| 1. |
- Angele, Kristian, 1971-
(författare)
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Experimental studies of turbulent boundary layer separation and control
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The object ofthe present work is to experimentally study thecase ofa turbulent boundary layer subjected to an AdversePressure Gradient (APG) with separation and reattachment. Thisconstitutes a good test case for advanced turbulence modeling.The work consists ofde sign of a wind-tunnel setup, developmentofP article Image Velocimetry (PIV) measurements and evaluationtechniques for boundary layer flows, investigations ofs calingofb oundary layers with APG and separation and studies oftheturbulence structure ofthe separating boundary layer withcontrol by means ofs treamwise vortices. The accuracy ofP IV isinvestigated in the near-wall region ofa zero pressure-gradientturbulent boundary layer at high Reynolds number. It is shownthat, by careful design oft he experiment and correctly appliedvalidation criteria, PIV is a serious alternative toconventional techniques for well-resolved accurate turbulencemeasurements. The results from peak-locking simulationsconstitute useful guide-lines for the effect on the turbulencestatistics. Its symptoms are identified and criteria for whenthis needs to be considered are presented. Different velocityscalings are tested against the new data base on a separatingAPG boundary layer. It is shown that a velocity scale relatedto the local pressure gradient gives similarity not only forthe mean velocity but also to some extent for the Reynoldsshear-stress. Another velocity scale, which is claimed to berelated to the maximum Reynolds shear-stress, gives the samedegree of similarity which connects the two scalings. However,profile similarity achieved within an experiment is notuniversal and this flow is obviously governed by parameterswhich are still not accounted for. Turbulent boundary layerseparation control by means ofs treamwise vortices isinvestigated. The instantaneous interaction between thevortices and the boundary layer and the change in the boundarylayer and turbulence structure is presented. The vortices aregrowing with the boundary layer and the maximum vorticity isdecreased as the circulation is conserved. The vortices arenon-stationary and subjected to vortex stretching. Themovements contribute to large levels ofthe Reynolds stresses.Initially non-equidistant vortices become and remainequidistant and are con- fined to the boundary layer. Theamount ofi nitial streamwise circulationwas found to be acrucial parameter for successful separation control whereas thevortex generator position and size is ofseco ndary importance.At symmetry planes the turbulence is relaxed to a nearisotropic state and the turbulence kinetic energy is decreasedcompared to the case without vortices.Keywords:Turbulence, Boundary layer, Separation,Adverse Pressure Gradient (APG), PIV, control, streamwisevortices, velocity scaling.
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| 2. |
- 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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| 3. |
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| 4. |
- Angele, Kristian, et al.
(författare)
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Instantaneous behavior of streamwise vortices for turbulent boundary layer separation control
- 2007
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Ingår i: Journal of Fluids Engineering. - : ASME International. - 0098-2202 .- 1528-901X. ; 129:2, s. 226-235
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Tidskriftsartikel (refereegranskat)abstract
- The present study investigates turbulent boundary layer separation control by means of streamwise vortices with focus on the instantaneous vortex behavior. A turbulent boundary layer is exposed to a pressure gradient that generates a separation bubble with substantial backflow. The separation bubble is controlled by conventional passive vortex generators creating pairs of counterrotating vortices. Quantitative information is achieved by applying Particle Image Velocimetry (PIV) to the cross-stream plane of the vortices. The characteristics of a pair of counter-rotating vortices shed from a vortex generator is investigated in the near-field downstream of the vortex generator. The vortices were found to grow with the boundary layer in the downstream direction, and the maximum vorticity decreases as the circulation is conserved. The vortices are nonstationary, and the movements in the spanwise direction are larger than those in the wall-normal direction, due to the presence of the wall. The vortices fluctuate substantially and move over a spanwise distance, which is approximately equal to their size. The most probable instantaneous separation between the two vortices shed from one vortex generator equals the difference between their mean positions. The unsteadiness of the vortices contributes to the observed maxima in the Reynolds stresses around the mean vortex centers. The instantaneous vortex size and the instantaneous maximum vorticity are also fluctuating properties, and the instantaneous vortex is generally smaller and stronger than the mean vortex. A correlation was found between a large instantaneous vortex size and a low instantaneous maximum vorticity (and vice versa), suggesting that the vortices are subjected to vortex stretching.
