Blunt Sampler Research Articles

On the efficiency and correction of vertically oriented blunt bioaerosol samplers in moving air

The aspiration efficiency of vertical and wind-oriented Air-O-Cell samplers was investigated in a field study using the pollen of hazel, sweet chestnut and birch. Collected pollen numbers were compared to measurements of a Hirst-type Burkard spore trap. The discrepancy between pollen counts is substantial in the case of vertical orientation. The results indicate a strong influence of wind velocity and inlet orientation relative to the freestream on the aspiration efficiency. Various studies reported on inertial effects on aerosol motion as function of wind velocity. The measurements were compared to a physically based model for the limited case of vertical blunt samplers. Additionally, a simple linear model based on pollen counts and wind velocity was developed. Both correction models notably reduce the error of vertically oriented samplers, whereas only the physically based model can be used on independent datasets. The study also addressed the precision error of the instruments used, which was substantial for both sampler types.

A Numerical Study of Calm Air Sampling with a Blunt Sampler

The performance of an idealized spherical sampler facing both vertically upwards and downwards in calm air is studied numerically. To describe the air flow around the sampler, both potential and viscous flow models have been adopted. The equations of particle motion are then solved to calculate the aspiration efficiency. The dependence of the aspiration efficiency upon the various parameters of importance in calm air sampling are investigated and compared where possible with the experimental work of Su and Vincent (2003, 2004a, b). It is found that in the case of upwards sampling the bluntness of the sampler only has a significant effect upon aspiration for large sampling velocities, values that would not generally be physically realistic. In the case of downwards sampling an important non-dimensional quantity, B 2 R C , is identified, where B represents the sampler bluntness and R C represents the gravitational effects. This quantity determines the physical conditions for which aspiration will not occur ...

Experimental measurements of aspiration efficiency for idealized spherical aerosol samplers in calm air

In a recent previous paper (Su & Vincent, J. Aerosol Sci. 33 (2002) 103) we described a new method by which to investigate the relationships between aspiration efficiency, particle inertia, gravitational effect and sampling orientation for aerosol sampling in perfectly calm air. All previous experimental work to elucidate the basic nature of aerosol sampling in calm air described has been carried out for thin-walled tubes, and none has yet been reported in relation to blunt samplers. To begin to fill this important gap, the present paper describes the application of our new method towards acquiring new measurements of aspiration efficiency for simple blunt samplers. Experiments were carried out to determine the aspiration efficiencies of simple, idealized, spherical blunt samplers for a range of sampling scenarios, for two sizes of blunt samplers and different sampling inlet diameters, and for upwards and downwards sampling scenarios (and hence a range of governing dimensionless physical quantities). It was shown that aspiration efficiency decreased both with increasing inertia (as represented by the Stokes’ number) and with increasing gravitational effect (as represented by the ratio of particle settling velocity to the air velocity at the sampler inlet). The results enabled qualitative physical explanation of the difference between what was observed for upwards and downwards-facing sampling, respectively, in terms of (a) the role of the sampler body in deflecting the air flow in the region close to the body of the sampler (in turn influencing the performance of the sampler), and (b) the interception of particles in the downwards-facing scenario falling within the ‘shadow’ projected upwards by the sampler body. The significant contribution of this work has been the acquisition of a definitive set of new experimental data. These data will be valuable in the future development of understanding of the physics underlying aerosol sampler performance. Such knowledge will be of practical value because (a) blunt samplers are generally the most representative of the types of instruments used in practical occupational, and (b) calm or slowly moving air is characteristic of many indoor situations.

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

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.

Aerosol Aspiration Efficiency of Blunt and Thin-Walled Samplers at Different Wind Orientations

The measured aerosol aspiration efficiency of a thin-walled probe and 5 blunt (thick-walled) samplers was used to compare the aspiration efficiency calculated using the models developed by Belyaev and Levin (1972, 1974) and by Vincent and his colleague (1987, 1995). At 0° wind direction, the model developed by Belyaev and Levin agrees quite well with the experimental value for both the thin-walled probe and the blunt samplers. In this case, the blunt sampler body diameter ( D b ) was used to replace the inlet diameter ( D i ) for the calculation of the Stokes number (St). At 90° and 180° wind directions, the model developed by Vincent and his colleague agrees fairly well with the experimental value. This study shows that the model developed by Belyaev and Levin, at 0° wind direction, can be modified to calculate the aerosol aspiration efficiency by using personal samplers as area samplers. For the model developed by Vincent and his colleague, in addition to personal sampler application, at 90° and 180° wi...

