On the Simultaneous Measurement of the Diameter of Soot Particle and Its Optical Constant in Flames

Abstract

The laser light scatter transmission methods of G. Mie’s theory have been used to study the behavior of soot particles in diffusion flames. These methods have made it possible to measure with some accuracy the diameter, number and density of soot particles in steady and non-steady flames. However the optical constant of soot particles is an uncertain element in determining their diameters. A soot particle is an absorptive substance, its chemical composition is unclear, and its optical constant differs according to the temperature and the wave length of the incident beam. It is very difficult to determine accurately the complex refractive index (the optical constant). This index has long been estimated by measuring the energy reflecting power or by theoretical equations of dispersion models. Most typically, Senftleben et al. [1] actually measured the index within a visual region by using the highly accurate Drude’s method. Dalzell et al. [2] made the soot pellets with acetylene and propane gas flames and obtained the index by measuring the reflected energy within a wave length range of 0.4358μm to 10.0μm and by calculations based on a dispersion model. The two results agree well with each other. Prado et al. [3], after examining the soot particle diameters in a methane and oxygen premixed flame using argonion laser beams and various optical constants, reported that the particle diameter is estimated to be smaller as the refractive index becomes larger and the absorption factor smaller, and that the particle diameters according to Dalzell et al. is about 20% larger than that according to Chippet et al. [4] They concluded that the former measurement is better. Under these circumstances, it is necessary to develop and establish a method of simultaneously measuring the diameter of soot particles and their complex refractive index in a non-contact way using some means which can enhance the time and space resolution in flame. This study has established a method for measuring both of them. It is based on multiple pieces of information obtained from the combined use of the light scatter and transmission methods. This method has been tested in actual flames.