Heat and mass transfer during a sudden loss of vacuum in a liquid helium cooled tube - Part I: Interpretation of experimental observations

Understanding heat and mass transfer processes involved in a sudden catastrophic loss of vacuum is important for many cryogenic systems around the world for cost, damage prevention, and safety reasons. Continuing research in our lab focuses specifically on studying the sudden vacuum break in beam-line tubes of liquid helium cooled superconducting particle accelerators. In our experiments, loss of vacuum is simulated by venting nitrogen gas from a buffer tank to a liquid helium cooled vacuum tube. Previous experiments with normal helium (He I) have revealed that the gas front propagation rate decreases exponentially (Dhuley and Van Sciver, 2016). This slowing down was attributed to the condensation of the nitrogen gas on the tube inner wall, but a quantitative analysis of the gas dynamics and condensation was lacking. In this paper, we extend the previous experimental work by examining the gas propagation in a longer helical tube system cooled by both He I and superfluid helium (He II). We discuss how the cold section of the tube above the liquid helium bath may affect the gas propagation and can lead to an apparently stronger slowing down effect in the He II cooled tube. We also discuss some limitations in the result interpretation in the previous research. A new theoretical model that systematically describes the gas dynamics and condensation is presented. Preliminary simulation results using the model reproduce some key experimental observations well. (C) 2018 Elsevier Ltd. All rights reserved.