Removal of aerosol particles from stationary air within porous media

Abstract

Aerosol particles suspended in stationary air within a closed system are subject to diffusional and/or gravitational deposition. In the present study monodisperse droplets in the diameter range between 0.09 and 1.5 μm and with a density of 0.912 g cm −3 were inhaled by a normal subject and the losses of particles in the pulmonary air spaces were determined as a function of the period of breath holding t p . In an analogous way various granular bed filters were filled with aerosol and emptied again after a certain pause t p . For particle diameters d < 0.15 μm, particle losses in the lung and in granular bed filters were found to be a function of the diffusion length (Dt p) 1 2 ( D: diffusion coefficient). For particle diameters d > 0.8 μm sedimentation dominates and the removal of particles in the lung (filter) is a function of the sedimentation length vt p ( v = settling velocity). The functional relationships found between particle losses, residence time, diffusion coefficient, settling velocity and air-space dimensions agree with the formula given by Landahl (1950). Particles between 0.15 and about 0.8 μm diameter undergo combined diffusional and gravitational deposition. Experimental results obtained for this intermediate range show that an independent superimposition of diffusion and sedimentation overestimates particle deposition; in order to fit the experimental points an interactional term had to be subtracted from the superposition of the two mechanisms as independent effects.