Improving Virtual Impactor Performance via Nozzle Optimization
2023-05
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Improving Virtual Impactor Performance via Nozzle Optimization
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2023-05
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Abstract
Virtual impactors are aerosol-concentrating devices composed of a nozzle and downstream receiving tube. The majority aerosol flow accelerated through the nozzle is
diverted from transmitting through the receiving tube, yet particles inertially maintain
trajectory into the tube, thereby increasing their concentrations. This study investigated
the effect of nozzle geometry on particle focusing, a phenomenon wherein particles not
only inertially enter the receiving tube but are radially confined to center streamlines.
Specifically, the study aimed to understand how modifications to the nozzle's geometry
can improve particle focusing. For this purpose, around 300 nozzles were simulated, with
both fluid flow and particle trajectories modeled. The extent of focusing was quantified
via a score metric determined by the fraction of particles confined to the inner 10% of the
nozzle exit. The study found that shorter nozzles and simple geometries, such as those
with a ratio of inlet radius to radius of curvature of around two, tend to have higher scores
for particle focusing. The study also suggested the development of a two-stage nozzle to
focus larger and smaller particles separately but concluded that the two stages cannot be
selected independently of each other. It is suggested that the addition of a straight section
between the two stages may allow for independent selection. Two focusing-optimized
virtual impactor geometries were experimentally tested and compared to a reference
geometry not optimized for focusing. All three impactor geometries were also simulated,
and their penetration curves were compared to experimental data. Experimental data
points were found to match well with their simulated curves, indicating accurate
predictions of impactor performance. One of the optimized geometries showed slightly
worse performance at small particle diameters, but no overfocusing at larger particle
diameters, when compared to the reference geometry. The other optimized geometry
showed similar performance at small particle diameters, but significantly improved
penetration at larger particle diameters. Comparing the overfocusing via the receiving
tube collection efficiencies, the two optimized virtual impactors showed 60% and 80%
reductions in receiving tube losses, indicating improved particle focusing. The study
concludes that the trajectory of particles can be controlled solely via the modification of
nozzle geometry, which can be used in conjunction with a virtual impactor to improve the
overall ability of the impactor to concentrate particles.
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University of Minnesota M.S.M.E. thesis. May 2023. Major: Mechanical Engineering. Advisor: Chris Hogan. 1 computer file (PDF); vii, 60 pages.
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Relling, Mckenna. (2023). Improving Virtual Impactor Performance via Nozzle Optimization. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/256942.
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