Interfacial Instability on Vapor Bubble Exposed to Subcooled Pool
A special attention is paid to the growing and condensing/collapsing processes of vapor bubble injected into a subcooled pool. We try to extract the vapor-liquid interaction by employing a vapor generator that supplies vapor at designated flow rate to the subcooled pool through an orifice instead of using a immersed heated surface to realize a vapor bubble by boiling phenomenon. This system enables ones to detect a spatio-temporal behavior of a single bubble of superheated vapor exposed to a subcooled liquid. In the present study, a series of experiments are conducted with a distilled water as a test fluid. The injection rate of the vapor to the pool is ranged up to about 60 mm3/s. The diameter of the orifice is of 2.0 mm. The degree of subcooling of the pool is ranged from 0 K to 70 K, and the vapor temperature is kept constant at 101 ?C. The behaviors of the injected vapor are captured by high-speed camera at frame rate up to 0.3 million frame per second (fps).We evaluate a temporal variation of the vapor bubble in the pool in the growing/condensing processes under the assumption of the axisymmetric shape of the bubble. We indicate the correlation between the growth/condensation processes of the vapor bubble and the onset of the instability over the vapor surface. The condensation process clearly exhibits a signature of the occurring condition of the instability resulting in the abrupt collapse. We also evaluate the wave number of the instability arisen on the free surface of the vapor bubble. Fine deformation on the interface emerges under the condition that the degree of subcooling is greater than around 30 K, and then the wave number increases as the degree of subcooling. It is found that the occurring condition of the instability and the variation of the wave number are almost of the injection rate. We also indicate that the occurring condition of the abrupt collapse and the wave number of the instability are described as a function of the second derivatives of the interface position.