Wind tunnel testing, numerical analyses and shape optimization of a vertical axis wind turbine

Abstract

In this work the aerodynamic efficiency of a small commercial vertical axis wind turbine is investigated experimentally and numerically. The turbine is a Darrieus type with three vertical airfoil blades having helical twist of 78 degrees, height 1.45m and diameter of 1.45m. The airfoils have a chord of 222 mm and a thickness of 35 mm. The experimental studies were made in a wind tunnel where a hot-wire anemometer was used to measure the wind speed. The power curves of the turbine were extracted using a generator connected with an inverter able to vary the resistant moment applied to the turbine and to measure its resulting angular speed. The energy produced by the turbine itself at various wind speeds using the original generator and inverted provided by the manufacturer was also evaluated. 2D and 3D computational fluid dynamics (Cfd) analyses were carried out to investigate the complex aerodynamic of the the blades during their rotation; due to the continuous variation of the angle of attack in their rotary motion, the blades experiments the phenomena of dynamic stall overall at low tip speed ratios when the variation of angle of attack is higher. To perform the simulations the commercial code Fluent was used; it resolves the Unsteady Reynolds-Average Navier-Stokes equations (U-RANS) by means of the finite element method. In the analyses the algorithm SIMPLE was used to achieve the pressure-velocity coupling, while spatial discretization was obtained using a second order upwind scheme. All the simulations were carried out till periodic result were obtained. A mesh of hexahedral elements made with the software Icem was used . The volume was divided into three different domains, the domain containing the blades was rotating at a fixed angular speed with the blades. These analyses were made at different wind speeds and, for each of them, different tip speed ratios were considered allowing the computation of the power curves at different wind speeds. The performance coefficient obtained by 2D analyses resulted higher than those of 3D analyses since the presence of tip vortex, not considered in 2D simulation, causes a reduction of the aerodynamic performance. After investigating the behavior of the commercial wind turbine, a new turbine having the same dimensions but different airfoils for the blades was designed and analyzed. In order to choose the best shape among many airfoils of the 4-digit NACA and 6-digit NACA, the effects of 3 different parameters of the airfoil were examined: chord, chamber and thickness. The effects of the variation of these parameters were analyzed in terms of moment generated, rotational speed and in terms of power coefficient obtained. After having varied these parameters individually, the effects of the simultaneous variation of 2 parameters were studied in order to evaluate the synergy among the parameters. At the end of this process of optimization the airfoil able to give the best result in terms of power coefficient was identified. This work shows once more that numerical approaches provide scientists and industry with a valuable tool for designing and optimizing wind turbines.