UBC Theses and Dissertations
A fractal analysis of premixed turbulent flames Das, Aurobindo K.
In this experimental study, the fractal behavior of lean premixed wrinkled turbulent flames was investigated over a range of turbulence conditions. For this purpose a turbulent combustion rig was designed and built in which the turbulence intensity and scale could be independently varied by means of a set of grids. The relative turbulence intensity varied from 0.5 to 8.6 times the laminar flame speed. The turbulent Reynolds number range was 10 to 360. The premixed lean flame was stabilized by means of a small pilot burner. A cross section of the wrinkled flame was visualized by an Argon-ion laser sheet and the flame contour was recorded at high shutter speeds by a 35 mm camera. The images were digitized at a high resolution and analyzed to determine their fractal parameters. The experimental technique of this study represents considerable improvements over previous studies. It was observed that premixed wrinkled flames exhibit fractal behavior over a range of turbulence conditions. The measured values of fractal dimension were observed to increase with turbulence intensity and were within 5% of the values reported by others. The inner cutoff scale, representing the smallest scale of surface wrinkling, was determined to be approximately three and a half times the laminar flame thickness. However, for turbulence levels below three times the laminar flame speed, the inner cutoff was found to increase with decreasing turbulence. This provided experimental verification of an earlier hypothesis regarding the smoothing effect of flame propagation at the smaller scales of wrinkling. The turbulent flame speeds based on the fractal parameters measured in this study were compared to those measured directly from the flame cone-angle determined from long-exposure photographs. It was found that, generally, flame speeds estimated from fractal analysis were in poor agreement with the directly measured values, as well as those predicted from a correlation of previous experimental data. There was fair agreement at values of turbulence intensity less than the laminar flame speed, but at higher values of intensity the fractal-based predictions severly [sic] underestimate the flame speed. It appears that the flame surface may not be an isotropic fractal object as presumed in the application of fractal concepts to wrinkled flames. To estimate this anisotropy, fractal analysis of flame contours obtained from simultaneous images in orthogonal planes is, perhaps, necessary. The role of turbulent transport of heat and mass from the flame front is not taken into account in fractal analysis. At higher turbulence levels, the predominant effect on flame propagation may be this transport of heat and mass rather than the relatively small increase in flame area predicted from fractal analysis.
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