CHARACTERIZATION OF FLAME FRONT SURFACES IN TURBULENT PREMIXED METHANE AIR COMBUSTION/

A detailed experimental investigation of the application of fractal ge ometry concepts in determining the turbulent burning velocity in the w rinkled flame regime of turbulent premixed combustion was conducted. T he fractal dimension and cutoff scales were determined for six differe nt 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. F rom this individual data, the pdf distributions of all three fractal p arameters, along with the distribution of the predicted increase in su rface area, may be determined. The analysis of over 300 flame images f or each flame condition provided a sufficient sample size to accuratel y define the pdf distributions and their means. However, the predicted S-T/S-L, calculated using fractal parameters, was significantly below the measured values. For conical flames, a geometrical modification f actor was employed to predict S-T/S-L, however, this did little to imp rove the predictions. There appeared to be no dependence of the predic ted S-T/S-L On the approach flow turbulence. The cutoffs did not seem to vary significantly with any of the length scales in the approach ho w turbulence, although the fractal dimension did appear to have a weak dependence on u'/S-L and Re-A. The probable reasons that fractal geom etry does not correctly predict S-T/S-L are that S-T/S-L = A(w)/A(o) d oes not hold in wrinkled turbulent premixed flames, that the flame fro nt surface cannot be described by a single scaling exponent, or that t hese are not wrinkled flames even though they are within conventional definitions of the wrinkled flame regime.