Swipe-based explosive trace detectors rely on thermal desorption to vaporize explosive particles collected on a swipe. The vaporized material is carried by air flows from the desorption unit to the inlet of the chemical analyzer, typically an ion mobility spectrometer. We have observed that the amount of explosives detected from a swipe varies with the location at which the explosives are collected on the swipe. There are two issues which may contribute to this effect: inhomogeneous or insufficient heating of the swipe during desorption and low velocity air flows that poorly transport desorbed vapor during the instruments analysis time. To better characterized this effect, we have simulated the air movements within a generic desorption unit using commercially available computational fluid dynamics software. Simulations are three dimensional, symmetric and solved under steady, laminar flow conditions. The calculated velocity field correlates directly with experimental detector response with the high explosive RDX. Results suggest that the limiting factor in this generic thermal desorption unit is not heating the swipe to a sufficient temperature, but the flow-field around the swipe and flow rate into the detector. Buoyancy effects due to heating dominate the flow-field and produce a vertical bulk fluid motion within the domain which opposes much of the flow drawn into the analyzer.
Citation: Journal of Fluids Engineering-Transactions of the Asme
Pub Type: Journals
thermal desorption, trace detection, inkjet printing, explosives