Fuel cells typically produce low-voltage high-current output because their individual cell voltage is low, and it is nontrivial to balance for a high-voltage stack. In addition, the output voltage of fuel cells varies depending on load conditions. Due to the variable low voltage output, the energy produced by fuel cells typically requires power conditioning systems to transform the unregulated source energy into more useful energy format. When evaluating power conditioning systems, efficiency and reliability are critical. The power conditioning systems should be efficient in order to prevent excess waste of energy. Since loss is dissipated as heat, efficiency directly affects system reliability as well. High temperatures negatively affect system reliability. Components are much more likely to fail at high temperatures. In order to obtain excellent efficiency and system reliability, low-voltage high-current power conditioning systems should be carefully designed. Low-voltage high-current systems require carefully designed PCB layouts and bus bars. The bus bar and PCB trace lengths should be minimized. Therefore, each needs to be designed with the other in mind. Excessive PCB and bus bar lengths can introduce parasitic inductances and resistances which are detrimental to system performance. In addition, thermal management is critical. High power systems must have sufficient cooling in order to maintain reliable operation. Many sources of loss exist for converters. For low-voltage high-current systems, conduction loss and switching loss may be significant. Other potential non-trivial sources of loss include magnetic losses, copper losses, contact and termination losses, skin effect losses, snubber losses, capacitor equivalent series resistance (ESR) losses, and body diode related losses. Many of the losses can be avoided by carefully designing the system. Therefore, in order to optimize efficiency, the designer should be aware of which components contribute significant amounts of loss. Loss analysis may be performed in order to determine the various sources of loss. The system efficiency can be improved by optimizing components that contribute the most loss. This thesis surveys some potential topologies suitable for low-voltage high-current systems. One low-voltage high-current system in particular is analyzed in detail. The system is called the V6, which consists of six phase legs, and is arranged as a three full-bridge phase-shift modulated converter to step-up voltage for distributed generation applications. The V6 converter has current handling requirements of up to 120A. Basic operation and performance is analyzed for the V6 converter. The loss within the V6 converter is modeled and efficiency is estimated. Calculations are compared with experimental results. Efficiency improvement through parasitic loss reduction is proposed by analyzing the losses of the V6 converter. Substantial power savings are confirmed with prototypes and experimental results. Loss analysis is utilized in order to obtain high efficiency with the V6 converter. Considerations for greater current levels of up to 400A are also discussed. The greater current handling requirements create additional system issues. When considering such high current levels, parallel devices or modules are required. Power stage design, layout, and bus bar issues due to the high current nature of the system are discussed. / Master of Science

No description available.

The oil and gas industry need for high performing and low cost multiphase meters is ever more justified given the rapid depletion of conventional oil reserves. This has led oil companies to develop smaller/marginal fields and reservoirs in remote locations and deep offshore, thereby placing great demands for compact and more cost effective soluti8ons of on-line continuous multiphase flow measurement. The pattern recognition approach for clamp-on multiphase measurement employed in this research study provides one means for meeting this need. Cont/d.

