Highly subcooled pool boiling heat transfer at various gravity levels

Abstract

A microscale heater array 2.7 mm×2.7 mm in size along with a video camera was used to provide subcooled pool boiling heat transfer measurements at gravity levels ranging from 1.8 g to 10 −6 g. The fluid was FC-72 at 1 atm and subcooled by nominally 36 °C for all cases. A Terrier–Orion sounding rocket and the KC-135 aircraft were used to provide the microgravity and low- g environments, respectively. The data indicate that there is little effect of gravity on boiling heat transfer at wall superheats below 25 °C, even though there are large differences in bubble behavior between gravity levels. At higher superheats, the low- g data fell significantly below the 1 g and 1.8 g data due to a large part of the heater surface drying out. A large primary bubble was observed to form and move over the surface in low- g, occasionally causing nucleation to occur. This primary bubble was surrounded by smaller bubbles which eventually merged with it. The primary bubble initially formed by the coalescence of smaller bubbles generated on the surface, but then remained constant in size for a given superheat, indicating a balance between evaporation at the bubble base and condensation on the bubble cap. The size of the primary bubble increased with increasing wall superheat. Most of the heaters under the primary bubble indicated low heat transfer, consistent with dryout on the heater surfaces. Strong Marangoni convection around the bubble was observed to develop in low- g, forming a “jet” of heated fluid into the bulk fluid.