Wind Tunnel Evaluation of an Aircraft-Borne Sampling System

Abstract

The U.S. Environmental Protection Agency (EPA), the Florida Department of Environmental Protection (FLDEP), and Texas A&M University collaborated in the design, construction, and testing of a unique, highly crosslinked, Teflon-coated inlet and manifold gas and aerosol sampling system that is being used in EPA aircraft atmospheric pollution characterization studies. The aircraft-borne ambient sampling system, which consists of a Teflon-coated shrouded probe coupled to a Teflon-coated aluminum manifold, is designed to collect reactive gases (e.g., mercury and halide species) and aerosols for subsequent analysis and characterization. The shrouded inlet probe was tested for particle transmission ratios in a high-speed aerosol wind tunnel. An existing wind tunnel was upgraded from a maximum wind speed of 13.4 m/s (48 km/h or 30 miles/h) to 50.5 m/s (182 km/h or 113 miles/h) to test this probe. The wind tunnel was evaluated for compliance with the criteria of ANSI 13.1 to establish the acceptability of its use in testing probes. The results demonstrated that the velocity and tracer gas concentration profiles were within the specified limits. A wellcharacterized ThermoAndersen Shrouded Probe (Model RF-2-112) was also tested to check tunnel performance and test methodology. The results obtained from these tests are in close agreement with earlier published data. When operated at a sampling flow rate of 90 L/min, the aircraft-borne shrouded probe showed a transmission ratio of about 0.76 at 45 m/s (162 km/h or 100 miles/h) for 10 μ m aerodynamic diameter particles. To improve the transmission ratio of the sampling probe, the sampling flow rate was reduced to 80 L/min and the air speed increased to 50.5 m/s, which increased the transmission ratio to about 0.9 for 10 μ m particles. Further reduction of the flow rate to 60 L/min increased the transmission to 1.2. The Teflon-coated manifold, which is located downstream of the shrouded probe, was statically tested for transmission ratio at flow rates of 90 L/min and 30 L/min. The results were a transmission ratio of about 0.80 for 10 μ m aerodynamic diameter particles. The combination of the shrouded probe operated at 60 L/min with a transmission ratio of 1.2 and the manifold with its transmission of 0.8 will give an overall transmission of about unity for 10 μ m aerodynamic diameter particles at a flight speed of 50.5 m/s. These findings suggest that shrouded probes can be used for low speed (∼ 100 miles/h) aircraft applications. The transmission ratio of these probes is a significant improvement over the conventional aircraft-mounted, sharp-edged isokinetic diffuser-type inlets.