On coalescence of bubble in branching microchannel with phase change

Understanding the bubble dynamics is essential to various two-phase applications. The three-dimensional numerical study on bubble coalescence with phase change in different heated branching microchannels (15 degrees, 90 degrees, and 180 degrees) is performed. The detailed bubble dynamics, flow structures, and phase change heat transfer characteristics are respectively presented. Results show that during coalescence process, the bubble successively experiences three stages including liquid film formation, neck growth, and oscillation. It is interesting to point out that the sandwich structure (droplet exists within bubble) may occur in large -angle branching microchannels due to the classical Rayleigh-Plateau instability. Besides, the unique encircling coalescence phenomenon is found to occur in 90 degrees branching microchannel, leading to the sudden increase in neck growth rate. Two distinct flow structures can be recognized, namely the smooth flow occurring in small -angle branching microchannels and the colliding flow occurring in large -angle branching microchannels. The fluid flow becomes quite chaotic under colliding flow structure, which significantly facilitates the convective heat transfer. The heat transfer enhancement for main channel is found to be limited because of the counter-balance mechanism of phase change heat transfer involved. The main channel produces the better heat transfer enhancement than the branching channel, and the best heat transfer enhancement occurs on the outer wall of main channel. This fundamental research sheds new light on the physical mechanisms of bubble dynamics in confined branching structures.