Simulation of aerosol agglomeration in the free molecular and continuum flow regimes

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Abstract

The formation of high temperature aerosol agglomerates is simulated by following the Langevin trajectory of each particle with the boundary condition that the particles stick upon collision. Both the free molecular and continuum flow are treated. A new derivation of the friction force of an agglomerate in the continuum limit is developed based on the evaluation of the surface momentum flux at the Oseen flow limit. The agglomerates can be described as a fractal, at least in regard to the power law relationship between mass and size, with a dimensionality of 1.7–1.9 independent of the flow regime. The particle growth is shown to be much more rapid in the free molecular regime than in the continuum. The global kinetics are shown to be consistent with a similarity analysis of the coagulation equation with a modified coagulation coefficient. Comparison between the simulation and coagulation theory at small time suggests a slight fluctuation enhancement in the free molecule case and a small-time enhancement of the coagulation rate at high concentration for the continuum case.

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      Assuming a fractal shape, additional parameters such as the fractal dimension and the prefactor can be used to characterize the morphology of these fractal-like clusters. In order to approximate the ballistic and diffusive collision length scales, early work by Mountain et al. (1986), Meakin (1987) and Mulholland et al. (1988) simply used the agglomerate’s radius of gyration (dc = 2 Rg) based on scaling arguments for the geometric cross sections. Rogak and Flagan (1992) proposed a more elaborate model for collision length scales by considering the cross sectional areas, the outer and the mobility diameters of the agglomerates.

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