A flow visualization study on self-ignition of high pressure hydrogen gas released into a tube

Hydrogen is known as one of the green energy sources for fuel cells and hydrogen-fueled cars in the next generation. The storage of high-pressure hydrogen gas conditions is preferred to its storage in cryogenic liquid state. However, cases of unidentified self-ignitions were reported, notably when the high-pressure hydrogen gas suddenly leaked out. Only a few of numerical simulations have shown visually the processes of the self-ignition inside a tube. This paper presents a flow visualization study to investigate the self-ignition mechanism in a test tube i.e. how the ignition process is initiated and the flame propagates. In addition to visualization, measurement of a number of pressure and light sensors installed in the tube supported the analysis of the self-ignition and flame propagation. The test result showed that self-ignition takes place at the boundary layer behind the front center of mixing zone at first, and the flame propagates to the front of mixing zone and tail of the mixing zone along the boundary layer. It showed that self-ignition is accompanied with complex mixing induced by shock interaction with the mixing front. It is also suggested that the self-ignition boundary has a certain critical threshold of static pressure at the boundary layer, based on various burst pressures of hydrogen.