Gas Helicity Yields New Insights into Gas–Liquid Reactor Design: Enhanced Oxygen Transfer Rate of Coaxial Mixers at Low-Speed Ratio

Abstract

The gas helicity isosurface plots (i.e., the projection of the gas velocity onto the gas vorticity) were used to visualize the degree of gas entrainment in the impeller vortices. We discovered an interesting link between the gas helicity and the volumetric mass transfer coefficient. The gas helicity can be negatively or positively depending upon the alignment of the entrained gas motion with the impeller rotation. The 1 wt % CMC and xanthan gum solutions obey the power-law and Herschel-Bulkley rheological models, respectively. The dissolved oxygen concentration profile was investigated both experimentally and numerically using coaxial mixers consisting of a central impeller and a wall-scraping anchor. The local gas helicity near the impeller blades revealed that the 1 wt % CMC and xanthan gum solutions experienced a different mass transfer capacity at the critical anchor impeller speed of 30 rpm. This study provides insights into how new energy-efficient impellers can be designed.