Analysis of cavitation for the optimized design of hydrokinetic turbines using BEM

Hydrokinetic turbines are a promising technology for renewable energy production from river, tidal and marine currents. This paper proposes an innovative approach applied to optimization of horizontal axis hydrokinetic turbines (HAHTs) considering the possibility of cavitation. The minimum pressure coefficient is the criterion used for identifying cavitation on blades. Blade Element Momentum (BEM) theory is employed for the rotor design. During the optimization procedure, chord length at each blade section is corrected by a modification on the local thrust coefficient in order to prevent cavitation. The hydraulic parameters as lift, drag and minimum pressure coefficients are calculated by XFoil. Additionally, Computational Fluid Dynamics (CFD) techniques are used to validate the proposed methodology. Cavitation volume in the water flow through the rotor, with and without geometrical modifications, is evaluated using a Reynolds Averaged Navier–Stokes (RANS) approach coupled to the Rayleigh-Plesset model to estimate the vapor production rate. The methodology is applied to the design of a 10m diameter Hydrokinetic Turbine (HT) rated to 250kW output power, for a flow velocity of 2.5m/s. The flow around the optimized rotor presents a reduction of the vapor volume without a major variation upon the turbine output power. A comparison with the Horizontal Axis Rotor Performance Optimization (HARP_opt) code was carried out, demonstrating good behavior. CFD simulations revealed that the proposed design method minimizes cavitation inception, yielding a useful tool for efficient HT design at rated conditions.

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