IMPROVING DRINKING WATER QUALITY
Flow pattern and residence time contours at the surface
Quality of the drinking water supply is a prime requirement for every water supplier.
Water quality can be adversely affected by a poor flow pattern in a reservoir, for instance if there are any significant stagnant zones or short-circuiting pathways. For security reasons, experimental access to potable-water reservoirs is not usually possible. However, CFD has the potential to provide valuable design insight into the hydraulics of reservoirs, but until recently applications of CFD in this field have been relatively rare.
New Plymouth District Council have acted as pioneers in New Zealand by commissioning Massey University and Flowsolve to perform CFD studies of their new potable water storage reservoir at Inglewood, Taranaki.
The reservoir is circular in shape. Water enters from a bellmouth entry just above water level, close to the perimeter. The momentum of the water cascading down from the inlet sets up a complex three-dimensional flow pattern in the reservoir which is simulated by the model. Quality can be assessed from plots of mean residence time, together with a hydraulic residence-time distribution profile at the outlet, the latter being generated from a transient simulation of the motion of a virtual tracer.
The height of the outlet of this reservoir is determined by the requirement to maintain some water content in the event of pipe rupture due to earthquake. Within this constraint, the exact outlet location could be varied in the model, and a number of simulations were performed to optimise the hydraulic performance of the reservoir. Valuable new insights were derived from the study.
The graphical plot above shows the flow pattern at the surface, with velocity vectors superimposed on contours of residence time. Water entering the reservoir (downwards, on the left) impacts on the bottom, and splays out in all directions. It is then “gathered up” by the circular side wall, and gradually returns to the surface, and as indicated by the graphic, flows around both sides of the reservoir towards the outlet (on the right). Only a small proportion of the water is drawn off at the outlet, and the remainder flows directly back along the diameter towards the inlet. This complex three-dimensional flow pattern effects considerable mixing of the water, as demonstrated by the near-uniform value of the residence time contours of around 28 hours.
