The loss of the insulating vacuum is a critical accidental scenario in cryogenic systems, as it induces high heat fluxes to the cryogenic fluid and a rapid pressure rise. To ensure the safe operation of such systems, all cryogenic circuits must be equipped with appropriately sized pressure relief devices, such as safety valves or rupture discs. The design of these devices relies on accurate knowledge of heat fluxes derived from experimental data.
For helium systems discharging above the critical pressure, available experimental data have been established for storage tanks, where heat transfer is dominated by natural convection at the helium-wall interface.  However, large scale facilities, such as fusion devices and particle accelerators, also include extensive vacuum-insulated piping networks for helium distribution. In these systems, helium flows at high mass flow rates under forced convection conditions, for which reference values of heat flux remain scarce.
As part of a PhD thesis at CEA/DSBT, this work used the experimental facility HELIOS at CEA Grenoble to study heat transfer during a loss of vacuum around a cryoline and address the lack of reference heat flux data. A dedicated test section enabled controlled venting of the insulating vacuum surrounding a one-meter-long pipe carrying supercritical helium, with mass flow rates ranging from 5 to 50 g/s. Both the HELIOS facility and the test section were instrumented to measure the heat flux transferred to the helium flow and to characterize the transient dynamics of the vacuum loss, including nitrogen ingress and condensation.
Experimental results show a clear influence of the helium flow regime on heat transfer. At low Reynolds numbers, the heat transfer is limited by internal forced convection, with stabilized heat fluxes remaining below 2 W/cm². In contrast, for Re > 4·10⁵ , a marked enhancement is observed. In this regime, a solid nitrogen layer forms along the entire pipe and the measured heat flux systematically exceeds 2 W/cm², the value currently recommended by the EN17527 standard for the sizing of safety valves for helium vessels. In the most severe case, the peak heat flux reaches 5.2 W/cm² before stabilizing at 2.3 W/cm².

 

Keywords
Loss of insulating vacuum|Supercritical helium flow|forced convection|Heat flux|Condensation