Similitude analysis and critical conditions for spontaneous ignition of hydrogen release into the atmosphere through a tube

Hydrogen is an emerging energy carrier for green hydrogen and fuel cell vehicles. The compressed gaseous hydrogen is selected by car manufacturers as a preferred option for onboard storage. One possible accident scenario is release of high-pressure hydrogen to the atmosphere through piping and tubes. The spontaneous ignition of accidently released hydrogen can be accompanied by pressure and thermal effects on humans and facilities. However, despite several previous studies, the critical conditions for spontaneous ignition to take place are not yet defined for an arbitrary case. Most of earlier studies present only a qualitative interdependence between the critical burst pressure for ignition and the ignition influencing factors, i.e. tube length and diameter, burst disk opening time, etc. The quantitative correlation is not yet available to underpin inherently safer engineering design of hydrogen systems and infrastructure. This paper focuses on quantitative analysis of the phenomenon of spontaneous ignition during high-pressure hydrogen release through a tube filled with air into the atmosphere. Similitude analysis of the problem is carried out. It is found that the ratio of shock pressure inside the tube to the atmospheric pressure, P_s/P_a, is a function of the dimensionless parameter, which is the product of the initial storage to the atmospheric pressure ratio, P_b/P_a, and the ratio of the characteristic shock propagation time to the burst disk opening time, (D/u_s)/t. Pressure transducers and light sensors are used in the experiments to record the shock propagation and ignition time and location. The correlation to quantitatively define the critical conditions for spontaneous ignition of released hydrogen inside a tube with air is derived for the first time using the similitude analysis. The correlation is calibrated against experiments carried out in this study and by other authors, where four identical triangular petals are formed upon rupture of inertial burst disk made of metal.