Study On Aerodynamic Property Of Aft-swept Wind Turbine Blade

The aft-swept wind turbine blade is a new type of smart blade formed by stacking airfoils along a line cured towards the trailing edge. It has been demonstrated to be able to dramatically reduce the thrust loads on the turbine while keeping the output power. Which means that the aft-swept blade provides promising approach to save the manufactural cost of wind turbine blade and make wind power more competitive in the financial aspect. However the aerodynamic characteristic of the swept blade is seldom studied especially the corresponding flow mechanism is not completely clear now. Therefore it is necessary to analyze the detailed flow field structure and separation characteristics of the swept blade from the special geometry parameters of the swept blade, such as the starting position of sweep and the local sweep angle etc.The swept blade can dramatically affect the aerodynamic of wind turbine. From the power comparison of NREL Phase VI straight blade and the corresponding swept one, computed by Computational Fluid Dynamics (CFD) method, we can see that:(1) at low wind speed, the output power of the swept blade is slightly less than straight blade;(2) when wind speed increases, the swept blade exhibits more stall delay char-acteristic and obtain higher power;(3) at high speed, when the flow are both separated, the aerodynamic performance of the swept blade decreases evidently. After deep anal-ysis on the three-dimensional (3D) flow filed, it was demonstrated that the span wise flow is speed by the swept profile, as the flow at the suction side around the airfoil leading edge slightly turn toward the rotating center and the flow at the suction side on the trailing edge is dramatically bending to the blade tip direction, which is greatly en-hanced at the flow separation condition. Further analyzing of the pressure distribution of local airfoil shows that the character of swept blade is mainly due to the influence of the centrifuge force, Coriolis force, Coanda effects and the interaction between them.For the separation aspect, comparing the local flow field of2D airfoil and3D swept blade, it can be seen that due to rotation the separation point of the3D swept blade is nearer to the trailing edge. But this condition is different when comparing the flow field of the swept blade and straight blade. This means that the swept blade has less stall delay effect than the straight one and this is not good for the load reduction of the swept blade and this is the why the swept blade has a poorer performance at deep separation.For the main flow aspect, based on the3D CFD results and using the wall cir-culation method, we analyzed the2D local flow field along the span of the swept and straight blades and obtained the inflow angle of attack (AOA). Comparing the flow fields, little difference of the AOA distribution between the swept and the straight blades was observed, and for the swept blade the reduction of load is realized from the change of the local pressure distribution.At last, the aerodynamic quality of the swept blade influenced by its geometry parameters is analyzed. Considering the complex geometry parameters of the swept blade may have an influence on the aerodynamic performance and the operation state of the wind turbine, we designed an orthogonal test, in which a group of swept blades has been included which has different geometry characteristic. Then the aerodynamic performance of these blades are computed by CFD. From the comparing of the thrust coefficients we can see that because the pressure difference between the suction side and the pressure side is reduced by Coanda effects, the swept blade can reduce the thrust load even not considering the excellent aeroelastic performance of which. Mak-ing range analysis of the geometry parameters describing the swept blade, we can see that the power coefficient of the swept blade is dramatically influenced by the starting point of the sweep and the local sweep angle. From the comparing the average aerody-namic performance influenced by these different geometry parameters it can be seen that the influence of the geometry parameters obey some certain rules, which provides the theoretical basis for the design of swept blade.