MARKKU YLÖNEN CAVITATION EROSION CHARACTERIZATION OF A FRANCIS TURBINE RUNNER BLADE MATERIAL. Master of Science Thesis - PDF

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MARKKU YLÖNEN CAVITATION EROSION CHARACTERIZATION OF A FRANCIS TURBINE RUNNER BLADE MATERIAL Master of Science Thesis Examiners: Professor Pentti Saarenrinne and Docent Juha Miettinen Examiners and topic

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MARKKU YLÖNEN CAVITATION EROSION CHARACTERIZATION OF A FRANCIS TURBINE RUNNER BLADE MATERIAL Master of Science Thesis Examiners: Professor Pentti Saarenrinne and Docent Juha Miettinen Examiners and topic approved by the Faculty Council of the Faculty of Natural Sciences on 7 th October 2015 i ABSTRACT Markku Ylönen: Cavitation Erosion Characterization of Francis Turbine Runner Blade Material Tampere University of Technology Master of Science Thesis, 116 pages, 2 Appendix pages March 2016 Master s Degree Program in Environmental and Energy Engineering Major: Power Plant and Combustion Engineering Examiners: Professor Pentti Saarenrinne and Docent Juha Miettinen Keywords: cavitation erosion, acoustic emission, cavitation detection, material cavitation resistance, Francis turbine In this study, the cavitation erosion resistance was characterized for a steel type used in Francis turbine runner blades. The goal of the study was to define the cavitation erosion rate of the runner blade steel and to compare the results to those of previously studied materials. The previously studied materials were aluminium alloy 7075, nickel aluminium bronze alloy C95400 and stainless steels A2205 and 304L. The material was tested in a cavitation tunnel in which sheet and cloud cavitation appears in the test section. Acoustic emission signal was measured from the sample experiencing cavitation erosion. Cavitation erosion can be divided into four distinct stages; the incubation period, the acceleration period, the deceleration period and the steady state period. Each of these periods is connected to the material response of a sample in a cavitation field. The material response is linked to the stage of work hardening and material cavity formation, as cavitation modifies the material surface. The cavitation tunnel used in this study circulates water and it has variable upstream and downstream pressures. The test section is a radially diverging channel in which cavitation inception occurs in the beginning of the radial section and cavitation closure occurs several millimeters downstream. The sample was cylindrical with one face experiencing cavitation. The cavitation erosion evolution was measured with a contact profilometer. The volume loss and the maximum depth of penetration were calculated from the surface profiles and they were compared to results from other materials. The acoustic emission results were compared with the erosion evolution. Material characteristics were obtained by elemental analysis, by macroscopic compression tests, by nanoindentation tests and by split Hopkinson pressure bar tests. The results show that the studied steel is as resistant to cavitation erosion as aluminium alloy 7075 and less resistant to cavitation erosion than nickel aluminium bronze alloy C95400 and stainless steels A2205 and 304L. The reason for the lower resistance compared to the stainless steels is corrosion, lower quality level or both. The voltage root mean square value of the acoustic emission signal reduced with increasing cumulative erosion time. Acoustic emission monitoring was found to be a potential method in estimating cavitation erosion evolution in hydraulic machines. ii TIIVISTELMÄ Markku Ylönen: Kavitaatioeroosion karakterisointi Francis-turbiinin juoksupyörän materiaalille Tampereen teknillinen yliopisto Diplomityö, 116 sivua, 2 liitesivua Maaliskuu 2016 Ympäristö- ja energiatekniikan diplomi-insinöörin koulutusohjelma Pääaine: Voimalaitos- ja polttotekniikka Tarkastajat: professori Pentti Saarenrinne ja dosentti Juha Miettinen Avainsanat: kavitaatioeroosio, akustinen emissio, kavitaation havaitseminen, materiaalin kavitaatiokestävyys, Francis-turbiinin juoksupyörä Tässä työssä tutkittiin Francis-turbiinin juoksupyörän siivessä käytettävän terästyypin kavitaatiokestävyyttä. Työn tavoitteena oli tutkia materiaalin eroosionopeutta sekä verrata sitä aiemmin tutkittuihin materiaaleihin. Aiemmin tutkitut materiaalit olivat alumiiniseos 7075, nikkeli alumiini pronssiseos C95400 sekä ruostumattomat teräkset A2205 ja 304L. Materiaalia tutkittiin kavitaatiotunnelissa joka tuottaa taso- sekä pilvimäistä kavitaatiota testausosiossaan. Näytekappaleesta mitattiin akustista emissiota kun se oli kavitaatioeroosion vaikutuksen alaisena. Kavitaatioeroosiolla on neljä toisistaan erotettavaa vaihetta: alkuvaihe, kiihtymisvaihe, hidastumisvaihe sekä tasaisen eroosionopeuden vaihe. Jokainen vaihe on yhteydessä materiaalin kavitaatiovasteeseen. Tämä vaste on yhteydessä materiaalin muokkauslujittumisen sekä onkaloiden syntymisen tasoihin, koska kavitaatio muokkaa materiaalin pintaa. Tutkimuksessa käytetty