Dynamic-Stall-Driven Vertical Axis Wind Turbine: An Experimental Study

Abstract

The driving torque on vertical axis wind turbines with large chord-to-radius ratio blades, typically greater than 0.5, is generated mainly by the phenomenon of dynamic stall. In order to determine the optimum configuration, an extensive experimental study was conducted on two-bladed, H-rotor test turbine, by systematically varying the blade chord-to-radius ratio, blade profile (NACA 0012 and NACA 0021), preset angle, strut-blade offset and Reynolds number. Unlike low solidity turbines, peak power coefficients were virtually independent of the chord-to-radius ratio in the range of 0.6 to 0.85 for both blade profiles. At low Reynolds numbers (~10⁵), turbine performance with NACA 0021 blades was vastly superior to that with NACA 0021 blades, producing approximately 90% greater power and torque coefficients at blade tip-speed to wind speed ratios between 0.8 and 1.6. The attainment of peak power and torque coefficients greater than 30% at low rotational speeds and Reynolds numbers renders these turbines highly applicable to urban and low wind speed applications. On the basis of blade-mounted phase-resolved tufts images, it was concluded that an aft dynamic stall vortex precedes the conventional leading-edge vortex on the NACA 0021, resulting in substantially greater post-stall dynamic lift. Preset angles away from zero degrees and variations in strut-blade offset away from the half-chord position reduced turbine performance, due to changes in the nature of dynamic stall. The similarity between preset and offset effects was explained by a new approach of a virtual preset angle that depends on the offset point.