Gas turbine blade/vane cooling is obtained by circulating the high pressure air from compressor to the internal cooling passage of the blade/vane. Heat transfer and cooling effect in the rotating blade is highly affected by rotation. The typical rotation number for the aircraft engine is in the range of 0~0.25 and for the land based power generation turbine in the range of 0~05. Currently, the heat transfer data at high rotation numbers are limited. Besides, the investigation of heat transfer phenomena in the turn region, especially near hub portion is rare. This dissertation is to study the heat transfer in rectangular channels with turns in the tip or the hub portion respectively at high rotation numbers close to the engine condition. The dissertation experimentally investigates the heat transfer phenomena in a two-pass rectangular channel (AR=W/H=2:1) with a 180 degree sharp turn in the tip portion. The flow in the first passage is radial outward and after the turn in the second passage, the flow direction is radial inward. The hydraulic diameter (Dh) of the channel is 16.9 mm. Parallel square ribs with an attack angle (alpha) of 45 degrees are used on leading and trailing surfaces to enhance the heat transfer. The rib height-to-hydraulic diameter ratio (e/Dh) is 0.094. For the baseline smooth case and the case with rib pitch-to-height ratio (P/e) 10, channel orientation angles (beta) of 90 degrees and 135 degrees were tried to model the cooling passage in the mid and rear portion of the blade respectively. Two other P/e ratios of 5 and 7.5 were studied at beta=135 degrees to investigate their effect on heat transfer. The data are presented under high rotation numbers and buoyancy parameters by varying the Reynolds number (Re=10,000~40,000) and rotation speed (rpm=0~400). Corresponding rotation number and buoyancy parameter are ranged as 0~0.45 and 0~0.8 respectively. The dissertation also studies the heat transfer in a two-pass channel (AR=2:1) connected by a 180 degree U bend in the hub portion. The flow in the first passage is radial inward and after the U bend, the flow in the second passage is radial outward. The cross-section dimension of this channel is the same as the previous one. To increase heat transfer, staggered square ribs (e/Dh=0.094) are pasted on leading and trailing walls with an attack angle (alpha) of 45 degrees and pitch-to-height ratio (P/e) of 8. A turning vane in the shape of half circle (R=18.5 mm, t=1.6 mm) is used in the turn region to guide the flow for both smooth and ribbed cases. Channel orientation angles (beta) of 90 degrees and 135 degrees were taken for both smooth and ribbed cases. The heat transfer data were taken at high rotation numbers close to previous test section.

Orientador: Luiz Eduardo de Ângelo Sanchez / Banca: Vicente Luiz Scalon / Banca: Alisson Rocha Machado / Resumo: A crescente produtividade de aumento na produtividade em operações de usinagem toma cada vez mais importante o desenvolvimento de novas ferramentas de corte e novos métodos de manufatura, os quais devem ter a capacidade de preencher a demanda atual. Deste modo, muitos esforços têm sido direcionados para permitir a utilização de velocidade de corte cada vez maiores. Um grande desafio é controlar a temperatura durante o processo de usinagem, uma vez que a temperatura aumenta com o aumento da velocidade de corte, reduzindo a dureza a quente da ferramenta e alimentando os mecanismos de desgaste. Para minimizar estes efeitos, vários métodos de refrigeração têm sido propostos, cada um com suas vantagens e desvantagens. Os métodos convencionais de refrigeração, que utilizam fluidos de corte, embora possuam eficiência reconhecida, adicionam custos ao processo, além de serem causadores de problemas relacionados com o meio ambiente e com a saúde dos operadores. Neste contexto a usinagem a seco, associada com o emprego de ferramenta com alta dureza a quente, tem sido um bom método para evitar os problemas mencionados. Outra opção é a usinagem criogênica, que utiliza ferramentas de metal duro em temperaturas abaixo de -150ºC, utilizando, para isso, nitrogênio líquido como fluido refrigerante. Entretanto, este método traz alguns problemas, como a necessidade de equipamentos especiais com tamanho significante ao lado da máquina-ferramenta. Neste estudo é proposto o desenvolvimento e a construção de um sistema de refrigeraçã de ferramenta para o processo de tornemaneto, com baixo custo e manutenção simples, composto por um porta-ferramenta, com um fluido refrigerante passando internamente ao seu corpo em um circuito fechado, onde o fluido evapora em uma câmara abaixo do inserto de usinagem, removendo assim calor da ferramenta. O fluido refrigerante passa então através... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The growing need of increase in productivity in machining operations emphasizes the importance of the development of new cutting tools and new manufacturing methods, which have the capacity to fulfill the present demand. In this way, many efforts are directed to enable the utilization of higher cutting speeds. One great challenge is to control the temperature during the machining process, since the temperature rises with the increase of the cutting speed, reducing the hot hardness of the cutting tool and accelerating the tool wear mechanism. To minimize these effects, many cooling methods have been proposed, each one with advantages and disadvantages. The conventional cooling methods, which use cutting fluids, although have recognized efficiency, add costs to the process, besides to cause problems regarding to the environment and operators health. In this context, dry machining, associated with the employment of tools with high hot hardness, has been a good method to avoid these problems. Another option is the cryogenic machinig, which utilizes carbide tools in temperatures lower than - 150ºC, using, for this, liquid nitrogen as cooling fluid. However, this method brings some problems, like the need of special devices with significant size around the machine-tool. In this work, it is proposed the development and the construction of a cooling tool system for turning process, with low cost and simple maintenance, composed by a tool-holder, with a cooling fluid flowing within its body in a loop circuit, where the fluid evaporates just under the insert location, removing heat from it. The cooling fluid passes through a heat exchanger where it condensates and a new cyble is started. As result the development system provides a tool life equal or better than with the cutting fluid application, with clear economic and environmental advantages / Mestre

Made available in DSpace on 2014-06-11T19:28:20Z (GMT). No. of bitstreams: 0 Previous issue date: 2010-06-17Bitstream added on 2014-06-13T19:16:07Z : No. of bitstreams: 1 vicentin_gc_me_bauru.pdf: 1398083 bytes, checksum: b02b474f8b8ff324c644909a8dd89c24 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / A crescente produtividade de aumento na produtividade em operações de usinagem toma cada vez mais importante o desenvolvimento de novas ferramentas de corte e novos métodos de manufatura, os quais devem ter a capacidade de preencher a demanda atual. Deste modo, muitos esforços têm sido direcionados para permitir a utilização de velocidade de corte cada vez maiores. Um grande desafio é controlar a temperatura durante o processo de usinagem, uma vez que a temperatura aumenta com o aumento da velocidade de corte, reduzindo a dureza a quente da ferramenta e alimentando os mecanismos de desgaste. Para minimizar estes efeitos, vários métodos de refrigeração têm sido propostos, cada um com suas vantagens e desvantagens. Os métodos convencionais de refrigeração, que utilizam fluidos de corte, embora possuam eficiência reconhecida, adicionam custos ao processo, além de serem causadores de problemas relacionados com o meio ambiente e com a saúde dos operadores. Neste contexto a usinagem a seco, associada com o emprego de ferramenta com alta dureza a quente, tem sido um bom método para evitar os problemas mencionados. Outra opção é a usinagem criogênica, que utiliza ferramentas de metal duro em temperaturas abaixo de -150ºC, utilizando, para isso, nitrogênio líquido como fluido refrigerante. Entretanto, este método traz alguns problemas, como a necessidade de equipamentos especiais com tamanho significante ao lado da máquina-ferramenta. Neste estudo é proposto o desenvolvimento e a construção de um sistema de refrigeraçã de ferramenta para o processo de tornemaneto, com baixo custo e manutenção simples, composto por um porta-ferramenta, com