kavitaatiotunneli kierrättää vettä ja sen ylä- sekä alavirran paineita voidaan muuttaa. Testausosio on radiaalisesti laajeneva kanava, jossa kavitaatio syntyy radiaalisen osion alussa ja romahtaa muutaman millimetrin päässä alavirtaan. Näytekappale oli sylinterimäinen ja sen yksi sivu altistui kavitaatioeroosiolle. Kavitaatioeroosion eteneminen mitattiin kontaktiprofilometrillä. Pintaprofiileista laskettiin tilavuushäviö sekä suurin eroosiosyvyys ja näitä tuloksia verrattiin toisten materiaalien vastaaviin tuloksiin. Akustisen emission mittaustuloksia verrattiin eroosion kehittymiseen. Materiaalin ominaisuudet selvitettiin alkuaineanalyysillä, makroskooppisilla puristustesteillä, nanoindentaatiolla sekä split Hopkinson pressure bar -testeillä. Tulokset osoittavat, että tutkittu teräslaatu on kavitaatiokestävyydeltään yhtä hyvä kuin alumiiniseos 7075, mutta huonompi kuin nikkeli alumiini pronssiseos C95400 tai ruostumattomat teräkset A2205 sekä 304L. Syy ruostumatonta terästä heikompaan kestävyyteen on joko korroosiossa, matalammassa laatutasossa tai molemmissa. Akustisen emission signaalin ulostulojännitteen neliöllisen keskiarvon kokonaiskeskiarvon havaittiin laskevan kavitaatioeroosion edetessä. Tämän havainnon vuoksi akustisen emission arvioitiin olevan potentiaalinen menetelmä hydraulisten koneiden kavitaatioeroosion vaiheiden tunnistamiseen. iii PREFACE This Master of Science thesis was written as a part of a larger project called Nopeutettu elinkaaren määritys or Accelerated life cycle estimation. The project was managed by Tampere University of Technology with Professor Pentti Saarenrinne and Docent Juha Miettinen as the responsible persons. The project was funded by Tekes the Finnish Funding Agency for Technology and Innovation, by companies and by Tampere University of Technology. The companies that funded the project are: Fortum Power and Heat Oy, Sandvik Mining and Constructions Oy, Teollisuuden Voima Oyj and Valtra Oy Ab. Neurovision Oy and Oliotalo Oy supported the project with knowledge and technology. The work in this thesis was carried out with a significant collaboration with the French National Center for Scientific Research (CNRS). The author did all the experiments linked to this thesis in the LEGI (Laboratoire des Ecoulements Géophysiques et Industriels) and the SIMaP (Science et Ingénierie des Matériaux et Procédés) laboratories in Saint-Martin d Hères, France. I want to thank Professor Pentti Saarenrinne and Docent Juha Miettinen for trusting me enough to hire me for the project and for their help and support in the process of completing my thesis. I want to thank Jean-Pierre Franc, the director of research of LEGI and Marc Fivel, the director of research of SIMaP-GPM2 for their invaluable aid and support in the scientific content of my research. Especially I want to thank Michel Riondet, the technical engineer responsible for the test setup, for the practical every day aid in using and understanding the cavitation tunnel used in this study. I also want to thank all the funders of the project and especially Fortum Power and Heat Oy for providing the interesting research topic. I want to thank my colleagues in LEGI for the everyday scientific and non-scientific communication we had. My special thanks go to my future wife Jenni Niska, who carried me through the exhausting process of finishing my thesis. Saint Martin d Hères, France, Markku Ylönen iv CONTENTS 1. INTRODUCTION CAVITATION Basic concepts Definition and occurrence Cavitation number Cavitation nuclei and bubble formation Bubble collapse Cavitation erosion Cavitation impulses and loadings Numerical methods: a brief description Re-entrant jet dynamics Bubble stand-off distance effects Material response Pit formation Erosion modelling Cavitating flow characterization Transient isolated bubbles Attached or sheet cavities Cavitating vortices Periods in cavitation erosion Incubation period Acceleration, deceleration and steady-state periods Erosion measurement and calculation ACOUSTIC EMISSION Definition Acoustic emission sources Acoustic emission wave propagation Important features in acoustic emission signals Acoustic emission measurement EXPERIMENT SETUP Experiment goals Sample preparation PREVERO high speed cavitation tunnel Machine operation Measurement parameters Surface profile measurement Acoustic emission measurement Measurement setup Sensor characteristics... 55 v 4.6 Material characterization tests Material density Compression tests Nanoindentation tests Split Hopkinson pressure bar tests Alternative cavitation erosion testing setups ASTM G32 vibratory cavitation apparatus ASTM G134 cavitating liquid jet Rotational disk apparatus and vortex cavity generator RESULTS Introduction to results Material characterization Elemental analysis Compression tests Nanoindentation tests Split Hopkinson pressure bar tests Erosion evolution Sample photographs Surface profiles Volume loss Maximum erosion depth Acoustic emission results ANALYSIS Erosion evolution and volume loss Volume loss and erosion depth Volume loss and erosion depth rates Material characteristic comparison Acoustic emission and erosion stages CONCLUSIONS REFERENCES APPENDIX 1: PREVERO cavitation tunnel measurement log vi LIST OF FIGURES AND TABLES Figure 1.1. Cavitation erosion damage in the Imatra power plant G2 unit runner blades Figure 2.1. Static equilibrium curves for several initial values of R 0 and for a constant P g0. [29]... 