Capability Assessment of URANS Turbulence Models for Simulation of Vortex Rope-Related Flow Defect in a Model Francis Turbine

Off-design operation of hydraulic turbines is necessitated for the prevention of load imbalance in the electric power system. In particular, the part load operation of the turbine introduces a flow anomaly in the draft tube. The anomalous flow, a consequence of the vortex breakdown phenomenon, is comprised of a centrally located helical vortex structure that self-rotates as well as precesses about the draft tube central axis. This structure known as the rotating vortex rope, wrapped by a normal flow toward the draft tube periphery, is causative of low-frequency pressure fluctuation at a large scale. Disadvantage accompanies as loss of draft tube pressure recovery, reduction in turbine efficiency, and possible mechanical vibrations in event of resonance between frequencies of system and rotating vortex rope. This work assesses in detail the applicability of URANS-based standard k-ε and SST k-ω turbulence models for the characterization of a part load vortex rope. Complete turbine transient simulations at PL, based on Francis-99 test case, are performed using the commercially available ANSYS CFX software. Obtained axial and radial velocity fields are compared with results from the experiment. A detailed spectral analysis is carried out using Fast Fourier Transforms to identify the dominant frequencies of pressure fluctuation in numerical and experimental data. Moreover, the pressure signal is decomposed into synchronous and asynchronous modes, the distribution of frequency within which is analyzed through Fast Fourier Transforms. A mismatch between experiment and simulation is identified and quantified to evaluate the utility of URANS models for investigations on RVR.