A Flow Visualization Study on Self-ignition of High Pressure Hydrogen Gas Released into a Tube

Abstract

Hydrogen is known as one of the green energy sources for fuel cells and hydrogen-fueled cars in the next generation. The flammable range of hydrogen is the widest fuel than any other fuel source, so handling of hydrogen to preventing the leakage is very important for ground based systems. Even though liquid hydrogen has a higher energy contents per volume, there are complex gasifying devices to use as a fuel for hydrogen-fueled systems. If the hydrogen gas can be carried safely with high pressure up to 700 atm., this can be comparable usage to the liquid hydrogen. Therefore, 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. A few experimental study using flow visualization inside a tube have been reported to investigate a mechanism of the self-ignition in the tube. This thesis 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 at the boundary layer has a certain critical threshold of static pressure at the boundary layer, based on various burst pressures of hydrogen. The effect of the rupture diaphragm holding diameter and inlet section shape of the rupture frame was also investigated. The opening time of diaphragm and energy density per rupture diaphragm deformed area were also major parameter for the self-ignition boundary based on thesis experiments.