AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Aggregation and Growth Kinetics in the Transition Regime
THASEEM THAJUDEEN, Hui Ouyang, Ranganathan Gopalakrishnan, Christopher Hogan Jr., University of Minnesota
Abstract Number: 85 Working Group: Aerosol Physics
Abstract Aerosol particle growth is driven by particle-particle collisions (coagulation/aggregation) in many instances, most notably high temperature combustion and synthesis systems. Described in this presentation, through dimensionless analysis of the collision process and Langevin based mean first passage time calculations, we have derived a universal collision rate coefficient/kernel (expressed in dimensionless form H) valid for aerosol particles of arbitrary shape across the entire diffusive Knudsen number (Kn$_D: the ratio of the mean persistence path of the colliding entities to an appropriate collision length scale) range for particle-particle collisions, and further extended this collision kernel to account for the influence of van der Waals potentials. Using this collision kernel for spheres, we have used constant number Monte Carlo simulations to examine particle growth via collisions in the transition regime, in which both the average diffusive Knudsen number and momentum transfer Knudsen number (gas molecule mean free path/particle length scale) evolve over time, decreasing monotonically. A universal curve for spherical particles is found, describing the rate of change in diffusive Knudsen number relative to the change in Knudsen number, and this curve is further found valid when van der Waals potentials are considered.
After examining spherical particle growth, we have used Brownian dynamics calculations to examine the evolution in average diffusive Knudsen number and Knudsen number of aggregating particles, i.e. particles which do not coalesce upon collision and therefore do not remain spherical. Through these simulations a distinctly different evolution through the transition regime is found; this indicates that the degree of sphericity of particles undergoing isothermal collisional growth can be determined simply through monitoring the average Knudsen numbers.