Thesis presented November 17, 2023

Abstract:
Several large scientific instruments require refrigeration capacity to reach very low temperatures, around 4 K. Helium is cooled by a cryogenic plant and distributed to the various elements by cryogenic lines forming the cryodistribution system.
Like all cryogenic installations, cryodistribution systems have to be protected against accidental overpressure by installing pressure relief devices such as valves or rupture discs. Sudden loss of vacuum is often considered as the most serious accident. The condensation of air gases on the cold walls following the loss of the insulating vacuum leads to transfer high heat flux to the cryogenic fluid. The sudden change in helium density generates overpressure in the system, and a large mass flow has to be evacuated.
Up to now, the design of safety devices was based on the use of experimentally measured standard heat flux values. For supercritical helium, measurements have only been carried out for the tank configuration with natural convection. Nevertheless, to ensure a reliable sizing of pressure relief devices for cryogenic facilities equipped with cryolines, measurements have also to be carried out for forced convection cases.
The aim of this work is to perform experimental measurement of the heat flux received by supercritical helium flowing in tubes, following the loss of vacuum. For this purpose, the experimental platform HELIOS has been modified to include a test section and perform loss of vacuum test. The experimental set up has been modelled with the thermal hydraulic code CATHARE in order to improve the understanding of the heat transfer phenomena involved. Two experimental campaigns have been carry out to the measure the heat flux.

Keywords:
Cryogenics, Loss of the insulating vacuum, Accidental heat inputs, Supercritical helium, heat flux, thermal-hydraulics, System code, Safety

On-line thesis.​