The document discusses numerical simulation of cavitation in Francis turbines. It presents (1) different forms of cavitation that can occur, such as leading edge cavitation, (2) methods used to model cavitation including the homogeneous model and Zwart mass transfer model, and (3) results showing the model can predict the shape and extent of cavitation and its effects on efficiency and pressure pulsation compared to experiments. The research was funded by the European Union and Slovenian government.

1. ANSYS Convergence Conference
Ljubljana, 25th of May 2016
Numerical simulation of different forms of cavitation in Francis
turbines
D. Jošt, A. Škerlavaj, M. Morgut, R. R. Stopar, E. Nobile
Kolektor Turboinštitut, Ljubljana, Slovenia
University of Trieste, Italy

2. • Turbines
– Development of water turbines
– Model acceptance testing in accordance with IEC
60193 standard
– Site testing
• Pumps
– Development of pumps
– Production, refurbishment and consultancy
• Small Hydro Power Plants
– Design, manufacturing and installation of small
turbines and electro mechanical equipment
Kolektor Turboinštitut
ACTIVITIES
Numerical simulation of different forms of cavitation in Francis turbines
D. Jošt, A. Škerlavaj, M. Morgut, R. R. Stopar, E. Nobile
ANSYS Convergence Conference, Ljubljana, 25th of May 2016

3. Primary aim: to develop reliable, high-fidelity methods for accurate
prediction, and optimization, of the performances of hydro-machinery
and marine propellers
Kolektor Turboinštitut, Slovenia
- Company
- Development & testing of hydro-
machinery (water turbines & pumps)
- Manufacturing of small water turbines
- CFD/HPC (supercomputer with 2000 cores)
University of Trieste, Italy
- Experts in num. prediction of
cavitation on marine propellers
- Experts in optimization
- HPC computing
ACCUSIM project = Accurate Simulations in
Hydro-Machinery and Marine Propellers
www.accusim.eu; 1/2/2014-31/1/2018
Knowledge
Research
Community
&
Public
Dissemination of knowledge
+
Numerical simulation of different forms of cavitation in Francis turbines
D. Jošt, A. Škerlavaj, M. Morgut, R. R. Stopar, E. Nobile
ANSYS Convergence Conference, Ljubljana, 25th of May 2016

4. Forms of cavitation in Francis turbines:
• Leading edge cavitation
• Travelling bubble cavitation
• Draft tube swirl
• Inter-blade vortex cavitation
Consequences of cavitation:
• Instable flow conditions
• Vibrations
• Noise
• Damage of material surface
• Reduction of efficiency
Cavitation in Francis turbines
Cavitation coefficient:
H
HHH vsa 

Numerical simulation of different forms of cavitation in Francis turbines
D. Jošt, A. Škerlavaj, M. Morgut, R. R. Stopar, E. Nobile
ANSYS Convergence Conference, Ljubljana, 25th of May 2016
Damages of blades due to
cavitation

5. 
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U
τUU
U
U
volumetotal
vapourvolume
volumetotal
liquidvolume
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Homogeneous two-phase
model
Flow equations:
Liquid and vapour volume fractions:
Density and dynamic viscosity of the
vapour-water mixture:
Numerical simulation of different forms of cavitation in Francis turbines
D. Jošt, A. Škerlavaj, M. Morgut, R. R. Stopar, E. Nobile
ANSYS Convergence Conference, Ljubljana, 25th of May 2016

6. Cavitation
Zwart mass transfer model
Rayleigh-Plesset equation:

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





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l
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vnuc
PP
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F-
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rnuc=5×10-4,
RB=2×10-6 m,
Fe=50,
Fc=0.01
Fe=300,
Fc=0.03
Nucleation site volume fraction:
Radius of a nucleation site:
Default evaporation and condensation constants:
On NACA66 hydrofoil calibrated constants:
(Morgut, M., Nobile, E. and Biluš, I. Comparison of mass transfer
models for the numerical prediction of sheet cavitation around a
hydrofoil. Int. Journal of Multiphase Flow. Vol. 37. No. 6. 2010. pp. 620-
626.
Numerical simulation of different forms of cavitation in Francis turbines
D. Jošt, A. Škerlavaj, M. Morgut, R. R. Stopar, E. Nobile
ANSYS Convergence Conference, Ljubljana, 25th of May 2016

7. Cavitation at full load:
• leading edge cavitation,
• traveling buble cavitation
• draft tube swirl
Absolute
pressure Experiment
The shape and extend of extent of cavitation
(Iso-surface of Vapour Volume Fraction = 0.3)
σ = 0.154
σ = 0.115
σ = 0.098
Numerical simulation of different forms of cavitation in Francis turbines
D. Jošt, A. Škerlavaj, M. Morgut, R. R. Stopar, E. Nobile
ANSYS Convergence Conference, Ljubljana, 25th of May 2016
The shape and extent of cavitation
Isosurfaces of Vapour Volume Fraction =0.3

