Experiments and simulations related to the sudden application of a heat pulse onto a supercritical helium flow

Abstract

Vacuum loss is a major accidental scenario in cryogenic facilities that use vacuum for thermal insulation from the surrounding environment. An experimental facility, called HELIOS, has been set up to study vacuum loss around a cryoline in which supercritical helium is flowing. Before carrying out real vacuum loss experiments, a preliminary test campaign was performed using an electrical test section inserted into a cryogenic loop within the HELIOS facility, at the exact location of the future vacuum loss test section. The electrical section is equipped with an electrical heater that was used to simulate the heat pulse that would be experienced by the supercritical helium flow in the event of a real vacuum loss accident. The consequences of the abruptly applied heat pulse have been studied, both in real and simulated conditions. The main objectives of this first experimental campaign are multiple: to assess the design of the HELIOS facility, verify the accuracy of the heat flux measurement methods, and use the results of the experimental test campaigns to improve the qualification of the thermal hydraulic code employed to size the experimental setup. The simulation was based on the thermal–hydraulic code CATHARE 3 and includes two-phases cryogenic flows. Thermal-hydraulic parameters of the supercritical helium flow as pressure, temperature and mass flow rate, have been recorded for several heat loads running from 0.45 kW to 2.5 kW, with durations of up to 2 min. Heat transfer in the supercritical loop and the liquid helium bath, as well as corresponding energy balances, have been calculated. The resulting estimations of the heat applied to the test section correspond with an excellent accuracy to the real thermal loads. For a heat flux of 1.35 kW, the data were also simulated with the CATHARE 3 code. The results of the simulation match the measured and calculated data with high accuracy. This preliminary work and its associated conclusions provide a firm basis for forthcoming experiments, which will be devoted to the study of heat transfer following a real breakdown of the insulating vacuum in the supercritical helium loop.