To improve the performance of a 3.5 kW small H‐type vertical axis wind turbine (VAWT) with the airfoil of NACA0015, a design method for installing split flaps at the trailing edge of a blade is proposed. By solving the two‐dimensional URANS equations, the simulation of VAWTs is carried out. The influences of the length (10% c‐30% c), arrangement positions (90% c‐70% c), and deflection angles (10°‐30°) of the split flaps on the performance of VAWT are determined by orthogonal design when Reynolds number is 2 × 105 and 1 < TSR < 3. In accordance with the influence degrees of these parameters and adopting the maximum wind energy utilization ratio CPmax and the maximum efficient operating range Δλmax as research objectives, the optimum flap length is determined as 22% c, the optimum deflection angle is 10°, and the optimum arrangement position is 92% c. Further studies show that the CPmax is increased by 5.8% and its Δλmax is increased by 25.9% after installing split flaps. Meanwhile, split flaps can change the Kutta‐Joukowsky trailing edge boundary condition, which increases the flow circulation around the airfoil. Also, split flaps can increase the positive pressure difference in the upwind area and reduce the negative pressure difference in the downwind area, which significantly improves the aerodynamic performance of VAWTs.

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