Quantifying Residence Times in Neutralizers to Improve Temporal Resolution of Measurements

ROBERT T. NISHIDA, Mino Woo, Tyler J. Johnson, Jason S. Olfert, University of Alberta

     Abstract Number: 443
     Working Group: Instrumentation and Methods

Abstract
Many aerosol instruments utilize bipolar charge conditioners (neutralizers), including the widely-used scanning mobility particle sizer (SMPS). To capture transient changes in aerosol size and concentration, new ‘fast scanning’ techniques (e.g. <15 s scans) have been developed creating new demands for temporal response of the system, including the neutralizer. In this work, we quantify residence times of particles in a common Kr-85 neutralizer (TSI 3077A; TSI Inc.) at two flow rates (0.3 and 1.5 L/min), with and without a flow insert recommended by the manufacturer. We find that at 1.5 L/min nearly all particles pass through the neutralizer in <10 s. At 0.3 L/min, most particles (>80%) pass through the neutralizer in <10 s, however, the remaining particles take significantly longer (>100 s). The wide distribution of residence times indicates significant jetting and recirculation of flow within the neutralizer. The large disparity in residence times would significantly affect ‘fast scanning’ when attempting to capture transients in the aerosol. The relatively long (>100 s) delay for some particles means the temporal resolution of the SMPS is limited by the flow field in the neutralizer rather than the voltage scan time. Thus, better SMPS temporal resolution can be obtained with better neutralizer design. Furthermore, particle charge may be affected since particles would have drastically different exposure time to ions even in the same neutralizer. We perform detailed computational fluid dynamics (CFD) modelling of the neutralizer and quantify residence time distributions numerically. The simulation results agree well with the experimental data and confirm jetting and recirculation of flow in a sudden expansion (a common fluid dynamics problem). Finally, we present a new neutralizer design, optimized using CFD, which minimizes recirculation and achieves the desired charge levels, and we experimentally demonstrate its suitability for measurements with high temporal resolution.