Combining performances of E(m)-corrected LII and absorption for in situ measurements of the volume fraction of 2–4 nm soot particles.

Abstract

Determining the soot volume fraction (f_v) in combustion environments requires detailed knowledge of the optical properties of the soot particles, and in particular of their absorption function E(m). This study addresses a fundamental lack of information on the optical properties of 2–4 nm soot particles. Recent works based on the modeling of the photoelectron emission yields and UV-vis-NIR-absorption measurements found a sharp decrease of E(m) with the particle size in the vis-NIR spectral region, which is inconsistent with the in situ detection of 2–4 nm particles in the near-infrared region by laser-induced incandescence (LII) or sensitive absorption methods like cavity ring-down extinction (CRDE). The objective of this study is twofold: first, an original method for the determination of E(m) of soot particles, including 2–4 nm particles is proposed. Then, the dynamic of two widespread in situ diagnostics, LII and CRDE, are compared over three orders of magnitude of f_v in atmospheric premixed ethylene/air flames with different flow rates and C/O. The determination of the absolute value of $E\left(m\right)$ and of its variation in the flames is derived from an original analysis, which does not require complex LII modeling. This analysis is based on the comparison between the experimental and calculated LII/LII_max signals in the low fluence regime, LII_max being the plateau value of the fluence curve, which is reached at fluence larger than 1 J/cm² for the smallest C/O. E(m) is found to vary between 0.15 at low C/O up to 0.36 for the richest flames. Concerning the comparison of the dynamics of LII and CRDE, an excellent agreement is found above a threshold (C/O)_limit, while LII exhibits a stronger decrease with C/O below (C/O)_limit. This discrepancy is attributed to the spectral dependence of E(m) which is negligible above (C/O)_limit, but increases when C/O decreases below (C/O)_limit. The particle size distribution function (PSD), measured by scanning mobility particle sizing, reveals monomodal or bimodal PSDs with soot having mobility diameter in the range 2.3–7.5 nm depending on the flame conditions. It is suggested that the particles contained in the first PSD mode, which is dominant in the low C/O range, could be affected by a significant spectral dependence of E(m) in comparison with the second PSD mode.