• Energy Research

When discussing potential resonances in hydraulic turbine runners, cavitation effects are usually neglected. Nevertheless, recent studies have experimentally proved, that large cavitation volumes in the proximity of flexible simple structures, such as hydrofoils, greatly modify their natural frequencies. In this paper, we analyze a resonance case in a Francis runner that leads to multiple fractures on the trailing edge of the blades, after just one day of operation at deep part load. If simple acoustic Fluid-Structure-Interaction (FSI) simulations are used, where the runner’s surrounding fluid is considered as a homogenous acoustic medium (water), the risk of structural resonances seems to be limited as the predicted natural frequencies are far enough from the excited frequencies by the flow. It is shown that the only hydraulic phenomenon which could have produced such fractures in the present case is the Rotor Stator Interaction (RSI). In order to analyze possible cavitation effects on the natural frequencies of the turbine runner, CFD simulations of the deep part load conditions have been performed, which predict large inter-blade vortex cavities. These cavities have been then introduced in the acoustical FSI model showing that under such conditions, natural frequencies of the runner increase approaching to some of the RSI excited frequencies. In particular, a possible resonance of the four-nodal diameter (4ND) mode has been found which would explain the fast behavior of the crack propagation. Furthermore, the shape and the position of the real fracture found agree with the local maximum stress spots at the junction between the trailing edges and the crown.

Abstract The assessment of the remaining useful life due to fatigue in Francis turbine runners implies complex measurements with strain gauges that have to be installed in a submerged and rotating structure, which is excited with high pressure pulsations and strong turbulent flows. Furthermore, the conditioning, storage and transmission of these signals to the stationary frame involves complicated technical solutions. In order to avoid such complex and expensive measurements, in this paper we explore the feasibility of obtaining the strain on the runner with stationary sensors, which can be easily installed and used for a continuous monitoring of the machine. Based on the experimental strain tests performed in a Francis turbine unit, strain on the runner blade is correlated with relevant indicators obtained with stationary sensors. The correlation within indicators is obtained considering linear regression models and improved with artificial intelligence techniques.

The widespread adoption of intermittent wind and solar energy sources disrupts the stability of power grids, necessitating frequent load rejection processes for pump-turbine. This jeopardizes the stability of pump-turbine units. To investigate the operating behavior of pump-turbine units during load rejection, this study focuses on a Francis reversible pump-turbine. Employing the results from a one-dimensional characteristic line method as boundary conditions, three-dimensional numerical simulations are conducted. The entropy production theory was employed to quantify energy loss in different components, including direct dissipation, turbulent dissipation, and wall shear stress. And the pressure load curves of runner blades at different spans are extracted. The flow characteristics, entropy production losses, and pressure loads within the passage are analyzed at various moments during the load rejection process. Results demonstrate that when a unit undergoes load rejection, it repeatedly transitions into and out of the S-characteristic region. The flow state within the passage is most unfavorable under reverse pump conditions, with the entropy production loss reaching its maximum value of 339 160 W/K. When the unit's rotational speed increased beyond 1.1nd, the unit's water thrust and torque underwent drastic changes. Moreover, the pressure load undergoes significant variations under runaway conditions, with the pressure load difference among various blades reaching up to 33 000 kPa. These findings provide a scientific basis for ensuring the safe and reliable operation of pump-turbine units during load rejection processes.

Abstract The natural frequencies of a turbine can be calculated from numerical methods. By comparing these natural frequencies with excitation sources, one can know the danger of a resonance and a possible failure in a component of the turbine. Therefore, it is often very important to have an accurate numerical model of the turbine to determine these natural frequencies. There are not many publications on the determination of the natural frequencies of reduced-scale models of Kaplan turbines. More papers exist for pump turbines or Francis turbines. For real Kaplan turbines, very few experiments can be found to determine mode shapes and natural frequencies. In this paper a Kaplan turbine of 37MW (maximum power), 12.5m (maximum head) and 50 m3/s (maximum flowrate) was tested. The turbine was equipped to determine the natural frequencies of the runner in air. For this purpose, one accelerometer in each blade of the runner was installed and a total of 16 impacts were done in each blade. Frequencies and mode shapes were obtained. In parallel, a numerical model was obtained. Numerical and experimental results were compared and an accurate numerical model is presented. With this numerical model the natural frequencies of the runner in water were calculated.

At present, hydropower plants play an important role because of their flexibility in the generation of electricity. Today, the required operating range of turbine units is expected to extend to deep part load. At part load the draft tube vortex rope is generated and, in some cases, can produce large pressure pulsations limiting the operation of the turbine. The draft tube vortex rope has been much studied. In this paper, the effects of the part load behavior on the hydraulic system in existing power plants has been studied. Different hydropower units with Francis turbines has been selected for this purpose. All of them present strong vibrations in penstocks and other machine components at part load. Vibrations, pressure fluctuations and other parameters were measured in different positions of the turbine, the draft tube and the penstock. Signals were acquired during transients and for several operating loads. The signals in time and frequency domain and other signal treatments for these cases are shown Peer Reviewed Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant

Experimental tests in prototype are necessary to understand the dynamic behaviour of the machine during different operating points. Hydraulic phenomena as well as its effect on the structure need to be studied in o rder to avoid instabilities during operation and to extend the life - time of the different components. For this purpose, a complete experimental study of a large Francis turbine prototype has been performed installing several sensors along the machine. Pres sure sensors were installed in the penstock, spiral case, runner and draft tube, strain gauges were installed in the runner, vibration sensors were used in the stationary parts and different electrical and operational parameters were also measured. All the se signals were acquired simultaneously for different operating points of the turbine.

Due to the massive entrance of new renewable energies such as wind or solar, hydraulic turbines have to work far from its designed point and withstanding multiple transients, such as starts and stops, that shorten the useful life of the machine and cause fatigue damages. The present paper reviews the complex problem of fatigue in Francis turbines particularly focused on the experimental data available for static and dynamic stresses. For this purpose, many researches, which include different Francis turbines covering a wide range of design head and power, have been considered. The experimental stresses characteristics measured with strain gauges installed on the turbine runner and obtained from previous works have been analyzed for the different operating conditions and transient states occurring in the normal life of actual Francis units. The actual computational capabilities and techniques typically used to estimate such stresses have been discussed in detail. Potential future techniques to simplify complex strain measurements on the turbine runner, computational and statistical methods to estimate turbine stresses are reviewed in this paper. Finally, the relative damage of the different operating conditions and useful life estimation of the turbine, based on past strain measurements of the runner, are addressed.