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- Angele, Kristian, et al.
(författare)
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Flow Mixing Inside a Control-Rod Guide Tube : Part 2—Experimental Tests and CFD-Simulations
- 2010
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Ingår i: 18th International Conference on Nuclear Engineering. - 9780791849323 ; , s. 655-663
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Konferensbidrag (refereegranskat)abstract
- Alarge number of control rod cracks were detected during therefuelling outage of the twin reactors Oskarshamn 3 and Forsmark3 in the fall of 2008. The extensive damage investigationfinally lead to the restart of both reactors at theend of 2008 under the condition that further studies wouldbe conducted in order to clarify all remaining matters. Also,all control rods were inserted 14% in order to locatethe welding region of the control rod stem away fromthe thermal mixing region of the flow. Unfortunately, this measureled to new cracks a few months later due toa combination of surface finish of the new stems andthe changed flow conditions after the partial insertion of thecontrol rods. The experimental evidence reported here shows an increasein the extension of the mixing region and in theintensity of the thermal fluctuations. As a part of thecomplementary work associated with the restart of the reactors, andto verify the CFD simulations, experimental work of the flowin the annular region formed by the guide tube andcontrol rod stem was carried out. Two full-scale setups weredeveloped, one in a Plexiglass model at atmospheric conditions (inorder to be able to visualize the mixing process) andone in a steel model to allow for a highertemperature difference and heating of the control rod guide tube.The experimental results corroborate the general information obtained through CFDsimulations, namely that the mixing region between the cold crud-removalflow and warm by-pass flow is perturbed by flow structurescoming from above. The process is characterized by low frequent,high amplitude temperature fluctuations. The process is basically hydrodynamic, causedby the downward transport of flow structures originated at theupper bypass inlets. The damping thermal effects through buoyancy isof secondary importance, as also the scaling analysis shows, howevera slight damping of the temperature fluctuations can be seendue to natural convection due to a pre-heating of thecold crud-removal flow. The comparison between numerical and experimental resultsshows a rather good agreement, indicating that experiments with plantconditions are not necessary since, through the existing scaling lawsand CFD-calculations, the obtained results may be extrapolated to plantconditions. The problem of conjugate heat transfer has not yetbeen addressed experimentally since complex and difficult measurements of theheat transfer have to be carried out. This type ofmeasurements constitutes one of the main challenges to be dealtwith in the future work.
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| 7. |
- Odemark, Ylva, et al.
(författare)
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High-Cycle Thermal Fatigue in Mixing Tees: New Large-Eddy Simulations Validated Against New Data Obtained by PIV in the Vattenfall Experiment
- 2009
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Ingår i: Proceedings of the 17th International Conference on Nuclear Engineering - 2009. - New York : American Society of Mechanical Engineers. - 9780791838525 ; , s. 775-785
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Konferensbidrag (refereegranskat)abstract
- New data was obtained for a previously studied T-junction experimental setup [1] for a range of flow ratios between hot and cold flows in order to validate new Large Eddy Simulations (LES). The instantaneous velocity field downstream of the T-junction was measured with two-component Particle Image Velocimetry (PIV) in several horizontal and vertical planes at the centre line downstream of the T-junction. The generated PIV database enables a thorough validation of CFD turbulence statistics. The turbulence statistics are shown to be well predicted despite the fact that the mesh in the LES is rather coarse. By usage of time resolved PIV the temporal evolution of the predominant low frequent large-scale structures, responsible for much of the mixing and the high amplitude temperature fluctuations on the walls, were captured. Those structures are, however, weaker in LES than in PIV, being in line with the fact that the wake region behind the penetrating vertical hot jet is underpredicted in LES. Tests regarding the influence of the LES-results to the shape of the inlet boundary conditions (developed or flat symmetric mean-velocity profiles) were carried out and the sensitivity in the results was shown to be small. Furthermore, the results show good agreement with the experimental data independent of the flow ratio between the hot and the cold flows.
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| 13. |
- Odemark, Ylva, et al.
(författare)
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The effects of a model forest canopy on the outputs of a wind turbine model
- 2014
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Ingår i: Journal of Physics, Conference Series. - : Institute of Physics Publishing (IOPP). - 1742-6588 .- 1742-6596. ; 555:1
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Tidskriftsartikel (refereegranskat)abstract
- The effects of a rough surface boundary layer on the outputs of a wind turbine model were investigated experimentally in a wind tunnel. The very rough surface consisted of cylindrical pins, in order to model a forest canopy. The hub height of the turbine model was varied in order to see the effect of the presence of the model forest in the power and thrust coefficients. A small effect of the hub height was observed in the averaged power coefficient, where the turbine produced less for the lowest hub height. The difference was however reduced when scaling the power output with the available power in the wind instead of using the velocity at hub height. Consistent trends were present in the standard deviation of the thrust coefficient and the rotational speed, which both increased by decreasing the hub height. This underlines the fact that not only the rotor but also the tower and the bearings of a wind turbine must withstand to increased loads when operating close to a canopy.
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| 14. |
- Odemark, Ylva, et al.
