OpenFOAM modelling of air ingress into the high vacuum for fusion reactor safety

The underexpanded jet induced by loss of vacuum accidents (LOVA) can significantly impact the fusion reactor management, since it poses safety problems associated with hydrogen risks and radioactivity to the environment. To characterize the air ingress process into the high vacuum environment, simulations were performed with OpenFOAM modelling in a representative small-scale fusion facility. The general features of the thermodynamic parameters, such as density, pressure, velocity, and temperature, were analyzed, and also characterized the wall friction velocity and the formation of the Mach disk within the underexpanded jet. Proper orthogonal decomposition (POD) method was applied to compare the fluid dynamic behaviors of the jet subjected to different thermodynamic conditions. The results show that the rapid expansion and acceleration of the jet leads to a decrease in density and an accumulation of the gas along the centerline. The high jet velocity will result in a lower jet temperature and raise the temperature of the surroundings, which also triggers the formation of recirculation zones, and the gradual development of Mach disk structure, also, the higher wall friction velocity will further contribute to the complex air ingress dynamics. Moreover, it is observed that the ideal gas and real gas model appear similar fluid dynamic structures and energy modes during the pressure and velocity development, and only subtle differences appear in the low energy contribution POD modes. The main differences of the energy modes are captured in the momentum field between these different thermodynamic conditions. The observations can contribute to fusion safety management and appropriate thermodynamic modelling selection in such applications.