Wind energy is the most promising renewable energy source. Wind power generation is the main form of wind energy utilization. The development of offshore and low wind speed regional wind farms has made the application of multi-megawatt horizontal axis wind turbines mainstream. The development of a special airfoil family with excellent performance is the key to improving the energy capture rate of horizontal axis wind turbine blades.
Based on the intrinsic needs of wind turbine blades, researchers have developed a series of dedicated airfoil families to replace the application of aviation airfoils on horizontal axis wind turbines since the 1980s. The relatively small thickness of the airfoil described above is limited to airfoil designs that are suitable for the root zone of the blade. With the significant increase in the size of newly installed wind turbine blades and the increasingly complex and harsh operating environment, the development of high-performance, high-thickness airfoils has become necessary.
The use of a blunt trailing edge design for thick airfoil can effectively improve its aerodynamic performance and structural strength. Timmer, Hoerner, van Dam and others have done a lot of research on this, including the modeling of thick-tailed edges and the effect of passivation of trailing edges. However, the current thick airfoil design criteria cannot be close to the operational requirements of multi-megawatt wind turbines, and there is also a lack of available large-thickness blunt trailing edge airfoil families.
Since 2007, the Institute of Engineering Thermophysics of the Chinese Academy of Sciences has developed wind turbine-specific airfoil families suitable for the distribution of Chinese wind resources. It contains four large thickness airfoils with relative thickness of 45% to 60%. However, these early large-thickness airfoil trailing edge thicknesses are small and the XFOIL design cannot accurately predict performance at large angles of attack. Recently, the researchers studied the design criteria and design methods for large-thickness blunt-end airfoil with a 5-megawatt wind turbine operating characteristics system and successfully developed four new large-thickness blunt-end trailing edge airfoils.
Studies have shown that 45% - 60% of the relative thickness of the airfoil is mainly applied to the inside of the blade 10% -20%. Due to the limitation of the leaf torsion angle, the angle of attack of this part is large, and the flow is mostly in the state of turbulent boundary layer, which is significantly different from the airfoil flow in the middle and outside of the blade. The design attack angle domain and design Reynolds number must be redefined. Based on the calculation results of the referenced 5MW wind turbine, the actual operating angle of attack for this part of the airfoil is in the range of 15°-25°, which is much higher than its stall angle of attack. The operating Reynolds number varies with the relative thickness and is determined accordingly. The design attack angle domain and design Reynolds number that meet the operating characteristics of MW wind turbines are shown in Table 1.
In terms of aerodynamic performance, the design index of the large-thickness airfoil with the level of the lift coefficient within the range of operating angle of attack is innovatively proposed, and the smooth characteristics of the lift coefficient within the angle of attack and the stability of the variable conditions of the lift curve are the major constraints. This is due to the fact that the relative thickness of multi-megawatt blades and airfoils is usually greater than 45%, resulting in a much slower attack angle than the thin airfoil and a large distance from the actual operating angle of attack. The maximum lift coefficient at this time obviously can no longer effectively characterize the performance requirements for large thickness airfoils. The lift coefficient changes smoothly with the angle of attack, and the change with Re changes can effectively ensure the stability of the output of the wind turbine.
Due to the larger angle of attack of the thicker airfoil, the flow is mostly under the turbulent boundary layer or even separated flow conditions. The prediction of thick airfoil flow, especially regarding the prediction of the flow transition and the separation point, is always a constraint on the large thickness of the aerodynamics. The key issue of design. Therefore, prior to design, it was verified by measurements at the Beijing University of Aeronautics and Astronautics D-4 wind tunnel (relative to the general CFD method) that the 45%-thick airfoil RFOIL can more accurately give lift characteristics up to 25° range of attack angle. . Therefore, RFOIL is designed using a hybrid design method. During the design process, the geometrical compatibility of the airfoil family is ensured by ensuring that the basic geometry of the airfoil (front radius, relative camber, and maximum relative thickness position) is similar; by delaying the separation of the turbulent boundary layer or delaying the separation point from the trailing edge The speed of the forward edge moves to achieve a high and gentle lift feature. The design results are shown in Figure 1 below.
Based on RFOIL aerodynamic analysis, the lift coefficient of the large-thickness blunt trailing edge airfoil in the range of high angle of attack continues to rise steadily and at a high level: between 15° and 30° angle of attack, the lift of the airfoil family. The coefficient approximately linearly varies from 1.0 to 1.7, and at the same time, it has excellent Reynolds number stability within the range of the angle of attack, which facilitates the wind turbine generator to obtain a more efficient and stable output and achieves the design goal. The integrated airfoil evaluation method proposed by the Wind Turbine Blade R&D Center shows that the overall performance of the new 45% relative thickness airfoil is better than that of the widely used DUV developed by Delft University of the Netherlands for the inner demand of MW-class wind turbine blades. - W2-401 airfoil. Therefore, researchers have used this airfoil as an alternative design for the development of multi-megawatt wind turbine blades. Wind tunnel measurement experiments on this airfoil family are currently underway.
The above research work has been supported by the National “863†Plan Project “Big Thickness, Blunt Trailing Edge, Low Noise Airfoil Design and Application Technology†(No.2012AA051303). The relevant research results have been presented at the 2013 Academic Conference of the Chinese Society of Engineering Thermophysics ( Hohhot) will be exhibited and will be published in the Journal of Engineering Thermophysics.
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