Study On The Aerodynamic Performance Optimization Of Large Scale Wind Turbine Dedicated Airfoils

The development of human societies is restricted by the problem of energy shortage and environment pollution. Compared with traditional energy, wind energy is a kind of abundant, geographically ubiquitous renewable energy. In one hand, the effective exploitation and utilization of wind energy can reduce the dependence on fossil fuels and the degree of contamination caused by using fossil fuels, in the other hand, compared with traditional thermal power and nuclear power generation, wind power generation can work without water, thus can save precious water resources. The exploitation and utilization of onshore wind energy of high quality has become saturated and more attention has been paid to the exploitation of offshore wind energy. Compared with onshore wind energy, the quality of offshore wind energy is better than onshore wind energy due to less restrict to wind on the sea. In order to capture more and more offshore wind energy, the size of horizontal axis wind turbines designed for offshore windfarms becomes large-scale and giant-scale. With the increase of the length of wind turbine blade, new demands for airfoils design at different radii should be considered, what’s more, the increase of the length of blade can reduce the stiffness and increase the flexibility of blade. So, deep research on the performance of airfoils at different radii of blade and the design of airfoils for wind turbines with high performance should be carried out according to the operating requirements of airfoils at different radial positions of large-scale wind turbine blade. These works will play an important role in both theoretical and industrial application.To study the performance of airfoils at different radial positions of large-scale wind turbine blade, the dissertation proposes a study entitled "Study on the aerodynamic performance optimization of large scale wind turbine dedicated airfoils", sponsored by funds of international science and technology cooperation program of China:Study on the aerodynamic characteristics and performance of wind turbines based on MEXICO experiment data (No.2010DFA6046) and the Danish council for strategic project:Develop of offshore wind turbines for China (No.12-130590). In this dissertation, according to the new operation characteristics of large-scale wind turbine blades, deep research of rotating and compressible effects on the performance of wind turbine airfoils and the performance of blunt trailing edge airfoils are carried out. Based on the research results, a new designed model of wind turbine airfoil is put forward and new airfoils for different radial positions are designed. A new blade is designed by using the newly designed airfoils. What’s more, an aeroelastic code for horizontal wind turbines is developed based on the newly established aeroelastic model for horizontal wind turbine. The detailed work and results of this dissertation are summarized as follows:(1) The aerodynamic and structural characteristics of blunt trailing edge airfoils are analyzed. Then, a method called mixed function of index is used to enlarge the thickness to both sides of the middle line of DU series airfoils from the location of maximum thickness to generate blunt trailing edge airfoils. The continuity of the trailing edge enlarged airfoils is analyzed. The steady, transient CFD methods as well as panel method are used to calculated the aerodynamic performance of trailing edge enlarged DU series airfoils at free and fixed transition conditions, the calculated lift and drag coefficients curves of blunt trailing edge airfoils with different maximum thicknesses are analyzed, the relationship between the lift characteristics and the thickness of trailing edge of blunt trailing edge airfoils at steady and transient conditions is also analyzed. Moreover, based on the theory of boundary layer, the mechanism of the change of blunt trailing edge airfoils’lift characteristics to common airfoils. The analyzed results lay a good foundation for the design of blunt trailing edge airfoils.(2) The rotor model of MEXICO experimental wind turbine is set up to carry out CFD simulation of the flow around MEXICO rotor. The calculated pressure coefficients on blades and the axial, radial and tangential components of velocity are compared with experimental data to validate the reliability of computation. The calculated three-dimensional lift and drag characteristics of RIS(?)-A1-21 airfoil in the middle part and NACA 64-418 airfoil in the tip part are compared with three-dimensional experimental data, two-dimensional experimental data, corrected data based on two-dimensional experimental data respectively, and the change law of the performance of airfoils at different part of the blade under rotating condition is summed up. In order to study the aerodynamic performance of blunt trailing edge airfoils further, the DU 97-W-300 and DU 97-W-300-05 airfoils are used to design a blade with constant chord length and angles of attack along the spanwise direction, so as to study the aerodynamic performance of blunt trailing edge airfoils under rotating condition.(3) In order to accommodate the operation condition that the tip speed of blade becomes larger, the effect of compressibility of air on the performance of wind turbine airfoil should be studied. The aerodynamic performance of DTU-LN218 airfoil was tested in a two-dimensional wind tunnel at Reynolds numbers of 1.5×106、3×106、4×106、5×106 and 6×106,and the experimental data at Reynolds number of 6×106 and 4×106 which correspond to Mach number of 0.3 and 0.2 is used for analysis. The data tested in wind tunnel directly is corrected into free air with consideration of compressibility effects. The prediction accuracy of FLUENT, XFOIL and Q3UIC for the aerodynamic performance of DTU-LN218 and DU 91-W2-250 airfoil with and without consideration of compressibility effects is validated by comparing calculated results with experimental data. What’s more, the correction accuracy of Prandtl-Glaueret, Karman-Tsien and Laitone compressibility correction rules in forward and reverse direction is compared and analyzed. In addition, the change law of the performance of airfoil only when the Mach number is changed is studied, the Reynolds number is fixed by changing the chord length when the inflow wind speed is changed. This study provide reference for the design and performance prediction of airfoils work in compressible flow.(4) The design requirements of wind turbine airfoils at different radii are analyzed. Based on the change law of the lift characteristics with the thickness of blunt trailing edge airfoil and the effect of compressibility on the performance of airfoil, a new design model for wind turbine dedicated airfoils with the aim to get maximum lift-to-drag ratio or two-dimensional power coefficient at a certain range of angles of attack under free and fixed transition conditions with consideration of aerodynamic, structure, geometric compatibility and easy for manufacture is presented. The geometric shape of airfoil is constructed by four Bezier curves so as to modify the local shape and generate the blunt trailing edge easily. At last WTA series airfoils with 18%,21%,24%,27% and 30% relative thickness are designed and whose aerodynamic performance is also validated. The design method of wind turbine airfoils is enriched by the presented design model in this dissertation.(5) By replacing the airfoils of the original 5 MW reference wind turbine with WTA airfoils, the output power is increased apparently. Then, one design model for horizontal axis wind turbine blade with the aim to get the maximum two-dimensional power coefficient is presented. By using this new design model with three-dimensional corrected data of WTA airfoils, a new blade with better power performance than the reference blade is designed, so the reliability of the design method and the good performance of WTA series airfoils is validated and which can provide foundation for engineering application. At last, the aeroelastic model of horizontal axis wind turbines is established. The transient BEM method is applied to calculate the load on blade, then yaw model is introduced into the aerodynamic model to correct the induced velocity, the principle of virtual work is used to establish the mass and stiffness matrix of the structure motion differential equation, and the Runge-Kutta-Nystrom scheme can be used to solve the differential equation. Based on the derived aeroelastic model of horizontal axis wind turbines, one software for predicting the aeroelastic performance of horizontal axis wind turbines is developed. By using the newly developed aeroelastic software, the aeroelastic performance of the 5 MW reference wind turbine and the wind turbine generated by replacing the original reference wind turbine with WTA airfoil is studied.