A Study on the Induced Liquid Velocity in Plumes by Tiny Bubbles

Abstract

The effect of the bubble-induced liquid velocity on the mass transfer performance in the bubble plume is analyzed quantitatively with numerical simulations. A two-way coupling Eulerian-Lagrangian approach is used in the modeling of the bubble plumes with mass transfer. The dissolution of oxygen in bubble plumes with the initial bubble diameters from 100μm to 1mm is simulated. The results show that when a single bubble generator is used with the gas flux rate equals 10−8 cubic meter per second, for the plume with 100μm bubbles inside a 0.1m height cubic tank the maximum of the bubble-induced liquid velocity is over 10 times larger than the bubble’s terminal velocity, and the averaged residence time of bubbles in the plume is around one-tenth of the rising period estimated with the terminal velocity of a single bubble. The result suggests that for bubble plumes in a shallow bulk of water, the benefits of using smaller bubbles for high mass transfer efficiency will be overestimated without considering the reduction of the residence time of bubbles because of the bubble-induced liquid velocity. The present simulation shows that the dissolution efficiency of oxygen for the bubble plume with 100μm bubbles in 0.1m tank is around 1/2 of the theoretical value estimated with a single bubble rising with negligible diameter shrink. Compared with a plume in a 0.1m tank, the shrink of bubble diameter and the scattering of bubbles from the center of plume during their rising in a 0.4m tank attenuate the reduction of the averaged residence time because of the acceleration process as shown in a 0.1m tank. The effect of bubble-induced liquid velocity on the mass transfer efficiency for plumes with initial bubble diameter smaller than 160μm does not present obviously in a 0.4m tank as it does in the shorter tank.