Recent studies have focused on mixing behavior at cross junctions, and incomplete mixing at cross junctions in water distribution systems was verified. Nevertheless, the research results on mixing at other junction configurations, such as double-Tee junctions, were insufficient. Double-Tee junctions can potentially be misrepresented as cross junctions because of network skeletonization. Hence, the diffusion and dilution of the contaminants at junctions were largely underestimated. To examine the mixing phenomenon and collect accurate mixing data at the double-Tee junction, a series of laboratory experiments was carried out with various Reynolds number ratios at the inlets and outlets combined with different dimensionless connecting pipe lengths (L/D). Results showed that the dimensionless connecting pipe length served an important function in mixing at double-Tee junctions. The cross junction was the special case of the double-Tee joint when L/D=0. The complete mixing state occurred when L/D→∞. The mixing degree of the double-Tee junction was between the cross junction and the complete mixing state. A conceptual model that described the mixing behavior at double-Tee junctions was developed. The model included the use of the dimensionless parameter φ, which defined the degree of departure from complete mixing.

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