Evaluating mixture rules and combustion implications for multi-component pressure dependence of allyl + HO2 reactions

Abstract

Many of the reactions important to combustion proceed through rovibrationally excited complexes, whose evolution depends on energy-transferring collisions with molecules in the surrounding mixture—yielding a strong dependence of rate constants on pressure and collisional energy transfer characteristics of the mixture. In nearly all realistic combustion environments, a substantial fraction of multiple species of distinct collisional energy transfer characteristics are present—thus requiring a “mixture rule” to interpolate kinetic data from individual single-component bath gases. While mixture effects for reactions proceeding through a single potential well to a single channel have been extensively investigated, mixture effects and mixture rules for multi-well and/or multi-channel reactions are significantly less characterized. The present study presents an investigation of the mixture effects, suitability of mixture rules for representing them, and their impacts on combustion predictions for the allyl + HO2 reaction system. The performance of different mixture rules for representing multi-component pressure dependence of rate constants for allyl + HO2 was evaluated through comparisons against master equation calculations for the mixture. The comparisons revealed that the classic linear mixture rule (LMR,P) yields deviations reaching a factor of two for typical combustion mixtures, whereas newly proposed mixture rules based on reduced pressure exhibit lower deviations of generally less ∼ 3%. Simulations of shock tube ignition delay times and jet-stirred reactor species profiles for C3H6 combustion suggest that mixture effects for allyl + HO2 on combustion predictions can be significant and may serve as a plausible explanation of the rate parameter adjustments employed in modeling studies to reproduce combustion measurements. Newly proposed mixture rules based on the reduced pressure are shown to accurately capture these mixture effects within combustion simulations.