Internal flow stability of a Francis turbine under wide load operating range

Modern power systems, energy sources, often necessitate that hydropower turbines function across a broad load range. This requirement presents significant challenges to the stable operation of turbines designed based on rated operating parameters. This study utilizes numerical simulations and experimental tests to investigate the flow characteristics, pressure pulsation, and vibration of a Francis turbine in a hydropower station within a hydro-photovoltaic hybrid system under varying load conditions. The findings indicate that with a rated head of 132 m, an intermediate head of 145 m, and a maximum head of 156.5 m, the internal flow pattern of the turbine becomes chaotic when the opening degree is between 20% and 40%. In contrast, when the opening degree ranges from 70% to 100%, the extent of eddy currents around the runner blades decreases, leading to a more stable flow pattern. Severe cavitation of the runner blades occurs at 20% opening when the water head is between 132 and 145 m. However, at a water head of 156.5 m, only minor cavitation is observed at the runner's inlet edge. As the guide vane opening increases, the primary cavitation zone shifts from the interior of the runner to the lower end of the discharge cone. Pressure pulsation and vibration tests suggest that operation should be minimized in the 55% to 70% opening range when the rated head is 132 m. To maintain efficient and stable operation of the power station unit, it is recommended that operating conditions be maintained within the 70% to 80% opening range. These findings offer a technical support and a theoretical framework for expanding the stable operation range of hydraulic turbines.