In this thesis, the flow and heat transfer in dimpled channels have been investigated using the Reynolds-averaged Navier-Stokes (RANS) approach. The primary objective of this investigation is to identify the capabilities of RANS models to reproduce the characteristics of the flow and heat transfer in dimples. The flow in dimpled channels has been chosen as the test case due to their relevance to gas turbine cooling applications, as well as the fairly complex flow features over dimples, which poses a challenge to turbulence modelling. Five turbulence models have been tested in the present work. These include: the Launder and Sharma k-epsilon model, both the Craft et al. (1996) and (2000) cubic k-epsilon models, the Hanjalic and Jakirlic Reynolds stress model (RSM), as well as the Craft (1998) two-component limit (TCL) RSM. The models have been chosen such that all three classes of RANS closure were tested. The tested models have been applied to two dimpled channel configurations with increasing complexity. In the first, the flow over a single dimple in a channel has been considered, while in the second, the case of a staggered array of dimples has been examined. Moreover, across these two configurations, the effect of the dimple depth, the channel height and the Reynolds number have also been investigated. The results show that all models produce a physically viable solution for the problem of the flow in dimpled channels. Nevertheless, the Craft et al. (1996) and (2000) cubic k-ε models, as well as the Craft (1998) TCL RSM, predicted dimple flow structures that deviate from the expected state. In general, the main flow characteristics are reproduced by the RANS models, and the predicted mean velocity profiles are in good agreement with the data. All models report an overall enhancement in heat transfer levels when using dimples in comparison to those of a plane channel.

Time-accurate calculations are used to investigate the three-dimensional flow structure and understand its influence on the heat transfer in a channel with concave indentations on one wall. A dimple depth to channel height ratio of 0.4 and dimple depth to imprint diameter ratio of 0.2 is used in the calculations. The Reynolds number (based on channel height) varies from Re = 25 in the laminar regime to Re = 2000 in the early turbulent regime. Fully developed flow and heat transfer conditions were assumed and a constant heat flux boundary condition was applied to the walls of the channel. In the laminar regime, the flow and heat transfer characteristics are dominated by the recirculation zones in the dimple with resulting augmentation ratios below unity. Flow transition is found to occur between Re = 1020 and 1130 after which both heat transfer and friction augmentation increase to values of 3.22 and 2.75, respectively, at Re = 2000. The presence of large scale vortical structures ejected from the dimple cavity dominate all aspects of the flow and heat transfer, not only on the dimpled surface but also on the smooth wall. In all cases the thermal efficiency using dimples was found to be significantly larger than other heat transfer augmentation techniques currently employed. / Master of Science

This work presents direct and large eddy simulations of a wide range of heat augmentation surfaces roughened by modified dimples/protrusions. The dissertation is composed of two main parts: Part I (Chapters 2-4) for compact heat exchangers and Part II (Chapter 5) for internal cooling of rotating turbine blades. Part I consists of three phases: Phase I (Chapter 2) investigates flow structure and heat transfer distribution in a channel with dimples/protrusions; Phase II (Chapter 3) studies the application of dimples as surface roughness on plain fins; and Phase III (Chapter 4) considers a new fin shape, the split-dimple fin, that is based on modifying the conventional dimple shape. Chapter 2 presents direct and large eddy simulations conducted of a fin bank over a wide range of Reynolds numbers, ReH=200-15,000, covering the laminar to fully turbulent flow regimes and using two channel height geometries. While the smaller fin pitch channel has better performance in the low to medium Reynolds number range, both channel heights show similar trends in the fully turbulent regime. Moreover, analysis of the results shows that vortices generated in the dimple cavity and at the dimple rim contribute substantially to heat transfer from the dimpled surface, whereas flow impingement and acceleration between protrusions contribute substantially on the protrusion side. Chapter 3 considers applying dimples as surface roughness on plain fin surfaces to further enhance heat transfer from the fin. Three fin geometries that consider dimple imprint diameter effect and perforation effect are considered. The dimple imprint diameter has a minimal effect on the flow and heat transfer of the fin. However, the introduction of perforation in the dimple significantly changes the flow structure and heat transfer on the dimple side of the fin by eliminating recirculation regions in the dimple and generating higher intensity vortical structures. Chapter 4 presents a novel fin shape, the split-dimple fin, which consists of half a dimple and half a protrusion with an opening between them. The split dimple provides an additional mechanism for augmenting heat transfer by perturbing continuous boundary layer formation on the fin surface and generating energetic shear layers. While the protruding geometry of the split dimple augments heat transfer profoundly, it also increase pressure drop. The split dimple fin results in heat conductance that is 60–175% higher than a plain fin, but at a cost of 4–8 times the frictional losses. Chapter 5 studies the employment of dimples/protrusions on opposite sides for internal cooling of rotating turbine blades. Two geometries with two dimple/protrusion depths are investigated over a wide range of rotation numbers, Rob=-0.77 to 1.10. Results show that the dimple side is more sensitive to the destabilizing forces on the trailing surface, while both react similarly to the stabilizing effect on the leading side. It is concluded that placing the protrusion on the trailing side for low rotation number, |Rob|<0.2, provides better performance, while it is more beneficial to place the dimple side on the trailing side for higher rotation numbers. / Ph. D.

