Experimental study of the performance of microtabs for load alleviation in wind turbines

Abstract

The goal of this thesis is to carry out an experimental study of the performance of smart blades reducing extreme and/or cyclic loads for their use in wind turbines. First of all, theoretical research about wind turbines and smart blades was done. Some of this information is explained in the thesis in order to set the background and make it more readable and understandable (Chapters 1 to 3). At the same time, the available wind tunnel was studied, in order to know its characteristics, decide whether it was appropriate for the experiments that would take place, and make the improvements needed for better results (Chapter 4). The next step was to choose a blade profile that would fit well the requirements of the experiments and the manufacture of the set up. Besides, one of the possible smart devices had to be chosen according to its expected performance and its feasibility. It had to be taken into account the technologies and tools that could be used for the construction of the set up, and the characteristics of the set up itself. Therefore, the main features of the set up had to be thought at this point, even though the final design was not decided. Before starting the experiments, theoretical calculations were accomplished in order to have information to compare with the results that would be obtained. These calculations made also possible the choice of the measuring devices, and to have an approximate idea of the pressures that the blade would have to face (Chapter 5). By doing the calculations before, the materials and the design of the set up would be perfectly able to carry out the tests. After all the preceding work, the set up was defined and built (Chapter 6). It consisted in a GU25(5)8-11, a profile designed by the University of Glasgow that is quite thick, and has a CL graph quite linear. The core was made out of foam to make it light, and covered with fiberglass and epoxy, in order to give it stiffness. For the pressure readings, holes were drilled on its surface. These holes go into tubes that run inside the wing, and exit from one side to be connected with the desired pressure measuring device. The wing was mounted in a test section, with a controller for the angle of attack and a protractor. The accuracy of the controller was 0.1 degrees. The experiments (Chapter 7) consisted in measuring the pressure distribution over the wing with and without microtab for different angles of attack, with a solidity ratio of 100%. Also different speeds were tested, as the behavior of the profile changes extremely depending on the Reynolds number. Two different microtabs were tried, a metal cylinder of 2 mm of diameter, and a wooden rectangular prism with section of 2.5 x 4 mm, using the 4 mm as height. Besides, the microtab was placed in three different positions. Firstly, in order to study the load alleviation, microtabs were installed in the suction side at 90% and 95% of the chord. Secondly, they were also tested in the pressure side, at 95% of the chord, obtaining measurements of the lift enhancement produced by the device. This last position was tried in order to have a reference of the effect of microtabs in both sides.