Sensitivity Analysis of Zwart-Gerber-Belamri Model Parameters on the Numerical Simulation of Francis Runner Cavitation

A series of numerical simulations of the 3D cavitation development in the GAMM Francis turbine runner were carried out with the Zwart-Gerber-Belamri (ZGB) two-phase model available in ANSYS® CFX v16.2. The aim of the investigation was to evaluate the influence of the ZGB parameters on the location, size and shape of the simulated cavitation. To begin, a sector of the entire fluid domain comprising a single blade was created taking profit from the rotational symmetry of the geometry and a stage simulation was set to simulate the flow field without cavitation. A grid independence analysis was carried out using the shear-stress transport (SST) turbulence model. Results such as the pressure drop and the torque at the best efficiency point (BEP) compared reasonably well with the experimental values, thus proving the model validity. The cavitation inception and development was simulated at BEP using the ZGB default parameters by progressively decreasing the sigma value. As expected, the onset of cavitation took place at the blade suction side close to the junction with the band, and the length and area of the blade cavity increased for decreasing sigma values. The cavity shape also showed a good agreement with the observations at the GAMM turbine model. Finally, the values corresponding to the mean nucleation site diameter, the nucleation site volume fraction and the empirical coefficients of evaporation and condensation were modified individually while keeping the rest constant in order to observe the changes in cavitation behavior. As a result, it was concluded that all the parameters have a significant effect in the cavity length, the pressure distribution and the torque. Moreover, extreme values leading to unrealistic results were also found.