Abstract
Experimental study on bouncing and coalescence of a pair of bubbles rising side by side in a quiescent liquid has been performed. A novel bubble generator has been developed to generate pairs of bubbles precisely. High-speed imaging technique has been employed to investigate the trajectory and shape of bubbles in detail. We observed that the trajectory of rising bubble varies with Reynolds number. When the Reynolds number is over the critical region, a pair of bubbles approaches each other and then collides. After the collision, two types of motion are observed: coalescence and bouncing. We investigate the critical Reynolds number and Weber number, based on the rising velocity, over which bubbles bounce. As a result, we clarify that the critical Weber number is around 2 regardless of Morton number. The critical Reynolds number decreases with increasing Morton number. In addition, we have found that the bouncing of a pair of bubbles differs from that of single bubbles at a wall; the two bubbles perform attracting and bouncing motion continuously with a rising velocity being kept almost constant, and the amplitude of bouncing motion of bubbles is seven to eight times larger than that of a single bubble at a wall.
