TRANSLATIONAL BROWNIAN DIFFUSION-COEFFICIENT OF LARGE (MULTIPARTICLE)SUSPENDED AGGREGATES

Aggregates (composed of large numbers of ''primary'' particles) are pr oduced in many engineering environments. One convenient characterizati on is the fractal dimension D-f, the exponent describing how the numbe r of primary particles in each aggregate scales with radial distance f rom its center of mass. By viewing each ensemble of aggregates of fixe d size N as a radially nonuniform but spherically symmetric ''porous s olid'' body, we describe a finite-analytic, pseudocontinuum prediction of the drag, and corresponding translational Brownian diffusivity, fo r a fractal aggregate containing N (>>1) primary particles in the near -continuum (Kn << 1) regime. While Stokes' equation is used to define the creeping Newtonian flow outside the aggregate, Brinkman's equation is used inside, with suitable matching conditions imposed at R(max) = [(D-f + 2)/D-f](1/2)R(gyr), where R(gyr) is the familiar gyration rad ius. A rational/accurate correlation technique is developed to rapidly estimate drag for an aggregate with any self-consistent combination o f N, D-f, and Kn(2R1). Our numerical results/rational correlations all ow prediction of aggregate deposition rates via the mechanisms of Brow nian diffusion and/or inertial impaction, modeling sol reaction engine ering systems involving aggregate Brownian coagulation and in interpre ting dynamic light scattering measurements on aggregate populations.