Two experimental configurations of linear slot nozzle virtual impactors wereconstructed and experimentally investigated for use as bioaerosol concentrators. In oneconfiguration, the Linear Slot Virtual Impactor (LSVI), the nozzle was a straight slothaving a length of 89 mm (3.5"). In the second configuration, the Circumferential SlotVirtual Impactor (CSVI), the nozzle was curvilinear following a circular path having adiameter of 152.4 mm (6.0") and the resulting total slot length was 479 mm (18.8").Multiple prototypes of the two configurations were constructed having nozzle widthsthat varied from 0.508 mm (0.015") to 0.203 mm (0.008"). Optical and physicalmeasurements were made of the nozzle dimensions in the critical region of the virtualimpactor units. For the LSVI units the misalignment between the acceleration nozzleand the receiver nozzle was measured between 6 ?m (0.00025") and 29 ?m (0.00114").This represented a range of 2% to 10% misalignment relative to the acceleration nozzlewidth. The CSVI Unit 1 and 2 misalignments were measured to be 15 ?m (0.00061")and 9 ?m (0.00036"), or 10% and 1.8% relative misalignment, respectively. The virtualimpactors were tested with liquid and solid monodisperse aerosol particles. Foroperation at flow rate conditions predicted from the literature to produce a cutpoint of 0.8 ?m AD, an acoustic resonance was observed, corresponding to significant nozzlewall losses of particles and an absence of normal particle separation in the virtualimpactor. The onset of the resonance phenomenon was observed to begin at a nozzleReynolds number of approximately 500 for the LSVI configuration, and 300 for theCSVI configuration. For flow rates just below the onset of resonance, normal virtualimpactor behavior was observed. The value of Stk50 was 0.58 for both devices,corresponding to a particle cutpoint size of 1.1 ?m AD for the LSVI configuration and2.2 ?m AD for the CSVI. The collection efficiency was greater than 72% for all particlesizes larger than twice the cutpoint up to the largest particle size tested (? 10 ?m AD).The peak collection efficiency for both concentrators was greater than 95%.
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