Development of a boiling regime map and gravity scaling parameter for pool boiling heat transfer
Virtually all data to date regarding parametric effects of gravity on pool boiling have been inferred from experiments performed in low- g, 1g, and 1.8g conditions. The current work is based on observations of boiling heat transfer obtained over a continuous range of gravity levels (0g-1.8g) and varying heater sizes under subcooled liquid (FC-72/n-perfluorohexane) conditions. Variable gravity pool boiling heat transfer measurements were made during the parabolic flight campaigns organized by the European Space Agency (ESA) and NASA. Heater size was varied by using two (2.7x2.7 mm 2 and 7.0x7.0 mm2) constant temperature microheater arrays consisting of 96 platinum resistance heaters deposited in a 10x10 configuration onto a quartz substrate. The ability to selectively power a subset of heater elements (1, 4, 9, 16, 25, 36, 64, and 96) in a square pattern out of the 10x10 configuration allowed a variation in heating area from 0.27x0.27 mm 2 to 7.0x7.0 mm2. A parametric study on the effects of fluid properties, wall superheat, liquid subcooling, and dissolved gas concentration on boiling heat transfer was also performed.
Based on the heater sizes and the gravity levels investigated, two pool boiling regimes were identified. For large heaters and/or higher gravity conditions, buoyancy dominated boiling and heat transfer results were heater size independent. Under low gravity conditions and/or for smaller heaters, surface tension forces dominated and heat transfer results were heater size dependent. A first ever pool boiling regime map differentiating buoyancy and surface tension dominated boiling regimes was developed. The non-dimensional ratio of heater size L h and capillary length Lc was found suitable to differentiate between the boiling regimes. Transition between the regimes was observed to occur at a threshold value of Lh/Lc ∼2.1.
Pool boiling data in the buoyancy dominated boiling regime ( Lh/Lc>2.1) was used to develop a gravity scaling parameter for pool boiling heat transfer. A non-dimensional temperature was defined in order to derive a gravity scaling parameter independent of dissolved gas concentrations and liquid subcooling. The power law coefficient for the gravity effect was observed to be a function of the non-dimensional wall temperature. The predicted results were found to be in good agreement with the heat transfer data over a wide range of gravity levels (0g-1.8g), dissolved gas concentrations, subcoolings, and heater surface morphologies. Use of this scaling parameter to obtain heat transfer at varying gravity levels is expected to save considerable experimental resources required to validate the performance of phase change based systems under different gravity conditions.