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

Planar laser induced fluorescence from the hydroxyl radical in spark-i gnited, freely propagating turbulent flame kernels of methane/air mixt ures has been used to study the early growth rate under varying turbul ence conditions and mixture stoichiometry and to determine the inner c utoff of the fractal flame surfaces. Methane/air mixtures were spark-i gnited in a vertical wind tunnel and expanding turbulent flame kernels were studied in a grid-generated decaying isotropic turbulent flow. T he cold flow turbulence structure was determined by laser Doppler anem ometry. An extensive use of the computational image processing and ana lysis techniques has been made to determine the kernel sizes and the f ractal characteristics. Inner cutoff values of the self similarity of the flame surfaces representing the smallest scale of the flame wrinkl ing have been measured to be 20-30 times larger than the flame thickne ss and 13-15 times larger than the Kolmogorov length scale. Results su ggest 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 consis tent with it being unaffected by the flow turbulence for kernel sizes smaller than the integral length scale of turbulence. For larger kerne l sizes and the fully wrinkled flame geometries the kernel growth tend s to become higher than the unstretched laminar values showing a weak dependence on the turbulence intensity.