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In this study, hydrogen was leaked using a nozzle that simulated an actual leak port (with varied materials and diameters), and the possibility of ignition was verified to collect data useful for establishing standards for the allowable flow rate of hydrogen leakage on receptacle. With the flow rate of a hydrogen leak set at 250 mL/h(NTP) (hereinafter mL/h is NTP condition) or less, ignition of leaked hydrogen with an electric spark and a small methane-fueled flame was attempted. The results confirmed that ignition of 200 mL/h of hydrogen was not achieved under tested conditions. In some cases, hydrogen at a flow rate of 250 mL/h was ignited. Tissue paper placed in contact with the flame at a flow rate of 250 mL/h combusted, resulting the flame went out almost immediately. Therefore, it was determined that a hydrogen leak at approximately 200 mL/h that occurred in this test is a very low possibility of ignition or spreading.

Gas behavior during fast filling of a compressed gaseous hydrogen storage tank (Type 3, 35MPa) was simulated numerically to investigate in detail the resulting unsteady temperature distribution and its correlation with the storage tank conditions. The governing equations for the gas phase are the mass, momentum, and energy equations; these equations were discretized using the finite volume method (FVM) in three-dimensional space. The numerical results were carefully compared with the experiment results and have been validated. Consequently, the temperature distributions in space, the time histories of temperature at the measured points, and the filling time to the target pressure were all in good agreement. Furthermore, the unsteady gas and its thermal behavior were clearly visualized in three-dimensional space.

The environmental temperature where vehicles are used varies significantly by region and season, so this study investigated the influence of environmental temperature on the fatigue life of compressed hydrogen tanks for vehicles (Type 3). Pressure-cycle tests with varying environmental temperature were conducted on tanks until the tank was failed. Results indicated that fatigue life decreased in low-temperature environments and improved in high-temperature environments. We investigated the cause of such results using the strains inside and outside the tank and other indexes and found that the effect of autofrettage varied as environmental temperature changed, due to the difference between the thermal expansion rate of CFRP and that of aluminum alloy. Thus, we concluded that fatigue life changed according to changes in environmental temperature.

The high-pressure hydrogen gas cylinder of a fuel-cell vehicle is equipped with a pressure relief device (PRD) to prevent the rupture of the cylinder due to heating by fire. Flame exposure tests (bonfire tests) are conducted to evaluate the safety of the cylinder with the PRD, specifically, cylinder resistance to fire and performance of the PRD. In this study, however, fire tests of vehicles equipped with high-pressure cylinders were not required for this test method. We implemented released-hydrogen flame tests by performing bonfire tests and fire tests on vehicles equipped with hydrogen-filled high-pressure gas cylinders (20,35MPa) to examine safety measures for fuel-cell vehicles. We then investigated the following: the characteristics of the released-hydrogen flame, radiation heat flux from the jet flame, combustion noise, the rate of pressure rise in the cylinder, the venting direction of the PRD, and behavior of fire in conjunction with a gasoline flame.

To determine the appropriateness of specifying the allowable amount of hydrogen leakage upon collision based on the amount of leakage with generated heat equivalent to that of gasoline vehicles and CNG vehicles, we investigated the safety of each type of fuel when flame ignites. Our results confirm that the flame lengths for hydrogen and methane are almost equal, and there is no remarkable difference between them in terms of the distance for assuring safety. Furthermore, we confirmed that the irradiant heat flux from the mixed burning of hydrogen flame with liquid flammable materials is almost equal to that of the spray flame of gasoline. Thus, no clear difference was found between various types of fuel. Therefore, it is appropriate to specify the allowable amount of hydrogen leakage based on the amount of leakage with generated heat equivalent to that of other types of fuel.

To investigate methods of conducting flame exposure tests (bonfire tests) on high-pressure hydrogen gas cylinders that are safe and have high accuracy across repeated tests, we used numerical simulation and experiments to analyze the feasibility of using substitutive gases for filling as well as the effects of the burners used as the fire source. Through a series of virtual experiments using substitutive gases, flame scales, and filling pressure as parameters, we examined the maximum internal pressure, the rate of pressure rise, and the starting time of Pressure Relief Device (PRD) activation. Because substitutive gas properties differ from those of hydrogen gas, we concluded that using substitutive gases would be inappropriate. In addition, we observed that when the flame scale was small, the cylinder's internal pressure before the thermal-activated PRD activation, the rate of pressure rise, and the starting time of PRD activation all increased rapidly.