Assessment of kinetic modeling for lean H₂/CH₄/O ₂/diluent flames at high pressures

Experimental measurements of burning rates, analysis of key reactions and kinetic pathways, and modeling studies were performed for H₂/CH ₄/O₂/diluent flames spanning a wide range of fuel-lean conditions: equivalence ratios from 0.30 to 1.0, flame temperatures from 1400 to 1800 K, pressures from 1 to 25 atm, CH₄ fuel fractions from 0 to 0.1. The experimental data show negative pressure dependence of burning rate at high-pressure, low-flame-temperature conditions for all equivalence ratios and with CH₄ addition. Substantial differences are observed between literature model predictions and the experimental data as well as among model predictions themselves-up to a factor of four at high pressures. Similar to our previous work that demonstrated that none of the recent kinetic models reproduced the measured pressure dependence of the mass burning rate for all diluent concentrations and medium to high equivalence ratios, here it is demonstrated that none reproduce the measured pressure dependence for very low equivalence ratios. The effect of pressure on the kinetics of lean flames is largely driven by competition of both H + O₂(+M) = HO₂(+M) and HO₂ + O/OH/HO₂ with the main branching reactions, in contrast to rich mixtures that are largely driven by competition of both H + O₂(+M) = HO₂(+M) and HO₂ + H with the main branching reactions. Methane addition is shown to influence the pressure dependence mainly through reactions of CH₃ with H and HO₂. Given the nature of the modeling problem for high-pressure flames, it appears that a rigorous solution to improving predictive capabilities will require both empirical adjustments of multiple rate constant parameters as well as improved characterization of the functional temperature and pressure dependence of certain highly sensitive reactions. Furthermore, many of the reactions responsible for uncertainties in the pressure dependence of H₂/O ₂ flames at high pressures are shown to contribute significantly to uncertainties in the pressure dependence of flames of hydrocarbon fuels.