While the gas turbine research community is continuously pursuing development of higher cyclic efficiency designs by increasing the combustor firing temperatures and thermally resistant turbine vane / blade materials, a simultaneous effort to reduce the emission levels of high temperature driven thermal NOX also needs to be addressed. Lean premixed combustion has been found as one of the solutions to these objectives. However, since less amount of air is available for backside cooling of liner walls, it becomes very important to characterize the convective heat transfer that occurs on the inside wall of the combustor liners. These studies were explored using laboratory scale experiments as well as numerical approaches for several inlet flow conditions under both non-reacting and reacting flows. These studies may be expected to provide valuable insights for the industrial design communities towards identifying thermal hot spot locations as well as in quantifying the heat transfer magnitude, thus aiding in effective designs of the liner walls. Lean premixed gas turbine combustor flows involve strongly coupled interactions between several aspects of physics such as the degree of swirl imparted by the inlet fuel nozzle, premixing of the fuel and incoming air, lean premixed combustion within the combustor domain, the interaction of swirling flow with combustion driven heat release resulting in flow dilation, the resulting pressure fluctuations leading to thermo-acoustic instabilities there by creating a feedback loop with incoming reactants resulting in flow instabilities leading to flame lift off, flame extinction etc. Hence understanding combustion driven swirling flow in combustors continues to be a topic of intense research. In the present study, numerical predictions of swirl driven combustor flows were analyzed for a specific swirl number of an industrial fuel nozzle (swirler) using a commercial computational fluid dynamics tool and compared against in-house experimental data. The latter data was obtained from a newly developed test rig at Applied Propulsion and Power Laboratory (APPL) at Virginia Tech. The simulations were performed and investigated for several flow Reynolds numbers under non-reacting condition using various two equation turbulence models as well as a scale resolving model. The work was also extended to reacting flow modeling (using a partially premixed model) for a specific Reynolds number. These efforts were carried out in order investigate the flow behavior and also characterize convective heat transfer along the combustor wall (liner). Additionally, several parametric studies were performed towards investigating the effect of combustor geometry on swirling flow and liner hear transfer; and also to investigate the effect of inlet swirl on the jet impingement location along the liner wall under both non-reacting as well as reacting conditions. The numerical results show detailed comparison against experiments for swirling flow profiles within the combustor under reacting conditions indicating a good reliability of steady state modeling approaches for reacting conditions; however, the limitations of steady state RANS turbulence models were observed for non-reacting swirling flow conditions, where the flow profiles deviate from experimental observations in the central recirculation region. Also, the numerical comparison of liner wall heat transfer characteristics against experiments showed a sensitivity to Reynolds numbers. These studies offer to provide preliminary insights of RANS predictions based on commercial CFD tools in predicting swirling, non-reacting and reacting flow and heat transfer. They can be extended to reacting flow heat transfer studies in future and also may be upgraded to unsteady LES predictions to complement future experimental observations conducted at the in-house test facility. / Ph. D. / While the gas turbine research community is continuously pursuing development of higher cyclic efficiency designs by increasing the combustor firing temperatures and thermally resistant turbine vane / blade materials, a simultaneous effort to reduce the emission levels of high temperature driven thermal NOX also needs to be addressed. Lean premixed combustion has been found as one of the solutions to these objectives. However, since less amount of air is available for backside cooling of liner walls, it becomes very important to characterize the convective heat transfer that occurs on the inside wall of the combustor liners. These studies were explored using laboratory scale experiments as well as numerical approaches for several inlet flow conditions under both non-reacting and reacting flows. These studies may be expected to provide valuable insights for the industrial design communities towards identifying thermal hot spot locations as well as in quantifying the heat transfer magnitude, thus aiding in effective designs of the liner walls. Lean premixed gas turbine combustor flows involve strongly coupled interactions between several aspects of physics such as the degree of swirl imparted by the inlet fuel nozzle, premixing of the fuel and incoming air, lean premixed combustion within the combustor domain, the interaction of swirling flow with