Characterisation of impeller driven and oscillatory mixing by spatial and temporal shear rate distributions

Computational fluid dynamics (CFD) technique was used to model spatial and temporal behaviour of flow patterns in an impeller-driven stirred vessel (IDSV) and in ail oscillatory flow baffled vessel (OFBV). The spatial and temporal shear rate distributions were assessed for IDSV based on k-epsilon model, appropriate for classical mixing, while the distributions for OFBV were calculated based on large eddy simulation suitable for oscillatory flow mixing. In addition, the real parameters of the vessels such as geometry, physical dimensions, impeller speed, oscillation frequency and amplitude, and density and viscosity of the fluid (water), were taken into account. Under given operating conditions the spatial shear rate distribution appears to be quite distinct for two different methods of mixing. For OFBV, the volume-averaged shear rate was found to be of one order of magnitude larger than that of IDSV. In addition, a marked distinction between the temporal shear rate distributions Was observed. In OFBV, the modelling shows that particles spend most of their residence time in the high shear regions, while in IDSV, particles reside mainly in the region of considerably lower shear rates. (C) 2004 Elsevier Ltd. All rights reserved.