Fluid-structure interaction mechanisms leading to dangerous power swings in Francis turbines at full load

Hydropower plants play an important regulatory role in the large scale integration of volatile renewable energy sources into the existing power grid. This duty however requires a continuous extension of their operating range, provoking the emergence of complex flow patterns featuring cavitation inside the turbine runner and the draft tube. When the power output is maximized at full load, self-excited pressure oscillations in the hydraulic system may occur, which translate into significant electrical power swings and thus pose a serious threat to the grid stability as well as to the operational safety of the machine. Today's understanding of the underlying fluid structure interaction mechanisms is incomplete, yet crucial to the development of reliable numerical flow models for stability analysis, and for the design of potential countermeasures. This study therefore reveals how the unsteady flow inside the machine forces periodic mechanical loads onto the runner shaft. For this purpose, the two-phase flow field at the runner exit is investigated by Laser Doppler Velocimetry and high-speed visualizations, which are then compared to the simultaneously measured wall pressure oscillations in the draft tube cone and the mechanical torque on the runner shaft. The results are presented in the form of a comprehensive, mean phase averaged evolution of the relevant hydro-mechanical data over one period of the instability. They show that the flow in the runner, and thus the resulting torque applied to the shaft, is critically altered by a cyclic growth, shedding and complete collapse of cavitation on the suction side of the runner blades. This is accompanied by a significant flow swirl variation in the draft tube cone, governing the characteristic breathing motion of the cavitation vortex rope.

Fluid–structure interaction mechanisms leading to dangerous power swings in Francis turbines at full load

François Avellan, Arthur Tristan Favrel, Christian Landry, Andres Müller

Hydropower plants play an important regulatory role in the large scale integration of volatile renewable energy sources into the existing power grid. This duty however requires a continuous extension of their operating range, provoking the emergence of complex flow patterns featuring cavitation inside the turbine runner and the draft tube. When the power output is maximized at full load, self-excited pressure oscillations in the hydraulic system may occur, which translate into significant electrical power swings and thus pose a serious threat to the grid stability as well as to the operational safety of the machine. Today's understanding of the underlying fluid–structure interaction mechanisms is incomplete, yet crucial to the development of reliable numerical flow models for stability analysis, and for the design of potential countermeasures. This study therefore reveals how the unsteady flow inside the machine forces periodic mechanical loads onto the runner shaft. For this purpose, the two-phase flow field at the runner exit is investigated by Laser Doppler Velocimetry and high-speed visualizations, which are then compared to the simultaneously measured wall pressure oscillations in the draft tube cone and the mechanical torque on the runner shaft. The results are presented in the form of a comprehensive, mean phase averaged evolution of the relevant hydro-mechanical data over one period of the instability. They show that the flow in the runner, and thus the resulting torque applied to the shaft, is critically altered by a cyclic growth, shedding and complete collapse of cavitation on the suction side of the runner blades. This is accompanied by a significant flow swirl variation in the draft tube cone, governing the characteristic breathing motion of the cavitation vortex rope.

Physical Mechanisms governing Self-Excited Pressure Oscillations in Francis Turbines

Andres Müller

The importance of renewable energy sources for the electrical power supply has grown rapidly in the past decades. Their often unpredictable nature however poses a threat to the stability of the existing electric grid. Hydroelectric powerplants play an important role in regulating the integration of renewable energy sources into the network by supplying on-demand load balancing as well as primary and secondary power network control. Therefore, the operating ranges of hydraulic machines has to be continuously extended, which potentially produces undesirable flow phenomena involving cavitation. An example is the formation of a gaseous volume in the swirling flow leaving a Francis turbine runner at off-design operating conditions. At high load, this so called vortex rope is shaped axisymmetrically and may enter a self-excited oscillation, measurable through significant fluctuations of the pressure throughout the system and the mechanical torque transferred to the generator. The main objective of the present work is the identification of the physical mechanisms governing this self-sustained, unstable behavior by measurement. Furthermore, the key parameters of numerical approaches using one-dimensional hydroacoustic flowmodels or CFD require experimental validation. For this purpose, the measurements provide a comprehensive data base of various flow and system parameters at varying operating conditions. Two test cases are studied, a small scale hydraulic circuit with a micro-turbine as well as a reduced scale physical model of an existing Francis turbine. On the first test case, the study of the flow rate fluctuations up- and downstream of the oscillating vortex rope in the draft tube, together with the volume of the cavity, revealed the destabilizing effect of the flow swirl in the draft tube inlet. The second test case accurately simulates the behavior of an actual hydraulic power plant. Investigations range from a local study of the flow field in the draft tube cone bymeans of LDV, PIV, high speed visualization and wall pressure measurements to a global analysis, considering the response of the hydraulic and mechanical system to the excitation by the vortex rope oscillation. Among the main observations is a periodical variation of the flow swirl in the draft tube, synchronized with the pressure oscillations. This is likely to be caused by a cyclically appearing volume of cavitation on the runner blades, modifying the relative flow angle at the outlet. The interaction of the blade cavitation and the vortex rope oscillation via the flow swirl is found to play a crucial role in the occurrence of self-excited pressure oscillations in Francis turbines.

EPFL2014

Dynamics of the cavitation precessing vortex rope for Francis turbines at part load operating conditions

Arthur Tristan Favrel

The massive penetration of the existing electrical grid by renewable energy sources requires a continuous extension of the operating range of hydroelectric powerplants, which can lead to cavitation flow instabilities inducing undesirable mechanical vibrations and large fluctuations of pressure and output power, putting at risk the structural integrity of the machine and ultimately the grid stability. A typical example is the development of a cavitation precessing vortex rope at the outlet of a Francis turbine runner operating at part load conditions. It acts as an excitation source for the hydraulic system, leading to the propagation of pressure fluctuations in the hydraulic circuit which are greatly amplified in case of resonance. Therefore, the assessment of the stability of hydropower plants operating at part load is crucial in order to ensure the safe extension of their operating range. The accurate prediction and transposition of pressure fluctuations from the model scale to the prototype scale by means of one-dimensional hydro-acoustic models represents a major challenge, as the physical mechanisms driving the excitation source and its interaction with the hydraulic system remain partially unclear. The main objective of this research work is to experimentally investigate the influence of the operating conditions on the dynamics of the cavitation precessing vortex rope and the excitation source it induces, as well as the interaction with the system. The test-case is a reduced scale physical model of a Francis turbine, accurately reproducing the behaviour of a real size machine. Experimental investigations include study of the flow field in the draft tube cone by means of Particle Image Velocimetry (PIV) and Laser Doppler Velocimetry (LDV), high-speed visualizations of the cavitation vortex and pressure measurements performed at various part load operating conditions, including cavitation-free and cavitation conditions. PIV measurements of the tangential flow field in the draft tube cone in cavitation-free conditions highlight the influence of the flow discharge on the vortex characteristics in terms of trajectory, circulation and structure, as well as the link between the vortex dynamics and the intensity of the excitation source. The effect of cavitation on the vortex rope dynamics and its interaction with the surrounding system is also studied, with a particular focus on resonance conditions. Flow velocity measurements performed for different values of the Thoma number reveal that the axial and tangential velocity fields in the draft tube cone are not impacted by the propagation of large synchronous pressure fluctuations. Among the other observations is a phenomenon of frequency lock-in between the excitation source and the system's oscillations occurring for low values of the Froude number. This phenomenon is assumed to be a consequence of the non-linear coupling taking place in resonance conditions between the excitation source and the oscillation of the cavitation volume. Finally, it is shown that the convective component of the pressure fluctuations at the precession frequency represents the main source of mechanical excitation for the runner.