土木工程与力学学院知识库

风能由于洁净可再生的特点，近年来得到了快速发展。目前世界装机容量已超过486.7GW，且在未来10年全球将会持续增大。逐渐增加的风电场建设规模和装机容量，为了追求高风能利用效率，受风能资源和建设选址的制约，迫使大量风电场向人烟稀少的近海或内陆荒漠地区发展，常受到飓风、暴雨、冰雹、沙尘暴、盐渍等的冲击与侵蚀，伴随高的叶尖旋转速度，很容易造成叶片表面的磨蚀。前缘磨蚀问题是许多风电企业运行中难以克服的关键问题，对风力机的发电效率和结构稳定都带来不利影响，在风电企业运行管理和维护方案制定中必须加以考虑。但是，目前关于磨蚀对风力机影响机理的认识还很有限，缺乏对磨蚀引起的翼型和叶片流场结构变化及气动载荷的定量研究。因此，本文围绕不同类型、不同程度磨蚀对风轮气动载荷的影响进行研究，旨在获得磨蚀对风力机气动特性的影响规律，为风电企业的运行管理、叶片维护、高效发电和安全生产提供理论依据，为不同磨蚀程度风力机的发电功率预估提供指导。

本文的主要研究内容与成果包括：采用CFD方法，系统分析了不同程度的凹槽磨蚀、点蚀、剥蚀对S809翼型绕流流场结构、压力系数和气动性能的影响，得出了不同磨蚀类型中磨蚀参数变化对翼型气动性能和流场结构的影响规律，结果表明当凹槽磨蚀深度/厚度的比值超过0.5时，随着磨蚀深度的增加翼型气动性能基本保持不变；对于点蚀模型，当点蚀深度超过0.5mm后翼型升阻比变化较小，点蚀各特征参数对翼型气动性能影响的次序为点蚀密度>点蚀深度>点蚀面积>点蚀位置；对于剥蚀模型，1%c的剥蚀长度对翼型流场结构和气动性能影响最大，随着剥蚀长度的增加，翼型尾缘分离区尺度逐渐减小，翼型的气动力系数反而增大，但随着剥蚀厚度的增加，翼型的气动力系数下降。对传统的BEM模型进行了修正，采用修正的BEM模型和二维磨蚀翼型的CFD气动力数据，对NREL Phase Ⅵ风力机在叶片前缘均匀剥蚀、均匀点蚀和均匀槽蚀时风轮气动力变化的物理作用机理和变化规律进行了详细阐述，得出了磨蚀参数变化时引起的叶片各断面轴向诱导因子、切向诱导因子、法向力系数和切向力的变化，以及对风轮转矩系数C_M、推力系数C_T、输出功率与推力的影响规律；通过量纲分析法建立了剥蚀、点蚀和槽蚀的数学模型，并通过数值试验标定了所建模型的磨蚀系数，得到了磨蚀系数标定图，通过在标定图中查取磨蚀系数并采用所建立的磨蚀模型可快速计算不同磨蚀程度时风轮气动载荷变化量；通过比较叶片前缘均匀剥蚀、点蚀和槽蚀对风轮气动性能的影响，发现了其影响程度为槽蚀＞剥蚀＞点蚀。通过将叶片划分为10个区域研究每个区域局部发生磨蚀时对风轮气动载荷的影响，结果表明当磨蚀分别发生在叶片外侧0.7R～1.0R、叶片中部0.4R～0.6R及叶片内侧0～0.4R时，所引起的损失约占叶片前缘整体剥蚀所引起损失的60%、30%和10%。据此进一步将叶片划分为内侧（I区）、中部（II区）和外侧（III区）三个区域进行局部磨蚀影响效应研究，结果表明所有磨蚀类型下I区发生局部磨蚀对风轮气动载荷的影响较小；对于局部剥蚀和槽蚀而言，III区和II区剥蚀对风轮气动载荷影响同等重要；对于局部点蚀而言，III区点蚀对风轮气动载荷影响大于II区。通过计算I区、II区、III区发生剥蚀、点蚀、槽蚀时对风力机功率和推力影响的权重因子、，采用分配权重的方法建立了完整的剥蚀、点蚀和槽蚀数学模型，并对不同磨蚀程度下的权重因子进行了标定，根据磨蚀系数、权重系数和磨蚀模型可计算不同磨蚀位置、磨蚀程度对风轮气动载荷的影响，通过叶片前缘非均匀磨蚀算例验证了所建磨蚀模型的可靠性。基于小波分析理论研究了阵风和湍流来流对风轮气动性能的影响，发现两种来流条件下风轮气动载荷波动的极大值均为极小值的2倍左右，与平均量相比，阵风引起的气动载荷波幅要大于湍流；随着平均风速的增大，来流中湍动能TKE和相干湍动能CTKE幅值增高，推力系数C_T和功率系数C_P的波动增加，气动载荷的极值交替频率增加；叶片非均匀磨蚀引起的风轮功率系数变幅远大于叶片均匀剥蚀，非均匀磨蚀时功率系数最大变幅可达99%，而均匀磨蚀时功率系数最大变幅为42.2%。通过CFD方法模拟了均匀来流、切变来流和极端运行阵风条件下叶片前缘磨蚀对风力机三维流场结构及气动载荷的影响，结果表明在均匀来流条件下，随着叶片前缘磨蚀程度的增大，流动分离点前移，叶片失速区扩大；与光滑叶片相比，磨蚀导致叶片吸力面压力的峰值明显下降，叶片整体压差减小；风轮转矩和推力均随磨蚀程度的增大而减小，随着风速的增大，风轮气动载荷减小的幅度先增大后减小。切变来流条件下，叶根处风轮气动性能受方位角变化影响较小，而在叶尖位置，当风轮旋转至最高点时前缘磨蚀对功率损失的影响增大，但随着风速的增大，磨蚀对风轮气动载荷的影响随叶片方位角的变化而减小前缘磨蚀对风轮转矩系数和推力系数的影响明显，在10m/s风速时，风轮旋转一周转矩系数C_My的平均降低42.54%，而20m/s风速时降低达54.19%。在极端阵风运行条件下，前缘磨蚀对风轮推力系数的影响较小，但对转矩系数的影响较大，随着磨蚀程度的增大，风轮的转矩系数降低。通过对比BEM与CFD计算结果，验证了BEM计算结果的可靠性。

Wind power industry has developed significantly during last decades for its clean and renewable characteristics. The global cumulative installed wind capacity has already exceed 486.7GW, which will continue growing in the following decades. Driven by the requirements in energy capture and wind turbine technology, the scope and size of modern wind turbine have grown considerably and single turbine power can reach a capacity of 8-10MW with rotor diameters in excess of 160m. However, various harmful environmental conditions, such as rain drops, windblown sand and dust particles among others could be of great threatens to the mechanical integrity of the wind turbine blade, especially at the leading edge. Leading edge erosion has been stated as a challenge and main issue for manufacturers and operators of wind turbines in many articles and reports. Although the detrimental effects of erosion on wind turbine performance have been investigated in some studies, their quantitative influences have not been thoroughly revealed, especially the flow field contours. Therefore, a thoroughly study on the problem of leading edge erosion must be conducted for a deep understanding of the influence mechanism. The dissertation focuses on the influence of different kinds of leading edge erosion and erosion thicknesses or erosion heights and erosion lengths on the aerodynamic force and power output of the horizontal axis wind turbine. The study can provide some theoretical basis for the management, blade maintanence, safe operation and power generation of the wind industrial manufacturers.

