Flamelet LES of turbulent premixed/stratified flames with [formula omitted] addition

Abstract

Large-eddy simulations are conducted for Darmstadt turbulent methane-hydrogen flames (MHFs, 20%H2+80%CH4 in volume fraction). Two different flames are investigated: one without stratification at the fuel inlets (MHF5) and the other with stratification (MHF7). Two different flamelet libraries are generated: one based on the unity Lewis number assumption (Le1) and the other based on the mixture-averaged (MA) approach. Further, two different techniques are adopted to access the flamelet libraries: one solving the transport equations for the six Raman-accessible species mass fractions (M1), and the other solving the transport equations for the three trajectory variables directly (M2). The heat loss effects in the pilot tube are investigated by comparing the adiabatic and non-adiabatic flamelet models. The suitability of the different modeling approaches is evaluated by comparing the simulation results with the available experimental data. The results show that the heat transfer in the pilot tube has significant effects on the variation trends of the gas temperature and the flame position. Overall, M2 performs better than M1 in predicting the thermo-chemical quantities, regardless of diffusion modeling. M2 coupled with different flamelet libraries makes good predictions for both flames in the upstream and inner downstream regions. However, in the outer downstream region only M2 coupled with the unity Lewis number assumption generates good predictions for MHF7, while the other modeling approaches over-predict the gas temperature and CO2 mass fraction for both MHF5 and MHF7. The reason for the different performances of the flamelet models is explored by analyzing the differential diffusion parameter.