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| 5. |
- Angele, Kristian P., et al.
(författare)
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A simple model for the effect of peak-locking on the accuracy of boundary layer turbulence statistics in digital PIV
- 2005
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Ingår i: Experiments in Fluids. - : Springer Science and Business Media LLC. - 0723-4864 .- 1432-1114. ; 38:3, s. 341-347
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Tidskriftsartikel (refereegranskat)abstract
- A simple model was constructed to study the effect of peak-locking on the accuracy of particle image velocimetry (PIV) turbulence statistics. A crucial parameter is the ratio between the root-mean-square (rms) velocity and the discretization velocity, which reflects the number of peaks distributed over the velocity probability density functions. When the ratio of the discretization velocity, which is set by the PIV setup parameters, to the rms, given by the flow, is larger than two, the maximum errors introduced in the mean and rms values become significant ( larger than 1%). The errors introduced also depend on the amplitude, or severity, of the peak-locking, and whether the mean displacement corresponds to an integer or a fractional number of pixels. The peak-locking affects the statistical moments of different order in such a way that the errors are phase shifted. The proposed model can be used to predict errors in the turbulence statistics in a laboratory PIV experiment. According to our model predictions, the most significant influence of peak-locking in a boundary layer type of flow is an overall underestimation of the wall-normal rms. Our predictions are in good agreement with our experimental results from turbulent boundary layers and the recent experimental results from a turbulent channel flow by Christensen (Exp Fluids 36: 484 - 497, 2004) for a case of moderate peak-locking.
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| 6. |
- Angele, Kristian P., et al.
(författare)
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PIV measurements in a weakly separating and reattaching turbulent boundary layer
- 2006
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Ingår i: European journal of mechanics. B, Fluids. - : Elsevier BV. - 0997-7546 .- 1873-7390. ; 25:2, s. 204-222
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Tidskriftsartikel (refereegranskat)abstract
- A high Reynolds number flat plate turbulent boundary layer was studied in a wind-tunnel experiment using particle image velocimetry (PIV). The flow is subjected to an adverse pressure gradient (APG) which is designed such that the boundary layer separates and reattaches, forming a weak separation bubble. With PIV we are able to get a more complete picture of this complex flow phenomenon. The view of a separation bubble being composed of large scale coherent regions of instantaneous backflow occurring randomly in a three-dimensional manner in space and time is verified by the present PIV measurements. The PIV database was used to test the applicability of various velocity scalings around the separation bubble. We found that the mean velocity profiles in the outer part of the boundary layer, and to some extent also the Reynolds shear-stress, are self-similar when using a velocity scale based on the local pressure gradient. The same can be said for the so called Perry-Schotield scaling, which suggests that the two velocity scales are connected. This can also be interpreted as an experimental evidence of the claimed relation between the latter velocity scale and the maximum Reynolds shear-stress.
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| 7. |
- Angele, Kristian P., et al.