combustion driven heat release resulting in flow dilation, the resulting pressure fluctuations leading to thermo-acoustic instabilities there by creating a feedback loop with incoming reactants resulting in flow instabilities leading to flame lift off, flame extinction etc. Hence understanding combustion driven swirling flow in combustors continues to be a topic of intense research. In the present study, numerical predictions of swirl driven combustor flows were analyzed for a specific swirl number of an industrial fuel nozzle (swirler) using a commercial computational fluid dynamics tool and compared against in-house experimental data. The latter data was obtained from a newly developed test rig at Applied Propulsion and Power Laboratory (APPL) at Virginia Tech. The simulations were performed and investigated for several flow Reynolds numbers under non-reacting condition using various two equation turbulence models as well as a scale resolving model. The work was also extended to reacting flow modeling (using a partially premixed model) for a specific Reynolds number. These efforts were carried out in order investigate the flow behavior and also characterize convective heat transfer along the combustor wall (liner). Additionally, several parametric studies were performed towards investigating the effect of combustor geometry on swirling flow and liner hear transfer; and also to investigate the effect of inlet swirl on the jet impingement location along the liner wall under both non-reacting as well as reacting conditions. The numerical results show detailed comparison against experiments for swirling flow profiles within the combustor under reacting conditions indicating a good reliability of steady state modeling approaches for reacting conditions; however, the limitations of steady state RANS turbulence models were observed for non-reacting swirling flow conditions, where the flow profiles deviate from experimental observations in the central recirculation region. Also, the numerical comparison of liner wall heat transfer characteristics against experiments showed a sensitivity to Reynolds numbers. These studies offer to provide preliminary insights of RANS predictions based on commercial CFD tools in predicting swirling, non-reacting and reacting flow and heat transfer. They can be extended to reacting flow heat transfer studies in future and also may be upgraded to unsteady LES predictions to complement future experimental observations conducted at the in-house test facility.

Tesis por compendio / [ES] El empuje actual de las normativas de emisiones y una conciencia social cada vez más crítica en este aspecto, ha llevado a la industria automotriz a elevar sus estándares en eficiencia a cimas nunca antes vistas. Con el mayor peso de las nuevas normativas puesto sobre los vehículos Diesel, la presión ejercida sobre esta tecnología es, si cabe, aún más crítica. Dada la necesidad de mantener este tipo de plantas propulsivas en determinadas aplicaciones, como son el transporte terrestre pesado, maquinaria o en el transporte marítimo, es también necesario mantener su desarrollo. Como parte fundamental de los motores Diesel, el sistema de inyección interviene directamente en la generación de la energía. La mejora y optimización de su funcionamiento repercute sobre la cadena de eficiencias del sistema. Esta tesis pretende contribuir al desarrollo de las plantas propulsivas Diesel en este aspecto y, concretamente, en el estudio de las geometrías de toberas Diesel de inyección directa. A lo largo del texto, este tipo de geometrías son estudiadas tanto desde la perspectiva del flujo interno como del flujo externo. Los estudios combinan modelos numéricos Eulerianos (para flujo interno o interno-externo acoplado), modelos Lagrangianos discretos (para el estudio del chorro), junto con medidas experimentales diversas que avalan los análisis ejecutados. La exploración de las geometrías propuestas no queda acotada solamente a formas circulares, más convencionales, sino que también se ha extendido a toberas de morfologías más innovadoras como son las elípticas. Las metodologías presentadas demuestran ser eficaces en el estudio de estos sistemas y una herramienta a tener en cuenta en la mejora de su diseño. Los distintos resultados obtenidos defienden, además, como la geometría de la tobera es un condicionante del desarrollo posterior de la mezcla y puede ser utilizada como elemento de optimización de la misma. / [CA] L'actual impuls de les normatives d'emissions i una consciència social cada vegada més crítica en aquest aspecte ha portat a la indústria automobilística a elevar els seus estàndards d'eficiència a cotes mai vistes abans. Amb el major pes de les noves normatives imposades als vehicles dièsel, la pressió exercida sobre aquesta tecnologia és, si cap, encara més crítica. Donada la necessitat de mantenir aquest tipus de plantes propulsives en determinades aplicacions, com el transport terrestre pesat, maquinària o el transport marítim, és també necessari mantenir el seu desenvolupament. Com a part fonamental dels motors dièsel, el sistema d'injecció intervé directament en la generació d'energia. La millora i optimització del seu funcionament repercuteix en la cadena d'eficiències del