Francis turbines are the most common water turbines in use today. The Francis turbine is a type of reaction turbine, a category of turbine in which the working fluid comes to the turbine under immense pressure and the energy is extracted by the turbine blades from the working fluid.
The desire to accurately comprehend the flow features within turbines has been an area of much concern. This is result of the need to predict their performance characteristics, determine the cavitation properties of the turbine and to further improve their efficiency.
The objective of the CFD study was to analyze the flow through a Francis turbine’s runner using computational fluid dynamics to evaluate the flow behaviour inside the runner, determine and locate the area with minimum pressure on the blade surface (areas susceptible to cavitation), and to predict the hydraulic efficiency of the runner.
To ensure the accurate transfer of the fluid pressure field obtained in CFD the finite vol. mesh of the fluid domain is used as finite element mesh of blades in stress analysis. The finite vol. mesh obtained from CFD contains only shell elements at all faces of blades this shell mesh is converted into 8 nodes tetrahedral parabolic elements.
As per the details provided by the client the 3D model for different parts of Francis Turbine was modeled. After preparing the 3d model of the geometry, tetrahedral and hexagonal mesh with mesh sizes of 0.001 m to 0.1 m were created and total mesh cells of 8.85 million were obtained.
Three simulation cases were considered for this study
Runner blades with maximum erosion.
Runner blades with minimum erosion.
Optimized Runner blades.
Modal Analysis, Fatigue and Stress Analysis were carried out for various scenarios like:-
Fluid pressure load and centrifugal loads of runner running at a speed of 300RPM.
Centrifugal loads running at speed of 545RPM during Free running condition.
Crack analysis and Life Estimation for runner blades were also analyzed for static and dynamic loading to identify the crack initiation and propagation.
CFD simulation was carried out to determine the hydraulic efficiency of Francis turbine for maximum, minimum and optimized runner condition.
The hydraulic efficiency for maximum, minimum and optimized runner condition is 92.02, 93.3 and 93.6 respectively. In order to achieve the rated power output of 255 MW the flow rate should be increased 3from the rated discharge 64.0 m /s. The flow rate at which rated power output of 255MW will be achieved was calculated from CFD analysis.
It was observed a decrease in natural frequencies of a runner in air and surrounded by water. The mode shapes in both cases were similar. It should be ensured that any disturbing frequencies should be separated by a margin of 10 % on either side of natural frequencies as obtained from modal analysis.