Hot-wire measurements of mean velocity and turbulence parameters were collected at the mid-point of a rod-wall gap and in the centre of a triangular subchannel in accelerating and decelerating flows through a large-scale (12.9:1) model of a 60\degree ~section of a CANadian Deuterium Uranium (CANDU) nuclear reactor 37-rod bundle. A method was developed to correct the axial velocity fluctuations for the effects of cycle-to-cycle variations, which were particularly significant during deceleration. Compared to values in stationary flows, the gap vortex street Strouhal number was slightly larger during accelerating flow and smaller during decelerating flow. The integral length scales of the axial velocity at both locations during both acceleration and deceleration were larger than the corresponding values in stationary flow. The Taylor microscale during transients was slightly larger than the stationary values. The turbulent kinetic energy dissipation rate was larger for accelerating flow and smaller for decelerating flow. The opposite was found to be true for the Kolmogorov microscale.

Two topics from nuclear engineering field are included in this dissertation. One study is the air-ingress phenomenon during a loss of coolant accident (LOCA) scenario, and the other is a 5-by-5 bundle assembly problem under a design of PWRs. The objectives are to investigate the Kelvin-Helmholtz instability of the gravity-driven stratified flows inside a coaxial pipe and the effects caused by two types of spacers at the downstream of the rod bundle problem. Richardson extrapolation is used for the grid independent study. Simulation results give good agreements with the experiments. Wavelet analysis and Proper Orthogonal Decomposition (POD) are used to study the flow behaviors and flow patterns. For the air-ingress phenomenon, Brunt-Vaisala frequency, or buoyancy frequency, predicts a frequency of 2.34 Hz, which is confirmed by the dominant frequency of 2.4 Hz obtained from the wavelet analysis between times 1.2 s and 1.85 s. For the rod bundle study, the dominant frequency at the center of the subchannel is given as 2.4 Hz with a secondary dominant frequency of 4 Hz and a much minor frequency of 6 Hz. Generally, wavelet analysis has much better performance than POD in the air-ingress phenomenon that is a strongly transient scenario; they both appropriate for the rod bundle study. Based on this study, when the fluid pair in a real condition is used, the time which air intrudes into the reactor is predictable.

In a pressurized water reactor (PWR), pressurized water flows over fuel rods containing radioactive uranium. Potential failure of these nuclear fuel rods is a primary concern, as fuel rod failure typically results in power generation losses and reactor downtime. Thermal parameters such as critical heat flux have traditionally been utilized as performance metrics to ensure that the reactor core remains stable even during failure events. Recently, fuel leaking events have occurred which have resulted in excess debris buildup on fuel rods and fuel grid array mixing devices. Understanding the flow field surrounding these nuclear fuel rods is critical in predicting where crud could deposit. Although CFD simulations have been conducted to characterize the fluid flow around fuel rod bundles, limited experimental data characterizing the mechanics of this fluid flow exists in the current literature. This study will present experimental data collected detailing the fluid flow around a rod bundle geometry using a novel matched refractive index particle tracking velocimetry (PTV) technique over a 3D volume cross section of a prototypical nuclear fuel rod bundle. Velocimetry tracking will be performed in order to characterize the mechanics of the fluid flow. Using optical distortion mitigation techniques and various image processing methods, data from multiple cameras was used to assemble 3-dimensional velocity information of a turbulent fluid region. Results are compared to the solution of a k-epsilon unsteady RANS numerical simulation.

