Abstract
For Francis turbines at part load operation a helical vortex rope is formed due to the swirling nature of the flow exiting the runner. This vortex creates pressure fluctuations which can lead to power swings, and the unsteady loading can lead to fatigue damage of the runner. In the case that the vortex rope cavitates there is the additional risk that hydro-acoustic resonance can occur. It is therefore important to be able to accurately simulate this phenomenon to address these issues. In this paper an unsteady, multi-phase CFD model was used to simulate two part-load operating points, for two different cavitation conditions. The simulation results were validated with test-rig data, and showed very good agreement. These results also served as an input for FEA calculations and fatigue analysis, which are presented in a separate study.
Highlights
The stochastic nature of production of new renewable energy technologies such as wind and photovoltaic has created the need for hydro power plants to be operated in a more flexible way to stabilise the grid
An excellent agreement is observed between the CFD and experimental results, which confirms that the correct operating point was being modelled with a reasonable level of accuracy
Virtual pressure sensors were placed in the CFD model, in the same positions as on the scale model used for experimental measurements on the test rig
Summary
The stochastic nature of production of new renewable energy technologies such as wind and photovoltaic has created the need for hydro power plants to be operated in a more flexible way to stabilise the grid. At part load operation – typically between 50 % and 85 % of the optimal power of the turbine, the flow exiting the runner and, entering the draft tube has an increasing tangential component This swirling flow creates instability in the flow field, and a helical vortex rope is formed in the draft tube cone. This vortex rope induces pressure fluctuations that can cause power swings and increase fatigue loading on turbine components due to dynamic stresses. To study the part load vortex rope two operating points were identified These have been named PL1 (corresponding to 80 % optimum discharge) and PL2 (corresponding to 64 % optimum discharge) respectively. Both were at rated head, and two different cavitation (sigma) numbers were chosen to study the influence of cavitation, as well as the accuracy of the CFD
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