Flame kinetics at scramjet-engine-relevant conditions: Role of prompt dissociation of weakly-bound radicals

Abstract

Combustion in high-speed ram-based propulsion engines occurs under distinct thermodynamic conditions of high reactant temperatures (greater than 1000 K) and relatively low pressures (<5 atm). There is a lack of fundamental flame measurements at such conditions that result in adiabatic flame temperatures (T_ad) exceeding 2500 K. In this work, we have measured laminar flame speeds of oxygen-enriched CH₄/oxidizer mixtures at sub-atmospheric conditions to probe kinetics at high T_ad using the isobaric spherically expanding flame approach. Simulations with recent kinetic models revealed increasing differences between data and model predictions with increasing T_ad, reaching up to 25 %. Kinetic analyses reveal that at the thermodynamic conditions in these O₂-enriched flames, i.e., lower pressures and higher T_ad, the effects of HCO prompt dissociation are accentuated. In addition to HCO, the prompt dissociations of CH₂OH and C₂H₅ are also considered. The prompt dissociations of all three radicals were evaluated and their effects considered in flame speed simulations. Reaction path analysis for the present flames revealed that approximately half of the reaction flux for HCO formation undergoes prompt dissociation to H + CO. Furthermore, these analyses also revealed that the pathways and sensitive reactions are similar between oxygen-enriched fuel/oxidizer mixtures and preheated fuel/air mixtures, if both have similar T_ad. Thus, flames of oxygen-enriched mixtures could be a surrogate to probe the flame chemistry of highly preheated mixtures at relatively low pressures that are often encountered in ram-based propulsion engine combustors.