Airflow and Particle Velocities Near a Personal Aerosol Sampler with a Curved, Porous Aerosol Sampling Surface

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Abstract
The airflow and particle-velocity patterns in the vicinity of a curved, porous sampling surface constituting the inlet of an aerosol sampler, have been determined by Laser-Doppler Velocimetry. The tests were performed on a personal aerosol sampler with a concave sampling surface, referred to as the “button” sampler. Measurements with the button sampler were conducted in a low-speed, open-circuit, horizontal wind tunnel using two types of monodisperse aerosols: 2 μm essentially inertialess propylene glycol particles (their trajectories represent the flow streamlines) and 44 μm inertial fly ash particles. The tests were performed at two free stream velocities (0.5 and 4 m/s) and two sampler orientations (facing the wind and facing downward at 90° to the wind). The sampling flow rate was 2 L/min. Sampling through the curved surface was conducted while the button sampler was freely suspended in the flow or while it was attached to a vertical stagnation plate simulating the human torso. The results indicated that the nondimensionalized airflow patterns and particle-velocity fields near the curved inlet surface were similar at different wind velocities, as long as the button sampler faced the wind. When a stagnation plate was added, while facing the wind, the airflow patterns and particle trajectories were not significantly affected. Based on limiting streamline considerations, it was concluded that only a small part of the incoming flow was aspired into the inlet. This limited transparency of the inlet to the incoming airflow caused the deflection of the airflow along the curved surface and explained the low dependence of the airflow patterns on the magnitude of the free stream velocity when the curved inlet surface faced the wind. However, the effects of wind velocity and plate on the airflow and particle velocity became significant when the inlet orientation was changed to 90° to the wind. This suggests that a curved, porous surface is beneficial as an aerosol sampling inlet when the wind direction is perpendicular or close to perpendicular to the surface. However, this benefit is reduced or eliminated when sampling at 90° to the wind.