Aspiration characteristics of idealized blunt aerosol samplers at large angles to the wind

The research described in this paper extends the body of work contained in earlier publications in which aerosol sampling aspiration efficiency was studied in a small wind tunnel for simple, idealized sampler geometries. These studies are aimed at providing generic knowledge about the physical and functional behaviors of such systems that can then be generalized to sampling systems of more practical interest. In this paper, attention is focused on the little-researched case of a blunt sampler facing away from the prevailing wind velocity. The results show, that for angles greater than 90 ∘ with respect to the wind, aspiration efficiency and particles losses inside the sampler close to the entry are constant as a function of α. This in itself may have some practical usefulness since it enables the definition of a regime of aerosol performance that is relatively “well-behaved”. These results are in marked contrast to what has been learned from the same body of work for sampler behavior when facing forward with respect to the wind. Here not only does aspiration efficiency vary sharply with orientation, but so too does the particle entry loss. From this it is concluded that no general model of aspiration efficiency can be available so long as experimental results embody unknown entry loss biases. With this in mind, caution is recommended in field sampling or in laboratory experiments where the measurement does not explicitly include such entry losses.

Aspiration efficiency for thin-walled nozzles facing the wind and for very high velocity ratios

The study of thin-walled aerosol sampling probes facing the wind was originally driven by practical applications (e.g., stack sampling). But more recently it has provided useful scientific insights into the performances of more complicated blunt aerosol samplers. Previous work has been carried out for a relatively narrow range of conditions, especially for R (the ratio of freestream air velocity to probe suction velocity), leading to the widely accepted model of Belyaev and Levin (J. Aerosol Sci. 5 (1974) 325). This model has been shown to predict aspiration efficiencies (for sharp-edged, thin-walled samplers facing the wind) that agree well with experimental data, for R ranging from 0.03 to 11. Recently, there has been renewed interest in studying aspiration for a much wider range of R, driven primarily by the desire for a new generation of particle size-selective blunt samplers, including the need to sample at lower flowrates in order to use lighter sampling pumps for industrial hygiene applications. The study of thin-walled sampling for wider ranges of R, especially in the range above about 10 is the first step towards improved understanding of blunt sampling as it might be applied to practical samplers under such conditions. This is because much of what is currently known about the aspiration of aerosols into blunt samplers derives from what is known about the less complex thin-walled sampler configurations. New experiments have been conducted for thin-walled cylindrical sampling probes for R ranging from 0.5 to 50. The experiments were conducted in a small wind tunnel (cross-section 0.30 m×0.30 m) using narrowly graded fused alumina powders to produce aerosols with mass median particle aerodynamic diameters up to 90 μm . The external wind velocity was held constant at 1 m/ s , so that Stokes’ number ( St) ranged from about 0.05 to 3.7. An extensive, highly repeatable set of experimental results showed that, for R increasing above about 6, the Belyaev and Levin model increasingly overestimated aspiration efficiency compared to what was measured. An improved model for aspiration efficiency of thin walled sampling probes facing the wind is proposed that describes what happens over this much wider range of conditions.

Effect of orientation and freestream velocity on the sampling efficiency of a blunt body sampler.

Effect of orientation and freestream velocity on the sampling efficiency of a blunt body sampler.

Experimental investigation of air flow around blunt aerosol samplers

To evaluate better the aerosol collection behavior of blunt samplers, the flow field around a two-dimensional cylindrical sampler and a spherical sampler is investigated experimentally. The velocity along the symmetric axis of the cylindrical sampler is measured with LDV. The measurements are in very good agreement with viscous numerical calculations. The stagnation distance and the location of the separation point on the spherical sampler is studied by flow visualization. Smoke is employed to illuminate the fluid flow around the sampler. When the sampler faces the wind and the ratio of suction velocity to free-stream velocity U s/ U 0 > 1, the experimental measurements are in excellent agreement with theoretical inviscid calculations. For U s/ U 0 ≤ 1, the theoretical predictions are higher than the measurements. When the sampler is oriented at an angle to the oncoming flow, the stagnation distance on a symmetric plane is consistent with the distance predicted by a theoretical inviscid calculation. Unfortunately, on an asymmetric plane, the experimental method breaks down and accurate results cannot be obtained. An appropriate experimental technique for this asymmetric study remains to be developed. The separation point on a spherical sampler at different orientations and different velocity ratios is also experimentally examined. With suction, the farther the sampler rotates, the stronger the influence of the suction on the location of the separation point. After rotating to angles greater than 90 °, the separation point reaches an asymptotic value. This asymptotic value depends on the velocity ratio. At different velocity ratios, higher suction rates push the separation point further back, and the location of the separation point and its asymptotic value moves backward with an increase of the velocity ratio. When the sampler opening locates past the separation point, the flow near the opening entrance becomes turbulent, and laminar or inviscid flow models are inadequate to calculate aspiration efficiency

