Since Darrieus turbine is omnidirectional against the wind, it has a great essential advantage in turbulent wind environment. However, a disadvantage on the starting up is known in low velocity conditions of the wind compared with propeller turbines. Darrieus turbine has a low efficiency problem at low tip-speed ratios due to this disadvantage. Some studies have done to overcome this problem, but these have been on the beginning of the way toward the solution. The reason is that the angle of attack for Darrieus blades is changed largely during the rotation relative to the wind direction. In this study, we found a new technique to improve the efficiency. The technique is the providing a large internal cylinder rotating together with the blades, which can control the angle of attack of the blades. We evaluated the influence of cylindrical diameter on the blade performance by the time-averaged velocity distribution and the intensity of the velocity fluctuation comparing between with cylinder and without cylinder. The flow field around the blades is measured with two kinds of instruments. One is Hot Wire Anemometer for analyzing temporal variations of flows, and the other is Particle Image Velocimetry for analyzing the spatial flow structure in detail. Angles of attack and lift coefficients are calculated from the measured relative velocity vector field. As a result, the angles of attack and the lift coefficients are improved in some rotational angles, which are also confirmed by numerical simulations.

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