MORPHOLOGY AND DYNAMICS OF AGGLOMERATED PARTICULATES IN COMBUSTION SYSTEMS USING LIGHT-SCATTERING TECHNIQUES

Knowledge of the morphology and dynamics of particulates in combustion systems is important because of its implications in critical research and practical areas as: (a) studies of the anisotropic characteristics of particulates in reacting systems, (b) particle growth and oxidation studies in flame systems, (c) production and control of ceramic aerosols with specific sizes and shapes, (d) studies of the agglomeration mechanisms and in situ determination of the characteristic fractal dimensions in fractal systems (e) mixing rates in multiphase systems and prediction of radiative transport from such systems, and (f) fine particle control in the electrons industry. The in situ light scattering and extinction techniques in the context of particulate sizing and morphology determination are reviewed and compared. The classical light scattering, extinction and scattering intensity ratios techniques may yield accurate particle size when the refractive indices of the particles are accurately known. However, the inferred particle size from the measured scattering and extinctions ratios may change by 75% or more when the real part of the refractive index varies in the range 1.3-2.0. On the other hand, the inferred total particle mass may change by a factor of two or more for the same range of values of the real part of the refractive index. Furthermore, the scattering intensity ratios that have been used for size measurements vary by 30% or more for particle diameters larger than 0.15-mu-m and the changes in the real part of the index by 15% or higher. The applications of the single particle light scattering theory to the characterization of agglomerates that consists of Rayleigh size units are reviewed and the limitations are discussed. Since apart from the classical scattering and extinction techniques the dynamics isotropic light scattering has been used for sizing of spherical particles based on their translational diffusion in flame systems, its advantages and limitations are also presented. Furthermore, it should be noted that most particulates in reacting systems depart from sphericity because of coagulation and/or surface growth and become optically anisotropic. Since in such cases the depolarized light scattering is suitable for particle shape characterization, comparisons are made between the gaseous molecular anisotropic scattering and the scattering corresponding to the anisotropic particulates. The suitability of the technique for particle morphology characterization in flames is assessed. In addition, the new approach of characterizing flame particulates using fractal analysis is summarized and its limitations are discussed.