Denna studie har undersökt om förvärmning av luft är en bra metod för att undvika frostbildning i motströmsplattvärmeväxlare i luftbehandlingsaggregat. Frostbildning i värmeväxlare för ventilation är ett problem i kalla klimat och sänker den återvunna energimängden när potentialen för energiåtervinning är som högst.   Tidigare forskning i området har utförts för att bland annat: 1) Definiera säkra förhållanden utan frostbildning i värmeväxlare med laminär luftströmning.  2) Jämföra olika frostkontrollstrategier. 3) Analysera frostbildningens egenskaper och effekt på värmeväxlare i ventilation. Att forskningen inte är entydig för vilken frostkontrollstrategi som är bäst belyser vikten av detta arbete.   I en fallstudie kvantifieras empiriska värmeöverföringskoefficienter som används i en analytisk modell som förutspår energianvändningen för luftvärmning under olika driftfall för ett luftbehandlingsaggregat. Värmeöverföringskoefficienterna tas fram genom mätningar i ett luftbehandlingsaggregat lokaliserat i ett flerbostadshus. Mätobjektet har ett installerat värmebatteri före värmeväxlaren vilket värmer inkommande uteluft till konstant lufttemperatur.  Tillverkaren proklamerar att det sker turbulent luftströmning i den studerade värmeväxlaren. Vid olika lufthastigheter ändras värmeöverföringskoefficienten mer vid turbulent strömning än vid laminär strömning. Att olika lufthastigheter har betydlig inverkan på värmeöverföringskoefficienten överensstämmer med resultatet från mätningarna. Effektiviteten av den aktuella styrningen av förvärmaren har granskats och brister har upptäckts. En ny optimerad styrning föreslås för frostfri drift och minimerad energianvändning. Den optimerade styrningen jämförs sedan med en vanlig avfrostningsmetod och utförs med hjälp av flödesbalansstatistik från ett aggregat med sektionsavfrostningsstyrning. Resultatet visar att förvärmning av luft för frostfri drift använder mindre energi än vid sektionsavfrostningsstyrning.  Den föreslagna styrningen innebär att bypass-spjället öppnas då förvärmaren går på full effekt. Detta för att förhindra påfrostning vid extrema utetemperaturer, då förvärmaren inte ger tillräcklig effekt för att undvika påfrostning. Den föreslagna styrningen regleras utifrån daggpunkten, temperaturen på tilluften innan värmeväxlaren och temperaturen på avluften. Den framtagna värmeöverföringskoefficienten i studien tar inte i beaktning ökad koefficient under fuktiga förhållanden då daggpunkten i frånluften är över fryspunkten och det sker kondens. En framtida studie kan hitta värmeöverföringskoefficienter som förutspår yttemperaturen under kondensutfällning då daggpunkten är över 0°C vilket ger medel för att ta fram en ännu mera energieffektiv styrning. Detta kan utföras genom mätning i ett luftbehandlingsaggregat med hög temperaturverkningsgrad placerat i en fuktig miljö. / This study has analysed if preheating of air is a good method to prevent frost formation in a counter flow plate heat exchanger used in Air Handling Units. Frosting in heat exchangers used in AHU-systems is a problem in cold climates and lowers the energy recovery when its potential is the highest.   Previous research has been done to 1) Define safe operating conditions without frost formation in heat exchangers with laminar flow. 2) Compare different frost control strategies. 3) Analyse frost formation properties and its effects on heat exchangers in AHU-systems. That the research is not unambiguous for which frost control method is the best highlights the importance of this work. Empirical heat transfer coefficients are quantified in this study and are used in the creation of an analytical model of a counter flow heat exchanger that predicts the energy use for heating the air at different operating modes. The heat transfer coefficients are produced by measurements in an AHU-systems located in an apartment building. The AHU has an installed air heater before the heat exchanger which heats the outdoor inlet air to constant temperature.   The manufacturer of the AHU-system proclaims turbulent flow in the heat exchanger. Change of airflow has greater impact on the heat transfer coefficients during turbulent flow compared to laminar flow conditions. This is also derived from the results of the measurements. The effectiveness of the current control system is analyzed, and flaws are discovered. A new control system is therefore proposed for frost free operation and higher energy efficiency. The proposed control system is compared to a frequently used frost control system which uses bypass-dampers to redirect the inlet air to the reheater, to let the warm air stream melt formed frost on the warm air side. This comparison is accomplished by analyzing the usage of the bypass-dampers during different outdoor temperatures in a comparable AHU-system that have the mentioned frost control method. The results show that preheating of inlet air for frost free operation uses less energy than usage of the bypass-dampers to melt formed frost on the warm air side. The proposed control system needs the bypass-dampers to be used when the preheater is on full operation to prevent frost formation at extreme outdoor temperatures when the preheating may not be enough. The proposed control system is regulated by the dewpoint and the temperatures of the exhaust air stream and the supply air stream. The derived heat transfer coefficient that is used to predict the plate-temperature take no account for condensation on the warm air side during humid conditions when the dew point is above the freezing point. Future studies can derive the heat transfer coefficient during condensation which will improve the prediction of the plate-temperature. This would make the system more energy efficient during humid air conditions. This can be done by measurements in an AHU-system with high temperature efficiency placed in a humid environment.

