LEWIS NUMBER EFFECTS ON PREMIXED FLAMES INTERACTING WITH TURBULENT KARMAN VORTEX STREETS

The effects of Lewis number on the global and local structure of premi xed flames interacting with turbulent Karman vortex streets are experi mentally investigated using OH planar-laser-induced fluorescence (PLIF ). The OH PLIF results show that over the range of Lewis numbers studi ed, i.e., Le = 0.21, 0.94 and 1.79, the flame area increases and the f lame front is oriented more randomly as Lewis number decreases, while the flame curvature pdfs are unchanged. The relationship between the l ocal flame structure and the local flame curvature is found to be cons istent with the results of stretched laminar flame theory. The correla tion 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-theta/S-L increases, indicating that the response of the fl ame to large flame stretch may be non-linear at high U-theta/S-L. The pdfs of peak OH LIF intensity suggest that the mean burning rate of th e H-2/He/air flame at U-theta/S-L = 3.3 is increased approximately by 10% in comparison to the undisturbed laminar flame. The present result s imply that even though the local flame curvature may strongly influe nce the local structure and burning rate of nonunity Lewis number flam es through the effect of flame stretch on the local burning rate, thes e 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 a nd zero mean of the curvature distribution. Therefore, the main effect of turbulence and Lewis number is to wrinkle the flame and produce fl ame area, while increasing the mean burning rate per unit surface area by relatively small amount through flow strain effects.