9 Figure 2.2. Static bubble re-entrant jet development and impact for nondimensional time-steps: a: t *= b: t *= c: t *= d: t *= The arrows in the figures represent the liquid flow relative velocity. The velocity increases with increasing arrow length. [40] Figure 2.3. Static bubble collapse near a wall with four different values of wall elasticity modelled by a spring-backed membrane model. The wall is located in the z = 0 level indicated in the figures. [61] Figure 2.4. Impact pressure for varying bubble collapse driving pressure, modelled by the incompressible and compressible link model. [50] Figure 2.5. Normal stress component of the first plate element below material surface in a cavitation impact simulated with a FEM-model. Bubble initial radius 400μm, stand-off distance 400μm and collapse driving pressure rise from 0.1 MPa to 10 MPa. [50] Figure 2.6. The typical erosion and erosion rate curve for an ASTM G32 device. Curve (a) is for the volume loss and curve (b) is for the volume loss rate. [44] Figure 2.7. Visualization of the geometric effect on bubble number density. Filled circles present initial bubbles and the non-filled circles present two potential bubble collapse patterns Figure 3.1. The difference between discontinuous and continuous acoustic emission signals. [84] Figure 3.2. The differences between longitudinal and transverse stress waves. Transverse waves are also called as shear waves. [90] Figure 3.3. Typical extracted parameters from an acoustic emission event. [92] Figure 3.4. The structure of an acoustic emission sensor. [93] Figure 4.1. Cavitation erosion test sample details. [96] Figure 4.2. Basic operation principle of the PREVERO cavitation tunnel. The arrows depict the flow direction. Downstream is defined to be after the test section until the pump and upstream is defined to be after the pump until the test section Figure 4.3. PREVERO cavitation tunnel test section details. [6] Figure 4.4. Sample surface after 2 minutes of cavitation erosion in the PREVERO cavitation tunnel vii Figure 4.5. Acoustic emission measurement setup with the sample and the sample holder Figure 4.6. Frequency responses for the Brüel & Kjær 8313 and 8314 AEsensors. [98] Figure 4.7. Frequency response for the Fujicera 1045S AE-sensor. [99] Figure 4.8. Photograph of a non-compressed sample and a compressed sample Figure 4.9. True stress in function of true strain from the compression test with a strain rate of /s. The curve begins from zero loading and the load is increased up to 75 kn load. After this, the load is discharged. This corresponds to the curve data after the maximum true stress Figure An example of a load-displacement curve obtained from the nanoindentation tests Figure Schematics of a typical SHPB test device. [105] Figure 5.1. Runner blade steel elemental analysis Figure 5.2. True stress in function of true strain for the strain rate of /s Figure 5.3. True stress in function of true strain for the strain rate of /s Figure 5.4. True stress in function of true strain for the strain rate of 1 1/s Figure 5.5. Nanoindentation sample magnified 20 times. Indent depth 2µm, distance between indents 40 µm Figure 5.6. Load in function of displacement for indent number 20. S=1.003 mn/nm Figure 5.7. Load in function of displacement for indent number 26. S=1.848 mn/nm Figure 5.8. True stress in function of true strain in a SHPB test Figure 5.9. True strain rate in function of true strain in a SHPB test Figure Normalized yield stress in the function of relative strain rate Figure Sample photographs from the incubation period. Increasing amount of pits is observed when the cumulative cavitation erosion time increases Figure Sample photographs from the advanced erosion periods. In the three photographs with the largest cumulative erosion time, significant material loss is observed Figure Profile evolution along measurement line Figure Profile evolution along measurement line Figure Profile evolution along measurement line Figure Profile evolution along measurement line Figure Volume loss in function of time for different measurement lines. Line 3 going to zero value in time of 1500 minutes means that the measurement is out of the range of the profilometer viii Figure Volume loss rate in function of time for different measurement lines. Line 3 going to zero value in time of 1380 minutes means that the measurement is out of the range of the profilometer Figure Averaged volume loss in function of cumulative erosion time Figure Averaged volume loss rate in function of cumulative erosion time Figure Maximum erosion depth in function of cumulative erosion time for different measurement lines Figure Maximum erosion depth rate in function of