Performance characterization of air mixing devices for square ducts

•Static air mixing devices designed for square ducts were investigated.•Mixing and hydraulic performance of air mixing devices were evaluated.•Effects of design and operating parameters on mixing performance were determined.•Mixing patterns for the mixers tested were examined. Considering that square ducts are widely utilized for HVAC system performance testing, air mixing devices designed for such ducts were experimentally investigated. The results of these studies can also inform field application of mixers, such as in airside economizer applications of rooftop air conditioning equipment. Three types of air mixing devices were considered; one was a louvered-type mixer designed based on ASHRAE Standard 41.2, and the others were an orifice mixer and an orifice-target combination mixer. With thermally maldistributed airflow, the mixing and hydraulic performance of the mixing devices were evaluated for various testing parameters including flowrate, flowrate ratio, overall mixing length, mixer spacing, mixer orientation, orifice diameter, and orifice-target spacing. The results indicate that all mixing devices tested were capable of reducing stratification of the airflow. It was found that the mixing effectiveness, in general, increases as the overall mixing length and/or for dual mixers, spacing, increase. For an overall mixing length of 2.0 duct diameters (Dh), which is the maximum mixing length for industrial use, the mixing effectiveness values of the louvered mixer, orifice mixer, and orifice-target mixer were 76.3, 75.1, and 69.9%, respectively. The pressure drop of the orifice-type mixers was significantly larger than for the louvered mixers. The results also indicate that in order to achieve more than 70% mixing effectiveness, the louvered mixers require a mixing length of 2.0 Dh while the mixing length required for orifice-type mixers varies from 1.0 to 3.0 Dh depending on the orifice hole diameter (0.4–0.6 Dh). Furthermore, it was found that the louvered-type mixers are less affected by total flowrate and the level of velocity maldistribution of incoming airflow than orifice-type mixers are. Additionally, orifice mixers cause substantial velocity maldistribution downstream of their location. This study provides insight into the comprehensive information on the performance and design of air mixing devices available for square ducts in addition to the literature in which most of the data are based on studies conducted under round ducts and relatively