H₂/Air Autoignition Dynamics around the Third Explosion Limit

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.