Flame surface densities in premixed combustion at medium to high turbulence intensities

The surface densities of flame fronts in turbulent premixed propane/air flames were determined experimentally. The instantaneous flame fronts were visualized using laser induced fluorescence (LIF) of OH on two Bunsen type burners of 11.2 and 22.4 mm diameters. Non-dimensional turbulence intensity, u'/S-L, was varied from 0.84 to 15, and the Reynolds number, based on the integral length scale, varied from 34 to 467. These flames are in the flamelet combustion regime as defined by the most recent turbulent premixed combustion diagrams. From 100 to 800 images were recorded for each experimental condition. Flame surface densities were obtained from the instantaneous maps of the progress variable, which is zero in the reactants and unity in the products. These flame surface densities were corrected for the mean direction cosines of the flame fronts, which had a typical value of 0.69 for the Bunsen flames. In the non-dimensional turbulence intensity range of up to 15, it was found that the maximum flame surface density and the integrated flame surface density across the flame brush do not show any significant dependence on turbulence intensity. This was discussed in the framework of a flame surface density-based turbulent premixed flame propagation closure model. The implication is that the conceptual increase in flame surface density with turbulence may not be the dominant mechanism for flame velocity enhancement in turbulent combustion in the region specified as the flamelet combustion regime by the current turbulent premixed combustion diagrams. Small-scale transport of heat and species may be more important and chemistry may not be decoupled from turbulence. Further, the applicability of the flamelet approach may be limited to a much smaller range of conditions than presently believed.