On rotor hub design for shrouded hydrokinetic turbines

Abstract

The effect of hub length and axial location on the performance of shrouded hydrokinetic turbines has been investigated using axisymmetric actuator disk simulations. Five systems with different hubs (or without a hub, for reference) are considered with a common shroud design. Flow separation on the hub was found to be detrimental to performance, and to be sensitive to rotor loading, leading to unacceptable off-design performance. When the thrust coefficient, CT, is below the optimal value of 8/9, increasing CT promotes flow separation on the central hub; continued increases beyond this value facilitate reattachment. Flow separation on the hub was avoided altogether when most of the hub was placed upstream of the rotor plane—contrary to the convention for unshrouded turbines—where the shroud geometry lends a favorable pressure gradient. This latter design achieved performance in close agreement with the open-centered design, which is recommended when feasible to avoid the risks of hub-flow separation. With a central hole diameter of 0.228D, where D is the rotor diameter, a penalty of <5% in optimal performance was found compared to an actuator disk spanning the entire shroud throat. Lastly, a discussion of the relationship between total system thrust and power extraction is offered.