Under microgravity conditions, the heat transfer is considered to be different from that in normal gravity because of 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 across the vapor-liquid interface because stable large-scale vapor bubbles are formed. In the present study, the behavior of a

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... spherical bubble generated by a micro heater was observed under microgravity condition during 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 of the 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.