Study on Condensation Process of Vapor Bubble Containing Noncondensable Gas

A visualized investigation was performed to study the condensation and collapse processes of vapor bubbles at high liquid subcooling with a high-speed video camera (Fastcam SA5). Experiment results show that a vapor bubble will collapse to many microbubbles rapidly when the initial volumetric fraction of noncondensable gas in the bubble, x₀, is less than 2.5%. Instead, a large bubble will split into just several tiny bubbles when x₀ is between 2.5% and 7.5%. Further, x₀ increases exceeding 7.5%, the bubble surface will be very stable, and no collapse or fragmentation occurs. The noncondensable gas in a vapor bubble reduces the condensation at the bubble surface, which weakens the inertial shock of the liquid on the surface and makes the bubble more stable. This may elucidate that the presence of noncondensable gas will inhibit the bubble collapse in microbubble emission boiling and deteriorate the heat transfer performance consequently.