In cross-flow water turbines, the flow channel between the lower nozzle wall and the guide vane acts as a diffuser for partial load operation with a small guide vane opening. Energy loss occurs here due to flow separation and increasing turbulence. The turbine cannot be applied for high heads because an increase in flow velocity causes cavitation below the guide vane. Therefore, the authors propose an energy loss suppression method that consists of sucking air through a hole on the lower nozzle wall upstream of the diffuser region. This study is focused on the air intake method's effect at various guide vane opening ratios of the cross-flow turbine.

It has been confirmed by using CFD that the enlarged flow path between the guide vanes and the nozzle wall causes energy loss due to the pressure decrease at the upstream contraction section. The authors propose a method by the naturally intake of air through a hole in the lower wall of the nozzle to facilitate flow separation and to prevent pressure drop and turbulence of water flow in the lower region of the guide vanes in low guide vane opening ratio. However, it was confirmed that water leaked out of the hole in case of large guide vane opening ratio. In order to reduce the amount of leakage from the intake hole, the hole geometry was changed, and the flow in the nozzle with the hole geometry were explored using CFD simulations. It was confirmed that slits with proper chamfering at the corners across the full width of the nozzle wall can bring less leakage of water, and can cause flow separation with less turbulent free surface. Furthermore performance of cross-flow turbines installed these hole is investigated experimentally, thus the turbine installed the slit which sealed the runner chamber side and atmospheric air is supplied naturally into the slit directly through the side wall, the turbine efficiency could be increased by 2.8% compared to a turbine without holes in the case of the guide vane opening 30%.

Unstable flow and pressure fluctuations due to diffuser rotating stall（DRS）are one of the important industrial issues because limit the operating range of pumps. In this study, the effect of DRS suppression by different diffuser vane slit widths was analyzed by experiment and CFD. As a result of the experiment, it was found that the DRS can be suppressed in all flow rate range with diffuser vane slits over a certain width. The result of relationship between the DRS suppression effect and the flow near the slit flow was analyzed by CFD, it was found that the momentum of the slit flow was an important indicator of the DRS suppression effect.