Servo valves are used in a broad variety of flow modulation applications. In the field of aerospace, servo valves are used in aero engines to meter fuel flow. The existing valves are labour intensive to manufacture and highly optimised such that to achieve improvements in performance requires a novel design. This research investigates smart material actuators and valve concepts. Specifically, a prototype pilot stage nozzle flapper valve is developed for the purpose of actuating a main stage spool. The typical nozzle flapper type servo valve uses a torque motor to actuate the flapper. In this research project, the torque motor has been substituted for two piezoelectric ring bender actuators. A novel mounting mechanism has been developed to secure the ring benders within the valve. Analytical and finite element models have been made to understand the displacement mechanism of a ring bender and the effects of the mount on the displacement and force from a ring bender, and the results were compared with experiment. The mounting stiffness at the inner and outer edges was found to decrease the displacement of the ring bender and it was found that the stiffness of the mount at the outer edge has a greater negative effect on displacement than the stiffness of the mount at the inner edge. The displacement of a ring bender was tested across the operational temperature range of an aero engine. It was found that the displacement of the ring bender is reduced at low temperatures and increases at high temperature. The variation of stiffness of the elastomeric mount was also tested with temperature and it was found that the displacement of a ring bender is significantly reduced when the mounting elastomer approaches its glass transition temperature. A prototype valve was built to test the pressures and flows that could be achieved at two control ports by using a ring bender as actuator. A single ring bender and two ring benders, mounted in tandem to provide redundancy, have been tested. An analytical model was developed and the predictions are compared with experimental results for pressures and flow. The full stroke of the valve was 300m when mounted and reduced to 150m when mounted in tandem with an inactive ring bender. The hysteresis of the valve is +/-10%. The pressures and flow at and between the control ports of the valve are consistent with the predictions.

Les boucles diphasiques à pompage thermo-capillaire de type LHP (pour Loop Heat Pipe, en anglais), dont le fonctionnement s’apparente à celui d’un caloduc, permettent un transfert de chaleur particulièrement efficace et entièrement passif entre une source chaude et une source froide. Ce transfert s’effectue au moyen d’un fluide diphasique, mû grâce à la force motrice capillaire générée par un matériau poreux contenu dans l’évaporateur/réservoir de la LHP. Outre son rôle de barrière hydraulique entre les phases liquide et vapeur, ce matériau doit assurer une fonction de barrière thermique afin de favoriser l’évaporation du liquide. L’aptitude du matériau à remplir ses fonctions dépend étroitement de sa microstructure, elle-même liée à la méthode de fabrication. Dès lors, l’objectif de la thèse est d’associer la science des matériaux à celle de la thermique, pour améliorer les procédures de fabrication de structures capillaires existantes ou tester de nouvelles méthodes, et aboutir à des structures dont les caractéristiques sont en adéquation avec celles qui sont recherchées. / The capillary pumped loops (CPL) or loop heat pipes (LHP), whom the operating principle is similar to classic heat pipes, enable an efficient heat transfer between a hot source and a cold source without additional energy sources. Indeed, a porous structure provides a capillary force that enables a two-phase fluid to circulate around the loop, transferring the heat from the evaporator to the condenser. The porous structure acts as a hydraulic barrier between the two phases and as a thermal barrier enabling the liquid evaporation. The ability of the capillary structure to fulfil its mission depends on its microstructure, and thus on the manufacturing process. Therefore, the objective of the present thesis is to join the thermal sciences with the material sciences in order to improve the existing manufacturing procedure or even to test new ones. It aims at obtaining capillary structures corresponding to heat transfer applications.

