Dispersion Tests on Concentration and its Fluctuations for 40MPa Pressurized Hydrogen

Hydrogen is one of the important alternative fuels for future transportation. At the present stage, research into hydrogen safety and designing risk mitigation measures are significant task. For compact storage of hydrogen in fuel cell vehicles, storage of hydrogen under high pressure, up to 40 MPa, at refuelling stations is planned, and safety in handling such high-pressure hydrogen is essential. This paper describes our experimental investigation into dispersion of high-pressure hydrogen gas which leaks through pinholes in the piping to the atmosphere. First, in order to comprehend the basic behaviour of the steady dispersion of high-pressure hydrogen gas from the pinholes, the time-averaged concentrations were measured. In our experiments, initial release pressures of hydrogen gas were set at 20 MPa or 40 MPa, and release diameters were in the range from 0.25 mm to 2 mm. The experimental results show that the hydrogen concentration along the axis of the dispersion plume can be expressed as a simple formula which is a function of the downwind distance X and the equivalent release diameter. This formula enables us to easily estimate the axial concentration (maximum concentration) at each downstream distance. However, in order for the safety of flammable gas dispersion to be analyzed, comparisons between time-averaged concentrations evaluated as above and lower flammable limit are insufficient. This is because even if time-averaged concentration is lower than the flammability limit, instantaneous concentrations fluctuate, and a higher instantaneous concentration occasionally appears due to turbulence. Therefore, the time-averaged concentration value which can be used as a threshold for assessing safety must be determined considering concentration fluctuations. Once the threshold value is determined, the safe distance from the leakage point can be evaluated by the above-mentioned simple formula. To clarify the phenomenon of concentration fluctuations, instantaneous concentrations were measured with the fast-response flame ionization detector. A small amount of methane gas was mixed into the hydrogen as a tracer gas for this measurement. The relationship between the time-mean concentration and the occurrence probability of flammable concentration was analyzed. Under the same conditions, spark-ignition experiments were also conducted, and the relationship between the occurrence probability of flammable concentration and actual ignition probabilities were also investigated. The experimental results show that there is a clear correlation between the time-mean concentration, the occurrence probability of flammable concentration, flame length and occurrence probability of hydrogen flame.