Erosive Effects of Hydrogen Jet Fires on Tunnel Structural Materials

This paper presents work undertaken as part of the Hytunnel-CS project, a consortium investigating safety considerations for fuel cell hydrogen (FCH) vehicles in tunnels and similar confined spaces. This test programme investigated erosive effects of an ignited high pressure hydrogen jet impinging onto tunnel structural materials, specifically concrete as used for tunnel linings and asphalt road surfacing for the road itself. The chosen test conditions mimicked a high-pressure release (700 bar) from an FCH car as a result of activation of the thermal pressure relief device (TPRD) on the fuel tank. These devices typically have a release opening of 2 mm and thus, a nozzle diameter of approximately 2 mm was used. The resultant releases were ignited using a propane pilot light and test samples were placed in the jet path at varying standoff distances from the release nozzle.
An initial characterization test of a free unimpeded ignited jet demonstrated a rapid and intense temperature increase, up to 1650 °C, lasting in the order of 3 - 5 minutes for that fuel inventory (4 kg hydrogen). Five tests were carried out where the ignited jet was impinged onto five structural samples. It was found that erosion occurred in the concrete samples where no fire mitigation, namely addition of polypropylene fibres, was applied. The road-surface sample was found to become molten but did not progress to combustion.
Post-test material analysis, including compressive strength and thermal conductivity measurements, was carried out on some of the concrete samples to investigate whether structural deformities had occurred within the sample microstructure. The results suggested that the erosive damage caused by the hydrogen jet was mostly superficial and as such, did not present an increased fire risk to the structural integrity to that of conventional hydrocarbon fires, i.e. those that would result from petrol or diesel fuel tank releases. In terms of fire resistance standards, it is suggested that current fire mitigation strategies and structural testing standards would be adequate for hydrogen vehicles on the road network.