Soot maturity and absorption cross sections

We used time-resolved laser-induced incandescence (LII) to compare optical and physical properties of soot at the edge and in the center of two co-flow diffusion flames in common use for soot optical-diagnostics development. We made these measurements over a wide range of laser fluences using a laser wavelength of either 532 or 1064 nm. Our results, combined with the previous results, suggest that the 532:1064 nm absorption cross-section ratio is ≤1.8 for mature soot and increases with increasing hydrogen content and decreasing soot maturity. The absolute absorption cross sections at 532 and 1064 nm, on the other hand, increase with soot maturity. Differences in the signal decay rates between center and edge regions further suggest that the thermal-accommodation coefficient decreases with increasing soot maturity. These results were analyzed using an energy- and mass-balance model that accounts for the effects of soot maturity on the absorption cross section, thermal-accommodation coefficient, and particle density. Given the sensitivity of LII to these parameters, it may be possible to use a combination of in situ measurements of pulsed LII fluence curves and temporal profiles to gain information about soot maturity.