The Effects of Turbulence on the Entry of Airborne Particles into a Blunt Dust Sampler

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Abstract
The behavior of blunt dust samplers in turbulent air flow has received relatively little attention. This paper describes the development of a theoretical model which aims to identify the important controlling parameters and to form the basis of an experimental inquiry. Physically, the model describes the competition between inertial effects in the distorted mean flow which tend to set up concentration gradients and turbulent diffusion which tends to counteract such gradients. Experiments were carried out in a wind tunnel using a simple axisymmetric disk-shaped blunt sampler facing the wind. Its aspiration efficiency was investigated as a function of particle aerodynamic diameter, turbulence intensity, and turbulence length scale (for fixed sampler dimensions, sampling flow rate and wind speed). Experiments were also carried out to investigate the effects of turbulence on the performances of sharp-edged isokinetic sampling probes. The results of all these experiments were broadly consistent with the theoretical model. The main conclusions are that (a) the performances of blunt samplers are relatively independent of the turbulence properties of the moving air for particles of small aerodynamic size but exhibit an increasing effect with increasing particle size, particularly above about 40 μm; and (b) the performances of sharp-edged probes used as isokinetic samplers should, in most cases, be independent of turbulence.