An effect of damper configurations on turbulent mixing for air conditioning systems optimization

Concern for indoor air quality (IAQ) is having a profound impact on the design and operation of heating, ventilating and air conditioning (HVAC) systems. The component that affects the performance of such systems significantly is the flow control damper. It is often assumed that sufficient mixing of the outdoor air and return air occurs if dampers are properly aligned. The perception today is that some mixed air dampers do not provide proper mixing of air streams. If the thermal mixing is not adequate to prevent coil freezing, there may also be inadequate mixing of oxygen and gaseous contaminants that usually lead to poor indoor air quality.; In the present work, a mathematical model was used to investigate the mixing characteristics occurring in an air-handling unit (AHU). In order to obtain the mixing effectiveness of thermal mixing and CO₂ mixing, as well as to trace the contaminants, computational fluid dynamics (CFD) has been used to simulate the AHU at different damper angles with various flow rates. The computed results were compared with available experimental data, the agreement was very good for velocity distribution in the mixing chamber. Computed temperature distributions were also compared with available experimental data with reasonable agreement. The CFD model also provided information for species concentration and for movements of contaminant particles, which were more difficult to obtain reliable experimental data.; A parametric study on damper positions was conducted for a specific air-handling unit with return air passing straight through the mixing chamber and the outdoor air entering the mixing chamber perpendicular to the return air. A series of five velocity ratios of return air to outdoor air, and five damper angles for return air as well as for outdoor air has been investigated. It was found that the worst effectiveness thermal mixing was achieved when the return air and outdoor air dampers were in the parallel position at 90 degrees and the return air to outdoor air velocity ratios were equal to one. The best damper configuration to achieve the highest mixing effectiveness at each velocity ratio has been recommended. It is noted opposed-blade dampers used for the outdoor air with a higher inlet velocity could provide the highest mixing effectiveness.