Large Eddy Simulation of the Rotating Stall in a Pump-Turbine Operated in Pumping Mode at a Part-Load Condition

The investigation of the rotating stall phenomenon appearing in the HYDRODYNA pump-turbine reduced scale model is carried out by performing a large-scale large eddy simulation (LES) computation using a mesh featuring approximately 85 x 10(6) elements. The internal flow is computed for the pump-turbine operated at 76% of the best efficiency point (BEP) in pumping mode, for which previous experimental research evidenced four rotating stall cells. To achieve an adequate resolution near the wall, the Reynolds number is decreased by a factor of 25 than that of the experiment, by assuming that the flow of our interest is not strongly affected by the Reynolds number. The computations are performed on the supercomputer PRIMEHPC FX10 of the University of Tokyo using the overset finite-element open source code FrontFlow/blue with the dynamic Smagorinsky turbulence model. It is shown that the rotating stall phenomenon is accurately simulated using the LES approach. The results show an excellent agreement with available experimental data from the reduced scale model tested at the EPFL Laboratory for hydraulic machines. The number of stall cells as well as the propagation speed agree well with the experiment. Detailed investigations on the computed flow fields have clarified the propagation mechanism of the stall cells.

High-resolution LES of the rotating stall in a reduced scale model pump-turbine

François Avellan, Chisachi Kato, Olivier Pacot

Extending the operating range of modern pump-turbines becomes increasingly important in the course of the integration of renewable energy sources in the existing power grid. However, at partial load condition in pumping mode, the occurrence of rotating stall is critical to the operational safety of the machine and on the grid stability. The understanding of the mechanisms behind this flow phenomenon yet remains vague and incomplete. Past numerical simulations using a RANS approach often led to inconclusive results concerning the physical background. For the first time, the rotating stall is investigated by performing a large scale LES calculation on the HYDRODYNA pump-turbine scale model featuring approximately 100 million elements. The computations were performed on the PRIMEHPC FX10 of the University of Tokyo using the overset Finite Element open source code FrontFlow/blue with the dynamic Smagorinsky turbulence model and the no-slip wall condition. The internal flow computed is the one when operating the pump-turbine at 76% of the best efficiency point in pumping mode, as previous experimental research showed the presence of four rotating cells. The rotating stall phenomenon is accurately reproduced for a reduced Reynolds number using the LES approach with acceptable computing resources. The results show an excellent agreement with available experimental data from the reduced scale model testing at the EPFL Laboratory for Hydraulic Machines. The number of stall cells as well as the propagation speed corroborates the experiment.

Iop Publishing Ltd2014

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.

EPFL2016

Use of compact Laser Doppler Velocimetry in reduced scale model testing of hydraulic machines

Loïc Andolfatto, François Avellan, Vincent Berruex, Andres Müller, Masashi Sakamoto

Laser based flow survey methods such as Laser Doppler Velocimetry (LDV) or Particle Image Velocimetry (PIV) have long been used for studying the flow in hydraulic machines. Even if the results have crucially improved our understanding of potentially destabilizing flow patterns in the draft tube at off-design conditions and the prediction of the stable operating range, these techniques are not systematically used for reduced scale model measurements in the frame of customer acceptance tests or development projects. In a not too distant past, the price, size and complexity of the necessary equipment and procedures exceeded the acceptable range for these projects. Recent developments however made them more accessible. Furthermore, the collaboration with experienced research institutes provides the necessary know-how as well as an added academic value by providing researchers with realistic test cases. This work reports a case study for which a compact LDV probe was used to measure the velocity field in the draft tube cone of a reduced scale physical model of a Francis turbine. The axial and tangential velocity profiles were established by performing measurements at several radial positions between the turbine centreline and the cone wall. The overall objective was the validation of numerical flow simulations across the entire operating range of the turbine, from partial to full load.

2018

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

Arthur Tristan Favrel

EPFL2016