Several nozzles have been designed to aerodynamically focus aerosol particles into a small-diameter jet, so that individual particles can be illuminated by a laser beam and their light scattering and/or laser-induced fluorescence (LIF) spectra can be measured. An additional nozzle can aerodynamically puff selected particles out of the airstream so that they can be sorted and collected.
In single-particle LIF and elastic scattering measurements, it is desirable that the particles be focused into as narrow a stream as possible (as small as 20 μm diameter), and for the particles to remain collimated for a distance of a few millimeters. Particles having different sizes and shapes should flow at the same speed and the same trajectory in the particle stream as it moves away from the nozzle. To help the particles flow in a collimated stream, an eduction tube was used a short distance (about 1 cm) below the nozzle.
The first aerodynamic focusing nozzle was a single-piece nozzle that looks similar to a 30° cone. Originally, plastic glass was used to make the nozzle, but it did not work well, possibly because of static charges. The first nozzle that worked well was machined from aluminum. This nozzle produces a laminar aerosol flow with an aerosol jet diameter of a few hundred microns at a flow rate of 0.6 to 2.1 L/min. Individual aerosol particles (1 to 10 mm size) within the jet move at about 10 m/s when the flow is nominally 1 L/min.
The second-generation nozzle was designed to measure two-dimensional angular optical scattering (TAOS) over very large angles using an elliptical mirror. In this setup, the laser and particle interrogation region (located at the mirror focal point) is located well below (more than 1") the nozzle exit. At this distance, the aerosol stream is no longer well focused. The nozzle assembly was modified so that it could be inserted into the relatively small (0.40") opening of the mirror and close to the mirror focal point. This nozzle functions similarly to the first-generation nozzle, but with a better focusing capability.
The third-generation nozzle assembly was developed with a sheath flow. The inner nozzle of the assembly has similar design to the second-generation nozzle, but with a separate outer nozzle for a clean-air sheath flow. This nozzle can produce a tightly focused aerosol jet of particles having relatively uniform speed over distances of more than 5 mm. The nozzle provides for a well-defined interrogation region, and also prevents the contamination of optics by preventing sampled aerosol from circulating in the optical cell.
The machining process posed a particular challenge, because the inner surfaces needed to be joined smoothly, with no abrupt changes in curvature, and the exit hole needed to be small (0.9-mm diameter). The external portions of the nozzles were machined in a more conventional manner using computer numerical control (CNC) lathes and milling machines running programs written by computer-aided machining (CAM) software. The close tolerance of concentricity of the two nozzles was achieved by placing a perforated ring at the end of the inner nozzle. This ring formed a close sliding fit to the outer nozzle. Fabrication of the nozzle with the desired shape was accomplished using EDM technology. First, a copper tungsten electrode was turned on a CNC lathe. The geometry of the electrode matched that of the inner surface to be machined. Next, the electrode was precisely aligned over the nozzle, and the EDM process initiated. Roughing and finishing electrodes were used to produce the desired finish on the inside surface.
This work was done by Yong-Le Pan, John Bowersett, Steven C. Hill, Ronald G. Pinnick, and Richard K. Chang of the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Mechanics/Machinery category. ARL-0099
This Brief includes a Technical Support Package (TSP).
Nozzles for Focusing Aerosol Particles
(reference ARL-0099) is currently available for download from the TSP library.
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