Turbulent flow of shear-thinning liquids in stirred tanks-The effects of Reynolds number and flow index

LDA measurements are reported on the turbulent velocity fields in vessels agitated by a Rushton turbine and containing Newtonian as well as non-Newtonian, shear-thinning fluids. Ten different liquids were investigated, with flow indices varying from 1.00 down to 0.56. Experiments were performed in three vessel sizes, viz. 28.6, 44.1, and 62.7 cm in diameter, at various impeller speeds. The main issue of the paper is the question whether or not, and if so to what extent, turbulent flow of shear-thinning fluids differs from that of Newtonian liquids, or - in other words - whether and when turbulent flow of shear-thinning liquids exhibits Reynolds number similarity.

The experimental data presented comprise profiles of the mean velocity components and the rms fluctuating velocity components as a function of the radial position in the tank at the height of the impeller disc as well as similar profiles in the impeller outflow near to the impeller tip. The effects - if any - of both Reynolds number and flow index on these profiles are assessed.

Fit equations are presented for the various profiles in the various liquids. These fit equations are claimed to be valid throughout the ranges of Reynolds numbers and flow indices covered by the experiments presented. The idea is that these fit equations may be used to validate Computational Fluid Dynamics (CFD) simulations of the Reynolds Averaged Navier-Stokes (RANS) type for agitated shear-thinning liquids, even for liquids and conditions not investigated in this study as long as falling within the Reynolds number and flow index ranges investigated. (C) 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.