(2017) Investigation of the size of the incandescent incipient soot particles in premixed sooting and nucleation flames of nbutane using LII, HIM, and 1 nm-SMPS, Aerosol Science and Technology, 51:8, 916-935, DOI: 10.1080/02786826.2017 Laser-induced incandescence (LII) measurements were conducted to explore the ability of LII to detect small soot particles of less than 10 nm in two sooting flat premixed flames of n-butane: a socalled nucleation flame obtained at a threshold equivalence ratio F D 1.75, in which the incipient soot particles undergo only minor soot surface growth along the flame, and a more sooting flame at F D 1.95. Size measurements were obtained by modeling the time-resolved LII signals detected using 1064 nm laser excitation. Spectrally-resolved LII signals collected in the nucleation flame display a similar blackbody-like behavior as mature soot. Soot particle temperature was determined from spectrally-resolved detection. LII modeling was conducted using parameters either relevant to those of mature soot or derived from fitting the modeled results to the experimental LII data. Particle size measurements were also carried out using (1) ex situ analysis by helium-ion microscopy (HIM) of particles sampled thermophoretically and (2) online size distribution analysis of microprobe-sampled particles using a 1 nm-SMPS. The size distributions of the incipient soot particles, found in the nucleation flame and in the early soot region of the F D 1.95 flame, derived from time-resolved LII signals are in good agreement with HIM and 1 nm-SMPS measurements and are in the range of 2-4 nm. The thermal and optical properties of incipient soot were found to be not radically different from those of mature soot commonly used in LII modeling. This explains the ability of incipient soot particles to produce continuous thermal emissions in the visible spectrum. This study demonstrates that LII is a promising in situ optical particle sizing technique that is capable of detecting incipient soot as small as about 2.5 nm and potentially 2 nm and resolving small changes in soot sizes below 10 nm.

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. This article is part of the topical collection "Laser-Induced Incandescence", guest edited by Klaus Peter Geigle and Stefan Will. C. Betrancourt: on leave from Univ. Lille.