Transient response of an axially-grooved aluminum-ammonia heat pipe with the presence of non-condensable gas

The lifetime of axially-grooved ammonia heat pipes (HP), which are widely used in satellites for temperature homogenization in structural panels can be affected by the generation of non-condensable gas (NCG), causing the gradual deterioration of the HP's performance. In order to better understand the influence of different quantities of NCG on steady-state and transient HP behavior, a mathematical model was developed and thoroughly explored to simulate the performance of heat pipes with and without the presence of NCG. The model was validated and correlated with experimental results from two identical axially-grooved aluminum-ammonia heat pipes, one without NCG, and the other with 0.014% mass ratio of argon as the NCG. Experiments were conducted under 75 W heat load and with condensation provided by a combination of natural and forced convection cooling. The behavior of the HPs was analyzed numerically and experimentally during start-up and shutdown phases. The results revealed how the presence of NCG distorted the pipe's temperature profile and how the NCG density distribution behaved at start-up. The proposed method of NCG detection, based on the analysis of temperature change rate, provided a more sensitive detection of small NCG amounts than usual methods of steady-state temperature profile analysis.