Dynamically evaluating mixture effects on multi-channel reactions in flames: A case study for the CH3 + OH reaction

Abstract

Complex-forming reactions, whose rate constants depend on pressure and collisional energy transfer characteristics of the surrounding bath gas, play a major role in the kinetics of combustion. In most realistic combustion environments, multiple species of distinct collisional energy transfer characteristics are present in significant quantities and thus contribute to collisional energy transfer involved in such systems. Recent studies have indicated that certain representations of multi-component pressure dependence (i.e. “mixture rules”) and/or a failure to implement a mixture rule can result in errors reaching an order of magnitude, whereas recently proposed mixture rules yield errors less than 10%. The present study compares the performance of various mixtures rules for representing multi-component pressure dependence of the multi-channel CH3 + OH reaction in flames, using a novel dynamic procedure for evaluating mixture effects as a function of reaction progress (viz. local temperature, pressure, and mixture composition). This procedure enables mixture effects to be simulated in current combustion codes despite codes not yet having functional forms intended to capture these mixture dependence effects. Results from this procedure, combining master equation simulations and kinetic-transport simulations, indicate that recently proposed mixture rules based on the reduced pressure provide a considerably more accurate representation of mixture effects for CH3 + OH than previous mixture rules based on the absolute pressure. Furthermore, the present results demonstrate that mixture effects for the CH3 + OH reaction, which are not accounted for in many models, have a significant effect on predictions of the laminar flame speed – of comparable magnitude to differences motivating parameter adjustments in model development studies.