Performance improvement of the axial self-rectifying impulse air-turbine for wave energy conversion by multi-row guide vanes: Design and experimental results

Self-rectifying impulse turbines are a popular alternative to the Wells turbine for oscillating-water-column wave energy converters. Self-rectifying impulse turbines have two sets of guide-vanes placed symmetrically on each side of the rotor, instead of a single set as in unidirectional turbines. The efficiency of self-rectifying turbines with fixed guide-vanes is severely affected by the significant aerodynamic losses due to the inherent misalignment between the outflow from the rotor and the downstream guide-vane system. The paper presents a new design method for high-deflection guide-vane systems arranged into multiple, rather than single, rows. The design method aims to increase the turbine efficiency by reducing the losses caused by aerodynamic outflow stalling at the exit guide-vane system while ensuring the required inlet guide-vane system inflow deflection. The new design method was validated by testing and comparing detailed experimental results for a pair of single-row guide-vane stators, with conventional camber line and thickness distribution vane sections, and a five-row guide-vane stator, with constant thickness circular-arc profiles, on each side of the rotor. A Reynolds number effect correction methodology is presented to estimate the performance of a full-scale self-rectifying turbine from model testing results.