Experimental studies of stall delay on the blade of horizontal axis wind turbine
Lee, Hsiao Mun
Date of Issue2014
School of Mechanical and Aerospace Engineering
This study is to experimentally investigate the physics of stall delay phenomenon using a down-scaled model of the blade of a 5kW small-scale horizontal axis wind turbine (HAWT) which is designed here using the popular Blade Element Momentum (BEM) method. The design tip speed ratio (TSR) is 6. At the beginning, a wind tunnel tests were carried out to obtain the lift ( Ct) and drag ( Cd) coefficients of the S805 airfoil. The C1 and Cd were corrected under wind tunnel experimental condition. The effects of wind tunnel correction on C1 and Cd are not significant at low angles of attack (AOAs) and the effects increase with the increasing of AOA. With the C1 and Cd obtained, BEM method was used to design the 5kW wind turbine. Volumetric velocity fields are measured using Tomographic Particle Image Velocimetry (Tomo-PIV) on the 1:10 down-scaled model of the rotating blade at two different global TSR of 3 and 5 with Reynolds number (Re) of 4800 and 4300, respectively. Static pressures are also measured and results illustrate higher suction peaks on the rotating blade than those on the static airfoil, which is typically observed for stall delay. Rather than the recirculation bubbles with strong reversed flows for the static airfoil at stall, attached flows are observed on the suction surface of the rotating blade. Radial flows from rotating blade's root to tip are also found with strong spanwise velocity component located in the vicinities of the vortices and close to the rotating blade's suction surface. In contrast to the case of static airfoil, the vortices that shed from the rotating blade's edges are not found to break down into small pieces. Surface streamlines of rotating blade are also presented. At large AOAs, Coriolis forces are found to be larger than centrifugal forces in vertical direction in all three measurement volumes, which contributes to the reduction of the adverse pressure gradient. In addition, the effects of freestream turbulence levels (FTLs) at 0.4%, 4% and 13% on stall delay are studied using Tomo-PIV system. Static pressure measurements illustrate that FTL has stronger effect on the surface pressures of the static airfoil than those of the rotating blade. Magnitudes of the absolute velocities within the separated flows above the static airfoil's suction surface increase significantly with higher FTL, while, in contrast, the change of these velocities above the rotating blade's surface is less obvious. At the root and middle sections of the rotating blade, the radial flows become wider with higher FTL near the rotating blade's leading edge when the AOAs arc large. At large AOAs, the strength and size of the vortices that shed from the rotating blade's leading and trailing edges decrease significantly with higher FTL. However, at small AOAs, the size and coherence of the vortices near the rotating blade's trailing edge increase significantly with higher FTL. Surface streamlines of the rotating blade illustrate that at the rotating blade's root region and at large AOAs, the streamlines tend to lean toward the rotating blade's trailing edge at higher FTL.
DRNTU::Engineering::Mechanical engineering::Motors, engines and turbines