Safety oriented analysis of cold helium–air mixture formation and stratification

Cryogenic installations of superconducting accelerators contain significant amount of high density helium, mostly filling the cryostats and headers located in underground tunnels and caverns. In the case of emergency relief of the helium, oxygen deficiency hazard arises. The paper proposes a description of helium–air mixture formation based on turbulent jet physics. A condition of stratified flow onset is given resulting from dimensional analysis. A performed experiment of cold helium discharge to an instrumented test tunnel enabled identification of different stratified flow types together with corresponding measurements of oxygen concentration and temperature drop. The results can be used for safety analysis of oxygen deficiency hazard in confined areas, especially in the tunnels.