um fluido refrigerante passando internamente ao seu corpo em um circuito fechado, onde o fluido evapora em uma câmara abaixo do inserto de usinagem, removendo assim calor da ferramenta. O fluido refrigerante passa então através... / The growing need of increase in productivity in machining operations emphasizes the importance of the development of new cutting tools and new manufacturing methods, which have the capacity to fulfill the present demand. In this way, many efforts are directed to enable the utilization of higher cutting speeds. One great challenge is to control the temperature during the machining process, since the temperature rises with the increase of the cutting speed, reducing the hot hardness of the cutting tool and accelerating the tool wear mechanism. To minimize these effects, many cooling methods have been proposed, each one with advantages and disadvantages. The conventional cooling methods, which use cutting fluids, although have recognized efficiency, add costs to the process, besides to cause problems regarding to the environment and operators health. In this context, dry machining, associated with the employment of tools with high hot hardness, has been a good method to avoid these problems. Another option is the cryogenic machinig, which utilizes carbide tools in temperatures lower than - 150ºC, using, for this, liquid nitrogen as cooling fluid. However, this method brings some problems, like the need of special devices with significant size around the machine-tool. In this work, it is proposed the development and the construction of a cooling tool system for turning process, with low cost and simple maintenance, composed by a tool-holder, with a cooling fluid flowing within its body in a loop circuit, where the fluid evaporates just under the insert location, removing heat from it. The cooling fluid passes through a heat exchanger where it condensates and a new cyble is started. As result the development system provides a tool life equal or better than with the cutting fluid application, with clear economic and environmental advantages

Die Kolbenstangeninnenkühlung (KSIK) ist eine Kühltechnologie für Kreuzkopfverdichter, die in verschiedenen Experimenten gezeigt hat, dass sie ein beträchtliches Potenzial zur Wärmeabfuhr aus den thermisch beanspruchten Komponenten im Zylinder- und Packungsbereich besitzt. Dass sie in der Praxis noch nicht zum Einsatz kommt, liegt unter anderem daran, dass ihre Wirkungsweise und insbesondere die thermo- und fluiddynamischen Vorgänge des zweiphasigen Kühlfluids im Stangeninneren nicht ausreichend erforscht sind, um die Wärmeabfuhr für verschiedene Verdichter und Förderaufgaben zu quantifizieren. Die vorliegende Arbeit hat daher zum Ziel, einen Teil dieser Wissenslücke für Kreuzkopfverdichter mit vertikaler Ausrichtung der Kolbenstange zu schließen. Um die relevante Wissensbasis zu Kreuzkopfverdichtern zu vermitteln, werden einleitend ihr Aufbau, ihre Funktionsweise und die Thematik ihrer Kühlung behandelt. Dabei wird unter anderem gezeigt, wie sich die Wärmeabfuhr auf die Energieeffizienz und die auftretenden Maximaltemperaturen auswirkt und welche Wärmemengen durch Reibung an den Dichtungselementen entstehen. Anhand einer exemplarischen Betrachtung von Verdichtern unterschiedlicher Druckniveaus und Förderströme wird deutlich, dass die KSIK vor allem bei niedrigen Drücken und für die Abfuhr der Reibleistungen effektiv eingesetzt werden kann. Die Größe bzw. Leistungsklasse erweist sich dabei nicht als limitierender Faktor. Anschließend erfolgt die Darstellung konventioneller Kühlverfahren sowie die ausführliche Vorstellung der KSIK mit den Themenschwerpunkten: Funktionsweise, Einflussgrößen, Stand der Technik und Einsatzgrenzen. Um die Wärmeabfuhr zu quantifizieren, die mit einer innengekühlten Kolbenstange eines stehenden Kreuzkopfverdichters erreicht wird, werden Messungen an einem Versuchsstand mit einer vertikal-oszillierenden Hohlstange vorgestellt. Insbesondere die Wahl des Kühlfluids sowie der eingefüllte Flüssigvolumenanteil beeinflussen den Wärmetransport, sind aber für den Fall der KSIK bislang nicht untersucht worden. Daher erfolgt zunächst eine Vorauswahl der vier bestgeeigneten Fluide anhand ihrer thermodynamischen Eignung für das Einsatzgebiet der Kreuzkopfverdichter. Bei den Messungen zeigt sich Wasser, insbesondere als Reinstoff-Füllung, aber auch als Gemisch mit Luft, als deutlich bestes Kühlfluid. Mit einer Dampf-Wasser-Füllung kann erwartungsgemäß eine bessere Wärmeabfuhr erzielt werden als mit einer Luft-Wasser-Füllung. Hinsichtlich des optimalen Flüssigvolumenanteils zeigte sich in den Messergebnissen mit Dampf-Wasser-Füllung eine optimale Wärmeabfuhr im Bereich zwischen 30 Vol.