Modelling flow and concentration field in rectangular water tanks

Water storage tanks are usually designed with two objectives, i.e. to provide a specified maximum storage volume and to fix a certain total head over the water supply network. Only more recently, some attention has been paid to the water quality inside the storage tank since this can significantly affect the quality in the distribution systems. Sometimes conceptual models were proposed in the literature, but computational fluid dynamics (CFD) methods can be considered as the best analysis and predictive tool in storage tanks water quality studies. In this paper, FLUENT®, a commercial CFD code, was applied to model flow and concentration field of a tracer within a 3D rectangular water tank. In the numerical study an approach based on the Reynolds Averaged Navier-Stokes (RANS) equations was applied, where the closure problem was solved by using the concept of turbulent viscosity. Particularly, the classical two-equation k-ε model was used. The transport of a tracer inside the tank was also simulated using advection-diffusion equation. Some indicators of mixing efficiency within a drinking water storage tank were presented and discussed. Finally, numerical results demonstrated that about 82% of the tank volume was under complete mixing conditions.