Evaluation of Mixing Elements in an L-Shaped Configuration for Application to Single-Point Aerosol Sampling in Ducts

Abstract

Experimental data for velocity and tracer gas concentration profiles were collected at several downstream locations of an L-shaped configuration with different mixing elements. Results were presented in the coefficients of variation (COVs) to help determine appropriate locations for single-point sampling downstream of each duct configuration. Comparisons between experimental data and numerical simulations for velocity COVs were also presented. Different mixing elements were applied to the L-shaped configuration to create additional mixing to help provide an acceptable sampling location with low pressure drop for single-point sampling in a duct at less than four duct diameters from the mixing element. The mixing elements included a 90° elbow, a tee, a horizontal generic-tee-plenum (H-GTP) system, and a vertical generic-tee-plenum (V-GTP) system. The COVs for velocity and gas concentration for the two GTP systems were all determined to be less than 13% as compared to a range of 6% to 89% for the 90o elbow and a tee at four duct diameters in round and square ducts. Tests with two different sizes of GTPs were conducted and the results showed the performance of the GTPs to be relatively unaffected by either size or velocity as reflected by the Reynolds number. The pressure coefficient was approximately 0.82 for the H-GTPs, as compared to 5.2 for a previously designed generic mixing system (GMS). The GTPs can be useful in the design of biological and chemical sampling systems in air-conditioning ducts and for mixing in aerosol wind tunnels where uniform aerosol concentrations are needed at the test section.