Experimental analysis of the swirling flow in a Francis turbine draft tube: Focus on radial velocity component determination

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. In the case of machine refurbishment of an existing power plant, most of the time only the runner and the guide vanes are currently modified. For financial and safety reasons, the spiral casing and the draft tube are seldom redesigned, even if these components present some undesirable behaviour. In some cases, the installation of an upgraded runner leads to a peculiar and 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 swirling flow exiting the runner is complex and highly turbulent. The radial velocity is rarely measured because a quite complicated measurement setup is needed. However, this velocity component is greatly needed in order to properly initialize the numerical simulations, and its influence is important in spite of its small magnitude. Velocity measurements downstream of the runner include radial component made at CREMHyG (Grenoble) by LDV, and PIV techniques are presented. An analytical formulation for this velocity component based on the formulation for the conical diffuser and on the three vortices structure is proposed and compared with measurements.