The HYPERBOLE FP7 European project, which means HYdropower plants PERformance and flexiBle Operation towards Lean integration of new renewable Energies, aims at improving the flexibility of the hydropower plants.
The main reason that impulses this project, is the increase in renewable energies among the sources of electric production in Europe. Such sources of energy are stochastic by nature and can lead to electrical grid instabilities. Hydropower plants can be used to stabilize the electricity grid due to their capacity to be quickly started.
Nevertheless, the use of the hydropower plants to stabilize the electricity grid, requires operating the power units outside of their best efficiency point. In this case, the efficiency drops drastically. Therefore, the project focuses on the electrical, hydraulic and mechanical developments that can enlarge the operating area with a good efficiency.
The HES-SO, through the hydraulic laboratory, is in charge of a sub-task, which concerns the study of the flow developing in the draft tube of a Francis turbine at full load (beyond the best efficiency point). In this case, a vortex rope at the outlet of the runner appears. This structure enhanced the formation of cavitation. The presence of cavitation is a strong drawback since it induces instabilities (hydro-acoustic waves) that can lead to severe damages (for example bursting of mechanical elements).
Numerical simulations of the flow, using the Ansys CFX software, will be performed in a simplified draft tube including cavitation. The goal is to provide an identification of the hydro-acoustic parameters which will be then implemented in a one-dimensional solver to carry out stability analysis of the entire hydraulic system.
Experimental visualisation of a vortex rope at full load.
Courtesy S. Alligné PhD. EPFL n° 5117 (2011)