Vibration propagation due to vibro-acoustic resonance exemplified

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


Outline

1. Introduction

2. Observed Vibration Phenomena

3. Possible Excitation Mechanisms

4. Vibro-Acoustic Phenomena

5. Summary - Solution - Conclusion


Introduction


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)


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.


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


Observed Vibration Phenomena


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.


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.


Possible Excitation Mechanisms


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



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!


• 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


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!



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!



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


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



Modal analysis: k=2 mode at f=301Hz

Pressure field in the fluid domain Axial runner displacement



Modal analysis: k=3 mode at f=325Hz




Harmonic response: Spectra for k=3 excitation

Pressure amplitude at runner inlet Displacement amplitude at trailing edge

295 Hz 295 Hz



Harmonic response: k=3 excitation at f=295Hz




Harmonic response: Spectra for k=7 excitation

Pressure amplitude at runner inlet Displacement amplitude at trailing edge

306 Hz 306 Hz



Harmonic response: k=7 excitation at f=306Hz



Summary - Solution - Conclusion


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.


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.


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.


Björn Hübner

Phone +49 7321 37 6693

[email protected]

Voith Hydro Holding GmbH & Co. KG

Basic Development − hab

Heidenheim − Germany

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Vibration propagation due to vibro-acoustic resonance exemplified