Lagrangian Analysis of the Thermochemical Trajectories in High-Speed, Turbulent, Premixed Methane-Air and Jet-Fuel-Air Flames

In a recent study of the dynamics of high-speed turbulent premixed methane-air and jet-fuel-air flames, we found that methane and heavy hydrocarbons exhibit opposite trends in terms of the turbulent burning velocity at low and high turbulent intensities. In particular, direct numerical simulations of statistically planar turbulent flames in a canonical “flame-in-the-box” configuration were performed spanning Karlovitz numbers from 10 to 10⁴. These calculations showed that while the normalized burning speed of methane S_T/S_L is significantly higher than that of jet fuels (n-dodecane, Cat A2, and Cat C1) at Ka ~ 10, at high intensities corresponding to Ka ≥ 10³, jet fuels exhibit significantly larger values of S_T/S_L compared to methane. Here we extend this study by performing a Lagrangian analysis of the thermochemical trajectories of fluid parcels traversing the flame for Ka = 10 – 10⁴. In particular, we discuss the variation in the characteristic residence times in different flame regions, as well as the non-monotonicity of the thermochemical trajectories previously observed in fast H₂/air flames. Finally, we discuss the implications of these results for the reduced chemical kinetics used to represent hydrocarbon fuels, as well as for the Large Eddy Simulation combustion models.