The main conclusions and results are as follows:A thorough investigation on the influence of leading edge cavity erosion, pits erosion and delamination on the flow field structures, surface pressure coefficients and aerodynamics of S809 airfoil was conducted, in which different erosion lengths and heights were considered and a lot of influential rules were obtained through the study. Results showed that the value of C_l/C_d remains a constant when the cavity erosion length/thickness ratio is greater than 0.5, and the length/thickness ratio of 0.5 can be considered as the critical value for the leading edge cavity erosion. For the pits erosion, the lift and drag coefficients of the airfoil change little when pits depth is larger than 0.5 mm, also the distance between two erosion pits is greater than 8 times of the pits diameter. Meanwhile, the lift drops sharply if the first 5%c of the airfoil is eroded, so the leading edge should be protected in the practical engineering. The results of path coefficient analysis showed that pits density is the most influential parameter, followed by pits depth. For the leading edge delamination, the delamination length of 1%c has the greatest influence on the flow field and aerodynamics of the airfoil, the aerodynamic coefficients of the airfoil grow gradually with the increase of the delamination length, while decrease with the increase of delamination thickness. Therefore, the delamination length of 1%c can be considered as the key influential erosion length for this kind of leading edge erosion.Some corrections have been conducted to improve the calculation accuracy of the traditional Blade Element Momentum Theory(BEM). Based on the improved BEM and the aerodynamic data of the eroded S809 airfoils obtained with CFD method in the third chapter, the physical mechanism and influence rules of the aerodynamic forces of the NREL Phase Ⅵ wind turbine blades with different erosions distributed uniform along the blade were elaborated, in which three kinds of leading edge erosion were investigated, that is cavity erosion, pits erosion and delamination. The change rules of the axial and circumferential factors on the sections of the blade span under different erosion parameters, and also the influence rules on torque coefficients and thrust coefficients, power outputs and thrusts of the wind turbine were obtained. With the dimensional method, three leading edge erosion mathematical models were built for calculate the aerodynamic forces variations under the condition of the wind turbine blades eroded with cavity, pits or delamination uniform along the spanwise. After the models building, the erosion coefficients are calibrated and described in figures, and the power output and thrust variations can easily got by finding the erosion coefficients in figure and put them into the models. Compared the influences of the investigated three kinds of leading edge erosion on the aerodynamics of the wind turbine, results show that leading edge cavity erosion has the most serious performance impact, followed by the leading edge delamination and pits erosion. To explore the influence of local erosion on the aerodynamics of the wind turbine, the blade was divided into 10 regions and set the erosion parameters to each region, and the aerodynamic forces of the wind turbine were calculated with the BEM method. Results showed that the leading edge erosion at the outboard(0.7R～1.0R), midspan(0.4R～0.6R) and inboard(0～0.4R) of the blade can lead the aerodynamic forces drop about 60%, 30% and 10% of the whole aerodynamic loss, respectively. Accordingly, the blade was divided in to three regions: inborad(I region), midspan(II region) and outboard(III region), to investigate the local erosion effects. Results showed that the