Intensification of mixing of shear-thinning fluids possessing yield stress with the coaxial mixers composed of two different central impellers and an anchor

Abstract

The performance of different configurations of coaxial mixers composed of a wall scraping anchor impeller in combination with two different or identical central high speed impellers in the agitation of the shear-thinning fluids with yield stress (e.g. xanthan gum solution) was investigated. These multi-impeller mixers are more compact for the larger scale of mixing operations. The coaxial mixers employed in this study were the Scaba-Scaba-anchor, Scaba-Rushton-anchor, Rushton-Scaba- anchor, Scaba-pitched blade-anchor, and pitched blade-Scaba-anchor. The quality of mixing achieved by these five coaxial mixers was assessed for the Reynolds numbers in the range of the laminar to the transitional regime in the co-rotating mode through computational fluid dynamic (CFD) and electrical resistance tomography (ERT) techniques. A new correlation was introduced for these complex configurations of the coaxial mixers by incorporating the Metzner-Otto constants (Ks) of the different types of the central impellers into the Reynolds number. The experimental and CFD data were employed to evaluate the mixing intensification attained by these five coaxial mixers in regard to the power consumption, mixing time, velocity profiles, shear strain rate profiles, flow number, power number, and pumping effectiveness. The analysis of the collected data indicated that the intensification of mixing of the highly viscous non-Newtonian fluids achieved by the Scaba-pitched blade-anchor coaxial mixer was the highest among the mixing systems explored in this study.