An approach for the optimization of diffuser-augmented hydrokinetic blades free of cavitation

Due to the Venturi effect caused by a diffuser, which speed-up the velocity through the rotor, shrouded turbines are able to exceed the Betz-Joukowsky limit if the power coefficient is based on the rotor diameter. However, on hydrokinetic turbines this increased velocity may also promote cavitation on the blade. As this subject is still not clear on the current literature, this work presents a novel approach for optimizing hydrokinetic turbines free of cavitation under diffuser effect. The model uses the minimum pressure coefficient as the criterion to keep the pressure near blade tip above water vapor pressure. It includes an extension of Vaz & Wood's optimization in order to take into account the influence of the diffuser speed-up ratio regarding cavitation effect. A changing on the thrust coefficient is assumed to optimize chord and twist angle distributions along the blade. As a result, the proposed model shows that cavitation is indeed sensitive to the diffuser speed-up ratio, demonstrating that such a phenomenon needs to be considered in the design of diffuser-augmented hydrokinetic turbines. Also, the optimization method corrects the chord without relevant changing in the turbine power coefficient, where the increased power output is about 42% higher than the bare turbine for a water velocity of 2.5 m/s. In this case, the model is assessed through comparisons using a 3-bladed hydrokinetic turbine with 10 m diameter, in which the diffuser speed-up ratio is varied. Furthermore, an evaluation is made with models available in the literature, suggesting good performance concerning the cavitation analysis on shrouded rotor design with the proposed optimization procedure.