Analysis of growth of non-spherical silica particles in a counterflow diffusion flame considering chemical reactions, coagulation and coalescence

The evolution of non-spherical silica particles in a counterflow diffusion flame has been studied considering the effects of convection, diffusion, thermophoresis, chemical reactions, coagulation and coalescence. The counterflow geometry provides a one-dimensional flow field along the stagnation point streamline which greatly simplifies the analysis of non-spherical particle growth. Flame analysis of multi-step chemical reactions of hydrogen/oxygen including both oxidation and hydrolysis of SiCl₄ has been done to predict flame temperatures, concentrations of gas species and particle generation. The present prediction of flame temperatures was in good agreement with the previous experimental data. Two-dimensional aerosol dynamics in which both particle volume and surface area are independent variables has been then analyzed to obtain the evolution of non-spherical particles which has been compared with the previous experimental data. Several different models of coalescence of silica particles were studied; viscous flow sintering, atomistic diffusion sintering, fast sintering and hybrid sintering models. The use of hybrid sintering model yielded the best agreement with the previous experimental data. Since the collision cross section of non-spherical particles is larger than that of spherical particles having the same volume, coagulation of particles was obviously shown to be enhanced. The important role of axial particle diffusion has been identified in the counterflow diffusion flame. Bi-modal size distributions were obtained at some flame heights.