Mechanism of detonation development in hydrogen (H₂)/methane (CH₄) - air mixtures in the presence of non-thermal reactivity

The binary fuel blend of H₂/CH₄ is one of the most favored hydrogen-enriched hydrocarbon fuels in spark-ignition (SI) engines. Yet, the undesirable phenomenon of “superknock”, which can severely and instantaneously damage an SI engine, limits its widespread adoption. Moreover, there is still a lack of consensus on the precise mechanism by which this phenomenon occurs i.e. via flame acceleration or spontaneous ignition, despite numerous previous investigations. Meanwhile, recent chemistry studies have demonstrated a high probability of occurrence of non-thermal reactions in practical flames due to the presence of non-trivial concentrations of reactive radicals including H, O and OH in addition to O₂. In this study, the mechanism of detonation formation in different blends of H₂/CH₄ in air under SI engine conditions was examined through fully resolved, constant volume 1D simulations with and without non-thermal reactivity. Non-thermal reactions were included in the macroscopic kinetic model as chemically termolecular reactions facilitated by radical-radical recombination and radical-molecule association reactions. Sensitivity analysis was performed to quantify the effects of non-thermal reactions on the duration of heat release and thereby the mechanism of detonation formation.