Summary
The draft tube of a hydraulic turbine is the component where the flow exiting the runner is decelerated, thereby converting the excess of kinetic energy into static pressure. However, in some refurbishment cases, the installation of an upgraded runner with an old draft-tube leads to an undesirable efficiency drop as the discharge is increased above the best efficiency point value. It is found to be related to a corresponding sudden variation in the draft tube pressure recovery coefficient at the same discharge. The model of a recent refurbishment which presents this instability is installed in the CREMHyG test rig. Steady and unsteady measurements of velocity and pressure fields of the complex and highly turbulent swirling flow exiting the runner have been carried out at CREMHyG (Grenoble)
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