Numerical Study of the Effects of Tunnel Inclination and Ventilation on the Dispersion of Hydrogen Released from a Car

Hydrogen cars are expected to play an important role in a decarbonised, clean-transport future. Safety issues arise though in tunnels, due to the possibility of accidental release and accumulation of hydrogen. This Computational Fluid Dynamics (CFD) study focuses on the effect of tunnel inclination and ventilation on hydrogen dispersion. A horseshoe shaped tunnel of 200 m length is considered in all seventeen cases examined. In most cases, hydrogen is released from the bottom of a car placed at the center of the tunnel. Various inclinations, in-tunnel wind speeds and fuel tank Pressure Relief Device (PRD) diameters were considered in order to assess their influence on safety. It was found that even if the long-term influence of the inclination is positive, there is no systematic effect at initial stages, nor at the most dangerous ‘nearly-stoichiometric’ cloud volumes (25% - 35% v/v). Adverse effects may also exist, like the occasionally higher flammable cloud (4% - 75% v/v). Regarding ventilation, it was found that even low wind speeds (e.g. 1 m/s) can reduce the flammable cloud by several times. However, no significant effect on the total nearly-stoichiometric volumes was found for most of the cases examined. Ventilation can also cause adverse effects, as for example at mid-term of the release duration, in some cases. Concerning the PRD diameter, a reduction from 4 mm to 2 mm resulted in about five times smaller maximum of the nearly-stoichiometric cloud volume. In addition, the effect of release orientation on hydrogen cloud was examined and it was found that the downwards direction presents drawbacks compared to the backwards and upwards release directions.