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Generation of Supersaturated Total Dissolved Gas at Submerged Hydropower Low-level Outlets
- Author / Creator
- Li, Pengcheng
Supersaturation of total dissolved gasses (TDG) downstream of hydropower dams and their consequences for fish have been identified as one of the key potential impacts of hydropower operations on fish biodiversity and fishery productivity. It has been recorded that fish exposed to high TDG levels may suffer gas-bubble disease. The generation of high level TDG has been mostly studied downstream of spillways with very little attention to its generation related to low-level outlets, which is typically considered to be free from air entrainment and TDG issues. This research aims at developing prediction models that can have a full understanding of the processes of TDG generation at low-level outlets, including the mass transfer of rising bubbles, air demand of a hydraulic jump in a closed conduit with a submerged outlet, and the TDG generation of low-level outlets and its influencing factors.
Dissolution of gases from air bubbles to water (mass transfer from air bubbles to water) directly leads to the generation of TDG. To have a good understanding of bubble-water mass transfer, a model for bubble–water mass transfer was established and validated using the experimental measurements from rising carbon dioxide bubbles. The effects of bubble horizontal velocity, bubble release depth, initial bubble size, background dissolved gas concentration in the water, and bubble swarms on the bubble–water mass transfer were examined. The bubble size changed along the rising path in response to both the mass transfer and the local hydrostatic pressure, whereas their relative importance varied under different conditions. Compared with previous models, the proposed model improved the prediction accuracy by including the influence of changing dissolved gas concentration in the water and mass transfer across the water surface.
Air demand of a hydraulic jump in a closed conduit is the source of the dissolved gas and TDG generation at low-level outlets. Physical experiments were conducted to study flow regimes and the air demand of a hydraulic jump in a closed conduit with various submerged outlet depths. Flow regimes with a submerged outlet were defined following previous studies based on the outlet depth. In a closed conduit, free surface supercritical flow without a hydraulic jump can induce a relative air demand (air flow rate to water flow rate) of about 36 - 90% when the Froude number is between 4 and 10, which decreases to about 3 - 14% for a free surface flow with a hydraulic jump. If the hydraulic jump is followed by pressurized pipe flow, the air demand decreases with the increasing submerged outlet depth. If the hydraulic jump is partially submerged, the relative air demand is significantly reduced to less than 1%. Field measurements of the air demand were consistent with the experimental measurements when the hydraulic jump was partially submerged. When the air supply was constrained by a nozzle of various sizes placed on the top of the air vent, the air pressure in the closed conduit deceased and the hydraulic jump moved upstream.
Supersaturated TDG was observed and evaluated through field measurements at Hugh Keenleyside Dam, B.C, Canada focusing on two groups of the low-level outlets (south and north low-level outlets). With an air entrainment amount of as small as 1%, a TDG level as high as 130% could be generated in the south low-level outlets. Numerical modelling was also adopted to obtain turbulence and flow field details downstream of the low-level outlets. Good agreement between model results and field data is found in the tailrace of the low-level outlet.
Stronger turbulence in the stilling basin can result in larger mass transfer coefficient across bubbles and produce smaller bubbles, which will substantially enhance gas transfer and TDG generation. Due to the shallower water depth, the south low-level outlet can generate stronger turbulence flow, with more efficient gas transfer and TDG generation compared with the deeper north low-level outlet (110% TDG).
- Graduation date
- Fall 2021
- Type of Item
- Doctor of Philosophy
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