Numerical Study of Single Phase Liquid Mixing in Stirred Tanks Fitted With Rushton Turbine and Flotation Impeller

Abstract

Mixing is a complex process and usually involves continuous reduction of length and time scales associated with fluid(s) being mixed. Mixing is an essential process and finds widespread application in a range of industries. Due to lack of understanding of the mixing process, industries lose a significant amount of money contributed by increased power consumption and longer process times. In this work a thorough comparison of flow, mixing, and turbulence characteristics of Rushton turbine (RT) and a flotation impeller, variation of disc turbine, is performed for single phase flows using Computational Fluid Dynamics (CFD). The fluid used is water. Base case validation and model verification is performed by comparing our CFD results with widely accepted Laser Doppler Anemometry (LDA) experimental results for the Rushton Turbine. Multiple reference frame (MRF) technique, a pseudo-steady modeling method, is used to model the impeller motion on flow characteristics at different Reynolds numbers (Re). Turbulence closure is provided using RANS based two equation realizable k-ε turbulence model. Grid independence studies are carried out a sufficiently fine grid is selected to capture the fine flow structures close to the impeller, though radial velocity close to impeller was under-predicted compared to experimental results. Effects of finite impeller blade and disc thicknesses on the local flow field, which are commonly modeled as thin surfaces, are explored. Various tank geometric variations, like different impeller clearances, and impeller diameter to tank diameter ratios (DI/DT), are also investigated. The numerical results will help in understanding the effect of impeller design on local and bulk flow characteristics and turbulence anisotropy close to the impeller. The results from this work will direct the tank and impeller design choices for two phase solid-liquid flows for future investigations.