Tidal Turbine Rotor Spacing Influence On Power Performance: Simulating A Scaled Dual-Rotor Axial Flow Turbine

Abstract

Power performance and turbulent wake characteristics of a scaled current-driven marine turbine were simulated using unsteady 3D RANS with the k-ω SST turbulence model and sliding mesh technique. The turbine is an axial flow, dual rotor tidal turbine with counter-rotating rotors, each with two blades and a diameter of d_T = 0.5 m, representing an approximately 1:40 scale system based on the U.S. Department of Energy’s Reference Model 1 (RM1) tidal turbine. Validation of numerical results for three tip speed ratios was performed by comparison with experimental data. The influence of rotor cross-stream spacing on power production was also studied by modeling three distinct lateral rotor separations, equal to 1.2d_T, 1.4d_T, and 1.6d_T. Numerical results showed a good correlation ranging within ±3.8% of turbine performance to experimental measurements for all tip-speed ratios studied, validating the numerical results for power estimation and demonstrating the advantages of this model when dealing with high-flow detachment. Inflow dynamics were captured well, exhibiting a difference of less than 5% compared to experimental data. However, wake dynamics showed a significant difference between numerical results and experimental data, ranging from 16% error at approximately 〖X/d〗_T=4, up to 170% error at 〖X/d〗_T=8. Finally, numerical results indicated a tendency for higher power production as the rotors are spaced farther apart, with the resulting power coefficient values of C_p = 0.449, 0.461, and 0483 for lateral rotor spacings of 1.2d_T, 1.4d_T, and 1.6d_T, respectively. This behavior was accredited to the reduction of the flow through the swept area of the rotors, causing what is known as 'choking effect’.