Optical measurement of volume fraction and organic mass fraction of ultra-fine soot particles emitted from inverse diffusion flames

Soot particles of various organic carbon (OC) contents were generated with a triple co-flow burner. TEM images showed that the primary particle boundaries were more ambiguous when OC contents were high. A set of diode lasers of various wavelengths and a white light source were used to measure light extinction over a wide range of wavelengths (450-850 nm), and the logarithm of the light intensity ratio was plotted as a function of the inverse of the various wavelengths. The slopes of the plot were taken from the both ends of the wavelength range, and their ratio was calculated. The results show that when the OC contents are high, the ratio increases to approximately 10, whereas the ratio remains nearly constant at a value of 1 when the OC contents are minimal. The analysis of the slope ratios shows that the wavelength-dependent absorption behavior of soot does not depend on the soot volume fraction or the soot particle size; rather, the slope ratio depends only on the soot's refractive index, which in turn is related to its OC contents. In addition, the soot volume fraction and OC contents can be simultaneously identified without any chemical analysis based on the pre-determined relationship between the slope ratio and the organic fraction of soot.