Wavelength dependence of extinction in sooting flat premixed flames in the visible and near-infrared regimes

A diode-laser-based setup was used for extinction measurements at various heights in two flat premixed sooting ethylene/air flames in order to investigate the wavelength dependence of the extinction. Twelve laser wavelengths were used in the interval 405-1064 nm, and with this setup, a sensitivity of similar to 10(-4) was achieved for the extinction. Soot volume fractions inferred from the extinction measurements were always higher in the visible spectral region than in the infrared, an effect associated with additional absorption by large hydrocarbons, such as polycyclic aromatic hydrocarbons. For wavelengths greater than around 700 nm and at high heights above burner (HAB), the extinction shows an inverse dependence with wavelength, in concurrence with wavelength-independent optical properties of soot. In the same spectral region at lower HAB, the wavelength dependence is observed to be stronger, indicating that the soot optical properties show variation with wavelength. Furthermore, a continuous change in the wavelength dependence of the absorption function E(m) is shown from nascent soot at lower heights to more mature soot at higher heights. The presented results are of importance for soot diagnostics relying on the efficiency of light absorption by soot such as laser-induced incandescence (LII).