Diffuser technology placed around hydrokinetic rotors may improve the conversion of
the fluid’s kinetic energy into shaft power. However, rotor blades are susceptible to the
phenomenon of cavitation, which can impact the overall power efficiency. This paper
presents the development of a new optimization model applied to hydrokinetic blades
shrouded by a diffuser. The proposed geometry optimization takes into account the effect
of cavitation inception on the rotor blades surface. The main contribution of this work
to the state-of-the-art is the development of an optimization procedure that takes into
account the effects of diffuser efficiency, ηd, and thrust, CT d. The model uses the Blade
Element Momentum Theory to seek optimized blade geometry in order to minimize or
even avoid the occurrence of cavitation. The minimum pressure coefficient is used as a
criterion to avoid cavitation inception. Also, a Computational Fluid Dynamics investigation
was carried out to validate the model based on the Reynolds Averaged Navier-Stokes
formulation, using the κ-ω Shear-Stress Transport turbulence and Rayleigh-Plesset models,
to estimate cavitation by means of water vapor production. The methodology is applied
to the design of a 10 m diameter hydrokinetic rotor, rated at 250 kW of output power at a
flow velocity of 2.5 m/s. An analysis of the proposed model with and without a diffuser is
carried out to evaluate the changes in the optimized geometry in terms of chord and twist
angle distribution. It is found that the flow around a diffused-augmented hydrokinetic
blade doubles the cavitation inception relative to the unshrouded case. Additionally, the
proposed optimization model can completely remove the cavitation occurrence, making it a
good alternative for the design of diffuser-augmented hydrokinetic blades free of cavitation.
|
