Pressure Effects of an Ignited Release from Onboard Storage in a Garage with a Single Vent

This work is driven by the need to understand the hazards resulting from the rapid ignited release of hydrogen from onboard storage tanks through a thermally activated pressure relief device (TPRD) inside a garage-like enclosure with low natural ventilation i.e. the consequences of a jet fire which has been immediately ignited. The resultant overpressure is of particular interest. Previous work [1] focused on an unignited release in a garage with minimum ventilation. This initial work demonstrated that high flow rates of unignited hydrogen through a thermally activated pressure relief device (TPRD) in ventilated enclosures with low air change per hour can generate overpressures above the limit of 10- 15 kPa, which a typical civil structure like a garage could withstand. This is due to the pressure peaking phenomenon. Both numerical and phenomenological models were developed for an unignited release, and this has been recently validated experimentally [2]. However, it could be expected that the majority of unexpected releases through a TPRD may be ignited; leading to even greater overpressures and to date, whilst there has been some work on fires in enclosures, the pressure peaking phenomenon for an ignited release has yet to be studied or compared with that for an equivalent unignited release. A numerical model for ignited releases in enclosures has been developed and computational fluid dynamics has then been used to examine the pressure dynamics of an ignited hydrogen release in a real scale garage. The scenario considered involves a high mass flow rate release from an onboard hydrogen storage tank at 700 bar, through a 3.34 mm diameter orifice, representing the TPRD in a small garage with a single vent equivalent in area to small window. It is shown that whilst this vent size, garage volume, and TPRD configuration may be considered “safe” from overpressures in the event of an unignited release, the overpressure resulting from an ignited release is two orders of magnitude greater and would destroy the structure. Whilst further investigation is needed, the results clearly indicate the presence of a highly dangerous situation which should be accounted for in regulations, codes and standards. The hazard relates to the volume of hydrogen released in a given timeframe, thus the application of this work extends beyond TPRDs and is relevant where there is a rapid, ignited release of hydrogen in an enclosure with limited ventilation.