Ce mémoire présente l’étude du comportement d’un flotteur bord à quai. Ce flotteur est constituéd’une bouée de section rectangulaire située à proximité d’une digue verticale. Un modèle analytique linéaire basé sur la théorie de l’écoulement potentiel est réalisé en profondeur d’eau intermédiaire. Un modèle numérique résolvant les équations de Navier-Stockes est développé. Des essais expérimentaux sont également mis au point pour des flotteurs de différentes dimensions et servent de références aux deux autres modèles. La comparaison de ces trois modèles permet de déduire le domaine de validité du modèle analytique, ainsi que la présence d’effets non-linéaires agissant sur le comportement du flotteur.Dans un premier temps, l’impact des dimensions du système sur le comportement du flotteur est quantifié. Les effets non-linéaires sont quantifiés en fonction des grandeurs du système puis une correction du modèle analytique est proposée. Par la suite, la correction du modèle est appliquée au comportement du système incluant un récupérateur d’énergie.Dans un second temps, l’effet du flotteur sur les franchissements de digue est étudié. Les franchissements de digue sont exprimés grâce à la hauteur de houle entre la digue et le flotteur (exprimée par le modèle analytique) et à la formule de Van Der Meer (2002). Ces franchissements sont également estimés de manière expérimentale. Il est montré que lorsque la longueur d’onde de la houle est importante et que le dégagement entre la digue et le flotteur est faible, les franchissements de digue sont réduits par la présence du flotteur. / The study of a quayside wave energy converter is proposed. A simplified model composed ofa rectangular buoy oscillating in heave motion and installed close to a vertical dike is developedusing three different models. An analytical model based on potential flow theory is developed inintermediate water depth. A numerical model resolving the Navier-Stokes equation is developed.Experimental tests are performed for different floater dimensions and are used as references for the two other models. The comparison between the three models presents the validity domain of the analytical model. The existence of non-linear effects acting on the floater behaviour is shown.At first, the impact of the system dimensions on the floater behaviour are highlighted and quantified. According to these results a correction of the analytical model is proposed. Moreover, the corrected analytical model is applied to the wave energy converter behaviour. Secondly, the overtoppings are expressed using the wave height between the dike and the floater (provided by analytical model) and the Van Der Meer formula (2002). These overtoppings are also estimated using experimental tests. The comparison of these two models shows the presence of strong load losses when the floater is close to the dike. In those cases the analytical model needs a correction taking into account these load losses. It is also shown that when at the same time the wave length is high and the toe clearance between the dike and the floater is small, the dike overtopping are reduced by the presence of the floater.

