Experimental Study on the Transition of He II Boiling Modes with the Hydrostatic Pressure Around the Lambda Temperature

Abstract

The transition of film boiling modes in He II with the change of the hydrostatic pressure is studied experimentally. Special attention is paid to the pressure dependence of boiling modes around the λ temperature. For this purpose, detailed investigation is carried out with the aid of optical visualization and the pressure oscillation and the temperature oscillation measurements. All the experiments are conducted by using a single cryostat over the entire pressure range from saturated vapor pressure to subcooled states because the excited pressure oscillation is highly affected by the geometrical configuration of the test space in a cryostat. In the case of subcooled boiling, flickering of bulk vapor film is seen on the planar heater instead of repeated growth and crush of a vapor bubble as observed in the noisy film boiling. It is an interesting finding that a regular pressure oscillation is excited at slightly lower temperature than the λ-temperature for a moderate value of the hydrostatic pressure, which is obviously different from rather irregular ones of noisy and subcooled film boiling modes. With the decrease in the temperature and the increase in the heat flux in the temperature range close to the λ-temperature, low frequency spiky oscillations are superposed on the regular oscillation. At still lower temperatures than λ-temperature, the regular oscillation disappears and boiling turns to noisy film boiling.