Conference PaperPDF Available

A challenging puzzle to extend the runner lifetime of a 100 MW Francis turbine

Authors:
  • HES-SO Valais-Wallis, Sion, Switzerland
  • HES-SO Valais-Wallis (Sion)
  • Kraftwerke Oberhasli AG

Abstract and Figures

The study is motivated by the fact that cracks on the runner blades of a 100 MW Francis turbine prototype have been noticed. The possible origin: a drastic increase of the number of daily starts/stops in the recent years. To this end, an in-situ experimental investigation using two synchronised sets of measurements has been conducted: the first one consists on onboard instrumentation while the other one consists on external non-intrusive instrumentation. The focus is put on the identification of the main stress-full operating condition responsible for these cracks, on a complete start-to-stop cycle including the full operating range from deep part-load up to full load. To complete the investigation of this challenging phenomenon puzzle, a complementary numerical approach has been provided. CFD of the fluid flow through the turbine for different operating conditions and FEM structural and modal analysis of the runner have been carried out and seem to confirm the observation.
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A challenging puzzle to extend the runner lifetime
of a 100 MW Francis turbine
V. Hasmatuchi J. Decaix M. Titzschkau C. Münch-Alligné
HES-SO Valais//Wallis HES-SO Valais//Wallis Kraftwerke Oberhasli AG HES-SO Valais//Wallis
School of Engineering School of Engineering Grimsel Hydro School of Engineering
Route du Rawyl 47 Route du Rawyl 47 Grimselstrasse 19 Route du Rawyl 47
CH-1950 Sion CH-1950 Sion CH-3862 Innertkirchen CH-1950 Sion
Switzerland Switzerland Switzerland Switzerland
Abstract
The study is motivated by the fact that cracks on the runner blades of a 100 MW Francis turbine prototype have
been noticed. The possible origin: a drastic increase of the number of daily starts/stops in the recent years. To
this end, an in-situ experimental investigation using two synchronised sets of measurements has been conducted:
the first one consists on onboard instrumentation while the other one consists on external non-intrusive
instrumentation. The focus is put on the identification of the main stress-full operating condition responsible for
these cracks, on a complete start-to-stop cycle including the full operating range from deep part-load up to full
load. To complete the investigation of this challenging phenomenon puzzle, a complementary numerical
approach has been provided. CFD of the fluid flow through the turbine for different operating conditions and
FEM structural and modal analysis of the runner have been carried out and seem to confirm the observation.
1. Background
In the framework of the development and the integration of new renewable energy sources such as wind or solar,
the electrical grid undergoes sometimes instabilities. Hydraulic turbines and pump-turbines are key technologies
to stabilize the grid. However to reach this objective, the hydraulic machines have usually to operate in extended
range (Lowys et al. [1]). Such operating conditions require dealing with frequent start-up and stand-by
conditions (Seidel et al. [2], Robert et al. [3], Coutu and Chamberland-Lauzon [4]), leading often to a reduction
of the machines lifespan. Indeed, this is indirectly linked to an increase of the number of high mechanical stress
cycles to which the machine has to make face (Sick et al. [5]).
From the electrical grid regulation point of view, pumped-storage power plants are one of the most suitable
solutions to balance the electrical power at a large scale. Since most of these power plants have been built
several decades before the integration of the new renewable energies, the stresses induced by the large number of
daily start and stop procedures have not been taken into account during the design phase of the machines.
Therefore, prototype experimental measurements must be performed in order to better predict the remaining
residual lifetime of the machine (Drefs et al. [6]) or to investigate the source of harsh conditions that may
conduct, in the worst case, to cracks (Decaix et al. [7]). In such conditions, one of the most affected component
of the turbine is the runner. Experimental measurements during the operation directly on the runner can give the
most direct responses when conducting such investigations. In the recent years, with the actual technological
possibilities, several successful attempts of measurements in the rotating frame during operation have been
performed on prototype by Gagnon et al. [8] & [9] or by Egusquiza et al. [10].
