Oxygenation using a draft tube deflector : ultrasonic deaeration optimized to obtain initial dissolved oxygen level*

Hydro power plants are a renewable energy source. However, due to the thermal stratification of the reservoirs, the dissolved oxygen levels can drop below the minimum level (3-6 mg/l) needed for aquatic life, thus turbine manufacturers are required to meet legislated minimum levels. Draft tube aeration is considered a retrofit option not unduly impacting operation. Draft tube deflectors naturally aspirate air forming a bubble swarm that is entrained into the flow dispersed ensuring a high rate of oxygen mass transfer. A physical model study will optimize the elbow deflector design to maximize the oxygenation rate and minimize the associated energy losses. An ultrasonic deaeration method, using a 20kHz probe, was optimized to produce the low dissolved oxygen water required for the test rig. A parameter study varied the water temperature [18 to 38°C] and vacuum pressure applied to the free surface [27, 54, 68 kPa]. At the optimum vacuum pressure of 68 kPa, the dissolved oxygen level was reduced to 2.5-3 mg/l in 60 mins. Temperature had an insignificant impact. A simplified deflector was studied to determine the oxygenation rate from initial dissolved oxygen levels of [4, 5, 6 mg/L] and the energy loss during the air injection.