Wind energy stands out as one of the most promising renewable energy sources, with wind power generation being its primary application. As offshore and low-wind-speed regions become more viable for wind farms, multi-megawatt horizontal-axis wind turbines have become the industry standard. To enhance the efficiency of these turbines, developing high-performance airfoils has become crucial.
Since the 1980s, researchers have been working on specialized airfoil families tailored for wind turbine blades, moving away from traditional aviation-based designs. Initially, these airfoils had relatively small thicknesses, making them suitable only for the root sections of the blades. However, as wind turbine blades grow larger and face more challenging environments, there's a growing need for high-thickness airfoils that can perform reliably under various conditions.
One effective approach is the use of blunt trailing edge designs, which improve both aerodynamic performance and structural strength. Researchers like Timmer, Hoerner, and van Dam have contributed significantly to this area, exploring how these designs affect flow characteristics and performance. Despite these efforts, current thick airfoil design standards still fall short of meeting the demands of modern multi-megawatt turbines, and there remains a gap in the availability of large-thickness, blunt-edge airfoil families.
In response, the Institute of Engineering Thermophysics at the Chinese Academy of Sciences developed a series of wind turbine-specific airfoils tailored for China’s wind resources starting in 2007. These include four large-thickness airfoils with relative thicknesses ranging from 45% to 60%. However, their early designs lacked sufficient trailing edge thickness, leading to inaccuracies in XFOIL predictions at high angles of attack. Recently, researchers refined the design criteria for large-thickness, blunt-edge airfoils, successfully creating four new options optimized for 5-megawatt turbines.
Studies show that these thick airfoils (45%-60% thickness) are mainly used in the inner part of the blade (10%-20% span). Due to blade twist limitations, the angle of attack in this region is high, often resulting in turbulent or separated flow—conditions quite different from those in the middle and outer parts of the blade. This necessitates redefining the design angle of attack and Reynolds number. Based on 5MW turbine data, the actual operating angle of attack for these sections ranges from 15° to 25°, much higher than the stall angle, requiring new design parameters.
Aerodynamically, the lift coefficient must be carefully managed within the operational range. The smooth variation of lift with angle of attack and stability across different Reynolds numbers are key constraints. For thick airfoils, the maximum lift coefficient is no longer a reliable indicator of performance, as they operate far from stall conditions. Instead, consistent lift behavior and Reynolds number stability are essential for efficient and stable wind turbine output.
To address these challenges, the RFOIL airfoil was designed using a hybrid method, ensuring geometric compatibility and delaying boundary layer separation. Wind tunnel tests at Beijing University of Aeronautics and Astronautics confirmed its accuracy in predicting lift up to 25°, making it ideal for high-angle-of-attack applications.
The new 45% thickness airfoil outperforms existing designs like the DUV-W2-401, showing better overall performance for multi-megawatt turbines. Ongoing wind tunnel experiments are validating these results, and the research has been supported by the National "863" Plan Project (No.2012AA051303). The findings were presented at the 2013 Chinese Society of Engineering Thermophysics Conference and will be published in the Journal of Engineering Thermophysics.
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