3C3: Cavitation as limiting factor for plant refurbishment – CFD calculations and test rig results
July 13, 2022
Equipment and Technology
The Krippau run-of-river hydropower plant on the river Enns is being revitalized and a new runner have been developed for this purpose. The power plant (commissioned in 1965) is a cavern power plant with a nominal power of P=24 MW and is equipped with only one unit. Each standstill generates a high production loss so that a standstill of the machine is avoided as far as possible. The aim of the refurbishment is to increase the efficiency in the relevant operating range of the turbine by replacing the runner while maintaining the runner diameter and to increase the maximum discharge from 120 m³/s to 135 m³/s. The new design of the runner with a diameter of D=4.1 m was developed using computational fluid dynamics simulations performed by Stellba Hydro and finally checked by a model test according to IEC 60193. For the new runner the blade number was reduced from 6 to 5, the hub ratio was decreased and an evolutionary optimization algorithm was used to gain a suitable hydraulic shape of the runner blades and meridional shape. With this solution the planned increase of the annual production by 4 % is possible and the whole Kaplan envelope curve is shifted to higher flow rates. Cavitation performance was given special focus due to given plant sigma values, which normally do not allow for such an extension in the operation range. Extensive numerical investigations were carried out to calculate the risk of cavitation of the turbine configuration. For this purpose, single-phase calculations, analyzing cavitation performance with the help of the statistical histogram method, and multiphase calculations with a cavitation model from Zwart were realized. These calculations were carried out both for the model size of the acceptance test and for the size of the machine engineered. These CFD investigations were then compared with the results of the model test. Primarily the full load operation point was investigated. The design shows, that in CFD as well as in the model test for the larger area of the blade surface no cavitation occurs, only limited runner tip cavitation with little streaks could be detected. For higher flow rates surface cavitation occurs, but these flow rates are outside of the guaranteed operation range of the turbine. At the same time cavitation within the part-load operation range is not in the main focus due to limited operation of only a few hours per year.