Experimental investigation of draft tube flow instability

Abstract

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). They included the three components of the velocity. The flow in the draft tube is rich in secondary flows and possible flow separation due to the elbow as well as to the divergent shape of this geometry. Pressure and velocity field measurements in the draft-tube have been performed. They let us identify some phenomena that may explain the origin of the unusual behavior of the draft tube. Finally, the same model, with a modified draft tube geometry that was designed to reduce the efficiency drop phenomenon has been installed. Velocity measurements were made to validate the benefit of such geometry modification.