Effects of equivalence ratios on the oblique detonation initiation in ammonia/hydrogen/air mixtures

This paper presents two-dimensional numerical simulations of oblique detonation waves (ODWs), employing Navier-Stokes equations coupled with detailed chemical reaction mechanisms. We explored the effects of equivalence ratio on initiation characteristics, including the transition type from oblique shock waves (OSWs) to ODWs and the induction length in pure ammonia and hydrogen-ammonia blend fuels. Results indicate that, in pure ammonia fuel, a wave structure transition from OSW₁ to OSW₂ and finally to ODW is formed. As the ammonia equivalence ratio increases, the induction length grows linearly and the transition from OSW to ODW becomes more abrupt. Hydrogen addition significantly shortens the induction length in ammonia-based oblique detonation, with low ammonia concentrations resulting in an induction length even shorter than that of pure hydrogen fuel. Chemical explosion mode analysis identifies O, H, OH, NH₂ as key species contributing to detonation process in the induction region, with ammonia playing a more significant role than hydrogen at initial stages. A predictive method for the OSW-ODW transition in hydrogen-ammonia blend fuels is proposed, offering insights into practical applications of ammonia in ODEs.