Lewis number effects on premixed flames interacting with turbulent Kármán vortex streets

The effects of Lewis number on the global and local structure of premixed flames interacting with turbulent Kármán vortex streets are experimentally investigated using OH planar-laser-induced fluorescence (PLIF). The OH PLIF results show that over the range of Lewis numbers studied, i.e., Le = 0.21, 0.94 and 1.79, the flame area increases and the flame front is oriented more randomly as Lewis number decreases, while the flame curvature pdfs are unchanged. The relationship between the local flame structure and the local flame curvature is found to be consistent with the results of stretched laminar flame theory. The correlation between the local maximum OH fluorescence intensity and the local curvature tends to level off for large positive curvature (H > 0.5 mm^-1) as U_θ S_L increases, indicating that the response of the flame to large flame stretch may be non-linear at high U_θ S_L. The pdfs of peak OH LIF intensity suggest that the mean burning rate of the H₂/He/air flame at U_θ S_L = 3.3 is increased approximately by 10% in comparison to the undisturbed laminar flame. The present results imply that even though the local flame curvature may strongly influence the local structure and burning rate of nonunity Lewis number flames through the effect of flame stretch on the local burning rate, these variations tend to cancel in the mean due to the linear relationship between local burning rate and curvature for the most probable values of curvature (-0.5 mm^-1 < H < 0.5 mm^-1) and due to the symmetry and zero mean of the curvature distribution. Therefore, the main effect of turbulence and Lewis number is to wrinkle the flame and produce flame area, while increasing the mean burning rate per unit surface area by relatively small amount through flow strain effects.