This thesis presents an investigation on multiphase reactor hydrodynamics using magnetic resonance imaging (MRI). The study demonstrates experimental techniques by which computational and quasi-analytical fluid models may be validated. Three types of industrially-important multiphase reaction vessels are considered: a co-current upflow gas-liquid-solid bed, a co-current downward trickle bed (gas, liquid, solid), and a gas-solid fluidised bed. These reactors were selected as they commonly demonstrate local hydrodynamic anisotropy which affects the global performance of industrial units. MRI was used to obtain 2D velocity images of the gas and liquid phases in the packed beds, and of the gas and the solid phases in the fluidised bed. This study reports the first spatially resolved velocity measurements of both the gas and liquid phases in a co-current upflow bed, and the gas and solid phases of an isolated bubble in a fluidised bed. The experimental vessels were: 52 mm in diameter using 5 mm glass spheres in the upflow bed at 8 bara, 27 mm with 5 mm glass spheres in the trickle bed at 6.75 bara, and 52 mm using 1.2 mm poppy seeds as the fluidised particles at 8.5 bara. The experiments were conducted at a laboratory temperature of 25.0 ± 3.0 °C. In the upflow bed, time-averaged velocity images were acquired over a 2.5 h experimental time. This was done to capture the steady state behaviour of the vessel operating in the pulsing flow regime. The temporally-stable trickle flow state in the trickle bed was imaged over 15-100 minutes. In both packed beds, severe spatial anisotropy in the distribution of flow between pores was revealed. Furthermore, the data were used to determine classical design features such as catalyst wetting and liquid holdup which compared well with literature models. The trickle bed data were further analysed using a morphological algorithm which unambiguously identified the gas-liquid and liquid-solid interfaces. The interfacial flow fields were found to be similar to the bulk flow, with most voxels exhibiting static behaviour. The amount of interaction between the phases was found to be minimal, which is typical of the low interaction regime. A single bubble injection system was employed in the fluidised bed which allowed the injection of isolated bubbles into the incipiently fluidised bed. It also enabled the triggered acquisition of NMR data at precise time intervals. The bubble was found to be an indented ellipsoidal shape, which rose with atypical behaviour which caused it to collapse. Rise velocity was found to be consistent with theory, and the injected bubbles were sufficiently spatially reproducible to acquire 2D velocity images using single-point imaging. These velocity images showed flow behaviour characteristic of a 'fast' rising bubble, with a gas recirculation cloud 37 mm in diameter. The particle field was shown to have very high flow in the bubble wake, revealing the mechanism of bubble collapse. The flow data were compared to classical two-phase fluidisation theory, which revealed noteworthy differences in the division of flow between the particulate and bubbling regions.

Cette thèse est centrée sur le couplage entre les phénomènes d’écoulement, d’ablation et de rayonnement au voisinage du point d’arrêt de véhicules d’entrée atmosphérique pourvus d’un système de protection thermique de type carbonephénolique. La recherche est divisée en trois parties : 1) le développement de méthodes numériques et d’outils pour la simulation d’écoulements hypersoniques hors équilibre autour de corps émoussés, 2) la mise en oeuvre d’un nouveau modèle de transport du rayonnement hors équilibre dans ces écoulements, y compris dans les couches limites contaminées par les produits d’ablation, et 3) l’application de ces outils à des conditions réelles de vol.Les effets du couplage entre l’ablation et le rayonnement sont étudiés pour les rentrées terrestres. Il est démontré que les produits d’ablation dans la couche limite peuvent augmenter le blocage radiatif à la surface du véhicule. Pour les conditions de flux maximum d’Apollo 4, les effets de couplage entre le rayonnement et l’ablation réduisent le flux conductif de 35%. L’accord avec les données radiométriques est excellent, ce qui valide partiellement la méthode de couplage et la base de données radiatives. L’importance d’une modélisation précise du soufflage du carbone dans la couche limite est également établie. / This thesis focuses on the coupling between flow, ablation, and radiation phenomena encountered in the stagnation region of atmospheric entry vehicles with carbon-phenolic thermal protection systems. The research is divided into three parts : 1) development of numerical methods and tools for the simulation of hypersonic, non equilibrium flows over blunt bodies, 2) implementation of a new radiation transport model for calculating nonequilibrium radiative heat transfer in atmospheric entry flows, including ablation contominated boundary layers, and 3) application of these tools to study real flight conditions.The effects of coupled ablation and radiation are studied for Earth entries. It’s shown that ablation products in the boundary layer can increase the radiation blockage to the surface of the vehicle. An analysis of the Apollo 4 peak heating condition shows coupled radiation and ablation effects reduce the conducted heat flux by as much as 35% for a fixed wall temperature of 2500 K. Comparison with the radiometer data shows excellent agreement, partially validating the coupling methodology and radiation database. The importance of accurately modeling the amount of carbon blown into the boundary layer is demonstrated by contrasting the results of other researchers.

