Single-step mechanisms for the autoignition of fuel-air mixtures in constant volume batch reactors

International audience; A new methodology to build simple and irreversible Arrhenius single-step mechanisms has been developed to accurately reproduce autoignition properties of fuel-air mixtures in constant volume applications. The main idea of the method is to define virtual chemical species, designed such as the global heat release is properly recovered, bypassing the need to account for the minor species in recovering the adiabatic temperature. A classical asymptotic expression, relating the ignition delay time with the reaction rate constant, is then used to fit the Arrhenius coefficients accordingly with the computations made with the complete mechanism .The main difference with previous procedures, is that a single parameter is required to recover ignition properties for a wide range of initial conditions (temperature, pressure, and equivalence ratio). The minimum amount of species were used to match the equilibrium properties of the completed mechanism and were tabulated as a polynomial function for the initial conditions, which speed up the procedure of finding the proper correlation between the initial-final compositions. This methodology also allows recovering the original species by simply substituting the virtual species mass fraction in the post processing stage. The following procedure has been extended to develop a two-step mechanism where a new step for the fuel degradation has been added and coupled with the combustion step. This extension points out the robustness of the approximation done in this work and their flexibility to be adapted to different situations.