sistema. Aquesta tesi pretén contribuir al desenvolupament de les plantes propulsives dièsel en aquest aspecte i, concretament, en l'estudi de les geometries de bussons dièsel d'injecció directa. Al llarg del text, aquest tipus de geometries són estudiades tant des de la perspectiva del flux intern com del flux extern. Els estudis combinen models numèrics Eulerians (per a flux intern o intern-extern acoblat), models Lagrangians discrets (per a l'estudi del corrent), juntament amb mesures experimentals diverses que avalen els anàlisis realitzats. L'exploració de les geometries proposades no queda acotada només a formes circulars, més convencionals, sinó que també s'ha estès a bussons de morfologies més innovadores com les el·líptiques. Les metodologies presentades demostren ser eficaces en l'estudi d'aquests sistemes i una eina a tenir en compte en la millora del seu disseny. Els diferents resultats obtinguts també argumenten que la geometria del busó és un condicionant del desenvolupament posterior de la barreja i pot ser utilitzada com a element d'optimització de la mateixa. / [EN] The current push for emissions regulations and an increasingly critical social awareness in this regard has led the automotive industry to raise its efficiency standards to unprecedented heights. With greater emphasis on new regulations placed on Diesel vehicles, the pressure on this technology is even more critical. Given the need to maintain such propulsion systems in specific applications like heavy land transport, machinery, or maritime transportation, it is also necessary to continue their development. As a fundamental part of Diesel engines, the injection system directly affects energy generation. Improving and optimizing its operation has an impact on the overall efficiency of the system. This thesis aims to contribute to the development of Diesel propulsion systems in this regard, specifically in the study of direct injection Diesel nozzle geometries. Throughout the text, these types of geometries are examined from both internal and external flow perspectives. The studies combine Eulerian numerical models (for internal or coupled internal-external flow), discrete Lagrangian models (for jet analysis), along with various experimental measurements that support the conducted analyses. The exploration of proposed geometries is not limited to conventional circular shapes but has also extended to more innovative morphologies such as elliptical nozzles. The presented methodologies prove to be effective in studying these systems and serve as a valuable tool in improving their design. The different results obtained also argue that nozzle geometry is a determining factor in the subsequent mixture development and can be used as an optimization element for it. / Las investigaciones de esta tesis han sido respaldadas por el Ministerio de Ciencia, Innovación y Universidades el Gobierno de España y mis estudios de doctorado han sido financiados por la Agencia Estatal de Investigación del gobierno de España y el Fondo Social Europeo. Dichas ayudas se concretaron dentro del marco del proyecto "Desarrollo de modelos de combustión y emisiones HPC para el análisis de plantas propulsivas de transporte sostenibles" (TRA2017-89139-C2-1-R) a través del "Subprograma Estatal de Formación del Programa Estatal de Promoción del Talento y su Empleabilidad en I+D+i". / Martínez-Miracle Muñoz, EC. (2024). Estudio sobre el impacto de la geometría de toberas diesel en el desarrollo del chorro, la formación de la mezcla y la combustión [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/203126 / Compendio

With the advent and increasing influence of lithium-ion batteries, it is imperative to devise the means for realizing a safe operation of the same. Battery modelling of lithium-ion cells is a means to comprehend the cell behaviour. A battery model includes both physical and mathematical modelling with each having their individual pros and cons. Depending on the scale of the battery model resolution the complexity and computational time increases. This thesis work deals with the development of a thermal model for 18650 cylindrical lithium-ion cells. This model encapsulates the thermal behaviour of the STB4 product at Atlas Copco and the battery model development has been done in conjunction with a 3D Computational Fluid Dynamic (CFD) analysis in Ansys Fluent to comprehend the heat distribution over the product surface and identify thermal bottlenecks. The model selection has been done based on the required level of functionality and its effect on the computational time. The proposed NTGK electrochemistry model in this thesis based on the Circuit Network solution method, provides an optimal balance between computational efficiency and model accuracy. The consolidated battery model and analysis results provide crucial insight towards the thermal patterns of the lithium cells and help indicate deviations in case of potential unsafe operation. The simulation model has been validated against the physical experimental tests performed on the battery pack and the STB4 provided by Atlas Copco. Finally, an approach towards a future software implementation of the developed model is outlined in this thesis. / Med tillkomsten och ökande