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Courtesy of ITT Flygt
The Columbia and Snake Rivers in the Pacific Northwest of the U.S. are
home to about twenty hydroelectric dams. While these provide inexpensive
power to the local communities, they also pose a threat to migrating fish,
particularly the Pacific salmon. Early in their life cycle, the salmon
migrate downstream to the sea. After maturing and spending two or three
years in the ocean, the fish return to their birth-place to spawn. While
the challenge of upstream migration of mature salmon past dams has been
addressed with moderate success through fish ladders for the better part
of a century, ensuring the safe down-stream migration of juvenile salmon
remains a difficult task. Some estimates indicate that when the finger-sized
fish swim through the hydroturbines, there is a mortality rate of up to
10% at each dam, resulting from the sudden pressure drop at the runner.
In an effort to help channel juvenile fish through a safer bypass route
around the dam, a unique 10 x 3 pump array is currently under construction.
The ultra low head, low noise, submersible propeller pumps to be used
were created specifically for this purpose by engineers at ITT Flygt of
Stockholm, Sweden and Trumbull, CT, USA. The pumps will be used to draw
water through a fish attraction channel and recirculate it into the headwater
above the dam.
A section of the array of Flygt's P7900 ultra low head pumps
FLUENT has been used for two facets of the project. First, it was used
in the design phase to optimize the geometry of pump components such as
the inlet, propeller, guide vane, and exit diffuser. Beginning in 1998,
designs were tested using CFD and compared to data from a 1/6 scale model
in the Flygt hydraulics lab in Stockholm.
A wall vortex generated by a single pump in the full size prototype test
rig in Pewaukee, Wisconsin
Second, FLUENT has been used in the application phase to understand better
the inlet and discharge flows as well as the interaction between neighboring
pumps in the array. The first CFD analyses on the discharge from a single
stack of three pumps suggested that the individual pump discharges did
not diffuse adequately after exiting from the pumps. Any high velocity
in the pump discharge area is undesirable because it might attract and
confuse the current-sensitive migrating fish. The CFD results also indicated
that considerable recirculation was taking place above, below, and in
between the pump discharge jets, thereby exacerbating the problem. Several
discharge baffle arrangements were tested using FLUENT to determine a
practical means of reducing the recirculation and making the discharge
from each set of pumps more uniform. The most effective of these had baff
les added above and below the set of pumps and diverging walls on either
side of the column. The resulting discharge was characterized by a flatter
velocity profile across the three pumps and only minimal recirculation
above and below the set.
Vertical and diffusing baffles on a set of pumps (top) and the resulting
flow field (bottom)
The first installation of the pump array is planned to go on line in
the spring of 2003 at the Rocky Reach Dam on the Columbia River in Washington.
When it occurs, the project will make life better for the local fish,
the local utility companies, and ITT Flygt, as similar installations are
planned for the future.
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