Complementary, numerical tools are often necessary to investigate complex phenomena in hydraulic machines
coupling both fluid and structure (Sick et al. [5], Münch et al. [11] or Hübner et al. [12]). Indeed, Computational
Fluid Dynamics (CFD) has already proved to be a reliable tool for assessing pressure fluctuations as for instance
due to the Rotor-Stator Interaction or during speed-no load (SNL) conditions (Casartelli et al. [13], Nennemann
et al. [14]). Then, the Finite Element Method (FEM) is commonly used to provide modal and harmonic analyses
of the runner in air or in water necessary to detect the structure Eigen frequencies and modes (Lais et al. [15]),
and further the fluid-induced stresses (Nennemann et al. [14], Seidel et al. [16]).
The present work focuses on a complementary experimental (Hasmatuchi et al. [17]) and numerical (Decaix et
al. [7]) investigation of the main source of runner blades cracks identified on a high-head turbine prototype,
previously investigated by Müller et al. [18]. The methodology comes with the case study, followed by a short
introduction of problematic and the applied investigation strategy. The experimental setup details the challenging
synchronised rotating-stationary frames measurements architecture. Then, the CFD and FEM numerical setups
are provided. The experimental results are mainly focused on the evidence of harsh runner blades loading
fluctuations during the start-up and shut down procedures. The numerical results come to sustain the
experimental analysis. Finally, a discussion and concluding remarks complete the work.
2. Methodology
The present project focuses on a 100 MW Francis turbine prototype, part of one of the four horizontal ternary
groups of Grimsel 2 pumped-storage power plant, in Switzerland (Schlunegger & Thöni [19]). Due to a drastic
increase of the number of daily starts/stops in the recent years, cracks on the runner blades of the Francis
turbines have been noticed, without a clear explanation regarding the phenomenon responsible for their onset.
The turbine runner, composed by 17 blades, has a specific speed of ν = 0.247, whilst the governor is equipped by
24 guide vanes. The speed and discharge factors of the turbine at Best Efficiency Point (BEP) are nED = 0.271
and QED = 0.183 respectively.
Fig. 1. Strategy of complementary experimental and numerical investigation.
The main purpose of this study is to identify the main stress-full operating condition responsible for these cracks,
on a complete start-to-stop cycle including the full operating range from deep part-load up to full load. The
applied strategy of this complementary experimental and numerical investigation is illustrated in Fig. 1. The full
study is divided in 3 main steps. The first one consists on mapping the harmful sources on the full turbine
operating range, including the start-up and shut down phases. Then, the effort is put on the identification of a
possible alternative start-up path, or speed no-load (SNL) operating point (OP) encountered during the
synchronisation of the generator with the grid. The third step consists on tests of an alternative start-up path
(and/or an alternative SNL operating point). The final objective of the study is to establish a protocol to mitigate
these harsh operating conditions on a different test case. To this end, an in-situ experimental investigation using
two synchronised sets of measurements has been conducted on the turbine of group no. 2: the first one consists
on onboard measurements while the other one consists on external non-intrusive measurements. The puzzle is
completed with results from numerical simulations including both CFD and FEM tools. The CFD analysis are
conducted using steady and unsteady flow simulations, while the FEM consists on modal analysis of the runner
and calculation of stresses.
3. Experimental setup
3.1 Synchronised rotating-stationary frames measurements architecture
The architecture of the experimental instrumentation is provided in Fig. 2. The philosophy of the employed
instrumentation to identify the possible harmful structural conditions for the turbine runner consists on
synchronised measurements between the rotating and the stationary frames of the machine. The onboard
instrumentation, which is the most challenging, includes strain gauges located on the runner blades,
accelerometers and tachometers. An autonomous acquisition system supplied by batteries has been installed in a
sealed chamber into the nozzle of the turbine runner. The stationary frame instrumentation used to capture the
source of the instabilities in the same time with the rotating frame instrumentation consists mainly on two
synchronised digitizers connected to several non-intrusive monitoring sensors: pressures sensors, flowmeter,
tachometer, proxymeters, microphone and accelerometers. The final purpose is to succeed to identify the sources
of instabilities (possible easily detectable with the onboard measurements) while using only non-intrusive
measurements in the stationary frame (as done by Botero et al. [20]). This approach should provide a smart
diagnostic tool useful for other machines suffering of similar problems.
The synchronisation between the onboard and the stationary frame systems has been ensured by hammer impacts
on the structure of the turbine registered simultaneously by the accelerometers connected on each acquisition
system. The synchronisation with the data coming from power plant’s SCADA control system and from the
dedicated control/monitoring system of the turbine governor is based on the identification of different events
such as the runner start-up, guide vanes opening and so on.
