Mixing in Water Storage Tanks. II: With Buoyancy Effects

Experiments on jet-induced mixing in water storage tanks with various single- and multiple-nozzle inlet configurations with positive and negative density differences between the inflow and stored water were conducted. Mixing was observed and quantified by a three-dimensional laser-induced fluorescence system. Small density differences can prevent mixing, and criteria to predict whether mixing occurs were presented. It depends primarily on the total inflow momentum flux, density difference, water depth, and nozzle locations and orientations. When the tanks did mix, their mixing times were generally within 20% of the corresponding times with no density differences. Some flows with negative buoyancies mixed significantly more quickly, possibly due to the destruction of the organized gyres that occurred with no buoyancy. Good mixing can be achieved by injecting positively buoyant inflows horizontally at the bottom, or negatively buoyant inflows horizontally near the water surface, resulting in long jet entrainment path lengths. Mixing of vertical negatively buoyant inflows in standpipes can be improved by the use of a draft tube. Mixing can usually be accomplished by relatively simple nozzle configurations, provided they are suitably configured, and overly elaborate mixing devices are probably unnecessary. An example is given that shows that the hydraulic head, and therefore power cost, required for jet mixing is generally quite low.