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By Karl Kuehlert, Fluent Inc.
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During hot summer months, lakes upstream of hydro turbine
power plants can experience stratification of dissolved oxygen.
The surface water is well oxygenated, but the deeper layers are
not. In most existing hydro stations, the turbine intakes are far below the
surface, so the water that is passed to rivers downstream contains little
oxygen. Low dissolved oxygen levels in the downstream water have an
impact on fish and other aquatic species, since they depend on sufficiently
high levels of dissolved oxygen for survival.
Pathlines colored by velocity
magnitude in the aeration system
Hydroturbine aeration systems address the issue of depressed levels of
dissolved oxygen in rivers and lakes. As part of a Department of Energy
(DOE) study into improved turbine aeration, the consulting group at
Fluent partnered with American Hydro Corporation to further develop
American Hydro’s Retrofit Aeration System (RAS). AmerenUE’s Osage
hydroelectric plant in Lakeside, Missouri was used as a basis for the project,
in which FLUENT was used to study the effects of aeration before
and after installing an RAS. CFD tools provided insight into the characteristics
of both air and water flow, allowing for targeted RAS application.
Pathlines colored by volume fraction of air
in the aeration system and turbine runner
Pathlines colored by
dissolved oxygen in the draft tube
Hydroturbine aeration systems often provide a flow passage for air in
the head cover. Air is drawn naturally through these flow passages
because of the below-atmospheric pressures downstream of the turbine
runner, where air bubbles become dispersed in the water. The water-air
mixture then travels through a draft tube before being injected into the
downstream river flow. When designing the aeration system, it is desirable
to first predict the turbine performance with and without aeration.
Using the predicted air flow rate for the applied operating conditions,
the uptake of dissolved oxygen in the water can be estimated. Insight
into the air dispersion in the draft tube can also be gained.
The complex technical problem of calculating the flow of water and
air into the runner and down through the draft tube was solved by combining
sophisticated computational tools with pragmatic engineering
judgment. Comprehensive two- and three-dimensional CFD models
were set up to include the interaction of air and water. The 2D models
were mainly used to explore different modeling options before running
the more time-consuming 3D model. The mixture model was found to
provide the most stable solution process while capturing all of the necessary
physics in the flow. The water flow rate and the water level in the
downstream river were supplied as boundary conditions. The model
then predicted the pressure distribution and air flow rate through the
aeration system. When downstream water levels rise, the air flow rates
decrease as a result. A new model for tracking dissolved oxygen was
developed and implemented. The overall computational scheme provided
reliable results within reasonable computational times.
The developed methodology was used to simulate a hydroturbine
with a retrofit aeration system for a wide range of operating conditions.
The results from these simulations are in good agreement with available
field data and contribute significantly to a better understanding of the
air-water interaction in the draft tube. With this increased insight, several
design modifications are now being considered to increase the air flow
rate and the dissolved oxygen uptake in the downstream water.
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