Abstract
A ducted Darrieus-type turbine has been proposed for extra-low head hydropower utilization of total head less than 2m. In the case of installing the runner in the duct, a narrow intake type brings about higher efficiency than that of the parallel intake with the same width of runner shaft section. In the latter application, the clearance between the casing wall and the runner pitch circle roles an important factor for turbine performance as flow distortion occurs toward the side wall and the main flow is decreased through the center passage of duct. Then, turbine performance is severely deteriorated. On the other hand in the former application, effects of the clearance on turbine performance are not clarified yet, though the flow distortion is supposed to become weakened because of the narrow intake before the runner. In the present paper, the effects on turbine performance are experimentally examined with help of flow measurement and the results are compared with the results of parallel walls. Then, it is shown that there is little influence of the casing clearance on the efficiency and without deteriorating turbine efficiency the structure of turbine enables to become simplified by removing the side-wall at the runner section and the draft tube downstream of runner.