Fig. 2. Experimental instrumentation architecture.
3.2 Rotating frame instrumentation
A Gantner Q.Station T autonomous digitizer (acquiring continuous synchronized signals at 10 kHz) equipped
with 4 dedicated acquisition boards, all mounted on a compact Q.brixx chassis, has been installed in a sealed
chamber into the nozzle of the turbine runner (see Fig. 3). The chamber is designed with 5 inserts that allows
mounting the different necessary connectors between the inside and the outside of the chamber. The rotating
frame instrumentation has been supplied with power by two LiPo batteries connected in parallel. Apart from the
operation during the rotation of the runner, once the machine stopped, the system had to be accessible in terms of
power switch on/off, programming and recovery of data as well as the batteries recharging. The system must
operate autonomously during minimum 12 hours, the different components being subject to centrifugal forces
due to the rotation of the runner (from 0 to 750 rpm) and due to the high level of vibrations, particularly
pronounced during the turbine and pump start-up phases. Finally, two single-axis IEPE accelerometers
(Wilcoxon 726T), two Sick IM18-10BNS-NC1 inductive tachometers and eight HBM 1-LY11-3/350 strain
gauges fixed on four consecutive runner blades have been connected to the acquisition system.
Fig. 3. Experimental instrumentation setup: rotating frame.
3.3 Stationary frame instrumentation
The stationary frame instrumentation consists of two synchronised National Instruments (NI) acquisition systems
(NI PXIe-1073 and NI cDAQ-9174) equipped with several dedicated acquisition modules (see Fig. 4). One
specific interface has been developed under Labview® environment to drive the continuous acquisition using a
10 kHz rate for all connected sensors. Concerning the instrumentation, two Kistler IEPE accelerometers (one tri-
axial and one mono-axial) have been placed on the turbine draft tube wall close to the outlet of the runner. A
GRAS IEPE microphone, directed again towards the outlet of the runner, has also been installed. The signals of
the two existing Bruel&Kjaer bearing eddy-current proximity sensors monitoring the turbine shaft bearing have
been also acquired. A Sick optical tachometer has been installed to recover the rotational speed of the runner
during operation.
The stationary frame instrumentation has been completed with a Balluff single-axis inclinometer installed on the
tip of the shaft of one guide vane. In addition, two pressure sensors have been installed to recover the head of the
turbine, respectively at the inlet of the spiral casing and at the outlet of the draft tube, along with an absolute
pressure sensor (dedicated to recover the atmospheric pressure during the tests). An ultrasonic flowmeter has
been added on the pipe upstream the spiral casing. Finally, two mono-axial IEPE accelerometers have been
installed on the draft tube of the pump close to the inlet of the impeller.
Fig. 4. Experimental instrumentation setup: stationary frame.
4. Numerical setup
Numerical simulations of unsteady incompressible turbulent flow (CFD) inside the turbine have been performed
using the Ansys® CFX v17.2 commercial code. The computational domain, illustrated in Fig. 5, takes into
account the whole turbine prototype from the inlet of the spiral casing up to the draft tube outlet. An adapted
Fig. 5. CFD (top) and FEM (bottom) numerical setup, Decaix et al. [7].
mesh using hexahedral cells, counting about 15 millions of nodes in total, with 2.8 millions of nodes in the
runner and 4 million of nodes in the guide vanes, has been generated with the Ansys ICEM commercial
software. Concerning the boundary conditions, the mass flow rate corresponding to the SNL operating condition
(at 2° of guide vanes opening angle), encountered by the turbine during the synchronisation of the generator with
the network, is imposed at the inlet of the spiral casing. An opening pressure condition has been set at the outlet
of the draft tube. A transient rotor/stator algorithm has been used for the interfaces between the stationary and
the rotating domains. Then, the flow is computed by solving both the Unsteady Reynolds-Averaged Navier-
Stokes (URANS) equations in their conservative form and the mass conservation equations using the finite
volume method (see Launder & Spalding [21]). The set of equations is closed and solved using a two-equation
turbulence model: the Shear Stress Transport (SST) model, Menter [22]. An advection scheme (2nd order in
space) and the backward Euler implicit scheme (2nd order in time) are used. The total computed physical time
corresponds to 12 runner revolutions with a timestep smaller than 1° of the runner revolution.
