THE SOOT OPTICAL PROPERTIESIN PREMIXED ACETYLENE/AIR FLAME MEASURED BY LASER-INDUCED INCANDESCENCE

To understand the behavior of the optical properties of soot in the visible and near-infrared spectral range, it is crucial to determine the magnitude of their emission and absorption. Such information is necessary for a 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 and 1064~nm in a premixed acetylene/air flame (equivalence ratio~1.8) using laser-induced incandescence (LII). The results have shown that E(m,1064) increased with the flame height from~0.19 to~0.53. Depending on the height above the burner, the ratio of E(m,1064)/E(m,532) increased from~1.12 to~1.55. These changes in the optical properties are attributed to differences in the soot structure, such as the different distances between graphene planes inside soot crystallites, which decreased from~0.43 to~0.36~nm as the height above the burner (HAB) increased from~5 to~20~mm. The presented data provide comprehensive information about the optical properties of soot in a premixed acetylene/air flame depending on the HAB and make it possible to compare the measurements of the soot optical properties in various target flames.