Deposition of Particles in a Chamber as a Function of Ventilation Rate

Abstract

ABSTRACT Particle deposition in indoor air is due primarily to turbulent diffusion to the boundary layer at macroscopic surfaces within the room. About 15 years ago, Crump and Seinfeld derived an equation for turbulent diffusion and deposition in an arbitrarily shaped vessel. They assumed that turbulent diffusivity near the wall is proportional to a power of the distance from the wall surface and an estimation of the turbulence intensity. Their values were found by fitting an equation with two parameters. Subsequently they showed that this theoretical result agreed with experimental data. However, other studies in which the turbulence intensity has been directly estimated, there have been problems fitting the measured deposition rates with the integer exponent needed for dimensional consistency. To eliminate this problem, Benes and Holub have recently proposed a new expression for the eddy diffusion coefficient based on the rules of dimensional analysis. However, their formulation did not include the gravitational sedimentation factor and is good only for very small particles. When a simple sedimentation term is added to this formula, the applicable size range can apparently be extended to the micrometer size range. The purpose of this present study is to investigate the deposition of particles as a function of size and exhaust ventilation rate, and also to test the ability of the Benes and Holub formulation to model the observed behavior. It appears that the approach of Benes and Holub provides reasonable fits to the data and thus, provides a useful extension to the Crump and Seinfeld model. These present results are still insufficient to fully describe turbulent deposition of particles in enclosed spaces, but does provide an alternative starting point for developing more complete models that can take convective flows, wall roughness, and ventilation into account in estimating wall deposition of particles.