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

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 Row 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 pm; and (b) the performances of sharp-edged probes used as isokinetic samplers should, in most cases, be independent of turbulence.

Sampling Errors in Cylindrical Nozzles

“…(25) at u,(t) = (u&(t) -t tga = ~o l ( t ) /~o l l ( t ) -The use of a thick-walled probe can be expected to radically change the situation: the external flow turbulence will effect the flow around the sampler varying the spatial distribution of particles at its orifice. This suggestion has also been recently made by Vincent et al (1985) and confirmed by them in their experiments. There are good reasons to believe that, even with thin-walled probes at yaw orientation with respect to the wind, the aspiration distortions will also be influenced by the turbulence since the A?)…”

“…Mainly, aspiration distortions have been studied experimentally with axisymmetric samplers facing the wind (e.g., Zenker, 1971;Levin, 1972, 1974;Gibson and Ogden, 1977;Jayasekera and Davies, 1980;Vincent and Gibson, 1981;Davies and Subari, 1982;Grinshpun et al, 1985;Vincent et al, 1985;Chung and Ogden, 1986;Lipatov et al, 1986a;and others). According to Boothroyd (1971), Cadle (1975), and Belyaev et al (1981), these samplers are widely used in practical aerosol sampling.…”

Use of Numerical Calculations to Simulate the Sampling Efficiency Performance of a Personal Aerosol Sampler

“…Turbulence intensity at the inlets and within the sampler, provided it is less than about 10%, should have little effect on the sampling efficiency for aerosol particles with Stokes numbers less than 1 (Vincent et al 1985;Wiener et al 1988), which is the case for these simulations. Turbulence intensity in the simulations was maintained at 5% at the inflow of the computational domain, and 3% inside the sampler based on experimental values of similar flows (Smith and Bird 2002).…”

“…Turbulence intensity in the simulations was maintained at 5% at the inflow of the computational domain, and 3% inside the sampler based on experimental values of similar flows (Smith and Bird 2002). The range of potential length scales was defined by estimates of the Kolmogorov microscales and experimental values (Vincent et al 1985). They were defined to be 0.01 m at the inflow to the domain (maximum), and equivalent to the minimum grid size at the respective sampler outlet, which was 0.0005-0.0010 m. These lengths are expected to cover the range of possible scales that will evolve for these flow scenarios.…”

Comparison of coarse coal dust sampling techniques in a laboratory-simulated longwall section

“…[12] For turbulent conditions characteristic of high-velocity mine air, isokinetic sampler performance may not be affected by turbulence for sampling probes with sharp-edge inlets. [13] However, blunt inlets may decrease aspiration efficiency for large particles, especially those above 40 μm. [13]…”