Explosion characteristics of hydrogen-air mixtures in a spherical vessel

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Hydrogen has been proposed as a potential fuel to replace fossil fuels and to reduce carbon emissions. This paper presents experimental data on the explosion characteristics of hydrogen-air mixtures in a 20-L sphere. This includes the maximum explosion pressure, deflagration index, the exponent parameter of the burning velocity, and the burning parameter. Methods and equations are provided to estimate these parameters. The experimental maximum explosion pressure agrees with the theoretical value estimated using a chemical equilibrium program if the concentration of hydrogen is from 10 to 75% in air, but not close to the flammable limits. Therefore, the maximum pressure can be estimated conservatively by the equilibrium program regardless of the size of the explosion vessel. The deflagration index for hydrogen-air mixtures, even if normalized by the cube root of the volume of explosion vessel, is found to be sensitive to the vessel volume. The maximum deflagration index in our 20-L explosion vessel is measured as 970 bar-m/sec at 36 vol. % of hydrogen in air. Other investigators have shown that it increases with the volume of the explosion vessel. We propose a theoretical model to estimate the deflagration index. This model shows that the experimentally determined value depends highly on the fraction of burnt gas just before the flame contacts the vessel wall. This model can be used to estimate a maximum deflagration index assuming the flame propagates to and arrives at the vessel wall at the same time without any previous quenching. The deflagration index estimated using this approach has a value as high as 1,700 bar-m/sec. The apparent burning velocity increases as the pressure increases during the explosion. The exponent parameter is a constant to correlate the variation of the apparent burning velocity with the change in pressure during the explosion, which we determined to be about 0.45 for hydrogen-air mixtures. A burning parameter was also calculated from our experimental data. The burning parameter is directly related to the laminar burning velocity of the gas mixture and the wrinkling of the flame front. It is a single kinetic parameter that can be used with a flame growth model to calculate the pressure-time history from initial ignition up to the maximum explosion pressure. An empirical equation for the burning parameter for hydrogen-air mixtures is developed from this work. The maximum value of the burning parameter occurs at about 40 vol. % of hydrogen in air, which is above the stoichiometric concentration of 29.6 vol. %. The proposed equations and methods for estimating the maximum explosion pressure, burning parameter, and deflagration index will be useful for safely using hydrogen to prevent catastrophic accidents in the application of hydrogen gas. © 2009 American Institute of Chemical Engineers.

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Process Safety Progress