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

Abstract

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.