Experimental investigation of the flow dynamics of rheologically complex fluids in a Maxblend impeller system using PIV

The Maxblend® mixer is used in processes involving Newtonian, shear-thinning or viscoelastic fluids and, as for many impellers, little is known regarding the relationship between the rheological behavior and the flow and mixing properties. An experimental study of the hydrodynamics in the Maxblend was carried out using Newtonian and non-Newtonian fluids in laminar and early transitional regimes. Flow fields were measured by two-dimensional particle image velocimetry (PIV) to evaluate the effect of highly shear-thinning and viscoelastic behavior on the performance of the mixer. The experimental setup consisted of a 35-L tank equipped with two baffles. A total of 100 measurements were proven to be sufficient in order to attain convergence of the velocity components for all studied regimes. Isolated zones in the highly shear-thinning case were observed by means of a decolorization technique. Elasticity in the laminar regime produced a reversal of the flow and a solid body rotation in the bottom region of the tank. Fluid velocity magnitude and spread were observed to be reduced by the presence of elastic forces.

► Circulation of Newtonian fluids can be improved by increasing the impeller speed.
► Segregated zones may appear in the case of shear-thinning fluids.
► Axial flow can be weak for viscoelastic fluids.