimetry (DPIV) measurement. It was shown that CFD simulations using k-ε model and LES approach agreed well with the DPIV measurement. Fluctuations of the radial and axial velocity were well predicted at different frequencies by the LES simulation. Velocity fluctuations of high frequencies were observed in the impeller region, while low frequencies velocity fluctuations were observed in the bulk flow. Flow circulation patterns predicted by the LES simulation were asymmetric, stochastic and complex, spanning a large portion of the tanks and varying with time, while circulation patterns obtained by the simulation using k-ε model were symmetric and simple. This shows that the LES simulation performs better than simulation using k-ε model for the investigation of flow instabilities in stirred tanks since large turbulent scales were directly calculated and a finer grid was used in the LES simulation, although the LES simulation demanded much more computational time and computer memories. The results of the present work give better understanding to the mixing mechanisms in the mechanically agitated tank.