Microgravity heat transfer mechanisms in the nucleate pool boiling and critical heat flux regimes using a novel array of microscale heaters

摘要

Boiling as a heat transfer mechanism is becoming of increasing importance to space based hardware due to increases in the amounts of heat that must be removed with little increase in temperature. An understanding of microgravity effects on boiling mechanisms is therefore critical to the proper design of heat removal equipment for use in space-based applications. Microgravity effects on critical heat flux levels (the maximum heat flux level that can be attained without a catastrophic increase in heater temperature) is another very important area that must be addressed if boiling is to be used reliably as a heat removal mechanism. Although critical heat flux is known to decrease with decreasing gravity, very little quantitative information is available due to the use of constant wall heat flux heaters (as opposed to constant wall temperature heaters). There are two primary objectives to this study: (1) to determine the relative contributions of various heat transfer mechanisms to the overall heat flux in subcooled, nucleate pool boiling of a fluorinert on a flat plate in terrestrial gravity, and (2) to obtain quantitative data regarding local heat transfer levels at critical heat flux in terrestrial gravity. To accomplish these objectives, a novel heater surface consisting of a two-dimensional array of microscale heaters is used to measure the heat transfer from a surface at many points underneath a bubble in conjunction with visual studies. This heater array enables the heat transfer to a surface during the bubble growth and departure process to be measured with very high temporal and spatial resolution. Furthermore, data can be obtained in the critical heat flux and transition boiling regions without the danger of heater burnout because the heaters are operated in the constant temperature mode.