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

TL;DR

This study investigates the resilience of variable conductance heat pipes in space-like conditions, focusing on frozen ammonia blockages, through computational modeling and long-term low-temperature exposure experiments.

Contribution

It introduces a modified Flat-Front Approach for predicting VCHP temperature profiles and validates the effects of frozen ammonia blockages through extensive testing.

Findings

Frozen ammonia can form blocks in the inactive section of VCHPs.

The presence of frozen ammonia affects the correlation between adiabatic and reservoir temperatures.

The computational routine effectively predicts temperature profiles and blockage locations.

Abstract

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 hours, 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.