Effects of Small-Scale Turbulence on NO_x Formation in Premixed Flame Fronts

摘要

A flamelet-based approach that accounts for turbulence-chemistry interaction has been formulated to simulate NO_x formation in turbulent lean premixed combustion. In the simulations, the species NO is transported and solved with the chemical source term being modelled through its formation in flame fronts and its formation rate in post-flame regions. The flame-front NO and post-flame NO formation rate are obtained from 1-D freely propagating premixed flames with detailed chemical kinetics. For turbulent premixed flames located in the thin-reaction-zones regime, small-scale eddies could penetrate into the preheat zone of the flame and enhance the mixing process. In this study, their effects on the flame-front NO are investigated through the incorporation of turbulence induced diffusion in the preheat zone of the 1-D premixed flames. One-dimensional methane/air premixed flames are simulated with the 53-species GRI-Mech 3.0 mechanism at both atmospheric and engine conditions with different turbulence intensities. It is found that the NO generated in flame fronts deceases with increased intensity of small-scale turbulence and the effect is more profound at high pressures. At high pressures, the turbulence induced diffusion in the preheat zone can reduce the flame-front NO by more than 40%.