Improving Virtual Impactor Performance via Nozzle Optimization

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.