EXPERIMENTAL AND COMPUTATIONAL STUDY OF MULTIPLE IMPELLER FLOWS

A computational and experimental study is conducted of viscous Bow in a stirred reactor with multiple impellers. The vessel is cylindrical i n shape with a stack of four 45 degrees pitched blade impellers, four rectangular side-wall baffles and an ellipsoidal shaped bottom. The fl ow is computed with an incompressible Navier-Stokes solver which uses the pseudocompressibility technique of coupling the velocity and press ure fields. The laminar viscous flow field is solved using an approxim ate steady-state technique which neglects relative motion between the impellers and baffles and solves the flow at a single impeller positio n in a rotating frame of reference. The resulting velocity field is sp atially averaged and compared with time-averaged experimental results. Computed results for the velocity field are shown to agree very well with experimental laser Doppler velocimetry (LDV) data fbr two differe nt impeller configurations. This work illustrates the utility of the n umerical method for studying complex multiple impeller flows at low Re ynolds number. A variety of different impeller configurations are stud ied numerically and the effect of relative impeller sizing, impeller s pacing and baffling on flow distributions within the stirred vessel is investigated. It is shown that global circulation patterns within the tank are strongly dependent on relative impeller size and spacing. It is concluded that obtaining good global circulation and mixing perfor mance is sensitive to relative impeller sizing and spacing. Improper i mpeller spacing or sizing can result in compartmentalization of the fl ow inside the vessel and hence poor global circulation. (C) 1997 Elsev ier Science Ltd.