Prediction of flow fields in a dual-impeller stirred vessel

Numerical simulations were conducted for the flow field in a baffled tank stirred by a dual Rushton impeller. For this geometry, LDV measurements show a characteristic dependence of the flow patterns upon the position of the impellers. Two advanced modeling approaches were tested. In the first, the vessel was divided into two concentric blocks, coupled by a sliding grid technique, and simulations were conducted in time-dependent mode. In the second approach, the vessel was modeled as two partially overlapping regions, the inner one rotating with the impeller and the outer one stationary; simulations were run in steady-state mode for each of the two regions, while information was iteratively exchanged between them after azimuthally averaging and transforming for the relative motion. A third set of simulations was conducted for comparison purposes by using a more conventional method, in which the impellers were not explicitly simulated, while their effects were modeled by imposing suitable values of velocities and turbulence quantities (derived from single-impeller experiments) at the blade periphery. The first two techniques gave similar predictions and successfully reproduced the dependence of the flow patterns on the position of the impellers. The latter method required far less computational effort. On the other hand, the impeller boundary conditions technique failed to reproduce the experimental flow patterns, because of the inadequacy of single-impeller boundary conditions for the present geometry.