A PLANAR LASER-INDUCED FLUORESCENCE STUDY OF TURBULENT FLAME KERNEL GROWTH AND FRACTAL CHARACTERISTICS

Planar laser induced fluorescence from the hydroxyl radical in spark-ignited, freely propagating turbulent flame kernels of methane/air mixtures has been used to study the early growth rate under varying turbulence conditions and mixture stoichiometry and to determine the inner cutoff of the fractal flame surfaces. Methane/air mixtures were spark-ignited in a vertical wind tunnel and expanding turbulent flame kernels were studied in a grid-generated decaying isotropic turbulent flow. The cold flow turbulence structure was determined by laser Doppler anemometry. An extensive use of the computational image processing and analysis techniques has been made to determine the kernel sizes and the fractal characteristics. Inner cutoff values of the self similarity of the flame surfaces representing the smallest scale of the flame wrinkling have been measured to be 20-30 times larger than the flame thickness and 13-15 times larger than the Kolmogorov length scale. Results suggest that the laminar flames are unaffected by the flow turbulence in a greater region than suggested by the criteria of turbulent Reynolds number < 1. The flame kernel growth for freely expanding methane/air flames in moderately turbulent flows (U'/U(F) = 1.62 - 2.73) is consistent with it being unaffected by the flow turbulence for kernel sizes smaller than the integral length scale of turbulence. For larger kernel sizes and the fully wrinkled flame geometries the kernel growth tends to become higher than the unstretched laminar values showing a weak dependence on the turbulence intensity.