Flamelet LES of turbulent premixe d/stratifie d flames with H_2 addition

摘要

Large-eddy simulations are conducted for Darmstadt turbulent methane-hydrogen flames (MHFs,20 H_2 + 80 CH_4 in volume fraction). Two different flames are investigated: one without stratificationat the fuel inlets (MHF5) and the other with stratification (MHF7). Two different flamelet libraries aregenerated: one based on the unity Lewis number assumption (Le1) and the other based on the mixtureaveraged(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 theother solving the transport equations for the three trajectory variables directly (M2). The heat loss effectsin the pilot tube are investigated by comparing the adiabatic and non-adiabatic flamelet models. Thesuitability of the different modeling approaches is evaluated by comparing the simulation results withthe available experimental data. The results show that the heat transfer in the pilot tube has significanteffects on the variation trends of the gas temperature and the flame position. Overall, M2 performs betterthan M1 in predicting the thermo-chemical quantities, regardless of diffusion modeling. M2 coupledwith different flamelet libraries makes good predictions for both flames in the upstream and inner downstreamregions. However, in the outer downstream region only M2 coupled with the unity Lewis numberassumption generates good predictions for MHF7, while the other modeling approaches over-predict thegas temperature and CO 2 mass fraction for both MHF5 and MHF7. The reason for the different performancesof the flamelet models is explored by analyzing the differential diffusion parameter.