Comparison of Experimental and Numerical Studies of the Performance Characteristics of a Pumped Counterflow Virtual Impactor

Abstract

Experiments and Computational Fluid Dynamic (CFD) simulations were performed to evaluate the performance characteristics of a Pumped Counterflow Virtual Impactor (PCVI). The diameter at which 50% of the particles were transmitted was determined for various flow configurations. Experimentally determined 50% cut sizes varied from 2.2 to 4.8 micrometers and CFD predicted diameters agreed within ±0.4 micrometers. Both experimental and CFD results showed similar transmission efficiency (TE) curves. CFD TE was always greater than experimental results, most likely due to impaction losses in fittings not included in the simulations. Ideal transmission, corresponding to 100% TE, was never realized in either case due to impaction losses and small-scale flow features such as eddies. Areas where CFD simulations showed such flow recirculation zones were also found to be the locations where particulate residue was deposited during experiments. CFD parametric tests showed that PCVI performance can be affected by the nozzle geometry and misalignment between the nozzle and collector orifice. We conclude that CFD can be used with confidence for counterflow virtual impactor (CVI) design. Modifications to improve the performance characteristics of the PCVI are suggested.

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“…The maximum TE observed is also similar to that reported by both Boulter et al (2006) and Kulkarni et al (2011), with an imperfect TE being attributed to losses on the internal fittings of the PCVI. However, the D 50 measured was consistently higher than that measured for very similar flow settings by Kulkarni et al (2011) (case number 5), with a D 50 of 4.9 µm in our characterization, as compared to 3.21 µm in Kulkarni et al (2011). As it is of the utmost importance to remove all interstitial particles from the sample flow, the higher cut-off size is not detrimental for our purposes, and based on our laboratory characterization the performance of the PCVI was deemed adequate.…”

“…The measured transmission pattern is in line with the idealized TE curves derived from CFD simulations in Kulkarni et al (2011), to which the reader is referred for an in-depth discussion of the curve morphology. The maximum TE observed is also similar to that reported by both Boulter et al (2006) and Kulkarni et al (2011), with an imperfect TE being attributed to losses on the internal fittings of the PCVI.…”

“…While misalignment does not have a negative effect on rejection of small particles (i.e. interstitial aerosol and droplet residues would still be removed from the sample flow), it has been shown to cause a substantial decrease in transmission efficiency if it should occur (Kulkarni et al, 2011). Nonetheless, the particles detected and characterized downstream of the ISI still represent residues of small ice crystals, albeit only a small fraction of all available.…”

“…Characterization of the PCVI transmission efficiency has been performed in several previous studies (Boulter et al, 2006;Kulkarni et al, 2011). As a result, we have focused solely on validating the characterization for those flow settings that were deemed relevant to our inlet setup.…”

“…Downstream of the PPD-2K, interstitial particles and residual particles released from the droplets in the droplet evaporation unit are removed from the sample flow with the use of the commercially available pumped counterflow virtual impactor (PCVI, model 8100, Brechtel Manufacturing Inc. (BMI), USA; Boulter et al, 2006;Kulkarni et al, 2011) which separates particles based on their inertia. Particles with insufficient inertia to overcome a counterflow are removed, while particles above a certain aerodynamic cut size are transmitted.…”