The structure of turbulence in the subchannels of a large-scale 60 degree section of a CANDU 37-rod bundle was studied at Reynolds numbers equal to 50,000, 100,000 and 130,000. Measurements were conducted at roughly 33.81 rod diameters from the inlet of the rod bundle using single-point, two-component hot-wire anemometry. Analysis of the axial velocity signal indicated a weak effect of Reynolds number on the axial velocity distribution and a bulging of axial velocity contours toward the narrow gaps. The normalised normal Reynolds stresses and the normalised turbulent kinetic energy were found to decrease as the Reynolds number increased. The radial Reynolds shear stress varied linearly with radial distance from the rod, crossing zero at the location of local maximum of the axial velocity. This stress was symmetric about the central rod whereas the azimuthal Reynolds shear stress was anti-symmetric. The Reynolds number effect was weak but measurable on the integral length scales of the axial and radial velocity fluctuations but negligible on the integral length scale of the azimuthal velocity fluctuations, especially in the gap regions. The Taylor and Kolmogorov microscales increased from the wall toward the centre of the subchannel and decreased as the Reynolds number increased. The wall shear stress stress distribution around the central rod indicated no effect of Reynolds number, when normalized by the corresponding average. The wall shear stress reached local minima at rod-wall and rod-rod gaps and local maxima in the open flow regions. Vortex streets were generated within the subchannels very close to the inlet of the rod bundle. The convection speed and frequency of the vortex street were found to increase proportionately to Reynolds number, whereas the vortex spacing was not affected by the Reynolds number.

Rod bundle is a typical constitutive element of a very wide range of nuclear reactor designs. This thesis describes the investigation of such geometry with wall-resolved Large Eddy Simulation (LES). In order to alleviate the mesh constraint, imposed by the near wall resolution, the usage of embedded refinements and polyhedral meshes is analysed firstly with a inviscid laminar case (Taylor Green vortices) and secondly with a fully turbulent case (channel flow only with embedded refinement). The inviscid test case shows that the addition of embedded refinements decreases the conservation properties of the code. Indeed the accuracy decreases from second order in a structured conformal mesh, to something in between first and second order depending on the quality of the unstructured mesh. Better results are obtained when the interface between refined and coarse areas presents a more regular and structured pattern, reducing the generation of skewed and stretched cells. The channel flow simulation shows that the Reynolds stresses, of some embedded refined meshes, are affected by spurious oscillations. Surprisingly this effect is present in the unstructured meshes with the best orthogonal properties. Indeed analysis of Reynolds stress budgets shows that terms, where the gradient in the wall normal direction is dominant, have a largely oscillatory behaviour. The cause of the problem is attributed to the convective term and in particular in the method used for the gradient reconstruction. As a consequence of these contradictory signs between the inviscid and the fully turbulent cases, the rod bundle test case is analysed using a conventional body fitted multiblock mesh. Two different Reynolds numbers are investigated reporting Reynolds stresses and budgets. The flow is characterised by an energetic and almost periodic azimuthal flow pulsation in the gap region between adjacent sub-channels, which makes turbulent quantities largely different from those in plane channel and pipes and enhances mixing. Experiments found that a constant Strouhal number, with the variation of the Reynolds number, characterises the phenomenon. The frequency analysis finds that present simulations are distinguished by three dominant frequencies, the first in agreement with the experimental value and two higher ones, which might be due to the correlation of the azimuthal velocity in the streamwise direction. Several passive temperature fields are added at the simulations in order to study the effects of the variation of the Prandtl number and the change in boundary conditions (Neumann and Dirichlet). A simplified case where an imbalance of the scalar between adjacent sub-channels is also investigated in order to evaluate the variation of the heat fluxes with respect to the homogeneous case. An alternative solution, to reduce the mesh constraint imposed by the wall, is to hybridize LES with RANS. The main achievement of this work is to integrate the heat transfer modelling to the already existing model for the dynamic part. Further investigations of the blending function, used to merge the two velocity fields, are carried out in conjunction with a study of the model dependency on the mesh resolution. The validation is performed on a fully developed channel flow at different Reynolds numbers and with constant wall heat flux. On coarse meshes the model shows an improvement of the results for both thermal and hydraulic parts with respect to a standard LES. On refined meshes, suitable for wall-resolved LES, the model suffers from a problem of double counting of modelled Reynolds stresses and heat fluxes because the RANS contribution does not naturally disappear as the mesh resolution increases.

