The complex refractive index of particles in a flame

To understand heat transfer within and from a particle laden flame requires the knowledge of the radiation absorption and scattering properties of the particles. These properties can be defined in terms of the complex refractive index. A literature survey failed to reveal measurements of the complex refractive index of high temperature particles of any material. The most suitable experimental method appeared to be that of Avery (1952) and the variations of the method (discussed by Humphreys-Owen 1961) were investigated. The complex refractive indices were measured on a flat, macrosized sample of spectral graphite, on particles of SiO₂ and pulverized coal, all at room temperature and on pulverized coal in a flame at about 1700 K. The experimental method adopted used a photomultiplier and amplifying system to detect the modulated monochromatic light reflected by the sample, the polarized components of the reflected light being selected by a motor driven polarizing chopper. Two methods of calculation were compared and it was concluded that the least squares method of calculation using a computer was preferred to the more generally used graphical method. Some of the assumptions made and the limitations to the theory are discussed, including a modified definition of the complex refractive index for particles of various shapes.