Characterization of flame front surfaces in turbulent premixed methane/Air combustion

A detailed experimental investigation of the application of fractal geometry concepts in determining the turbulent burning velocity in the wrinkled flame regime of turbulent premixed combustion was conducted. The fractal dimension and cutoff scales were determined for six different turbulent flames in the wrinkled flame regime, where the turbulence intensity, turbulent length scale, and equivalence ratio were varied. Unlike previous reports, it has proved possible to obtain the fractal dimension and inner and outer cutoffs from individual flame images. From this individual data, the pdf distributions of all three fractal parameters, along with the distribution of th predicted increase in surface area, may be determined. The analysis of over 300 flame images for each flame condition provided a sufficient sample size to accurately define the pdf distributions and their means. However, the predicted ST/SL, calculated using fractal parameters, was significantly below the measured values. For conical flames, a geometrical modification factor was employed to predict ST/SL, however, this did little to improve the predictions. There appeared to be no dependence of the predicted ST/SL on the approach flow turbulence. The cutoffs did not seem to vary significantly with any of the length scales in the approach flow turbulence, although the fractal dimension did appear to have a weak dependence on u′/SL and ReΛ. The probable reasons that fractal geometry does not correctly predict ST/SL are that ST/SL = Aw/A0 does not hold in wrinkled turbulent premixed flames, that the flame front surface cannot be described by a single scaling exponent, or that these are not wrinkled flames even though they are within conventional definitions of the wrinkled flame regime.