Study On The Method Of Overall Performance Optimization Of Dedicated Airfoils For Horizontal Axis Wind Turbines

The increasing of the capacity of wind turbines and diversity of wind fields emphasizes multi-scale factors that influence the flow of blades. And the airfoils’flow conditions become more diverse and uncertain, which bring a great challenge for the study of flow control of the rotor blades and design of wind turbine airfoils. Based on complicated requirements of multi-megawatt (MW) wind turbine blades, this thesis performs systemic investigations on the optimization of overall performance of wind turbine airfoils, which are as follows:Firstly, according to the actual operating features of rotor blades, the thesis proposes the concept of the stability of airfoil performance under variable conditions, and divides the airfoil’s aerodynamic characteristics into design performance, off design performance, stall feature and performance stability. Then these characteristics are further parameterized. Combined with geometrical parameters and related weight coefficients, an evaluation method that the overall performance is weighted sum of each performance parameter is established. Based on tests, the CAS-W1 airfoils’overall performance is evaluated with the reference of some DU airfoils. Results show that, compared with DU airfoils, thin CAS-W1 airfoils have good design and off-design performance, and could keep stable with the variation of surface roughness and the Reynolds number. But three thick inboard airfoils need further optimization.Then, the optimization method on the overall performance of wind turbine airfoils is studied. Based on complex flow characteristics of rotor blades and different requirements along span-wise of a blade, the design principles of dedicated airfoils for MW rotor blades are investigated, and the aerodynamic design philosophy "high efficiency, low load, stable and wide operating region of angle of attack" is put forward. Considering other requirements of improving structural property and reducing aerodynamic noise, the thesis builds a mathematic model of the multidisciplinary design optimization on wind turbine airfoils. The objective function and constraint condition consist of aerodynamic, structural and acoustical parameters; the design variables are airfoil geometrical parameters. The functional modules of airfoil’s geometrical design, multidisciplinary analysis and set of optimization scheme are integrated and an optimization design platform for wind turbine airfoils is founded, which is suited for airfoils’ automation designs with different relative thicknesses. Otherwise, for the aerodynamic designs of airfoils with complicated flows or deep optimization, the thesis put forward a composite design method which combines the inverse design method and numerical optimization, to perform more delicate design for airfoils’aerodynamic performance.Furthermore, the designs of dedicated airfoils suited for multi MW wind turbines are conducted according to the operating characteristics of a 5MW wind turbine. For the inner part of the blade, the operating angle of attack is high, and the flow boundary layer is more easily separated. In addition to improve the structural property, the design objectives focus on the aerodynamic performance in operating ranges of angle of attack at blade root. Four large thickness airfoils with blunt trailing edge are designed by the composite design method. Predictions through RFOIL show that the new-designed airfoils exhibit high lift coefficient in operating angles of attack and stable with the variation of Reynolds number. During the design process, it found that at suction surface of the airfoil, the peak of pressure coefficient and its location, the gradient of pressure recovery after the peak point significantly affect the airfoil’s performance. And then, based on the design platform, the overall optimization on the aerodynamic performance for the middle blade part airfoils and on the multidisciplinary performance for outboard airfoils are performed. The former obtains a new 35% relative thickness airfoil with high maximum lift to drag ratio and design lift coefficient, stable performance with the variation of surface roughness and Re to substitute thinner airfoils used in middle part of a blade. And the later design get a high overall performance airfoil of 21% relative thickness with aerodynamic features of "high efficiency, low load, stable and wide operating region of angle of attack", good structural property and satisfactory noise characteristic. The approximate models between airfoil performance and design variables are reached by response surface model based on the database produced during the airfoil optimization.At last, an experimental investigation is performed to determine the effects of Reynolds number on the aerodynamic performance of CAS-W1 airfoils. Results show that the airfoils’ characteristics exhibit different variations with increasing Reynolds number. As Reynolds number increasing, the maximum lift coefficient increases for the airfoil of relative thickness no more than 21% which is consistent with the test results of aviation airfoils. But For thicker CAS-W1 airfoils, the maximum lift coefficient decreases significantly with increasing Re. It is deduced that thin airfoils’maximum lift coefficient reaches in turbulent flow with trailing edge separation at a certain extent. The increasing Reynolds number will lead to more stable turbulent boundary layer and the separation will be delayed. And thick airfoil’s lift coefficient reaches its peak at low angle of attack in laminar flow with trailing edge separation at a certain extent. The increasing Re will destroy the stability of the laminar boundary layer, and bring out early separation, consequently result in a decrease of maximum lift coefficient.