With increasing demand for producing clean and pollution free energy, special attention has been paid to wind turbines and improving their performance. Reducing the effect of wingtip vortices on the wind turbine performance can be achieved by using winglets which work to weaken the impact of wingtip vortices by diffusing them away from the blade tips. The general trend of the literature has considered winglets as diffusers of the wingtip vortices. However, extending the span of the turbine rotor by attaching winglet could improve the potential of a rotor to capture more kinetic energy from moving air. Accordingly, the winglet planform and airfoil play vital roles in wind turbines performance.
The present work reports on the study of the effect of winglet planform and winglet airfoil on the wind turbine performance using Computational Fluid Dynamics (CFD) tools. The National Renewable Energy Laboratory (NREL) phase VI rotor is used as a baseline rotor and the CFD results are validated with the experimental data in terms of torque, pressure and normal force coefficients for different wind speeds.
In this study, two turbulence models are used, which are the SST k-ω and the Spalart-Allmaras models, which can be used to predict the properties of the fluid flow in the computational domain. Both of the models show a good match of the numerical results when compared to the experimental data, at a range of low wind speeds from 5m/s to 8m/s, due to the absence of stalled flow. At higher wind speeds of 10m/s, the SST k-ω model shows a better match between the calculated torque and the experimental measurements. Consequentially, the SST k-ω model is implemented to predict the behaviour of fluid flow in all the CFD calculations in the present study.
The aerodynamic behaviour of two winglet planforms is investigated. These are rectangular and elliptical winglets to increase the NREL phase VI rotor performance. The performances of four winglet configurations are assessed when compared to the baseline power, at the range of wind speeds from 5m/s to 25m/s. The configurations are obtained by changing the winglet planforms and airfoils using the S809 and PSU 94-097 airfoils.
In this regard, the elliptical planform causes a minimizing of the wingtip vortices, more than the rectangular planform, due to the reduction of the elliptical tip by 75% when compared to the rectangular tip. A rectangular planform shows a better performance than the elliptical planform in percentages of power increase. The highest percentage in the power
increase is achieved by attaching the rectangular planform that tilted by a cant angle of 45o and extended by 15cm. This improvement is slightly more than 9%, at the range of low wind speeds from 5m/s to 10m/s, since the flow is almost attached.
Considering the effect of winglet airfoil, the study reports that, choosing a suitable winglet airfoil is mainly dependent on the aerodynamic coefficients of the selected airfoil, such as lift coefficient (Cl), drag coefficient (Cd) and moment coefficient (Cm). For this purpose, a preliminary analysis is conducted using the Xfoil code to predict the aerodynamic coefficients of selected airfoils (S801, S803, S805A and S806A airfoils). The S806A and S805A airfoils are chosen to create two different configurations. The 3D calculations show more increase in the NREL phase VI power is achieved by attaching the configuration that created using the S806A airfoil since this airfoil has less drag coefficient.