Chemical Engineering Journal, Vol.265, 164-175, 2015
Effects of a bubble and the surrounding liquid motions on the instantaneous mass transfer across the gas-liquid interface
To understand the mass transfer mechanism from a bubble to the surrounding liquid, one must consider the relation between the mass transfer, gravity-center and surface motions of the bubble. Bubbles in chemical and bio-reactors usually exhibit a zigzag motion. Simultaneously, such bubbles accompany periodic surface oscillation. Knowledge about the correlation between the mass-transfer mechanism and bubble motion is as yet incomplete. In this study, we used experimental results from highly precise measurements of bubble volume to clarify the instantaneous mass-transfer coefficients of a zigzagging CO2 bubble. We visualized the zigzagging motion and surface oscillation 3-dimensionally and simultaneously, using two high-speed cameras and mirrors. We also visualized the CO2 dissolution (mass transfer) process from the bubble to the surrounding liquid using the LIF/HPTS method. To obtain the precise instantaneous mass transfer coefficient and bubble motions, single bubbles were visualized in three sections: the linear-ascent, the first-inversion, and the second-inversion of the zigzag motion. The instantaneous mass-transfer coefficients in an interval of 6 ms in these sections were calculated from the bubble-volume shrinkage with ms time-resolution. The instantaneous mass-transfer coefficients increased in acceleration areas i.e., in the linear-ascent and second-inversion sections. Interestingly, in the latter, the gravity-center velocity of the bubble reached terminal velocity and was constant, but the velocity of the bubble hemisphere was accelerated due to the zigzag motion. This partial acceleration of the bubble hemisphere led to a high renewal rate of the liquid on the bubble interface. The effect of the partial acceleration on the instantaneous mass transfer was significant for bubbles categorized into zigzag motion. (C) 2014 Elsevier B.V. All rights reserved.