Freeze/thaw characteristics of room temperature heat pipes-incorporating thermal history effects and non-condensible gas charging

Abstract

The freeze/thaw characteristics of room temperature heat pipes have been investigated experimentally and analytically to determine the effects of non-condensible gas loading and thermal history. Experimentally, the freeze/thaw characteristics of a copper/water heat pipe of rectangular cross-section were examined to determine the effect of variations in the amount of non-condensible gases present. The transient internal temperature profiles in the liquid channel are presented along with contours of the frozen fluid configuration obtained through visual observation. Several interesting phenomena were observed including total blockage of the vapor channel by a solid plug, evaporator dryout during restart, freezing blowby, and working fluid migration by sublimation. In addition, the restart characteristics are shown to be strongly dependent upon the shutdown procedure used prior to freezing, indicating that accurate prediction of the startup or restart characteristics requires a complete knowledge of the thermal history. An analytical investigation was presented to examine the effects of non-condensible gas loading on restart. The model utilized regional energy balances and assumed uniform regional temperatures, independent movement of the vapor/gas front and the liquid melt front, and working fluid sublimation in the frozen state. The model compared well with experimental data for a uniformly distributed working fluid along with no non-condensible gases and with sufficient gas charging to result in a gas-controlled restart. To investigate the effects of microgravity on the freeze/thaw characteristics, a microgravity flight experiment (acronym FROZEPIPE) was constructed and qualified for the first flight of the Commercial Experiment Transporter (COMET).