Insights into the fluid dynamics of the Bach impeller

Abstract

The Bach impeller is a novel impeller that was purposefully designed to reduce power consumption as well as shear stress levels in stirred bioreactors. Contrary to conventional impellers where stirring is achieved by the pushing action of the blades, the Bach impeller relies on a central spiral element which induces an up-pumping central vortex and discharges the flow in the tangential direction through vanes. This feature is new and relevant to the field of biochemical engineering where cell culture and microbial fermentation often make use of shear-sensitive, living organisms to produce biopharmaceuticals and medicines. This article offers a thorough characterisation of the fluid mechanics of this impeller by means of 2D phased- and time-resolved particle image velocimetry (PIV) experiments. Key flow dynamics parameters including velocity magnitudes, kinetic energy, and shear stresses were estimated for different combinations of impeller size, clearance, and speed. The standard impeller size, D/T=0.52, performed most promisingly in combination with an off-bottom clearance of C=0.6 T, as the elevated impeller height allowed to pull high momentum fluid to the upper part of the tank and therefore improved overall transport phenomena in the entire reactor volume. For this impeller diameter-clearance combination peak ensemble-averaged shear stresses at the vessel base were lower due to the impeller greater distance from the bottom. A distinctive feature of the Bach impeller was its "smooth operation" mode, where velocity fluctuations due to the vane passages are nearly negligible (∼5 % of ensemble-averaged kinetic energy). These aspects make of the Bach impeller an appealing option in the context of stem cell bioprocess applications.