Simulation of a shut-down transient in the Francis-99 turbine model

Abstract

Due to the new intermittent electric energy sources, hydropower is forced to run more and more at off-design conditions and to regulate the operating conditions. This causes flow instabilities with pressure fluctuations, and load variations that may deteriorate the machine. Previous numerical research on the flow in water turbines has mainly focused on steady operating conditions at BEP and off-design. More knowledge is needed on transients between operating conditions, and start-up and shut-down procedures. This requires dynamic meshes that both rotate the runner, using a coupling interface to the non-rotating part of the mesh, and morph the guide vane mesh due to the change in guide vane angle during the transient. Special care needs to be taken to apply boundary conditions that resemble the experimental condition to be used for validation. The present work addresses a shut-down transient of the Francis-99 turbine (https://www.ntnu.edu/nvks/francis-99), using OpenFOAM-2.3.x. Turbulence is modelled by LES based on the dynamic one-equation eddy viscosity sub-grid model and the cube root volume filter width, although the temporal and spatial resolution requirements have not been assessed in these preliminary results. The inlet and outlet boundary conditions are set using pressure conditions. The outlet static pressure is taken from the experiment at BEP. The inlet total pressure is adjusted to yield the same flow rate as in the experiment at BEP. The flow rate is given by the solution. The runner region of the mesh rotates with a solid-body rotation, while the guide vane region of the mesh is morphing due to the continuous change in guide vane angle. Rotating and stationary parts of the mesh are coupled with an arbitrary mesh interface (AMI). The results show that the methodology can be used to capture the experimentally observed flow features during transients.