The rehabilitation of a Francis runner in an existing power plant is described. Fluid dynamics optimization of new blade profiles and meridional channel is reached using Ansys CFX. A trial and error approach is used to modify geometric shapes so as to increase blade effectiveness and to contain total pressure loss and cavitation phenomena in the new runner.
Monte Sant'Angelo power plant was built in Italy at the end of '60s and it was put on trial in 1971. It has two Francis turbines with a maximum power of 84 MW for each. The rehabilitation consists in the replacement of all rotating components of turbine, like runner, synchronous alternator and guide vanes, while stationary parts, like spiral casing with stay vanes and draft tube, are preserved.
That is one of the most important problem you can encounter in the rehabilitation. A new runner has to be adapted in an existing power plant and, often, that is not simple and ad hoc design is needed.
At first a CFD steady analysis of existing runner is made. So it is possible to observe the 3D flow field inside the original turbine, blade loading, cavitation phenomena so as to find improvement area.
Then the main problems meet in the preliminary design of the new runner are described. A higher rotational speed is choosen for the new runner to have a cheaper synchronous alternator with two poles less. In this way, for the same values of net head and mass flow rate, the new turbine has a higher specific speed. That choice has created many problems about design of new runner, because of geometric constrains imposed from existing spiral casing like the stay vanes height and their location, above all to keep down cavitation phenomena on the leading edge of the blades. After many CFD analysis and geometric modifications it was possible to guarantee good values for effectiveness and to avoid vapour bubble making up.
At the end a comparison between the existing and the new design runner is made to show the real increase in performance achieved with CFD optimization. |
