Thermal response of high-aspect-ratio hydrogen cylinders undergoing fast-filling

Abstract

Fast-filling can lead to excessive heating of a hydrogen cylinder. Gas cylinders with large length-to-diameter aspect ratios are prone to developing hot-spots that may cause the temperature of the structure locally to exceed specified limits. The hot-spots develop because convective heat transfer into the cylinder wall is not uniform along the length of the cylinder. Computational fluid dynamic simulations of fast-filling reveal that the flow within large aspect ratio cylinders is characterised by two distinct flow patterns. The region near to the inflow nozzle exhibits an axi-symmetric recirculating flow pattern that extends to approximately three cylinder diameters downstream from the inlet. Wall heat transfer in the recirculation region of the flow is driven strongly by the jet of turbulent fluid from the nozzle. The wall heat transfer in the recirculation region rapidly develops quasi-steady behaviour in which the Nusselt number is approximately proportional to the inlet jet Reynolds number. Downstream of the recirculation zone there is a region of axial flow, in which wall heat transfer occurs at a slower rate, driven by decaying turbulence transported from the recirculation zone. The reduced heat transfer in the end gas leads to a higher gas temperature but also to a lower structural temperature downstream of the recirculation zone. Analysis shows that wall heat transfer can be enhanced and peak gas temperatures reduced by dividing the hydrogen injection between separate nozzles. Directing the nozzles axially, ideally at intervals of around 4 cylinder diameters, increases heat transfer and prevents the formation of hot regions of stagnant fluid. This approach allows more hydrogen to be stored for a given pressure or gas temperature limit. However the analysis across various length-to-diameter ratios and nozzle configurations reveals substantially different trends for the gas temperatures appearing in ISO safety specifications, and the temperature of the structural materials they seek to safeguard.