(författare)
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The effect of streamwise vortices on the turbulence structure of a separating boundary layer
- 2005
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Ingår i: European journal of mechanics. B, Fluids. - : Elsevier BV. - 0997-7546 .- 1873-7390. ; 24:5, s. 539-554
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Tidskriftsartikel (refereegranskat)abstract
- A high Reynolds number flat plate turbulent boundary layer is investigated in a wind-tunnel experiment. The flow is subjected to an adverse pressure gradient which is strong enough to generate a weak separation bubble. This experimental study attempts to shed some new light on separation control by means of streamwise vortices with emphasize on the change in the boundary layer turbulence structure. In the present case, counter-rotating and initially non-equidistant streamwise vortices become and remain equidistant and confined within the boundary layer, contradictory to the prediction by inviscid theory. The viscous diffusion cause the vortices to grow, the swirling velocity component to decrease and the boundary layer to develop towards a two-dimensional state. At the position of the eliminated separation bubble the following changes in the turbulence structure were observed. The anisotropy state in the near-wall region is unchanged, which indicates that it is determined by the presence of the wall rather than the large scale vortices. However, the turbulence in the outer part of the boundary layer becomes overall more isotropic due to an increased wall-normal mixing and a significantly decreased production of streamwise fluctuations. The turbulent kinetic energy is decreased as a consequence of the latter. Despite the complete change in mean flow, the spatial turbulence structure and the anisotropy state, the process of transfer of turbulent kinetic energy to the spanwise fluctuating component seems to be unchanged. Local regions of anisotropy are strongly connected to maxima in the turbulent production. For example, at spanwise positions in between those of symmetry, the spanwise gradient of the streamwise velocity cause significant production of turbulent fluctuations. Transport of turbulence in the spanwise direction occurs in the same direction as the rotation of the vortices.
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| 8. |
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| 9. |
- Burman, Anton J., et al.
(författare)
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Case Study of Transient Dynamics in a Bypass Reach
- 2020
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Ingår i: Water. - : MDPI. - 2073-4441. ; 12:6
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Tidskriftsartikel (refereegranskat)abstract
- The operating conditions of Nordic hydropower plants are expected to change in the coming years to work more in conjunction with intermittent power production, causing more frequent hydropeaking events. Hydropeaking has been shown to be detrimental to wildlife in the river reaches downstream of hydropower plants. In this work, we investigate how different possible future hydropeaking scenarios affect the water surface elevation dynamics in a bypass reach in the Ume River in northern Sweden. The river dynamics has been modeled using the open-source solver Delft3D. The numerical model was validated and calibrated with water-surface-elevation measurements. A hysteresis effect on the water surface elevation, varying with the downstream distance from the spillways, was seen in both the simulated and the measured data. Increasing the hydropeaking rate is shown to dampen the variation in water surface elevation and wetted area in the most downstream parts of the reach, which could have positive effects on habitat and bed stability compared to slower rates in that region.
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| 10. |
- Green, Torbjörn
(författare)
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Particle image velocimetry in practice
- 2009
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Experimental fluid mechanics has for a long time been used to visualize flow phenomenon. An early pioneer was Ludwig Prandtl who used aluminum particles in water flumes to describe the flow in a qualitative manner. In line with the rapid development of Computational Fluid Dynamics, CFD, the need for new validation tools has increased. By combining Prandtls attempt to trace particles and contemporary tools in laser and computer technologies a quantitative non intruisiv whole field technique, so called Particle Image Velocity (PIV) has been developed. The PIV technique has been improved and grown in popularity through recent decades with the increase in computer capacity. This thesis describes three rather different areas of application of PIV measurements. In the first case PIV is used as pure measurement technology tool to describe the flow field inside an attraction channel in connection to fish migration. In the second case, PIV is applied as a validation tool for CFD calculations with Large Eddy Simulation (LES) including an extensive analysis of the results. Finally, a description of how PIV technique can be adopted to study the flow of complex fluids in small geometries by means of microscopy is given.The attraction channel is a U-shaped channel designed to facilitate salmonoid like fihes to migrate upstream to their spawning grounds. The attraction channel has a restriction in the downstream outlet that provides an acceleration of the attraction water up to 38% of the sourunding water velocity according to the PIV measurements. With PIV measurements it is also shown that the depth of displacement over the restriction is significant for how far downstream the acceleration is perceptible. CFD technology is constantly evolving and new methods will become the future standard in the industry. In the current situation Reynolds Avereaged Numerical Simulations (RANS) is the most used method in CFD. But development is approaching LES technology. This is, for instance, motivated by energy production units which has many applications with high turbulence and temperature fluctuations. In the current situation it is required to extend the service life of existing power plants. Therefore it is desirable to be able to estimate these fluctuations impact on thermal loads on the materials inside the plant, for example pipe walls. An LES approach is superior to applying to RANS since the large eddies are resolved. However, LES is still not mature enough to be used without validation in critical applications. Therefore, PIV has been used to create a validation database for a generic T-junction.Double Restriction Sealings (DRS) have been used in bearings and other lubricated applications since the 1940's. A DRS is intended to prevent contamination from entering and is therefore used to increase the life span of lubricated parts, i.e. hinder polutants to reach the rolling elements in bearings for example. Although it is widly applied little is known about the actual function and mechanism of the DRS. To learn more about the flow and particle tracks within a DRS, a new method to visualize and quantify grease flow within a DRS has been developed based upon micro PIV. The main result from this study is that it is possible to make quantititative measurement of the flow within a DRS.
