Prediction and analysis of the cavitating performance of a Francis turbine under different loads

The variations in the operating conditions of Francis turbines may lead to cavitation due to pressure pulsations in vanless space. In order to combat the fluctuations in power demand and input flow conditions such as net head and discharge, the turbine needs to be operated at off-design conditions. Due to the higher demand of medium-specific speed Francis turbine at most of the hydropower plants, it is necessary to predict the low-pressure zone and vapor bubbles formation on the surface of turbine runner blades. Under the present study, it is proposed to investigate the cavitation and performance characteristics of a medium-specific speed Francis turbine. In order to resemble the fluctuations in load, three different operating regimes, i.e. part-load, rated load and over-load, were considered to especially highlight the performance under cavitation and without cavitation conditions of the turbine. Based on investigations, it has been revealed that turbine experiences a minimum drop in hydraulic efficiency corresponding to rated-load operation and the maximum drop in efficiency was observed during the over-load operation. The pressure variations across the runner blades were observed at mid-span of the blade in stream-wise direction. The cavitation characteristics of the turbine were derived by using the sigma curve and the vapor volume fraction. The critical values of sigma for part-load, rated-load, and over-load regimes were obtained as 0.18, 0.12, and 0.16, respectively. The results obtained under the present investigation were verified by the experimental model testing results of the turbine.