Experimental study of pool boiling of FC-72 over smooth surface under microgravity

Experiments of subcooled pool boiling of FC-72 on a small scale smooth silicon chip with the dimensions of 10 mm × 10 mm × 0.5 mm were studied in short-term microgravity utilizing Drop-Tower Beijing with duration of about 3.6 s. The smooth silicon chip was heated by means of controlling heating current for the desired heat flux. The results indicate that steady or quasi-steady nucleate pool boiling was observed in the experiments in low and intermediate heat flux regions. The performance of boiling heat transfer can be maintained unchanged. It is found that the oscillation due to coalescence of adjacent bubbles is the primary reason of bubbles departure in microgravity condition in intermediate heat flux regime. At high heat fluxes, a large coalesced bubble forms quickly and covers the heater surface completely in microgravity, followed by shrinking to an oblate in shape and smooth in contour due to the highly subcooled condensation. The mean heater surface temperature increases significantly, which indicates the deterioration of boiling heat transfer. It is possible for the occurrence of local dry-out or transition to film boiling at the bottom of the large coalesced bubble.