Enhanced aeration efficiency in non-Newtonian fluids using coaxial mixers: High-solidity ratio central impeller with an anchor

Abstract

One of the major challenges in bioreactors is maintaining a critical level of dissolved oxygen concentration in highly viscous fluids. In this perspective, we investigated how the aeration efficiency (gas holdup per unit power) can be enhanced by operating the downward-pumping high-solidity ratio central impeller at a low critical speed (290 rpm) with a slowly-rotating anchor impeller (10–30 rpm). The non-Newtonian fluid used in this study was carboxymethyl cellulose (1 wt% CMC) which obeys the power law model. Electrical resistance tomography (ERT) and computational fluid dynamics (CFD) were employed to characterize the gas dispersion in non-Newtonian fluids. For the first time, we also explored the effect of anchor impeller hydrodynamics on gas dispersion in non-Newtonian fluids both numerically and experimentally in a co-rotating mode. At a fixed central impeller speed of 290 rpm, a significant drop in the local gas holdup was observed when the anchor impeller speed increased from 30 to 50 rpm. Through air velocity vector obtained via CFD, this study shed light on how the anchor impeller hydrodynamics affect gas dispersion. With an increasing solidity ratio (SR) of the central impeller, a superior aeration efficiency was observed at the speed ratio of 29 compared to that at 43. The research findings hold promise for enhancing the aeration efficiency in non-Newtonian fluids under laminar flow regime.