1 Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05
Vibration propagation due to vibro-acoustic
resonance exemplified at a Francis turbine
B. Hübner, U. Seidel, A. D`Agostini Neto / ACUM 2014 / Nürnberg / 2014-06-05
2 Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05
Outline
1. Introduction
2. Observed Vibration Phenomena
3. Possible Excitation Mechanisms
4. Vibro-Acoustic Phenomena
5. Summary - Solution - Conclusion
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Introduction
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Introduction (1)
Acoustic resonances in hydro turbines
Example of rotor-stator
interaction in a pump turbine
(compressible CFD analysis)
• Pressure amplification due to
acoustic resonance effects in
- water ways
- runner channels
- spiral case
(phase resonance)
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Introduction (2)
Vibro-acoustic resonances
Vibro-acoustic coupling
in a pump turbine
(acoustic FSI analysis)
• Natural frequencies are
governed by structural
and acoustic properties.
• Coupled vibro-acoustic
mode shapes include
runner displacement and
acoustic pressure field.
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Introduction (3)
Vortex shedding lock-in
• If the vortex shedding frequency fs is close to a natural
frequency of the structure, vortex shedding locks in at
this natural frequency (here: ft for the torsional mode).
• This lock-in effect leads to resonance conditions and
may cause large amplitude vibrations. Images: LMH - EPFL - Lausanne
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Observed Vibration Phenomena
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Measured guide vane vibrations at
medium head Francis turbine (1)
Vibration spectra of all guide vanes
Full load operation of unit A
Observed vibration behavior
• Distinct frequencies within
a narrow frequency band
around 300 Hz.
• All guide vanes of a unit
vibrate with exactly equal
frequencies.
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Measured guide vane vibrations at
medium head Francis turbine (2)
Short time FFT for a single guide vane
Load ramp from 75% to max. power at unit B
Observed vibration behavior
• Strong vibrations start at
90% power output.
• Vibration intensity increases
by approaching max. power
and remains stable at full
load.
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Possible Excitation Mechanisms
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Possible excitation mechanisms (1)
Guide vane resonance
• Natural frequencies of guide vanes in water does not exist
close to observed vibrations around 300 Hz.
➜ Guide vane resonance is not present!
x
z
y
Guide vane modal analysis in water: 92 Hz 175 Hz 377 Hz 433 Hz
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Possible excitation mechanisms (2)
Self-excited guide vane vibrations
Well-known instability
• Self-excited vibrations of
pump turbine guide vanes
at the beginning of pump
mode operation.
• Overlapping guide vanes
at small opening must be
present.
➜ Hydroelastic instability of fully opened guide vanes in turbine mode not known!
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• RSI freq. in stationary frame
• RSI freq. in rotating frame
• RSI pressure modes are
characterized by the
number k of diamterical
node lines according to
Possible excitation mechanisms (3)
Rotor-stator interaction (RSI)
n · Zg + k = m · Zr
fR = n · Zg · N = n · GPF
fS = m · Zr · N = m · BPF
➜ Only the 8th harmonic of BPF is in the range of 300 Hz!
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Possible excitation mechanisms (4)
Vortex shedding at guide vanes or stay vanes
• v. Karman vortex shedding at guide
vanes or stay vanes are a likely
source of guide vane vibrations.
• However, designed trailing edge
shapes are proven to surely prevent
vortex shedding, and unsteady CFD
analyses do not reveal any vortex
shedding below 1000 Hz.
➜ It is quite unlikely that the observed
vibrations are induced by vortex
shedding at guide or stay vanes!
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Possible excitation mechanisms (5)
Vortex shedding at runner blades
• Unsteady CFD analyses reveal vortex shedding at runner blades
with and without originally applied chamfering of trailing edges.
➜ Only possible excitation source found in the range of 300 Hz!
chamfered TE
fs 370 Hz
blunt TE
fs 220 Hz
16 MyPresentation2011.ppt | HDH_BHn | VHZ-hab | 2011-07-07
Vibro-Acoustic Phenomena
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Vibro-acoustic phenomena (1)
Finite element model of simplified geometry
FE model of the runner Simplified water domain (rotating frame)
Modal analysis: Wall BC (do nothing)
Harmonic response: Non-reflecting BC
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Vibro-acoustic phenomena (2)
Modal analysis: k=2 mode at f=301Hz
Pressure field in the fluid domain Axial runner displacement
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Vibro-acoustic phenomena (3)
Modal analysis: k=3 mode at f=325Hz
Pressure field in the fluid domain Axial runner displacement
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Vibro-acoustic phenomena (4)
Harmonic response: Spectra for k=3 excitation
Pressure amplitude at runner inlet Displacement amplitude at trailing edge
295 Hz 295 Hz
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Vibro-acoustic phenomena (5)
Harmonic response: k=3 excitation at f=295Hz
Pressure field in the fluid domain Axial runner displacement
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Vibro-acoustic phenomena (6)
Harmonic response: Spectra for k=7 excitation
Pressure amplitude at runner inlet Displacement amplitude at trailing edge
306 Hz 306 Hz
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Vibro-acoustic phenomena (7)
Harmonic response: k=7 excitation at f=306Hz
Pressure field in the fluid domain Axial runner displacement
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Summary - Solution - Conclusion
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Summary
• Comparable strong vibrations of all guide vanes at equal
frequencies were observed in a medium head Francis turbine.
• Vortex shedding at runner blade trailing edges is the only
excitation phenomena close to observed vibration frequencies.
• Both modal and harmonic analyses with different BCs reveal
vibro-acoustic resonance conditions in the range of 300 Hz.
• Corresponding mode shapes exhibit large trailing edge
deflections and high pressure pulsations in vaneless space.
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Solution
• Vortex shedding excitation at all runner blades locks in at
resonance frequencies of coupled vibro-acoustic mode shapes.
• Pressure pulsations of vibro-acoustic mode shapes induce
forced vibrations of all guide vanes with equal frequencies.
➜ By minimizing and de-tuning vortex shedding at runner blades
with an appropriate trailing edge shape, runner smoothness
and guide vane vibrations were reduced considerably.
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Conclusion
• Lock-in effects based on coupled vibro-acoustic resonance
conditions may synchronize and amplify vortex shedding.
• Vibro-acoustic mode shapes may propagate and amplify
pressure pulsations and vibrations within rotating and
stationary parts of turbines.
➜ The source of the excitation and the point of maximum
measured response may differ completely.
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Björn Hübner
Phone +49 7321 37 6693
Voith Hydro Holding GmbH & Co. KG
Basic Development − hab
Heidenheim − Germany