Coherent fine-scale eddies in turbulent premixed flames

Abstract

Direct numerical simulation of a hydrogen/air turbulent premixed flame propagating in three-dimensional homogeneous isotropic turbulence was conducted to investigate fine-scale structure of turbulent premixed flames. A detailed kinetic mechanism including 12 reactive species and 27 elementary reactions was used to represent the H2/O2 reaction in turbulence. The fine-scale structure of turbulent premixed flames is significantly affected by the coherent fine-scale eddies in turbulence. Relatively strong coherent fine-scale eddies can survive behind the flame front and are perpendicular to the flame front. The local heat release rate increases where the coherent fine-scale eddy impinges on the flame front with large axial velocity to the direction of the burned side. The direction of many coherent fine-scale eddies tends to be parallel to the flame front and to enhance the chemical reaction. In this case, the distribution of the high heat release rate shows a tubelike structure similar to the coherent fine-scale eddies of turbulence. The probability density function of the curvature of the flame front is far from Gaussian and shows exponential tails for large curvatures. Most of the flame elements are stretched by turbulent motion in the tangential direction. The flame elements that are convex toward the burned side with large curvature tend to have a high heat release rate. The statistical nature of the local flame elements strongly depends on the second invariant of the velocity gradient tensor at the flame fronts, which means that the coherent fine-scale eddies are an important structure which determines the local flame structure of turbulent premixed flames.