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

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 950 K 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 850 K, 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.