Investigation of the Time Resolution Set Up Used to Compute the Full Load Vortex Rope in a Francis Turbine

The flow in a Francis turbine at full load is characterised by the development of an axial vortex rope in the draft tube. The vortex rope often promotes cavitation if the turbine is operated at a sufficiently low Thoma number. Furthermore, the vortex rope can evolve from a stable to an unstable behaviour. For CFD, such a flow is a challenge since it requires solving an unsteady cavitating flow including rotor/stator interfaces. Usually, the numerical investigations focus on the cavitation model or the turbulence model. In the present works, attention is paid to the strategy used for the time integration. The vortex rope considered is an unstable cavitating one that develops downstream the runner. The vortex rope shows a periodic behaviour characterized by the development of the vortex rope followed by a strong collapse leading to the shedding of bubbles from the runner area. Three unsteady RANS simulations are performed using the ANSYS CFX 17.2 software. The turbulence and cavitation models are, respectively, the SST and Zwart models. Regarding the time integration, a second order backward scheme is used excepted for the transport equation for the liquid volume fraction, for which a first order backward scheme is used. The simulations differ by the time step and the number of internal loops per time step. One simulation is carried out with a time step equal to one degree of revolution per time step and five internal loops. A second simulation used the same time step but 15 internal loops. The third simulations used three internal loops and an adaptive time step computed based on a maximum CFL lower than 2. The results show an influence of the time integration strategy on the cavitation volume time history both in the runner and in the draft tube with a risk of divergence of the solution if a standard set up is used.

Evaluation sur modèle réduit et prédiction de la stabilité de fonctionnement des turbines Francis

Stability of operation of Francis turbines Hydraulic disturbances often come with the operation of Francis turbines outside of design head and flow. In some cases, the excessive amplitude of the dynamic system response to these disturbances leads to restriction in the normal operation of the plant. Disturbances largely depend upon operating conditions and the particular turbine design. The response amplitude is under a strong influence from the feed pipe dynamics. Technical and economic consequences of a faulty stability of operation are such that the prediction of stability from model tests is one of the major challenges in present research on hydraulic machinery. Acceptance tests performed on a scale model in a hydraulic laboratory may provide the necessary elements for a full description of characteristics significant for the stability of operation of the full-size turbine in its piping system. This thesis is discussed in the report. 2. Dynamic characterization on the scale model The diagnosis for the dynamic behaviour associated with a machine design and given operating conditions is based on the observation of pressure fluctuations in various positions on the model and test circuit. Auxiliary measurements and surveys of cavitation, noise and vibrations support this data. This study presents a set of evaluation criteria for the elaboration of a diagnosis. The method is illustrated using practical examples from published works and from the numerous experiments performed by the author. A discussion of similitude shows that a diagnosis set using this method provides a good characterisation of the dynamic behaviour associated with the particular turbine design. Adapting acoustic power methods to Francis turbine tests provides a quantitative evaluation of disturbance sources. This tool makes it possible to look forward to an actual prediction of oscillation amplitudes associated with the full-size turbine operation. 3. Elements for the prediction The test laboratory provides a description of the dynamic behaviour associated with a particular turbine design in a guaranteed range of operation. This description is based on three elements of evaluation: discussion of the various dynamic phenomena according to operating conditions: relative frequency and amplitude of the part load precession, of 80 % load oscillation, relative frequency of draft tube column free oscillations, vortex-free region, organisation of the full load pulsation; survey of the draft tube cavitation compliance, lumped at the runner outlet, according to operating conditions; relative emission of the acoustic power toward the piping system, according to operating conditions. These three elements allow an adequate representation of the Francis turbine for a computation of stability of operation involving the piping system dynamics. However, this is the limit of what the test laboratory can say; the prototype piping system layout is liable to change after the turbine model acceptance tests. Once the turbine is adequately described the prediction of stability is performed using known methods and is done under responsibility of the technical consultant. 4. Conclusions The proposed method complies with technical and economic requirements of a laboratory acceptance test on a scale model. Following a simple experimental procedure, if yields a good description of the dynamic behaviour associated with a particular Francis turbine design and its guaranteed range of operation. It provides the elements for computations of stability of operation. Systematic use of the method and comparisons with field observations are the logical sequel to this research. This necessary phase of development will make the stability computations finer, for instance in handling damping in the connecting pipes. It will also provide the basis for a definition of scale effects to be used for the correction of similitudes.

EPFL1993

Large-eddy simulation of atmospheric boundary layer flow through wind turbines and wind farms

