Investigation of Different Airfoils on Outer Sections of Large Rotor Blades PDF
Preview Investigation of Different Airfoils on Outer Sections of Large Rotor Blades
School of Innovation, Design and Engineering Bachelor Thesis in Aeronautical Engineering 15 credits, Basic level 300 Investigation of Different Airfoils on Outer Sections of Large Rotor Blades Authors: Torstein Hiorth Soland and Sebastian Thuné Report code: MDH.IDT.FLYG.0254.2012.GN300.15HP.Ae Sammanfattning Vindkraft står för ca 3 % av jordens produktion av elektricitet. I jakten på grönare kraft, så ligger mycket av uppmärksamheten på att få mer elektricitet från vindens kinetiska energi med hjälp av vindturbiner. Vindturbiner har använts för elektricitetsproduktion sedan 1887 och sedan dess så har turbinerna blivit signifikant större och med högre verkningsgrad. Driftsförhållandena förändras avsevärt över en rotors längd. Inre delen är oftast utsatt för mer komplexa driftsförhållanden än den yttre delen. Den yttre delen har emellertid mycket större inverkan på kraft och lastalstring. Här är efterfrågan på god aerodynamisk prestanda mycket stor. Vingprofiler för mitten/yttersektionen har undersökts för att passa till en 7.0 MW rotor med diametern 165 meter. Kriterier för bladprestanda ställdes upp och sensitivitetsanalys gjordes. Med hjälp av programmen XFLR5 (XFoil) och Qblade så sattes ett blad ihop av varierande vingprofiler som sedan testades med bladelement momentum teorin. Huvuduppgiften var att göra en simulering av rotorn med en aero-‐elastisk kod som gav information beträffande driftsbelastningar på rotorbladet för olika vingprofiler. Dessa resultat validerades i ett professionellt program för aeroelasticitet (Flex5) som simulerar steady state, turbulent och wind shear. De bästa vingprofilerna från denna rapportens profilkatalog är NACA 63-‐6XX och NACA 64-‐6XX. Genom att implementera dessa vingprofiler på blad design 2 och 3 så erhölls en mycket hög prestanda jämfört med stora kommersiella HAWT rotorer. Abstract Wind power counts for roughly 3 % of the global electricity production. In the chase to produce greener power, much attention lies on getting more electricity from the wind, extraction of kinetic energy, with help of wind turbines. Wind turbines have been used for electricity production since 1887 and have since then developed into more efficient designs and become significantly bigger and with a higher efficiency. The operational conditions change considerably over the rotor length. Inner sections are typically exposed to more complex operational conditions than the outer sections. However, the outer blade sections have a much larger impact on the power and load generation. Especially here the demand for good aerodynamic performance is large. Airfoils have to be identified and investigated on mid/outer sections of a 7.0 MW rotor with 165 m diameter. Blade performance criteria were determined and investigations like sensitivity analysis were made. With the use of XFLR5 (XFoil) and Qblade, the airfoils were made into a blade and tested with the blade element momentum theory. This simulation gave detailed information regarding performance and operational loads depending on the different airfoils used. These results were then validated in a professional aero-‐elastic code (Flex5), simulating steady state, turbulent and wind shear conditions. The best airfoils to use from this reports airfoil catalogue are the NACA 63-‐6XX and NACA 64-‐6XX. With the implementation of these airfoils, blade design 2 and 3 have a very high performance coefficient compared to large commercial HAWT rotors. Carried out at: Statoil ASA, R&D NEH OWI Advisor at MDH: Sten Wiedling (KTH) Advisor at Statoil ASA: Andreas Knauer, Dr.