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The laser-induced incandescence (LII) and cavity ring-down extinction (CRDE) optical techniques offer excellent sensitivity to allow in situ soot volume fraction \(({f_{\text{v}}})\) measurements over a wide dynamic range. The objective of this work is to quantitatively measure the axial \({f_{\text{v}}}\) profiles in two n-butane premixed flames with very different stages of soot maturity and very different levels of \({f_{\text{v}}}\). 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 \({f_{\text{v}}}\) 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 \({f_{\text{v}}}\) in the range of 0.013–9.7 ppb.

applied physics. b, lasers and optics

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