Subgrid scale modelling for MILD combustion

A simple closure for filtered reaction of a reaction progress variable is analysed in this study using explicitly filtered DNS data of turbulent MILD combustion of methane for Large Eddy Simulation (LES). The conditional averages of major and minor species mass fractions, and reaction rate constructed from the DNS data along with those obtained using flamelet and Perfectly Stirred Reactor (PSR) models suggest that the PSR can serve as a good canonical reactor for MILD combustion modelling. The flamelet predictions of reaction rate are observed to be poor because it does not include effects of flame interactions, which are abundant in the MILD combustion. The PSR solution obtained over a wide range of residence time along with presumed beta sub-grid PDF seems a reasonable closure for the filtered reaction rate for the LES filter size greater than three flame thermal thicknesses. Both spatial variations and joint PDF of modelled and DNS values of filtered reaction rates are analysed.