Size-Dependent Collection Efficiency of an Airborne Counter flow Virtual Impactor

ABSTRACT

A three-dimensional numerical model of an airborne counterflow virtual impactor (CVI) was made, and its steady state airflow velocity field was calculated using a commercially available fluid dynamics code (STAR-CD). Size-dependent impaction and front-end (first 0.2 m) collection efficiencies were determined from the analysis of non-evaporating droplet trajectories calculated from the velocity solutions, assuming the droplets stick upon impaction with inlet surfaces. The general features of the velocity field solutions agree well with a two-dimensional model with idealized geometry. Models were made for two values of counterflow, 1.0 lmin⁻¹ and 3.0 lmin⁻¹, in order to investigate different cut sizes. The locations of the stagnation planes in both models agreed with those calculated from simple geometrical considerations. The impaction efficiency curves show cut sizes of 10.1 μm and 17.0 μm for 1 lmin⁻¹ and 3 lmin⁻¹ counterflows, respectively. Calculated collection efficiencies are in general agreement with results from CVI field measurements.