By Coen van Gulijk, TNO Prins Maurits Laboratory, The Netherlands

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Gas masks are prime protectors of emergency responders in toxic environments. Researchers at the Prins Maurits Laboratory of TNO build, test, and improve gas masks for protection against chemical and biological warfare agents. Gas masks protect the wearer by purifying air in filter canisters. After filtration, the air is distributed inside the gas mask to minimize the thermal load on the face and condensation of water on the eyepieces. TNO develops new designs of gas masks using CAD/CAM methods, and its researchers use CFD to study the masks without the need for building costly prototypes. The term "digital gas mask" has been given to gas masks that are developed and studied in this manner.

3D scanning, Courtesy of Vitronic GmbH., Germany

The first digital gas mask was based on an existing gas mask supplied by the Dutch Ministry of Defense, who funded the project. Creating the computational mesh was a challenge, since a digital CAD/CAM model was unavailable. To generate the gas mask geometry, a different strategy was followed instead: a 3D scan of the gas mask was made using tools from the German company, Vitronic GmbH. The geometry could not be scanned directly because of the complex internal shape of the gas mask while worn on the face. The geometry was therefore built up by scanning three parts: the face piece, which covers the entire face; the nose piece, a smaller mask piece surrounding the nose and mouth that is located inside the face piece; and a mannequin head. Special software was used to unify the parts in a stereolithography (STL) file format. Engineers from Fluent Europe imported the STL file into GAMBIT for meshing. The end result was an unstructured mesh comprised of 290,000 cells. Using this mesh, FLUENT simulations were initiated to study the flow patterns inside the mask that are normally hidden from view, and to study the residence time distribution (RTD).

The studies of flow inside the mask were focused on the vicinity near the eyepieces during the breathing cycle. Because of moisture in the breath, there is a likelihood that water will condense on these surfaces. CFD calculations were used to show that during inhalation, flow from the inlet rapidly introduces a supply of fresh air to the region around the eyepieces in a swirling pattern. This periodic freshening of the eyepieces prevents water condensation from developing.

real gas mask (top left) digital gas mask (top right)
CFD model of a face mask after scanning; external features are omitted and Close-up of flow patterns near the eyepiece

The RTD calculations showed that during inhalation, 25% of the tracer particles (injected at the inlet) leave the face mask at the outlet after the average residence time is reached (computed as volume divided by volumetric flow rate). This simple exercise indicates that there are some dead zones or recirculation regions inside the mask. Future efforts will focus on locating and minimizing these regions using CFD. Over the next year, plans are in place to improve the CFD techniques for the digital gas mask. This tool will help TNO optimize the design of gas masks that are currently being developed using CAD/CAM methods.

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