Mixing and Stagnation in Drinking Water Storage Tanks

Abstract: This thesis contributes to studies of the assessment of mixing conditions in drinking water storage tanks and analyses of parameters governing mixing rates and formation of stagnant zones. The work also contributes to a validation of Computational Fluid Dynamics (CFD) models of jet-forced mixing in storage tanks under neutral and thermally stratified conditions.

Measurements of temperatures and flow rates were conducted in two drinking water storage tanks in the field to collect data on the mixing conditions in cases of lighter and denser inflows. The local conditions in the field made it possible to treat the temperature as a conservative tracer and quantify the effects of the inflow temperature on the water exchange of a fill-and-draw cycle. The field study also provided basic data to estimate the critical conditions for mixing and to validate the numerical model.

The numerical model employed the eddy viscosity concept and the standard κ - ε model of turbulence. The results confirm that the model can be used to simulate jet-forced mixing processes in drinking water storage tanks and to predict when stagnation caused by thermal stratification is initiated.

Application of the model in a parametric study gave detailed information about mixing characteristics in cylindrical tanks with neutral and denser inflows directed vertically and diagonally through the tank. The study adopted a mixing criterion from a previous study and showed that the critical value, which separates mixed and stagnant cases, depends on the height-to-width ratio of the tank and the inflow direction. The calculations also showed that the risk of stagnation increases with increasing water depth and temperature of the stored water. The results can be used to estimate the risk of stagnation in jet-mixed storage tanks and determine which inflow velocity is required to avoid stagnation for a certain temperature difference between the incoming and the stored water.