Effect of opening area on the suppression of self-ignition of high-pressure hydrogen gas leaking in the air by an extension tube.

The most influential factor for self-ignition of high-pressure hydrogen is known to be the strength of the shock. Thus, the self-ignition can be suppressed by weakening the shock strength, which is possible by reducing the area where the hydrogen is ejected in this study. To confirm the possibility of this method, experiments were done by controlling the burst pressure of up to 302 bar and the ratio of the opening area. The experimental results showed that the minimum burst pressure of self-ignition is increased exponentially as the opening area is reduced. This confirmed that reducing the opening area under the same burst pressure conditions has an effect on the suppression of self-ignition. However, it was also found that the minimum shock speed that causes self-ignition gradually decreases as the opening area becomes smaller, which results from an increasing in mixing. The CFD simulation results showed that the volume of the flammable region in the tube was increased and the hydrogen-air mixing efficiency also increased when the opening area became smaller. The results suggest that reduction of the opening area can suppress a self-ignition by weakening the shock strength, but it should be noted that an increase in mixing effect also occurs. Image 1 • The self-ignition of hydrogen can be effectively suppressed by reducing the opening area. • The burst pressure for self-ignition increases exponentially with the reduction of opening area. • The small opening area ratio results in weaker shock strength, but stronger air/hydrogen mixing. • Strong mixing was observed to increase the possibility of self-ignition even with weak shock wave. [ABSTRACT FROM AUTHOR]