Performance analysis of a small-scalewind turbine at variable pitch and withpitch unbalance

University essay from KTH/Strömningsmekanik och Teknisk Akustik

Abstract: When it comes to design a wind turbine rotor, several parameters have to be taken into account. The present work focuses on the inclination of the blades with respect to the rotor plane, namely the pich angle. The main goal of the project was to design a small-scale wind turbine rotor for wind tunnel tests and in this thesis the optimization of a first prototype is presented. The characterization of the performances was carried out by coupling two different approaches: an experimental and a numerical one. For the experimental part, a proper setup was built and the wind turbine model was tested in a wind tunnel. The results were compared with a Blade Element Momentum theory code developed in Python, that involved also CFD simulations to assess the aerodynamic properties of the blade sections. The analysis characterized the performances at variable collective pitch in terms of power and thrust coefficient, showing that the intitial blade design was not the optimal one. Therefore, the optimal pitch angle that maximize the power porduction was found for variable conditions. The second part of the experiments focused on the case of pitch unbalanced and the potential risks connected to it. As a result, the analysis demonstrated that any kind of pitch unbalance generates losses in the power production and may lead to a possible increase of the thrust. To compare the results, a modified BEM code was developed by assuming an axisymmetric axial induction factor. Finally, an additional analysis on the wind turbine oscillations was made, finding a connection between lateral vibrations and rotor unbalance and revealing the resonance frequency of the structure.

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