The Local Dependence of Bubble Breakup for Different Impeller Geometries in a Stirred Tank

This work presents an investigation into the effect of impeller geometry on the local bubble breakage behavior in a stirred tank. The relative breakage location and local number of produced daughter bubbles (fragments) were investigated using a high-speed imaging method. Three impeller geometries were used for a range of impeller Reynolds numbers (Re) (18380–40830). The impellor geometries used were: 4-flat blades, 4-twisted blades, and 2-flat blades. The results revealed that the number of breakages around the impeller is dependent on the impeller geometry and Re. At lower Re, most breakages occurred close to the tip of the blades in locations near the entry to the impellor region. For higher Re, the rotational motion of the fluid drives the mother and daughter bubbles into different regions around the impeller before breaking up. The highest number of daughter bubbles was observed to be generated close to the blade’s tips of 4-flat blades impeller (average of 8.3). The lowest number of daughter bubbles was observed to occur for the 4-twisted blades (average 2.7). The highest percentage of local bubble breakages and the highest number of resulting daughter bubbles were observed for the 4-flat bladed impeller for all Re (reached up to 90% for highest Re).