Fractal characterisation of high-pressure and hydrogen-enriched CH4-air turbulent premixed flames

Flame surface characteristics were measured in turbulent premixed flames of hydrogen-enriched methane-air mixtures at elevated pressures. The equivalence ratio range was from 0.6 to 0.8 and the hydrogen mole fraction in the fuel was varied from 0 to 0.2. The pressure range covered from atmospheric to 0.9 MPa. Planar Mie scattering measurements were performed to obtain the flame front images, which were further analyzed for fractal parameters, flame front curvature statistics, and flame surface density. Non-dimensional turbulence intensity, u'/S-L, changed in proportion to the pressure and was 1.53 at atmospheric pressure and 6.51 at 0.9 MPa. With increasing pressure the flame images displayed finer structures indicating that the flame surface area was increasing with pressure. This was also indicated by the probability density function of the flame front curvature as a function of the pressure. Fractal dimension showed a strong dependence on pressure and increased from about 2.1 at atmospheric pressure to about 2.25 at 0.9 MPa. Fractal parameters, including inner and outer cut-offs, agree to a certain extend with the previous experimental data obtained at atmospheric and elevated pressures. The sensitivity of the fractal dimension to u'/S-L was found to depend on the way u'/S-L Was varied. A strong correlation between the inner cut-off scale and the average flame radius of curvature was demonstrated. The implications of these results for flame surface density estimations are discussed. (C) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.