Theoretical and experimental investigation of the startup characteristics of cryogenic loop heat pipes

Cryogenic loop heat pipes (CLHPs) are widely used in satellites. To ensure the successful startup of a CLHP, the gas reservoir volume is typically designed to be 10 to 90 times larger than that of the CLHP itself. With the future miniaturization of satellites, it is crucial to reduce the total weight of the CLHP to lower the resources consumed on the thermal control subsystem. In this study, the relationship between the evaporation temperature of the secondary evaporator and the gas reservoir volume during the CLHP startup was theoretically analyzed. The validity of the relationship was demonstrated by comparing the theoretical results with the experimental data reported in the literature. Additionally, an ethylene charged CLHP with a gas reservoir volume of 3.6 times the CLHP volume was fabricated and tested. It was found that the CLHP can successfully start up by increasing the evaporation temperature of the secondary evaporator to 230 K, which agrees well with the theoretical calculation. The factors influencing the evaporation temperature of the secondary evaporator during the startup process were analyzed theoretically. A dimensionless parameter representing the minimum gas reservoir volume was established, which could guide the lightweight design of the gas reservoir volume.