Time-dependent inception of vortex rings in a Francis turbine during load variation: large eddy simulation and experimental validation

Abstract

The present study investigates a transient condition, i.e. load variation from the best efficiency point to the part load, of a Francis turbine. A complete turbine is considered for the study. The dynamic mesh approach is implemented to close the guide vanes and to perform load variation numerically. The primary goal is to investigate the inception, propagation and interaction of vortex rings as load on the turbine changes. The secondary goal is to determine characteristic frequencies. The continuous change in flow angle, due to the closing of the guide vanes, induces strong boundary layer separation in the runner. Flow decelerates along the blade and establishes an adverse pressure gradient due to steep curvature of the blades. Separated flow from the boundary layer creates two swirling regions attached to the blade suction side. Fully developed vortex rings exiting from the adjacent blade channels interact and induce high-amplitude stochastic fluctuations at the runner outlet.

Keywords:

• Hydraulics of renewable energy systems
• large eddy simulations
• rotating and swirling flows
• rotational flows
• vortex interactions

Acknowledgements

The author acknowledges Einar Agnalt (PhD student) and the laboratory staff for the work with the measurements used in this study.

ORCID

Chirag Trivedi http://orcid.org/0000-0002-2198-8981

Supplemental data

Supplemental data for this article can be accessed https://doi.org/10.1080/00221686.2019.1671514.

Notes

1 An angular position (ϕ) of 0° indicates that the reference stay vane, guide vane and blade are in-line. Flow field in a turbine is dependent on the relative position of the guide vane/blade; hence, the reference/fixed position helps to compare the flow condition at different time instants but at the same spatial location of the runner.

Additional information

Funding

The computational resources to perform the simulations were used under the Notur/Norstore project – Numerical investigations of a Francis turbine [project NN9504K]. The partial measurements were conducted under the HiFrancis research project – High Head Francis Turbine [grant 254987]. Norges Forskningsråd.