Mixing of complex fluids with flat-bladed impellers: effect of impeller geometry and highly shear-thinning behavior

Abstract

Mixing of rheological complex fluids was investigated using flat-bladed impellers as close-clearance agitators in the laminar regime. Two Newtonian and six highly shear-thinning fluids were used. The non-Newtonian fluids were adequately described by a power-law model with a flow index n between 0.1 and 0.4. Power draw analysis was used to explore the combined influence of pseudoplasticity and impeller geometry. Geometry was studied first by varying the column-to-impeller diameter ratio, and then by combining several similar mixing elements on the same shaft. For pseudoplastic fluid, the Rieger–Novak and the power curve methods as well as an original Couette analogy were used for estimating the effective shear rate and the proportionality constant K S. A good agreement was obtained between these three methods. K S was shown to be nearly independent of n: the Metzner–Otto assumption was shown to be valid for all the geometries studied. A generalized dimensionless power draw curve which took pseudoplasticity into account was obtained by shifting the non-Newtonian results to the Newtonian curve. The effectiveness of flat-bladed impellers for dispersive mixing in complex fluids proved in previous works was explained by the fact that the effective shear rate remained high even when power consumption dramatically decreased with n.