Standardmäßig erfolgt die Datenaufbereitung der Neutronenwirkungsquerschnitte für Reaktorkernrechnungen mit 2D-Zellcodes. Ziel dieser Arbeit war es, einen 3D-Zellcode zu entwickeln, mit diesem Code 3D-Effekte zu untersuchen und die Notwendigkeit einer 3D-Datenaufbereitung der Neutronenwirkungsquerschnitte zu bewerten. Zur Berechnung des Neutronentransports wurde die Methode der Erststoßwahrscheinlichkeiten, die mit der Ray-Tracing-Methode berechnet werden, gewählt. Die mathematischen Algorithmen wurden in den 2D/3D-Zellcode TransRay umgesetzt. Für den Geometrieteil des Programms wurde das Geometriemodul eines Monte-Carlo-Codes genutzt. Das Ray-Tracing wurde auf Grund der hohen Rechenzeiten parallelisiert. Das Programm TransRay wurde an 2D-Testaufgaben verifiziert. Für einen Druckwasser-Referenzreaktor wurden folgende 3D-Probleme untersucht: Ein teilweise eingetauchter Regelstab und Void (bzw. Moderator mit geringerer Dichte) um einen Brennstab als Modell einer Dampfblase. Alle Probleme wurden zum Vergleich auch mit den Programmen HELIOS (2D) und MCNP (3D) nachgerechnet. Die Abhängigkeit des Multiplikationsfaktors und der gemittelten Zweigruppenquerschnitte von der Eintauchtiefe des Regelstabes bzw. von der Höhe der Dampfblase wurden untersucht. Die 3D berechneten Zweigruppenquerschnitte wurden mit drei üblichen Näherungen verglichen: linearer Interpolation, Interpolation mit Flusswichtung und Homogenisierung. Am 3D-Problem des Regelstabes zeigte sich, dass die Interpolation mit Flusswichtung eine gute Näherung ist. Demnach ist hier eine 3D-Datenaufbereitung nicht notwendig. Beim Testfall des einzelnen Brennstabs, der von Void (bzw. Moderator geringerer Dichte) umgeben ist, erwiesen sich die drei Näherungen für die Zweigruppenquerschnitte als unzureichend. Demnach ist eine 3D-Datenaufbereitung notwendig. Die einzelne Brennstabzelle mit Void kann als der Grenzfall eines Reaktors angesehen werden, in dem sich eine Phasengrenzfläche herausgebildet hat.

<div>With the rapid development of the advanced two-phase flow experimental technologies, more experimental databases with extended measurement ranges have been established to support the two-phase flow model development. The advantage of the Two Fluid model in modeling the complex two-phase flow phenomena over the mixture models stands out. One key aspect in the Two Fluid model development is the accurate modeling of the interfacial area between phases, which is strongly related to the interfacial mass, momentum, and energy transfer. As a closure relation of interfacial area concentration (interfacial area per unit volume) for the Two Fluid model, the Interfacial Area Transport Equation (IATE) provides dynamic predictions on the interfacial area change. It substantially solves the shortcoming of using flow-regime-dependent empirical correlations that can introduce numerical discontinuities between flow regimes. </div><div><br></div><div>The IATE has been extensively developed over the past twenty-five years. Many studies targeted on improving its prediction capability by developing bubble interaction source terms based on their experimental data. </div><div>The existing models are usually based on medium and large flow channels, yet the models may not be physically fit the small flow channels. The major reason is that the wall effect can have a larger influence on the two-phase flow in a small flow channel, as the surface area to volume ratio greatly increases. Therefore, the primary objectives of this study are to physically investigate the wall effect on two-phase flow and develop a generalized IATE by extending the application range of existing IATE from large and medium flow channels to small flow channel.</div><div><br></div><div>To achieve the objective, this study established a rigorous database of air-water two-phase flows in a small diameter pipe with its inner diameter of 12.7 mm, focusing on the bubbly-to-slug transition regime. The experimental analysis was performed on the pipe wall effect on the interfacial characteristics, based on the current experimental database and the existing experimental database collected on vertical pipes of different sizes. It is observed that 1) the pipe wall effect can alter the non-uniform radial two-phase distribution; 2) the bubbly-to-slug flow regime transition in a small diameter pipe happens in a smaller void fraction than in a large diameter pipe; 3) the bubble coalescence phenomenon can be more dominant for small pipe flow, and an intensive intergroup transfer can happen for the two-group interfacial area transport in two-phase flows. </div><div>As the interfacial area transport is directly related to the two-phase geometrical configuration, the two-phase geometrical parameters, void fraction and relative bubble size, are identified as the key parameters for modeling.</div><div><br></div><div>In the modeling of IATE source terms, the high geometrical scalability of the model is realized by properly including the wall effect into the modeling consideration. The following major improvements on the existing models are: 1) the inertia subrange assumption on the turbulent-driven interaction is properly improved; 2) the bubble-induced turbulent-driven interactions such as wake entrainment is revised by considering the wall effect on the wake region. In summary, models of bubble interaction due to random collision, wake entrainment, turbulent impact, and shearing-off are revised based on the existing studies on the IATE source terms development. The newly proposed interfacial area transport models are evaluated against an experimental database with 112 test conditions in total from a wide range of experimental pipe diameters from 12.7 mm to 304.8 mm. The new models can accurately capture the drastic intergroup transfer of void fraction and interfacial area concentration between two groups in transition flows. Overall, the relative error of void fraction and interfacial area concentration comparing with the experimental data are within ±15\% and ±10\%, respectively.</div>

