Experimental Measurements and Numerical Calculations of Aspiration Efficiency for Cylindrical Thin-Walled Aerosol Samplers in Perfectly Calm Air

Abstract

A large experimental study has been conducted to make definitive measurements of the aspiration efficiencies of idealized cylindrical thin-walled aerosol samplers in perfectly calm air, using a method that involved the direct visual observation of falling streams of particles of well-defined size for sampling under specific conditions of sampling flowrate and orientation (downwards facing and horizontal). These data augment an earlier set of experimental data for upwards-facing sampling, and those results are included again in this article for the sake of completeness. In addition to the experimental study, a numerical study was also carried out, first for the purpose of comparison with the experimental results in ranges where such results are available, and second for providing information in ranges where experiments could not be conducted satisfactorily. From this combined approach, a very comprehensive set of new data was generated. In general, the experimental and numerical results were seen to be in very good agreement for the ranges of conditions where data were obtained using both methods. For all cases it was shown in general that aspiration efficiency decreases with increasing Stokes' number (representing inertial forces) but increases with decreasing ratio of particle settling velocity to sampling inlet velocity (representing gravitational forces). From all the data it was seen that the detailed relationship between aspiration efficiency for the various sampler orientations is different for various particle inertia regimes. In the small St_c regime, aspiration efficiency is highest for upwards-facing sampling, with that for downwards-facing and horizontal sampling being about the same. In the intermediate St_c regime, aspiration efficiency for horizontal sampling is greater than for downwards facing, which in turn is greater than for upwards facing. In the large St_c regime, aspiration efficiency for upwards-facing sampling is greater than for horizontal, which in turn is greater than aspiration efficiency for downwards facing.