La présente thèse vise à caractériser le comportement mécanique à la ruine de poutres métalliques avec ouvertures, de formes et de tailles variables, renforcées ou non par raidisseurs. L’étude s’intéresse à des poutres de dimensions réalistes et s’appuie sur une combinaison de différentes approches : essais, modèles éléments finis et modèles analytiques. Les raidisseurs sont généralement utilisés pour corriger certaines faiblesses au niveau du comportement mécanique autour des ouvertures. Autour des ouvertures rectangulaires, ces raidisseurs, disposés verticalement ou horizontalement, ont fait l’objet de peu d’études scientifiques publiées. Généralement, le comportement des poutres à ouvertures d’âmes est considéré comme étant similaire à celui d’une poutre échelle (ou poutre Vierendeel) chargée aux noeuds. Cette hypothèse permet de considérer que les membrures de l’ouverture sont soumises à un moment fléchissant local bi-triangulaire sur la longueur de l’ouverture. Cependant, la distribution du moment fléchissant, et de ses contraintes axiales, le long d’une membrure peut s’avérer être plus complexe et dépend de la forme de l’ouverture (rectangulaire, hexagonale, circulaire, sinusoïdale ou quelconque), de la position de l’ouverture le long de la poutre et enfin, du type de chargement (concentré ou uniformément réparti). Plusieurs études numériques et expérimentales ont déjà été menées sur les poutres à ouvertures d’âmes et ont permis de développer différentes approches analytiques en vue de décrire leurs comportements. Cependant, ces modèles montrent certaines imprécisions ou insuffisances pour différents types et dimensions d’ouvertures notamment les ouvertures rectangulaires. Les travaux présentés dans cette thèse font le point sur les principaux modèles existants de calcul de résistance des poutres avec ouvertures d’âmes. Ils présentent notamment leurs avantages et limites. Pour conforter les descriptions basées sur les analyses bibliographiques, une campagne expérimentale est réalisée pour valider un modèle éléments finis développé dans le cadre de l’étude. Les essais s’appuient sur des tronçons de poutres de dimensions réalistes avec différentes configurations d’ouvertures isolées renforcées ou non par des raidisseurs. Les mesures autour de l’ouverture utilisent des jauges de déformation pour analyser le profil des contraintes et son évolution, au cours du chargement, dans différentes sections critiques. Les essais sont menés jusqu’à la ruine qui s’est développée généralement par une combinaison de comportement élasto-plastique et d’instabilité locale autour des ouvertures. Certaines ruines se sont produites par rupture en traction-cisaillement de l’acier dans des zones critiques. Les analyses numériques et expérimentales ont permis d’étudier l’influence de différentes conditions sur la distribution des contraintes, notamment axiales, au sein d’une membrure d’ouverture, en vue d’évaluer son potentiel de voilement local qui reste un phénomène complexe à appréhender. Elles ont permis aussi de comprendre le fonctionnement mécanique des ouvertures renforcées par des raidisseurs de différentes dimensions et positions. L’étude paramétrique menée, en utilisant le modèle éléments finis validé par essai, a permis d’évaluer la pertinence des hypothèses retenues dans les approches analytiques existantes. Ces analyses ont permis aussi de proposer un modèle analytique tenant compte du comportement d’ouvertures isolées avec ou sans raidisseurs. / The present thesis aims to characterize the mechanical behavior to failure of steel beams with openings, with variable shapes and sizes, reinforced or not by stiffeners. The study considers beams of realistic dimensions and combines different approaches: tests, finite element models and analytical models. The stiffeners are generally used to improve some weaknesses in the mechanical behavior around the openings. Around rectangular openings, these stiffeners, in vertical or horizontal arrangements, have been the subject of limited number of scientific publications. In general, the behavior of beams with web openings is considered similar to that of Vierendeel beams with loads applied at the nodes. This hypothesis allows to consider that each frame around the opening is characterized by a bi-triangular local bending moment along the frame elements. In fact, the bending moment distribution, and their resultant axial stresses, along a frame can be more complex and depends on the shape of the opening (rectangular, hexagonal, circular, sinusoidal or any), the position of the opening along the beam and finally, the type of loading (concentrated or uniformly distributed). Several numerical and experimental studies have been conducted on the beams with web openings and different analytical approaches have been developed to describe and predict the behavior of these beams. However, these models show some inaccuracies or inadequacies depending on the types and dimensions of openings including rectangular openings. The work presented in this thesis examines the main existing models predicting the resistance for beams with web openings. It shows their advantages and limits based on the existing but limited results. To obtain additional and specific results to be used in the comparisons and to validate a finite element model developed within the framework of the study, an experimental campaign is carried out. The tests are focused on beams of real dimensions with different configurations of isolated openings reinforced or not by stiffeners. The measurements around the opening use strain gauges to analyze the stress profile and its evolution, during loading, in different critical sections. The tests are carried out until failure generally characterized by a combination of elastic-plastic behavior and local instability around openings. Some final failures occurred by tensile-shear fracture of steel in some critical zones. Numerical and experimental analyzes are performed to study the influence of various conditions on the stresses distributions, in particular axial stresses, within the frames around the openings. The stress distribution is mainly observed to evaluate the local buckling potential that remains a complex phenomenon difficult to predict. The results are also analyzed to understand the mechanical behavior of the openings reinforced by stiffeners of different dimensions and positions. The parametric study conducted using the validated finite element model allows evaluating the relevance of the assumptions considered in the existing analytical approaches. These analyzes made it possible to propose an analytical model taking into-account the behavior of isolated openings with or without stiffeners.

