Performance of a Counterflow Virtual Impactor in the NASA Icing Research Tunnel

Abstract

A counterflow virtual impactor (CVI) designed for aircraft use was evaluated at the NASA Icing Research Tunnel in Cleveland, Ohio. Tests were conducted for tunnel speeds of 67 and 100 m s?1, for liquid water contents of 0.23?1.4 g m?3, and for a wide range of droplet median volume diameters (MVDs). For droplet distributions with MVDs between about 30 and 240 ?m, liquid water content (LWC) measured by the CVI agreed with reference values within the uncertainty of the measurements. For a range of LWCs at 30-?m MVD, the relationship was near 1:1, and no systematic dependence of CVI results on LWC or airspeed was observed. For smaller MVDs, the CVI underestimated LWC. Decreased collection efficiency for small droplets can partially explain this effect, but the difference from reference values was larger than expected based on previous calibrations and comparisons with in situ data. Tunnel runs conducted with a flow-straightening shroud around the CVI inlet produced approximately 20% enhancements in LWC at small MVDs, which are expected for these speeds based on previous modeling studies. The effect of large drop breakup on CVI droplet number concentration was evaluated both theoretically and experimentally; drop breakup was predicted to occur for drops larger than 169 ?m at 67 m s?1 and larger than 76 ?m at 100 m s?1. Enhancement in number concentration measured by the CVI was found to be strongly related to observed large drop concentrations, particularly to those in the 312?700-?m-diameter range.