Combustion modeling of ethylene /oxygen and allene /oxygen mixtures: Chemical oxidation mechanisms and numerical solution of the non-adiabatic flame equations
The objective of this work is to improve the understanding of combustion chemistry by modeling the chemical and physical characteristics of ethylene and allene reacting mixtures in a wide variety of combustion systems. This is of critical importance for environmental and economic reasons today: formation of air toxics whose emissions are regulated by the U.S. Clean Air Act Amendments, optimization of engines output, and reformulation of fuels, for example.
First, a reaction set containing 731 reactions of 85 species was assembled from the literature and theoretical calculations. The mechanism focuses on the C2 and C3 hydrocarbons reactions but it also includes heavier species up to C6 hydrocarbons such as benzene and phenyl. New rate constants for the reactions of the key system C2H 3+O2 were calculated theoretically and rate constants for the C3H3 recombination reaction were inferred from the recent literature.
This reaction set was then used in simulations of shock-tubes, flow reactors and especially flame experiments in various conditions. The predictions obtained throughout the study are generally in good agreement with the experimental data and establishes the overall high-quality of the reaction set. However, in the case of a fuel-lean (&phis; = 0.70) ethylene flame, the predicted profiles of key radicals are not within the experimental uncertainty. The modeling of important reactions involving radicals such as H+O2 in this environment may still be inaccurate.
A novel approach to flat-flame modeling is also introduced. External heat losses by radiation are included in the energy conservation equation for the first time. This improvement of the model allows the temperature profile to be computed in the case of real, non-adiabatic flames. The analysis of the predictions shows that temperature is greatly affected by the presence of soot in fuel-rich flames. In fuel-lean flames, insufficient heat is generated by the mechanism at low temperature. Finally, net heat flux analysis is used to demonstrate the very good agreement of the predictions with data of the fuel-rich flame and quantify the discrepancy in the case of the fuel-lean flame.