Some Computational Issues on Fluid Dynamics in Large-Scale Hydraulic Turbines

The large Reynolds number, flow unsteadiness and complicated geometry give rise to complex flow phenomena in large scale hydraulic turbines and make such a field challenging for the numerical and modeling communities [1]. The verification of codes, validation of the turbulence models and quality of the simulations are essential [2]. The goal of the paper is to asses the turbulent model, grid density and computation code in complex flows in hydraulic turbines. A large-scale Francis turbine at the Three-Gorges Hydropower Plant in China was studied by using various numerical schemes. By comparing with the experimental results [3], it is found that the single fluid passage model gives more accurate energy characteristics than the entire fluid passage model. The RANS based method, Reynolds Stress Model (RSM), described the swirling flow more effectively than the Large Eddy Simulation approach. The flow structure in the draft-tube was represented by using RSM, as shown in Figure 1. The predicted frequency of the vortex rope rotation in the draft-tube was consistent with those of pressure fluctuation measured. On the other hand, the one fluid model with density change was employed to simulate the cavitation flow in the draft-tube. It is found that this model is more accurate at great flow rate condition than at low flow rate condition. Figure 1 Predicted vortex rope in draft-tube and its pressure distribution on a section