Relation between flow pattern and blending in stirred tanks

Abstract

In the present work, the relationship between the flow pattern and blending has been investigated. The flow patterns generated by around 40 axial flow impellers have been examined. The impellers differed in blade angle, blade twist, blade width, impeller diameter, impeller location, and pumping direction. The mean-flow and turbulence characteristics generated by all of the impellers have been measured using laser doppler velocimetry (LDV). On the basis of available LDV data, the flow pattern throughout the vessel was established by employing computational fluid dynamics (CFD) and subsequently used for the simulation of the blending process. The predicted mixing times were found to be in excellent agreement with the experimental measurements. It has been shown that the dimensionless mixing time (θ ) varies inversely with the secondary flow number of the impeller. Comparison of the impellers on the basis of equal power consumption per unit mass has shown that θ_mix ∝ N_P^1/3T^2/3/N_QS. The present CFD model has shown the possibility of reducing the eddy diffusivity to about 20% of the actual value and still achieving the same mixing time. This reduction in eddy viscosity represents substantial savings in operating costs.