Study On Power Characteristics And Load Reduction Mechanism Of Swept Blades Based On Vortex Lift Line Model

In order to improve the utilization rate of wind energy and reduce the cost of wind power,it is an inevitable trend for modern wind turbine to be large.Although the increase of the diameter of the wind turbine can improve the power of the blade,it inevitably increases the weight of the blade.The wind conditions that the blades face are also more complex.The aerodynamic load,gravity load and inertial load increase sharply the diameter of the wind turbine,which increases the cost of manufacturing and installation of the wind turbine.To improve the utilization rate of wind energy and reduce the load of the blade,the cost of wind turbine manufacturing and installation,it is urgent to explore new aerodynamic layout and shape structure of the blades of wind turbines.The back-swept blade of horizontal axis wind turbines is a blade designed by bending the airfoil located in the span direction to the trailing edge.Existing researches on wind turbine blades point out that the use of swept-back blades can not only maintain the power,but also reduce the load on the blades and the production cost of the wind turbine,which is an effective way to improve the competitiveness of wind power.In this paper,a nonlinear aeroelastic coupling program based on vortex lift line theory is used to optimize the output power of back-swept blades and study the influence of sweep parameters on power characteristics and load reduction mechanism.(1)Due to the prominent geometrical nonlinearity and blend-twist coupling of back-swept blades,it is essential to build a reasonable aeroelastic coupling model to calculate the aeroelastic coupling response of blades.In this paper,the helicoidal wake vortex lift line model is selected to the blade aerodynamic model.This model has good three-dimensional flow field analysis capabilities and is suitable for the three-dimensional flow field analysis of the swept blades.The " superelement" method is used to discretize the swept blades into a multi-body system connected by a kinematic joint(universal joint,rotating joint),spring and damper,applying multi-body dynamics theory,integrated with the aerodynamic model,established the nonlinear aeroelastic coupling program of the blades.(2)Aiming at the three-dimensional flow characteristics of back-swept blades,the operating parameters of blades were optimized to improve the utilization rate of wind energy.In order to accurately simulate the aeroelastic characteristics of back-swept blades,applying of nonlinear aeroelastic coupled blade program,The steady-state responses of 5MW straight blades and back-swept blades in National Renewable Energy Laboratory(NREL)were calculated to verify the validity and accuracy of the prograom.The operating pitch Angle was taken as the design variable;the annual energy production was taken as the objective function,and the rated power,flapwise tip deflection and axial thrust were taken as the constraint conditions.A 5MW back-swept blade was selected as the prototype to seek the optimal pitch angle corresponding to each wind speed within the wind speed range of5m/s～25m/s,and the results were compared and analyzed.The results show that compared with the prototype blade and the straight blade,the annual energy production of the back-swept blade optimized with pitch angle is greatly increased.(3)To study the effect of blade sweep parameters on power characteristics and load reduction mechanism,firstly,a new stacking line equation of blades was drawn into establishing four groups of back-swept blade models with starting point at the root of the blade and the tip sweep of 2 m,4 m,6 m and 8 m,and three groups of back-swept blade models with the tip swept of 4m and the starting point of 0.25,0.5 and 0.75 was at the blade spread position to avoid the effect of the swept amount of blade on the tower barre of wind turbine.Secondly,according to the operation data of the 5MW wind turbine of the National Renewable Energy Laboratory,the aeroelastic responses of the straight blade and each back-swept blade at wind speeds 5m/s,11.4m/s and 25m/s were calculated by simulating blades with a tip speed ratio of 8.7,rated speed of 12.1rpm and considering wind shear and gravity load.The effect of back-swept parameters on aerodynamic performance of back-swept blades and straight blade with the same power and total torque was analyzed and compared.The results show that the back-swept blades using the new stacking line has the functions of load reduction and efficiency increase.The research work of this paper applied the nonlinear aeroelastic coupling program of the blades.The aerodynamic performance of back-swept blades was improved by optimizing the operating pitch angle.A new stacking line was introduced to study the influence of back-swept parameters on blade load and power.The research results will lay a theoretical basis for the optimization design of back-swept blades shape and provide a scientific support and theoretical guidance for the research on load reduction mechanism of back-swept blades.