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Two-dimensional images of OH fluorescence, polycyclic aromatic hydrocarbons (PAHs) fluorescence, and laser-induced incandescence (LII) from soot were measured in a sooting diffusion flame. To obtain an accurate OH fluorescence image, two images were taken with the laser wavelength tuned to (“on”) and away from (“off”) the OH absorption line. An accurate OH fluorescence image was obtained by subtracting the off-resonance image from the on-resonance image. For the PAH fluorescence and LII measurements, temporally resolved measurements were used to obtain the individual images; the LII image was obtained by detecting the LII signal after the PAH fluorescence radiation had stopped and the PAH fluorescence image was obtained by subtracting the LII image from the simultaneous image of PAH fluorescence and LII. Based on the obtained images, the relative location of OH, PAH, and soot in the flame was discussed in detail. To investigate the PAH size distribution in a sooting flame using LIF, an estimation strategy for PAH size is proposed. Emission spectra were measured at several heights in the flame using a spectrograph. Since the emission wavelength of PAH fluorescence shifts toward longer wavelengths with increasing PAH size, the main PAH components in the emission spectra could be estimated. The results suggest that PAH grows and the type of PAH changes as the soot inception region was approached. Near the soot inception region, we estimated that the PAHs, which have over 16 carbon atoms, mainly constituted the emission spectrum.

Spontaneous Raman spectroscopy with KrF excimer laser was applied to obtain a fuel concentration distribution in a sooting flame. In the case of sooting flame, fluorescence from polycyclic aromatic hydrocarbons (PAH) and laser-induced incandescence (LII) from soot particles appeared with Raman scattering. These background emissions overlapped on the Raman scattering. In order to separate the Raman scattering and the background emissions, polarization property of laser-induced emissions was utilized. Since the background emissions were depolarized whereas the Raman scattering was highly polarized, it is possible to subtract the background emissions from the overlapping signal of the Raman scattering and the background emissions. Subtracting the emission signals for the electric vector of the laser light perpendicular and parallel to the direction of observation allows to extract the precise Raman signals. By using this technique, detailed fuel concentration distribution in sooting flames could be obtained based on Raman scattering.

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