Abstract
We present the design, simulation, and characterization of the radial opposed migration ion and aerosol classifier (ROMIAC), a compact differential electrical mobility classifier. We evaluate the performance of the ROMIAC using a combination of finite element modeling and experimental validation of two nearly identical instruments using tetra-alkyl ammonium halide mass standards and sodium chloride particles. Mobility and efficiency calibrations were performed over a wide range of particle diameters and flow rates to characterize ROMIAC performance under the range of anticipated operating conditions. The ROMIAC performs as designed, though performance deviates from that predicted using simplistic models of the instrument. The underlying causes of this non-ideal behavior are found through finite element simulations that predict the performance of the ROMIAC with greater accuracy than the simplistic models. It is concluded that analytical performance models based on idealized geometries, flows, and fields should not be relied on to make accurate a priori predictions about instrumental behavior if the actual geometry or fields deviate from the ideal assumptions. However, if such deviations are accurately captured, finite element simulations have the potential to predict instrumental performance. The present prototype of the ROMIAC maintains its resolution over nearly three orders of magnitude in particle mobility, obtaining sub-20 nm particle size distributions in a compact package with relatively low flow rate operation requirements.
Highlights
Particles within a narrow range of mobilities migrate across a channel between two electrodes in the time required to transit the length from the entrance port to a downstream sample extraction port in the counter-electrode, where they exit in a classified sample flow; others deposit on the walls of the differential mobility analyzer (DMA) or are discharged in an exhaust flow
This study demonstrates the ability of the radial opposed migration ion and aerosol classifier (ROMIAC) to classify particles ranging from 1.16 to 20 nm in diameter over a wide range of classifier operating resolutions, while using cross-flow rates less than 40 lpm in a compact package
Experimental results validate empirical relationship dependencies derived from finite element simulations when geometry, flows, and electric field details are accurately captured
Summary
The differential mobility analyzer (DMA) has long been the primary instrument used to measure size distributions of aerosol particles smaller than 1 mm in diameter. This instrument separates charged particles according to their electrical mobilities, Z, in an electric field that is transverse to a particle-free sheath flow (Knutson and Whitby 1975). The Knutson-Whitby DMA (KWDMA) is usually operated with sheath and exhaust flow rates (Qsh and Qex, respectively) 10 times those of the aerosol and classified sample flows (Qa and Qc). DMAs are typically operated with balanced flows, i.e., Qa D Qc and d D 0
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