Effects of tip injection on the performance and near wake characteristics of a model wind turbine rotor

This paper presents an investigation of the effects of tip injection on the performance and near wake characteristics of a model wind turbine rotor. Experiments are conducted by placing a three-bladed horizontal axis wind turbine rotor at the exit of an open-jet wind tunnel. The rotor blades are non-linearly twisted and tapered with NREL S826 airfoil profile. The nacelle, hub and the blades are specifically designed to allow pressurized air to pass through and get injected from the blade tips while the rotor is rotating. Measurements of torque and thrust coefficient variations with the Tip Speed Ratio (TSR) as well as wake flow field surveys using Constant Temperature Anemometry are performed with and without tip injection. Results show that power and thrust coefficients both increase due to injection especially at TSR values higher than 3.5. Wake characteristics show a tip flow region that is radially pushed outwards with increased levels of turbulence occupying wider areas compared to the no-injection case. Up to two diameters downstream within the wake, the boundary between the wake and the freestream, which is normally dominated by the presence of the tip vortices, gets more diffused and turbulence levels are significantly increased due to tip injection.