Visualization Study of Growth of Spherical Bubble in He II Boiling under Microgravity Condition

Under microgravityconditions, the heat transfer is considered to bedifferent from that in normal gravity becauseof zero subcooling due to zero hydrodynamic pressure in saturated He II. Thus the heat transfer in He II under microgravity is an interesting research target. Microgravity experiment is expected to reveal some hidden mechanism of boiling heat transfer acrossthe vapor-liquid interface because stable large-scale vapor bubblesare formed. In the present study, the behavior of a single spherical bubble generated by a micro heater was observed under microgravity conditionduring free fall in a drop tower for about 1.3 second. The visualized images taken by a high-speed camera were analyzed to examine the time variation of a large vapor bubble ofthe order of 10 mm. It was seen that the sizes of a single bubble increased with decreasing He II temperature for fixed heat input. The bubble size near the lambda temperature was smaller than that at 1.9 K though the effective thermal conductivity is quite small. The magnitude of the saturated vapor pressure seems to be a dominant factor to determine the bubble size. For the case of He I, the vapor bubble growth can be predicted by a simple consideration in terms of the latent heat and the gas density in film boiling state.