Influence of Aeration Microporous Aperture on Oxygen Mass Transfer Efficiency in Terms of Bubble Motion Flow Field

Abstract

Microporous aeration has been widely used to restore eutrophic water bodies. The gas–liquid mass transfer in the aeration process has a significant influence on the improvement of water quality. Therefore, the influence mechanism of oxygen mass transfer is worth studying. However, the influence of bubble movement characteristics on oxygen mass transfer has not been systematically studied. Thus, the present study explored the influence mechanism of microporous apertures on oxygen mass transfer in terms of bubble motion characteristics by investigating the oxygen mass transfer process and the feature of bubble movement under different aeration microporous aperture sizes. The results showed that the mass transfer efficiency was reduced as the micropore aperture increased from 200 to 400 μm. and the reduction rate was 7.17% when the aperture increased from 200 to 300 μm, which was lower than that from 300 to 400 μm (19.17%). Furthermore, the micropore aperture showed a positive correlation with the time-averaged velocity field. With the increase in aperture, the bubble velocity gradient (from the center to both sides of the edge) increased from about 0.2 to 0.4 m/s, which increased the oxygen mass transfer effect. The increase of micropore aperture caused the increase of average Sauter bubble diameter and the decrease of specific surface area of bubbles. In addition, the negative effects of the reduction of specific surface area and the shortening of bubble residence time on oxygen mass transfer efficiency were greater than the positive effects of the increase of turbulent kinetic energy. When the aperture changes from 300 to 400 μm, the shortening of bubble residence time should have played a major role. This study provides some theoretical parameters for investigating the mechanism of oxygen mass transfer in microporous aeration.