Experimental validation of heat switch capability of cryogenic loop heat pipe

Abstract

The nitrogen-charged cryogenic loop heat pipe (CLHP) was utilized as an effective cryogenic heat switch operating at around 100 K by thermally connecting or blocking a cryogenic heat source and a cryogenic heat sink. The switching-off function of the CLHP heat switch was achieved by eliminating heat load on the secondary evaporator without any dedicated support of additional components or mechanisms during a normal operation. The removal of heat load on the secondary evaporator leads to the dry-out of the primary evaporator, which causes the thermal switching-off ultimately. The on/off thermal conductance ratios of the CLHP heat switch were obtained in various operational conditions such as different heat loads on the primary and secondary evaporators and different initial charging-pressures of nitrogen. The switching-off process divided into the on-state, transient state, and off-state was investigated experimentally, and the characteristics in the unsteady off-state were specially analyzed in detail. The obtained on/off thermal conductance ratios of the CLHP heat switch were in the range from 400 to 3900 in the experiments which are exceedingly larger than those of the conventional heat switches. The ratio becomes larger when the initial charging-pressure of working fluid is low but the heat loads on the primary evaporator and/or the secondary evaporator are large because the effective thermal conductance in the on-state is dominantly affected by the mass flow rate of working fluid. Between the heat loads on the primary and secondary evaporators, the on/off thermal conductance ratio of the CLHP heat switch is more sensitively affected by the heat load on the PE.