Study of mixing and exchange in a drinking water reservoir using CFD modeling

Sammanfattning: This thesis examines water mixing and exchange in a drinking water reservoir operated by themunicipal association Norrvatten. Recent water samples from the reservoir’s outgoing waterhave shown an increase in culturable bacteria during late summer and fall. This thesis utilizesComputational Fluid Dynamics (CFD) modeling and analysis in OpenFOAM to simulatereservoir inflow and outflow, analyzing mixing processes and their relationship to operationalstrategies. The objective is to understand the correlation between the residence time of waterand microbial growth and propose operational improvements to increase the exchange of waterin order to achieve improved water quality. A trace element was implemented in the CFDmodel to simulate the residence time of water. Initial simulations were based on the reservoir’shistorical operational data, utilizing temperature and water level measurements providedby Norrvatten. After the initial simulations, four alternative simulations were performed,comparing different operational strategies by modifying inflow parameters. Inflow parametersthat were changed were the volumetric inflow rate, water level variation, and the temperatureof the inflowing water. The post­processing in ParaView focused on the thermal stratificationand residence time distribution near the outlet during each mixing process. The study revealeda complex relationship between flow conditions and microbial growth, making it challengingto identify a clear pattern. However, based on the simulations with the alternative operationalstrategies it was concluded that the set of operational strategies called ”Strategy 1” generated themost optimal flow conditions. This strategy involves a three times larger volumetric inflow rate(an increase from 0.05 to 0.15 m^3/s) and a water level that is kept at the same values comparedto the original simulation. Strategy 1 resulted in a 3.6 % higher water exchange compared to theoriginal simulation. In comparison to the other simulated strategies, Strategy 1 generates thehighest water exchange, with a 63.6 % increase compared to the worst­-case scenario involvingcolder inflow. The conclusion that could be drawn is that the most favorable operationalstrategies involve higher volumetric inflow rates, lower water levels, and an incoming watertemperature that is higher than the initial reservoir temperature.