Measurement of local bubble diameters and analysis of gas dispersion in an aerated vessel with disk-turbine impeller

An improved photoelectric capillary method was used to determine the characteristics of the gas dispersion in a stirred tank by disk-turbine impellers with various numbers of blades to examine the effect of the number of blades. The results show that the bubble size distribution in the system has three different characteristics according to the patterns of liquid flow. In the impeller discharge region the sizes of bubbles are dominated by the strength of the trailing vortex and the cavity structure behind the blades. No difference is observed among impellers with various numbers of blades when the region-averaged Sauter mean diameter D₃₂is correlated with Pg/V and Q_g. In the upper circulation region, bubble size is affected by both the cavity structure and the circulating flow. In the lower circulation region, bubble size is controlled mainly by the pumping capacity of the impeller. These transitional rotation speeds, N_c and N_R, were obtained from the relationship between D₃₂and gas flow number Fl, which can serve not only as the turning points of the status of gas dispersion, but also can be used to explain the characteristics of the bubble size distributions for each region.