Deposition of aerosols from turbulent flow through pipes

Abstract

The factors are summarized which control the transfer to an absorbing wall of diffusing substances conveyed through a pipe. When the boundary layer is turbulent the problem is generalized by using dimensionless variables based on the friction velocity of the turbulent flow. The part of the velocity boundary layer across which most of the gradient of concentration of a diffusing substance is established is relatively large for gases but extremely thin in the case of particles. The latter, however, may possess sufficient inertia to allow them to be projected, by the radial resolute of the fluctuation velocity, over distances which range from nearly zero up to the tube radius, according to the size of the particle. Hence, although the diffusion boundary layer may be very thin, the projection distance eliminates the dependence of the radial distribution of concentration upon Brownian diffusion and transfers it to eddy diffusion in the central region of the pipe. In order to calculate the resulting velocity of deposition it was necessary to derive an expression for the eddy diffusion coefficient at all points between the centre of the pipe and its wall; an expression was also deduced relating the normal resolute of fluctuation velocity to distance from the wall. The expression finally obtained for the velocity of deposition is valid over a wide variety of conditions which are limited at the small end of the particle size range by Brownian diffusion becoming appreciable and, at the large end, by the particles having so much inertia that they fail to respond to the velocity fluctuations of the fluid which sweeps them along.