HYDRODYNAMICS OF FRACTAL AGGREGATES WITH RADIALLY VARYING PERMEABILITY

Hydrodynamic properties of fractal aggregates with radially varying pe rmeability are investigated in terms of two parameters: radius of equi valent solid sphere experiencing the same drag as the aggregate (Omega radius of aggregate) and fluid collection efficiency (eta) of the agg regate. Resistance to the fluid flow through the aggregate is predicte d to increase with increasing fractal dimension, while the fluid colle ction efficiency is expected to decrease. It is shown that a reasonabl e estimate of the fluid drag force on a fractal aggregate may be obtai ned by assigning a constant volume-averaged porosity to the aggregate and using any of the expressions available in the literature for aggre gates with uniform permeability. The two hydrodynamic parameters, Omeg a acid eta, are used to modify the existing expressions for interactio ns between solid spheres to account for the porous nature of aggregate s and thus calculate the collision rate kernels for interacting aggreg ates. The ratio of hydrodynamic radius of an aggregate to its radius o f gyration predicted by the proposed model was in reasonable agreement with an experimental value reported in the literature. (C) 1996 Acade mic Press, Inc.