Influence of guide vane clearance on internal flow of medium-specific speed Francis turbine

In Francis hydro turbines, a small clearance between the guide vane blade and the facing plate is crucial in pivoting the guide vane blade and controlling the flow rate of the turbine. This clearance-to-blade height ratio is inversely proportional to the scale of the hydro turbine. Smaller hydro turbines have higher clearance-to-blade height ratio than larger ones. Most of the time, this clearance is not included in the simulation model for it can cause inconsistencies between the computation and model turbine. This paper is focused on the various guide vane clearance and its influences on the flow. Firstly, steady numerical calculation of the turbine model (casing, stay vanes, guide vanes and draft) with and without guide vane reference clearance was carried out using the commercial code ANSYS CFX. Experiments were conducted on model turbine of specific speed 180 [m-kW] with guide vane reference clearance. The inter-blade pressure near the leading edge (LE), mid-chord and the trailing edge (TE) of the guide vane were obtained using the pressure sensor located along the circumference of the turbine casing. The result shows a good agreement between numerical computation and experiment. Furthermore, guide vane models with different clearance heights were simulated and the impact on runner inlet energy loss was investigated. In conclusion, it can be clarified that the roll-up flow from the guide vane clearance interacted with the main flow upstream of the runner which then caused loss to the turbine performance.