Pour augmenter les performances des MOSFET, il est indispensable de comprendre les différents phénomènes physiques qui dégradent la mobilité apparente des électrons et trous traversant le canal et qui limitent l’amélioration obtenue par réduction de sa longueur. Pour cela, une étude précise du transport par des simulations Monte-Carlo a été effectuée. Cette méthode de simulation semi-classique permet de résoudre l’équation de transport de Boltzmann en prenant en compte à la fois le régime quasi-balistique, les interactions avec les phonons, les impuretés ionisées, la rugosité de surface, et le confinement quantique, par génération aléatoire des électrons et de leurs interactions, décrites selon les lois de la mécanique quantique.Un modèle simple de mobilité a alors pu être établi et validé par les simulations. Il est basé sur trois paramètres importants : la mobilité à canal long, la résistance d’accès et la résistance balistique. Ce modèle de mobilité s’est avéré compatible avec des résultats expérimentaux, ce qui suggère que la résistance d’accès est déterminante dans la réduction de mobilité apparente.Par ailleurs, la contribution du transport balistique dans la mobilité a été calculée en tenant compte précisément du confinement quantique et des fonctions de distribution des différentes sous-bandes, ce qui a ainsi permis d’améliorer le modèle de mobilité apparente de Shur qui sous-estime (d’environ 50 Ω.µm) la résistance balistique. Cette résistance balistique est inférieure à la résistance d’accès mais elle pourrait avoir une incidence sur les dispositifs ultimes. / To improve the MOSFET performances, it is necessary to understand the physical phenomena contributing to the apparent mobility of electrons and holes crossing the channel, and limiting the improvement obtained by reducing the channel length. Therefore, a precise study of transport using Monte Carlo simulations was performed. This semi-classical simulation method allows for solving the Boltzmann transport equation, taking into account the quasi-ballistic regime, phonon and Coulomb scattering, surface roughness, as well as the quantum confinement, by randomly generating electrons and their scattering events described by the laws of quantum mechanics.A simple mobility model has been established and validated by the simulations. It is based upon three important parameters: the long channel mobility, the access resistance, and ballistic resistance. This mobility model proved compatible with experimental results, suggesting that the access resistance is determining in the apparent mobility reduction.By the way, the ballistic transport contribution in the mobility was calculated by taking into account the quantum confinement accurately and the distribution functions of the different subbands, allowing for an improvement of Shur’s apparent mobility model, which underestimates (of about 50 Ω.µm) the ballistic resistance. The latter is lower than the access resistance but it could have an incidence on the ultimate devices.Keywords: MOSFET, FDSOI, mobility degradation, analytical model, contact resistance, ballistic, multi-subband Monte Carlo, simulation.

