Gas-liquid hydrodynamics and mass transfer in microreactors under ultrasonic oscillation

Integration of microreactors with ultrasound is a promising way to intensify gas-liquid mass transfer. The present study locally visualized and characterized the O₂ mass transfer around Taylor bubbles flowing in a square microchannel reactor under ultrasonic oscillation. The effect of acoustic field on the gas-liquid hydrodynamics and mass transfer were investigated. A reciprocating phenomenon of the bubble contraction and expansion was observed, and the evolution of bubble length was found to be a periodic harmonic process. It was also revealed that the cavitation microstreaming generated by the bubble surface oscillation, as well as the periodic bubble length variation, could strongly modify and twist the recirculation loops in the liquid slug, thus intensify the mass transfer. As the bubble surface oscillation during the bubble formation was not as fierce as during the bubble flowing, the enhancement factors of mass transfer by the ultrasonic oscillation range from 1.04 to 1.85 for the formation and from 1.27 to 3.17 for the flowing stage. Semi-empirical equations were established to predict the overall mass transfer coefficient in terms of the acoustic power density and Reynolds number.