(7d) Simulation of the Hydrodynamic Behaviour of Aggregated Particles

Nanoparticles produced in aerosol processes typically show a highly structured morphology, i.e. they are comprised of primary particles which may be partially sintered. The hydrodynamic behaviour of such aggregates is of great relevance in many applications, e.g. classification in electrical mobility analyzers, gravitational or centrifugal settling, separation by low-pressure impaction, etc. Therefore, it is of great interest to investigate the hydrodynamic behaviour, i.e. the drag forces on aggregates in motion relative to the fluid and the complete motion of such particles in different flow fields, e.g. shear flow or elongational flow. A recently developed method allows simulating realistic aggregate morphologies for given operating conditions during the aerosol formation process accounting for all relevant basic formation mechanisms, i.e. coagulation, sintering and surface growth. In general, one has to distinguish the free molecular regime (typical for nanoscaled particles in gases, especially at reduced pressures), the continuum regime (i.e. particles in fluids) and the transition regime. We will present systematic simulations of the hydrodynamic forces on aggregates for the free molecular as well as the continuum regime. The results for the drag force on aggregates show a significant scatter for single aggregates for changing direction of flow. If one considers an ensemble of aggregates formed at identical conditions and with identical mass their mean drag force (i.e. averaged for each aggregate over all orientations which gives the mean drag force on such an aggregate performing rotational Brownian motion) again show considerable variation of more than 20%. In turn, this means that a monomobile fraction of aggregates may exhibit a substantial width of the volume equivalent diameters. Furthermore, the influence of aggregate size and structure is evaluated for both regimes. In the free molecular regime it is shown that for a given aggregate size there is a unique representation of the drag with respect to the fractal dimension. However, there is no simple unique correlation of the drag with respect to particle volume or other cumulative parameters as e.g. the radius of gyration. Nevertheless, a universal correlation can be derived by a combination of separate correlations for aggregate size and structure. In the case of continuum regime Stokesian Dynamics as well as Lattice-Boltzmann simulations of aggregates in fluids have been performed. Both methods give comparable results but have their distinct advantages in different cases. It is shown that there is no unique representation of the drag force with simple geometrical parameters describing the aggregate geometry. The influence of aggregate structure on the drag forces is discussed in detail. It will be especially emphasized that variations in aggregate structure may either occur by the stochastic process of coagulation even if the aggregates are formed under identical conditions or by discrepancies in the operating conditions. Finally, the potential of agglomerate dispersion in flow fields is investigated by performing simulations of particle motion in defined flow fields including full calculation of all intra-agglomerate forces. This way the disintegration of aggregates can be predicted.