by Andrej Lipej, Turboinstitut, Republic of Slovenia
For the past 50 years, Turboinstitut has been involved in the research and development of hydraulic machines. CFD has become an indispensable tool in our activities, and this is especially true for our turbine refurbishment projects, which are usually realized entirely with CFD. Using CFX-TASCflow, we can analyze the flow throughout the whole turbine, taking into account the stator-rotor-draft tube interaction and avoiding potentially inaccurate boundary conditions between components. However, compared to individual simulations for each part, such calculations require large models and long run times. Separate analysis is much faster, but it is essential to know when coupled analysis must be performed and what are the differences between separate and coupled analyses. At the Institute, by comparing results for coupled and separated calculations, we have found that the separate analysis usually overpredicts flow energy losses in all turbine parts and only coupled analysis is suitable for accurate prediction of turbine efficiency.