Aspiration Efficiencies Of Aerosol Samplers Research Articles

Towards a general semi-empirical model for the aspiration efficiencies of aerosol samplers in perfectly calm air

In previous papers we have described a body of experimental work, backed up by numerical studies, to acquire a large set of new data for the aspiration efficiencies of aerosol samplers of simple geometries in perfectly calm air and for a range of orientations. The samplers studied were idealized thin-walled and spherical blunt probes, and experiments were conducted for upwards, downwards and horizontal sampling. Taken as a whole, this is believed to be the most comprehensive data set yet obtained for aerosol sampling in calm air. In this paper, we have examined this complete data set—including both experimental and numerical data—and used it to construct a set of semi-empirical models for aspiration efficiency. The starting point for these is the basic physical model of Levin (1957) for the entry of particles into a point sink in calm air. Additional terms are introduced that account for the role of the sampler having finite dimensions and defined shape. The models are expressed in terms of groups of dimensionless combinations of variables that embody the effects of particle inertia, gravitational settling, sampler bluntness and sampler orientation with respect to the vertical. They are semi-empirical in that, although they thus embody physical mechanisms and ideas, the added terms (beyond the Levin-model starting point) containing those dimensionless quantities are incorporated into mathematical constructs that are entirely empirical. The constructs are such that they may be combined together in a single universal form that reverts back to the individual specific cases when those conditions are inserted. Agreement is generally good across the whole range of conditions studied. The small number of areas where agreement is less good may be explained in terms of experimental error or technical limitations in acquiring the experimental data.

New experimental studies to directly measure aspiration efficiencies of aerosol samplers in calm air

Typical air movements in many indoor living and working places are as low as, or less than, 0.1 m/ s , and so conditions might best be described in terms of calm, as opposed to moving air—or perhaps some combination of the two. However, the history of aerosol sampling science has been dominated by studies of the aspiration efficiencies of sampling devices in moving air, and there have been very few definitive studies of calm air sampling, even for very simple sampling systems. With this in mind, this paper sets out the basis for the development of a larger body of experimental work, and describes an experimental method by which unambiguous measurements may be made for a range of simple aerosol sampling systems (i.e., shape and orientation). The method, referred to as the “direct method”, owes much to the ‘thread-of-droplets’ approach proposed by Lipatov et al. ( J. Aerosol Sci. 17 (1986) 763) for studies of aspiration efficiency in moving air. In the new method, monodisperse large droplets of olive oil with aerodynamic diameter in the range from 40 to 70 μm , were generated using a vibrating orifice aerosol generator and the experimental conditions were controlled very carefully so that a well-defined, vertically-falling droplet stream was obtained. By the application of appropriate tracking apparatus, visual observation of this droplet stream was used to map the pattern of particle trajectories near a given sampling inlet and to identify limiting particle trajectories in order to calculate aspiration efficiency. In an initial application of the new method, measurements of aspiration efficiency were carried out for upwards-facing thin-walled sampling tubes, and the results were found to be in satisfying agreement with the numerical results of Agarwal and Liu (1980, in the Am. Ind. Hyg. Assoc. J. 41 (1980) 191) and the numerical and experimental results of Yoshida et al. ( Kagaku Kogaku Ronbunshu 4 (1978) 123). The results of this initial study provide confidence that the method described may now be applied to a wider range of sampling systems.

Impaction Model for the Aspiration Efficiencies of Aerosol Samplers in Moving Air under Orientation-Averaged Conditions

This paper applies what is already known about the aspiration efficiency of blunt aerosol samplers at large angles to the interesting case of sampling under “orientation-averaged” conditions. This is particularly relevant to practical aerosol sampling in workplaces and the atmospheric environment. The resultant semiempirical model of aspiration efficiency contains a number of coefficients which are fitted by nonlinear regression to data sets for the human head and for two rotating-head inhalable aerosol samplers (a 3 L/min sampler intended for applications in workplaces and a 70 L/min sampler intended for particle sampling in the ambient atmosphere). Agreement for all three data sets (with 88, 64, and 37 records respectively) is generally good, with an overall R 2 corr of 68%. Such semiempirical models can be useful for predicting and interpreting sampler performance until practical versions of more rigorous mathematical and numerical models become available.

Impaction model for the aspiration efficiencies of aerosol samplers at large angles with respect to the wind

This paper starts off by recognising that the development of a full theoretical description of the aspiration efficiencies of samplers like those used in real-life situations is still some way off. As an interim contribution to solving the problem, it describes how the simple impaction model approach, which has previously been widely applied to thin-walled sampling probes and to blunt samplers under very idealized conditions, can be extended semi-empirically to describe the performances of contrasting types of sampler—thin-walled probes and the human head—oriented at 90° and 180° to the wind, respectively. Physically-based models for aspiration efficiency at these orientations are suggested and their coefficients found by non-linear regression against the limited amount of relevant data available in the literature. Agreement between such models and the experimental data is found to be quite good. Such models have the potential to provide useful insight into the performances of practical sampling devices. Furthermore they can provide valuable guidelines for the development of more rigorous approaches to blunt sampler theory.

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.