Effects of blade number on the aerodynamic performance and wake characteristics of a small horizontal-axis wind turbine

Abstract

Small-scale horizontal-axis wind turbines (SHAWTs) are powerful equipments for wind energy conversion. Furthermore, the wake vortices generated by SHAWTs are used in structural engineering to potentially mitigate vibrations. The blade number of SHAWTs is an important parameter for determining the wake vortices, and thus, in addition to its impact on power generation, it significantly influences structural vibration control. Consequently, the aerodynamic performance and wake characteristics of SHAWTs, with blade numbers from 2 to 5 and tip speed ratio from 1 to 6, were systematically investigated using large-eddy simulations and wind tunnel tests. A superior SHAWT with a high power coefficient (>35 %) was designed. The near-wake characteristics and wake evolution were highly sensitive to blade number. An increased number of rotor blades resulted in a (i) wider near wake, (ii) faster decrease in streamwise vorticity, and (iii) greater velocity deficit (VD) and turbulence intensity (TI) in the near wake. Furthermore, in the far wake, the unstable positions of the tip vortices and peak positions of the TI were closer to the rotor and the VD recovered faster, causing the wind-speed distribution to transition from W-shaped to V-shaped. The results have significant implications for the design optimisation of wind energy harvesting. Moreover, they are potentially beneficial for the control of structural vibration using SHAWTs.