A review on heat transfer characteristics of cryogenic heat pipes

Heat pipes are broadly applied for thermal management of devices with high heat flux. Due to the dependency of their efficient operating range on the boiling point of the working fluids, employing special types of fluids is necessary to have applicable heat pipes at very low temperatures. Hydrogen, neon and nitrogen are among the most attractive working fluids for heat pipes in order to reach efficient heat transfer in cryogenic conditions. According to the reviewed studies on cryogenic heat pipes, similar to the medium or high-temperature heat pipes, their thermal performance depends on different factors such as filling ratio, input power, structure and type of heat pipe and working fluid. Employing appropriate fluids, applicable for the required operating temperature, provides heat pipes with high effective thermal conductivity in cryogenic conditions. The effective thermal conductivity of the cryogenic heat pipes is several orders of magnitudes higher than that of the conductive metals in the similar temperature which makes them as desirable thermal management devices for low-temperature applications such as cooling superconductors. In this paper, a comprehensive review is carried out on the researches focused on the cryogenic heat pipes and the factors influencing their heat transfer features to provide a useful and applicable reference for the future studies with relevant topics. This review study is categorized based on the type of heat pipes: conventional heat pipes, pulsating heat pipes and thermosyphons (gravity-assisted heat pipes). The reviewed studies have demonstrated that the operating fluid and heat load have substantial role in the performance of heat pipes. Finally, some recommendations for future studies are provided to reach further enhancement in the performance of cryogenic heat pipes.