Extinction and autoignition of n-heptane in counterflow configuration

A study was performed to elucidate the mechanisms of extinction and autoignition of n-heptane in strained laminar flows under non-premixed conditions. A previously developed detailed mechanism made up of 2540 reversible elementary reactions among 556 species was the starting point for the study. The detailed mechanism was previously used to calculate ignition delay times in homogeneous reactors, and concentration histories of a number of species in plug-flow and jet-stirred reactors. An intermediate mechanism made up of 1282 reversible elementary reactions among 282 species and a short mechanism made up of 770 reversible elementary reactions among 159 species were assembled from this detailed mechanism. Ignition delay times in an isochoric homogeneous reactor calculated using the intermediate and the short mechanism were found to agree well with those calculated using the detailed mechanism. The intermediate and the short mechanism were used to calculate extinction and autoignition of n-heptane in strained laminar flows. Steady laminar flow of two counterflowing streams toward a stagnation plane was considered. One stream, made up of prevaporized n-heptane and nitrogen, was injected from the fuel boundary, and the other stream, made up of air and nitrogen, was injected from the oxidizer boundary. Critical conditions of extinction and autoignition given by the strain rate, temperature, and concentrations of the reactants at the boundaries were calculated. The results were found to agree well with experiments. Sensitivity analysis was carried out to evaluate the influence of various elementary reactions on autoignition. At all values of the strain rate investigated here, high-temperature chemical processes were found to control autoignition. In general, the influence of low-temperature chemistry was found to increase with decreasing strain. A key finding of the present study is that strain has more influence on low-temperature chemistry than the temperature of the reactants.