In-situ optical measurement of additive effects on particulates in a sooting diffusion flame

Abstract

Mie scattering of visible laser light by soot particulates has been used to determine their size, number density, and volume fraction in a laminar diffusion flame with and without metallic additives present. In general, additives alter the preceding particulate properties. Under certain circumstances, the volume fraction is reduced significantly below that which occurs in the unseeded flame, and soot abatement results. Data have been obtained for all the alkali chlorides, which were present initially in solutions of varying molar concentration, and which were then aspirated in propane/oxygen flames diluted with nitrogen. The size, number density, and volume fraction of soot present are evaluated from scattered light intensities at 45 and 135°. The scattering results from argon-ion laser radiation at 5145Å incident on the particulates in the flame. The number density of alkali atoms is determined from the flame temperature and the excited state emission intensities, where the intensities are first corrected for quenching due to collisions of the excited alkali atoms with other flame species. The number density of alkali cations is evaluated as well from an assumed Saha equilibrium among atoms, cations, and electrons, wherein the previously mentioned atom density and flame temperature are used. The soot volume fraction is shown to be strongly correlated with the number of additive cations. A qualitative explanation of the additives action is given with reference to an ionic theory of soot formation.