Assessment of disparities in estimating filtered chemical reaction rates in LES using DNS of turbulent premixed flames

Abstract

An a priori assessment of the disparities in estimating the filtered chemical reaction rates in Large-Eddy Simulations (LES) is performed using two Direct Numerical Simulations (DNS) of turbulent premixed flames. DNS of turbulent premixed flames with the same turbulent Reynolds number but different Karlovitz numbers are carried out using semi-detailed finite-rate chemistry to represent different flame conditions. The sensitivities of the differences between the LES-filtered chemical reaction rates and those constructed using LES-filtered thermochemical variables (referred to as ‘disparity’ below) to reaction types, species characteristics, and flame conditions are systematically investigated. The sensitivity to reaction type is examined by quantifying the disparity in the filtered temperature-dependent reaction rate constant as a function of kinetic rate parameters. The sensitivity to species characteristics is examined by quantifying the disparity in the filtered law of mass action as a function of species chemical time scales and formation location. The sensitivity to flame conditions is examined by contrasting the results obtained from the two simulations. These disparities are compared to characterise distinct reaction/species categories with different levels of disparity in estimating the filtered chemical reaction rates in LES. For both flames, it is found that the rate constants estimated using LES-filtered temperature severely under-predict the LES-filtered rate constants, especially for reactions with high activation energies. The law of mass action estimated using LES-filtered species concentrations severely over-predicts the LES-filtered law of mass action, especially for reactions involving reactants with small chemical time scales. The combined effects are found to lead to considerable differences between the LES-filtered chemical reaction rates and those constructed using LES-filtered thermochemical quantities for all reaction types involving different classes of reactants typically included in combustion reaction mechanisms. The details and explanations of these differences provide test cases useful to the improvement of key LES modelling approximations.