-‐Ing. Examiner: Mirko Senkovski Nomenclature B – Number of Blades BEM – Blade Element Momentum theory c – Chord length (m) C – Section Drag Coefficient d C – Total Drag Coefficient D C – Section Lift Coefficient l C – Total Lift Coefficient L Cl/Cd – L/D – Lift to drag ratio C – Pitching Moment Coefficient m C – Pressure Coefficient / Performance Coefficient P H , H – Shape factor 12 32 HAWT – Horizontal-‐Axis Wind Turbine M – Mach number NACA – National Advisory Committee for Aeronautics P – Power output (W) p – Pressure (Pa) R – Global radius (m) r – Local radius (m) RPM – Revolutions Per Minute S.U., S.L. – Separation Upper and Separation Lower T.U., T.L – Transition Upper and Transition Lower t/c – Thickness to chord ratio (%) V – Free stream wind speed (m/s) W – Relative blade velocity (m/s) x/c – Location along the chord (m) α – AoA – Angle of Attack (degrees °) β – Inflow angle (degrees °) Γ – Circulation γ – Twist angle (degrees °) δ , δ , δ – Displacement, Momentum and Energy thickness 1 2 3 η – Efficiency λ -‐ TSR – Tip Speed Ratio μ – Dynamic viscosity (𝑃𝑎⋅𝑠) ρ – density (kg/m3) Ω – Angular velocity (rad/s) SAMMANFATTNING 2 ABSTRACT 3 NOMENCLATURE 5 1. INTRODUCTION 9 2. HISTORICAL PERSPECTIVE 10 3. AIRFOILS 15 3.1 National Advisory Committee for Aeronautics (NACA) 15 3.2 National Renewable Energy Laboratory (NREL) 17 4. METHODS 18 4.1 Historical Turbines 18 4.2 General Blade Design Criteria 18 4.2.1 Blade Performance Criteria 19 4.2.2 Inner Root Section Criteria 20 4.2.3 Middle Section Criteria 21 4.2.4 Outer Section Criteria 21 4.2.5 Blade Section Calculation 21 4.2.6 Specific Blade Design 22 4.2.7 Blade Design Procedure 23 4.3 Airfoil Catalogue and Roughness Insensitivity Analysis 24 4.3.1 Airfoil Design for Wind Turbines With Roughness Insensitivity 25 4.3.2 Boundary Layer Theory 28 4.4 Blade Element Momentum (BEM) Theory 32 4.4.1 Momentum Theory 32 4.4.2 Blade Element Theory 33 4.5 Qblade 38 4.5.1 General Validation of Simulation Results 38 4.6 Javafoil 39 4.6.1 Roughness analyses 39 4.6.2 Limitations 40 4.7 Flex5 41 5. RESULTS 42 5.1 Historical Turbines 42 5.1.1 Gedser Wind Turbine 42 5.1.2 MOD-‐2 Turbine 44 5.2 Blade Design Criteria 47 5.3 Roughness Insensitivity Analysis 48 5.4 Qblade Blade Design and Turbine Simulation 50 5.4.1 Blade Design 1 50 5.4.2 Blade Design 2 56 5.4.3 Blade Design 3 61 5.5 Flex5 66 5.5.1 Blade Design 2 67 5.5.2 Blade Design 3 71 6. DISCUSSION 74 6.1 Historical Turbines 74 6.1.1 Gedser Wind Turbine 74 6.1.2 MOD-‐2 Turbine 74 6.2 Roughness Insensitivity Analysis 76 6.3 Qblade 76 6.3.1 Blade Design 1 76 6.3.2 Blade Design 2 77 6.3.3 Blade Design 3 78 6.4 Flex5 79 6.4.1 Blade Design 2 80 6.4.2 Blade Design 3 81 6.5 Comparison of Qblade and Flex5 82 7. CONCLUSION 83 8. FURTHER WORK 84 APPENDIX A 85 Airfoil Catalogue 85 APPENDIX B 104 Use of Qblade 104 APPENDIX C 108 Airfoil Catalogue 108 APPENDIX D 111 Example of Aerodynamic Data and Blade Geometry Input for Flex5 111 APPENDIX E 113 Wind Shear Simulation in Flex5 113 Turbulence Simulation in Flex5 115 REFERENCES 117 1. Introduction The selection of airfoil shape directly influences the efficiency and loading of wind turbine rotors. In this graduate project at Mälardalens Högskola and carried out at Statoil Research Center in Bergen, Norway, several airfoils have been investigated for use in offshore wind turbine operation. The selected airfoils are for the use on a 7.0 MW turbine with a diameter of 165 m. Statoil, primary an oil company, is also involved in the offshore wind turbine industry, especially as an operator of wind farms. Statoil has an interest in the trends in turbine size and airfoils being used. The first part of the report is a study of performance criteria for airfoils and blade design. Since wind turbine operation is somewhat different to aircraft operation, a literature study was performed. An introduction to the history of wind energy and development trends is also included. Historical wind turbine blades were studied and analyzed, so that operational/test data and software data could be compared to newer turbines. The second part consists of airfoil analyses, primarily for the middle and outer sections of a large rotor blade, based on performance criteria. An airfoil catalogue was developed including aerodynamic performance data and roughness