The modal analysis system available in the Ansys Workbench 17.2. has been employed to study the turbine
runner. The computational domain is shown in Fig. 5 (bottom-left side). The runner and the surrounding water
volume are meshed with approximately 300’000 tetrahedral elements (see the bottom-centre figure). A fixed
support condition (illustrated on the bottom-right figure) has been set at the junction between the runner and the
shaft. The FEM has been carried out for the runner at rest surrounded by water. Indeed, this allows us focusing
on the influence of the added mass effect (due to the presence of water) on the modes and Eigen frequencies.
5. Results
5.1 Experimental evidence of harmful turbine start-up and shut down procedures
For a given upper and lower dams level, the full turbine hill chart has been covered during the measurement
campaign including start-up, speed no-load, deep part load, best efficiency, full load and shut-down operating
conditions. In Fig. 6, focusing on a complete start-to-stop cycle, the signals of two strain gauges installed on the
blade #17 of the turbine runner close to the hub and to the shroud (aligned with the blade trailing edge, see Fig.
3) along with the signal of one onboard accelerometer are provided. Chronologically, one may easily identify on
the fluctuations of the strain and of the vibrations the turbine filling, the turbine start-up, followed by the phase
of synchronization of the generator with the electrical network, then the stable part load operation and finally the
turbine shut down and the draining. The noticed f/fn = 48.9 frequency (fn stands for the runner nominal rotational
frequency) of strain fluctuations, is very close to the first harmonic of the blade passing frequency, being actually
the response of one of the runner Eigen modes. The fluctuations amplitude of this strain direction is larger at the
shroud than at the hub blade side. Then, the strain fluctuations show also a sub-synchronous frequency,
compared to the runner rotational frequency.
Fig. 6. Evidence of harsh strain fluctuations on the runner blades during start-up
and shut down procedures, Hasmatuchi et al. [17].
5.2 Strain and vibration fluctuations chart full operating range
Then, a hydro-structural stability diagnosis diagram of the prototype has been established for the whole
operating range. In Fig. 7, the fluctuations standard deviation (STD) of the runner blades strain (left) and of the
runner vibrations (right) at SNL, deep part load, and the full normal operating range of the turbine are provided.
At the normal operating range, the guide vanes opening is larger than around 12°, with the best efficiency point
(BEP) at around 18° for the tested head. As already observed in Fig. 6 from the history of fluctuations during one
start-to-stop cycle, the amplitude of blades loading fluctuations as well as of the vibrations are several times (up
to 6 times) larger than on the full operating range and even than on the deep part load regime, particularly
interesting in terms of operation flexibility.
Fig. 7. Fluctuations standard deviation of the runner blades strain (left) and the runner vibrations (right)
at SNL, deep part load, and the full normal operating range of the turbine.
5.3 Numerical investigation results
To complete the investigation and, at the end, to provide a solution, complementary CFD numerical simulations
of the fluid flow through the turbine for different operating conditions, along with FEM structural and modal
analysis of the runner have been carried out.
For a combination of 17 runner blades and 24 guide vanes, applying the theory of Tanaka [24], the resonance of
the runner could occur if the excitation source due to the Rotor-Stator Interaction (RSI) has k = 3 nodal
diameters (ND), with an excitation frequency of 48·fn in the rotating frame and 51·fn in the stationary frame. In
the right side of Fig. 8, the resulting three bending modes, obtained by FEM modal analysis, are illustrated.
According to Tanaka, the 3 ND bending mode should not be excited due to the RSI since the dimensionless
frequency of the bending mode f/fn = 38.5 does not match the f/fn = 48 dimensionless frequency of the RSI.
However, the frequency of the bending modes with respectively 4 and 5 nodal diameters are close to the one
observed on the measurements.
Finally, in the left side of Fig. 8, the resulting turbine flow field at SNL operating point along with the pressure
contour on the runner as well as the iso-surface of the Q-criterion at the trailing edge of the blades are illustrated.
The velocity streamlines show a flow organisation typical for an off-design operating point. Then, a high-
pressure region close to the blades trailing edge can be noticed. Moreover, a large spanwise vortex and a smaller
streamwise vortex are located at the junction between the hub and the blade surround this region, where cracks
have been observed.
