Design of a Tapered and Twisted Blade for the NREL

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April 1999 NREL SR 500 26173,Design of a Tapered and. Twisted Blade for the NREL,Combined Experiment Rotor. March 1998 March 1999,P Gigu re and M S Selig,Department of Aeronautical. and Astronautical Engineering,University of Illinois at Urbana Champaign. Urbana Illinois,NREL Technical Monitors James L Tangler and.
David A Simms,Prepared under Subcontract No XAF 4 14076 03. This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof nor any of their employees makes any. warranty express or implied or assumes any legal liability or responsibility for the accuracy completeness. or usefulness of any information apparatus product or process disclosed or represents that its use would. not infringe privately owned rights Reference herein to any specific commercial product process or service. by trade name trademark manufacturer or otherwise does not necessarily constitute or imply its. endorsement recommendation or favoring by the United States government or any agency thereof The. views and opinions of authors expressed herein do not necessarily state or reflect those of the United States. government or any agency thereof,Available to DOE and DOE contractors from. Office of Scientific and Technical Information OSTI. P O Box 62,Oak Ridge TN 37831,Prices available by calling 423 576 8401. Available to the public from,National Technical Information Service NTIS. U S Department of Commerce,5285 Port Royal Road,Springfield VA 22161.
703 605 6000 or 800 553 6847,DOE Information Bridge. http www doe gov bridge home html, Printed on paper containing at least 50 wastepaper including 20 postconsumer waste. Previous phases of experimenting with the Combined Experiment Rotor CER of the National. Renewable Energy Laboratory NREL have provided test results from two constant chord blade sets. The first blade set had no twist whereas the second had twist As the next step the design of a. tapered twisted blade for the CER was contracted out to the Department of Aeronautical and. Astronautical Engineering of the University of Illinois at Urbana Champaign This blade design work. consisted of a systematic trade off study where many blade configurations were compared to determine. how much the design constraints affected blade performance Based on the results of the tradeoff study a. blade having a linear taper and nonlinear twist and that uses the S809 airfoil was selected as the new. CER blade An extended version of this blade was also designed for a two bladed rotor configuration. The new CER blades are presently being built by an independent blade manufacturer NREL plans to test. the new blades under constant and variable speed operations. James L Tangler, National Renewable Energy Laboratory National Wind Technology Center. 1617 Cole Boulevard,Golden Colorado 80401 USA,E mail james tangler nrel gov. Phone 303 384 6934,Fax 303 384 6901, A tapered twisted blade set was designed for the Combined Experiment Rotor CER of the National.
Renewable Energy Laboratory The objective was to build on the knowledge base of the previous CER. tests conducted with constant chord untwisted blades and constant chord twisted blades Such CER. tapered twisted blades will yield performance that is more representative of commercial blades In. addition these new blades will continue to satisfy the scientific needs for fundamental research in rotor. aerodynamics, This blade design work for the CER was performed during the summer of 1997 while the first author was. at the National Wind Technology Center The authors would like to thank NREL for providing funding. under subcontract XAF 4 14076 03 and the opportunity to design new blades for the CER Also the. authors would like to thank James L Tangler David A Simms and Lee J Fingersh of NREL for their. feedback and suggestions throughout this blade design work The comments of Dr Michael C Robinson. and C P Sandy Butterfield of NREL were also appreciated. A tapered twisted blade was designed to operate on the Combined Experiment Rotor CER of the. National Renewable Energy Laboratory NREL which is a stall regulated downwind wind turbine. having a rated power of 20 kilowatt The geometry of the new blade set was optimized based on annual. energy production subject to the constraints imposed on the design These constraints were mainly. related to scientific needs for fundamental research in rotor aerodynamics A trade off study was. conducted to determine the effect of the different design constraints Based on the results of this study. which considered nonlinear twist and taper distributions as well as the NREL S809 S814 S822 and S823. airfoils a blade having a linear taper and a nonlinear twist distribution that uses the S809 airfoil from root. to tip was selected This blade configuration is the logical continuation of the previous constant chord. twisted and untwisted blade sets and will facilitate comparison with those earlier blades Despite the. design constraints based on scientific needs the new blade is more representative of commercial blades. than the previous blade sets, The new blade was designed to be applicable for three and two bladed rotor configurations To enhance. the performance of the new blade in a two bladed rotor configuration instead of the baseline three bladed. rotor an increase in blade span was investigated which led to the design of an extended blade having a. 10 increase in span Furthermore an increase in rotor speed was also investigated A two bladed rotor. making use of extended blades and rotating at a speed 8 faster than the baseline speed or revolution per. minute setting was found to yield comparable power output to that of the new blades in a three bladed. rotor configuration Results for the CER equipped with the new blades baseline and extended blades in. terms of mechanical power output rotor thrust as well as lift coefficient and axial inflow distributions. along the blade span are presented Even though the new blades were designed for constant speed. operations they can also be used for fundamental research in variable speed operations To facilitate the. selection of the most appropriate rotor configuration for the NREL variable speed test bed using the new. blades results showing the power coefficient as a function of the tip speed ratio for various pitch settings. are presented Finally recommendations for future blade sets for the CER are also given. Foreword iii,Preface iv,Nomenclature vii,Introduction 1. Design Constraints 2,Design Approach 3, Design Trade offs and Blade Geometry Optimization 4. Performance Predictions 8,Conclusions and Recommendations 14.
