Verification of large eddy simulation (LES) applied in Francis hydro-turbine under partial flow conditions

To precisely predict the internal flow characteristics and external features of Francis hydro-turbine when running off the optimal flow conditions, numerical computation based on large eddy simulation (LES) method was performed. For LES method, the results from both single-phase model and cavitation model were compared. It is revealed that LES is capable of providing better depiction for inner flow field of Francis hydro-turbine, and also, at partial flow conditions, LES method accompanying with cavitation model can capture more accurate structures for channel vortices and draft tube vortices. Moreover, LES method shows advantages for predicting the external characteristics, including the trends of flow rate and working efficiency, which are verified by the experimental results. Based on the transient simulation on the internal flow, the cavitating vortices in the runner and draft tube are captured by using Q method, and their evolutions with certain frequency are re-generated. For the condition of a=10 mm and n₁₁=64.9 r/min, the frequencies of channel vortex and vortex rope are 15 times and 0.4 times of rotating frequency, respectively. The results are in agreement with the experimental ones, thus the LES method is proved to be accurate for the complicated three-dimensional simulation.