Coherent structures decomposition of the flow field in Francis turbine runner under different working conditions

Abstract

The dynamic mode decomposition (DMD) method is adopted to do flow field decomposition in the runner of Francis turbine. The coherent structures corresponding to main frequency components in runner passage under different working conditions are extracted, and the relative error of the reconstructed flow field and the predicted flow field are analyzed. The results show that the DMD method successfully decomposes the position and shape of the coherent structure corresponding to a single frequency in the unsteady flow, and its temporal and spatial evolution. Under part load (PL), best efficiency point (BEP), and high load (HL) working conditions, the coherent structures with high frequency induced by rotor-stator interaction (RSI) are the most stable, which located at the entrance of the runner passage. The low-frequency coherent structures mostly appear inside the runner passage, which is mainly caused by the short blade wake and flow separation under off-design conditions. The averaged flow field contains the most flow energy (larger than 90%), and unsteady coherent structures consume or transport less energy. The flow field in runner passage predicted by the DMD method is consistent with the computational flow field, which relative error is stable at 10−2. The study of coherent structures in runner passage of Francis turbine is beneficial to achieve a better design of runner and more stable operation of power plants.