Local flame structure in the well-stirred reactor regime

Direct numerical simulations of hydrogen/air turbulent premixed flame propagating in the three-dimensional turbulence are conducted to investigate local flame structures in the well-stirred reactor regime. A detailed kinetic mechanism including 12 reactive species and 27 elementary reactions is used to represent the H-2/air reaction in turbulence. Although the flame condition is classified into the well-stirred reactor regime, the geometry of the regions with high heat release rate shows thin sheetlike structure. The fluctuation of the heat release rate along the flame surface is relatively high, and the maximum heat release rate reaches up to 1.3 times the corresponding laminar flame. The heat release rate tends to be high in the regions convex toward the burned side. The flame structure in the case of the well-stirred reactor regime shows a double-layered feature. One may conclude that reaction zone becomes thick in the well-stirred reactor regimes only from temperature, H, and OH distributions, while the heat release rate, mass fraction of 0 atoms, and reaction rates of 0 atoms and OH radicals are fluctuating significantly in that region in fact. Specifically, reaction rates of 0 atoms and OH radicals show characteristic behaviors in the burned side due to their chemical characteristics. In the preheat zone, mass fraction and reaction rate of HO2 show quite thin and smooth distributions compared to other properties such as the heat release rate. The distribution of H2O2 reaction rate reflects the double-layered feature of the well-stirred reactor regime very well. It is shown that the double-layered feature can be explained by discussing the balances of the elementary reactions in detail. As the flame front can be defined even in the well-stirred reactor regime, statistics of the local flame elements are also discussed.