Detailed heat transfer coefficient distributions have been obtained for narrow converging and diverging channels with and without enhancement features. The enhancement feature considered for this study is dimples (inline and staggered) on the main heat transfer surfaces. All the measurements are presented at Reynolds numbers of 3500, 8900, 18000, and 7000, 14000, 28000 for converging and diverging channels respectively. Pressure drop measurements for the overall channel are also presented to evaluate the heat transfer enhancement geometry with respect to pumping power requirements. The test models were studied for wall heat transfer coefficient measurements using the transient liquid crystal technique. The modeled wall inner surfaces were sprayed with thermochromic liquid crystals, and a transient test was used to obtain the local heat transfer coefficients from the measured color change. Analysis of results shows that dimples, in general, have very good enhancement capabilities and staggered dimpled surfaces provide considerably higher heat transfer coefficients and a reasonable pressure drop compared to inline dimpled configuration. Additionally, this study was extended to understand the effect of strategic placement of dimples (staggered) at various locations along the channel to understand regions that contribute significantly to the overall enhancement. / Master of Science

Film cooling effectiveness on a gas turbine blade tip on the near tip pressure side and on the squealer cavity floor is investigated. Optimal arrangement of film cooling holes, effect of a full squealer and a cutback squealer, varying blowing ratios and squealer cavity depth are also examined on film cooling effectiveness. The film-cooling effectiveness distributions are measured on the blade tip, near tip pressure side and the inner pressure and suction side rim walls using a Pressure Sensitive Paint (PSP) technique. A blowing ratio of 1.0 is found to give best results on the pressure side whereas the other tip surfaces give best results for blowing ratios of 2. Film cooling effectiveness tests are also performed on the span of a fully-cooled high pressure turbine blade in a 5 bladed linear cascade using the PSP technique. Film cooling effectiveness over the entire blade region is determined from full coverage film cooling, showerhead cooling and from each individual row with and without an upstream wake. The effect of superposition of film cooling effectiveness from each individual row is then compared with full coverage film cooling. Results show that an upstream wake can result in lower film cooling effectiveness on the blade. Effectiveness magnitudes from superposition of effectiveness data from individual rows are comparable with that from full coverage film cooling. Internal heat transfer measurements are also performed in a high aspect ratio channel and from jet array impingement on a turbulated target wall at large Reynolds numbers. For the channel, three dimple and one discrete rib configurations are tested on one of the wide walls for Reynolds numbers up to 1.3 million. The presence of a turbulated wall and its effect on heat transfer enhancement against a smooth surface is investigated. Heat transfer enhancement is found to decrease at high Re with the discrete rib configurations providing the best enhancement but highest pressure losses. Experiments to investigate heat transfer and pressure loss from jet array impingement are also performed on the target wall at Reynolds numbers up to 450,000. The heat transfer from a turbulated target wall and two jet plates is investigated. A target wall with short pins provides the best heat transfer with the dimpled target wall giving the lowest heat transfer among the three geometries studied.

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.

Les méta-matériaux sont une nouvelle classe de matériaux composites artificiels quiprésentent des propriétés inédites. Ils sont typiquement sous divisés en unité appelées méta-atomes.Un design approprié de ces méta-atomes, architecturés à l’échelle nanométrique, permet d’induire despropriétés aussi extraordinaires qu’un indice de réfraction négatif. Dans ce contexte, nous avonsdéveloppé des particules à patchs, capable de développer des interactions selon des directionsprédéterminées. Des clusters multipodiques fait de ces particules (diélectrique) entourées d’un nombrecontrôlé de satellites plasmoniques (or) ont été développés. Nous nous sommes focalisés sur desclusters isotropes, dérivant de géométries tétraédriques, octaédriques et icosaédriques (trois des cinqsolides de Platon). Pour cela, nous avons utilisé des clusters silice/polystyrène, obtenus parpolymérisation ensemencée en émulsion, qui ont servi de préformes. Ils ont ainsi permis d’obtenir desparticules dont les patchs sont en fait des fossettes au fond desquelles subsiste un résidu de chaînespolystyrène greffées. En modifiant chimiquement ces chaînes, nous avons permis soit l’accrochage aufond de ces fossettes de colloïdes d’or puis leur croissance, soit l’accostage de satellites de silice surlesquels nous avons ensuite fait croître une coquille d’or. La seconde voie à offert un meilleur contrôlede la morphologie des clusters et notamment de la distance entre les satellites d’or (quelquesnanomètres) qui est primordiale pour assurer un couplage plasmonique optimal. Les propriétés desclusters obtenus ont été modélisées et mesurées. / Metamaterials are a novel class of artificial composite materials, typically made of subunit called meta-atoms and exhibiting unusual properties. Such meta-atoms, have to be architecturedat the nanometric level, to induce as extraordinary properties as a negative refractive index. In thiscontext, we developed patchy particles, capable to create interactions along predetermined directions.Multipodic clusters made of those (dielectric) particles surrounded by a controlled number ofplasmonic satellites (gold) were developed. We focused on isotropic clusters deriving fromtetrahedral, octahedral and icosahedral geometry (three of the fifth Platonic solids). For that purpose,we used silica/polystyrene clusters, obtained from seeded emulsion polymerization, as template. Byderiving those clusters, patchy particles bearing dimples containing grafted residual polystyrene chainswere obtained. By chemically deriving those chains, we explored two synthetic pathways, thedecoration of the dimples with gold colloids subsequently grown or the anchoring of silica satellitesonto which gold shells were subsequently grown. The second one was prove to offer a better controlover the cluster morphology as well as the inter-satellites gap (few nanometer) which is pivotal toensure an optimal plasmonic coupling. Then, the optical properties of the as obtained clusters weresimulated and measured.