inflytande av litiumjonbatterier är det absolut nödvändigt att utarbeta medel för att förverkliga en säker drift av desamma. Batterimodellering av litiumjonceller är ett sätt att förstå cellens beteende. En batterimodell inkluderar både fysisk och matematisk modellering där var och en har sina individuella för- och nackdelar. Beroende på storleken på batterimodellens upplösning ökar komplexiteten och beräkningstiden. Detta examensarbete behandlar utvecklingen av en termisk modell för 18650 cylindriska litiumjonceller. Denna modell kapslar in det termiska beteendet hos STB4-produkten hos Atlas Copco och utvecklingen av batterimodeller har gjorts i samband med en 3D Computational Fluid Dynamic (CFD)-analys i Ansys Fluent för att förstå värmefördelningen över produktytan och identifiera termiska flaskhalsar. Modellvalet har gjorts utifrån den funktionalitet som krävs och dess effekt på beräkningstiden. Den föreslagna NTGK elektrokemimodellen i denna avhandling baserad på lösningsmetoden Circuit Network, ger en optimal balans mellan beräkningseffektivitet och modellnoggrannhet. Den konsoliderade batterimodellen och analysresultaten ger avgörande insikt om litiumcellernas termiska mönster och hjälper till att indikera avvikelser i händelse av potentiell osäker drift. Simuleringsmodellen har validerats mot de fysiska experimentella testerna som utförts på batteripaketet och STB4 från Atlas Copco. Slutligen beskrivs ett tillvägagångssätt för en framtida mjukvaruimplementering av den utvecklade modellen i denna avhandling.

Permeability is an important property of a porous medium and it controls the flow of fluid through the medium. Particle characteristics are known to affect the value of the permeability. However, experimental investigation of the effects of these particle characteristics on the value of permeability is time-consuming while analytical predictions have been reported to overestimate it leading to inefficient design. To overcome these challenges, there is the need for the development of new models that can predict permeability based on input variables and process conditions. In this research, data from experiments, Computational Fluid Dynamics (CFD) and literature were employed to develop new models using Multivariate Regression (MVR) and Artificial Neural Networks (ANNs). Experimental measurements of permeability were performed using high and low shear separation processes. Particles of talc, calcium carbonate and titanium dioxide (P25) were used in order to study porous media with different particle characteristics and feed concentrations. The effects of particle characteristics and initial stages of filtration as well as the reliability of filtration techniques (constant pressure filtration, CPF and constant rate filtration, CRF) were investigated. CFD simulations were also performed of porous media for different particle characteristics to generate additional data. The regression and ANN models also included permeability data taken from reliable literature sources. Particle cluster formation was only found in P25 leading to an increase of permeability especially in sedimentation. The constant rate filtration technique was found more suitable for permeability measurement than constant pressure. Analyses of data from the experiments, CFD and correlation showed that Sauter mean diameter (ranging from 0.2 to 168 μm), the fines ratio (x50/x10), particle shape (following Heywood s approach), and voidage of the porous medium (ranging from 98.5 to 37.2%) were the significant parameters for permeability prediction. Using these four parameters as inputs, performance of models based on linear and nonlinear MVR as well as ANN were investigated together with the existing analytical models (Kozeny-Carman, K-C and Happel-Brenner, H-B). The coefficient of correlation (R2), root mean square error (RMSE) and average absolute error (AAE) were used as performance criteria for the models. The K-C and H-B are two-variable models (Sauter mean diameter and voidage) and two variables ANN and MVR showed better predictive performance. Furthermore, four-variable (Sauter mean diameter, the x50/x10, particle shape, and voidage) models developed from the MVR and ANN exhibit excellent performance. The AAE was found with K-C and H-B models to be 35 and 40%, respectively while the results of using ANN2 model reduced the AAE to 14%. The ANN4 model further decreased the AAE to approximately 9% compared to the measured results. The main reason for this reduced error was the addition of a shape coefficient and particle spread (fine ratio) in the ANN4 model. These two parameters are absent in the analytical relations, such as K-C and H-B models. Furthermore, it was found that using the ANN4 (4-5-1) model led to increase in the R2 value from 0.90 to 0.99 and significant decrease in the RMSE value from 0.121 to 0.054. Finally, the investigations and findings of this work demonstrate that relationships between permeability and the particle characteristics of the porous medium are highly nonlinear and complex. The new models possess the capability to predict the permeability of porous media more accurately owing to the incorporation of additional particle characteristics that are missing in the existing models.

