AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Aerosol Aggregation in the Transition Regime
THASEEM THAJUDEEN, Suhrid Deshmukh, Christopher Hogan Jr., University of Minnesota
Abstract Number: 24 Working Group: Aerosol Physics
Abstract As aggregates form frequently in aerosol synthesis processes, it is important to understand how the exact morphology of aggregates arises, such that their shape can be predicted in a given synthesis system. Unfortunately, most prior studies investigating the mechanisms of aerosol aggregation have focused solely to either the continuum or free molecular regimes, which are specific limits of transport. Owing to the characteristic sizes of the common aerosol particles, the mass and momentum transfer processes leading to aggregation occur in the transition regime for both momentum transfer (drag) and mass transfer (collisions). Therefore, the morphology of produced aggregates is dependent on the Knudsen number (the momentum transfer scaling parameter) and diffusive Knudsen number (the mass transfer scaling parameter). It is presently unknown how these numbers (which decrease as aggregation occurs) explicitly evolve and influence subsequent aggregate growth. In this study we model aggregate formation across different mass and momentum transfer regimes, with to date the most rigorously correct model of the physics involved. Through scaling arguments it is shown that the mass transfer processes involving aggregates can indeed be fully described using aptly defined Knudsen number and diffusive Knudsen number, both of which depend on two different size descriptors, namely the Smoluchowski radius and the orientationally averaged projected area. On non-dimensionalizing the equations of motion for aggregates, it is further found that both the diffusive Knudsen number and Knudsen number influence the motion of the aggregates/particles. Brownian dynamic-based cluster-cluster aggregation simulations are performed, for which the inputs are the initial Knudsen number, initial diffusive Knudsen number, and particle volume fraction. It is shown that these three parameters fully govern the time evolution of aggregates.