AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Improving the Resolution of Low Pressure Impactor
ANSSI ARFFMAN, Jaakko Yli-Ojanperä, Jorma Keskinen, Tampere University of Technology
Abstract Number: 126 Working Group: Instrumentation and Methods
Abstract Low pressure impactors in a cascade configuration are widely used to measure the aerodynamic particle size distribution of aerosol sample. Besides the particle size measurement, LPIs have been used for example in studying agglomerate particle bond strength by impacting nanoparticles in the LPI and observing the degree of disintegration as a function of particle impaction energy. In both applications it is beneficial to have an LPI with steep collection efficiency curve. Steep collection efficiency provides possibility to bring stage cutpoints closer to each other without interference of successive stages in the size distribution measurement. It also provides uniform impaction conditions and impaction energies for particles in the whole jet domain.
We studied experimentally and with numerical simulation methods the critical LPI parameters that affect the resolution or the steepness of the collection efficiency curve. Two LPI geometries, rectangular slit and round nozzle with 25nm cutpoint were investigated. Based on the simulation results, a new impactor stage with exchangeable nozzle throat length and jet-to-plate distance was designed, built and tested. The impactor was calibrated with four different configurations using monodisperse dioctyl sebacate aerosol. A good agreement between the simulated and the experimental resolutions and cutpoints was found. The best observed resolution was achieved with the slit type LPI that had very small nozzle throat length to width ratio. Compared to corresponding impactor stages of commercially available LPIs, the resolution of the new stage is by far the highest.