Cavitation vortex dynamics in a high-head model Francis turbine

Francis turbines play a critical role in maintaining grid stability owing to their flexibility in handling a wide range of operating conditions and their ability to rapidly adjust to variations in power demand. However, when operated under off-design conditions, such as part load or overload, unfavorable flow phenomena emerge, including cavitation, vortex rope formation, flow separation, and strong pressure fluctuations, all of which can significantly impair performance. Cavitation is particularly critical at deeper part load, where inter-blade vortices strongly modify the runner flow. In this study, numerical simulations are performed to investigate cavitation dynamics at two part-load conditions, such as flow rate 69.9% of best efficiency point (PL1) and 54.2% of best efficiency point (PL2). The results reveal that at PL2 prominent inter-blade cavities develops near leading edge while unsteady cavity structures are observed at trailing edge, while at PL1, the cavitation structures remain smaller and more stable near both leading edge and trailing edge. Variational Mode Decomposition of the pressure signals demonstrates that PL1 is dominated by low frequency on the order of 0.176 times runner rotation frequency, whereas PL2 exhibits frequency of 1.05 times runner rotation frequency and its harmonics. Furthermore, torque analysis indicates that fluctuations at PL2 are significantly higher in amplitude and less coherent compared to PL1, underlining the adverse impact of cavitation in part-load operation. Entropy production analysis is employed to quantify the irreversibility within the turbine, revealing significantly higher energy dissipation (30%–40%) at PL2 compared to PL1, primarily driven by cavitation-induced turbulence.