Abstract
We have developed an axial flow hydraulic turbine having a collection device that increases the output power by collecting the water flow and increasing its speed; however, in the flow field of this open channel-installed hydraulic turbine, the flow speed and water depth upstream change due to the resistance of the hydraulic turbine itself; also, the flow field contains a free surface that is highly complex. The purpose of this study is to reveal the performance characteristics of the hydraulic turbine in a free surface flow field. For this purpose, we performed a multiphase flow analysis that takes the performance characteristics and the free surface into account and a single-phase flow analysis that does not consider the free surface. It was found that the maximum power coefficient obtained by the multiphase flow analysis was significantly higher than that obtained by the single-phase flow analysis for the same reference flow speed. This was because, in free surface flow fields with low water depths, the collection device was not filled with water; moreover, despite no pressure recovery effect, the effect of the fall arising from the decrease in the backpressure coefficient was strong, significantly increasing the inlet velocity ratio.