Investigation of power consumption, torque fluctuation, and local gas hold-up in coaxial mixers containing a shear-thinning fluid: Experimental and numerical approaches

Abstract

The performance of a coaxial bioreactor in the intensification of gas dispersion in shear-thinning fluids in terms of the power consumption of the central and anchor impellers, torque fluctuation for the anchor impeller, global and local gas hold-up, and cavity size was assessed. Electrical resistance tomography and computational fluid dynamics methods were used to evaluate the effects of the central impeller type, impeller speed, pumping direction, and rotating mode on the efficacy of the aerated coaxial mixers. Unlike the coaxial mixer equipped with a radial-flow impeller, for the coaxial mixer furnished with an axial-flow impeller, the rotating mode had a pronounced effect on the relative power demand. Surprisingly, it was observed that increasing the central impeller speed increased the contribution of the anchor impeller to the total power consumption of the co-rotating coaxial mixer furnished with a down-pumping impeller. Furthermore, by increasing the central impeller speed the torque fluctuation of the coaxial mixer was diminished due to the uniform distribution of the energy dissipation rate. Unexpectedly, for a coaxial mixer furnished with a down-pumping impeller in both rotating modes and an up-pumping impeller in the counter-rotating mode, the gas hold-up decreased by a further increase in the central impeller speed.