Measurement of the optical extinction coefficients of post-flame soot in the infrared

Abstract

The optical extinction coefficients of post-flame soot have been measured in the wavelength range 2.8 to 4.1 μm. A laminar diffusion burner was combined with an infrared spectrograph and gravimetric measurements to determine the mass specific extinction coefficient, σ s, and the dimensionless extinction coefficient, K e. Using ethene gas as the fuel, the burner was operated at four global equivalence ratios (φ = 0.8, 1.0, 2.0, and 3.0) to examine the effect of the fuel-air ratio on the extinction coefficient. The extinction coefficient was found to decrease with increasing values of the global equivalence ratio for φ = 1.0, 2.0, and 3.0. The results for φ = 0.8 and φ = 1.0 were in agreement to within the uncertainty of the measurements. Measurements were obtained using propane gas as the fuel (φ = 1.0) and resulted in extinction coefficients equivalent to those of ethene. Transmission electron microscopy (TEM) images revealed differences in the morphology of the particles, consistent with the quantitative differences observed in the extinction data. The data indicate that the equivalence ratio has a strong effect on the optical properties of post-flame soot agglomerates.