As a result of growing concerns about the carbon emissions associated with the combustion of conventional hydrocarbon fuels, hydrogen is gaining more attention as a clean alternative. The combustion of hydrogen in air produces no carbon emissions. However, hydrogen-air combustion does have the potential to produce oxides of nitrogen (NOx), which are harmful pollutants. The production of NOx can be significantly curbed using lean premixed combustion, wherein hydrogen and air are mixed at an equivalence ratio (the ratio of stoichiometric to actual air in the combustion process) significantly less than 1.0 prior to combustion. Hydrogen is a good candidate for use in lean premixed systems due to its very wide flammability range. The potential for the stable combustion of hydrogen at a wide range of equivalence ratios makes it particularly well-suited to application in gas turbines, where the equivalence ratio is likely to vary significantly over the operating range of the machine. The strong lean combustion stability of hydrogen-air flames is due primarily to high reaction rates and the associated high turbulent burning velocities. While this is advantageous at low equivalence ratios, it presents a significant danger of flashback — the upstream propagation of the flame into the premixing device — at higher equivalence ratios. An investigation has been conducted into the operation of a specific hydrogen-air premixer design in a gas turbine engine. Laboratory tests were first conducted to determine the upper stability limits of a single premixer. Tests were then carried out in which eighteen premixers and a custom-fabricated combustor liner were installed in a modified Pratt and Whitney Canada PT6A-20 turboprop engine. The tests examined the premixer and engine operability as a result of the modifications. A computer cycle analysis model was created to help analyze and predict the behavior of the modified engine and premixers. The model, which uses scaled component maps to predict off-design engine performance, was integral in the analysis of premixer flashback which limited the operation of the modified engine. / Master of Science

Slope stability is an important aspect of geotechnical engineering. Input parameters for the analysis are the governing factors and they must be determined accurately and precisely. Field investigations, laboratory testing and back analyses are vital instruments for the input parameters. This study presents the results of slope stability analysis for the soil slopes at Elbistan-&Ccedil / &ouml / llolar lignite mine. After executing the drilling programme, samples taken from the drilling work, delivered to soil mechanics laboratory for testing. The basic input parameters, namely cohesion and friction angle determined at soil mechanics laboratory were compared to the parameters obtained from back analysis of a large scale slope failure. Input parameters for the analysis are determined by this way. After determining the input parameters, slope stability analyses were carried out both for the permanent and temporary slopes in AfSin-Elbistan lignite basin, &Ccedil / &ouml / llolar sector. The effect of ground water on the stability of slopes was investigated in detail and maximum safe slope angles were determined for different water levels. For limit equilibrium analysis, Rocscience SLIDE software, for finite difference analysis in 3-D, Itasca FLAC3D was used. In the limit equilibrium analyses both circular and composite failures were considered. Shear strength reduction method is used for the finite difference method. The results between limit equilibrium and 3-D finite difference methods were compared. When the failure surfaces obtained from the finite difference analyses were imposed to limit equilibrium analysis, computations are resulted in lower factor of safety values for limit equilibrium analysis.

Dentre os principais problemas que ocorrem na estampagem de chapas metálicas estão a fratura, e estricção (necking) e o enrugamento (wrinkling). Este último pode fazer com que a peça fabricada seja rejeitada por problemas que vão desde motivos estéticos até dificuldades de montagem destas peças e desgaste excessivo da ferramenta. Diversos pesquisadores tem estudado o problema de instabilidade à compressão que origina o fenômeno de enrugamento. Pretende-se estudar os principais métodos para predição do enrugamento, a saber, a teoria da bifurcação, a introdução de uma falha induzida e a aplicação do indicador proposto por Nordlund e Häggblad (coeficiente NH). O método escolhido será implementado junto a um programa de análise por elementos finitos de modo a obter diagramas limite de estabilidade, para então ser aplicado a peças mais genéricas. O procedimento é aplicado a peças de geometria relevante para serem ensaiadas experimentalmente de modo a validar as simulações numéricas propostas. / Among the main problems in sheet metal forming processes are the fracture, the necking and the wrinkling behavior of the material. The last one can produce rejected parts due difficulties in the assembly of these parts or excessive wear of the tooling. Many researchers have been studied the problem of the compression instability phenomenon that origins the wrinkling behavior. The objective of this work is the study of the main available methods to the wrinkling prediction, as the bifurcation theory, the induced defect and an indicator proposed by Nordlund and Häggblad (NH coefficient). The more suitable method is implemented into a finite element analysis software in order to obtain the stability limit diagrams that can be used in the design of generic shape parts. Experimental studies will be done in parts subjected to compressive forming loads in order to validate the numerical simulations.

