Shear and vortex instabilities at deep part load of hydraulic turbines and their numerical prediction

For all types of turbines, deep part load operation (DPL) poses a challenge. An example of a dynamic phenomenon due to vaneless space vortices occurring at DPL is presented for three turbine types: a diagonal, a propeller and a Francis turbine. The backflow region occurring at DPL is often considered to be a main factor in these phenomena. Our results confirm that these backflow regions play an important role, but other factors also seem to be significant in specific cases. In our example of a diagonal turbine, the intersection of the backflow region with the leading edge of the blades seems to generate particularly high pressure pulsations and vibrations. In the case of the propeller turbine, large vaneless space vortices are found in CFD, but vibrations on the prototype machine are well within acceptable levels. Inspecting the flow at an operating point before large vaneless space vortices occur, shows high shear levels near the inner head cover that generate the high-intensity vortices at an even lower load. In the Francis turbine, half the number of strong vaneless space vortices interact with equally strong inter-blade vortices with vorticity of opposite sign to result in high dynamic blade torque. CFD simulations are well capable of capturing these phenomena, allowing them to be considered in the mechanical design of the turbine components for safe operation.