Successful CFD optimisation

of a Francis turbine as part of a power plant refurbishment

For a hydroelectric power plant in Spain, we were commissioned about two years ago to conduct a CFD-based refurbishment study for a Francis turbine as part of the power plant refurbishment. 

Main data of the HPP and the Francis turbine:

• Net head (Design): 114.2 m
• Possible range of net head: 86 m – 138 m
• Max. discharge Qmax (Design): 8.68 m3/s
• Runner diameter at outlet: 980 mm
• Height of guide vanes: 243 mm
• Nominal turbine speed: 750 rpm
• Altitude of the power plant: ~ 800 m
• Max. mech. power 9000 KW
• Suction head: - 4.6 m (turbine below tail water)
• Number of guide vanes: 16
• Pitch circle diameter of guide vanes: 1300 mm
• Number of runner blades: 13

Initial situation

The original turbine had been in service for over 35 years and was operated over a wide head range. As the blades showed cavitation damage in several places (see photos), the original runner was to be replaced with a new, optimised design.

During operation, problems with vibrations in the runner and bearings also occurred, but these are typical for medium and low partial load operation. Depending on the flow phenomena that occur (cavitating helical partial load vortices, cavitating blade channel vortices), strong vibrations can be expected if the turbine is operated far away from its best efficiency point.

CFD study confirms existing damage

We analysed the existing turbine as part of a CFD study. This clearly confirmed both - the existing cavitation damage and the insufficient performance. The turbine, which we then optimised, has since been manufactured and is now successfully in operation.