Investigation of the detrimental effect of the rotational speed on gas holdup in non-Newtonian fluids with Scaba-anchor coaxial mixer: A paradigm shift in gas-liquid mixing

Abstract

Our research findings challenge the paradigm that a high rotational speed of impellers can intensify the gas dispersion. Interestingly, we observed the deterioration in the gas holdup at the higher rotational speeds. Electrical resistance tomography (ERT) and computational fluid dynamics (CFD) were extensively utilized to shed light on the hydrodynamics of Scaba-anchor coaxial mixer and its impact on the aeration efficiency in non-Newtonian fluids, for the first time. The working fluid was 1 wt% carboxymethyl cellulose solution, which is a non-Newtonian fluid and obeys the power law model. At a fixed central impeller speed of 290 rpm, as the anchor impeller speed was increased from 15 to 50 rpm, the decrease in the local gas holdup was attributed to the decline in the Scaba impeller pumping capacity and its aerated power consumption. On the other hand, at a constant anchor impeller speed of 10 rpm, when the Scaba rotational speed was increased from 290 to 430 rpm, the gas holdup decreased surprisingly even with an increasing Scaba’s pumping capacity at higher rotational speed. The sharp drop in the local gas holdup was the consequence of a poor axial gas dispersion. With a minimal fluid shear stress at low agitation speed, our research findings hold promise to optimize energy savings while enhancing the aeration efficiency in non-Newtonian fluids at low Reynolds numbers.