Influence of the boundary conditions on the dynamic behavior of large hydraulic machines

Resumen

Nowadays, hydropower plays an essential role in the energy market. With the massive entrance of new renewable sources such as wind or solar power, hydropower is the only renewable generating source that can provide fast response and regulation capacity to the electric grid. It can even store the surplus of energy when it is necessary using Reversible Pump-Turbine (RPT) power plants. However, this situation makes that hydraulic turbines are increasingly working at off-design conditions with a high number of start and stops in comparison with ten years ago. At these conditions, the forces and stresses over the structure are high, especially in the runner, documenting some failures along the time. Therefore, it is of paramount importance to study the dynamic behavior of the runner under operation in order to avoid resonance conditions and fatigue problems. To study the dynamic behavior of the runner, both excitation and response have to be determined. Excitation forces have been studied for many years and they can be predicted with good accuracy through computational methods. However, the dynamic response of the runner still needs to be studied in detail. To define this dynamic response, natural frequencies, damping ratios and mode-shapes of the runner have to be estimated under operating conditions and for the different boundary conditions found in a hydraulic turbine. In this thesis, the natural frequencies, damping ratios and mode-shapes of submerged structures under different boundary conditions are studied. As a hydraulic turbine runner is a complex structure where the boundary conditions are fixed, simplified models are used to study the influence of those boundary conditions on their dynamic response. Submerged and confined disks have been used to experimentally study the effects of axial and radial gaps to rigid walls, the effects of rotation and the effects of the acoustic modes of the surrounding fluid on their dynamic response. Moreover, experimental measurements in a large Pump-Turbine and a large Francis turbine prototype have been performed to confirm the knowledge acquired in the simplified models. Numerical models have been also developed and validated in the present work to study the dynamic response of hydraulic turbine runners. This is an Article-Based Thesis, so it is based on three Journal Papers that have been published during the thesis duration. These three Journal Papers are based on the simplified models research, and they are attached and commented though the whole document of this thesis. Moreover, a summary of the findings of the research on hydraulic turbine prototypes is also included to extend the application of the knowledge acquired with the simplified models to actual hydraulic turbine prototypes.