Oxygenation of flow at an elbow deflector: characterisation of air pocket and bubble plume formation and subsequent mass transfer

Hydropower, an important renewable energy source for climate change mitigation, presents downstream environmental impacts, particularly regarding low dissolved oxygen levels. Draft tube aeration using an elbow deflector as a retrofit solution to increase dissolved oxygen levels downstream is investigated. An experimental rig was built to investigate air pocket and bubble dynamics, oxygenation rates, aspiration potential and energy loss in a vertical conduit, in which the buoyancy force and the water flow are co-directional, for a range of relative flow velocities ( U/U 0 = 0.54 – 1, Re O ( 10 5 ) ) and air-water void ratios ( Φ= 0 – 4.4%). Increasing air flow rates results in larger bubbles, while increasing the water flow velocity results in a decrease in the bubble sizes shed from the air pocket. The oxygen transfer coefficient ( k L a ) represented by the Sherwood number increases linearly with relative water velocity and void ratio, due to smaller bubbles and increased surface area. Dimensional analysis is used to develop an equation expressing the Sherwood number as a function of Reynolds number (flow velocity) and air-water void ratio. Self-aspiration potential increases with velocity. Energy loss across the deflector increases with velocity and with air pocket formation. An increased understanding of the oxygenation processes in draft tube aeration techniques will lead to environmental benefits.