<p>This licentiate thesis describes development of modellingtools, experimental physical modelling and numerical modellingto simulate real combustion processes for advanced industrialutility boiler before and after retrofit.</p><p>The work presents extended study about formation,destruction and control of pollutants, especially NOx, whichoccur during combustion process.</p><p>The main aim of this work is to improve mixing process incombustion chamber. To do this, the optimization of placementand direction of additional air and fuel nozzles, the physicalmodelling technique is used. By using that method, it ispossible to obtain qualitative information about processes,which occur in the real boiler. The numerical simulationsverify the results from physical modelling, because duringmathematical modelling quantitative informations about flow andmixing patterns, temperature field, species concentration areobtained.</p><p>Two 3D cases, before and after retrofit, of pulverized fuelfired boiler at 125 MW output thermal power are simulated. Theunstructured mesh technique is also used to discretize theboiler. The number of grid was 427 656 before retrofit and 513362 after retrofit. The comparisons of results of numericalsimulation before and after retrofit are presented. The resultsfrom physical modelling and numerical simulation are alsoshown.</p><p>Results present that nozzles of additional air and fuel givea considerably better mixing process, uniform temperature fieldand CO2 mass fraction. The whole combustion chamber worksalmost as a "well stirred reactor", while upper part of boilerworks as a "plug flow reactor".</p><p>Differences between from measured of temperatures andpredicted temperatures are not too big, the maximum differenceis about 100 K. It seems, that calculated temperatures showgood agreement with measurement data.</p><p>The results illuminate the potential of physical andnumerical modelling methods as promising tools to deal with thecomplicated combustion processes, even for practicalapplication in the industry.</p><p><b>Keywords:</b>air staging, fuel staging, boiler, furnace,computational fluid dynamics, numerical simulation, pollutants,physical modeling, pulverized fuel combustion.</p>

In the process of engine design, it is important for the engine designer to predict the accurate component temperatures. Controlling the temperature of engine components requires a better understanding of the coolant behaviour in the coolant jacket of an engine which is critical to internal combustion engine design, The studies reported in the literature emphasize the influence of the cooling system on other engine operation such as exhaust emission, fuel consumption and engine wear. In this context, much work has been done with the purpose of improving the coolant jacket design and components of the cooling system to achieve higher performance. (Some of these studies) Previous researches have shown the possibility of achieving higher engine efficiency and performance with higher coolant temperature. This project aims at understanding the coolant flow behaviour in the coolant jackets of a diesel engine and investigating the possibility of running the engine at higher coolant temperatures by predicting the temperature distribution of the structure which is required for the assessment of the durability ofthe engine components. In this thesis, CFD (Computational Fluid Dynamics) and FE (Finite Element) techniques are used to study coolant flow in the coolant jackets and to predict the temperature distribution within the engine structure respectively. The objectives are to develop an FE model of the engine structure for thermal analyses and a CFD model of the fluid domain for the coolant flow CFD analyses. A number of case studies are carried out with the purpose of determining the most suitable technique for accurate temperature prediction. The methodology of manual coupling approach between CFD and FE analyses, which is more widely used in industry, and conjugate approach are demonstrated. Using these approaches, thermal analysis of the engine is conducted with the purpose of identifying the thermally critical locations throughout the engine. Furthermore, the influences of higher coolant temperature on these thermally critical regions of the engine are highlighted by carrying out four case studies with coolant inlet temperatures of 110°C, !ISOC, 117.5"C and !20°C. The temperature rise at the particular points around thermally critical regions is found to be in the range of 3-9 degrees at the higher coolant temperatures. This slight increase in temperature of critical locations may affect the durability of the structure. However, without carrying out the structural analyses it is not possible to comment on the durability of the engine structure. The effects of surface roughness and viscosity on heat transfer rate are also investigated and shown to be insignificant.

