Surface Density Function and Its Evolution in Homogeneous and Inhomogeneous Mixture n-Heptane MILD Combustion

Moderate or intense low-oxygen dilution (MILD) combustion is a combustion technique with potential to simultaneously improve thermal efficiency and reduce emissions. This paper focuses on the mean behavior of the reactive scalar gradient characterized by the surface density function (SDF = magnitude of the reaction progress variable gradient) and its evolution for exhaust gas recirculation (EGR) type, homogeneous and inhomogeneous mixture n-heptane combustion under MILD conditions using Direct Numerical Simulations (DNS) with reduced chemical mechanism. Two oxygen concentrations have been considered here for the homogeneous mixture case, namely, 3.0% and 4.5% by volume. The characteristics of the SDF and displacement speed were also analyzed in the reaction and propagating-flame-dominated regions of the domain to illustrate the effect of combustion mode on the mean variations of these quantities. The mean values of the SDF in turbulent MILD combustion cases are found to be smaller compared to the corresponding laminar flame cases. Moreover, the effect of mixture inhomogeneity on the SDF variation is found to be marginal for the parameters considered here. It is found that increasing the dilution factor reduces the percentage of flame thickening with respect to the corresponding laminar flame thickness under identical turbulence conditions. The effects of dilatation rate are found to be weak in the studied cases due to the expected low heat release under MILD conditions which leads to weak thermal expansion effects. The effective flame normal and tangential strain rates are found to be dominated by the additional flame normal and tangential strain rates due to flame propagation, respectively. The effective flame normal strain rate has positive values across the flame which promotes flame thickening and could explain the decrease in the SDF values, while the effective flame tangential strain rate is negative throughout the flame and indicates a fractional reduction in flame area that could be attributed to flame surface interactions. The reaction-dominated regions give rise to reduced level of the SDF in the homogeneous mixture cases and increased level of Sd in all cases compared to those calculated in the propagating-flame region.