CFD-based investigation of self-starting enhancement and unsteady aerodynamic response improvement in Darrieus vertical axis wind turbines using an optimized dual-row blade configuration

• Dual-row Darrieus VAWT is selected to enhance self-starting capability in low-TSR. • The optimum d o / d i is 1.3, resulting in 25% C p improvement over the simple rotor. • Different blade orientations for both secondary and primary blade rows is studied. • Considering both blade rows with positive camber blades resulted in the worst case. • Optimum performance occurs with a symmetric primary row and negative camber secondary. Darrieus vertical axis wind turbines (VAWTs) are regarded as reliable turbomachines; however, they encounter challenges with self-starting capability, as they require initial torque within the low-tip speed ratio (TSR) range. This study presents the dual-row Darrieus VAWT designed to overcome self-starting challenges, using a CFD method. The proposed design incorporates a symmetric primary blade row (NACA0021) alongside a secondary blade row featuring asymmetric, positively cambered blades (NACA4415). Firstly, an optimal diameter ratio between the two blade rows was identified at 1.3, which enhances the power coefficient ( C p ) by up to 25% within the low-TSR range at TSR=2, compared to the conventional rotor. This configuration also demonstrates a twofold increase in the moment coefficient ( C m ) and an 80% rise in the lift coefficient ( C l ), highlighting substantial improvements in self-starting capability. However, extreme spacing between blade rows and a reduction in the diameter ratio to 1.16 yielded no additional advantages. Following this, the impact of blade camber orientation was assessed for the control case with a diameter ratio of 1.3. Among the various configurations tested, Case 2—featuring a symmetric primary blade and a negatively cambered secondary blade—exhibited the most favorable performance, achieving a 50% enhancement in C p compared to the conventional rotor and a 20% improvement over the control case at TSR=2. Conversely, Case 5, employing positive camber in both blade rows, resulted in a 29% decrease in C p compared to the conventional rotor. Overall, the configurations that incorporated negative camber blades in one or both rows (Cases 2- 4-6) demonstrated superior equivalent torque, thereby confirming their advantages in enhancing self-starting.