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

摘要

The laser-induced incandescence (LII) and cavity ring-down extinction (CRDE) optical techniques offer excellent sensitivity to allow in situ soot volume fraction $$({f_{ext{v}}})$$ ( f v ) measurements over a wide dynamic range. The objective of this work is to quantitatively measure the axial $${f_{ext{v}}}$$ f v profiles in two n -butane premixed flames with very different stages of soot maturity and very different levels of $${f_{ext{v}}}$$ 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 $${f_{ext{v}}}$$ f v require the knowledge of the dependence of soot absorption function $$E(m)~$$ E ( m ) on wavelength and soot maturity. To this aim, two novel approaches were developed to evaluate the variation of $$E(m)~$$ 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 $${f_v}$$ f v by CRDE measurements. The performance of the combined techniques is demonstrated in the two investigated flames for $${f_{ext{v}}}$$ f v in the range of 0.013–9.7 ppb.