Effect of maturity on soot volume fraction measurements using the AC-LII technique in a laminar coflow ethylene diffusion flame

The auto-compensating laser-induced incandescence (AC-LII) technique has been frequently used to measure soot volume fraction (f_v) in flames. AC-LII relies on an assumption about the spectral variation of soot absorption function E(m,λ) at the two detection wavelengths for soot temperature determination. It has been a common practice to assume a constant E(m,λ) based on the believe that LII measures mature soot only. Since soot maturity in flames is a continuous phenomenon from young to mature, it is unclear how soot maturity affects the AC-LII measured f_v. In this study, a correction to the AC-LII measured f_v with the conventional hypothesis of mature soot is proposed based on the soot maturity coefficient β and absorption function E(m,λ) in a laminar coflow ethylene diffusion flame. The application of this correction is first demonstrated for synthetic time-resolved LII signals based on the spatial distribution of soot properties in the ethylene diffusion flame predicted by the CoFlame code. A significant decrease of the simulated incandescence signals S_LII and soot temperature T_eff is observed when the maturity coefficient is increased from 0 for fully mature soot to 1.2 for highly immature soot. Then the proposed correction is applied to AC-LII measured f_v in the same flame with the help of spatial distributions of β and ratio of spectrally-resolved E(m,λ) by multiwavelength line-of-sight attenuation and emission (MW-LOSA/EMI) measurements. The uncorrected AC-LII measured f_v is in general significantly lower than that measured by MW-LOSA/EMI, which inherently considers soot maturity. The corrected AC-LII f_v results are in good agreement, within the experimental error, with those measured by MW-LOSA/EMI. Therefore, the proposed methodology can quantify the effect of soot maturity on AC-LII measured f_v and largely eliminate the difference in the measurements of f_v by AC-LII and MW-LOSA/EMI.