8. σ = 0.180 σ = 0.154 σ = 0.128 σ = 0.115 σ = 0.098
Standard
constants
Tuned
constants
Figure 6: Distribution of water and vapour at the suction side of one runner blade
Regions of cavitation on suction side of runner blades
Standard constants (CFX): Evaporation coefficient Fe = 50, Condensation coefficient Fc = 0.01
Tuned constants: Fe = 300, Fc = 0.03
Numerical simulation of different forms of cavitation in Francis turbines
D. Jošt, A. Škerlavaj, M. Morgut, R. R. Stopar, E. Nobile
ANSYS Convergence Conference, Ljubljana, 25th of May 2016

9. Draft tube swirl - Cavitating vortex rope in the draft tube at different operating regimes
Part Load High Load
Q/QBEP = 0.66 Q/QBEP = 0.8 Q/QBEP = 0.85 Q/QBEP = 1.15
Experiment
Numerical simulation of different forms of cavitation in Francis turbines
D. Jošt, A. Škerlavaj, M. Morgut, R. R. Stopar, E. Nobile
ANSYS Convergence Conference, Ljubljana, 25th of May 2016
Numerical simulations without cavitation modeling
Previous results without cavitation modeling:
LIPEJ, Andrej, JOŠT, Dragica, MEŽNAR, Peter, DJELIĆ, Vesko. Numerical prediction of pressure pulsation amplitude for different operating
regimes of Francis turbine draft tube. V: 24th IAHR Symposium on Hydraulic Machinery and Systems, October 27-31, 2008, Foz do Iguassu,
Brazil.

10. Configuration Spiral casing, stay and
guide vanes
Runner Draft tube Total
1 1,400,000 1,500,000 900,000 3,800,000
2 1,400,000 12,600,000 3,000,000 17,000,000
3 - 12,600,000 12,400,000 25,000,000
Number of nodes in particular geometry configuration
Numerical simulation of different forms of cavitation in Francis turbines
D. Jošt, A. Škerlavaj, M. Morgut, R. R. Stopar, E. Nobile
Ljubljana, 25th of May 2016
Operating
point
Exp.
values
Numerical values for different
configurations
1 2 3
OP1 3,20 Hz 3,09 Hz 3,22 Hz -
OP2 4,00 Hz 4,01 Hz 3,95 Hz 3,80 Hz
OP3 4,10 Hz 3,80 Hz 3,82 Hz -
OP4 4,45 Hz - - -
Pressure pulsation at part load
Previous results without cavitation modeling:
LIPEJ, Andrej, JOŠT, Dragica, MEŽNAR, Peter, DJELIĆ, Vesko. Numerical prediction of pressure pulsation
amplitude for different operating regimes of Francis turbine draft tube. V: 24th IAHR Symposium on
Hydraulic Machinery and Systems, October 27-31, 2008, Foz do Iguassu, Brazil.
Pressure pulsation frequency

11. Cavitating vortex rope at part load
• Operating point: Q/QBEP = 0.8, y/yBEP = 0.97
• Turbulence model: SAS SST
• Advection scheme: HRS
• Time step: 2 deg. of runner revolution
• Homogeneous model, default evaporation and condensation constants
Input data: geometry, head, rotating speed
Output: flow rate, torque on the shaft, efficiency, pressure pulsation
Pressure
pulsation
measurement
Numerical simulation of different forms of cavitation in Francis turbines
D. Jošt, A. Škerlavaj, M. Morgut, R. R. Stopar, E. Nobile
ANSYS Convergence Conference, Ljubljana, 25th of May 2016
Number of nodes Spiral casing, stay and guide vanes Runner Draft tube Total
1,400,000 1,500,000 2,500,000 5,400,000

12. Experiment Simulation without
cavitation modelling
Simulation with
cavitation modelling
Numerical simulation of different forms of cavitation in Francis turbines
D. Jošt, A. Škerlavaj, M. Morgut, R. R. Stopar, E. Nobile
ANSYS Convergence Conference, Ljubljana, 25th of May 2016

13. Conclusions:
• Different types of cavitation in Francis turbines can be numerically
predicted.
• The effect of cavitation on turbine efficiency is quite accurately
predicted, with slightly premature drop of efficiency curve.
• The accuracy of calculated amplitudes of pressure pulsation,
caused by cavitating vortex rope at part load is improved if
cavitation is included in simulation.
Numerical simulation of different forms of cavitation in Francis turbines
D. Jošt, A. Škerlavaj, M. Morgut, R. R. Stopar, E. Nobile
ANSYS Convergence Conference, Ljubljana, 25th of May 2016

14. Acknowledgements
The research was funded by
the People Programme (Marie Curie Actions) of the European Union's
Seventh Framework Programme FP7/2007-2013/ under REA grant
agreement n°612279
and
Slovenian Research Agency ARRS, Contract No. 1000-09-150263
Numerical simulation of different forms of cavitation in Francis turbines
D. Jošt, A. Škerlavaj, M. Morgut, R. R. Stopar, E. Nobile
ANSYS Convergence Conference, Ljubljana, 25th of May 2016

15. Thank you for your attention !
Numerical simulation of different forms of cavitation in Francis turbines
D. Jošt, A. Škerlavaj, M. Morgut, R. R. Stopar, E. Nobile
ANSYS Convergence Conference, Ljubljana, 25th of May 2016