(författare)
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The stability and development of tip and root vortices behind a model wind turbine
- 2013
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Ingår i: Experiments in Fluids. - : Springer Science and Business Media LLC. - 0723-4864 .- 1432-1114. ; 54:9, s. 1591-
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Tidskriftsartikel (refereegranskat)abstract
- When designing new wind farms, one has to rely on models describing the flow field around and inside the farm, since direct numerical simulation is far too computationally expensive. In order to develop better models for power prediction of wind farms, knowledge about the flow field around turbines, the stability of the wakes and the interaction between them is essential. Since the conditions during field measurements are difficult to control, wind tunnel measurements play an important role when studying wakes behind wind turbines. Within the present work, an experimental methodology has been developed to study the evolution and stability of the tip vortices shed from the rotor blades of a small-scale turbine model. The stability of the tip vortices was studied by introducing a periodic disturbance to the flow, which is shown to have a clear effect on the development of the vortices. Prior to the vortex breakdown, clear signs of vortex pairing were also observed. A parameter study was performed by varying the amplitude and frequency of the forced disturbance, and the effect on the tip vortices was evaluated. This experiment is one of the first where the influence of a periodic disturbance on a wind turbine wake is studied, something that previously has been performed in a number of numerical studies.
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| 15. |
- Odemark, Ylva, et al.
(författare)
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Wake evolution and trailing vortex instabilities
- 2011
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The production losses and inhomogeneous loads of wind powerturbines placed in the wake of another turbine is a well-known problemwhen building new wind power farms, and a subject of intensive research.The present work aims at developing an increased understanding of thebehaviour of turbine wakes, with special regard to wake evolution andthe stability of the trailing vortices. Single point velocity measurementswith hot-wire anemometry were performed in the wake of a small-scalemodel turbine. The model was placed in the middle of the wind tunneltest section, outside the boundary layers from the wind tunnel walls.In order to study the stability of the wake and the trailing vortices,a disturbance was introduced at the end of the nacelle. This was ac-complished through two ori¯ces perpendicular to the main °ow, whichwere connected to a high-pressure tank and two fast-switching valves.Both varicose and sinusoidal modes of di®erent frequencies could be trig-gered. By also triggering the measurements on the blade passage, themeandering of the wake and the disturbance frequency, phase averagedresults could be computed. The results for di®erent frequencies as wellas studies of wake evolution will be presented.
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| 16. |
- Odemark, Ylva
(författare)
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Wakes behind wind turbines - Studies on tip vortex evolution and stability
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The increased fatigue loads and decreased power output of a wind turbine placed in the wake of another turbine is a well-known problem when building new wind power farms. In order to better estimate the total power output of a wind power farm, knowledge about the development and stability of wind turbine wakes is crucial. In the present thesis, the wake behind a small-scale model turbine was investigated experimentally in a wind tunnel. The velocity in the wake was measured with hot-wire anemometry, for different free stream velocities and tip speed ratios. To characterize the behaviour of the model turbine, the power output, thrust force and rotational frequency of the model were also measured. These results were then compared to calculations using the Blade Element Momentum (BEM) method. New turbine blades for the model was constructed using the same method, in order to get an estimate of the distribution of the lift and drag forces along the blades. This information is needed for comparisons with certain numerical simulations, which however remains to be performed.By placing the turbine at different heights in a turbulent boundary layer, the effects of forest turbulence on wind turbine outputs (power and thrust) could also be investigated.The evolution of the tip vortices shed from the turbine blades was studied by performing velocity measurements around the location of the tip vortex breakdown. The vortices' receptivity to disturbances was then studied by introducing a disturbance in the form of two pulsed jets, located in the rear part of the nacelle. In order to introduce a well-defined disturbance and perform phase-locked measurements, a new experimental setup was constructed and successfully tested for two different disturbance frequencies. The mean streamwise velocity and the streamwise turbulence intensity was found to scale well with the free stream velocity and the spreading of the wake was found to be proportional to the square root of the downstream distance. The comparison for power and thrust between measurements and BEM calculations showed good agreement in some cases but worse agreement when the pitch angle of the blades was small.The velocity measurements showed that the tip vortices can be susceptible to disturbances and an earlier breakdown could be detected. However, more measurements need to be made to fully investigate the dependance on disturbance frequency and amplitude.
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| 18. |
- Odemark, Ylva
(författare)
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Wind-turbine wake flows - Effects of boundary layers and periodic disturbances
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The increased fatigue loads and decreased power output of a wind turbine placed in the wake of another turbine is a well-known problem when building new wind-power farms and a subject of intensive research. These problems are caused by the velocity gradients and high turbulence levels present in the wake of a turbine. In order to better estimate the total power output and life time of a wind-power farm, knowledge about the development and stability of wind-turbine wakes is crucial.In the present thesis, the flow field around small-scale model wind turbines has been investigated experimentally in two wind tunnels. The flow velocity was measured with both hot-wire anemometry and particle image velocimetry. To monitor the turbine performance, the rotational frequency, the power output and the total drag force on the turbine were also measured. The power and thrust coefficients for different tip-speed ratios were calculated and compared to the blade element momentum method, with a reasonable agreement. The same method was also used to design and manufacture new turbine blades, which gave an estimate of the distribution of the lift and drag forces along the blades.The influence of the inlet conditions on the turbine and the wake properties was studied by subjecting the turbine to both uniform in flow and different types of boundary layer in flows. In order to study the stability and development of the tip vortices shed from the turbine blades, a new experimental setup for phase-locked measurements was constructed. The setup made it possible to introduce perturbations of different frequencies and amplitudes, located in the rear part of the nacelle. With a newly developed method, it was possible to characterize the vortices and follow their development downstream, using only the streamwise velocity component.Measurements were also performed on porous discs placed in different configurations. The results highlighted the importance of turbine spacings. Both the measurements on the turbine and the discs were also used to compare with large eddy simulations using the actuator disc method. The simulations managed to predict the mean velocity fairly well in both cases, while larger discrepancies were seen in the turbulence intensity.
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