Aggregate characteristics accounting for the evolving fractal-like structure during coagulation and sintering

Abstract

Coagulation and partial coalescence (or sintering) frequently results in fractal-like aerosol structures in natural and industrial processes. The asymptotic form of such structures is described reasonably well with the so-called fractal dimension, Df. Little is known, however, for its evolution, from spheres to fractal-like particles and, in particular, its effect on aerosol primary particle and collision diameters that determine the environmental impact or manufactured product performance. So the effect of a variable or constant Df on product crystalline (TiO2) and amorphous (SiO2) aerosol particle characteristics is elucidated over their process synthesis parameter space ( maximum temperature 1600–2000K, cooling rate 103–106K/s and precursor molar fraction 10−4–10−1). Aerosol dynamics by coagulation and sintering are simulated accounting for the evolving fractal-like structure by either a linear interpolation or detailed mesoscale simulations from spherical to asymptotic fractal-like structures. In addition, two sintering rates for SiO2 as well as expressions for the effect of particle structure on sintering rate are compared in terms of product particle characteristics. Neglecting the evolution of Df hardly affects the product primary particle and soft-agglomerate diameters but overestimates the agglomerate collision diameter growth rate during the hard- to soft-agglomerate transition. This underpredicts the hard-agglomerate diameter by 25–30% at high cooling rates (105–106K/s).