erosion at I region has a relative little influence on the aerodynamics of the wind turbine, while the erosion occurred at II and III regions has nearly equal influences on the aerodynamics of wind turbine under the condition of leading edge delamination and cavity erosion. For the pits erosion, the erosion at the outboard(III region) has the most influence on the aerodynamics of the wind turbine. Comparing the shaft power and thrust of the wind turbine with local erosion at I, II and III regions to the wind turbine with leading edge erosion uniform at the whole blade, the weight factors  and  of the local erosion of the investigated three kinds of erosion were obtained, and which were calibrated by numerical experiments. The complete delamination, pits erosion and cavity erosion models were built by assigning the weight factors to the models built in the fourth chapter. With the models, the aerodynamic force variations of wind turbine blade eroded with different extents and at different locations can be predicted quickly. To validate its accuracy of the models, the aerodynamics of the wind turbine with non-uniform erosions at the leading edge were calculated.The influence of gust and turbulence on the aerodynamics of wind turbine were analyzed based on the wavelet theory, and results showed that the gust and turbulence can cause fluctuation of the aerodynamic forces, the maximum of the shaft power and thrust are about twice as the minimum values. When compared to the average values, the amplitudes of the shaft power and thrust variations caused by gust are greater than that caused by turbulence. Also, the influence of turbulence on the aerodynamic forces of wind turbine with leading edge eroded nonuniformly at the wind speeds of 7m/s and 10m/s, results showed that fluctuation of the turbulence kinetic energy, coherent turbulence kinetic energy, thrust coefficient, power coefficient increase with the increase of wind speed, and the alternate frequencies of extreme values of the aerodynamic forces increase as well. The amplitude of the power coefficient of the wind turbine blades eroded non-uniform is greater that of the blade eroded uniformly, and the maximum amplitude of the power coefficient can reach 99% for the non-uniform eroded wind turbine.3D flow field structures and aerodynamic forces of wind turbine with different leading erosions were simulated with CFD method with the inflow of uniform wind, shear wind and extreme gust, results showed that the flow separation point moves forward and the flow separation region grows with the increase of leading edge erosion under the condition of uniform wind. The maximum pressure of the suction surface decreases tremendously and the differential pressure of the turbine blade decreases when compared the eroded blades to the smooth blade, which indicates that the leading edge erosion can decrease the aerodynamics of the wind turbine. The torque and thrust of the wind turbine decrease with the increase of erosion extents, and the reduction first increase and then decrease with the increase of wind speed. Under the condition of shear wind, the influence of leading edge erosion on the torque coefficient and thrust coefficient was significant, and the average torque coefficient decrease about 42.54% at the wind speed of 10m/s, and that is 54.19% for the wind turbine blade delaminated of 3mm at the wind speed of 20m/s when compared to the smooth blade. For extreme gust inflow, leading edge erosion has little influence on the thrust coefficient, but has great influence on the torque coefficient of the wind turbine, which decreases with the increase of leading edge erosion. Finally, the calculated results were validated by comparing the results obtained by BEM and CFD methods.