A DNS study of ignition characteristics of a lean iso-octane/air mixture under HCCI and SACI conditions

Abstract

The effect of thermal stratification, spark-ignition, and turbulence on the ignition of a lean homogeneous iso-octane/air mixture at constant volume and high pressure is investigated by direct numerical simulations (DNS) with a new 99-species reduced kinetic mechanism developed for very lean mixtures from a detailed mechanism. Two-dimensional DNS are performed in a fixed volume with two-dimensional isotropic velocity spectrums, temperature fluctuations, and an ignition source superimposed on the initial scalar fields. The influence of variations in the initial temperature field imposed by changing the variance of temperature, the ignition-timing by changing the time at which the ignition source is superimposed, and the turbulence intensity and length scale on ignition of a lean iso-octane/air mixture is elucidated. The mean heat release rate increases more slowly and ignition delay decreases with increasing thermal stratification under homogeneous charge compression-ignition (HCCI) conditions since the present mean temperature lies far outside of the negative temperature coefficient (NTC) regime. The spark-ignition induces relatively short ignition delay under spark-assisted compression ignition (SACI) conditions while mildly spreading out the mean heat release rate. For SACI combustion, high turbulence intensity decreases the ignition delay more by increasing the turbulent flame area. Displacement speed and Damkohler number analyses reveal that the high degree of thermal stratification induces deflagration at the reaction fronts, and hence, the mean heat release rate is smoother subsequent to thermal runaway occurring at the highest temperature regions in the domain. For SACI combustion, the heat release rate occurs solely by deflagration prior to the occurrence of the maximum heat release rate and subsequently by mixed modes of deflagration and spontaneous ignition. These results suggest that thermal stratification provides smooth operation of HCCI engines and moreover, spark-ignition can precisely control the ignition timing for SACI combustion.