Lorsqu'un hélicoptère vol au dessus de l'eau le constructeur se doit d'anticiper l'amerrissage d'urgence (ditching) pour la sécurité de l'équipage. La solution adoptée consiste en des systèmes de flottabilités gonflables. Ces systèmes pour EUROCOPTER ont deux buts : amortir l'impact et assurer la flottabilité le temps que l'appareil soit évacué. Des normes définissent les limites du ditching en terme de vitesses d'impact et en terme d'état de mer. Lors du dimensionnement de l'appareil et des flottabilités pour l'impact des centaines de calculs sont a réitérer pour converger vers la solution optimale.Cette thèse s'inscrit dans un projet d'optimisation d'un logiciel maison voué au Ditching. Elle fait suite à celle de N.Malleron(2009) focalisée sur le cas vertical. Elle apporte des contributions à la compréhension de cette IFS complexe via différentes approches expérimentales et l'exploration de différents modèles que l'on tente de corréler avec les résultats expérimentaux. / When operating on offshore conditions, helicopters may encounteremergency situations during which an immediate sealanding isrequired. In this case, the aircraft may experience a water impact event.Floatability systems are inflated to cushion the impact and toensure that the floatation is maintained while the crew evacuates.The dimensioning procedure of helicopter's floatability systemsremains very challenging. In this process, accurate prediction of thebehavior and the global loads on the helicopters floaters during ditchingis crucial. To obtain a better understanding of the floatability system,and to be able to develop more accurate models, the hydrodynamicalcharacteristics of the floaters, as flexible bodies, need to be furtherinvestigated. Experiments assessing the hydrodynamics characteristicsof these particular types of deformable bodies are rare. To this aim, anexperimental campaign has been carried out to identify the nature ofloads (global and local) and potential deformations. Numericalinvestigations are proposed to complete our understanding ofphenomena observed during experimental tests.An academic experimental campaign have been led in the flume of Ecole Centrale de Marseille. A baloon was drop from 0.5 to 2.4m with various inner pressure. Deformations and kinematics are measured and analysed before comparisons with a semi-analytical modelisation. Some results with the in-house software DITCHER are also proposed in this thesys.

<p>Severe accidents in a Light Water Reactor (LWR) have been a subject of the research for the last three decades. The research in this area aims to further understanding of the inherent physical phenomena and reduce the uncertainties surrounding their quantification, with the ultimate goal of developing models that can be applied to safety analysis of nuclear reactors. The research is also focusing on evaluation of the proposed accident management schemes for mitigating the consequences of such accidents.</p><p>During a hypothetical severe accident, whatever the scenario, there is likelihood that the core material will be relocated and accumulated in the lower plenum in the form of a debris bed or a melt pool. Physical phenomena involved in a severe accident progression are complex. The interactions of core debris or melt with the reactor structures depend very much on the debris bed or melt pool thermal hydraulics. That is why predictions of heat transfer during melt pool formation in the reactor lower head are important for the safety assessment.</p><p>The main purpose of the present study is to advance a method for describing turbulent natural convection heat transfer of a melt pool, and to develop a computational platform for cost-effective, sufficiently-accurate numerical simulations and analyses of Core Melt-Structure-Water Interactions in the LWR lower head during a postulated severe core-melting accident.</p><p>Given the insights gained from Computational Fluid Dynamics (CFD) simulations, a physics-based model and computationally-efficient tools are developed for multi-dimensional simulations of transient thermal-hydraulic phenomena in the lower plenum of a Boiling Water Reactor (BWR) during the late phase of an in-vessel core melt progression. A model is developed for the core debris bed heat up and formation of a melt pool in the lower head of the reactor vessel, and implemented in a commercial CFD code. To describe the natural convection heat transfer inside the volumetrically decay-heated melt pool, we advanced the Effective Convectivity Conductivity Model (ECCM), which was previously developed and implemented in the MVITA code. In the present study, natural convection heat transfer is accounted for by only the Effective Convectivity Model (ECM). The heat transport and interactions are represented through an energy-conservation formulation. The ECM then enables simulations of heat transfer of a high Rayleigh melt pool in 3D large dimension geometry.