-%fl und 70 Vol.-%fl. In dieser relativ großen Spanne treten lediglich geringe Unterschiede hinsichtlich des Wärmetransports auf. Für die untersuchte Versuchsstandkonfiguration und im untersuchen Drehzahlbereich (300 min−1 bis 600 min−1) liegen die zugehörigen axialen Wärmestromdichten zwischen ca. 40 W cm−2 und 80 W cm−2 und die Wärmewiderstände zwischen ca. 0,27 K W−1 und 0,37 K W−1. Für eine Luft-Wasser-Füllung stellt sich ein etwas ausgeprägteres Optimum der Wärmeabfuhr bei 25 Vol.-%fl ein, für das sich axiale Wärmestromdichten zwischen ca. 33 W cm−2 und 38 W cm−2 und Wärmewiderstände zwischen ca. 0,7 K W−1 und 1,3 K W−1 ergeben. Da der fluidgebundene Wärmetransport im Innenvolumen der KSIK maßgeblich von der Strömung des zweiphasigen Kühlfluids abhängig ist, schließen sich an die Analyse der Wärmeabfuhr optische Messungen des Strömungsverhaltens einer Dampf-Wasser- sowie einer Luft-Wasser-Füllung mittels Hochgeschwindigkeitskamera an. Hierfür wurde eine optisch zugängliche Hohlstange mit nahezu gleichen Abmessungen, gleicher oszillierender Bewegung, aber ohne Wärmezu- und -abfuhr an den Stangenenden verwendet. Den Aufnahmen der Luft-Wasser-Füllungen ist zu entnehmen, dass sich die Strömung für alle untersuchten Drehzahlen und Flüssigvolumenanteile durch eine gemeinsame Struktur auszeichnet. Hierbei tritt die flüssige Phase stets in zwei hauptsächlichen Erscheinungsformen auf: Zum einen ein Teil am unteren Stangenende, der in der vorliegenden Arbeit als Sumpf bezeichnet wird. Und zum anderen ein zweiter Teil, der durch die oszillierende Stangenbewegung den Sumpf verlässt und sich als Wandfilm im Innenvolumen zunächst nach oben bewegt und nach Erreichen einer maximalen Höhe wieder nach unten zurückfließt. Dieser Teil erhält daher die Bezeichnung Film. Die Bewegung der Luft im Stangeninneren ergibt sich im Wesentlichen durch die Verdrängung der Flüssigkeit. Die Strömung von Sumpf und Film kann auf der Basis vereinfachender geometrischer Annahmen mithilfe von drei zeitabhängigen, charakteristischen Strömungsparametern beschrieben werden: Der Sumpfhöhe, der Filmhöhe sowie der Filmdicke. Diese wurden für verschiedene Füllmengen zwischen 10 Vol.-%fl und 40 Vol.-%fl und im Drehzahlbereich zwischen 300 min−1 und 600 min−1 quantifiziert. Es zeigt sich, dass das Wärmetransportverhalten und die Strömung einer Luft-Wasser-Füllung eng gekoppelt sind und sich die Ergebnisse der optischen Untersuchung bei der Interpretation des thermodynamischen Verhaltens als hilfreich erweisen. Beispielsweise offenbaren sie die Gründe für den optimalen Flüssigvolumenanteil von 25 Vol.-%fl. Darüber hinaus wurde auch die Strömung verschiedener Dampf-Wasser-Füllungen untersucht, die in einigen wesentlichen Aspekten von den Ergebnissen mit Luft-Wasser-Füllung abweicht. Der wichtigste Unterschied zeigt sich in der Möglichkeit des Phasenwechsels zwischen flüssiger und gasförmiger Phase und vice versa, was bei einem Luft-Wasser-Gemisch in keiner Messung festgestellt werden konnte. Neben einer mutmaßlichen Verbesserung der Wärmeübergangskoeffizienten resultiert hieraus auch eine bessere Durchströmung des Innenvolumens sowie eine bessere Durchmischung der gasförmigen und flüssigen Phase, sodass nachvollziehbar wird, warum ein Dampf-Wasser-Gemisch eine bessere Wärmeabfuhr ermöglicht als mit Luft als zusätzlicher Komponente. Um die Berechnung der Strömung von Luft-Wasser-Füllungen in einer vertikal-oszillierenden Hohlstange für beliebige Konfigurationen zu ermöglichen, wurde ein Strömungsmodell entwickelt, mit dem die zeitlich abhängige Verteilung der flüssigen und gasförmigen Phase berechnet werden kann. Dabei sind u. a. die Drehzahl, der Flüssigvolumenanteil sowie der Innendurchmesser und die Länge der Hohlstange frei wählbar. Zur Beschreibung der Verteilung der flüssigen Phase wird die in den optischen Untersuchungen festgestellte Aufteilung in Sumpf- und Filmanteile