Numerical investigation of the ignition and annihilation of CH4/N2/O2 mixtures under MILD operative conditions with detailed chemistry

Abstract

The identification of controlling processes on the micro-scale level is critical in the elaboration of effective models with particular regard to MILD (moderate or intense low-oxygen dilution) combustion modelling. The objective of this study is to determine relevant features and controlling mechanisms in the ignition, interference and annihilation of diffusion layers in MILD combustion conditions. The two interfaces between the mixing layers are sufficiently close to cause interference phenomena. The inner region between the two interfaces is assumed to be air to mimic the engulfed air in a fuel jet. The temporal evolution has been analysed in dependence on the initial oxidant layer width and temperature; evidencing possible interactions between the two mixing layers. Interactions can be categorised according to the ignition and annihilation stages. Concerning the ignition, the minimum ignition delay for interfering double diffusive layers is significantly shorter than the corresponding isolated one. However, the ignition exhibits the same most reactive mixture fraction, and the ignition delay dependence on the oxidant layer width behaves similar to a stratified charge condition for the same most reactive mixture fraction. In the ignition region, the scalar dissipation rate is reduced due to expansion toward fuel sides due to heat release. The annihilation time delay scales with the initial layer thickness according to the canonical diffusion equation and does not significantly depend on the initial oxidant temperature. Oxygen depletion during annihilation limits oxidation in the rich side mixture, which produces a final oxidation level near equilibrium values.