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| 11. |
- Hang, Trieu
(författare)
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Performance of image-based velocimetry in river flow – Large Scale PIV and PTV
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- River flow velocity is critical information for hydraulic and hydrological applications. Monitoring flow fields in near-plant waterways and river reach has wide engineering applications insustainable hydropower generation. For instance, exploring eco-hydraulic concerns such as fish migration, pollutant transport, and river erosion and understanding river floating debris are a few examples of practical applications. Due to complicated geometry and large volumes of natural flows, the measurement task using traditional methods (e.g., velocity propellers, acoustic Doppler velocimetry, with acoustic Doppler current profilers) usually requires extensive investigative work. The measurement procedure also requires contact with waterbody, thus avoiding its use in severe flood conditions. Image analysis approach allows the measurement task to capture the surface-water velocity distribution over a large outdoor area. The main objectives of this research are to (1) evaluate the feasibility of employing multiple cameras in a single measuring system to estimate the flow surfacevelocity and (2) improve the capability to use natural floating materials in river flow observations.The properties of the camera system and particle tracking velocimetry (PTV) algorithm were investigated in a laboratory open channel flow measurement before being deployed for field measurements. The in situ camera calibration methods, which correspond to the two measurement situations, were used to mitigate the instability of the camera mechanism and camera geometry. The artificial tracer particles were deployed to seed the flows. Two photogrammetry-based PTV algorithms are presented regarding different types of employed seeding particles. The first algorithm uses the particle tracking method applied for individual particles, whereas the second algorithm employs correlation-based particle clustering tracking for clusters of small-size particles. The outcomes reveal that the method can offer a reliable and accurate assessment of 3D surface velocity.In river surface velocity measurements, flow seeding is unavoidable in some situations where the water flow is clear, and there are no occurrences of floating materials on the surface. This part of the study focuses on the application of this technique for river velocity measurements using natural surfacefloating patterns. The use of a multiple-camera system provides the ability to perform 3D measurements on the river surface, including surface velocimetry and water surface reconstruction. The pattern-based tracking approach is used to adapt the performance of image measurements on different types of naturalfloating tracers. A comparison of pattern-based tracking with particle tracking reveals that these two approaches are consistent. An analysis of the characteristics of floating patterns is performed to understand their influences on standard deviation of measured velocity. Considerations on practicing image velocimetry in river flows are also discussed.
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| 12. |
- Lögdberg, Ola, et al.
(författare)
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On the robustness of separation control by streamwise vortices
- 2010
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Ingår i: European journal of mechanics. B, Fluids. - : Elsevier BV. - 0997-7546 .- 1873-7390. ; 29:1, s. 9-17
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Tidskriftsartikel (refereegranskat)abstract
- The robustness of vane-type vortex generators (VGs) for separation flow control was studied in a separating turbulent boundary layer on a flat plate. VG arrays of different sizes and streamwise positions were positioned upstream of the separation bubble and their effect on the flow field was studied with the help of particle image velocimetry (PIV). The extent of the separated region was varied by changing the pressure gradient. Three different separation bubbles were produced and their extent was approximately doubled for each increase in pressure gradient. It was found that the sensitivity of the control effect to changes in the size of the separation bubble is small within the applied range of pressure gradients. Furthermore, the importance of the relative position of the VGs with respect to the separated region is small.
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| 13. |
- Lögdberg, Ola, et al.
