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

Abstract

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.

A Review of Terminology Used to Describe Soot Formation and Evolution under Combustion and Pyrolytic Conditions

“…132,[242][243][244][245] For combustion conditions near the sooting threshold, however, primary-particle monomers with diameters as small as 2-3 nm and optical properties consistent with mature soot have been observed. [246][247][248][249][250] Similar observations have been made of primary-particle monomers with very ordered crystalline fine structure and diameters of ~5 nm in a co-flow diffusion flame. 251 There has been one suggestion to refer to mature soot particles as "nano-sphere soot".…”

Experimental study of the influence of hydrogen as a fuel additive on the formation of soot precursors and particles in atmospheric laminar premixed flames of methane

“…The LII profiles were then calibrated into soot volume fraction profiles by an extinction measurement carried out by CRDS. To this aim, the soot absorption function E(m) was chosen equal to a constant value of 0.25 as previously determined for nascent soot particles studied in comparable nucleation flames [56]. A schematic diagram of this system is available in SM1 (Fig.…”

“…Details of the method can be found in numerous papers [56,[59][60][61][62]. Briefly, the measured LII signal, corresponding to the incandescent emission coming from the heated soot particles by a laser pulse, can be expressed according to the following expression:…”

Multi-technique physico-chemical characterization of particles generated by a gasoline engine: Towards measuring tailpipe emissions below 23 nm

“…It has been applied to, e.g., the measurement of the soot volume fraction in turbulent flames (Lemaire et al, 2010) or in the exhaust of aero-engines (Delhay et al, 2009). Recent efforts were devoted to the detection of very small soot nanoparticles (Betrancourt et al, 2017(Betrancourt et al, , 2019. The LII technique was adapted here to probe soot particles in an optical cell mounted on the exhaust duct (Fig.…”

“…These observations provide precious information for optimizing the simultaneous sampling of the particulates for off-line analyses. Further efforts will be devoted to calibrate f ν using an extinction technique at a given engine set-point (Betrancourt et al, 2019) and to combine LII with particle sizers as mentioned above in a context of very small particles (down to 10 nm). From the LII measurements, several set-points showing a high stability have been selected for the off-line characterization, Table 2.…”