Aeration of hydraulic turbines to increase downstream dissolved oxygen concentration

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The concentration of dissolved oxygen plays a vital role in biogeochemical cycling and in aquatic ecosystem function. In warm climates, warm temperatures cause thermal stratification in hydropower reservoirs inhibiting mixing and leading to deoxygenation of waters in the hypolimnion of the reservoir. Hydropower generation with turbines withdrawing water at depth results in low dissolved oxygen in the downstream flow negatively impacting the downstream riverine ecosystem. Legislation (Canada, USA and elsewhere) now requires minimum limits for dissolved oxygen concentration to be met. Technological approaches to re-aerate the flow with bubble streams is a solution that also reduces vibrations at part-load turbine operation increasingly used to balance intermittent loads as renewables are fed into the grid. Retrofit technologies to re-aerate flows are preferred, as they have smaller impacts on revenue streams. Andritz Hydro Canada implemented one of the first prototypes of draft tube aeration using elbow deflectors at the John H. Kerr power plant, demonstrating the viability of this retro-fit system. Building on an initial exploratory collaborative study, in this study we aim to optimize the elbow deflector technology, understand the physical processes of the two-phase flow of the aspirated bubble swarm in the draft tube flow and quantify the subsequent dissolution of oxygen into the flow. Experiments on a test rig will provide physical model results to examine the effect of design parameters on the bubble swarm dynamics in the two-phase flow, the rate of oxygen transfer and scale effects and to develop similitude laws. The results will be subsequently used to support development of a computational model to predict prototype operations. The project will benefit from the 15 years of experience in R&D of hydropower hydraulics of Prof. Gaskin. Andritz Hydro Canada will position itself as a leader for hydropower projects, in which environmental impacts of low dissolved oxygen are important (countries with warm climates, e.g. USA, Spain), able to guarantee meeting (or exceeding) the regulatory requirements for dissolved oxygen while optimizing turbine performance.