10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Simulations for Estimating Dynamic Shape Factors of Aerosol Aggregates
Aniket Talele, Y.S. MAYYA, Jyoti Seth, Indian Institute of Technology Bombay
Abstract Number: 1106 Working Group: Aerosol Modeling
Abstract Aerosol aggregates are a common occurrence in nature and in various technological contexts, due to the ever-present process of coagulation. Soot particles formed in flames, metallic particles synthesized from vapor phase, particles produced by spray drying of suspensions are some of the well-known examples of aggregated particles. It is crucial to estimate the dynamical shape factors of the aggregates for describing their coagulational and depositional characteristics, in the atmospheric or in the respiratory tract. Unlike single particle systems, the dynamical behavior of aggregates is strongly governed by the interaction between the fluid and the individual constituent units and their spatial distribution. There have been several experimental and theoretical efforts in the past to describe the hydrodynamic behavior of aggregates (Sorensen, 2011). Semi-analytical approaches such as Kirkwood-Riseman approximation and simulation techniques such as Lattice-Boltzmann method or Stokesian Dynamics are some of the approaches used for evaluating their hydrodynamic resistances. Among these Stokesian Dynamics formalism is a comprehensive approach that incorporates fluid-particle and particle-particle interactions among the large number of primary particles. Particles interact via hydrodynamic forces transmitted through the solvent as well as short range inter-particle forces.
In these simulations, clusters of different fractal dimensions are constructed using Diffusion Limited (DLA), Reaction Limited (RLA) and Cluster-Cluster Aggregation (CCA) algorithms. A cluster of particles is represented as an assembly of spheres held together by either rigid bonds or soft Hookean springs. The latter aspect takes into account the change in shape of the aggregates subject to stress fields. Using the Stokesian Dynamics formalism (Brady and Bossis, 1988) configuration dependent hydrodynamic interactions are calculated, which account for the motion of particles relative to the fluid. Using Faxen’s laws, far-field, many-body hydrodynamic forces are calculated by inverting a mobility matrix constructed from the first few terms of multipole expansions for two hydrodynamically interacting bodies. As this method is a good approximation only in the far field, close-range lubrication forces are calculated separately and accounted for in a pair-wise manner. Other non-hydrodynamic forces such as Brownian, colloidal, inter-particle, or external body forces can also be included.
In this talk, simulation estimates for the scaling exponent and pre-factors for drag forces experienced by rigid and deformable particle clusters will be presented. Apart from obtaining relationship between hydrodynamic radius and aggregate mass for different fractal dimensions, these simulations can also shed light on mechanisms of deformation of aggregates under various flow and settling regimes.
References: [1] Sorensen, C. M., Aerosol Science and Technology, 45: 765–779 (2011). [2] Brady J. F. and Bossis G., Ann. Rev. Fluid Mech., 20: 111-157 (1988).