Multiphase flows are of great interest to a large variety of industries because flows of two or more immiscible liquids are encountered in a diverse range of processes and equipment. However, the advent of high viscosity oil requires more investigations to enhance good design of transportation system and forestall its inherent production difficulties. Experimental and numerical studies were conducted on water-sand, oil-water and oilwater- sand respectively in 1-in ID 5m long horizontal pipe. The densities of CYL680 and CYL1000 oils employed are 917 and 916.2kg/m3 while their viscosities are 1.830 and 3.149Pa.s @ 25oC respectively. The solid-phase concentration ranged from 2.15e-04 to 10%v/v with mean diameter of 150micron and material density of 2650kg/m3. Experimentally, the observed flow patterns are Water Assist Annular (WA-ANN), Dispersed Oil in Water (DOW/OF), Oil Plug in Water (OPW/OF) with oil film on the wall and Water Plug in Oil (WPO). These configurations were obtained through visualisation, trend and the probability density function (PDF) of pressure signals along with the statistical moments. Injection of water to assist high viscosity oil transport reduced the pressure gradient by an order of magnitude. No significant differences were found between the gradients of oil-water and oil-water-sand, however, increase in sand concentration led to increase in the pressure losses in oil-water-sand flow. Numerically, Water Assist Annular (WA-ANN), Dispersed Oil in Water (DOW/OF), Oil Plug in Water (OPW/OF) with oil film on the wall, and Water Plug in Oil (WPO) flow pattern were successfully obtained by imposing a concentric inlet condition at the inlet of the horizontal pipe coupled with a newly developed turbulent kinetic energy budget equation coded as user defined function which was hooked up to the turbulence models. These modifications aided satisfactory predictions.

This thesis is based on the PhD work of investigating the Friction Stir Welding process (FSW) with numerical and Artificial Neural Network (ANN) modelling methods. FSW was developed at TWI in 1991. As a relatively new technology it has great advantages in welding aluminium alloys which are difficult to weld with traditional welding processes. The aim of this thesis was the development of new modelling techniques to predict the thermal and deformation behaviour. To achieve this aim, a group of Gleeble experiments was conducted on 6082 and 7449 aluminium alloys, to investigate the material constitutive behaviour under high strainrate, near solidus conditions, which are similar to what the material experiences during the FSW process. By numerically processing the experimental data, new material constitutive constants were found for both alloys and used for the subsequent FSW modelling work. Importantly no significant softening was observed prior to the solidus temperature. One of the main problems with numerical modelling is determining the values of adjustable parameters in the model. Two common adjustable parameters are the heat input and the coefficients that describe the heat loss to the backing bar. To predict these coefficients more efficiently a hybrid model was created which involved linking a conventional numerical model to an ANN model. The ANN was trained using data from the numerical model. Then thermal profiles were abstracted (summarised) and used as inputs; and the adjustable parameters were used as outputs. The trained ANN could then use abstracted thermal profiles from welding experiments to predict the adjustable parameters in the model. The first stage involved developing a simplified FE thermal model which represents a typical welding process. It was used to find the coefficients that describe the heat loss to the backing bar, and the amount of power applied in the model. Five different thermal boundary conditions were studied, including both convective and ones that included the backing bar with a contact gap conductance. Three approaches for abstracting the thermal curves and using as inputs to the ANN were compared. In the study, the characteristics of the ANN model, such as the ANN topology and gradient descent method, were evaluated for each boundary condition for understanding of their influences to the prediction. The outcomes of the study showed that the hybrid model technique was able to determine the adjustable parameters in the model effectively, although the accuracy depended on several factors. One of the most significant effects was the complexity of the boundary condition. While a single factor boundary condition (e.g. constant convective heat loss) could be predicted easily, the boundary condition with two factors proved more difficult. The method for inputting the data into the ANN had a significant effect on the hybrid model performance. A small number of inputs could be used for the single factor boundary condition, while two factors boundary conditions needed more inputs. The influences from the characteristics of the ANN model were smaller, but again thermal model with simpler boundary condition required a less complex ANN model to achieve an accurate prediction, while models with more complex boundary conditions would need a more sophisticated ANN model. In the next chapter, the hybrid method