From viewpoint of greenhouse gas emission reduction, recently utilization of renewable energy becomes the focus of attention. Among several renewable energy systems, hydro power generation can supply the most stable energy and Torishima has already supplied an axial micro hydro turbine [1].

In general, the low cost system is needed in the case of micro hydro turbine. And one solution is to apply a fixed blade and constant rotation speed type. However the operating range will be narrow because of a cavitation problem. As is well known, there are some kinds of cavitations in the hydro machinery and they cause adverse effects such as vibration, excessive noise, decreasing efficiency and affecting turbine life [2]. Furthermore, these effects are more serious at partial and/or overload condition, so that turbine allowable operating range is limited by them. Therefore, the purpose of this study is to improve the cavitation performance in wider range for satisfying this requirement.

In this paper, we focused on the influence of tip leakage vortex cavitation; one of the important phenomena for cavitation performance improvement as described in the references [3–5]. We tried to clarify the influence by both experimental and numerical investigation on the cavitation performance. In CFD (Computational Fluid Dynamics) analysis, tip clearance was applied for whole circumference model of runner and the full cavitation model [6] was used for the unsteady cavitation analysis. The calculation domain included guide vane was used to consider the effect of unsteady interference between runner and guide vane. In the CFD result, the tip leakage vortex cavitation was identified by void ratio and helicity [7] to analyze vortex structure with detail.

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