Dentre os principais problemas que ocorrem na estampagem de chapas metálicas estão a fratura, e estricção (necking) e o enrugamento (wrinkling). Este último pode fazer com que a peça fabricada seja rejeitada por problemas que vão desde motivos estéticos até dificuldades de montagem destas peças e desgaste excessivo da ferramenta. Diversos pesquisadores tem estudado o problema de instabilidade à compressão que origina o fenômeno de enrugamento. Pretende-se estudar os principais métodos para predição do enrugamento, a saber, a teoria da bifurcação, a introdução de uma falha induzida e a aplicação do indicador proposto por Nordlund e Häggblad (coeficiente NH). O método escolhido será implementado junto a um programa de análise por elementos finitos de modo a obter diagramas limite de estabilidade, para então ser aplicado a peças mais genéricas. O procedimento é aplicado a peças de geometria relevante para serem ensaiadas experimentalmente de modo a validar as simulações numéricas propostas. / Among the main problems in sheet metal forming processes are the fracture, the necking and the wrinkling behavior of the material. The last one can produce rejected parts due difficulties in the assembly of these parts or excessive wear of the tooling. Many researchers have been studied the problem of the compression instability phenomenon that origins the wrinkling behavior. The objective of this work is the study of the main available methods to the wrinkling prediction, as the bifurcation theory, the induced defect and an indicator proposed by Nordlund and Häggblad (NH coefficient). The more suitable method is implemented into a finite element analysis software in order to obtain the stability limit diagrams that can be used in the design of generic shape parts. Experimental studies will be done in parts subjected to compressive forming loads in order to validate the numerical simulations.

This thesis introduces spatial filtering, which is a technique to extend the time step size beyond the conventional stability limit for the Finite-Difference Time-Domain (FDTD) method, at the expense of transforming field nodes between the spatial domain and the discrete spatial-frequency domain and removing undesired spatial-frequency components at every FDTD update cycle. The spatially-filtered FDTD method is demonstrated to be almost as accurate as and more efficient than the conventional FDTD method via theories and numerical examples. Then, this thesis combines spatial filtering and an existing subgridding scheme to form the spatially-filtered subgridding scheme. The spatially-filtered subgridding scheme is more efficient than existing subgridding schemes because the former allows the time step size used in the dense mesh to be larger than the dense mesh CFL limit. However, trade-offs between accuracy and efficiency are required in complicated structures.

This thesis introduces spatial filtering, which is a technique to extend the time step size beyond the conventional stability limit for the Finite-Difference Time-Domain (FDTD) method, at the expense of transforming field nodes between the spatial domain and the discrete spatial-frequency domain and removing undesired spatial-frequency components at every FDTD update cycle. The spatially-filtered FDTD method is demonstrated to be almost as accurate as and more efficient than the conventional FDTD method via theories and numerical examples. Then, this thesis combines spatial filtering and an existing subgridding scheme to form the spatially-filtered subgridding scheme. The spatially-filtered subgridding scheme is more efficient than existing subgridding schemes because the former allows the time step size used in the dense mesh to be larger than the dense mesh CFL limit. However, trade-offs between accuracy and efficiency are required in complicated structures.