Hao Lu, Fernando Porté Agel, Yu-Ting Wu

Accurate prediction of atmospheric boundary layer (ABL) flow and its interactions with wind turbines and wind farms is critical for optimizing the design (turbine siting) of wind energy projects. Large-eddy simulation (LES) can potentially provide the kind of high-resolution spatial and temporal information needed to maximize wind energy production and minimize fatigue loads in wind farms. However, the accuracy of LESs of ABL flow with wind turbines hinges on our ability to parameterize subgrid-scale (SGS) turbulent fluxes as well as turbine-induced forces. This paper focuses on recent research efforts to develop and validate an LES framework for wind energy applications. SGS fluxes are parameterized using tuning-free Lagrangian scale-dependent dynamic models. These models optimize the local value of the model coefficients based on the dynamics of the resolved scales. The turbine-induced forces (e.g., thrust, lift and drag) are parameterized using two types of models: actuator-disk models that distribute the force loading over the rotor disk, and actuator-line models that distribute the forces along lines that follow the position of the blades. Simulation results are compared to wind-tunnel measurements collected with hot-wire anemometry in the wake of a miniature three-blade wind turbine placed in a boundary layer flow. In general, the characteristics of the turbine wakes simulated with the proposed LES framework are in good agreement with the measurements in the far-wake region. Near the turbine, up to about five rotor diameters downwind, the best performance is obtained with turbine models that induce wake-flow rotation and account for the non-uniformity of the turbine-induced forces. Finally, the LES framework is used to simulate atmospheric boundary-layer flow through an operational wind farm. (C) 2011 Elsevier Ltd. All rights reserved.

Elsevier2011

Large eddy simulation study of scalar transport in fully developed wind-turbine array boundary layers

Marc Calaf Bracons, Charles Vivant Ignacio Meneveau, Marc Parlange

become larger, they begin to attain scales at which two-way interactions with the atmospheric boundary layer (ABL) must be taken into account. Several studies have shown that there is a quantifiable effect of wind farms on the local meteorology, mainly through changes in the land-atmosphere fluxes of heat and moisture. In particular, the observed trends suggest that wind farms increase fluxes at the surface and this could be due to increased turbulence in the wakes. Conversely, simulations and laboratory experiments show that underneath wind farms, the friction velocity is decreased due to extraction of momentum by the wind turbines, a factor that could decrease scalar fluxes at the surface. In order to study this issue in more detail, a suite of large eddy simulations of an infinite (fully developed) wind turbine array boundary layer, including scalar transport from the ground surface without stratification, is performed. Results show an overall increase in the scalar fluxes of about 10%–15% when wind turbines are present in the ABL, and that the increase does not strongly depend upon wind farm loading as described by the turbines’ thrust coefficient and the wind turbines spacings. A single-column analysis including scalar transport shows that the presence of wind farms can be expected to increase slightly the scalar transport from the bottom surface and that this slight increase is due to a delicate balance between two strong opposing trends.

American Institute of Physics2011

Evaluation sur modèle réduit et prédiction de la stabilité de fonctionnement des turbines Francis

Stability of operation of Francis turbines Hydraulic disturbances often come with the operation of Francis turbines outside of design head and flow. In some cases, the excessive amplitude of the dynamic system response to these disturbances leads to restriction in the normal operation of the plant. Disturbances largely depend upon operating conditions and the particular turbine design. The response amplitude is under a strong influence from the feed pipe dynamics. Technical and economic consequences of a faulty stability of operation are such that the prediction of stability from model tests is one of the major challenges in present research on hydraulic machinery. Acceptance tests performed on a scale model in a hydraulic laboratory may provide the necessary elements for a full description of characteristics significant for the stability of operation of the full-size turbine in its piping system. This thesis is discussed in the report. 2. Dynamic characterization on the scale model The diagnosis for the dynamic behaviour associated with a machine design and given operating conditions is based on the observation of pressure fluctuations in various positions on the model and test circuit. Auxiliary measurements and surveys of cavitation, noise and vibrations support this data. This study presents a set of evaluation criteria for the elaboration of a diagnosis. The method is illustrated using practical examples from published works and from the numerous experiments performed by the author. A discussion of similitude shows that a diagnosis set using this method provides a good characterisation of the dynamic behaviour associated with the particular turbine design. Adapting acoustic power methods to Francis turbine tests provides a quantitative evaluation of disturbance sources. This tool makes it possible to look forward to an actual prediction of oscillation amplitudes associated with the full-size turbine operation. 3. Elements for the prediction The test laboratory provides a description of the dynamic behaviour associated with a particular turbine design in a guaranteed range of operation. This description is based on three elements of evaluation: discussion of the various dynamic phenomena according to operating conditions: relative frequency and amplitude of the part load precession, of 80 % load oscillation, relative frequency of draft tube column free oscillations, vortex-free region, organisation of the full load pulsation; survey of the draft tube cavitation compliance, lumped at the runner outlet, according to operating conditions; relative emission of the acoustic power toward the piping system, according to operating conditions. These three elements allow an adequate representation of the Francis turbine for a computation of stability of operation involving the piping system dynamics. However, this is the limit of what the test laboratory can say; the prototype piping system layout is liable to change after the turbine model acceptance tests. Once the turbine is adequately described the prediction of stability is performed using known methods and is done under responsibility of the technical consultant. 4. Conclusions The proposed method complies with technical and economic requirements of a laboratory acceptance test on a scale model. Following a simple experimental procedure, if yields a good description of the dynamic behaviour associated with a particular Francis turbine design and its guaranteed range of operation. It provides the elements for computations of stability of operation. Systematic use of the method and comparisons with field observations are the logical sequel to this research. This necessary phase of development will make the stability computations finer, for instance in handling damping in the connecting pipes. It will also provide the basis for a definition of scale effects to be used for the correction of similitudes.

EPFL1993

Large-eddy simulation of atmospheric boundary layer flow through wind turbines and wind farms

Hao Lu, Fernando Porté Agel, Yu-Ting Wu

Elsevier2011