Variation of Film Boiling Modes in He II from Strongly to Weakly Subcooled States

Film boiling modes in both subcooled and saturated superfluid helium (He II) were experimentally investigated. The visual observation and the transient pressure and temperature measurements were performed to extract some characteristics of each boiling mode. The classification of all four film boiling modes, strongly subcooled and weakly subcooled modes in subcooled He II (He IIp) and noisy and silent film boiling modes in saturated He II (He IIs), was drawn on the boiling mode map. It was found from the heater temperature measurement that the boiling heat transfer is enhanced in the weakly subcooled mode compared with in the strongly subcooled mode. In the weakly subcooled mode, the vapor behavior is much activated by the instability in vapor‐liquid interface. The boundary region between the strongly and the weakly subcooled modes becomes thick as the rise of He II temperature or the increase of the heat flux. The noisy film boiling does not occur at the pressure above 9 kPa, though it appears at the pressure above pλ. It is found that in the region adjacent to the lambda line He I film boiling mode occurs even in He II.