The pairwise interaction of coalescing air bubbles in water

There is a relative scarcity of experimental data relating to the interaction of a pair of millimetre-scale air bubbles rising having an offset configuration through stagnant water. To add to this dataset was the motivation behind this work. A series of experiments are presented, which can add to the understanding of bubble-bubble interaction. The trajectories of bubble pairs were tracked using a high speed camera. The diameter and relative positions of the nozzles were varied to produce different separation distances between the rising bubbles. It was found that when the trailing bubble is slightly smaller (as little as 2.3%) than the leading bubble it would approach the leading bubble. Hence, a greater tendency to coalesce between the rising pairs has been noticed. The initial relative angle between the coalesced bubbles, θi, was also correlated which has agreed well with others’ previous work. A proportional relationship has been presented to link the time required for coalescence with the ratio of the bubbles radii. A complementary set of numerical simulations, using a multiphase CFD model with adaptive meshing, have confirmed some of the experimental observations and added insights into the flow structures responsible for coalescence. Finally, a map for the boundaries of coalescence from the numerical and experimental observations is suggested which relates the separation of the bubbles and their radii ratio to the likelihood of coalescence.