A. Ungut et al., A PLANAR LASER-INDUCED FLUORESCENCE STUDY OF TURBULENT FLAME KERNEL GROWTH AND FRACTAL CHARACTERISTICS, Combustion science and technology, 92(4-6), 1993, pp. 265
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.