cumulative erosion time for different measurement lines Figure Averaged erosion depth in function of cumulative erosion time Figure Averaged erosion depth rate in function of cumulative erosion time Figure khz high-pass filtered AE signals y-axis is presented in 10-base logarithmic scale Figure khz high-pass filtered AE signals. y-axis is presented in 10-base logarithmic scale Figure khz high-pass filtered AE signals. y-axis is presented in 10- base logarithmic scale Figure Maximum erosion depth and cavitating region acoustic emission signal RMS-value with 10 khz filter in function of cumulative erosion time Figure Maximum erosion depth and cavitating region acoustic emission signal RMS-value with 100 khz filter in function of cumulative erosion time Figure Maximum erosion depth and non-cavitating region acoustic emission signal RMS-value with 10 khz filter in function of cumulative erosion time Figure 6.1. Volume losses of different materials in function of erosion time Figure 6.2. Erosion depths of different materials in function of erosion time Figure 6.3. Volume loss rates of different materials in function of erosion time Figure 6.4. Erosion depth rates of different materials in function of erosion time ix Table 3.1. Acoustic emission signal sources and intensities. [86] Table 4.1. Profilometer characteristics Table 4.2. Acoustic emission sensor characteristics Table 4.3. Compression test measurement parameters Table 5.1. Measurement time intervals and method details Table 5.2. Material characteristics derived from the compression tests Table 5.3. Averaged values of parameter S and Young s modulus E Table 5.4. Yield stress values and average strain rate from the SHPB tests Table 6.1. The incubation times of different materials the with 25 μm erosion depth definition Table 6.2. Materials classified by the final and averaged final volume loss and erosion depth rates Table 6.3. Mechanical properties of compared materials Table 7.1. Summary of the material characterization tests Table 7.2. Summary of the cavitation erosion evolution and the material properties for all materials x LIST OF SYMBOLS AND ABBREVIATIONS A Area A Amplitude A(h) Projected contact area in function of height C Strain rate dependency constant c Wave velocity C L Longitudinal wave velocity c L Speed of sound in a liquid C R Rayleigh wave velocity C T Transverse wave velocity D Pit diameter d Displacement in compression or traction testing d nozzle Nozzle diameter D* Characteristic pit diameter D Normalized pit diameter d, p Unit vectors for wave movement direction d dr d First derivative in respect to radius First derivative in respect to time dt E Young s modulus E r Reduced Young s modulus E ind Indenter Young s modulus E Steady state erosion rate F Force f Frequency Fr Felicity ratio f s Shedding frequency g Gravitation acceleration for free fall H Hydraulic head h Height H ro Runner outlet hydraulic head H T Total head over a hydraulic machine H ti Turbine inlet hydraulic head h k Erosion depth vector H Head loss H o-d Head loss between runner outlet and downstream setting level i, j, k Indexes K Spring stiffness per unit area k w Weibull distribution shape parameter K Strain hardening constant k Erosion rate-incubation time dependency constant L Sample height l c Sheet cavity length L H Maximum hardened layer thickness l H Hardened layer thickness L 0 Initial sample height m Mass per unit area xi N Pitting rate n Strain hardening exponent N tot Total amount of measurement points n V Amount of signal measurement points N* Characteristic pitting rate N Normalized pitting rate P Pressure P AE Structure loading when acoustic emission is observed P c Critical pressure P D Downstream setting pressure P g Gas pressure P g0 Initial gas pressure P initial Initial structure loading P ro Runner outlet pressure P l Liquid pressure P peak Bubble collapse peak pressure P ti Turbine inlet pressure P v Saturated vapour pressure P Reference pressure P Pressure difference r Radius r c Critical bubble radius r 0 Bubble initial radius r Radial resolution S Nanoindentation curve fit parameter S c Contact area S c,0 Initial contact area S st Structure length S st, 0 Initial structure length St Strouhal number S Structure length difference t Time T i Incubation period duration t* Characteristic time in volume loss t * Non-dimensional time in re-entrant jet formation t Normalized time t Time difference u Displacement vector u Acceleration vector V Velocity V D Downstream setting velocity V def Deformation velocity V ro Runner outlet velocity V t Test section flow velocity V ti Turbine inlet velocity V Reference velocity V loss Volume loss V i Signal output voltage Root mean square voltage V RMS xii V loss Characteristic volume loss V loss Normalized volume loss x General location variable x First time derivative of displacement i.e. velocity x Position vector x Profilometer x-resolution y General height variable Y D Downstream setting level Y ro Runner outlet level y radial Test section height Y ti
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