10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Effect of Eccentricity on the Performance of a Cylindrical Differential Mobility Classifier
THAMIR ALSHARIFI, Da-Ren Chen, Virginia Commonwealth University
Abstract Number: 268 Working Group: Instrumentation
Abstract Scanning/differential mobility particle sizers are now commonly used in aerosol community to characterize the size distribution of submicrometer particles in gases. A differential mobility classifier (DMC) is the key component in the sizers for sizing particles. In the DMC design, it is required to provide the tolerance at least on all the key dimensions of a DMC when the mechanical drawing is produced for machine shops. The tolerance requirement in a DMC design also affects how the DMC is constructed. It is therefore very important to understand the design/construction imperfection on the performance of a DMC. Differential mobility classifiers in various configurations have been designed and evaluated in the literatures. Among different configurations, DMCs in the cylindrical design is the most popular ones because of no side-wall effect encountered in parallel-plate DMCs and their successfully commercialization. As the first step in a series of investigation, this study focused on the effect of eccentricity on the performance of a cylindrical DMC. Numerical approach was taken in this study. COMSOL was applied in the numerical modelling.
In this study, we investigated the effect of eccentricity on the performance of different cylindrical DMCs operated at different aerosol and sheath flow rates. The geometrical parameters of studied DMCs include the diameters of inner and outer cylinders, the length of particle classification zone, and the angles of aerosol entrance and exit. Prior to the parametric study, our modelling result was validated by the comparison with the theoretical transfer function given by Knutson & Whitby (1975). The theoretical transfer function derived by (Knutson, 1972) for an eccentric DMC in the cylindrical design was also included in the comparison.
Our study shows that, because of the eccentricity, the DMC transfer function was deteriorated as the eccentricity increased for Nano-DMA. The peak of DMC transfer function would split into two peaks as the eccentricity (defined as the off-center distance to the annular spacing between two cylinders) is more than 5%. This two-peak appearance of the transfer function would occur at the less eccentricity as the classification length of DMCs. However, the effect of eccentricity would be reduced as the annular spacing between outer and the inner cylinders was increased. More details of our study will be presented at the conference.
References
Knutson, E. O. (1972). The distribution of electric charge among the particles of an artificially charged aerosol. Mechanical Engineering, Ph. D. dissertation.
Knutson, E. O., & Whitby, K. T. (1975). Aerosol classification by electric mobility: apparatus, theory, and applications. Journal of Aerosol Science, 6(6), 443–451.