(111c) Explosion Characteristics of Hydrogen-Air Mixtures in a Confined Space

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, and 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 shown to be sensitive to the vessel volume. The fraction of burnt gas just before the flame contacts the wall has a dominant effect on the deflagration index. The maximum deflagration index in a 20 l explosion vessel is observed as 970 bar-m/sec at 36 vol. % of hydrogen in air. It increases with the volume of the explosion vessel and may reach theoretically as high as 1,700 bar-m/sec. We propose a theoretical model to estimate the deflagration index.

A burning parameter was also calculated from the 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 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.