Role of metal additives in light scattering from flame particulates

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Abstract
The scattering and extinction behavior of multicomponent particles formed in flame systems is significant in a number of research areas and practical applications, such as the production of superconducting and other ceramic materials by means of the aerosol route as well as pollutant suppression through the use of metal additives. The objective of this study is to assess the role of iron pentacarbonyl vapor addition on the light scattering and extinction behavior of flame soot. Calculations were carried out by using the scattering models for homogeneous and coated spheres and comparisons were made between the particle diameters and volume fractions. In addition, scattering, absorption, and dynamic light-scattering measurements at the wavelength of 488 nm in a premixed propane–oxygen flame with a fuel-equivalence ratio of ϕ = 2.4 unseeded and seeded with iron pentacarbonyl vapor 0.32% by weight iron to fuel were performed. The refractive index and number densities of the soot particles in the unseeded flame were determined as functions of position above the burner by combining the scattering and absorption measurements with the particle size-distribution parameters determined from photocorrelation. In the seeded flames the soot particles were found to contain iron oxide throughout the flame. Thus the data were analyzed by using both the scattering–absorption model for coated spheres and the Maxwell-Garnett relation for the effective refractive index. Differences up to 131% in particle-volume fractions were found from the data analysis by using the constant and variable effective index of the mixture (soot plus iron oxide). The results of the coated-sphere analysis are discussed and the effects of particle agglomeration on the inference of particle-volume fractions are assessed. It is concluded that the effects of particle optical inhomogeneity in the analysis of scattering and absorption data from multicomponent particles cannot be neglected.