Quantitative measurement of volume fraction profiles of soot of different maturities in premixed flames by extinction-calibrated laser-induced incandescence

The laser-induced incandescence (LII) and cavity ring-down extinction (CRDE) optical techniques offer excellent sensitivity to allow in situ soot volume fraction (f(v)) measurements over a wide dynamic range. The objective of this work is to quantitatively measure the axial f(v) profiles in two n-butane premixed flames with very different stages of soot maturity and very different levels of f(v). The first flame is a nucleation flame in which soot particles undergo only minor growth and have diameters between 2 and 4 nm. The second is a normal sooting flame generating soot from inception to mature stage. Experiments were performed by combining LII and CRDE using a laser at 1064 nm. Quantitative measurements of f(v) require the knowledge of the dependence of soot absorption function E(m) on wavelength and soot maturity. To this aim, two novel approaches were developed to evaluate the variation of E(m) at 1064 nm along the flame centerline and between 532 and 1064 nm covering the entire spectral range used in this study. The axial LII profiles were converted to absolute f(v) by CRDE measurements. The performance of the combined techniques is demonstrated in the two investigated flames for f(v) in the range of 0.013-9.7 ppb.