An undershot cross-flow water turbine does not require a casing for open channels and hence considerable simplification is attained. However, because this water turbine does not have a casing and has a free surface, it is believed that the flow field considerably changes with the blade angle. In this study, the effects of blade angle on performance and flow field of undershot cross-flow water turbine at a constant flow rate are investigated by numerical analyses, and the following facts are clarified. At the outer circumference, as for the runner with a large blade angle, the upstream water depth becomes large and the downstream water depth becomes small. The reason is that a relative flow angle of the runner with large blade angle is different from the blade angle in the first stage inlet. Therefore, the resistance to flow is large. However, as for the runner with large blade angle, turbine efficiency is high. The reason is that a relative flow angle of the runner is close to the blade angle in the second stage inlet.
