Tabulation of NOx chemistry for Large-Eddy Simulation of non-premixed turbulent flames

Abstract

Detailed chemistry tabulations are mostly based on reaction progress variables used to track species evolutions along trajectories in composition space, which in some cases are obtained from simulations of prototype combustion problems. For hydrocarbon chemistry, these progress variables are usually built from major species, as CO, CO2, H2O to which reactants are sometimes added. Prediction of NO mass fraction is first evaluated by comparing results from single progress variable and flamelet based chemical look-up tables against simulations of reference flames performed with the detailed chemistry mechanism that was used to generate the tabulations. Nitric oxides mainly formed in the burnt gases still evolve once major species have almost reached their equilibrium levels, then large errors in the predictions of NO mass fractions are unavoidable when using a progress variable based on major species only. It is concluded that the time scale separation inherent to NOx species must be included in the progress variable definition. A novel definition of progress variable is proposed for flame based tabulation and tested, which allows for reproducing both major species as well as NO, NO2 and N2O. Large-Eddy Simulation (LES) of a jet flame in a vitiated coflow is performed with the new tabulation and results are compared against experiments.