Circumferential-Slot Virtual Impactors with Stable Flow

Flow instabilities in a virtual impactor designed for bioaerosol concentration (size range about 2 to 10 mu m AD) can seriously degrade performance. When the flow in a 100 L/min circumferential slot virtual impactor (CSVI) was unstable, the transmission efficiency was 30% for 7.2 mu m AD aerosol particles, but when the instability problems were corrected, the transmission efficiency was increased to above 90%. Three-dimensional CFD simulations have been used to examine flows in two CSVIs, a nominal 10 L/min device in which the flow was stable, and the 100 L/min device in which the flow was initially unstable. From the CFD flow patterns in the 100 L/min device, the principal instability was in the minor flow region and was caused by overly rapid flow deceleration, too large of a volume, and too low of a jet velocity in that region. Changes were made to the geometry of the CSVI, and CFD was used as the diagnostic tool to determine when stable flow was achieved. Also, for the 100 L/min unit, wake effects from alignment posts that hold together the two halves of the CSVI propagated into the receiver section, and CFD analyses were used to modify the post locations to optimize the transmission efficiencies for the stable units. Numerical and experimental results show the dynamic ranges (ratio of the largest Stokes number for which the transmission efficiency is 50% to the cutpoint Stokes number) are about 100 for both devices. The peak value of the transmission efficiency for the 100 L/min unit is 95% and that for the 10 L/min device is 97%.