Experimental investigation and numerical simulation of flow in the draft tube elbow of a Francis turbine over its entire operating range

Abstract

Hydraulic power plants are increasingly required to extend their operating range, enabling the seamless large scale integration of renewable energy sources into the electrical grid. In Francis turbines, the complex cavitation patterns appearing in the draft tube cone at off-design operation cause a decline in turbine performance and cyclic pressure pulsations that may lead to dangerous hydro-acoustic instabilities. The accurate prediction of the velocity distribution and the pressure pulsations by numerical simulation in the design phase is important for manufacturing stable and more efficient hydraulic machines over a wide operating range. This work seeks to improve the accuracy of numerical flow models for predicting the dynamic turbine characteristics through a systematic comparison of the velocity fields simulated with CFD and measured with Laser Doppler Velocimetry (LDV) in the draft tube cone. The numerical simulation and experimental measurements are performed in flow conditions for six values of discharge factor, four part load comprised 40%, 60%, 80%, 90% of the discharge value at the Best Efficiency Point (BEP), at the BEP, and one full load of 110%. The steady Reynolds-Averaged Navier-Stokes (RANS) simulation are performed for numerical analysis by the commercial flow solver ANSYS CFX. The LDV measurements are performed on a reduced scale model with a specific speed of nQE=0.20, installed the test rig of EPFL Laboratory for Hydraulic Machines.