Umströmte Rohr- bzw. Stabbündel sind als Übertrager von Wärmeenergie in einem breiten Spektrum von Anwendungsgebieten zu finden. Beispiele sind Heizkörper, Kühlaggregate, Heizpatronen, industrielle Wärmetauscher und Brennelemente in Kernreaktoren. Für jede dieser Anwendungen besteht die Anforderung, die Wärmeübertragung an den Wärmeübertragerflächen zu optimieren. Dabei besteht eine enge Kopplung zwischen Wärmetransport und Strömungsstruktur. Eine besonders effiziente Form der Wärmeübertragung ist die Verdampfung. Diese wird unter anderem bei Brennelementen in Druckwasserreaktoren genutzt. Hier siedet das Kühlwasser an der Brennstaboberfläche. Durch Kondensation der Dampfblasen in der unterkühlten Kernströmung wird die Wärme dann effizient in die Flüssigphase übertragen. Durch die hohe Verdampfungsenthalpie des Wassers wird beim Strömungssieden ein viel höherer Wärmestrom in das Kühlwasser übertragen, als bei rein einphasig-konvektivem Wärmetransport. Sicherheitstechnisch relevant für Brennelemente in Leichtwasserreaktoren ist der Übergang vom Blasensieden zum Filmsieden (kritischer Wärmestrom). Dieser muss unter allen Umständen vermieden werden, um die Integrität der Brennstabhüllen zu gewährleisten, die bei Überschreiten der kritischen Heizflächenbelastung aufschmelzen bzw. reißen können. Aus diesem Grund werden im Rahmen eines vom Bundesministerium für Bildung und Forschung geförderten Projektes (Förderkennzeichen 02NUK010A) numerische Strömungsberechnungsmodelle entwickelt, die bei der Beschreibung und numerischen Behandlung der Siedephänomene helfen sollen. Zur Validierung dieser Modelle anhand von Experimenten wurde ein Strömungskanal konstruiert, in dem ein vertikales Stabbündel von einem Kältemittel (RC318) aufwärtig durchströmt wird. Der Versuchsstand ist so konzipiert, dass ein optischer und messtechnischer Zugang zu den umströmten Einbauten gegeben ist. Damit sind Messungen in Zweiphasenströmungen ebenso möglich, wie Untersuchungen zur einphasigen Durchströmung. Für später erfolgende Zweiphasen-Experimente mit Stabbeheizung wurden zunächst Voruntersuchungen zur einphasigen Durchströmungen durchgeführt, welche insbesondere Aufschluss über die Homogenität der Strömung in den Unterkanälen sowie die Existenz von Querströmungen geben sollten. Als Messverfahren dafür wurde die Particle Image Velocimetry (PIV) ausgewählt, welche es ermöglicht, zweidimensionale Strömungsfelder aufzuzeichnen. Die experimentellen Studien erfolgten am Optical Multi-Phase Flow Research Laboratory des Nuclear Engineering Department der Texas A&M University in College Station, USA. Die Untersuchungen wurden für drei Volumenstromraten durchgeführt. Der vorliegende Bericht umfasst die Beschreibung des Versuchsstandes und der Messmethodik, eine Vorstellung des Auswerteverfahrens und relevanter Ergebnisse sowie eine Fehlerbetrachtung.