Revealing soot maturity based on multi-wavelength absorption/emission measurements in laminar axisymmetric coflow ethylene diffusion flames

A novel diagnostic is proposed to characterize the maturity of soot particles
in a laminar axisymmetric coflow ethylene diffusion flame in terms of the
spectral dependence of soot absorption function. The method relies on the
combination of line-of-sight attenuation (LOSA) and emission measurements
at four wavelengths (500, 532, 660 and 810 nm). The analysis of the measured signals enables the determination of soot temperature, soot volume
fraction, soot maturity and the contribution of soot scattering to extinction. The analysis of extinction and emission measurements considers the
spatial variation of soot optical properties. The introduction of a maturity
index allows the evaluation of soot maturity based on the spectral variation
of the soot absorption function. The maturity index is correlated with the
organic or the mature soot content and finally in terms of the absolute value
of absorption function at 810 nm. The methodology is validated using a set
of synthetic spectral LOSA and emission signals representing experimental
measurements based on numerical results obtained using the CoFlame code.
A sensitivity analysis of the Abel inversion is also performed to properly address the effect of deconvolution procedure. Finally, the proposed method is
applied to analyze the experimental data of spectrally-resolved LOSA and
emission acquired in a laminar axisymmetric coflow ethylene diffusion flame
established on a G¨ulder burner. The two-dimensional distributions of soot
temperature, soot volume fraction, soot maturity, and the ratio of total scattering to absorption are determined. Mature soot particles are found on the
top of the flame in the centerline region and also in the outer edge of the
flame wing displaying strong gradients.