Radial Lift-Based Cascade for Bi-Directional Wave Energy Air-Turbines

Over the last decades various turbine designs have been proposed capable of handling a bi-directional flow in an oscillating water column (OWC) ocean wave power plant. The radial bi-directional lift-based turbine proposed by Kentfield in the early 1980ies is one candidate for power take-off in OWC’s. Key element of this air-turbine is a radial cascade of non-staggered blades made of symmetrical airfoils, with their mean lines geometrically mapped on the rotor’s circumference. The objective of this study is to understand and quantify the performance behavior of an isolated cascade of unstaggered radial blades under bi-directional flow by means of a numerical experiment.3D boundary layer interaction in the corners where the blades are attached to back and front plate degrade cascade performance. But even with side wall effects neglected the through flow direction (centrifugal or centripetal) has a substantial impact. A variation of the radial cascade parameter ‘chord length/radius ratio’ revealed that the performance characteristics can be improved considerably in terms of stall margin by choosing large values of the chord length/radius ratio. This effect is more pronounced for the centrifugal but also existing for centripetal through flow direction. As one increases the chord length/radius ratio the mean flow, the cascade operates in, is modified from plane to curved.Chordwise pressure and friction distributions on the blade surfaces did not disclose a plausible mechanism. However, one hypothesis to explain to the most favorable stall behavior of the more curved cascades is a large stable vortex detected at the blade suction that generates a new ‘blade’ contour. This allows the flow to be attached on the blade suction side at large angles of attack and hence increases lift.© 2014 ASME