Abstract
Heat pipes are valuable heat transfer devices that can be used in space; however, when exposed to the extremely low temperature of space, the working fluid can freeze. Currently, there are different methods to help mitigate freezing effects, including non-condensable gas-charged heat pipes and understanding ice formation on surfaces (e.g., typically surfaces with hydrophobic coatings). However, there is limited research about ice formation on wicks. Different wicking structures may delay freezing or mitigate freezing effects. This paper will investigate ice formation on two surfaces — commercial sintered and grooved wicks. An indoor environmental chamber was used to control ambient air temperature (i.e., 22°C) and relative humidity (i.e., 60% RH) and a Peltier cooler was used to control the surface temperature (i.e., −5°C). The resulting condensation of water onto the surface and then freezing was recorded for an hour and analyzed for the time freezing began on the surface (i.e., ice is initially visible) and the time freezing was complete on the surface. Initial results indicate that the sintered wick begins to freeze first (on average at 10.73 minutes versus 13.66 for the grooved wick) and the freezing front propagates faster (taking on average 10.83 minutes versus 12.44 minutes for the grooved wick). From the analysis, it is seen that the wicking surface structure influences the initial freezing time and the rate the freezing front propagates across the surface. These differences and the causes are investigated in this paper. These differences can, in the future, be exploited to design an optimal freeze-tolerant heat pipe and heat pipe freezing models.
