The effect of the number of the blades on diffuser augmented wind turbine performance.

Abstract

Diffuser Augmented Wind Turbine (DAWT) has been acknowledged to accelerate the flow and increase the efficiency of the wind turbine exceeding the Betz limit. However, the flow separation that occurs on the inner walls of DAWT is the main problem unfavorably degrading the DAWT performance. Therefore, this study aimed to assess the possibility of using the number of rotor blades as a passive boundary layer control. In this research, the DAWT design consists of a small wind turbine with a diameter of 0.6m and a compact diffuser shroud. The ratio between the diffuser length and the rotor’s diameter is 0.21. This research was conducted using computational fluid dynamics (CFD) simulations via Ansys CFX wherein the result was validated using wind tunnel testing. Analyzing the velocity contours behind the rotor's plane showed that increasing the number of the rotor's blades would augment the kinetic energy at the rotor's tips which helped energize the flow close to the diffuser’s inner walls. Consequently, this helped the boundary layer stay attached to the diffuser's walls and avoid separation. Velocity vectors from CFD results showed that the 4-blade DAWT has the flow fully attached to the diffuser at an opening angle of 20° compared to the 3-blade and 2-blade DAWT, which had the flow completely separated at the same opening angle. In conclusion, the findings proved that the number of blades could be used as a passive boundary layer control.