A two-stage circumferential slot virtual impactor for bioaerosol concentration

Abstract

Slot virtual impactors provide an efficient low power method of concentrating aerosols. A circumferential slot virtual impactor (CSVI) is especially effective because it has a continuous slot, and, therefore, has no losses associated with the ends of the slot. A CSVI can also fit a longer slot in a smaller footprint than a linear slot virtual impactor. A two-stage circumferential slot virtual impactor system has been designed and tested. The CSVI units are similar in principle to that tested by Haglund and McFarland (2004). Specific geometric changes to the nozzle region were introduced based on the numerical models of Hari (2005). The greatest change to the nozzle geometry of Haglund and McFarland (2004) is the introduction of a radius on the accelerator nozzle. The radius on the accelerator section allows larger particles to make a smoother transition into the focused jet. The smoother transition reduces the amount of wall losses for larger particles. The geometric changes show a significant increase in the particle size range that the virtual impactor can effectively concentrate. The extension of the dynamic range of the improved geometry was evident in the results for both the 100 L/min first stage and the 10 L/min second stage CSVI units. The two stages were tested individually and in series where the nozzle Reynolds number was 250 for both units.The results of the experiments on the two stage CSVI system showed a peak collection efficiency of 90%. The first and second stage had a Stokes cutpoint of 1.2, corresponding to a particle size of about 2.5