Abstract
Hydrogen is being increasingly used as an alternative fuel for vehicles and aircraft propulsion. For long-distance transportation and large-scale energy storage, liquid hydrogen or cryo-compressed hydrogen with typical densities of 70 g/L, is more promising than warm compressed gaseous hydrogen. The use of cryogenic compressed hydrogen introduces new challenges for existing safety standards. In the event of potential leaks, the resulting jets are characterised by intense heat and mass transfer. Furthermore, given the storage pressure and leak diameters, these jets can acquire extremely high velocities, which, in combination with the ultra-low temperature prevailing, introduce considerable challenges to characterize the physics of complex cryogenic hydrogen jets. In this study, using a novel CFD framework specifically tailored to ultra cold dynamics, we analyse the warming and diffusion of a hydrogen cryogenic gas jet into atmospheric air. The simulated conditions are based on an experimental vertical jet. Stagnation conditions are characteristic of unintended leaks from pipe systems that connect cryogenic hydrogen storage tanks and could be encountered at a fuel cell refuelling station. Jets with pressure up to 5 bar and temperatures just above the saturation liquid temperature were examined. Comparisons are made to the centreline mass fraction and temperature decay rates. Satisfactory agreement with the measurements was found in terms of centreline mass fraction and temperature. To characterise the mixing and warming mechanisms of such cold jets, a principle component analysis (PCA) method, namely the proper orthogonal decomposition (POD) technique, is also performed on the simulated data sampled at a fixed domain but at different instants in time. The combined role of the mass and heat flux on heating the cold hydrogen jet is observed. The POD shows the role of turbulent fluctuations in warming the core of the jet. The heat flux observed in the outer plume instead is not captured by the low-order temporal fluctuations, which means that thermal diffusion is responsible for the warming in this region.
