FINE SCALE STRUCTURE OF H₂-AIR TURBULENT PREMIXED FLAMES

Direct numerical simulation of H₂-air turbulent premixed flame propagating in three-dimensional homogeneous isotropic turbulence is conducted to investigate fine scale structure of turbulent premixed flames. Detailed kinetic mechanism including 12 reactive species and 27 elementary reactions is used to represent the H₂-0₂ reaction in turbulence. The fine scale structure of turbulent premixed flames is significantly affected by the coherent fine scale eddies in turbulence. The relatively strong coherent fine scale eddies can survive behind the flame front and they are perpendicular to the flame front where heat release rate increases. Direction of the coherent fine scale eddies near the flame front tends to be parallel to the flame front and enhance the chemical reaction. In this case, the distributions of high heat release rate show tube-like structure similar to the coherent fine scale eddies in turbulence. The probability density function (pdf) of local heat release rate is nearly Gaussian with a peak at maximum heat release rate of a laminar flame. The pdf of the curvature of flame front is far from Gaussian and shows exponential tails for large curvature. Most of flame elements are stretched by turbulent motion in the tangential directions. Strong correlation exists between local heat release rate and curvature of flame front. The flame elements convex toward the burnt side with large curvature tend to have high heat release rate. The correlation between local heat release rate and tangential strain rate also exist, while this is not so strong.