Modeling and Simulation of Nanoparticle Coagulation in High Reynolds Number Incompressible Flows

摘要

Ultrafine particles play an integral role in a wide variety of physical/chemical phenomena and processes. These include but are not restricted to synthesis of nanostructured materials (nanoparticles and coatings). Nanostructured materials are expected to play an increasingly significant role in many major industries as we enter the new millennium. There are several technologies which can be employed in the manufacture of nanoscale materials (films, particles, etc). Vapor-phase methodologies are by far the most favored because of chemical purity and cost considerations. The formation of very fine particles from vapor encompasses a large number of physical/chemical phenomena. When driven from gas phase precursors (as is typical in many cases), one must address vapor phase chemistry, particle nucleation and growth (coagulation/coalescence, condensation, etc). Sundaram and Collins investigated the influence of particle parameters on collision frequencies in a turbulent particle laden suspension leading to coagulation and found that the magnitude of the minimum particle collision frequency was more strongly correlated with the turbulent motions at the integral scale. Reade and Collins considered the coagulation and growth of aerosol particles in an initially mono-disperse population of particles subject to isotropic turbulence. Other researchers have used sectional methods in modeling the particulate phase, including extending a one-dimensional sectional technique to two dimensions to obtain the evolution of both particle size and shape during gas phase production of titania and silica powders; Pyykonen and Jokiniemi employed the sectional method in conjunction with a Reynolds-averaged Navier-Stokes solver, to simulate aerosol formation via nucleation, condensation and coagulation. In this work we perform direct numerical simulation (DNS) of a coagulating aerosol in a two-dimensional, incompressible, isothermal planar jet. The evolution of the particle field is obtained by utilizing a sectional model to approximate the aerosol general dynamic equation (GDE). The GDE is written in discrete form as a population balance on each cluster or particle size and describes particle dynamics under the influence of various physical phenomena: convection, diffusion, and coagulation. This representation facilitates the capture of the underlying physics in a time-accurate manner. The goal is thus to facilitate better prediction of fluid-particle systems and to elucidate the underlying structure of vapor-phase particle growth processes.