References 15, Appendix A Tabulated Data for the Tapered Twisted CER blade 16. Appendix B Tabulated Results for Figures 4 8 17,List of Figures. 1 Planform of the baseline tapered twisted CER blade three bladed rotor configuration 6. 2 Planform of the extended tapered twisted CER blade two bladed rotor configuration 6. 3 Final twist distribution of the tapered twisted CER blade 7. 4 Power curves for the tapered twisted CER blade 8. 5 Rotor thrust curves for the tapered twisted CER blade 9. 6 Lift coefficient distribution along the span for the tapered twisted CER blade 10. 7 Axial inflow coefficient distribution along the span for the tapered twisted CER blade 11. 8 Power coefficient vs tip speed ratio for the tapered twisted CER blade 12. List of Tables, 1 Results of the blade configuration trade off study 5. Nomenclature,c Blade chord,HD Hub diameter,R Blade radius. r Radial distance along the blade span from the center of the rotor. Introduction, The Combined Experiment Rotor CER of the National Renewable Energy Laboratory NREL has a.
diameter of 10 06 m 33 ft and is composed of three blades This rotor is mounted on a Grumman Wind. Stream 33 horizontal axis wind turbine HAWT which is a stall regulated downwind machine having a. rated power of 20 kilowatt kW and operating at a speed of 72 revolutions per minute rpm 1 To date. two blade sets were tested with this wind turbine for fundamental research in rotor aerodynamics The. first blade set was composed of constant chord untwisted blades and the second set had constant. chord twisted blades Both of these blade sets were built with a chord of 457 mm 18 in and used the. NREL S809 airfoil along the entire span In addition one blade of each set was instrumented with. chordwise pressure taps and a 5 hole probe at five spanwise locations namely at 30 47 63 80. and 95 span These two blade sets were extensively tested and the results of those experiments can be. found in Refs 2 and 3, The objective of this work was to design a third blade set for the CER having both taper and twist In. contrast with the two other blade sets the blade geometry for the new set was to be designed for. maximum annual energy production Because of the need for fundamental research in rotor. aerodynamics and practical aspects constraints were imposed on the design An important part of this. work was to conduct a study of the design trade offs to determine the effect of those constraints on the. energy capture of the rotor Another objective was to determine the necessary modifications to the blade. geometry and operating rpm for a two bladed rotor configuration This report describes the approach and. process used to design a tapered twisted blade and provides performance predictions for the CER. equipped with the new blades The performance of the CER equipped with the new blade set was also. investigated under variable speed operation,Design Constraints. NREL provided the design constraints for the tapered twisted CER blades 4 Some practical constraints. were imposed to facilitate the instrumentation of the new blades in a manner similar to the previous CER. blade sets Other constraints were imposed to ensure consistency with the previous blade sets for ease of. comparison of the data The design constraints for the tapered twisted blades for the CER were as. A blade span of 5 03 m 16 5 ft including a 102 mm 4 in tip shape for the baseline three bladed rotor. A rated power of 20 kW,A cone angle of 3 4 degrees. Keep distance from pitch axis to the leading edge less than 584 mm 23 in so that the 5 hole probes do. not hit the tower, A minimum chord of 305 mm 12 in to allow for the installation of pressure taps and instrumentation. on the instrumented blade, A fixed chord length of 457 mm 18 in at 80 of the blade span for comparison of the pressure data.
with the previous blade sets, Use the S8095 airfoil for as much of the blade span as possible to facilitate comparison with the. previous blade sets, Transition to the S8145 root airfoil from the S809 airfoil not to begin before 47 of the blade span. Design Approach, The computer programs PROPID6 7 and PROPGA7 8 were used to carry out the blade design process. PROPID is an inverse design and analysis method for HAWTs that is based on the blade. element momentum theory PROP code 9 The inverse design capabilities of PROPID allow for the desired. performance and aerodynamic characteristics of the rotor to be prescribed from which the blade geometry. and corresponding operating conditions are determined In the present design work the inverse design. capability of PROPID was used to limit the mechanical power output of the CER to 20 kW Also the. Prandtl tip loss model was used and the Corrigan post stall model10 was incorporated into PROPID to. modify the two dimensional airfoil for three dimensional effects 11 PROPGA is a genetic algorithm. based optimization method for HAWTs that relies on PROPID for the analysis of the possible blade rotor. designs Given a set of design constraints requirements bounds for the parameters to be optimized and a. figure of merit objective function for the optimization PROPGA provides the optimum blade geometry. As a first and important step in the design process the trade offs between various blade configurations. and airfoils were investigated to determine the effect of the design constraints on the energy capture of the. rotor For each blade configuration considered the blade geometry was optimized for maximum gross. annual energy production GAEP using PROPGA Once the final blade geometry was selected. PROPID was used to finalize the blade twist distribution and pitch setting Particular attention was given. to obtaining smooth stall characteristics along the entire blade span Following the design of the blades. for the baseline three bladed rotor PROPID was also used to determine the necessary modifications to the. blade geometry and operating rotor speed for a two bladed rotor configuration In addition PROPID was. used to provide performance predictions with greater accuracy under constant speed and variable speed. operations, Throughout the design process the GAEP assuming a 100 generator efficiency was computed based. on a Rayleigh wind speed distribution having an average wind speed of 7 2 m s 16 mph which is. representative of the windy months at NREL Also the power output of the rotor was computed up to a. wind speed of 17 9 m s 40 mph using standard atmospheric conditions at the altitude of the National. Wind Technology Center 1 829 m or 6 000 ft The number of segments used along the blade span was. 10 for a PROPGA run and 20 when using PROPID outside the optimization scheme of PROPGA In. tapered twisted blade for the CER was contracted out to the Department of Aeronautical and Astronautical Engineering of the University of Illinois at Urbana Champaign This blade design work consisted of a systematic trade off study where many blade configurations were compared to determine how much the design constraints affected blade performance Based on the results of the tradeoff study

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