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

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, D_f. 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 D_f 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 D_f 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).