AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
A New Laminar-Flow Water Condensation Method
SUSANNE HERING, Steven Spielman, Gregory Lewis, Aerosol Dynamics Inc.
Abstract Number: 100 Working Group: Instrumentation and Methods
Abstract Our original, laminar flow water condensation technology uses a “pre-conditioner” followed by a single-stage “condenser”. As applied to the water-condensation particle counters (TSI WCPCs), flow passes through a single, wet-walled tube, the second portion of which is warmer than the first. The heating of the second section creates a large partial pressure of water vapor at the walls. As the water is a smaller molecule than either nitrogen or oxygen, it diffuses into the cooler entering flow more quickly than the flow warms, creating a region of water vapor supersaturation. This approach provides activation of particles as small as 3-5 nm, depending on operating temperatures. However, it has the disadvantage that the dew point of the air exiting the growth region is quite high. Typically WCPCs operate with the first section at 20°C, and the second portion around 60°C. The dew point at the end of the growth tube is close to the 60°C temperature of the heated wall.
To eliminate the high dew point at the exit of our water condensation cell, we developed a two-stage condenser system consisting of a warm-wet walled “initiator” followed by a cool-walled section. The initiator is 25-30% of the length of the original condenser, and the combined length of both sections is the same as for the single stage condenser. The initiator is hot and provides the water vapor that activates the condensational growth. The second, cooler walled section maintains the supersaturation and provides the time for droplet growth while lowering the exit dew point. Modeling shows the activation sizes are unchanged, and the resulting droplet size is only slightly smaller, yet the exiting dew point brings the flow to below saturation at room temperature.