für das Berechnungsmodell übernommen. Die Grundlage für die Berechnung der Strömung mittels Zeitschrittverfahrens stellt die Bewegungsgleichung für die Flüssigkeit in vertikaler Richtung unter Berücksichtigung verschiedener Beschleunigungsanteile dar, die die Filmbewegung hervorrufen. Die restlichen charakteristischen Strömungsparameter ergeben sich durch die Inkompressibilität der flüssigen Phase und durch die empirische Vorgabe der Filmdicke, sodass dadurch auch die zeitlich abhängige Verteilung des Sumpfs und der gasförmigen Phase ermittelt werden kann. Der Vergleich der Berechnungs- und Messergebnisse für die charakteristischen Strömungsparameter zeigt eine in den meisten Fällen zufriedenstellende Übereinstimmung und bestätigt die Herangehensweise und das Berechnungskonzept des Strömungsmodells.:1 Einleitung 2 Grundlagen der Kreuzkopfverdichter 3 Experimentelle Untersuchungen der Wärmeabfuhr 4 Experimentelle Untersuchungen der Strömung 5 Berechnungsmodell für die Strömung mit Luft-Wasser-Füllung 6 Zusammenfassung A Anhang

This study attempts to understand one of the most fundamental and challenging problems in fluid flow and heat transfer for rotating machines. The study focuses on gas turbines and electric generators for high temperature and high energy density applications, respectively, both which employ rotating cooling channels so that materials do not fail under high temperature and high stress environment. Prediction of fluid flow and heat transfer inside internal cooling channels that rotate at high rotation number and high density ratio similar to those that are existing in turbine blades and generator rotors is the main focus of this study. Both smooth-wall and rib-roughened channels are considered here. Rotation, buoyancy, bends, ribs and boundary conditions affect the flow inside theses channels. Ribs are introduced inside internal cooling channel in order to enhance the heat transfer rate. The use of ribs causes rapid increase in the supply pressure, which is already limited in a turbine or a generator and requires high cost for manufacturing. Hence careful optimization is needed to justify the use of ribs. Increasing rotation number (Ro) is another approach to increase heat transfer rate to values that are comparable to those achieved by introduction of ribs. One objective of this research is to study and compare theses two approaches in order to decide the optimum range of application and a possible replacement of the high-cost and complex ribs by increasing Ro. A fully computational approach is employed in this study. On the basis of comparison between two-equation (k-[epsilon] and k-[omega]) and RSM turbulence models, against limited available experimental data, it is concluded that the two-equation turbulence models cannot predict the anisotropic turbulent flow field and heat transfer correctly, while RSM showed improved prediction. For the near wall region, two approaches with standard wall functions and enhanced near wall treatment were investigated. The enhanced near wall approach showed superior results to the standard wall functions approach. Thus RSM with enhanced near wall treatment is validated against available experimental data (which are primarily at low rotation and buoyancy numbers). The model was then used for cases with high Ro (as much as 1.29) and high-density ratios (DR) (up to 0.4). Particular attention is given to how turbulence intensity, Reynolds stresses and transport are affected by Coriolis and buoyancy/centrifugal forces caused by high levels of Ro and DR. Variations of flow total pressure along the rotating channel are also predicted. The results obtained are explained in view of physical interpretation of Coriolis and centrifugal forces. Investigation of channels with smooth and with rib-roughened walls that are rotating about an orthogonal axis showed that increasing Ro always enhances turbulence and the heat transfer rate, while at high Ro, increasing DR although causes higher turbulence activity but does not necessarily increase Nu and in some locations even decreases Nu. The increasing thermal boundary layer thickness near walls is the possible reason for this behavior of Nu. The heat transfer enhancement for smooth-wall cases correlates linearly with Ro (with other parameters are kept constant) and