AN EXPERIMENTAL-STUDY TO DETERMINE FRACTAL PARAMETERS FOR LEAN PREMIXED FLAMES

In this study the fractal parameters of a lean, pre-mixed methane-air flame were determined over a range of turbulence conditions. The focus of the present work was to improve the experimental technique so as t o resolve the inner cutoff scale, the outer cutoff scale, and the frac tal dimension. By adjusting the how velocity through a set of three in terchangeable grids in a steady-flow combustion tunnel, a range of tur bulence intensities and scales tvas obtained within the test section. The integral scale varied from 2.5 to 5.5 mm and the turbulence intens ity varied from 0.5 to 3.8 times the laminar burning velocity, while t he equivalence ratio of the fuel-air mixture was 0.60. The flame was s tabilized inside a 51 mm square, open-ended test section by means of a small, centrally-located, pilot burner. A 60 mm x 45 mm cross section of the flame was visualized by means of an argon-ion laser sheet and titanium dioxide seeding, and was recorded on high-sensitivity black a nd white him by a 35 mm camera using a shutter speed of 1/8000 s. The film negatives were digitized at 60 pixels/mm, equivalent to a resolut ion of 12 pixels/mm (83 mu m per pixel) on the scale of the flame. Usi ng commercially available software, the images were analyzed to identi fy the position of the flame front; custom software was used to determ ine the fractal dimension and the inner and outer cutoff scales of the turbulent flame. In the range of conditions reported in this paper, i t was observed that the fractal dimension increased with turbulence le vel but the values were approximately 5% lower than those reported by others. The inner cutoff scale was found to increase with decreasing t urbulence, thus confirming an earlier hypothesis about the smoothing e ffect of flame propagation at low turbulence levels. The outer cutoff scale varied from 11 to 16 mm and its value tended to decrease with in creasing turbulence level.