Influence of turbulence on autoignition in stratified mixtures under compression ignition engine conditions

Abstract

The influence of turbulence on the autoignition characteristics of initially stratified mixing layers is studied using numerical simulations and multi-step kinetic mechanisms. The computations are carried out at a high initial pressure and a high initial temperature representative of conditions in compression ignition engines. n-Heptane is used as the surrogate fuel. It is shown that the influence of turbulence is to reduce the time for the first occurrence of ignition in the mixing layer in all cases. The initial gradient is modified by the turbulence, resulting in higher and lower gradients than the initial value. The lower gradients favor ignition, as pointed out in the literature. During a two-stage ignition process, the influence is primarily reflected in the second stage of ignition. The separate influences of the velocity scale and the length scale on autoignition are explored. An increase in the velocity and/or a decrease in the length scale increase(s) the spread of gradients in the domain and accelerate(s) ignition.