Mixing at Cross Junctions in Water Distribution Systems. I: Numerical Study

The present study investigates solute mixing phenomena at various flow rates within a cross junction, which is commonly found in municipal drinking water distribution systems. Simulations using computational fluid dynamics are employed to model the solute concentrations leaving the junction when one inlet is comprised of clean water while the other inlet carries a solute at $\mathsf{R}>\mathrm{10,000}$ . For a few exemplary cases, the resulting velocity vectors and contours of dimensionless concentration are presented to explain the detailed mixing mechanisms at the impinging interface. The turbulent Schmidt number $\left({\mathsf{Sc}}_t\right)$ , an important scaling parameter, is also evaluated. Experimental results were used to assess values of ${\mathsf{Sc}}_t$ for various flow conditions that accurately captured the detailed mixing processes within the junction. The present study clearly indicates that mixing at pipe cross junctions is far from “perfect.” Incomplete mixing results from bifurcating inlet flows that reflect off one another with minimal contact time. Improving the existing water quality model based on accurate mixing data and simulations is important not only to predict concentrations of chemical species such as chlorine in water distribution systems, but also to prepare for potential intentional and accidental contamination events.