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Research Papers: Flows in Complex Systems

Analysis and Prevention of Vortex Breakdown in the Simplified Discharge Cone of a Francis Turbine

[+] Author and Article Information
Romeo Susan-Resiga

Department of Hydraulic Machinery, “Politehnica” University of Timisoara, Bvd. Mihai Viteazu 1, RO-300222 Timisoara, Romaniaresiga@mh.mec.upt.ro

Sebastian Muntean

Center for Advanced Research in Engineering Science, Romanian Academy–Timisoara Branch, Bvd. Mihai Viteazu 24, RO-300223 Timisoara, Romaniaseby@acad-tim.tm.edu.ro

Vlad Hasmatuchi

Laboratory for Hydraulic Machines, École Polytechnique Fédérale de Lausanne, Avenue de Cour 33Bis, CH-1007 Lausanne, Switzerlandvlad.hasmatuchi@epfl.ch

Ioan Anton

Department of Hydraulic Machinery, “Politehnica” University of Timisoara, Bvd. Mihai Viteazu 1, RO-300222 Timisoara, Romaniaanton@acad-tim.tm.edu.ro

François Avellan

Laboratory for Hydraulic Machines, École Polytechnique Fédérale de Lausanne, Avenue de Cour 33Bis, CH-1007 Lausanne, Switzerlandfrancois.avellan@epfl.ch

J. Fluids Eng 132(5), 051102 (Apr 27, 2010) (15 pages) doi:10.1115/1.4001486 History: Received January 14, 2007; Revised March 15, 2010; Published April 27, 2010; Online April 27, 2010

We perform a numerical analysis of the decelerated swirling flow into the discharge cone of a model Francis turbine operated at variable discharge and constant head, using an axisymmetric turbulent swirling flow model and a corresponding simplified computational domain. Inlet boundary conditions correspond to velocity and turbulent kinetic energy profiles measured downstream the Francis runner. Our numerical results are validated against experimental data on a survey section further downstream in the cone, showing that the Reynolds stress turbulence model with a quadratic pressure-strain term correctly captures the flow field. It is shown that the diffuser performance quickly deteriorates as the turbine discharge decreases, due to the occurrence and development of vortex breakdown, with a central quasistagnant region. We investigate a novel flow control technique, which uses a water jet injected from the runner crown tip along the axis. It is shown that the jet discharge can be optimized for minimum overall losses, while the vortex breakdown is eliminated. This flow control method is useful for mitigating the Francis turbine flow instabilities when operating at partial discharge.

Copyright © 2010 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

FLINDT draft tube and the simplified straight conical diffuser, with the computational domain in a meridian half-plane

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Figure 2

Variation of the parameters in swirling flow analytical representation of Eqs. 3,4 versus the discharge coefficient on the survey section S0

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Figure 3

Variation of the parameters in turbulent kinetic energy analytical representation of Eq. 5 versus the discharge coefficient on the survey section S0

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Figure 4

Velocity components and turbulent kinetic energy for discharge φ=0.340

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Figure 5

Streamline pattern computed with RSM—upper half-plane, and with the realizable RKE—lower half-plane

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Figure 6

Velocity components and turbulent kinetic energy for discharge φ=0.368

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Figure 7

Velocity components and turbulent kinetic energy for discharge φ=0.380

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Figure 8

Velocity components and turbulent kinetic energy for discharge φ=0.410

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Figure 9

Streamline pattern for axisymmetric flow in the draft tube cone, with development of vortex breakdown as discharge decreases

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Figure 10

Velocity on the axis and swirl number variation in the discharge cone

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Figure 11

Potential CPR and kinetic CKR energy recovery coefficients versus turbine discharge

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Figure 12

Energy loss coefficient ζ versus turbine discharge

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Figure 13

Kinetic-to-potential energy conversion ratio χ in the draft tube cone versus turbine discharge

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Figure 14

Energy loss coefficient ζ and the potential energy recovery coefficient CPR

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Figure 15

Cross section through the Francis turbine model, with the control jet nozzle at the crown tip

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Figure 16

Modified axial velocity profile on the survey section S0 by injecting an axial control jet for turbine discharge φ=0.340

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Figure 17

Diffuser dissipation ΔE and jet power Pjet as fractions of the turbine hydraulic power P=ρgHQ versus jet discharge as fraction of the turbine discharge at φ=0.340

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Figure 18

Streamline pattern at turbine discharge φ=0.340, without control jet (upper half-plane) and with optimal control jet (lower half-plane)

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