Fig. 8. Resulting flow field (left) at SNL operating point along with pressure contour on the runner wall
and the iso-surface of the Q-criterion at the trailing edge off the blades;
Total displacement for bending modes (right) obtained by FEM simulations.
5.4 Discussion
The maximum amplitude of strain fluctuations (extremely high compared with the values at part load operation)
seems to occur at SNL operating point during the synchronization of the generator with the network as well as
during the shutdown phase. Indeed, such behaviour has been already noticed by Gagnon et al. [23] or Coutu and
Chamberland-Lauzon [4]. To complete, theses harsh operating conditions, that accelerate the shortening of the
runners remaining lifetime, are encountered each time the turbine started and stopped for several tens of seconds,
or, in the worst case, even up to few minutes, as noticed by the operators.
Considering the same SNL operating point, one may state that the same predominant oscillation (f/fn = 48.9) is
observed not only on the signal of the accelerometer installed in the runner, but also in the signal of the
accelerometer installed on the turbine casing, as well as in the ones coming from the microphone and from the
proximity sensor (see Decaix et al. [7]). This result is actually interesting in the way that the final purpose of this
study is to establish a diagnosis protocol based only on a simplified instrumentation set (basically non-intrusive)
to identify harsh operating conditions on different hydropower units in order to avoid them.
Conclusion
The present work has focused on a 100 MW Francis turbine prototype, part of one of the four horizontal ternary
groups of Grimsel 2 pumped-storage power plant, in Switzerland. Due to a drastic increase of the number of
daily starts/stops in the recent years, cracks on the runner blades of the Francis turbines have been noticed,
without a clear explanation regarding the phenomenon responsible for their onset.
To this end, an experimental investigation using two synchronised sets of measurements has been conducted.
The first one, which is the most challenging, consists on onboard measurements during the operation of the
machine, using strain gauges located on the runner blades, accelerometers and tachometers. The other one
consists on external non-intrusive measurements based on accelerometers, microphone, bearing proxymeters and
tachometer, along with an inclinometer, an upstream and a downstream pressure sensor as well as an ultrasonic
flowmeter. The main purpose of this study was to identify the main stress-full operating condition responsible
for these cracks, on a complete start-to-stop cycle including the full operating range from deep part-load up to
full load. To complete the investigation of this challenging phenomenon puzzle and, at the end, to provide a
solution, a complementary numerical investigation has been performed. CFD numerical simulations of the fluid
flow through the turbine for different operating conditions and FEM structural and modal analysis of the runner
have been carried out.
To conclude, the onboard measurements evidenced the highest mechanical stresses on the runner blades at speed
no-load operating condition during the synchronization of the generator with the network. The high stresses
frequency spectrum noticed on the runner blades, observed on the various signals as well, might suggest a
mechanical excitation of the runner blades possibly linked to the rotor/stator interaction between the runner and
the guide vanes and/or to a particular hydrodynamic flow instability developing in the runner. This conclusion is
supported by CFD and modal analysis, which put in evidence the possible excitation of one of the runner’s Eigen
frequency by the fluctuations of the pressure field.
Acknowledgements
The present work is part of the FlexSTOR research project (Innosuisse no. 17902.3 PFEN-IW-FLEXSTOR) standing for
“Solutions for flexible operation of storage hydropower plants in changing environment and market conditions”, bringing
together the SCCER-SoE research partners and Kraftwerke Oberhasli AG (KWO) to improve the climate and market
resilience, as well as eco-compliance of Switzerland. The study has been realised in the framework of SCCER - Supply of
Electricity Swiss program, supported by the Innosuisse Swiss Innovation Agency, in collaboration with EPFL Laboratory
for Hydraulic Machines and Laboratory of Hydraulic Constructions, with the support of the Kraftwerke Oberhasli AG -
Grimsel Hydro industrial partner.
