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 in shape with a stack of four 45 degrees pitched blade impellers, four rectangular side-wall baffles and an ellipsoidal shaped bottom. The flow is computed with an incompressible Navier-Stokes solver which uses the pseudocompressibility technique of coupling the velocity and pressure fields. The laminar viscous flow field is solved using an approximate steady-state technique which neglects relative motion between the impellers and baffles and solves the flow at a single impeller position in a rotating frame of reference. The resulting velocity field is spatially 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 different impeller configurations. This work illustrates the utility of the numerical method for studying complex multiple impeller flows at low Reynolds number. A variety of different impeller configurations are studied numerically and the effect of relative impeller sizing, impeller spacing 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 performance is sensitive to relative impeller sizing and spacing. Improper impeller spacing or sizing can result in compartmentalization of the flow inside the vessel and hence poor global circulation. (C) 1997 Elsevier Science Ltd.