The oil and gas industry need for high performing and low cost multiphase meters is ever more justified given the rapid depletion of conventional oil reserves. This has led oil companies to develop smaller/marginal fields and reservoirs in remote locations and deep offshore, thereby placing great demands for compact and more cost effective soluti8ons of on-line continuous multiphase flow measurement. The pattern recognition approach for clamp-on multiphase measurement employed in this research study provides one means for meeting this need. Cont/d.

The present study addresses an optimization strategy for maximizing the structural ductility of Fiber Reinforced Concrete (FRC) with long textile fibers. Due to material brittleness of both concrete and fiber in addition to complex interfacial behavior between above constituents the structural response of FRC is highly nonlinear. Consideration of this material nonlinearity including interface is mandatory to deal with this kind of composite. In the present contribution three kinds of optimization strategies based on a damage formulation are described. The performance of the proposed method is demonstrated by a series of numerical examples; it is verified that the ductility can be substantially improved.

Die chemische Zusammensetzung und die physikalischen Eigenschaften von troposphärischen Gasen, Partikeln und Wolken hängen aufgrund zahlreicher Prozesse stark voneinander ab. Insbesondere chemische Multiphasenprozesse in Wolken können die physiko-chemischen Eigenschaften der Luft und troposphärischer Partikel klein- und großräumig verändern. Diese chemische Prozessierung des troposphärischen Aerosols innerhalb von Wolken beeinflusst die chemischen Umwandlungen in der Atmosphäre, die Bildung von Wolken, deren Ausdehnung und Lebensdauer, sowie die Transmissivität von einfallender und ausgehender Strahlung durch die Atmosphäre. Damit sind wolken-chemische Prozesse relevant für das Klima auf der Erde und für verschiedene Umweltaspekte. Daher ist ein umfassendes Verständnis dieser Prozesse wichtig. Die explizite Behandlung chemischer Reaktionen in der Flüssigphase stellt allerdings eine Herausforderung für atmosphärische Computermodelle dar. Detaillierte Beschreibungen der Flüssigphasenchemie werden deshalb häufig nur für Boxmodelle verwendet. Regionale Chemie-Transport-Modelle und Klimamodelle berücksichtigen diese Prozesse meist nur mit vereinfachten chemischen Mechanismen oder Parametrisierungen. Die vorliegende Arbeit hat zum Ziel, den Einfluss der chemischer Mehrphasenprozesse innerhalb von Wolken auf den Verbleib relevanter Spurengase und Partikelbestandteile mit Hilfe des state‑of‑the‑art 3D-Chemie-Transport-Modells COSMO-MUSCAT zu untersuchen. Zu diesem Zweck wurde das Model um eine detaillierte Beschreibung chemischer Prozesse in der Flüssigphase erweitert. Zusätzlich wurde das bestehende Depositionsschema verbessert, um auch die Deposition von Nebeltropfen zu berücksichtigen. Die durchgeführten Modellerweiterungen ermöglichen eine bessere Beschreibung des troposphärischen Multiphasensystems. Das erweiterte Modellsystem wurde sowohl für künstliche 2D-Bergüberströmungsszenarien als auch für reale 3D-Simulationen angewendet. Mittels Prozess- und Sensitivitätsstudien wurde der Einfluss (i) des Detailgrades der verwendeten Mechanismen zur Beschreibung der Flüssigphasenchemie, (ii) der Größenauflösung des Tropfenspektrums und (iii) der Tropfenanzahl auf die chemischen Modellergebnisse untersucht. Die Studien belegen, dass die Auswirkungen der Wolkenchemie aufgrund ihres signifikanten Einflusses auf die Oxidationskapazität in der Gas- und Flüssigphase, die Bildung von organischer und anorganischer Partikelmasse sowie die Azidität der Wolkentropfen und Partikel in regionalen Chemie-Transport-Modellen berücksichtigt werden sollten. Im Vergleich zu einer vereinfachten Beschreibung der Wolkenchemie führt die Verwendung des detaillierten chemischen Flüssigphasenmechanismus C3.0RED zu verringerten Konzentrationen wichtiger Oxidantien in der Gasphase, einer höheren Nitratmasse in der Nacht, geringeren nächtlichen pH-Werten und einer veränderten Sulfatbildung. Darüber hinaus ermöglicht