The change of soot refractive index function along the height of premixed ethylene/air flame and its correlation with soot structure

It is crucial to understand the behavior of the optical properties of soot in the visible and near-IR spectral range to calculate the magnitude of their emission and absorption. Such information also helps in the development of diagnostic methods for soot. We measured the absolute value of the refractive index functions of soot nanoparticles E(m,1064) at a wavelength of 1064 nm, as well as the ratio of the refractive index functions at laser wavelengths of 532 nm and 1064 nm in a premixed ethylene/air flame using laser-induced incandescence. E(m,1064) increased with the flame height from 0.29 to 0.43. Depending on the height above the burner, the ratio of E(m,1064)/E(m,532) increased from 0.88 to 1. These changes in the optical properties are attributed to differences in the soot structure, such as the different sizes of graphene planes, which increased from 0.96 to 1.24 nm as the height above the burner increased from 10 to 20 mm. The presented data provide comprehensive information about the optical properties of soot in a premixed ethylene/air flame according to the height above a burner and make it possible to compare measurements of the soot optical properties in various target flames.