Wind tunnel investigation on the two- and three-blade Savonius rotor with central shaft at different gap ratio

The Savonius rotor seems to be a promising wind turbine as it not only has the simplest and cheapest design but also is capable of yielding a higher annual energy output at low wind speed than the Darrieus rotor. Moreover, the Savonius rotor can also be used in ventilation systems, for local electricity production, as the start-up device for the Darrieus rotor, and small hydrokinetic turbines operating at low velocity. As a two-blade Savonius rotor suffers from negative average static torque coefficient (AC_TS) at some azimuth angles and large-amplitude variation of AC_TS, several studies have been conducted in recent years to improve AC_TS. The three-blade rotor seems to be a potential candidate for AC_TS improvement. However, less research has been done on three-blade rotors with a 180° arc and central shaft at different gap ratios (GRs) for different wind speeds. Therefore, the focus of the present work is to compare the two- and three-blade rotor in terms of AC_TS and power coefficient (C_P) through a wind tunnel experiment. Results show that the wind speed had a small effect on AC_TS. However, negative azimuth angle range is narrowed and the negative azimuth angle range is moved upward as GR increased. Hence, the Savonius rotor with three blades could not only eliminate the negative range of AC_TS but also smooth AC_TS curves. In terms of the C_P, the maximum power coefficient of the two-blade configuration was approximately 1.5 times that of the three-blade configuration. The 1/6 GR test data exhibited the attainment of super performance for all wind speed and blade number.