Rate-ratio asymptotic analysis of autoignition of n-heptane in laminar nonpremixed flows

摘要

A rate-ratio asymptotic analysis is carried out to elucidate the mechanisms of autoignition of n-heptane (C_7H_(16)) in laminar, nonpremixed flows. It has been previously established that autoignition of n -heptane takes place in three distinct regimes. These regimes are called the low-temperature regime, the intermediate-temperature regime, and the high-temperature regime. The present analysis considers the high-temperature regime. A reduced chemical-kinetic mechanism made up of two global steps is used in the analysis. The reduced mechanism is deduced from a skeletal mechanism made up of 16 elementary reactions. The skeletal mechanism is derived from a short mechanism made up of 30 elementary reactions. The short mechanism is deduced from a detailed mechanism made up of 56 elementary reactions. In the reduced mechanism, the first global step represents a sequence of fast reactions starting from the rate-limiting elementary reaction between n-heptane and HO_2. In this global step C_7H_(16) is consumed and hydrogen peroxide (H_2O_2) is formed. The second global step represents a sequence of fast reactions starting from the rate-limiting elementary reaction in which H_2O_2 is consumed and OH is formed. A key aspect of the second global step is that the sequence of fast reactions gives rise to consumption of fuel only without net consumption of H_2O_2. This makes the chemical system autocatalytic. The unsteady flamelet equations are used to predict the onset of autoignition. In the flamelet equations a conserved scalar quantity, Z, is used as the independent variable. On the oxidizer side of the mixing layer Z = 0, and on the fuel side Z = 1. The practical case where the temperature of the oxidizer stream, T_2, is much greater than the temperature of the fuel stream is considered. Therefore autoignition is presumed to take place close to Z = 0. Balance equations are written for C_7H_(16) and H_2O_2. It is postulated that autoignition will take place when the gradient of mass fraction fuel with respect to Z, evaluated at Z = 0, is zero. The value of T_2 when autoignition takes place is obtained as a function of the strain rate. These critical conditions of autoignition obtained from asymptotic analysis agree well with those calculated using the detailed mechanism and the skeletal mechanism.