Experimental Study of the Heater Size Effect on Subcooled Pool Boiling Heat Transfer of FC-72 in Microgravity

作者: Xueli Wang, Yonghai Zhang, Baojin Qi, Jianfu Zhao, Jinjia Wei*
发表/完成日期: 2016-03-26
期刊名称: Experimental Thermal and Fluid Science, 2016, 76: 275-286.
期卷: 76
Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were conducted on a
large scale silicon chip (2  2  0.05 cm3, denoted as chip S 2  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  1  0.05 cm3, denoted as chip S 1  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  2 is wider than that for chip S 1  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  2 increased linearly with heat flux while later remained constant. At a same heat
flux, chip S 2  2 showed a bubble departure radius larger than that of chip S 1  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 qCHF of chip S 2  2 is 20% greater
than that of chip S 1  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  1, while on chip S 2  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  2, but it was in surface tension dominated boiling
regime for chip S 1  1, which proved the above speculation. Moreover, the range of transition heater size
criteria is 1:45 < LhLc< 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  2.
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