Limits of self-ignition in the process of hydrogen-methane mixtures release under high pressure into unconfined space

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The promising way for the storage of gaseous combustible mixtures based on hydrogen is the use of high-pressure vessels. However, as a result of accidental release of such mixtures under high pressure into atmosphere the self-ignition could arise. In the paper a series of numerical simulations of hydrogen/methane mixtures release from the high-pressure vessel into the open space is carried out in order to investigate safe conditions for storage and pumping of such mixtures. The storage pressure is varied in the range from 350 to 700 atm, that correspond to the pressure range promising for hydrogen storage. Main attention in the paper is paid to the variation of methane content in its mixture with hydrogen. It is shown that under any pressures considered there is a limit of methane content in its mixture with hydrogen providing the absence of self-ignition of combustible hydrogen-methane mixture. Herewith, the certain value of critical methane content depends on geometry of hydrogen/methane source (hole, slit, its size). In particular, results of the study demonstrate that in the case of small orifice self-ignition limits are wider when the combustible mixture is released through the slit than through the round hole. At the same time, when the large orifice is used this difference levels off. It is shown that the intensity of gas-dynamic processes decreases with the increase in the methane content, and thereby there is a certain decrease in the temperature of shock-heated air and self-ignition probability.