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Unsteady operation of the Francis turbine

Ng, TB 2007 , 'Unsteady operation of the Francis turbine', PhD thesis, University of Tasmania.

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Increasing interconnection of individual power systems into major grids has imposed
more stringent quality assurance requirements on the modelling of hydroelectric
generating plant. This has provided the impetus for the present study in which existing
industry models used to predict the transient behaviour of the Francis-turbine plants are
reviewed. Quasi-steady flow models for single- and multiple-turbine plants developed
in MATLAB Simulink are validated against field test results collected at Hydro
Tasmania's Mackintosh and Trevallyn power stations. Nonlinear representation of the
Francis-turbine characteristics, detailed calculation of the hydraulic model parameters,
and inclusion of the hydraulic coupling effects for multiple-machine station are found to
significantly improve the accuracy of predictions for transient operation. However, there
remains a noticeable phase lag between measured and simulated power outputs that
increases in magnitude with guide vane oscillation frequency. The convective lag effect
in flow establishment through the Francis-turbine draft tube is suspected as a major
contributor to this discrepancy, which is likely to be more important for hydro power
stations with low operating head and short waterway conduits.
To further investigate these effects, the steady flow in a typical Francis-turbine draft
tube without swirl is analysed computationally using the commercial finite volume code
ANSYS CFX. Experimental studies of a scale model draft tube using air as the working
medium are conducted to validate and optimise the numerical simulation. Surprisingly,
numerical simulations with a standard k-ε turbulence model are found to better match
experimental results than the steady-flow predictions of more advanced turbulence
models. The streamwise pressure force on the draft tube is identified as a quantity not
properly accounted for in current industry models of hydro power plant operation.
Transient flow effects in the model draft tube following a sudden change in discharge
are studied computationally using the grid resolution and turbulence model chosen for
the steady-flow analysis. Results are compared with unsteady pressure and thermal
anemometry measurements. The three-dimensional numerical analysis is shown to
predict a longer response time than the one-dimensional hydraulic model currently used
as the power industry standard. Convective lag effects and fluctuations in the draft tube
pressure loss coefficient are shown to largely explain the remaining discrepancies in
current quasi-steady predictions of transient hydro power plant operation.

Item Type: Thesis - PhD
Authors/Creators:Ng, TB
Keywords: Turbines, Draft tubes, Fluid dynamics, Hydroelectric power plants
Copyright Holders: The Author
Copyright Information:

Copyright 2007 the Author

Additional Information:

Thesis (PhD)--University of Tasmania, 2007. Includes bibliographical references

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