</p><p>In order to describe the phase-change heat transfer associated with core debris, a temperature-based enthalpy formulation is employed in the ECM (the phase-change ECM or so called the PECM). The PECM is capable to represent possible convection heat transfer in a mushy zone. The simple approach of the PECM method allows implementing different models of the fluid velocity in a mushy zone for a non-eutectic mixture. The developed models are validated by a dual approach, i.e., against the existing experimental data and the CFD simulation results.</p><p>The ECM and PECM methods are applied to predict thermal loads to the vessel wall and Control Rod Guide Tubes (CRGTs) during core debris heat up and melting in the BWR lower plenum. Applying the ECM and PECM to simulations of reactor-scale melt pool heat transfer, the results of the ECM and PECM calculations show an apparent effectiveness of the developed methods that enables simulations of long term accident transients. It is also found that during severe accident progression, the cooling by water flowing inside the CRGTs plays a very important role in reducing the thermal load on the reactor vessel wall. The results of the CFD, ECM and PECM simulations suggest a potential of the CRGT cooling as an effective mitigative measure during a severe accident progression.</p>

Severe accidents in a Light Water Reactor (LWR) have been a subject of intense research for the last three decades. The research in this area aims to reach understanding of the inherent physical phenomena and reduce the uncertainties in their quantification, with the ultimate goal of developing models that can be applied to safety analysis of nuclear reactors, and to evaluation of the proposed accident management schemes for mitigating the consequences of severe accidents.  In a hypothetical severe accident there is likelihood that the core materials will be relocated to the lower plenum and form a decay-heated debris bed (debris cake) or a melt pool. Interactions of core debris or melt with the reactor structures depend to a large extent on the debris bed or melt pool thermal hydraulics. In case of inadequate cooling, the excessive heat would drive the structures' overheating and ablation, and hence govern the vessel failure mode and timing. In turn, threats to containment integrity associated with potential ex-vessel steam explosions and ex-vessel debris uncoolability depend on the composition, superheat, and amount of molten corium available for discharge upon the vessel failure. That is why predictions of transient melt pool heat transfer in the reactor lower head, subsequent vessel failure modes and melt characteristics upon the discharge are of paramount importance for plant safety assessment.  The main purpose of the present study is to develop a method for reliable prediction of melt pool thermal hydraulics, namely to establish a computational platform for cost-effective, sufficiently-accurate numerical simulations and analyses of core Melt-Structure-Water Interactions in the LWR lower head during a postulated severe core-melting accident. To achieve the goal, an approach to efficient use of Computational Fluid Dynamics (CFD) has been proposed to guide and support the development of models suitable for accident analysis.   The CFD method, on the one hand, is indispensable for scrutinizing flow physics, on the other hand, the validated CFD method can be used to generate necessary data for validation of the accident analysis models. Given the insights gained from the CFD study, physics-based models and computationally-efficient tools are developed for multi-dimensional simulations of transient thermal-hydraulic phenomena in the lower plenum of a LWR during the late phase of an in-vessel core melt progression. To describe natural convection heat transfer in an internally heated volume, and molten metal layer heated from below and cooled from the top (and side) walls, the Effective Convectivity Models (ECM) are developed and implemented in a commercial CFD code. The ECM uses directional heat transfer characteristic velocities to transport the heat to cooled boundaries. The heat transport and interactions are represented through an energy-conservation formulation. The ECM then enables 3D heat transfer simulations of a homogeneous (and stratified) melt pool formed in the LWR lower head. In order to describe phase-change heat transfer associated with core debris or binary mixture (e.g. in a molten metal layer), a temperature-based enthalpy formulation is employed in the Phase-change ECM (so called the PECM). The PECM is capable to represent natural convection heat transfer in a mushy zone. Simple formulation of the PECM method allows implementing different models of mushy zone heat transfer for non-eutectic mixtures. For a non-eutectic binary mixture, compositional convection associated with concentration gradients can be taken into account. The developed models are validated against both existing experimental data and the CFD-generated data. ECM and PECM simulations show a superior computational efficiency compared to the CFD simulation method. The ECM and PECM methods are applied to predict thermal loads imposed on the vessel wall and Control Rod Guide Tubes (CRGTs) during core debris heatup and melting in a Boiling Water Reactor (BWR) lower plenum. It is found that during the accident progression, the CRGT cooling plays a very important role in reducing the thermal loads on the reactor vessel wall. Results of the ECM and PECM simulations suggest a high potential of the CRGT cooling to be an effective measure for severe accident management in BWRs. / <p>QC 20100812</p>