Experimental studies of the aspiration efficiencies of blunt aerosol samplers at large angles to the wind

Experimental studies of the aspiration efficiencies of blunt aerosol samplers at large angles to the wind

Re-entrainment of particles on the outer wall of a cylindrical blunt sampler

In this paper the effects of the operating conditions on the re-entrainment of particles on the front surfaces of a flat disc and a cylindrical blunt sampler have been investigated. The fluid flow around the sampler, or flat disc, is first assumed to be inviscid and the governing equation has been solved using a finite-difference technique. Then the boundary-layer flow on the sampler, or disc, wall has been numerically investigated. Attention is concentrated on the operating conditions under which a particle may not be re-entrained into the boundary-layer flow near the front surface of the body. Both a static and a rotational model have been developed to investigate the re-entrainment of the particle and details of the location of the edges of the deposition ring of particles on the front surface of the body has been presented. It is found that particles tend to be more likely to be displaced from the surface of the body by rotating rather than sliding on it. Given the operating conditions, it is possible to predict accurately the edges of the deposition ring of particles on the front surface of the blunt body and the present results show very good agreement with some related experimental data.

39 P 12 The effect of the filter on the turbulent flow around and inside a blunt sampler

39 P 12 The effect of the filter on the turbulent flow around and inside a blunt sampler

Use of the boundary element method to determine the sampling efficiency of blunt aerosol samplers

Blunt samplers are widely used in occupational hygiene and environmental air monitoring in order to determine the concentration of particles in the atmosphere. The rate of aspiration will depend on the features of the fluid flow, which are influenced by the sampler, and also the nature of the particles. Although past research has led to a greater understanding of the performances of blunt samplers, there is still a need for a better understanding of the factors which govern the performances of such devices. In this work the Boundary Element Method is adopted to model the problem. This allows for an accurate description of the fluid behaviour near the sampler to be obtained and hence the behaviour of the particles in the flow determined. This results in an accurate prediction of the aspiration efficiency of the sampler which is necessary when using such devices. The method is particularly useful for this problem as it can easily be adapted to any shape of sampler with sampling taking place at any location on the sampler body. It also makes it possible to include the finite size of the sampling orifice in the model which has not been possible in previous mathematical investigations.

Recent advances in aspiration theory for thin-walled and blunt aerosol sampling probes

In this paper, an attempt is made to generalize and extend what has already been learned about the physical basis of the aspiration efficiencies of aerosol samplers. It begins with the assumption that all such samplers are, in fact, aerodynamically blunt since their physical presence inevitably imposes some restriction to the movement of the surrounding air. The thin-walled probe, which has already been the subject of much interest, is just such a limiting case. Other forms of blunt sampler have received less attention. The paper describes an improved theoretical model for aspiration efficiency which is derived from knowledge of the broad features—rather than the details—of the distorted air flow near a blunt sampler. Such features include, for example, the location on the sampler body of stagnation, indicating where the airflow divides between that which is sampled and that which passes by outside the sampler. This is dependent not only on the sampling flow rate and wind speed but also on the aerodynamic ‘bluntness’ of the sampler body. The model was tested against experimental results already published for thin-walled probes and a range of less-idealized sampling systems. The comparison is good except for very blunt samplers at orientations other than facing the wind. This is believed to be associated with the fact that we do not yet know how to account for changes in aerodynamic bluntness with increasing yaw angle. In view of the wide practical importance of such work to the development of samplers with characteristics consistent with the latest particle size-selective recommendations for aerosols in workplaces and in the ambient atmosphere, the need for further experimental work is identified.

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

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.

Particle Blow-off: A Source of Error in Blunt Dust Samplers

From blunt sampler theory it is possible to predict over-sampling for simple single-orifice, axisymmetric “total dust” samplers (such as the Casella type T13032) arising from particle blow-off from external surfaces. Wind tunnel experiments confirmed the predicted broad trends, revealing the errors in question to be as large as 50% for the sampling of dry dusts under some conditions of practical relevance. The results suggest that the laboratory evaluation of dust samplers in general ought to be carried out using test aerosols whose physical characteristics approximate as closely as possible those expected in practice.