eine detaillierte Wolkenchemie erst Untersuchungen zur Bildung sekundärer organischer Partikelmasse in der Flüssigphase. Die größenaufgelöste Behandlung der Flüssigphasenchemie hatte nur geringen Einfluss auf die chemischen Modellergebnisse. Schließlich wurde das erweiterte Modell für Fallstudien zur Feldmesskampagne HCCT‑2010 genutzt. Zum ersten Mal wurde dabei ein chemischer Mechanismus mit der Komplexität von C3.0RED verwendet. Die räumlichen Effekte realer Wolken z. B. auf troposphärische Oxidantien oder die Bildung anorganischer Masse wurden untersucht. Der Vergleich der Modellergebnisse mit verfügbaren Messungen hat viele Übereinstimmungen aber auch interessante Unterschiede aufgezeigt, die weiter untersucht werden müssen. / In the troposphere, a vast number of interactions between gases, particles, and clouds affect their physico-chemical properties, which, therefore, highly depend on each other. Particularly, multiphase chemical processes within clouds can alter the physico-chemical properties of the gas and the particle phase from the local to the global scale. This cloud processing of the tropospheric aerosol may, therefore, affect chemical conversions in the atmosphere, the formation, extent, and lifetime of clouds, as well as the interaction of particles and clouds with incoming and outgoing radiation. Considering the relevance of these processes for Earth\'s climate and many environmental issues, a detailed understanding of the chemical processes within clouds is important. However, the treatment of aqueous phase chemical reactions in numerical models in a comprehensive and explicit manner is challenging. Therefore, detailed descriptions of aqueous chemistry are only available in box models, whereas regional chemistry transport and climate models usually treat cloud chemical processes by means of rather simplified chemical mechanisms or parameterizations. The present work aims at characterizing the influence of chemical cloud processing of the tropospheric aerosol on the fate of relevant gaseous and particulate aerosol constituents using the state-of-the-art 3‑D chemistry transport model (CTM) COSMO‑MUSCAT. For this purpose, the model was enhanced by a detailed description of aqueous phase chemical processes. In addition, the deposition schemes were improved in order to account for the deposition of cloud droplets of ground layer clouds and fogs. The conducted model enhancements provide a better insight in the tropospheric multiphase system. The extended model system was applied for an artificial mountain streaming scenario as well as for real 3‑D case studies. Process and sensitivity studies were conducted investigating the influence of (i) the detail of the used aqueous phase chemical representation, (ii) the size-resolution of the cloud droplets, and (iii) the total droplet number on the chemical model output. The studies indicated the requirement to consider chemical cloud effects in regional CTMs because of their key impacts on e.g., oxidation capacity in the gas and aqueous phase, formation of organic and inorganic particulate mass, and droplet acidity. In comparison to rather simplified aqueous phase chemical mechanisms focusing on sulfate formation, the use of the detailed aqueous phase chemistry mechanism C3.0RED leads to decreased gas phase oxidant concentrations, increased nighttime nitrate mass, decreased nighttime pH, and differences in sulfate mass. Moreover, the treatment of detailed aqueous phase chemistry enables the investigation of the formation of aqueous secondary organic aerosol mass. The consideration of size-resolved aqueous phase chemistry shows only slight effects on the chemical model output. Finally, the enhanced model was applied for case studies connected to the field experiment HCCT-2010. For the first time, an aqueous phase mechanism with the complexity of C3.0RED was applied in 3‑D chemistry transport simulations. Interesting spatial effects of real clouds on e.g., tropospheric oxidants and inorganic mass have been studied. The comparison of the model output with available measurements revealed many agreements and also interesting disagreements, which need further investigations.