In both the chemical and mechanical pulping process, the logs are cut into wood chips by a disc chipper before fibre separation. To make the wood chipping process more efficient, one have to investigate in detail the coupling between process parameters and the quality of the chips. One objective of this thesis was to obtain an understanding of the fundamental mechanisms behind the creation of wood chips. Another objective with the thesis was to investigate whether it was possible to, in a way tailor the chipping process so as to reduce the energy consumption in a following mechanical refining process. Both experimental and analytical/numerical approaches have been taken in this work. The first part of the experimental investigations, were performed with an in-house developed chipping device and a digital speckle photography equipment. The results from the experimental investigation showed that the friction between the log and chipping tool is probably one crucial factor for the chip formation. Further more it was found that the indentation process is approximately self-similar, and that the stress field over the entire crack-plane is critical for chip creation. The developed analytical model predicts the normal and shear strain distribution and to be more specific, the model can predict the compressive stresses parallel to the fibre direction for an assumed linear elastic and orthotropic material. The analytical distributions were found to be in reasonable agreement with the corresponding distributions obtained from a finite element analysis. To be able to study the chipping process under realistic conditions, which for example means to use chipping rates representative for a real wood chipper, a laboratory chipper was developed. Details regarding the chipper and how to evaluate the force measurements are given together with an example of how the force on the cutting tool (the knife) varies with time during cutting. To investigate the influence of a certain chipping process parameter, the chips were after production in the laboratory chipper, refined in a pilot refiner during conditions optimized for TMP (thermomechanical pulp) and CTMP (chemithermomechanical pulp) processes. It was concluded that the details concerning the chip process had a large impact on e.g. the energy consumption in both first stage and second stage refining. Results showing this are given in this thesis. / För både kemisk och mekanisk pappersmassa så tillverkas flis av trädstockar med hjälp av en skivhugg innan fibrerna separeras. För att göra flisningsprocessen mer effektiv, måste kopplingen mellan processparametrar och fliskvalitet studeras. Ett mål med denna avhandling är att ge fundamental kunskap om mekanismerna bakom bildandet av träflis. Både experimentella och analytiska/numeriska metoder har använts i detta arbete. De experimentella undersökningarna har gjorts med hjälp av egen utvecklad utrustning. Resultaten från den experimentella undersökningen visar att friktionen mellan stammen och flisningsverktyget har betydelse vid flisning. Vidare observerades det att inträngnings processen är approximativt självlik (self similar) och att det är spänningsfältet över hela sprickplanet som är kritiskt för bildandet av en flis. Den utvecklade analytiska modellen förutsäger normal- och skjuvspänningsfördelningen över sprickplanet och kan mer specifikt förutsäga den kompressiva belastning som verkar parallellt fiberriktningen i ett linjärt elastiskt och ortotropt material (trä). De analytiskt bestämda fördelningarna stämmer relativt väl överens med motsvarande fördelningar beräknad med finit element analys. För att kunna studera flisningsprocessen under realistiska förhållanden, vilket bl.a. betyder att skärhastigheter som är representativa för en verklig process skall användas, så utvecklades inom ramen för avhandlingsarbetet, en laboratoriesflishugg. Detaljer rörande flishuggen samt hur uppmätta lastsignaler skall utvärderas ges tillsammans med ett exmpel på hur kraften på skärverktyget (kniven) varierar under ett skärförlopp. Inverkan av en viss flisningsprocessparameter undersöktes genom att flis tillverkades i laboratorieflishuggen varefter de raffinerades i en pilotraffinör under förhållanden som var optimerade för TMP (termomekanisk massa) och CTMP (kemitermomekanisk massa) processerna. Det konstaterades att detaljer i flisningsprocessen hade stor inverkan på t.ex. energiåtgången i både första stegs – och andrastegsraffinering. Resultat som verifierar detta ges i avhandlingen. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Submitted. Paper 5: Submitted.</p>