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The Authors
V. Hasmatuchi graduated in 2007 at the Faculty of Mechanical Engineering, Hydraulic Machinery Branch from
“Politehnica” University of Timisoara, Romania. In the same year, Vlad Hasmatuchi joined the Laboratory for Hydraulic
Machines from the École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, to achieve a doctoral work in the field of
hydraulic turbomachinery. In 2012 he got his Doctoral Degree in Engineering from the EPFL. Since 2012 he is Senior
academic associate in the Hydroelectricity research team at the HES-SO Valais//Wallis, School of Engineering in Sion,
Switzerland. He is in charge mainly of experimental investigations, as well as of numerical simulations. His research interests
are the hydrodynamics of turbines, pumps and pump-turbines, including design and evaluation of hydraulic performance.
J. Decaix graduated in 2009 in Energy and Production at the INP Grenoble, France. In 2012 he got his Doctoral Degree in
Fluid Mechanics, Process and Energy from the University of Grenoble, France, with a doctoral work in numerical simulation
of cavitating flow performed in the Laboratory of Geophysical and Industrial Fluid Flows. In 2012, he joined the hydraulic
research team of the HES-SO Valais//Wallis, School of Engineering in Sion, Switzerland. Since 2016, he is Senior academic
associate in the same Hydroelectricity research team of HES-SO Valais. His main research activity is focused on numerical
simulation of hydraulic turbines.
M. Titzschkau studied mechanical engineering at the University of Karlsruhe (TH), nowadays KIT, with a focus on fluid
machinery and material sciences. He graduated with a diploma thesis about optimization of semi open impellers by using
CFD and stereo PIV in 2009.Since then he works for the KWO as a research engineer in the field of turbine rehabilitation and
optimization. From 2012-13 he was responsible for the submission of the electromechanical equipment of the new power
plants Handeck 2A and Innertkirchen 1A.
C. Münch-Alligné obtained an engineering degree from INPG, Grenoble, France, department of Numerical and Modelling
of Fluids and Solids, in 2002. She then obtained a grant from the CNRS and the CNES to start a PhD thesis on large eddy
simulations of compressible turbulent flows. She defended her doctoral degree in 2005 at the INPG. From 2006 to 2010, she
worked as a research associate in the Laboratory of Hydraulics Machines at EPFL on flow numerical simulations in hydraulic
turbines. Since 2010, she has been Professor at HES-SO Valais/Wallis, School of Engineering in Sion, Switzerland. She is
head of the Hydroelectricity research team. Her main research interests are hydraulic turbomachinery, numerical simulations,
performance measurements, turbulence and fluid-structure interactions.
... However, to provide frequent grid regulation services, partial damages caused by transient operation must carefully be analyzed. Frequent start and stop cycles of conventional hydropower plants may drastically IOP Publishing doi: 10.1088/1755-1315/1079/1/012079 2 accelerate fatigue damage of hydro-mechanical components like runners as experienced in [1]. To overcome such issues, modern technologies like variable speed drives [2], dedicated to the optimization of hydropower plants in terms of life cycle and flexibility operation, are investigated and demonstrated in the XFLEX HYDRO project [3]. ...
Article
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Variable speed hydropower units offer a large spectrum of grid regulation services and may therefore contribute to the stability of future power supply systems. Full Size Frequency Converters (FSFC) already found real world application in Pumped Storage Hydropower Plants up to a rated power of 100 MW and are even considered scalable up to a few hundred MW. Apart from the extension of the power range and grid regulation capacities, the FSFC technology also provides new control possibilities during transient operations such as start-up in generating mode. Thus, harsh conditions with damaging impact on the hydromechanical components may be avoided by tuning the operating point trajectory in the start-up phase. In this paper, runner fatigue damage during start-up in generating mode of a 5 MW variable speed Francis pump-turbine prototype equipped with a FSFC is numerically analyzed. The fixed speed solution is compared to a variable speed solution following a BEP tracking control strategy. 1D hydraulic transient simulations provide boundary conditions for detailed 3-D CFD/FEA simulations. Full and reduced numerical domains are used and compared. The overall outcome of the present numerical study indicates an important reduction of partial damages using variable speed drives for turbine start-up manoeuvres.
... During the last 15 years, several improvements have been achieved in the development of URANS and hybrid RANS/LES models [33][34][35]. Consequently, the present paper focuses on the influence of some "advanced" URANS turbulence models regarding the dynamics of the flow at the SNL operating condition of a high head Francis turbine already investigating at part load by Müller et al. [36] and at various operating points by the current authors [37][38][39]. ...
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