The presence of gas in engine liquid cooling systems can have severe consequences for engine efficiency and life. The presence of stagnant, trapped gases will result in cooling system hotspots, causing gallery wall degradation through thermal stresses, fatigue and eventual cracking. The presence of entrained, transient gases in the coolant flow will act to reduce its bulk thermal properties and the performance of the system s coolant pump; critically the liquid flow rate, which will severely affect heat transfer throughout the engine and its ancillaries. The hold-up of gas in the pump s impeller may cause the dynamic seal to run dry, without lubrication or cooling. This poses both an immediate failure threat should the seal overheat and rubber components melt and a long term failure threat from intermittent quench cooling, which causes deposit formation on sealing faces acting to abrade and reduce seal quality. Bubbles in the coolant flow will also act as nucleation sites for cavitation growth. This will reduce the Net Positive Suction Head available (NPSHA) in the coolant flow, exacerbating cavitation and its damaging effects in locations such as the cylinder cooling liners and the pump s impeller. This thesis has analysed the occurrence of trapped gas (air) during the coolant filling process, its behaviour and break-up at engine start, the two-phase character of the coolant flow these processes generate and the effects it has on coolant pump performance. Optical and parametric data has been acquired in each of these studies, providing an understanding of the physical processes occurring, key variables and a means of validating numerical (CFD) code of integral processes. From the fundamental understanding each study has provided design rules, guidelines and validated tools have been developed, helping cooling system designers minimise the occurrence of trapped air during coolant filling, promote its breakup at engine start and to minimise its negative effects in the centrifugal coolant pump. It was concluded that whilst ideally the prevention of cooling system gases should be achieved at source, they are often unavoidable. This is due to the cost implications of finding a cylinder head gasket capable of completely sealing in-cylinder combustion pressures, the regular use of nucleate boiling regimes for engine cooling and the need to design cooling channel geometries to cool engine components and not necessarily to avoid fill entrapped air. Using the provided rules and models, it may be ensured stagnant air is minimised at source and avoided whilst an engine is running. However, to abate the effects of entrained gases in the coolant pump through redesign is undesirable due to the negative effects such changes have on a pump s efficiency and cavitation characteristics. It was concluded that the best solution to entrained gases, unavoidable at source, is to remove them from the coolant flow entirely using phase separation device(s).

In order to preserve groundwater resources from contamination by volatile organic compounds and to clean up sites contaminated with the compounds, we should understand fate and transport of contaminants in the subsurface systems and physicochemical processes involving remediation technologies. To enhance our understanding, numerical studies were performed on the following topics: (i) multiphase flow and contaminant transport in subsurface environments; (ii) biological transformations of contaminants; (iii) in-situ air sparging (IAS); and, thermal-enhanced venting (TEV). Among VOCs, trichloroethylene (TCE) is one of the most-frequently-detected chemicals in the contaminated groundwater. TCE and its daughter products (cis-1,2-dichloroethylene (cDCE) and vinyl chloride (VC)) are chosen as target contaminants. Density-driven advection of gas phase is generated by the increase in gas density due to vaporization of high-molecular weight contaminants such as TCE in the unsaturated zone. The effect of the density-driven advection on fate and transport of TCE was investigated under several environmental conditions involving infiltration and permeability. Biological transformations of contaminants can generate byproducts, which may become new toxic contaminants in subsurface systems. Sequential biotransformations of TCE, cDCE, and VC are considered herein. Under different reaction rates for two bioreaction kinetics, temporal and spatial concentration profiles of the contaminants were examined to evaluate the effect of biotransformations on multispecies transport. IAS injects clean air into the subsurface below the groundwater table to remediate contaminated groundwater. The movement of gas and the groundwater as a multiphase flow in the saturated zone and the removal of TCE by IAS application were analyzed. Each fluid flow under IAS was examined in terms of saturation levels and fluid velocity profiles in a three-dimensional domain. Several scenarios for IAS systems were simulated to evaluate remedial performance of the systems. TEV was simulated to investigate its efficiency on the removal of a nonaqueous phase liquid in the unsaturated zone under different operational conditions. For numerical studies herein, the governing equations for multiphase flow, multispecies transport, and heat energy in porous media were developed and solved using Galerkin finite element method. A three-dimensional numerical model, called TechFlowMP model, has been developed.