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Research Papers: Multiphase Flows

On the Efficiency Alteration Mechanisms Due to Cavitation in Kaplan Turbines

[+] Author and Article Information
Sebastián Leguizamón

Laboratory for Hydraulic Machines (LMH),
Department of Mechanical Engineering,
Swiss Federal Institute of Technology (EPFL),
Avenue de Cour 33 Bis,
Lausanne CH-1007, Switzerland
e-mail: sebastian.legui@epfl.ch

Claire Ségoufin

Alstom Power Hydro,
82 Avenue Léon Blum,
Grenoble FR-38100, France
e-mail: claire.segoufin@power.alstom.com

Phan Hai-Trieu

Alstom Power Hydro,
82 Avenue Léon Blum,
Grenoble FR-38100, France
e-mail: hai-trieu.phan@power.alstom.com

François Avellan

Professor
Laboratory for Hydraulic Machines (LMH),
Department of Mechanical Engineering,
Swiss Federal Institute of Technology (EPFL),
Avenue de Cour 33 Bis,
Lausanne CH-1007, Switzerland
e-mail: francois.avellan@epfl.ch

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received November 9, 2015; final manuscript received December 3, 2016; published online April 5, 2017. Assoc. Editor: D. Keith Walters.

J. Fluids Eng 139(6), 061301 (Apr 05, 2017) (8 pages) Paper No: FE-15-1811; doi: 10.1115/1.4035928 History: Received November 09, 2015; Revised December 03, 2016

A transport-equation-based homogeneous cavitation model previously assessed and validated against experimental data is used to investigate and explain the efficiency alteration mechanisms in Kaplan turbines. On the one hand, it is shown that the efficiency increase is caused by a decrease in energy dissipation due to a decreased turbulence production driven by a drop in fluid density associated with the cavitation region. This region also entails an increase in torque, caused by the modification of the pressure distribution throughout the blade, which saturates on the suction side. On the other hand, the efficiency drop is shown to be driven by a sharp increase in turbulence production at the trailing edge. An analysis of the pressure coefficient distribution explains such behavior as being a direct consequence of the pressure-altering cavitation region reaching the trailing edge. Finally, even though the efficiency alteration behavior is very sensitive to the dominant cavitation type, it is demonstrated that the governing mechanisms are invariant to it.

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Figures

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Fig. 1

Meridional view (left) and three-dimensional render (right) of the computational domain. The blade-to-blade passage is composed of the inlet (1), one guide vane (2), one turbine blade (4), the rotor-stator interface (3), and the outlet (7). The typical Kaplan turbine gaps between the blade and the discharge ring (5) and the hub (6) are visible in the meridional view.

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Fig. 2

Surface mesh on the blade tip trailing edge. The surface mesh on the volume describing the gap between the blade and the discharge ring is also presented.

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Fig. 3

Visualization of the turbine blade suction side at σ = 0.41. The simulated cavitation region is visualized using an isosurfaces of vapor volume fraction = 0.35, 0.70, 0.85 (with decreasing level of transparency).

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Fig. 4

Relative efficiencies for case 1. The normalization is done with respect to the cavitation free condition, σ ∼ 3.

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Fig. 5

Isosurface of vapor volume fraction = 0.5 on the blade suction side for three values of Thoma number for case 1

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Fig. 6

Pressure coefficient at 9% blade span for case 1. The three horizontal lines (Cp = −1.48, −1.41, −1.18) correspond to the vapor pressure for σ = 0.76, 0.69, and 0.57, respectively.

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Fig. 7

Pressure coefficient at 75% blade span for case 1. The three horizontal lines (Cp = −0.45, −0.43, −0.36) correspond to the vapor pressure for σ = 0.76, 0.69, 0.57, respectively.

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Fig. 8

Evolution of span-averaged normalized energy losses for case 1, with the blade located between the streamwise positions 1.25 and 1.75. The normalization is done with respect to the total losses (i.e., at the turbine outlet, streamwise position = 2.0) of condition σ = 0.57.

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Fig. 9

Isosurfaces of vapor volume fraction = 0.35, 0.70, 0.85 (with decreasing level of transparency) in blue (light gray) and isosurfaces of turbulence kinetic energy = 1.5 (m2 s−2) in orange (dark gray) for several Thoma numbers, for case 2. It is clear that the arrival of the cavitation region to the trailing edge coincides with a sensible increase in turbulence.

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Fig. 10

Relative efficiencies for case 2. The normalization is done with respect to the cavitation free condition, σ ∼ 1.75.

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Fig. 11

Evolution of span-averaged normalized energy losses for case 2, with the blade located between the streamwise positions 1.25 and 1.75. The normalization is done with respect to the total losses (i.e., at the turbine outlet, streamwise position = 2.0) of condition σ = 0.41.

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