The internal fluid mechanics of explosive trace detectors using computational fluid dynamics

Efforts are underway in the Surface and Microanalysis Science Division at the National Institute of Standards and Technology to study the vapor transport mechanisms inside explosive trace detection instruments (ETD's) and produce standard test materials to verify their performance. In most swipe-based ETD's, a woven cloth is swiped across a surface to collect micrometer-sized particles from explosive contamination. The swipe is then introduced into a thermal desorption unit where it is rapidly heated to produce an explosive aerosol or vapor. This vapor is transported to a chemical detector, typically an ion mobility spectrometer, for analysis. Understanding the underlying physics of the flow fields within these instruments allows researchers to design better test materials for calibration and verification. In this work, several ETD thermal desorption units are modeled using computational fluid dynamics (CFD). With CFD, the governing equations of fluid motion are solved numerically for a given model geometry and boundary conditions. CFD allows one to visualize and animate flow patterns, streamlines, and recirculation zones, and reveals how vapor is transported from the surface of a swipe to the chemical analyzer. The flow-fields inside these complex geometries would otherwise be difficult, if not impossible, to observe with traditional experimental flow visualization techniques. The thermal desorption units presented here have geometries representative of what is used in ETD's today. Results suggest that the transport efficiency of desorbed explosives can be optimized if appropriate screening procedures are followed. Issues such as velocity magnitude, pressure differential, transient effects, and buoyancy effects will be discussed.