Microgravity and Earth Gravity Boiling Heat Transfer Mechansims: Investigations Using Microheater Arrays

Jung Ho Kim1

The use of two-phase thermal systems on spacecraft has been greatly hampered by the inability to predict with sufficient confidence their performance at various gravity levels (Earth, Mars, and lunar gravity and microgravity).  The limited number of experiments to date have shown that stable, subcooled boiling on flat plates in microgravity environments is possible with some alteration in heat transfer coefficients, but almost all of the research has been either of a qualitative nature (photographic studies) with some wall heat flux/wall temperature measurements, analytical work, or numerical simulations.  In the studies where heat transfer coefficients were measured, the heated surfaces were always comparable to or larger than the bubbles, so only average heat transfer rates over the entire heated surface were obtained.  Very little experimental data is available regarding the local heat transfer rates under and around bubbles as they grow and depart from the surface.  Better understanding of the boiling heat transfer mechanisms can be attained by pinpointing when and where in the bubble departure cycle large amounts of heat are removed from the wall, and correlating this information to visual observations of the state of the bubble at those times.  Such information can provide much needed data regarding the important heat transfer mechanisms during the bubble departure cycle, and can serve as benchmarks to validate many of the analytical and numerical models used to simulate boiling.  We have designed and built a microheater array that provides time and space resolved heat transfer under individual bubbles, and are slowly gaining an understanding of the true boiling heat transfer mechanisms in various gravity environments.