insensitivity. Experimental data and analysis tools, such as XFoil (XFLR5) and Javafoil were used. The third part of the report is the main part. Blade design optimization was developed in Qblade. By combining 2D airfoil aerodynamic performance coupled with the Blade element momentum theory and a 3D correction, a viable result was achieved. The last part is a rotor investigation of the results from part three. This was done in aero-‐elastic simulation with Flex5. The blade optimized in Qblade was verified by employing professional software. Since this is a public report in collaboration with industry, there were certain limitations to the use of airfoil geometry. Because of license and other limitations, only airfoil geometry found easily on the Internet was investigated. Therefore, a handful of different airfoils have not been studied in this project and entire airfoil families have been excluded, especially the Risøe A-‐ family, which is licensed, tailored wind turbine airfoils. Since the airfoils used were open source, there was no opportunity to validate the correctness of the airfoil geometry. An assumption was made that they are. XFoil, Qblade and Javafoil only account for steady state, incompressible laminar flow while the real operational state would differ from this. Compressibility was not taken account of, since the blade rotation will be less than Mach 0.3. As for 9 turbulent flow and wind shear, which a real turbine will encounter during normal operation, this is checked in Flex5. A wind turbine blade designer has to take account of structural limitations. Because of the limited time, the project did not include a structural investigation of the blades. Avoiding very sophisticated blade structures and keeping to industry standards, the structural limitations would presumably not need detailed investigation. General losses due to mechanical and electrical efficiencies have not been analyzed. Losses have been set at 3 % for calculations, except when other values were given. 2. Historical Perspective A wind turbine is a machine that converts kinetic energy into mechanical energy and the mechanical energy is then usually converted into electrical energy through a generator. There are two major types of wind turbines:1 Horizontal-‐ axis and Vertical-‐axis, the horizontal being the primary type used. The first use of windmills where in old Persia in the 7th century, introduced in Europe during the 15th century. The windmills got towers, twisted blades, tapered planforms and control devices to point the mill into the wind in the 17th century. The Dutch brought the windmill expertise to North America in the 18th century, where wind energy was used to pump water. The first horizontal-‐axis wind turbine (HAWT) for generating electricity was built in Scotland, in 1887. In the early 1890s, the Danish scientist Poul la Cour was the first to discover that fast rotating turbines with fewer rotor blades were more efficient in generating electricity over slow rotating drag or impulse wind turbines. In 1931 in Yalta, in the Soviet Union, a predecessor to the modern HAWT was built. It had a 30 m high tower producing 100 kW. The wind turbine had a maximum efficiency of 32 %, which is still respectable at today’s standards. Ten years later, the pioneering Smith Putman wind turbine was built in Pennsylvania and ran for four years, until it encountered a blade failure. The device had a two-‐bladed variable pitch rotor working downwind of the tower. The rotor was 53 meters in diameter with a rotational speed of 28 rpm, giving a peak output rating of 1.25 MW and was therefore the first producing in excess of 1 MW. The blades were untwisted and rectangular with a chord of 3.7m and consisted of NACA 4418 airfoil. Ulrich Hütter pioneered the industry in Germany during the 1950´s using innovative materials and designs for several different horizontal axis wind turbines. The turbines were medium sized with rotors made of glass fiber 10
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