2D in situ determination of soot optical band gaps in flames using hyperspectral absorption tomography

During the formation of soot, the particles undergo a fine structural transformation, akin to graphitization, from “young” to “mature” soot. This ageing process is accompanied by a change in optical properties, which manifests in the soot optical band gap and dispersion exponent, E_g and ζ. Although both quantities are regularly determined via broadband extinction measurements, they have a nonlinear relationship to the local extinction spectrum, so line-of-sight integrated measurements introduce errors into estimates of E_g and ζ. We report the development of an absorption tomography sensor that enables 2D in situ mapping of E_g and ζ. Our sensor employs broadband back-illumination, fiber-coupled collection optics, and an imaging spectrograph to simultaneously acquire 24 spectrally-resolved absorption signals, suitable for tomographic reconstruction. We perform proof-of-concept experiments to characterize soot formation in non-premixed steady flames from a Gülder burner, using ethylene and propane as fuels. Reconstructions based on axisymmetric and non-axisymmetric absorption tomography are compared, and we explore how our sensor can be adapted to perform temporally-resolved measurements in a turbulent flame.