was applied to a FSW process model developed for the Flexi-stir FSW machine. This machine has been used to analyse the complex phase changes that occur during FSW with synchrotron radiation. This unique machine had a complex backing bar system involving heat transfer from the aluminium alloy workpiece to the copper and steel backing bars. A temperature dependent contact gap conductance which also depends on the material interface type was used. During the investigation, the ANN model topologies (i.e. GFF and MFF) were studied to find the most effective one. Different abstracting methods for the thermal curves were also compared to explore which factors (e.g. the peak temperature in the curve, cooling slope of a curve) were more important to be used as an input. According to close matching between the simulation and experimental thermal profiles, the hybrid model can predict both the power and thermal boundary condition between the workpiece and backing bar. The hybrid model was applied to six different travel speeds, hence six sets of heat input and boundary condition factors were found. A universal set was calculated from the six outcomes and a link was discovered between the accuracy of the temperature predictions and the plunge depth for the welds. Finally a model with a slip contact condition between the tool and workpiece was used to investigate how the material flow behaviour was affected by the slip boundary condition. This work involved aluminium alloys 6082-T6 and 7449-T7, which have very different mechanical properties. The application of slip boundary condition was found to significantly reduce the strain-rate, compared to a stick condition. The slip condition was applied to the Flexi-stir FSW experiments, and the results indicated that a larger deformation region may form with the slip boundary condition. The thesis successfully demonstrates a new methodology for determining the adjustable parameters in a process model; improved understanding of the effect of slip boundary conditions on the flow behaviour during FSW and insight in to the behaviour of aluminium alloys at temperatures approaching the solidus and high strain-rates.

Together with the world economic growth is the increasing of energy generation demand. However, the upgrade of world power production capability could affect the environment negatively. Even the clean and renewable sources, such as hydroelectricity and wind powers have socio-economic and environmental disadvantages. For example, the required flooded area for a hydro power plant construction could devastate entire forests, and the installation of a wind farm power plant could affect migratory rotes of birds and generate high levels of noise. Hence, for the balancing of advantages and disadvantages of each power generation source, it is necessary to diversify, which requires investments in new power sources. In this context, the energy generation in the ocean is highlighted. The first point concerning the ocean energy is that there is no need of population removal from the installation area, such as the onshore based methods and the second point is that most of the population is concentrated in coastal areas. Therefore the production occurs near to the demand, decreasing the costs with energy distribution. The two main methodologies for harassing energy from oceans are based on gravity waves and in tides. And since the tidal cycles are governed mainly by the gravitational interaction between oceans, Moon and Sun, they are easily predictable, which increases the reliability of such systems. These works explores methodologies to analyse the power generation from a single axial tidal current turbine through a Steady State RANS methodology. Are discussed the effects of flow directionality, inlet velocity profile and turbulence levels and the results are compared with an experimental scheme. It is proposed an alternative methodology for free surface modelling in the CFD analysis. The usual methodology, VOF, it is based on a homogeneous, biphasic approach which requires an additional mesh refinement and is computationally expensive. This new methodology introduces an elastic wall approach in the free surface region in which the stiffness is calculated to provide the same restoring effect as gravity. In general, the results for open domain matched with the experimental results, validating the numerical model and the confined domain has shown a higher power and thrust coefficients if compared with the open domain, which is in accordance with the actuator disk theory approach. The elastic free surface presented convergence problems related to high Froude numbers and therefore to high deformations. However, a simulation with 10% of the original inlet velocity was performed, achieving reasonable results for both power and thrust coefficients evaluation. / O crescimento econômico mundial e o aumento na demanda pela geração de energia andam juntos. No entanto, uma maior capacidade de produção de energia poderia afetar negativamente o meio ambiente. Mesmo as fontes limpas e renováveis, como a hidrelétrica e a eólica acarretam em impactos socioeconômicos e ambientais. Por exemplo, a construção de uma usina hidrelétrica demanda uma imensa área alagada que pode devastar florestas inteiras e