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第21887号 / エネ博第388号 / 新制||エネ||75(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 齊藤 泰司, 教授 横峯 健彦, 准教授 伊藤 啓 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

In order to resolve the Al void formation originated from the severe stress issues in dynamic random access memory (DRAM), double-deposited-aluminum (DDA) layer process was proposed. This novel metallization process can be effectively and simply performed with the native oxide such as Al 2 O 3 between upper and lower Al metal layer by ex-situ deposition technique. We could effectively control the Al void by adapting the DDA layers with different grain structure. From this novel metallization process, we have confirmed the optimal thickness of Al barrier metal to 100Å to be free from Al voids, which makes it possible to improve the static refresh characteristics of DRAM by 17%.

Resin infusion of composite parts has continually been reaching to achieve laminate quality equal to, or exceeding, the quality produced with prepreg in an autoclave. In order for this to occur, developers must understand the key process variables that go in to producing a laminate with minimal void content. The purpose of this research is to continue efforts in understanding 1) the effect of process conditions on the resultant void content, with a focus on resin infusion flow rate, 2) applying statistical metrics to the formation, location and size of voids formed, and 3) correlate these metrics with the local mechanical properties of the composite laminate. The variation in dispersion and formation of micro-voids and macro-voids varied greatly between the rates of flow the infusion occurred, especially in the non-crimp carbon fiber samples. Higher flow rates led to lower volumes of micro-voids in the beginning section of the carbon fiber laminates with macro-voids being introduced approximately half-way through infusion. This was determined to have occurred decreasing pressure gradient as the flow front moved away from the inlet. This variation in void content per location on the laminate was more evident in the carbon fiber samples than the fiberglass samples. Micro-voids follow void formation modeling especially when coupled with a pressure threshold model. Macro-void formation was also demonstrated to correlate strongly to void formation models when united with void mobility theories and pressure thresholds. There is a quick decrease in mechanical properties after the first 1-2% of voids signaling strength is mostly sensitive to the first 0-2% void content. A slight decrease in SBS was noticed in fiberglass laminates, A-F as v0 increased but not as drastically as represented in the NCF laminates, G and H. The lower clarity in the exponential trend could be due to the lack of samples with v0 greater than 0% but less than 1%. Strength is not well correlated to void content above 2% and could possibly be related to void morphololgy.

No description available.

