Temperature measurement and flow visualization of cryo-compressed hydrogen released into the atmosphere

For assessing the risks of liquid hydrogen pump facilities, we conducted a test in which liquid hydrogen was pressurized to a maximum of 85 MPa and steadily released through a pinhole nozzle. In order to quantitatively evaluate the cooling effect of the released cryogenic hydrogen on the surrounding environment, the temperature of the hydrogen jet was measured while changing the supply pressure and temperature parameters. We applied shadowgraph flow visualization to understand the diffusion mechanism of the low-temperature hydrogen jet escaping the pinhole nozzle. The results showed that no liquid phase appeared during the cryo-compressed hydrogen leakage and that the hydrogen jet temperature could be accurately predicted using the Joule-Thomson expansion equation. The shadowgraph showed that a dense potential core was formed in the hydrogen jet even under a high-temperature condition far from the critical point (Tr = 2.4) and was characterized by a supercritical jet. In addition, it was confirmed that the boundary where the hydrogen jet becomes visible exists near the Widom line. Further consideration is required regarding the consistency of these results with the conclusions of existing studies that pseudo-boiling becomes negligible in the region wherein Pr > 10.