10th International Aerosol Conference
September 2 - September 7, 2018
America's Center Convention Complex
St. Louis, Missouri, USA

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Penetration Efficiency of Sub-3 nm Particles in the Sampling Line of an Electrical Mobility Size Spectrometer

YUEYUN FU, Mo Xue, Runlong Cai, Juha Kangasluoma, Jingkun Jiang, Tsinghua University

     Abstract Number: 278
     Working Group: Instrumentation

Abstract
Due to the high diffusivity of sub-3 nm particles, their diffusion loss through the instrument sampling line is large. In addition, the often used Gormely-Kennedy (G-K) formula for penetration efficiency through a straight cylindrical tube may fail for sub-3 nm particles. The G-K formula is an asymptotic analytical solution of the convection diffusion equation and is applicable when particle concentration at the inlet is uniform. However, the boundary conditions used to derive the G-K formula are usually violated in practical use, especially for sub-3 nm particles. In addition, when the flow field changes in the sampling line, particle concentration will redistribute in the cross-sectional profile that may result in larger concentration gradient near the wall and thus additional particle loss. Thus, penetration efficiencies through three-way connectors, elbows, bends, and core-sampling tubes can be different from values predicted by the G-K formula. Experiments and numerical simulation were performed to study the penetration efficiency of sub-3 nm particles through the sampling line of an aerosol electrical mobility spectrometer. Straight cylindrical tube, elbow, three-way connector, and core-sampling tube were characterized. The penetration efficiency through a soft X-ray neutralizer and a DMA were also evaluated. Strategies to improve the sampling efficiency of sub-3 nm particles were tested. For instance, properly designed core sampling tube (2 LPM core-sampling flow, 8 LPM transport flow, 60 cm in length) can increase the sampling efficiency of 1.5 nm particle by 26%, comparing to a straight cylindrical tube with the same length and total flow. The penetration efficiencies through each parts of the spectrometer sampling line were integrated together to properly recover size distributions of sub-3 nm particles.