Investigation of hydrodynamic performances of coaxial mixers in agitation of yield-pseudoplasitc fluids: Single and double central impellers in combination with the anchor

Abstract

The hydrodynamic performance of coaxial mixers, the single and double Scaba impellers in combination with an anchor impeller, was investigated in the mixing of yield-pseudoplastic fluids (xanthan gum solutions) in the laminar – transitional regime in the co-rotating mode. To explore and determine the efficiency of the coaxial mixers, both numerical and experimental approaches were adopted. The fluid rheology was described by the Herschel–Bulkley rheological model. Electrical resistance tomography (ERT) with seven planes of electrodes was applied to measure the mixing time and visualize the flow pattern inside the vessel. The flow domain of the fluid was simulated three-dimensionally applying the computational fluid dynamics (CFD). The developed model was then validated through experimentally measured torque and the mixing time. The performances of the investigated coaxial mixers in this work were compared at the constant power input and similar fluid rheology with respect to the mixing time, fluid velocity profiles, and mixing efficiency. Applying the previously published correlations for the power and Reynolds numbers of the coaxial mixers showed that Pakzad et al. (2013) model was valid for a wide range of the speed ratios for the double Scaba-anchor coaxial mixer, while Bao et al. (2011) model was only appropriate for the higher speed ratios. Considering the mixing efficiency criteria, it was found that the double Scaba-anchor coaxial system was more efficient than the single Scaba-anchor coaxial mixer in the mixing of yield pseudoplastic fluids with regard to the mixing time and power drawn.