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18589号 / 工博第3950号 / 新制||工||1607(附属図書館) / 31489 / 京都大学大学院工学研究科原子核工学専攻 / (主査)教授 功刀 資彰, 教授 中部 主敬, 講師 河原 全作 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Umströmte Rohr- bzw. Stabbündel sind als Übertrager von Wärmeenergie in einem breiten Spektrum von Anwendungsgebieten zu finden. Beispiele sind Heizkörper, Kühlaggregate, Heizpatronen, industrielle Wärmetauscher und Brennelemente in Kernreaktoren. Für jede dieser Anwendungen besteht die Anforderung, die Wärmeübertragung an den Wärmeübertragerflächen zu optimieren. Dabei besteht eine enge Kopplung zwischen Wärmetransport und Strömungsstruktur. Eine besonders effiziente Form der Wärmeübertragung ist die Verdampfung. Diese wird unter anderem bei Brennelementen in Druckwasserreaktoren genutzt. Hier siedet das Kühlwasser an der Brennstaboberfläche. Durch Kondensation der Dampfblasen in der unterkühlten Kernströmung wird die Wärme dann effizient in die Flüssigphase übertragen. Durch die hohe Verdampfungsenthalpie des Wassers wird beim Strömungssieden ein viel höherer Wärmestrom in das Kühlwasser übertragen, als bei rein einphasig-konvektivem Wärmetransport. Sicherheitstechnisch relevant für Brennelemente in Leichtwasserreaktoren ist der Übergang vom Blasensieden zum Filmsieden (kritischer Wärmestrom). Dieser muss unter allen Umständen vermieden werden, um die Integrität der Brennstabhüllen zu gewährleisten, die bei Überschreiten der kritischen Heizflächenbelastung aufschmelzen bzw. reißen können. Aus diesem Grund werden im Rahmen eines vom Bundesministerium für Bildung und Forschung geförderten Projektes (Förderkennzeichen 02NUK010A) numerische Strömungsberechnungsmodelle entwickelt, die bei der Beschreibung und numerischen Behandlung der Siedephänomene helfen sollen. Zur Validierung dieser Modelle anhand von Experimenten wurde ein Strömungskanal konstruiert, in dem ein vertikales Stabbündel von einem Kältemittel (RC318) aufwärtig durchströmt wird. Der Versuchsstand ist so konzipiert, dass ein optischer und messtechnischer Zugang zu den umströmten Einbauten gegeben ist. Damit sind Messungen in Zweiphasenströmungen ebenso möglich, wie Untersuchungen zur einphasigen Durchströmung. Für später erfolgende Zweiphasen-Experimente mit Stabbeheizung wurden zunächst Voruntersuchungen zur einphasigen Durchströmungen durchgeführt, welche insbesondere Aufschluss über die Homogenität der Strömung in den Unterkanälen sowie die Existenz von Querströmungen geben sollten. Als Messverfahren dafür wurde die Particle Image Velocimetry (PIV) ausgewählt, welche es ermöglicht, zweidimensionale Strömungsfelder aufzuzeichnen. Die experimentellen Studien erfolgten am Optical Multi-Phase Flow Research Laboratory des Nuclear Engineering Department der Texas A&M University in College Station, USA. Die Untersuchungen wurden für drei Volumenstromraten durchgeführt. Der vorliegende Bericht umfasst die Beschreibung des Versuchsstandes und der Messmethodik, eine Vorstellung des Auswerteverfahrens und relevanter Ergebnisse sowie eine Fehlerbetrachtung.

