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

Abstract Self-rectifying impulse turbines are a popular alternative to the Wells turbine for oscillating-water-column wave energy converter converters. Self-rectifying impulse turbines have two sets of guide-vanes, one set 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 known to be severely affected by the large aerodynamic losses due to the inherent misalignment between the outflow from the rotor and the downstream guide-vanes. The biradial turbine is an advanced, more efficient, version of the impulse self-rectifying turbine, as compared with the conventional axial-flow type. The paper presents a new topology for the radially-set guide-vane system arranged into multiple, rather than simple, rows, aiming to increase the turbine efficiency by reducing the losses by aerodynamic outflow stalling at the exit guide-vane system while ensuring the required inflow deflection by the inlet guide vanes. The design method combines a evolutionary optimisation algorithm with cascade-flow CFD RANS calculations. Experimental results are presented to validate the design method and to assess the performance and flow-losses of the single and double-row guide-vane system configurations.