Wind Tunnel Evaluation of a Rotating-Element Large-Particle Sampler

A large-particle sampler was tested in an environmental wind tunnel to characterize the efficiency as a function of particle size and type (solid or liquid). The sampler, which had been developed by another organization, has two slotted-cylinder collection elements which are rotated through air at a tangential speed of 39 m/s. Data from wind tunnel tests show the efficiency of liquid droplet collection to increase with particle size to approximately 50% at 25 μm and then decrease with a further increase in particle size. It is suggested that the air flow patterns created by the sampler produce this anomalous behavior. Results of tests with solid particles show higher efficiencies than are obtained with liquid droplets of the same size. It appears that solid particles, which rebound from collection sites other than a slot, can subsequently enter the slots. Supporting data were obtained on the performance of a single slotted cylinder that was fixed in the discharge region of a small free-jet tunnel. Those results show the impaction efficiency of all sizes of liquid droplets in the slotted cylinder follow the type of trend expected for a circular cylinder, i.e., a curve in which efficiency monotonically increases with size. However, 41-μ m diameter solid particles are re-entrained in the air stream and show reduced efficiency in comparison with liquid aerosol particles.