Experimental analysis of the clustering behavior of spherical bubbles rising along an inclined flat plate

Abstract

The behavior of mono-dispersed spherical bubbles rising along an inclined flat plate has been observed experimentally and the effects of the inclination angle and the air flow rate have been investigated. This study considers the clustering behavior of bubbles for the range of bubble Reynolds number Re from 100 to 200 (Re being based on the bubble diameter and bubble rising velocity along the flat plate). When the number density of bubbles becomes higher and the rising velocity along an inclined flat plate becomes slower, the bubbles tend to be horizontally arranged in line and such arrangements pile up to the bubble cluster. The spatial distribution of bubbles has been evaluated by the pair distribution function, which shows the frequent existence of side-by-side bubble pairs and the preferential accumulation within the range of about 8d (d is the average bubble diameter) corresponding to the size of the typical bubble cluster. Also, the statistics of the velocity difference of bubble pairs has been analyzed in order to evaluate the bubble-bubble interaction quantitatively. For the bubble pair moving in line, the trailing bubble becomes faster by the effect of the leading bubble, which leads to the tandem configuration of two bubbles. However, since the tandem configuration is unstable, the resultant rotational velocities around the other bubble centers cause the side-by-side configuration of the bubble pair. This process forms horizontally aligned bubbles.