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Research Papers: Flows in Complex Systems

URANS Models for the Simulation of Full Load Pressure Surge in Francis Turbines Validated by Particle Image Velocimetry

[+] Author and Article Information
J. Decaix

Institute of Systems Engineering,
University of Applied Sciences and Arts
Western Switzerland Valais,
Route du Rawyl 47,
Sion 1950, Switzerland
e-mail: jean.decaix@hevs.ch

A. Müller

Laboratory for Hydraulic Machines,
Ecole Polytechnique Fédérale de Lausanne,
Avenue de Cour 33 Bis,
Lausanne 1007, Switzerland
e-mail: andres.mueller@epfl.ch

A. Favrel

Laboratory for Hydraulic Machines,
Ecole Polytechnique Fédérale de Lausanne,
Avenue de Cour 33 Bis,
Lausanne 1007, Switzerland
e-mail: arthur.favrel@epfl.ch

F. Avellan

Laboratory for Hydraulic Machines,
Ecole Polytechnique Fédérale de Lausanne,
Avenue de Cour 33 Bis,
Lausanne 1007, Switzerland
e-mail: francois.avellan@epfl.ch

C. Münch

Institute of Systems Engineering,
University of Applied Sciences and Arts Western
Switzerland Valais,
Route du Rawyl 47,
Sion 1950, Switzerland
e-mail: cecile.muench@hevs.ch

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received March 1, 2017; final manuscript received June 6, 2017; published online September 1, 2017. Assoc. Editor: Riccardo Mereu.

J. Fluids Eng 139(12), 121103 (Sep 01, 2017) (14 pages) Paper No: FE-17-1124; doi: 10.1115/1.4037278 History: Received March 01, 2017; Revised June 06, 2017

Due to the penetration of alternative renewable energies, the stabilization of the electrical power network relies on the off-design operation of turbines and pump-turbines in hydro-power plants. The occurrence of cavitation is however a common phenomenon at such operating conditions, often leading to critical flow instabilities which undercut the grid stabilizing capacity of the power plant. In order to predict and extend the stable operating range of hydraulic machines, a better understanding of the cavitating flows and mainly of the transition between stable and unstable flow regimes is required. In the case of Francis turbines operating at full load, an axisymmetric cavitation vortex rope develops at the runner outlet. The cavity may enter self-oscillation, with violent periodic pressure pulsations. The flow fluctuations lead to dangerous electrical power swings and mechanical vibrations, dictating an inconvenient and costly restriction of the operating range. The present paper reports an extensive numerical and experimental investigation on a reduced scale model of a Francis turbine at full load. For a given operating point, three pressure levels in the draft tube are considered, two of them featuring a stable flow configuration and one of them displaying a self-excited oscillation of the cavitation vortex rope. The velocity field is measured by two-dimensional (2D) particle image velocimetry (PIV) and systematically compared to the results of a simulation based on a homogeneous unsteady Reynolds-averaged Navier–Stokes (URANS) model. The validation of the numerical approach enables a first comprehensive analysis of the flow transition as well as an attempt to explain the onset mechanism.

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Figures

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Fig. 1

Reduced-scale physical model of a Francis turbine installed on the Ecole Polytechnique Fédérale de Lausanne test rig PF3

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Fig. 2

Flow survey instrumentation on the reduced scale model from Ref. [1]

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Fig. 3

Streamwise location of flow survey cross sections

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Fig. 4

x–z plane view of mesh 1 (left) and mesh 2 (right)

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Fig. 5

PIV measurement setup and data-acquisition chain

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Fig. 6

Computational domain

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Fig. 7

Mesh along the blade suction side (top) and away from the blade trailing edge (bottom)

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Fig. 8

Maximum extension of the cavitating vortex rope visualized with the iso-surface of the liquid volume fraction αL = 0.9 on which contours of the radial velocity Cr are added. URANS CFD results.

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Fig. 9

Instantaneous picture of the cavitating vortex rope. Experimental data.

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Fig. 10

Cu2+Cr2 m s−1. Experimental measurements (left) and URANS CFD results (right). Horizontal sections S1 (top) and S2 (bottom). OP1. The dashed line refers to the line where the velocity profile is extracted.

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Fig. 11

Cz2+Cr2 m s−1. Experimental measurements (left) and URANS CFD results (right). Vertical section. OP1. The dashed lines refer to the lines where the velocity profiles are extracted.

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Fig. 12

Cu2+Cr2 m s−1. Experimental measurements (left) and URANS CFD results (right). Horizontal sections S1 (top) and S2 (bottom). OP2. The dashed line refers to the line where the velocity profile is extracted.

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Fig. 13

Cz2+Cr2 m s−1. Experimental measurements (left) and URANS CFD results (right). Vertical section. OP2. The dashed lines refer to the lines where the velocity profiles are extracted.

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Fig. 14

Circumferential velocity profile Cu. Sections S1 (top) and S2 (bottom). OP1.

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Fig. 15

Axial velocity profile Cz. Sections S1 (top) and S2 (bottom). OP1.

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Fig. 16

Circumferential velocity profile Cu. Sections S1 (top) and S2 (bottom). OP2.

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Fig. 17

Axial velocity profile Cz. Sections S1 (top) and S2 (bottom). OP2.

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Fig. 18

Time history of the local pressure coefficient Cp in section S1 of the draft tube. URANS CFD results (top) and experimental measurements (bottom).

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Fig. 19

Time history of the dimensionless volume of vapor in the complete domain Vv and in the runner VvRu and draft tube VvDt domains. OP2 (top) and OP3 (bottom). URANS CFD results.

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Fig. 20

Time history of the dimensionless flow discharge at the runner inlet Qrin, the draft tube inlet Qdtin, and the draft tube outlet Qdtout. OP2 (top) and OP3 (bottom). URANS CFD results.

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Fig. 21

Time history of the dimensionless terms of the continuity equation. OP2 (top) and OP3 (bottom). URANS CFD results.

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Fig. 22

Instantaneous iso-surface of the liquid volume fraction αL = 0.9 colored by the radial velocity Cr. OP2 (top) and OP3 at t*n = 2.14 (bottom). URANS CFD results.

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Fig. 23

Instantaneous contours of the axial vorticity at the runner outlet with velocity vectors colored by the axial velocity. OP2 (top) and OP3 at t*n = 2.14 (bottom). URANS CFD results.

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Fig. 24

Instantaneous contours of the secondary flow Cu2+Cr2 at the runner outlet. OP2 (top) and OP3 at t*n = 2.14 (bottom). URANS CFD results.

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Fig. 25

Swirl number in section S1 of the draft tube. URANS CFD results.

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Fig. 26

Instantaneous contours of the circumferential velocity Cu in the xz-plane. OP2 (top) and OP3 at t*n = 2.14 (bottom). URANS CFD results.

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