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

Abstract

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

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

“…We have described a new experimental method for acquiring unambiguous data for a variety of thin-walled and blunt probe situations (Su & Vincent, 2002), and have reported experimental data for those situations (Su & Vincent, 2003, 2004. Those data were shown to be consistent with those from the earlier studies for comparable sets of conditions, yet went further to include extended sets of conditions.…”

Evaluation of Aerodynamic Particle Sizer and Electrical Low-Pressure Impactor for Unimodal and Bimodal Mass-Weighted Size Distributions

“…Soft bends of 45 degrees were used in the transport lines to fit the instruments. Su and Vincent (2004) have reported experimental aspiration efficiencies in calm air for samplers positioned upwards with ratios of the sedimentation velocity to the sampling velocity (R c ) of 0.1 and 0.01. For a given inlet Stokes number, the aspiration increases towards 1.0 with decreasing R c .…”

A Numerical Study of Calm Air Sampling with a Blunt Sampler

“…As the model was developed for thin-walled samplers no account was taken for sampler bluntness. More recently, using their comprehensive experimental data, Su and Vincent (2004a) developed empirical formulae for thin-walled and blunt samplers operating in calm air. In the case of a blunt sampler pointing upwards the formula for A is given by:…”

“…Hence, in this work, a study is made of the performance of a blunt sampler operating in calm air. Recently experimental studies were undertaken into the sampling performance of thin-walled, and some simple blunt, samplers by Su and Vincent (2002, 2004a, 2004b. Such studies are invaluable in furthering our understanding on this topic as experimental data with which to validate numerical models is lacking due to the difficulties in obtaining it.…”

“…Such studies are invaluable in furthering our understanding on this topic as experimental data with which to validate numerical models is lacking due to the difficulties in obtaining it. In the case of the thin-walled samplers some numerical work was also undertaken by Su and Vincent (2004a). The experimental method, known as the direct method, involved the direct visual observation of the limiting particle trajectories and hence the results obtained do not include errors due to particle bounce or blow off from the external surface of the sampler.…”