a instalação de uma usina eólica pode afetar a migração de certas espécies de pássaros e produzir altos níveis de barulho. Portanto, para equilibrar as vantagens e desvantagens devidas a cada meio de produção de energia, é necessária a diversificação, que demanda de investimentos em novas fontes. Neste contexto, a geração de energia nos oceanos é destacada. O primeiro ponto a respeito desta fonte é de que não há a necessidade de remoção da população na área de instalação, tal como os métodos de geração dentro do continente. O segundo principal ponto é a respeito da distribuição de energia. A maior parte da população mundial vive em regiões costeiras, diminuindo, portanto, a distância entre a produção e demanda, reduzindo assim, seus custos. As duas principais metodologias para se explorar a energia proveniente dos oceanos são: Energia de Ondas e Energia de Marés. E considerando que os ciclos de mare são governados principalmente pela interação gravitacional entre os oceanos, lua e sol, eles são facilmente previsíveis, o que aumenta a confiabilidade dos sistemas de geração de energia baseados em marés. Este trabalho explora as metodologias para analisar a geração de energia a partir de uma única turbina axial de corrente de maré através de uma metodologia baseada nas equações de Navier-Stokes com a média de Reynolds, analisadas em regime permanente. São discutidos efeitos da direção do escoamento, perfil de velocidades na entrada e nos níveis de turbulência. Os resultados são comparados com experimentos. É proposta uma metodologia alternativa para a modelagem da superfície livre com CFD uma vez que a metodologia atual é baseada em um escoamento bifásico que demanda de um refinamento adicional da malha e é computacionalmente caro. A nova metodologia usa uma parede elástica na região da superfície livre com a rigidez ajustada para se obter o mesmo efeito de restauração que a gravidade. De maneira geral, os resultados para o domínio aberto se aproximaram dos resultados experimentais, validando o modelo numérico e além disso, o modelo considerando confinamento da turbine mostrou maiores valores para os coeficientes de potência e empuxo, estando portanto, de acordo com a teoria do disco atuador. O modelo com a superfície livre elástica apresentou problemas de convergência, relacionados com números de Froude elevados, uma vez que isto se relaciona com maiores deformações na região da superfície livre. Uma simulação com 10% da velocidade original foi realizada, obtendo-se resultados coerentes para ambos coeficientes de potência e empuxo.

Tidal energy has the potential to make a valuable contribution to meeting future global energy demands. Converting the energy of tidal streams into useful electricity can be achieved with use of tidal-stream turbines, such as the Momentum-Reversal and Lift (MRL) device. This turbine utilises a blade motion where each blade rotates continuously through 180° about its own axis for every 360° of turbine rotation. The aim of the design is to harness both useful lift and drag forces when rotating at relatively slow speeds. However, no detailed analysis of the time-varying fluid dynamic behaviour of the turbine has been undertaken before this study. The primary aim of this study has been to further understanding of the performance characteristics of the MRL turbine design, focusing on a laboratory- scale device. The study has analysed both the time-averaged and time-varying torque and power output, and the associated fluid-dynamic structure of flow through the turbine. A secondary aim was to generate data that can be used by other researchers who focus on the wake generation of the MRL tidal turbine. This study has used OpenFOAM to develop a time-dependent RANS CFD model and investigate the performance of the MRL turbine. To allow validation of the CFD model, experiments were firstly undertaken in order to measure the cycle-mean torque and power output of the turbine when operating in a laboratory flume. Measurements of the flow velocity at a number of upstream and downstream locations were also taken, in order to allow comparison with the CFD simulation results, where appropriate. Also, in order to allow validation of the CFD approach against time-varying data, the motion of the turbine blades was analysed. This allowed suitable experimental test cases to be identified from the literature and CFD simulation results have been compared to these. A detailed sensitivity analysis of the MRL turbine CFD model was carried out, followed by two-dimensional simulations of the turbine involving a single-blade and three-blades. Three-dimensional simulations were also undertaken, with results compared to the gathered experimental results. Finally, the effect of varying turbine solidity was investigated with the CFD model. Overall it was found that the CFD simulations successfully reproduce the rotational speed at which maximum torque and power are developed. However, the three-dimensional simulations significantly over-predict the magnitude of results in comparison to the gathered experimental results. Regardless, the two- and three-dimensional simulations have allowed detailed analysis of the flow behaviour and structures that are responsible for the development of blade forces and turbine torque.