An experimental study to determine fractal parameters for lean premixed flames

Abstract

 In this study the fractal parameters of a lean, premixed 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 to resolve the inner cutoff scale, the outer cutoff scale, and the fractal dimension. By adjusting the flow velocity through a set of three interchangeable grids in a steady-flow combustion tunnel, a range of turbulence intensities and scales was obtained within the test section. The integral scale varied from 2.5 to 5.5 mm and the turbulence intensity varied from 0.5 to 3.8 times the laminar burning velocity, while the equivalence ratio of the fuel–air mixture was 0.60. The flame was stabilized inside a 51 mm square, open-ended test section by means of a small, centrally-located, pilot burner. A 60 mm ×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 and white film 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 resolution of 12 pixels/mm (83 μm per pixel) on the scale of the flame. Using commercially available software, the images were analyzed to identify the position of the flame front; custom software was used to determine the fractal dimension and the inner and outer cutoff scales of the turbulent flame. In the range of conditions reported in this paper, it was observed that the fractal dimension increased with turbulence level but the values were approximately 5% lower than those reported by others. The inner cutoff scale was found to increase with decreasing turbulence, thus confirming an earlier hypothesis about the smoothing effect of flame propagation at low turbulence levels. The outer cutoff scale varied from 11 to 16 mm and its value tended to decrease with increasing turbulence level.