Aspiration efficiencies of disc-shaped blunt nozzles facing the wind, for coarse particles and high velocity ratios

Abstract

In the development of aerosol samplers with desired particle size-selective properties, it is important to understand and be able to predict the physical processes that determine the aspiration of aerosols into their nozzles. An idealized sampling scenario is one involving a disc-shaped blunt nozzle facing the wind. A few earlier studies have investigated this scenario but only for relatively narrow ranges of particle aerodynamic diameter ( d ae) and velocity ratio ( R, the ratio between the air velocity in the approaching freestream and that in the plane of the sampling orifice). The purpose of the current study is to expand this knowledge to larger d ae-values pertaining to the inhalable aerosol fraction and to larger R-values. The issue of sampling at large R is seen to be especially important in relation to our desire to develop low flowrate samplers for occupational hygiene applications. New experiments were conducted for disc-shaped blunt nozzles, varying in disc diameter and orifice diameter, for R ranging from 0.5 to 25. The experiments were conducted in a small wind tunnel using narrowly graded fused alumina powders to produce aerosols with mass median particle aerodynamic diameters up to 90 μm . A highly repeatable set of experimental results showed a broad trend of A increasing with R, with the magnitude of its increase proportional to particle size. This was expected from what is known about aerosol sampling theory. However, a comparison of these data with predictions from an earlier mathematical model showed that, for R increasing above about 2, the predictive model significantly overestimated aspiration efficiency in relation to what was measured in these new experiments. The model, therefore, was modified to include this more expansive set of data, including additional terms involving both R and the dimension ratio ( r, the ratio of the size of the sampling orifice to that of the sampler body). The new, modified model was found to agree very well indeed with the whole data set, as well as with the limited data sets reported earlier. Another notable observation was the relatively small change in A that resulted from a two-fold increase in r. This suggests that within a certain range, r may play only a minor role in determining A. The new model provides physical insights into the performance of blunt samplers when facing the wind, and it will ultimately aid our ability to design new samplers for specific applications.