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 Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 1 Outline 1. Introduction 2. Observed Vibration Phenomena 3. Possible Excitation Mechanisms 4. Vibro-Acoustic Phenomena 5. Summary - Solution - Conclusion Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 2 Introduction Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 3 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) Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 4 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. Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 5 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 Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 6 Observed Vibration Phenomena Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 7 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. Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 8 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. Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 9 Possible Excitation Mechanisms Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 10 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! Guide vane modal analysis in water: 92 Hz 175 Hz 377 Hz 433 Hz z y x Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 11 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! Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 12 Possible excitation mechanisms (3) Rotor-stator interaction (RSI) • RSI freq. in stationary frame fS = m · Zr · N = m · BPF • RSI freq. in rotating frame fR = n · Zg · N = n · GPF • RSI pressure modes are characterized by the number k of diamterical node lines according to n · Zg + k = m · Zr ➜ Only the 8th harmonic of BPF is in the range of 300 Hz! Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 13 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! Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 14 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 Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 blunt TE fs 220 Hz 15 VibroAcoustic Phenomena MyPresentation2011.ppt | HDH_BHn | VHZ-hab | 2011-07-07 16 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 Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 17 Vibro-acoustic phenomena (2) Modal analysis: k=2 mode at f=301Hz Pressure field in the fluid domain Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 Axial runner displacement 18 Vibro-acoustic phenomena (3) Modal analysis: k=3 mode at f=325Hz Pressure field in the fluid domain Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 Axial runner displacement 19 Vibro-acoustic phenomena (4) Harmonic response: Spectra for k=3 excitation Pressure amplitude at runner inlet 295 Hz Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 Displacement amplitude at trailing edge 295 Hz 20 Vibro-acoustic phenomena (5) Harmonic response: k=3 excitation at f=295Hz Pressure field in the fluid domain Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 Axial runner displacement 21 Vibro-acoustic phenomena (6) Harmonic response: Spectra for k=7 excitation Pressure amplitude at runner inlet 306 Hz Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 Displacement amplitude at trailing edge 306 Hz 22 Vibro-acoustic phenomena (7) Harmonic response: k=7 excitation at f=306Hz Pressure field in the fluid domain Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 Axial runner displacement 23 Summary - Solution - Conclusion Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 24 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. Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 25 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. Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 26 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. Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 27 Björn Hübner Phone +49 7321 37 6693 [email protected] Voith Hydro Holding GmbH & Co. KG Basic Development − hab Heidenheim − Germany Hübner et al. | ACUM 2014 | Nürnberg | 2014-06-05 28
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