Enhancement of Bubble Departure and Critical Heat Flux in Saturated Pool Boiling under Microgravity Conditions

摘要

All the correlations and analytical models of critical heat flux in saturated pool boiling predict that the critical heat flux under microgravity conditions becomes much smaller than the terrestrial value. In the present paper, aiming at the enhancement of the critical heat flux under microgravity conditions, a novel technique to shorten the bubble departure period τd in saturated pool boiling under microgravity conditions was proposed. The technique proposed is based on the self round--off motion of bubbles induced by surface tension. Such self round-off motion is expected to occur by setting ribs of an adequate height on the boiling surface. In the present experiment, to realize and simulate such a boiling system, saturated boiling experiments were conducted using a ribbon heater sandwiched by two glass plates. In the experiments, the glass plate height H was changed from 0.5 mm to 20 mm. Experimental data under microgravity conditions indicate the following results. In the cases of H=0.5-4.5 mm, the bubble departure period of ethanol under microgravity conditions' is as short as the terrestrial value and then the critical heat flux is as high as the terrestrial value. The critical heat flux is caused when the value of τdq reaches a maximum. The equivalent liquid- film thickness calculated from the experimental data of τdq is much smaller than the macrolayer thickness calculated from the model proposed by Haramura and Katto.