Experimental study of the heater size effect on subcooled pool boiling heat transfer of FC-72 in microgravity

Abstract

Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were conducted on a large scale silicon chip (2 Â 2 Â 0.05 cm 3 , denoted as chip S 2 Â 2) in short-term microgravity condition and normal gravity condition by utilizing the drop tower in Beijing respectively. The results were compared with published results of a smaller scale chip (1 Â 1 Â 0.05 cm 3 , denoted as chip S 1 Â 1) both in normal gravity condition and microgravity condition, to study the heater size effect on boiling heat transfer. It is indicated that in microgravity, the input heat flux range in which bubble departure took place for chip S 2 Â 2 is wider than that for chip S 1 Â 1, and an interesting phenomenon observed is that coalesced bubbles departed at a continuously smaller radii at a given heat flux. Initially, the average bubble departure radii for chip S 2 Â 2 increased linearly with heat flux while later remained constant. At a same heat flux, chip S 2 Â 2 showed a bubble departure radius larger than that of chip S 1 Â 1. By comparison, it is found that nucleate boiling performance deteriorates with increase in heater size in both earth gravity condition and microgravity condition. However, in microgravity the q CHF of chip S 2 Â 2 is 20% greater than that of chip S 1 Â 1, contrary to the CHF characteristic in earth ground condition. Moreover, boiling patterns in microgravity are different in high heat flux region: a smooth hemispherical bubble was generated on chip S 1 Â 1, while on chip S 2 Â 2 an oblate vapor blanket was formed, which indicates that different dominated boiling heat transfer mechanisms exist in both cases. It is found that in microgravity, the boiling was dominated by buoyancy for chip S 2 Â 2, but it was in surface tension dominated boiling regime for chip S 1 Â 1, which proved the above speculation. Moreover, the range of transition heater size criteria is 1:45 < L h Lc < 2:89 in the present work. By using the updated model developed by Raj and Kim, it is discovered that predicted values are generally lower than experimental values in both experiments. Moreover, differences were more evident for chip S 2 Â 2.

Submerged boiling and jet impingement for cooling high power electronics

“…The results were consistent with those of Park and Bergles [116]. Wang et al [117] reported a different trend by conducting experiments in subcooled FC-72 on the silicon chips with the areas of 20×20 mm 2 and 10×10 mm 2 . The NBHT coefficient of the smaller heater was significantly higher than that of the larger one, regardless of the heat flux.…”

“…Lu et al [119] and Mori et al [120] also found similar result on the silicon wafer and porous copper surface, respectively. It is worth mentioned that CHF was found to increase with the heater size under microgravity in the studies of Wang et al [117]. The reason of the opposite trends might be a result of the different heat transfer mechanism of various heater sizes under the microgravity condition.…”