Numerical Simulation on Hybrid Savonius Turbine with NACA-Airfoils as H-rotor Blades

Abstract

The demand for green and clean energy is increasing all over the world. Vertical Axial Wind Turbine (VAWT) is utilized to convert mechanical energy into electrical energy, which is commonly used in urban and residential areas. The combination of Darrieus and Savonius rotors is notified as a hybrid wind turbine. It has the Darrieus blades form as the airfoils, and the Savonius blades placed in the middle. Darrieus rotor has high power efficiency, but it is difficult to create a start-rotating rotation, as the inverse of the Savonius rotor, which is better self-starting capability, but it has low efficiency. The aims of this study is to compare the performance of the hybrid turbine with two diffferent configurations. Two shapes of H-Darrieus airfoils were investigated, which were the symmetric NACA-0018 airfoil and the asymmetric NACA-2415 airfoil. Both configurations were tested on varying tip speed ratios of 0.5 to 4 and wind velocities of 4 m/sec and 7 m/sec. Unsteady, two-dimensional numerical simulations were performed. A moving mesh was applied for the rotating part. The simulation was carried out by using a shear stress transport (SST), $\pmb{k}-\omega$ turbulent model. From the simulation results, the flow contours, coefficient of moment and coefficient of power are extracted. It is found that at the optimum TSR of 2.75, the maximum $\pmb{C}_{\pmb{P}}$ extracted for the hybrid turbine with the NACA-2415 profile is 36.62% and 40.9%, at a wind speed of 4 m/sec and 7 m/sec, respectively. Meanwhile, the hybrid turbine with the NACA-0018 profile has the maximum $\pmb{C}_{\pmb{P}}$ of 35.14% (optimum TSR of 2.75) and 37.42% (optimum TSR of 2.5) at wind speeds of 4 m/sec and 7 m/sec, respectively. Furthermore, the minimum value of the coefficient of moment for varying azimuth angle is approximately zero, suggesting better self-starting capability of the hybrid turbine.