Aspiration efficiency for thin-walled nozzles facing the wind and for very high velocity ratios

Abstract

The study of thin-walled aerosol sampling probes facing the wind was originally driven by practical applications (e.g., stack sampling). But more recently it has provided useful scientific insights into the performances of more complicated blunt aerosol samplers. Previous work has been carried out for a relatively narrow range of conditions, especially for R (the ratio of freestream air velocity to probe suction velocity), leading to the widely accepted model of Belyaev and Levin (J. Aerosol Sci. 5 (1974) 325). This model has been shown to predict aspiration efficiencies (for sharp-edged, thin-walled samplers facing the wind) that agree well with experimental data, for R ranging from 0.03 to 11. Recently, there has been renewed interest in studying aspiration for a much wider range of R, driven primarily by the desire for a new generation of particle size-selective blunt samplers, including the need to sample at lower flowrates in order to use lighter sampling pumps for industrial hygiene applications. The study of thin-walled sampling for wider ranges of R, especially in the range above about 10 is the first step towards improved understanding of blunt sampling as it might be applied to practical samplers under such conditions. This is because much of what is currently known about the aspiration of aerosols into blunt samplers derives from what is known about the less complex thin-walled sampler configurations. New experiments have been conducted for thin-walled cylindrical sampling probes for R ranging from 0.5 to 50. The experiments were conducted in a small wind tunnel (cross-section 0.30 m×0.30 m) using narrowly graded fused alumina powders to produce aerosols with mass median particle aerodynamic diameters up to 90 μm . The external wind velocity was held constant at 1 m/ s , so that Stokes’ number ( St) ranged from about 0.05 to 3.7. An extensive, highly repeatable set of experimental results showed that, for R increasing above about 6, the Belyaev and Levin model increasingly overestimated aspiration efficiency compared to what was measured. An improved model for aspiration efficiency of thin walled sampling probes facing the wind is proposed that describes what happens over this much wider range of conditions.