Effect of Blade Skew Strategies on the Operating Range and Aeroacoustic Performance of the Wells Turbine

Abstract

One of the most intensively studied principles of harnessing the energy from ocean waves is the oscillating water column (OWC) device. The OWC converts the motion of the water waves into a bidirectional air flow, which in turn drives an air turbine. The bidirectional axial Wells turbine as a candidate for OWC power takeoff systems was the object of considerable research conducted in the last decades. The vast majority of the investigations focused on the aerodynamic performance. However, aiming at minimizing the overall environmental impact of this technology requires a new effort to reduce the aeroacoustic noise associated with a Wells turbine's operation. As for other turbomachinery, rotor blade skew is hypothesized to affect aeroacoustic noise sources favorably. Because of the unique symmetry of the blade shape of any Wells turbine, skew here means an inclination of the stagger line exclusively in circumferential direction and hence incorporates a combination of blade sweep and dihedral. Based on a blade element momentum theory, a new blade design methodology for a Wells turbine with skewed blades is established. Then, the effect of blade skew is assessed systematically by numerical simulations and experiments. As compared to a state-of-the-art rotor with straight blades, optimal backward/forward blade skew from hub to tip delays the onset of stall and increases the range of unstalled operation by approximately 5% in terms of static pressure head. As a Wells turbine in an OWC power plant operates cyclically along its characteristic, any extension of stall-free operating range has the potential of improving the energy yield. The flow-generated sound in unstalled operation was decreased up to 3 dB by optimal backward/forward blade skew. However, the predominate noise benefit in terms of equivalent sound power along complete operating cycles is due to the extended operating range without excessive sound due to stall.