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Variable Conductance Heat Pipes (VCHP) are mainly employed to cool downelectronic systems in spacecraft applications, as they can handle hightemperature fluctuations in their cold source, preventing thus the systems fromdamaging. These fluctuations, as well as ultra-low temperatures, are alwayspresent in outer space, and one of the key steps in a VCHP design is thereforeto make sure that they endure these conditions. However, not much has beenwritten about their resilience during and after a long exposition tosubfreezing conditions, i.e. temperatures lower than the freezing point of theworking fluid. In this paper we implement and validate a computational routinebased on a modified Flat-Front Approach to predict the VCHP temperature profileand to determine the location of the gas-vapor front. Then we continuouslyexpose an ammonia/stainless-steel VCHP to temperatures below the ammoniafreezing point for 211 hours, to later examine the formation and subsequentdynamics of a thin block of frozen ammonia which is diffused into the inactivepart of the heat pipe condenser. We describe as well how a strong correlationbetween the adiabatic section and the reservoir temperatures is maintained (orbroken) upon the occurrence (or absence) of the blockage of frozen ammonia.

Experimental Study on the Detection of Frozen Diffused Ammonia Blockage in the Inactive Section of a Variable Conductance Heat Pipe