An Instrument for the Classification of Aerosols by Particle Relaxation Time: Theoretical Models of the Aerodynamic Aerosol Classifier

Abstract

A new aerosol particle classifier, the aerodynamic aerosol classifier (AAC), is presented and its classifying characteristics are determined theoretically. The AAC consists of two rotating coaxial cylinders rotating at the same angular velocity. The aerosol to be classified enters through a gap in the inner cylinder and is carried axially by particle-free sheath flow. The centrifugal force causes the particles between the rotating cylinders to move in the radial direction and particles of a narrow range of particle relaxation times exit the classifier through a gap in the outer cylinder with the sample flow. Particles with larger relaxation times impact and adhere to the outer cylinder and particles with smaller relaxation times exit the classifier with the exhaust flow. Thus, the aerosol is classified by particle relaxation time from which the aerodynamic equivalent diameter can easily be found. Four theoretical models of the instrument transfer function are developed. Analytical particle streamline models (with and without the effects of particle diffusion), like those often used for mobility classifiers, are developed for the case when the centrifugal acceleration field is assumed to be uniform in the radial direction. More accurate models are developed when this assumption is not made. These models are the analytical limiting trajectory model which neglects the effects of diffusion and a numerical convective diffusion model that does not. It is shown that these models agree quite well when the gap between the cylinders is small compared to the radii of the cylinders. The models show that, theoretically, the AAC has a relatively wide classification range and high resolution.

Generation of a Monodisperse Size-Classified Aerosol Independent of Particle Charge

“…Figure 3 shows the comparison of the theoretical and experimental response spectra of the AAC-DMA setup using balanced flows for three different AAC resolutions. Like the DMA, the ratio of the sheath flow to aerosol flow in the AAC changes the transfer function width as shown by Tavakoli and Olfert (2013). In general, the experimental results (shown here and in the Supplementary Information) agree fairly well with the theoretical response spectrum when the sheath to aerosol flow ratio is 10.…”

“…One AAC prototype was built, tested, and compared with the AAC particle streamline model developed by Tavakoli and Olfert (2013). The AAC prototype was tested over a range of particle sizes.…”

“…The particle streamline model developed by Tavakoli and Olfert (2013) showed that the particle relaxation time at the maximum of the transfer function, τ * , is…”

“…This letter describes the first prototype of the AAC and experimental results are used to validate the model of the transfer function developed by Tavakoli and Olfert (2013).…”

“…This can often lead to ambiguity (or error) in the interpretation of the results or require the use of charge-correction schemes. Tavakoli and Olfert (2013) have presented theoretical models of the transfer function of a new classifier called the aerodynamic aerosol classifier (AAC). The AAC classifies particles by their relaxation time, τ , defined as…”