hence it is possible to derive linear correlation for the increase in Nu as a function of Ro. Investigation of channels with rib-roughened walls that rotate about orthogonal axis showed that 4-side-average Nur correlates with Ro linearly, where a linear correlation for Nur/Nus as a function of Ro is derived. It is also observed that the heat transfer rate on smooth-wall channel can be enhanced rapidly by increasing Ro to values that are comparable to the enhancement due to the introduction of ribs inside internal cooling channels. This observation suggests that ribs may be unnecessary in high-speed machines, and has tremendous implications for possible cost savings in these machines. In square channels that rotate about parallel axis, the heat transfer rate enhances with Ro on three surfaces of the square channel and decreases on the inner surface (that is the one closest to the axis of rotation). However, the four-sides average Nu increases with Ro. Increasing wall heat flux at high Ro does not necessarily increase Nu on walls although higher turbulence activity is observed. This study examines the rich interplay of physics under the simultaneous actions of Coriolis and centrifugal/buoyancy forces in one of the most challenging internal flow configurations. Several important conclusions are reached from this computational study that may have far-reaching implications on how turbine blades and generator rotors are currently designed. Since the computation study in not validated for high Ro cases, these important results call for a experimental investigation.

Establishing a clean and renewable energy supply is the preeminent engineering challenge of our time. Turbines, in some form, are responsible for more than 98 percent of all electricity generated in the United State and 100 percent of commercial and military air transport. The operation of these engines is clearly responsible for significant consumption of hydrocarbon fuels and, in turn, emission of green house gases into the atmosphere. With such wide-scale implementation, it is understood that even the smallest increase in the operating efficiency of these machines can lead to enormous improvements over the current energy situation. These effects can extend from a reduction in the emission of greenhouse gases to lessening the nation's dependence of foreign energy sources to lower energy prices for the consumer. The prominent means of increasing engine efficiency is by raising the 'Turbine Inlet Temperature' ' the temperature of the mainstream flow after combustion, entering the first stage of the turbine section. The challenge is presented when these temperatures are forced beyond the allowable limits of the materials inside the machine. In order to protect these components, active cooling and protection methods are employed. The focus of this work is the development of more efficient means of cooling 'hot' turbine components. In doing so, the goal is to maximize the amount of heat removed by the coolant while minimizing the coolant mass flow rate: by removing a greater amount of heat with a lower coolant mass flow rate, more compressed air is left in the mainstream gas flow for combustion and power generation. This study is an investigation of the heat transfer augmentation through the fully-developed portion of a narrow rectangular duct (AR=2) characterized by the application of dimples to the bottom wall of the channel. Experimental testing and numerical modeling is performed for full support and validation of presented findings. The geometries are studied at channel Reynolds numbers of 20000, 30000, and 40000. The purpose is to understand the contribution of dimple geometries in the formation of flow structures that improve the advection of heat away from the channel walls. Experimental data reported includes the local and Nusselt number augmentation of the channel walls and the overall friction augmentation throughout the length of the duct. Computational results validate local Nusselt number results from experiments, in addition to providing further insight to local flow physics causing the observed surface phenomena. By contributing to a clearer understanding of the effects produced by these geometries, the development of more effective channel-cooling designs can be achieved.

