Leakage process and spontaneous ignition of hydrogen within a tube after releasing from the storage container with pressures up to 20 MPa

Albeit pressurized storage of gaseous hydrogen is the most common approach to hydrogen storage in the current industry, there is a risk of hydrogen leakage in current metal pipelines, following the potential of hydrogen spontaneous ignition during leakage. Under different burst pressures (from 4 MPa to 20 MPa, corresponding to the actual engineering pressure range in China's current high/ultra-high-pressure pipelines), the present work investigates the leakage process of hydrogen within a tube by validated models. Hydrogen has been observed to spontaneously ignite when the burst pressure is no less than 4 MPa. The time to spontaneous ignition exponentially declines as burst pressure rises, but it can hardly be reduced to less than 10 mu s since a period is required to prepare ignition conditions. Three modes of spontaneous ignition have been classified according to the locations at which initial flame kernels spontaneously appear. Mode I (burst pressure of fewer than 8 MPa) triggers spontaneous ignition near the tube's wall, Mode III (burst pressure of more than 8 MPa) generates initial flame kernels at the tube's central axis, while Mode II (burst pressure of 8 MPa) obtains the flame kernels at both locations. At lower burst pressures, the shockwave intensity alone cannot raise the hydrogen-air mixture's temperature to the ignition temperature; the help of boundary layer effects is essential to spontaneous ignition. At higher burst pressures, the shockwave intensity is dominant in raising the hydrogen-air mixture's temperature to reach the ignition condition. Furthermore, tulip flames expand rapidly under high-pressure conditions and form stable structures, indicating pressurized hydrogen exhibits a greater propensity for generating intense flames.