Experimental study on the detection of frozen diffused ammonia blockage in the inactive section of a variable conductance heat pipe

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Variable Conductance Heat Pipes (VCHP) are mainly employed to cool down electronic systems in spacecraft applications, as they can handle high temperature fluctuations in their cold source, preventing thus the systems from damaging. These fluctuations, as well as ultra-low temperatures, are always present in outer space, and one of the key steps in a VCHP design is therefore to make sure that they endure these conditions. However, not much has been written about their resilience during and after a long exposition to subfreezing conditions, i.e. temperatures lower than the freezing point of the working fluid. In this paper we implement and validate a computational routine based on a modified Flat-Front Approach to predict the VCHP temperature profile and to determine the location of the gas–vapor front. Then we continuously expose an ammonia/stainless-steel VCHP to temperatures below the ammonia freezing point for 211 h, to later examine the formation and subsequent dynamics of a thin block of frozen ammonia which is diffused into the inactive part of the heat pipe condenser. We describe as well how a strong correlation between the adiabatic section and the reservoir temperatures is maintained (or broken) upon the occurrence (or absence) of the blockage of frozen ammonia.