Detached Eddy Simulations (DES) is a hybrid URANS-LES technique that was proposed to obtain computationally feasible solutions of high Reynolds number flows undergoing massive separation with reliable accuracy. Since its inception, DES has been applied to a wide variety of flow fields, but mostly limited to unbounded external aerodynamic flows. This is the first study to apply and validate DES to predict the internal flow and heat transfer in non-canonical flows of industrial relevance. The prediction capabilities of DES in capturing the effects of Coriolis forces, which are induced by rotation, and centrifugal buoyancy forces, which are induced by thermal gradients, are also authenticated. The accurate prediction of turbulent flows is sensitive to the level of turbulence predicted by the turbulence scheme. By treating the regions of interest in LES mode, DES allows the unsteadiness in these regions to develop and hence predicts the turbulence levels accurately. Additionally, this permits DES to capture the effects of system rotation and buoyancy. Computations on a rotating system (a sudden expansion duct) and a system subjected to thermal gradients (cavity with a heated wall) validate the prediction capability of DES. The application of DES is further extended to a non-canonical, internal flow which is of relevance in internal cooling of gas turbine blades. Computations of the fully developed flow and heat transfer shows that DES surpasses several shortcomings of the RANS model on which it is based. DES accurately predicts the primary and secondary flow features, the turbulence characteristics and the heat transfer in stationary ducts and in rotating ducts, where the effects of Coriolis forces and centrifugal buoyancy forces are dominant. DES computations are carried out at a computational cost that is almost an order of magnitude less than the LES with little compromise on the accuracy. However, the capabilities of DES in predicting the transition to turbulence are inadequate, as highlighted by the flow features and the heat transfer in the developing region of the duct. But once the flow becomes fully turbulent, DES predicts the flow physics and shows good quantitative agreement with the experiments and LES. / Ph. D.

For the past several years there has been great effort in the analysis of internal duct cooling. The steady increase in power output and thermal efficiency requirements for gas turbine engines has called for significant advancement in turbine blade internal duct cooling technology. Numerical analysis of turbulent duct flow has been largely limited to Reynolds Averaged Navier-Stokes (RANS) simulations. This is because of the low computational requirements of such calculations relative to Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS). However, the tides have started to turn in favor of LES, partly because of the exponential increase in computer hardware performance in recent years. Three conference papers make up the contents of this thesis. LES is performed for fully developed flow and heat transfer in a staggered 45º ribbed duct in the first paper. The rib pitch-toheight ratio P / e is 10 and a rib height to hydraulic diameter ratio h e / D is 0.1. The Reynolds numberbased on the bulk flow rate and hydraulic diameter is 47,300. The overall heat transfer enhancement obtained was a factor of 2.3, which matched experimental data within 2%. The surfaces of highest heat transfer enhancement were the ribbed walls and the outer wall. Results from LES of an orthogonally rotating 90º ribbed duct are presented in the second paper for rotation numbers: Ro = 0.18, 0.35 and 0.67. The Reynolds number is 20,000. The P / e and h e / D were the same as in the first paper. Turbulence and heat transfer are augmented on the trailing surface and reduced at the leading surface. Secondary flows induced by Coriolis forces, increase heat transfer augmentation on the smooth walls. Finally, the third paper studies the same flow conditions of the second paper and goes further by including effects of centrifugal buoyancy forces using LES. Two buoyancy numbers are studied: Bo = 0.12 and 0.29. Centrifugal buoyancy does not have a large effect on leading side augmentation ratios for all rotation numbers, but increases heat transfer significantly on the trailing side. In all papers, mean flow and heat transfer results compare well with experimental data. / Master of Science

