| Summary: | 博士 === 國立中興大學 === 機械工程學系所 === 101 === This study investigates, via Ansys/Fluent, the effects of pitch angle and blade camber on the flow characteristics and performance of small-size Darrieus vertical axis wind turbine (VAWT). The user-defined function is employed to calculate the instantaneous moments produced by all the blades and drive the VAWT system to rotate from rest to the state of steady rotation. Further, a dynamic model is employed with reduced moment of inertia ( ) at varying resistant coefficient to explore the mechanism of multiple accelerating phenomenon of the VAWT system. The VAWT system includes three blades with a chord length 0.1m and a rotating radius 0.5 m, and is driven by a uniform wind speed (10 m/s). The blade profiles with three different cambers are NACA0012, 2412 and 4412, respectively, and the pitch angles vary between 10° and -10°. For the conditions studied herein, some important results are concluded as follows:
(1) The magnitudes of are mostly positive within one complete revolution for and the power coefficient for is the highest among all other pitch angles. While the pitch angle decreases from θ＝5° to θ＝-10°, the peak magnitude of decreases significantly, the interval (or azimuthal portion) of negative moment extends wider and the negative value of increases within one complete revolution.
(2) The self-starting ability is the best for pitch angle and is degraded monotonously while the pitch angles change from to . Further, the initial acceleration has the largest value for NACA 4412 implying the best self-starting ability of the VAWT system.
(3) The root mean square moment has a maximum value (0.175) at for NACA2412, and is much higher than those for NACA0012 and 4412 at all pitch angles.
(4) The mechanism for multiple accelerating stages is caused sequertially by the linear and nonlinear terms existed in the driving aerodynamic moment of the VAWT system. The first stage acceleration is doministed by the linear term, while the other acceleration is caused primarily by the nonlinear terms which are strong dependent on the rotating speed. The latter effect will first increase and then decreases as the tip speed ratio increases.
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