A Computational Analysis of Autoignition of H2/Air Mixture with Temperature Fluctuations Using Computational Singular Perturbation

Introduction
Ignition of fuel-air mixtures is an important issue in modern engine applications, such as homogeneous charge compression ignition (HCCI) or low-temperature combustion (LTC) engines, towards higher efficiency and lower emissions. These engines operate in compression ignition (CI) mode with early fuel injection, such that the overall combustion phasing behavior is mainly dictated by the autoignition of nearly uniform fuel-air mixture. The mixture composition and temperature distribution is not perfectly uniform, however, due to the incomplete fuel-air mixing, wall heat losses, and the presence of residual gases. Therefore, the first occurrence of autoignition is usually localized, and such ignition “kernels” [1] grow into the development of subsequent
autoignition or flame propagation in neighboring mixtures. From a Lagrangian standpoint, the evolution of chemical activities within the ignition kernel may be described as a reactive mixture pocket subjected to temporal fluctuations in temperature and composition during the induction period, where the timescales of the fluctuations depend on the level of turbulence and initial stratifications. For this reason, the response of the igniting mixture in the presence of imposed oscillations in temperature and fuel composition has been studied in the past [2,3]. For example, Bansal et al. [2] showed that a harmonic oscillation of temperature can lead to a net advancement or retardation of autoignition of the mixture depending on the frequency and phasing of the
oscillation. As an extension of these fundamental studies, the present work employs the computational singular perturbation (CSP) tools in order to provide a more detailed analysis of chemical pathways responsible for the autoignition of homogeneous mixture in the presence of temperature fluctuations.