H2/Air Autoignition Dynamics around the Third Explosion Limit

Abstract

This paper examines the influence of wall reactions on the generation of the explosive time scale that characterizes ignition delay around the third explosion limit of a stoichiometric H2/air homogeneous mixture. The only wall reactions exhibiting a sizeable influence are HO2→HO2(w) and H2O2→H2O2(w)—in both cases opposing the ignition process. The opposing influence of the former wall reaction complements that of 2HO2→H2O2+O2 in opposing H2O2+H←H2+HO2, which promotes ignition. However, the combined influence of these three reactions is not practically affected when the third explosion limit is crossed by increasing the initial pressure for a given initial temperature. The latter wall reaction opposes 2OH(+M)←H2O2(+M), which also promotes ignition. The combined influence of these reactions increases substantially as the third explosion limit is crossed, leading to significantly lower ignition delays. It is shown that around the third explosion limit the temperature has a strong influence on the explosive mode that leads to ignition. This influence is stronger when the wall reactions are accounted for.