Cavitation prediction in a Kaplan turbine using standard and optimized model parameters

The paper presents efficiency and cavitation prediction in a 6-blade Kaplan turbine. The study is a result of a collaboration between the University of Trieste (Italy) and Kolektor Turboinštitut (Slovenia), which recently joined in the ACCUSIM EU project with the aim to develop reliable, high fidelity methods for accurate predictions and optimization of the performances of hydro-machinery and marine propellers. Numerical simulations were done at one operating point for maximal runner blade angle and nominal head. Steady state results obtained with the SST (Shear Stress Transport) turbulence model were improved by transient simulations, where the SAS (Scale Adaptive Simulation) SST model was used. Cavitating flow was simulated using the homogeneous model. Mass transfer rate due to cavitation was regulated by the Zwart et al. model with default model constants used in ANSYS CFX commercial code and also with the evaporation and condensation parameters previously calibrated considering the sheet cavity flow around a hydrofoil. For a Kaplan turbine the numerical results were compared with the observation of cavity size on the test rig and with the measured sigma break curve. Steady state simulations predicted a significant too small efficiency level and too small extent of cavitation on the runner blades. With transient simulations, the shape and size of the predicted sheet cavitation agreed well with the cavitation observed on the test rig. In addition, also the predicted efficiency was more accurate, although the value of σ (cavitation or Thoma number) where the efficiency dropped for 1% was a bit too large. The difference between the results obtained with standard and calibrated model parameters of the Zwart mass transfer model was small.