Contaminant dispersion in ventilation ducts of air-conditioning systems: Mixing lengths under multiple factors

Knowledge of contaminant mixing in ventilation ducts is essential for rationally simplifying the calculation of contaminant concentrations in ducts and thus efficiently predicting the non-uniform distribution of contaminants inside multiple rooms of a building connected by a complex ventilation ductwork. However, few systematic studies have addressed contaminant mixing characteristics in building air conditioning and ventilation ducts. In this study, the mixing characteristics during contaminant transport in various representative ventilation ducts of air-conditioning systems are analyzed numerically using the index of mixing length. The effects of duct air velocity, aspect ratio of the duct cross-section, coverage of contaminant source relative to the cross-section, and duct components on contaminant mixing are revealed. An increase in air velocity from 3 to 8 m/s leads to a moderate increase in mixing length from 62 to 68 D (D is the hydraulic diameter). Square ducts can lead to a rapid reduction in mixing length from over 100 to 57 D compared to rectangular ducts. Partial coverage of the source in the cross-section is not conducive to rapid mixing of contaminants. The double elbows substantially intensify the mixing of contaminants. Instead, the mixing length may increase significantly after the contaminant passes through the tee.