Towards Fire Test Protocol for Hydrogen Storage Tanks

The reproducibility of fire test protocol in the UN Global Technical Regulation on Hydrogen and Fuel Cell Vehicles (GTR#13) is not satisfactory. Results differ from laboratory to laboratory and even at the same laboratory, when fires of different heat release (HRR) rate are applied. This is of special importance for fire test of tank without thermally activated pressure relief devise (TPRD), the test requested by firemen. Previously the authors demonstrated a strong dependence of tank fire resistance rating (FRR), i.e. time from fire test initiation to moment of tank rupture, on the HRR in a fire. The HRR for complete combustion at the open is a product of heat of combustion and flow rate of a fuel, i.e. easy to control in test parameter. It correlates with heat flux to the tank from a fire – the higher HRR the higher heat flux. The control of only temperature underneath a tank in fire test, as per the current fire test protocol of UN GTR#13, without controlling HRR of fire source is a reason of poor fire test reproducibility. Indeed, a candle flame can easily provide a required by the protocol temperature in points of control, but such test arrangements could never lead to tank rupture due to fast heat dissipation from such tiny fire source, i.e. insufficient and very localised heat flux to the tank. Fire science requires knowledge of heat flux along with the temperature to characterise fire dynamics. In our study published in 2018 the HRR is suggested as an easy to control parameter to ensure the fire test reproducibility. This study demonstrates that the use of specific heat release rate, HRR/A, i.e. HRR in a fire source divided by the area of the burner projection, A, enables testing laboratories to change freely a burner size depending on a tank size without affecting fire test reproducibility. The invariance of FRR at its minimum level with increase of HRR/A above 1 MW/m2 has been discovered first numerically and then confirmed by experiments with different burners and fuels. The validation of computational fluid dynamics (CFD) model against the fire test data is presented. The numerical experiments with localised fires under a vehicle with different HRR/A are performed to understand the necessity of the localised fire test protocol. The understanding of fire test underlying physics will underpin the development of protocol providing test reproducibility.