Planar Laser Induced Fluorescence Measurement of [ OH] in the Stabilization Stage of a Spray Jet Flame

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Abstract
Planar laser induced fluorescence of OH is quantitatively applied in the lift-off region of a spray jet flame. The spray is produced by a coaxial air-blast injector fed with liquid methanol. OH is laser excited through the (0,1) vibrational band so that the shifted fluorescence emitted via the ( 0,0) and ( 1,1) bands can be spectrally discriminated against elastic scattering by the methanol drops. The lift-off location is determined at each laser shot making it possible to investigate the reactive structures in their own coordinate system. Statistical analysis of the fluorescence images shows that the lifted flame may develop in the remarkable structure of two diverging diffusion-like fronts as was predicted by modeling of spray combustion in a counterflow laminar diffusion flame. Calibration of the fluorescence imaging system into absolute OH concentration is performed in a laminar diffusion flame of methanol vapor where [ OH] is determined in situ by absorption measurement with the same optical device. The OH concentration conditioned on the elevation above the lift-off is found to hold a constant superequilibrium value about four times the equilibrium level.