Numerical investigation of model Francis turbine for performance enhancement with geometrical modification

Abstract

Abstract Most of the hydropower plants in Nepal are run-of-river type and over 60% of them are operating with Francis turbines. There are huge fluctuations in flow during the dry and wet seasons, which force the power plant owner to operate the turbines at off-design conditions which can induce cavitation and fatigue loads on the turbines. This study focuses on numerical analysis of a model turbine designed for the Jhimruk hydropower plant in Nepal. A geometrical modification is made to the model turbine runner blades by attaching two pairs of vortex generators (VGs) to each blade surface on the pressure side at 0.5% span, at the leading edge. The flow on the blade breaks off and separates from the surface during the off-design conditions so VGs are introduced to mitigate swirls generated at the inlet of the turbine. VGs can help to reattach the flow and improve off-design performance of the turbine. The performance of model turbine is analyzed at different guide vane openings to simulate different operating conditions by keeping the operating head constant and varying the rotational speed of the turbine from 500 to 1500 rpm. Performance curves and hill diagrams of the turbine are generated for each case with and without VGs. The results show that VGs at off-design conditions show a maximum increment in efficiency by 12% and 1 to 5% on average. The power output is also boosted by 1 to 6 kW at off-design conditions. This objective has been achieved when compared to efficiency and power at low-discharge and high-speed regions. In conclusion, VGs are simple but effective tools to enhance the performance of hydraulic turbines as well.