AN EXPERIMENTAL INVESTIGATION OF SUBCOOLED FLOW BOILING HEAT TRANSFER UNDER MICROGRAVITY CONDITIONS
An experimental apparatus has been constructed and used to investigate subcooled flow boiling heat transfer under microgravity conditions. Freon (R-113) was circulated at a constant mass flux between 230 kg/m2s and 840 kg/m2s through a stainless steel tubular test section (L 914.4 mm, OJ3.12.5 mm, ID. 12.0 mm). The horizontal test section was heated externally up to 17 kW/m2 by a spirally wrapped nichrome foil heater. Thermocouples attached to the outer surface of the test section measured steady wall temperature profiles. The resulting two-phase flow was then condensed and cooled before being recirculated in the flow loop.
Experiments under microgravity were performed aboard NASA's KC-135 aircraft. Comparison with ground-based normal gravity data revealed that subcooled boiling heat transfer was enhanced during microgravity. Heat transfer coefficients were approximately 5 to 20% greater in microgravity, generally increasing with higher qualities. The greater movement of the vapour bubbles generated on the heater surface caused more localized turbulence, which is believed to be responsible for the increased heat transfer coefficients.