Boiling phenomena in near-critical SF6 observed in weightlessness

Abstract

Boiling phenomena in the two-phase region of SF6 close to its critical point have been observed using the high-quality thermal and optical environment of the CNES dedicated facility ALI-DECLIC on board the International Space Station (ISS). The weightlessness environment of the fluid, which cancels buoyancy forces and favorites the three-dimensional spherical shape of the gas bubble, is proven to be an irreplaceable powerful tool for boiling studies. To identify each key mechanism of the boiling phenomena, the ALI-DECLIC experiments have benefited from (i) the well-adapted design of the test cells, (ii) the high-fidelity of the ALI insert teleoperation when long-duration experiment in stable thermal and microgravity environment are required and (iii) the high repeatability of the controlled thermal disturbances. These key mechanisms were observed by light transmission and interferometry technique independently with two sample cells filled with pure SF6 at a near-critical density. The fluid samples are driven away from thermal equilibrium by using a heater directly implemented in the fluid, or a surface heater on a sapphire optical window. In the interferometry cell, the bulk massive heater distinguishes two symmetrical two-phase domains. The modification of the gas bubble shape is observed during heating. In the direct observation cell, the gas bubble is separated by a liquid film from the thin layered transparent heater deposited on the sapphire window. The liquid film drying and the triple contact line motion during heating are observed using light transmission. The experiments have been performed in a temperature range of 10K below the critical temperature Tc, with special attention to the range 0.1mK≤Tc−T≤3mK very close to the critical temperature. The unique advantage of this investigation is to provide opportunities to observe the boiling phenomena at very low heat fluxes, thanks to the fine adjustment of the liquid–vapor properties, (e.g. surface tension), by precise control of the distance to the critical point. We present the new observations of the gas bubble spreading over the heating surface which characterizes the regime where vapor bubbles nucleate separately and grow, as well as liquid drying, vapor film formation, triple contact line motion, which are the key mechanisms at the origin of the boiling crisis when the formed vapor film reduces the heat transfer drastically at the heater wall.