TABLE OF CONTENTS Diffusional transport in flowing fluids is called convective diffusion. Particle deposition on surfaces by this mechanism is of fundamental importance to the functioning of gas-cleaning equipment, such as scrubbers and filters, as well as aerosol measurement systems, such as the diffusion battery and certain types of filters. Convective diffusion contributes to the scavenging of small atmospheric particles by raindrops, as well as removal by vegetation and other surfaces, and is a significant mechanism of deposition in the lung. The particle size at which convective diffusion is effective depends on velocity and external force fields, but is usually in the submicron range.
The intensity of the Brownian motion increases as particle size decreases (Chapter 2). As a result, the efficiency of collection by diffusion for particles smaller than about 0.5 ?m increases with decreasing particle size; as shown in this chapter, certain gas-cleaning devices are most efficient for the removal of very small particles.
In what follows, the equation of diffusion derived in Chapter 2 is generalized to take into account the effect of flow. For point particles ( d p = 0), rates of convective diffusion can often be predicted from theory or from experiment with aqueous solutions because the Schmidt numbers are of the same order of magnitude. There is an extensive literature on this subject to which the reader is directed. For particle diffusion, there is a difference from the usual theory of convective diffusion because of the special boundary condition: The concentration vanishes...
