Increasing the power efficiency of rotors at transitional Reynolds numbers with synthetic jet actuators

Abstract

Increasing the power efficiency of rotating energy systems such as wind turbines and gas turbine rotors is critical to deriving more energy from wind or fossil fuels. In this experimental study, thrust-power and laser Doppler velocimetry measurements were made on a sub-scale three-bladed rotor containing an S809 airfoil and 60 piezoelectric-based synthetic jet actuators. The rotor was operated at rotational speeds between 250 and 1250 rpm (RPM) and blade pitch angles of 0, 3, and 6 degrees. It was found that activating 20 synthetic jets per blade reduces the net power input to drive the rotor by up to 10.6% at a rotational speed of 500 RPM and blade pitch angle of 3 degrees. It was also shown that rotor performance parameters; figure of merit and thrust coefficient also increased by up to 28% and 9% respectively. Chordwise velocity measurements reveal that synthetic jets reduce velocity deficit within the boundary layer improving the blade’s sectional lift coefficient and thrust. A correlation is made between the blade tip vortex Reynolds number and spanwise velocity measurements, which indicates that the tip vortex becomes more turbulent and induces an adverse pressure gradient with increasing rotor speed and blade pitch angle. The results indicate that synthetic jets are a viable means of improving the power efficiency of a rotor with vortex Reynolds numbers under 1.8 × 105.