Modeling and Simulations of Flow Pattern, Chlorine Concentration, and Mean Age Distributions in Potable Water Service Reservoir of Singapore

Abstract

Study of flow pattern and chlorine distribution is of major interest in the characterization, design, and evaluation of service conditions of a service reservoir. With the advancement of computational science and resources, the computational fluid dynamics (CFD) method could be a reliable alternative to flow and tracer tests. A salient feature of flow in the service reservoir is the changing inflow and outflow caused by variations in water demand and supply. To overcome the challenge of tracking water-level variations, two different numerical methods (i.e., multiphase analysis and dynamic meshes) were implemented in this study. The multiphase analysis demonstrates that sloshing of water is not significant because of the relatively low velocity in contrast to the large dimensions of the service reservoir. Therefore, it is valid to assume that the water-air interface remains as a horizontal plane. Dynamic meshes are then used to track the water-level variations to reduce the computational cost. The predicted chlorine concentration at the outlet matches well with that of on-site measurement. The study on a common service condition reveals short-circuiting of the flow. The subsequent formation of recirculation zones implies poor lateral and moderate vertical mixings, which result in nonuniform chlorine distribution in the reservoir. To provide the spatial distributions of the local mean residence time of the chlorine, the “mean age” concept was introduced in the present study. It can be used as a quantitative indicator to guide the design of service reservoirs for achieving a homogenous chlorine distribution.