Validation of a numerical model for an axial hydraulic turbine operating at upper and lower part-load conditions

The operation of hydraulic turbines has shifted towards lower loads to balance intermittency on the electrical grid. Since the turbines are designed to be operated at the rated flow rate, this presents plenty of challenges. Part-load (PL) operation is often associated with a vortical structure in the draft tube, known as a rotating vortex rope (RVR), which is extensively investigated for Francis turbines but not as much for axial ones. A numerical model of an axial turbine used to enhance the understanding of the flow field during the entire regime of PL operation is developed and validated against experimental data to bridge a knowledge gap. The RVR rotating mode (RM) and plunging mode (PM) are analyzed. The reasons behind the increased pressure pulsations and decreased hydraulic efficiency during lower PL (LPL) compared to upper PL (UPL) are investigated in terms of triple decomposition and head losses. Results show that the numerical model generally agrees well with the experiments. The RM dominates over the PM during LPL, where the numerical model reveals a double helix vortex rope, which is uncommon in axial turbines. Moreover, the double helix originates on the upper part of the runner cone, where the flow separates, and the stagnant region begins, unlike UPL, where it only separates on one side near the bottom of the runner cone. The pressure pulsations are more intense during LPL because the turbulent production from the mean flow is higher and spreads more, which increases the head losses and decreases the hydraulic efficiency. The PM is more intense during UPL, where the RVR extends into the draft tube elbow, causing a pattern of flow separation and reattachment from the draft tube elbow's inner radius bend.