Effect of turbulent mixing on the end gas auto-ignition of n-heptane/air mixtures under IC engine-relevant conditions

Abstract

The effect of turbulence on the end-gas auto-ignition (AI) of stoichiometric n-heptane/air mixtures under engine relevant conditions is numerically studied through a combined methodology of Large Eddy Simulation (LES) and one-dimensional stand-alone Linear Eddy Model (LEM). Similar end-gas auto-ignition process in super rapid compression machine experiments is first been qualitatively reproduced through LES. To further investigate the effect of small-scale turbulence-chemistry interaction on the end-gas auto-ignition in the near-wall region which consists of a core region and a boundary layer, 1-D LEM simulations are performed by extracting the thermal-chemical and turbulent parameters at top dead center from LES. The parametric study covers a range of representative fluctuation velocity values under engine-like conditions at two initial temperatures of 700K and 900K, which are below and within the Negative Temperature Coefficient regime respectively. It is generally found that, increased turbulence intensity delays AI formation and also reduces the kernel size at the onset of AI. At the initial temperature of 700K, intense turbulence can change the combustion mode of the end-gas following the initial spontaneous auto-ignition in the core region. Diffusive - reactive flame structures are observed and the kernel expansion is governed by turbulent flame propagation. At a high initial temperature of 900K, the formation of “cold” AI kernels is inhibited with increased turbulence intensity. A budget analysis reveals that the expansion processes of nascent kernels mainly are spontaneous ignitions and turbulence may enhance the local scalar mixing. However, turbulence won't change the nature of auto-ignition propagation wave driven dominantly by exothermicity during the end-gas combustion process.