Numerical prediction of flow fields in baffled stirred vessels: A comparison of alternative modelling approaches

Numerical simulations of the flow field in baffled mixing tanks, based on three alternative methods, are presented and discussed. In the first method, the impeller is not explicitly simulated, and its effects are modelled by imposing suitable, empirically derived, boundary conditions to the external flow. In the second method, the whole vessel volume is divided into two concentric, partially overlapping, regions. In the inner region, containing the impeller, the flow field is simulated in the rotating reference frame of the latter, while in the outer region simulations are conducted in the reference frame of the laboratory. Information is iteratively exchanged between the two regions after azimuthally averaging and transforming for the relative motion. In the third method, the tank volume is divided into two concentric blocks, the inner one rotating with the impeller and the outer one stationary. The two blocks do not overlap and are coupled by a sliding-mesh technique. Predictions are presented here for baffled tanks stirred either by single and dual Rushton turbines (radial impellers) or by a constant-pitch helical impeller (axial impeller), and are compared with experimental data from the literature. Satisfactory results can be obtained by the first method only if reliable empirical data are available for the flow near the impeller, while large errors may arise if this is not known with reasonable accuracy. The other two methods both yield satisfactory results while requiring no empirical information, and thus allow a much greater generality. (C) 1998 Elsevier Science Ltd. All rights reserved.