This technical reference is intended for users of the pumped counterflow virtual impactor and similar devices (Boulter et al., 2006).
The pumped counterflow virtual impactor (PCVI) is designed to separate large aerosols from an air stream and place them in a separate gas flow. Small aerosols and the original air are exhausted to a pump. So far, the main application has been separating ice crystals from aerosols for laboratory or field measurements. If the flow of added gas is warm and dry then the ice crystals evaporate and the residues are available for analysis (e.g. Cziczo et al., 2003; DeMott et al., 2003; Cziczo et al., 2006; Gallavardin et al., 2008)
Flow modeling of the PCVI was conducted by Mr. Afchine during an extended visit to the Chemical Sciences Division. Software and computer resources were provided by Forschungzentrum Juelich. The PCVI has several characteristics that make the modeling difficult and computer intensive. Flow velocities can be large enough to require compressible flow. The flow geometry is complicated and has large aspect ratios that are a challenge for mesh generation.
Flow modeling was performed in three dimensions using ANSYS CFX version 11. The mesh included approximately 2.5 million nodes. The flow field was calculated by solving the Navier-Stockes equation for a steady state, compressible, and turbulent flow. The Shear Stress Transport (SST) k-w based turbulence model was used, which is more accurate in solving stagnation flow and flow with separation. The flow field was calculated, then particles were injected into the assuming that they have no influence on the flow field because of their low concentration. Particles sizes from 1 to 5 microns were examined. The Schiller-Naumann drag force model was used. Additional forces on the particles are the turbulent dispersion forces in regions where the turbulent viscosity ratio is above the value 5. All particles were lost upon a wall collision with no bounce.
Refer to Boulter et al., 2006 for definitions of the termed input, output, pump and add flows as well as details of the flow geometry and experimental tests of the PCVI.
Flow modeling was performed for flows of 2.7 and 5.13 standard liters per minute (slpm) at a pressure of 400 mbar and temperature of 298 K. The add flows were 1.2 slpm for both cases and the exhaust flows were 3.6 and 6.84 slpm. These flows represent a fairly low-speed and medium-speed cases with Mach numbers at the tip of the inlet of about 0.25 and 0.55, respectively. A variety of cross sections of various flow parameters are appended to this description. Most results were common to both flow cases. View higher resolution images.
The modeling here suggests some improvements on the PCVI design:
On the other hand, the losses of particles in the pump line are so large that it seems that minor design changes are unlikely to improve particle transmission enough to allow quantitative sampling of particles smaller than the cut point in the pump flow.
This modeling is not complete. Some important effects that need to be investigated are near-sonic flows, the sharpness of the cut point, and the effects of misalignment from machining tolerances. Some of these have been modeled by Mikhail Pekour, Gourihar Kulkami, and Daniel Cziczo at Pacific Northwest Laboratory.
Another topic for future modeling is investigating the details of how the add flow is inserted in order to try to minimize the cut point. The existing design has outstanding rejection of particles smaller than the cut point, but it would be desirable to reduce the cut point. Could the length of the counterflow region be reduced (yielding a smaller cut point) without allowing transmission of a few smaller particles?
Boulter, J. E., D. J. Cziczo, A. M. Middlebrook, D. S. Thomson, and D. M. Murphy, Design and performance of a pumped counterflow virtual impactor, Aerosol Sci. Technol., 40, 969-976, 2006.
Cziczo, D. J., P. J. DeMott, C. Brock, P. K. Hudson, B. Jesse, S. M. Kreidenweis, A. J. Prenni, J. Schreiner, D. S. Thomson, and D. M. Murphy, A method for single particle mass spectrometry of ice nuclei, Aerosol Sci. Technol., 37, 460-470, 2003.
Cziczo, D. J., D. S. Thomson, T. L. Thompson, P. J. DeMott, and D. M. Murphy, Particle Analysis by Laser Mass Spectrometry (PALMS) studies of ice nuclei and other low number density particles, Int. J. Mass Spectrom., 258, 21-29, 2006.
Gallavardin, S. J., K. D. Froyd, U. Lohmann, O. Moehler, D. M. Murphy, and D. J. Cziczo, Single particle laser mass spectrometry applied to differential ice nucleation experiments at the AIDA chamber, Aerosol Sci. Technol., 42, 773-791, 2008.
DeMott, P. J., D. J. Cziczo, A. J. Prenni, D. M. Murphy, S. M. Kreidenweis, D. S. Thomson, and R. Borys, Measurements of the concentration and composition of nuclei for cirrus formation, Proc. Nat. Acad. Sci., 100, 14655-14660, 2003.