Numerical modelling of cold helium safety discharges from the cryogenic distribution line

Abstract: The European Spallation Source, ESS, is a project currently with 17 Partner Countries, and with Sweden and Denmark as host nations. The ESS main facility is being constructed in Lund, to ultimately become the world’s most powerful neutron source, providing around 30 times brighter neutron beams than any earlier facility. The Linear Accelerator, Linac, at the heart of ESS is fundamentally powered by superconducting cavities that require extremely low temperatures to operate, namely 2K (-271 °C). The cooling/cryogenic fluid, helium, is supplied to the cavities through a complex cryogenic distribution system (CDS). At abnormal failure modes, to protect against unwanted over-pressurisation, this system needs to be vented through pressure safety valves. To avoid helium discharges into the tunnel the safety valves will discharge cold helium into a vent line, which transfer the helium into the external environment. Usually the most critical failure mode is a loss of insulation vacuum, which exposes the CDS to a sudden rise of heat loads. This thesis investigates the most critical phenomena for the non-insulated vent line flows. Due to a lack of simple engineering descriptions of these unsteady flows, a set of complex numerical models will support a proper vent line sizing proposal. A commercial numerical software, ANSYS Fluent, is used and compared with simpler solution methods for differential equations derived from Fanno and Rayleigh flow theories.