Abstract
The flow in a Francis turbine at full load is characterised by the development of an axial vortex rope in the draft tube. The vortex rope often promotes cavitation if the turbine is operated at a sufficiently low Thoma number. Furthermore, the vortex rope can evolve from a stable to an unstable behaviour. For CFD, such a flow is a challenge since it requires solving an unsteady cavitating flow including rotor/stator interfaces. Usually, the numerical investigations focus on the cavitation model or the turbulence model. In the present works, attention is paid to the strategy used for the time integration. The vortex rope considered is an unstable cavitating one that develops downstream the runner. The vortex rope shows a periodic behaviour characterized by the development of the vortex rope followed by a strong collapse leading to the shedding of bubbles from the runner area. Three unsteady RANS simulations are performed using the ANSYS CFX 17.2 software. The turbulence and cavitation models are, respectively, the SST and Zwart models. Regarding the time integration, a second order backward scheme is used excepted for the transport equation for the liquid volume fraction, for which a first order backward scheme is used. The simulations differ by the time step and the number of internal loops per time step. One simulation is carried out with a time step equal to one degree of revolution per time step and five internal loops. A second simulation used the same time step but 15 internal loops. The third simulations used three internal loops and an adaptive time step computed based on a maximum CFL lower than 2. The results show an influence of the time integration strategy on the cavitation volume time history both in the runner and in the draft tube with a risk of divergence of the solution if a standard set up is used.
Highlights
The rapidly growing share of new renewable energy sources in the past few decades leads to new challenges for the electrical network stability
It has to be mentioned that the simulation Case 1 diverges when the vortex rope in the draft tube collapses
A standard set up Case 1 usually used for the URANS computation of a Francis turbine is not able to capture the flow due to the crash of the simulation after the first collapse of the cavitating vortex rope
Summary
The rapidly growing share of new renewable energy sources in the past few decades leads to new challenges for the electrical network stability. Due to their rapid response time, hydraulic power plants are often used to compensate load fluctuations in the grid. The flow leaving the runner possesses a significant residual swirl, leading to an inhomogeneous pressure distribution in the draft tube and eventually to the inception of cavitation. For Francis turbines, running at an operating point for which the flow rate is above the nominal one leads to the development of an axial vortex rope. If the pressure level inside in the vortex is sufficiently low, cavitation occurs.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
