Two-phase flow phenomena in hydraulic turbines and pump-turbines operating in synchronous condenser mode

Hydraulic turbines and pump-turbines are often required to operate in particular conditions at which they consume active power. To keep the power consumption at a minimum, it is necessary to dewater the runner to reduce the mechanical torque. One of these operating conditions is the synchronous condenser mode for the reactive power regulation of the grid. This operating mode requires the guide vanes closure and the lowering of the tail water below the runner by injecting pressurized air in the machine. Due to the air-water mixing, torque instabilities and air losses are observed in this operating mode causing the increase of the power consumption. In this thesis, the two-phase flow phenomena characterizing the operation in synchronous condenser mode are experimentally investigated; additionally, a theoretical framework is proposed to analytically describe and predict the observed flow phenomena and the corresponding power and air losses. These research challenges are addressed by means of four different contributions. First, the development of the sloshing motion of the air-water free-surface below the runner is qualitatively and quantitatively described by using high-speed visualization, pressure fluctuations measurements and automated image processing methods. Second, the development of an air-water ring in the vaneless gap between the runner and the guide vanes is studied by means of image processing and pressure fluctuations measurements to assess both the velocity and the pressure flow field. The pressure measurements in the vaneless gap make apparent a fluctuation due to the interaction of the rotating and stationary components and of the rotating two-phase flow. This interaction is revealed by the development of the Fourier series applied to the harmonic components of the system and the modal decomposition of the pressure signal. Third, the influence of the two-phase flow phenomena on the torque swings is investigated. Measurements of the resisting mechanical torque transmitted through the coupling of the runner and the shaft and corresponding to the absorbed mechanical power of the runner in the hydraulic turbine are performed; in addition, a study on the influencing operating parameters allows building an empirical predictive model of the torque. Finally, a study on the oxygen mass transfer in the water volume is performed by measuring the oxygen concentration in the spiral case and in the draft tube of the hydraulic turbine. A theoretical framework is developed on the equations governing the diffusion of oxygen in water and the diffusion coefficients are computed and validated with experimental results. Moreover, this study allows quantifying the air losses due to the diffusion of air in water and to model the diffusion process as a function of the operating parameters of the machine. The results allow the understanding of the two-phase flow mechanism in a turbine or pump-turbine operating in synchronous condenser mode causing instabilities and air losses. Furthermore, the predictive model of the power losses due to both torque and air consumption represents a novel achievement for the study of the two-phase flow in a turbine operating in condenser mode or, more in general, in dewatered condition, and for the control of the machine behavior and consumption on the full scale prototype.

On the sloshing free surface in the draft tube cone of a Francis turbine operating in synchronous condenser mode

Loïc Andolfatto, François Avellan, Elena Vagnoni

Hydropower plants may be required to operate in synchronous condenser mode in order to supply reactive power to the grid for compensating the fluctuations introduced by the intermittent renewable energies such wind and solar. When operating in this mode, the tail water in the Francis turbine or pump-turbine is depressed below the runner by injecting pressurized air in order to spin in air to reduce the power consumption. Many air-water interaction phenomena occur in the machine causing air losses and a consequent power consumption to recover the air lost. In this paper, the experimental investigation of the sloshing motion in the cone of a dewatered Francis turbine performed by image visualization and pressure measurements is presented. The developed image post processing method for identifying the amplitude and frequency of the oscillation of the free surface is described and the results obtained are illustrated and discussed.

2017

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

Experimental investigation of the sloshing motion of the water free surface in the draft tube of a Francis turbine operating in synchronous condenser mode

Loïc Andolfatto, François Avellan, Arthur Tristan Favrel, Elena Vagnoni

Hydropower units may be required to operate in condenser mode to supply reactive power. In this operating mode, the water level in the turbine or pump-turbine is decreased below the runner by closing the guide vanes and injecting pressurized air. While operating in condenser mode the machine experiences power losses due to several air–water interaction phenomena which cause air losses. One of such phenomena is the sloshing motion of the water free surface below the runner in the draft tube cone of a Francis turbine. The objective of the present work is to experimentally investigate the sloshing motion of the water free surface in the draft tube cone of a reduced scale physical model of a Francis turbine operating in condenser mode. Images acquisition and simultaneous pressure fluctuation measurements are performed and an image processing method is developed to investigate amplitude and frequency of the sloshing motion of the free surface. It is found that this motion is excited at the natural frequency of the water volume and corresponds to the azimuthal wavenumber m=1 of a rotating gravity wave. The amplitude of the motion is perturbed by wave breaking and it decreases by increasing the densimetric Froude number. The sloshing frequency slightly increases with respect to the natural frequency of the water volume by increasing the densimetric Froude number. Moreover, it results that this resonant phenomenon is not related to the torque perturbation.

2018

Physical Mechanisms governing Self-Excited Pressure Oscillations in Francis Turbines

Andres Müller

EPFL2014