Ignition in compositionally and thermally stratified n-heptane/air mixtures: A direct numerical simulation study

Abstract

This paper presents a direct numerical simulation (DNS) study of ignition characteristics in compositionally and thermally stratified n-heptane/air mixtures, relevant to homogeneous charge compression ignition (HCCI) engines. Nine two-dimensional constant volume cases are simulated. For each of three different mean temperatures, all within the negative temperature coefficient (NTC) region, three cases having different levels of mixture stratification are considered. Local temperature fluctuations, negatively correlated with mixture fraction fluctuations are also considered. It is shown that as the level of stratification increases, the heat release rate has a lower peak in the first stage for the mean temperature of 850K. An opposite behaviour is observed for the mean temperature of 950K. Inspection of the homogeneous ignition delay suggests that the distribution of ignition delay time at the initial conditions leads to the observed behaviour.In the second stage of ignition, stratification reduces the ignition delay time and the peak heat release rate relative to a homogeneous case for all three mean temperatures. However, a non-monotonic behaviour for the peak heat release rate in the second stage is observed for the mean temperatures of 850 and 950 K. This behaviour for cases with the mean temperature of 950K can be attributed to the mixture-fraction dependence of ignition delay times, which shows a local minimum at mixtures which are richer than the mean mixture fraction. Due to this effect, increased levels of stratification results in the introduction of a larger range of mixture fractions having similar ignition delay times. However, for cases with the mean temperature of 850K, the distribution of homogeneous ignition delay time does not explain the non-monotonic behaviour. For these cases, the analysis suggests that molecular mixing plays an important role in the steepening of the heat release rate at higher stratification levels.