In the past few decades, the operating temperatures of gas turbine engines have increased significantly with a view towards increasing the overall thermal efficiency and specific power output. As a result of increased turbine inlet temperatures, the hot gas path components downstream of the combustor section are subjected to high heat loads. Though materials with improved temperature capabilities are used in the construction of the hot gas path components, in order to ensure safe and durable operation, the hot gas path components are additionally supplemented with thermal barrier coatings (TBCs) and sophisticated cooling techniques. The present study focusses on two aspects of gas turbine cooling, namely augmented internal cooling and external film cooling. One of the commonly used methods for cooling the vanes involves passing coolant air bled from the compressor through serpentine passages inside the airfoils. The walls of the internal cooling passages are usually roughened with turbulence promoters like ribs to enhance heat transfer. Though the ribs help in augmenting the heat transfer, they have an associated pressure penalty as well. Therefore, it is important to study the thermal-hydraulic performance of ribbed internal cooling passages. The first section of the thesis deals with the numerical investigation of flow and heat transfer characteristics in a ribbed two-pass channel. Four different rib shapes- 45° angled, V-shaped, W-shaped and M-shaped, were studied. This study further aims at exploring the performance of different rib-shapes at a large rib pitch-to-height ratio (p/e=16) which has potential applications in land-based gas turbines operating at high Reynolds numbers. Detailed flow and heat transfer analysis have been presented to illustrate how the innate flow physics associated with the bend region and the different rib shapes contribute to heat transfer enhancement in the two-pass channel. The bend-induced secondary flows were observed to significantly affect the flow and heat transfer distribution in the 2nd pass. The thermal-hydraulic performance of V-shaped and 45° angled ribs were better than W-shaped and M-shaped ribs. The second section of the study deals with the analysis of film cooling performance of different hole configurations on the endwall upstream of a first stage nozzle guide vane. The flow along the endwall of the airfoils is highly complex, dominated by 3-dimensional secondary flows. The presence of complex secondary flows makes the cooling of the airfoil endwalls challenging. These secondary flows strongly influence endwall film cooling and the associated heat transfer. In this study, three different cooling configurations- slot, cylindrical holes and tripod holes were studied. Steady-state experiments were conducted in a low speed, linear cascade wind tunnel. The adiabatic film cooling effectiveness on the endwall was computed based on the spatially resolved temperature data obtained from the infrared camera. The effect of mass flow ratio on the film cooling performance of the different configurations was also explored. For all the configurations, the coolant jets were unable to overcome the strong secondary flows inside the passage at low mass flow ratios. However, the coolant jets were observed to provide much better film coverage at higher mass flow ratios. In case of cylindrical ejection, the effectiveness values were observed to be very low which could be because of jet lift-off. The effectiveness of tripod ejection was comparable to slot ejection at mass flow ratios between 0.5-1.5, while at higher mass flow ratios, slot ejection was observed to outperform tripod ejection. / Master of Science

Aircraft engine manufacturers strive for greater performance and efficiency by continually increasing the turbine inlet temperature. High turbine inlet temperatures significantly degrade the lifetime of components in the turbine. Modern gas turbines operate with turbine inlet temperatures well above the melting temperature of key turbine components. Without active cooling schemes, modern turbines would fail catastrophically. This study will evaluate a novel cooling scheme for turbine airfoils, called microcircuit cooling, in which small cooling channels are located extremely close to the surface of a turbine airfoil. Coolant bled from the compressor passes through the microcircuits and exits through film cooling slots. On further cooling benefit is that the microcircuit passages are filled with irregular pin fin features that serve to increase convective cooling through the channels. Results from this study indicate a strong interaction between the internal microcircuit features and the external film-cooling from the slot exit. Asymmetric cooling patterns downstream of the slot resulted from the asymmetric pin fin design within the microcircuit. Adiabatic effectiveness levels were found to be optimum for the slot design at a blowing ratio of 0.37. The pin fin arrangement along with the impingement cooling at the microcircuit entrance increased the area-averaged heat transfer by a factor of three, relative to an obstructed channel, over a Reynolds range of 5,000 to 15,000. / Master of Science