A Mathematical Study of the Sampling Characteristics of a Thin-walled Sampler Operating in Calm Air

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 ftowrate 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 Stc regime, aspiration efficiency is highest for upwards-facing sampling, with that for downwards-facing and horizontal sampling being about the same. In the intermediate Stc regime, aspiration efficiency for horizontal sampling is greater than for downwards facing, which in turn is greater than for upwards facing. In the large Stc regime, aspiration efficiency for upwards-facing sampling is greater than

Section: Discussionmentioning

confidence: 98%

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

2004

“…The physics of aerosol sampling in calm air is different from that of sampling in moving air as noted by Vincent (1989) and Dunnett (1992). In calm air, gravity plays a predominant role in particle movement, whereas in moving air, convective transport of particles often dominates and gravity can be neglected or treated as a secondary influence.…”

Section: Introductionmentioning

confidence: 92%

“…For particles much larger than this size, the value of settling velocity is large and U, is relatively small. In this case, U, is negligible and the equation of aspiration efficiency can be simplified to the form indicated by Vincent (1989) and Dunnett (1992).…”

Section: Aspiration Efficiency and Numerical Proceduresmentioning

confidence: 99%

The Effects of Bluntness and Orientation on Two-Dimensional Samplers in Calm Air

Chung

Dunn‐Rankin

1993

This article uses numerical simulation to investigate the effect of sampler bluntness, particle size, and sampler orientation on aspiration efficiency in calm air. The procedure is to first numerically solve the velocity field around the sampler in calm air and then to trace the particle trajectories and calculate the the aspiration efficiency. Two samplers are studied: a two-dimensional parallel plate and a two-dimensional -blunt cylinder. The variation of aspiration efficiency with particle size shows two minima between two asymptotic values. When the samplers are facing upward, the asymptotic values are 1 for very small particles and the ratio of particle settling velocity to suction velocity for very large particles. At other orientations, the horizontal-facing and the downward-facing, the asymptotic value for large particles is 0. The sampler bluntness has an important effect in the region of particle size where there is competition between the particle inertia and the fluid drag force (i.e., 5 y m < d < 100 y m in our case). A blunt sampler always has higher aspiration efficiency than does a sharp-edged sampler in this region of particle sizes. For very small particles and very large particles, the aspiration efficiencies approach asymptotic values and the sampler bluntness has little effect. The results also show that the sampler orientation affects the predicted aspiration efficiency.

“…The results of experimental studies of the aspiration efficiency for thin-walled and thick-walled tubes operating in still air were presented by Kaslow and Emrich (1974), Gibson and Ogden (1977), Yoshida et al (1978), Belyaev and Kustov (1980), Grinshpun et al (1989), and Su and Vincent (2002). Mathematical models for thin-walled tube sampling in still air were developed by Agarwal and Liu (1980) and Dunnett (1992). The aspiration efficiency for the twodimensional blunt sampler operating in calm air at horizontal and upward and downward vertical orientations was studied by Chung and Dunn-Rankin (1993).…”

Section: Introductionmentioning

confidence: 99%

Numerical Study of the RespiCon Sampler Performance in the Calm Air

Zaripov

Koch

2013

Results of a numerical study of the RespiCon sampler performance in the calm air are presented. The air flow is described by the Navier-Stokes equations of axisymmetric stationary viscous flow of incompressible fluid that are numerically integrated by the computational fluid dynamics (CFD) software FLUENT. The collection efficiencies of RespiCon impactor stages agree quite well with experimental data and curves of the European standards for the thoracic and respirable dust fractions. The aspiration efficiencies derived from the numerical model overestimate the experimental data in the range of particle sizes of 10 μm < d p < 40 μm; however, they correctly predict the value of maximal size of aspirated particles. A new design of the RespiCon sampler with a higher volume flow rate was developed.