AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
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Coagulation of Agglomerates with Polydisperse Primary Particles in the Free Molecular Regime
Eirini Goudeli, Maximilian L. Eggersdorfer, SOTIRIS E. PRATSINIS, ETH Zurich
Abstract Number: 78 Working Group: Aerosol Physics
Abstract Agglomerate coagulation in the free molecular regime is encountered in the early stage of aerosol formation in both atmospheric and industrial processes. Here ballistic agglomeration of fractal-like particles consisting of polydisperse primary particles (PP) is investigated by discrete element modeling (DEM). The DEM-extracted coagulation rate is compared systematically to that in the literature for spherical (aerosol textbooks) and agglomerate particles consisting of monodisperse PP (J. Colloid Interface Sci. 1986, 114:67−81; Aerosol Sci. Technol. 2012, 46:1174−1186; Langmuir 2015, 31:1320−1327). Increasing the width or polydispersity of the PP size distribution accelerates initially the coagulation rate of agglomerates but delays their attainment of the asymptotic fractal-like structure and self-preserving size distribution without altering these values provided that sufficiently large numbers of PPs are employed. For example, the standard asymptotic mass fractal dimension, Df of 1.91 is attained with clusters containing each, on the average, 50 monodisperse PPs and about 900 polydisperse PPs having geometric standard deviation equal to 3. This transition to the fractal regime with increasing agglomerate size is consistent with mass-mobility measurements. Even though the same asymptotic Df and mass-mobility exponent, Dfm, are attained regardless of PP polydispersity, the asymptotic prefactors or lacunarities of Df and Dfm increase with PP polydispersity. For monodisperse PPs the average agglomerate radius of gyration, rg, becomes larger than the mobility radius, rm when agglomerates consist of more than 40 PPs. Increasing PP polydispersity increases that number of PPs similarly to the required PPs per agglomerated for attainment of the asymptotic Df. Various options for calculation of that PP number as a function of PP polydispersity are compared.