(författare)
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On the scaling of turbulent separating boundary layers
- 2008
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 20:7, s. 075104-1-075104-4
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Tidskriftsartikel (refereegranskat)abstract
- This study focuses on the mean velocity distribution of turbulent boundary layers near, at, and after separation. The proper mean velocity scaling of the outer region in strong adverse pressure gradients and separated turbulent boundary layers is still under debate and over the years various different velocity scales have been proposed. Here the scaling proposed by Zagarola and Smits [J. Fluid Mech. 373, 33 (1998)] is applied to data from three different separated flows. In all cases the mean velocity defect profiles are self-similar in the region between separation and the position of maximum mean reverse flow. Downstream of the reverse flow maximum, the profiles change, but they are still self-similar within that region. It was also found that the mean velocity defect profiles of all three pressure gradients show similarity in the region between separation and the position of maximum mean reverse flow.
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| 14. |
- Michta, Edouard, et al.
(författare)
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NUMERICAL PREDICTIONS OF BUBBLY TWO-PHASE FLOWS WITH OPENFOAM
- 2011
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Ingår i: Proceedings of the 14th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics.
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Konferensbidrag (refereegranskat)abstract
- A new model for simulation of bubbly two-phase flows has been developed and implemented into an open-source Computational Fluid Dynamics (CFD) code OpenFOAM. The model employs the two-fluid framework with closure relationships for the interfacial momentum transfer. The bubble size is calculated based on the solution of the interfacial area concentration equations. The predictions are validated against a wide range of experimental data containing measured void fraction, the phasic velocity and the interfacial area concentration. The new model demonstrates the ability to capture the wall peaking of void fraction for small bubbles. The predicted levels of void fraction and phasic velocities are in good agreement with measured data.
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| 15. |
- 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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| 16. |
- Pegonen, Reijo, et al.
(författare)
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Investigation of Thermal Mixing in the Control Rod Top Tube Using Large Eddy Simulation
- 2014
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Ingår i: Journal of Power Technologies. - : Institute of Heat Engineering. - 2083-4187 .- 2083-4195. ; 94:1, s. 67-78
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Tidskriftsartikel (refereegranskat)abstract
- Thermal mixing and thermal fatigue has led to component failures in the nuclear industry. The thermal fatigue phenomenon is intimately linked with the mixing of streams of different temperatures in proximity to a solid wall. Due to conjugate heat transfer, temperature fluctuations are induced in the wall. One of the key issues is to predict the amplitude and the frequency of the fluctuations. This paper presents pre-calculations of the thermal mixing experiments that are under preparation at the KTH Royal Institute of Technology as part of the THEMFE project (Thermal Mixing and Fatigue Experiment).The proposed geometry is a simplification of a reactor control rod and consists of a top-tube and control rod stem, which are modeled as concentric cylinders. In addition there are only two hot inlet jets and two cold inlet jets, whereas in reality there are 8 upper inlets and 4 lower inlets for hot bypass water and the cold flow is annular.Thermal mixing was studied by using a transient Computational Fluid Dynamics (CFD) solver for the incompressible filtered Navier-Stokes equations and employing a Large Eddy Simulation model of turbulence implemented in OpenFOAM. The aim was to verify that the proposed simplified geometry and the flow conditions of the experiment will lead to low frequent temperature fluctuations of the order of 0.1-1 Hz, as seen in previous experiments with the real geometry. Such low frequencies are typical for the thermal fatigue phenomenon. The study was focused on the region near the control rod stem and therefore a refined grid was used in that region. The final mesh consisted of over one million cells.The results did indeed reveal low frequent temperature fluctuations in the lower part of the mixing region near the control rod stem. The results of this paper indicate that the length of the mixing region is 23 cm, which is large enough to be resolved in the experiment. It was also found that the most dangerous region, where the dominant high amplitude temperature fluctuations have a frequency of the order of 0.1 Hz, is 4 cm long. As expected, the instant flow field is asymmetric with large secondary flows. The present results verify that the proposed geometry and flow conditions can be applied in the experiment.
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