Aerosol Capture in Granular Beds in the Impaction Dominated Regime

Abstract

Single grain capture efficiencies η for high speed gas flow through clean granular beds are reported. The aerosol particles range in diameter D_p from about 0.6 to 4.5 μm; both liquid (dioctyl phthalate) and solid (potassium biphthalate) aerosol particles were used. The grains used were spheres of diameter D _G equal to 2 or 4 mm and pea gravel with an effective diameter of 4 mm. The superficial gas velocity U was varied from about 0.1 to 6 m/sec. Impaction is the dominant capture mechanism for these conditions. Consequently, the Stokes number Stk = p _p D _p2C _p U/18μD _G is an important dimensionless group for correlation of the data. Because the gas flow field depends on the fraction solids α_G in the bed and the grain Reynolds number Re = _iD _G U/μ, the single grain capture efficiency η also must depend on these two dimensionless groups. For conditions of greatest practical interest, the Stokes number is much less than unity. Theoretical considerations based on creeping flow and boundary layer flow theory suggest that the three independent dimensionless groups can be combined into a single effective Stokes number Stk_eff and, provided that StkRe^½ << 1, η should depend only on Stk_eff. The form suggested for Stk_eff is

where A_H(α_G) is a known function given by Happel's model for flow through packed beds. Our data are successfully correlated by this approach. At sufficiently high velocities, solid aerosol particles exhibit bouncing and attrition. The onset of bouncing appears to be correlated with the particle's kinetic energy.