Unsteady flow and cavitation during off-design and transients in water turbines

Forskningsprojekt, 2021 – 2022

Due to the new intermittent electric energy sources, hydropower is forced to run more and more at offdesign conditions and to continuously vary the operating conditions in order to regulate the electric grid.
This includes off-design operation, start/stop, speed no load, load rejection, etc. Such operation causes flow instabilities with pressure fluctuations, load variations, and cavitation, that may deteriorate the machine and reduce the efficiency. It is therefore of highest priority in the hydropower industry to learn more about the flow and cavitation during transients, so that the origins of the problems are identified and can be avoided while operating the power plants. The same knowledge is also of very high importance when refurbishing hydropower plants that were originally designed for base load production, for efficient and safe use in the present and future more flexible electric energy system. Such refurbishments typically involve model tests. Those have historically been done under steady operating conditions, but now it is also important to study transient operation. Vattenfall is therefore re-designing their lab rig for model
tests under transient operation, and plan to do extensive experimental tests the coming years. The proposed numerical studies will be validated with the experimental results and used to further extend the understanding of the flow and cavitation in the system. For such studies to be relevant, the expected operating procedures at prototype scale must be down-scaled to the model scale, and the model scale data must be up-scaled to the prototype scale. This scaling, in terms of fluctuation amplitudes and timescales, is yet not fully understood and will be addressed during the proposed project. Vattenfall intend to repeat experiments both in model scale and prototype scale, and the same can be done numerically to further extend the gained understanding. Many uncertainties will arise in this procedure, for which an uncertainty quantification and sensitivity analysis will be made. As part of the present project, we also intend to extend the already planned experimental activities with cavitation observations for validation of
the corresponding numerical results.