A pulsating heat pipe embedded radiator: Thermal-vacuum characterisation in the pre-cryogenic temperature range for space applications

The use of Pulsating Heat Pipes (PHPs) in the space field is still an open issue because of the lack of data obtained during actual operation in relevant environment. A considerable amount of data is available in the literature on the thermal response of PHPs to a variable gravity condition and on the operation in the cryogenic field but in both cases the heat load at the condenser is rejected by convection to a constant temperature sink. On the other hand, barely any work in the PHP's literature deals with thermal radiation to a low temperature sink as only heat transfer mode. The present work attempts to fill the gap by testing a PHP radiator (16 turns, 1.1 mm inner diameter, 50% filled with FC-72 at 293 K) in thermo-vacuum conditions in horizontal orientation at different heat loads and different environment (chamber) temperature. Fluid pressure measurements coupled with the frequency analysis characterised the effect of the cold source temperature on the device operational limits and efficiency. Results show that the device thermal performance in the radiative configuration is mostly affected by the lower operating temperatures needed to obtain a sensible heat rejection, rather than the heat transfer mode itself. The decrease of the environment temperature shortens the operational heat load range: the start-up occurs at higher heat input levels while the thermal crisis occurs at lower heat loads. The frequency analysis reveals that the equivalent thermal resistance is positively affected by higher values of the dominant frequency for all the cases.