<p>We propose a physically based model that describes the density and size of voids in silicon introduced via high dose helium ion implantation and subsequent annealing. The model takes into account interactions between vacancies, interstitials, small vacancy clusters, and voids. Void evolution in silicon occurs mainly by a migration and coalescence process. Various factors such as implantation energy and dose, anneal temperature, atmospheric pressure, and impurity level in silicon can influence the migration and coalescence mechanism and thus play a role in the void evolution process. Values for model parameters are consistent with known values for point defect parameters and assumed diffusion limited reaction rates. A single “fitting parameter” represents the rate of bubble migration and coalescence and is therefore related to surface diffusion of adatoms. Results obtained from simulations based upon the model were compared to our experimental results and to previously reported experimental results obtained over a wide range of conditions.</p> <p>Our own experiments involved the implantation of silicon samples and samples with a thin Si_1-xGe_x (x = 0.05, 0.09) epilayer on silicon with 30 keV, 5×10¹⁶ cm^-2 helium. Anneals were done in the range 960-1110°C for 15-30 minutes in nitrogen and dry oxygen. Void size distributions were measured from transmission electron microscopy images. Average void diameter and void density values and void size distribution did not show any significant differences between the samples annealed in nitrogen and dry oxygen. However, the presence of Si_1-xGe_x epilayer on silicon resulted in increased average void diameter and reduced average void density when compared with Si samples as well as more selective void size distribution.</p> <p>Data from the literature included experiments with helium ion implantation energies in the range 30 - 300 keV, doses of 1×10¹⁶ - 1×10¹⁷ cm^-2, subsequent annealing temperatures in the range 700 - 1200°C, and annealing duration in the range 15 minutes - 2 hours. Excellent agreement is found between the simulated results and those from reported experiments. The extracted migration and coalescence rate parameter shows an activation energy consistent with surface diffusivity of silicon. It shows a linear dependence on helium dose, and increases with decreased implantation energy, decreased ambient pressure, decreased substrate impurities, increased temperature ramp rate, or increased Ge fraction in cavity layer, all consistent with the proposed physical mechanism. Our mathematical model specifically ignores the long time saturation in void size, although we propose a simple explanation consistent with the physical picture. Similarly, we give physical reasons for a threshold implant dose resulting in the formation of small vacancy clusters during implant. But in modeling void growth we simply show that when such clusters exist voids will evolve according to our model.</p> <p>In our experiments, the presence of a Si_0.95Ge_0.05 epilayer on silicon resulted in retarded B diffusion when compared with Si samples. This phenomenon is correlated to the role of the Si_0.95Ge_0.05 epilayer on silicon in the void evolution mechanism and both are attributed to Ge interdiffusing from the epilayer into the Si bulk. The B diffusion data also allows us to predict conditions for the SiGe epilayer to modify the injection of interstitials from surface during dry oxidizing anneal.</p> / Doctor of Philosophy (PhD)

Fluidized beds have widespread industrial applications ranging from chemical industries to power plants. The flow inside a fluidized bed system consists of two main phases, a particle phase and the fluid phase. The two phases are strongly coupled to each other through various forces like drag and pressure. Capturing this multiphase phenomenon requires modeling strategies that possess good fidelity over a range of scales. Discrete Element Modeling (DEM) coupled with Computational Fluid Dynamics (CFD) provides a good platform to analyze the complex coupled multiphase hydrodynamics inside fluidized bed systems. Conventional DEM-CFD framework suffers from contradictory spatial resolution requirements for the particle and fluid phases, respectively. This prevents the conventional DEM-CFD method to be applied to geometries that have features comparable to the particle diameter of the solid phase. The novelty of this work lies in the development and validation of a two-grid formulation that removes the resolution restrictions of the conventional DEM-CFD framework. The results obtained from this new framework agree reasonably well with the experiments showing the capability of the new scheme to simulate conditions not possible with conventional DEM-CFD framework. In addition, this research also focuses on performing both 2D and 3D jetting fluidized bed simulations having millions of particles; validate/compare results with experiments and to perform heat transfer studies in a jetting fluidized bed system. The results suggest convective and diffusive mixing for a single jet at higher superficial velocity to be better than the mixing obtained in a multiple jet framework. The comparison with experimental results obtained in a multiple jetting setup shows that a 2D simulation captures the essential jet characteristics near the distributor plate reasonably well while a 3D simulation is needed to capture proper bubble dynamics near the freeboard of the bed. These results give insight into the detailed dynamics of fluidized bed systems and provide a foundation for a better design of these systems. / Ph. D.

The rate at which stars have formed throughout the history of theuniverse is not constant, it started out slow, increased until around redshift ∼ 2 when it reversed and became slower again. The reason for this behaviour is still being investigated with various models and simulations usually based upon dark matter halos. The aim of this study is to instead investigate whether there is a correlation between the cosmic star formation rate and the evolution of cosmic voids. This is achieved by comparing the total mass flow from voids with the amount of matter forming stars. A simple model of void mass flow is created and compared with observational data of star formation. The model is shown to exhibit the same behaviour as the star formation rate indicating that there is indeed a correlation between void evolution and star formation. This suggests it to be fruitful to create a more involved, alternative model of star formation based upon void evolution as opposed to the common halo evolution / Hur snabbt stjärnor bildas har genom universums historia förändrats över tid, det började långsamt och ökade sedan fram till rödförskutning ∼ 2 då trenden vände och saktade ner igen. Förklaringen till detta beteende utforskas fortfarande genom diverse modeller och simularingar som vanligtvis bygger på mörk materia halos. Syftet med detta arbete är att istället undersöka ifall det finns en korrelation mellan tomrumsutveckling och den kosmiska stjärnbildningen. Detta åstadkoms genom att jämföra det totala massflödet från tomrum med den massa som bildar stjärnorna. En simpel model för tomrumsutveckling skapas och jämförs med observationell data för stjärnbildningshastighet. Denna modell visar samma beteende som stjärnbildningen och tyder på att det finns en korrelation mellan denna och tommrumsutveckling. Som slutsats pekar denna studie på att det kan vara fruktbart att utveckla en mer anancerad modell för den kosmiska stjärnbildningen som bygger på tomrumsutveckling istället för mörk materia halos.

