Numerical investigation of a highly underexpanded jet with real gas effects during the loss of coolant accident in a cryostat: Flow and heat transfer characteristics

During a loss of coolant accident (LOCA) caused by a rupture on helium-cooled thermal shields, an underexpanded jet of cryogenic helium forms in the cryostat, increasing pressure in the cryostat and heat load on superconducting magnet walls. The real gas effect of helium at low temperatures has significant impacts on the fluid flow and heat transfer characteristics, which has not yet been fully investigated. In this work, simulations of the helium underexpanded jets during a LOCA with the ideal gas assumption and the NIST real gas model are compared. Besides, the effect of breach size on helium underexpanded jets is also investigated. Results show that the simulation with the ideal gas assumption significantly underestimates the average pressurization rate in the vacuum chamber and the average heat flux on the center solenoid wall (CS-1) compared to the NIST real gas model, with relative errors of 38.59 % and 30 % in 1 s, respectively. As the breach diameter increases from 120 mm to 240 mm, a non-proportional relationship exists between the breach diameter and the pressurization rate. For all the breach sizes discussed, there is no risk of overpressure in the vacuum chamber during the early stage of the LOCA, whereas the risk of liquid helium film boiling occurs in the superconducting magnet cooling system.