Numerical simulation of different forms of cavitation in Francis turbines

ANSYS Convergence ConferenceLjubljana, 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

• 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 turbinesD. Jošt, A. Škerlavaj, M. Morgut, R. R. Stopar, E. Nobile

ANSYS Convergence Conference, Ljubljana, 25th of May 2016

Primary aim: to develop reliable, high-fidelity methods for accurateprediction, and optimization, of the performances of hydro-machineryand 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 Propellerswww.accusim.eu; 1/2/2014-31/1/2018

Knowledge

ResearchCommunity

&Public

Dissemination of knowledge

+



http://www.accusim.eu/

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



Damages of blades due to cavitation

l

M

vl

m

t

SPt

m

)(

)()(

11

U

τUUU

U

volumetotal

vapourvolume

volumetotal

liquid volume

1

vl

vl

)1(

)1(

Homogeneous two-phase model

Flow equations:

Liquid and vapour volume fractions:

Density and dynamic viscosity of the vapour-water mixture:



Cavitation

Zwart mass transfer model

Rayleigh-Plesset equation:

v

l

v

B

v

v

l

v

B

vnuc

PPP

PP

R

PPP

PP

R

r

m

if3

23F

if3

2)1(3F-

c

e

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 transfermodels for the numerical prediction of sheet cavitation around ahydrofoil. Int. Journal of Multiphase Flow. Vol. 37. No. 6. 2010. pp. 620-626.



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



The shape and extent of cavitationIsosurfaces of Vapour Volume Fraction =0.3

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 = 0.01Tuned constants: Fe = 300, Fc = 0.03



Draft tube swirl - Cavitating vortex rope in the draft tube at different operating regimes

Part Load High LoadQ/QBEP = 0.66 Q/QBEP = 0.8 Q/QBEP = 0.85 Q/QBEP = 1.15

Experiment



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.

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


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 loadPrevious 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

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 speedOutput: flow rate, torque on the shaft, efficiency, pressure pulsation

Pressurepulsationmeasurement



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

Experiment Simulation without cavitation modelling

Simulation with cavitation modelling



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 ifcavitation is included in simulation.



AcknowledgementsThe 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



Thank you for your attention !



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Numerical simulation of different forms of cavitation in Francis turbines

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