AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Effect of Upstream Flow Mixing on Charging Capability of Aerosol Neutralizers
JAMES FARNSWORTH, Hans-Georg Horn, TSI Incorporated
Abstract Number: 213 Working Group: Instrumentation and Methods
Abstract Bipolar charge neutralizers (Kr-85, soft X-ray, Po-210, Am-241, Ni-63) are commonly used to predefine the charge distribution of an aerosol upstream of an electrostatic classifier. Charge equilibrium can be predicted depending on aerosol concentration, residence time, and the charger activity level. In practice however, charge equilibrium is assumed and not often verified, which can result in invalid data. In this study we show how a flow recirculation within a neutralizer of a typical experimental setup can result in inadequate neutralization, and how the issue can be mitigated by adding flow mixing elements to the flowpath.
A representative aerosol conditioning setup (generator, dryer, neutralizer) and SMPS were employed to compare the charging capacity of various neutralizers under typical experimental conditions (0.3-3 l/min aerosol flow, polydisperse NaCl, <10^6 #/cc total concentration). An apparent 2X difference in charging capability was observed at some flow rates when using a Kr-85 neutralizer (TSI Model 3077A) versus a soft x-ray neutralizer (TSI Model 3088). Further investigation revealed that internal geometry differences between neutralizer housings were the cause of the disparity; models indicated that particles along the edges of the flow profile were being recirculated by the Kr-85 neutralizer, while particles along the centerline were passed through at severely reduced residence times such that the exiting aerosol was insufficiently charged.
Various means were used to investigate the flow recirculation. By placing a flow restriction of dia. <2.5mm upstream of the neutralizer, or by adding a simple step change of tube diameter within 40mm of the neutralizer inlet, flow patterns inside the housing were sufficiently altered that flow recirculation did not occur.
Suggestions for the prevention of flow recirculation within neutralizers, from the perspective of both the manufacturer and the researcher, will be presented. Potential drawbacks of using flow mixing elements (i.e. diffusion losses) will also be discussed.