Thesis (MA)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Somalia continues to intrigue scholars and policy-makers around the world, due to the fact that it is enduring what few others have, total state collapse. Not only does the situation defy easy explanation, the degree of state failure is unprecedented. After more than two decades without a functioning central authority, Somalia is now the longest-running instance of state collapse in postcolonial history. While researching and understanding state weakness and state failure is critical, it is useless unless combined with devising ways to prevent state failure in the future. Somalia provides policy makers with a unique opportunity to study the consequences of state collapse. Understanding the complex dynamics of state weakness and state collapse could ultimately help save the lives of thousands of people on the African continent. This study focuses on the mechanisms of coping by analysing the actors who have stepped up to fill the political vacuum left behind by the collapsing state. The role played by both state and non-state actors will be explored throughout this study. As it is not possible to address every actor who has played a part in Somalia since the implosion of the state in 1991, only the three most important internal and external actors will be analysed. The role played by the United Nations (UN) and the African Union (AU) will be discussed at length. The contributions made by neighbouring country Ethiopia, will also be investigated. Although the contributions and the impact of these external actors have in some instances led to undesired results, it does not change the fact that these actors attempted to fill the void in Somalia. Non-state actors positions within Somalia will also be examined in detailed. These actors include regional authorities, the role of Al-Shabaab, as well as the Islamic Courts Union. The study does not excuse the behaviour of these actors but rather discusses the rise of these organizations in light of the collapsing state and the position which they have taken up in Somalia to fill the political void. Lastly the role of the “Somali coast guards” also known as the Somali pirates, will be discussed.ipti / AFRIKAANSE OPSOMMING: Somalië bly die brandpunt van belangstelling vir geleerdes en beleidmakers dwarsoor die wêreld. Die hoofrede hiervoor is dat hierdie land ervaar het wat min ander lande het, naamlik algehele ineenstorting van die staat en bestuur. ‘n Maklike verduideliking of vereenvoudiging van die situasie is daar nie, want die ongeëwenaarde ineenstorting van stuktuur en staat, van Somalië is uniek. Twee dekades later is hierdie land nogsteeds sonder ‘n funksioneerende gesetelde staatsgesag. Hierdie tydperk is die langste voorbeeld van staatsineenstorting sonder herstel in post-koloniale geskiedenis. Navorsing en insig in elke staat se inherente swakhede en probleme help om mislukte bestuur te voorspel en te voorkom. Hierdie inligting moet korrek geimplimenteer en geinterpreteer word om krissise en mislukkings binne staatsbestel te verhoed. Somalië bied beleidmakers ‘n unieke geleentheid om die gevolge van totale staatsineenstorting te bestudeer en daaruit te leer. Net deur middel van begrip vir die komplekse dinamika van ‘n staat wat ineenstort en deur wanbestuur verswak word, sal daar uikoms vir duisende mense op hierdie vasteland kom. Sodoende sal ontelbare lewens in Afrika gered kan word. Hierdie studie analiseer die tegnieke van aanpassing, gebruik deur die rolspelers wat die politieke vakuum vul, wat deur die staatineenstorting nagelaat is. Die ondersoek fokus ook op die impak en effekte van beide interne en eksterne rolspelers. Aangesien dit onmoontlik is om die aandeel wat elke rolspeler sedert die ineenstorting van Somalië in 1991 gehad het te bestudeer, word net die drie belangrikste interne en eksterne rolspelers bespreek. Die Verenigde Nasies (VN), Afrika Unie (AU) en die buurland Ethiopië se bydraes sal ondersoek word. Alhoewel die bydraes en insette van hierdie eksterne rolspelers soms tot ongewenste resultate gelei het, bly die feit staan dat hulle probeer het om die leemtes in Somalië te vul. Ander belangrike rolspelers wat nie deel van die plaaslike regering is nie, sal ook in diepte bespreek word. Hierdie invloede binne Somalië sluit streeksowerhede in, asook die rol van Al Shabaab en die Islamitiese Howe Unie. Die studie verskoon nie die gedrag van die rolspelers nie, maar bespreek eerder die opkoms van hierdie organisasies. Dit word alles gesien in die lig van die ineenstorting van die staat en die posisie wat hulle in Somalië geneem het om mag te bekom en die politieke leemte te vul. Ten slotte word die omstrede rol van die "Somaliese kuswagte", anders bekend as Somaliese seerowers, bespreek.