Virtually all data to date regarding parametric effects of gravity on pool boiling have been inferred from experiments performed in low-g, 1g, or 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) under subcooled liquid conditions (n-perfluorohexane, ΔTsub=26°C, and 1 atm), two gas concentrations (220 ppm and 1216 ppm), and three heater sizes (full heater-7×7 mm2, half heater-7×3.5 mm2, and quarter heater-3.5×3.5 mm2). As the gravity level changed, a sharp transition in the heat transfer mechanism was observed at a threshold gravity level. Below this threshold (low-g regime), a nondeparting primary bubble governed the heat transfer and the effect of residual gravity was small. Above this threshold (high-g regime), bubble growth and departure dominated the heat transfer and gravity effects became more important. An increase in noncondensable dissolved gas concentration shifted the threshold gravity level to lower accelerations. Heat flux was found to be heater size dependent only in the low-g regime.

Phase change technology is a science that is continually finding new applications, from passive refrigeration cycles to semiconductor cooling. The primary heat transfer techniques associated with phase change heat transfer are pool boiling, flow boiling, and spray cooling. Of these techniques, spray cooling is the least studied and the most recent to receive attention in the scientific community. Spray cooling is capable of removing large amounts of heat between the cooled surface and the liquid, with reported heat flux capabilities of up to 1000 W/cm 2 for water. Many previous studies have emphasized heat flux as a function of spray parameters and test conditions.Enhanced spray cooling investigations to date have been limited to surface roughness studies. These studies concluded that surface tolerance (i.e. variations in machined surface finish) had an impact upon heat flux when using pressure atomized sprays.Analogous pool boiling studies with enhanced surfaces have shown heat flux enhancement. A spray cooling study using enhanced surfaces beyond the surface roughness range may display heat flux enhancement as well.In the present study, a group of extended and embedded surfaces (straight fins, cubic pin fins, pyramids, dimples and porous tunnels) have been investigated to determine the effects of enhanced surface structure on heat flux. The surface enhancements were machined on the top surface of copper heater blocks with a crosssectional area of 2.0 cm 2 . Measurements were also obtained on a flat surface for baseline comparison purposes. Thermal performance data was obtained under saturated (pure fluid at 101 kPa), nominally degassed (chamber pressure of 41.4 kPa) and gassy conditions (chamber with N 2 gas at 101 kPa). The study shows that both extended and embedded structures (beyond the surface roughness range) promote heat flux enhancement for both degassed and gassy spray cooling conditions. The study also shows that straight fins provide the best utilization of surface area added for heat transfer. An Energy conservation based CHF correlation for flat surface spray cooling was also developed. CHF predictions were compared against published and non-published studies by several researchers. Results for the correlations performanceshow an average mean error of ±17.6% with an accuracy of ±30% for 88% of the data set compared against.

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Although the effects of microgravity, earth gravity, and hypergravity (>1.5 g) on pool boiling heat flux have been studied previously, pool boiling heat flux data over a continuous range of gravity levels (0–1.7 g) was unavailable until recently. The current work uses the results of a variable gravity, subcooled pool boiling experiment to develop a gravity scaling parameter for n-perfluorohexane/FC-72 in the buoyancy-dominated boiling regime (Lh/Lc>2.1). The heat flux prediction was then validated using heat flux data at different subcoolings and dissolved gas concentrations. The scaling parameter can be used as a tool to predict boiling heat flux at any gravity level in the buoyancy dominated regime if the data under similar experimental conditions are available at any other gravity level.