A study visualizing the collapse of vapor bubbles in a subcooled pool

The condensation processes of vapor bubbles in a subcooled pool were recorded and analyzed, which could help to illustrate the mechanism of collapse of vapor bubbles and emission of microbubbles during Microbubble Emission Boiling (MEB). Vapor bubbles were injected into the subcooled pool through an orifice with the inner diameter of 4 mm at different vapor volumetric flow rates. Visualized results showed that the bubble surface became rougher and more unstable with the increase in liquid subcooling and volumetric flow rate, during which four condensation features were confirmed and defined as smooth bubble regime, shape oscillation regime, transition regime and capillary wave regime, respectively. Fine surface wave formed after a while of condensation would enlarge the effective area of condensation, resulting in the acceleration of bubble condensation. Furthermore, the fine surface wave which might also lead to bubble collapse only appeared on bubble surface at liquid subcooling over 25 K and vapor volumetric flow rate higher than 0.74 m3/h, which corresponded to the occurrence conditions of MEB. After the sudden collapse of the vapor bubble with capillary wave, a great many microbubbles formed, with a greater velocity and much smaller radius with subcooling and vapor injection rate increasing. In the region of MEB, a strong surface wave usually appeared on vapor film before the film collapse and the emission of microbubbles. Nevertheless, the film surface formed in film boiling was relatively smooth compared with that formed in MEB. All of the experiments results and analysis illustrated that the strong surface wave introduced by condensation may be one of the reasons resulting in the formation of MEB.