L'analyse des écoulements au sein des faisceaux d'assemblages constitue un volet important des études des réacteurs à eau pressurisée. Une mauvaise répartition thermique au sein de ces écoulements peut conduire à une crise d'ébullition nuisible à la sûreté du réacteur. De nombreuses études ont montré l'existence de phénomènes de réorganisation de structures aux grandes échelles dans ces écoulements. Cette thèse vise à améliorer notre compréhension de ces phénomènes, l'objectif étant de développer des modélisations aux petits échelles adaptées. Un travail bibliographique a mis en évidence les difficultés rencontrées par les simulations pour reproduire ces phénomènes, ainsi que de nombreux questionnements concernant leur caractère physique. Des simulations 3D ont été réalisées et ont permis d'identifier deux mécanismes de réorganisation pour les structures aux grandes échelles : un changement de signe de la vitesse transverse entre les crayons ou du tourbillon dans un sous-canal. Il est apparu qu'il semblait pertinent d'adopter l'hypothèse de Taylor pour considérer que les grandes structures 3D évoluaient comme un écoulement 2D transporté. Un gros volet de la thèse a concerné la mise en œuvre d'un code basé sur une méthode statistique pour un champ 2D dans le but de déterminer les états thermodynamiquement stables dans des géométries avec obstacles. Des similarités ont été obtenues entre les structures en REP et les états stables en 2D. Des simulations 2D ont permis d'identifier deux bifurcations possibles pour l'écoulement, qui présentent un parallèle avec les mécanismes de réorganisations 3D, et permettent ainsi de poser les bases d'une explication physique du phénomène / The analysis of fuel rod bundle flows constitute a key element of pressurized-water reactors safety studies. Indeed, an insufficient flow thermal mixing can lead to a boiling crisis, which is nefarious for the reactor safety. Numerous studies have shown the existence of reorganisation phenomena in the flow large-scale structures. This thesis work aims at improving our understanding of these phenomena, with the long-term goal of developing small-scales models suited for this type of flow. A bibliographic study has brought to light the challenges faced by simulations attempting to capture these phenomena, as well as various questions regarding their physical meaning. 3D simulations have been performed in order to study this flow ; they allowed to identify two reorganisation mechanisms for the large-scale structures consisting in a sign change for either a transverse velocity in rod-to-rod gaps or for a subchannel vortex. It appeared relevant to adopt a Taylor hypothesis in order to consider the evolution of large-scale 3D structures as transported-2D. A statistical method has then been applied to the 2D field in order to determine its thermodynamically-stable states in geometries with obstacles using the resolution of an optimization problem with a numerical calculation tool. Interesting similarities have been obtained between the PWR coherent structures and the stable states in a simplified 2D geometry. Further, 2D numerical simulations allowed to identify two different possible flow bifurcations. A parallel is drawn between these bifurcations and the two reorganizations observed in 3D simulations, laying the foundations for a physical explanation of this phenomenon

Turbulent flows and coherent structures emerging within turbulent flow fields have been extensively studied for the past few decades and a wide variety of experimental and numerical techniques have been developed for measurement and analysis of turbulent flows. The complex nature of turbulence requires methods that can accurately estimate its highly chaotic spatial and temporal behavior. Some of the classical cases of turbulent flows with simpler geometries have been well characterized by means of the existing experimental techniques and numerical models. Nevertheless, since most turbulent fields are of complex geometries; there is an increasing interest in the study of turbulent flows through models with more complicated geometries. In this dissertation, characteristics of turbulent flows through two different facilities with complex geometries are studied applying two different experimental methods. The first study involves the investigation of turbulent impinging jets through a staggered array of rods with or without crossflow. Such flows are crucial in various engineering disciplines. This experiment aimed at modeling the coolant flow behavior and mixing phenomena within the lower plenum of a Very High Temperature Reactor (VHTR). Dynamic Particle Image Velocimetry (PIV) and Matched Index of Refraction (MIR) techniques were applied to acquire the turbulent velocity fields within the model. Some key flow features that may significantly enhance the flow mixing within the test section or actively affect some of the structural components were identified in the velocity fields. The evolution of coherent structures within the flow field is further investigated using a Snapshot Proper Orthogonal Decomposition (POD) technique. Furthermore, a comparative POD method is proposed and successfully implemented for identification of the smaller but highly influential coherent structures which may not be captured in the full-field POD analysis. The second experimental study portrays the coolant flow through the core of an annular pebble bed VHTR. The complex geometry of the core and the highly turbulent nature of the coolant flow passing through the gaps of fuel pebbles make this case quite challenging. In this experiment, a high frequency Hot Wire Anemometry (HWA) system is applied for velocity measurements and investigation of the bypass flow phenomena within the near wall gaps of the core. The velocity profiles within the gaps verify the presence of an area of increased velocity close to the outer reflector wall; however, the characteristics of the coolant flow profile is highly dependent on the gap geometry and to a less extent on the Reynolds number of the flow. The time histories of the velocity are further analyzed using a Power Spectra Density (PSD) technique to acquire information about the energy content and energy transfer between eddies of different sizes at each point within the gaps.