Flow in turbines during new operating procedures

Forskningsprojekt, 2019 – 2021

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. We have previously assessed state-of-the-art unsteady turbulence modeling (URANS, PANS, hybrid models and LES) for the use in hydro power applications under steady operating conditions. We have shown that it is possible to accurately predict the
unsteady flow features under such conditions. The next step is to include the transients when continuously regulating the electric grid, when changing from one operating condition to another, during start-up/shut-down, or at a sudden load rejection. Another situation is when the turbines are running at speed no-load, ready to connect to the electric grid at very short notice. Numerical simulations of such situations require further research on turbulence modeling, wall functions, and coupling to the entire hydraulic system. In most cases the transients involve changes in blade angles that must be taken into account. That requires developments in mesh generation, mesh morphing, special treatment of small gaps, mapping of
intermediate results, etc.
The numerical results will help gaining new knowledge on the flow physics during unsteady flow and transients in hydraulic turbines. The unsteady forces on the machine may be predicted and their origin may be understood. Such knowledge is beneficial for an expanded knowledge of the rotor dynamics of the machines. Eventually, a coupling between rotor dynamic models and efficient unsteady flow modeling may be accomplished.