Extinction and Autoignition of n-Heptane in Counterflow Configuration

A study is performed to elucidate the mechanisms of extinction and autoignition of n-heptane in strained laminar flows under nonpremixed conditions. A previously developed detailed mechanism made UP of 2540 reversible elementary reactions among 557 species is 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 160 species are assembled from this detailed mechanism. Ignition delay times in an isochoric homogeneous reactor calculated using the intermediate and the short mechanism are found to agree well with those calculated using the detailed mechanism. The intermediate and the short mechanism are used to calculate extinction and autoignition of n-heptane in strained laminar flows. Steady laminar flow of two counter flowing Streams toward a stagnation plane is considered. One stream made up of prevaporized n-heptane and nitrogen is injected from the fuel boundary and the other stream made up of air and nitrogen is 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, are calculated. The results are found to agree well with experiments. Sensitivity analysis is 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 are found to control autoignition. In general, the influence of low temperature chemistry is 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.