Engine development has been in the direction of increased turbine inlet temperatures to improve efficiency and power output. Secondary flows develop as a result of a near-wall pressure gradient in the stagnating flow approaching the inlet nozzle guide vane as well as a strong cross-passage gradient within the passage. These flow structures enhance heat transfer and convect hot core flow gases onto component surfaces. In modern engines it has become critical to cool component surfaces to extend part life. Bypass leakage flow emerging from the slot between the combustor and turbine endwalls can be utilized for cooling purposes if properly designed. This study examines a three-dimensional slot geometry, scalloped to manipulated leakage flow distribution. Statistical techniques are used to decouple the effects of four geometric parameters and quantify the relative influence of each on endwall cooling levels and near-wall total pressure losses. The slot geometry is also optimized for robustness across a range of inlet conditions. Average upstream distance to the slot is shown to dominate overall cooling levels with nominal slot width gaining influence at higher leakage flow rates. Scalloping amplitude is most influential to near-wall total pressure loss as formation of the horseshoe vortex and cross flow within the passage are affected. Scalloping phase alters local cooling levels as leakage injection is shifted laterally across the endwall. / Ph. D.

A higher demand in power output from modern land based gas turbines has resulted in an increase in combustor exit temperatures. High temperatures in turn have resulted in flatter profiles at the combustor exit warranting the need for sufficient cooling of the endwall region. Endwall cooling is affected by the coolant flow through certain design features. A typical endwall design includes a leakage slot at the interface between the combustor and the vane, a leakage slot at the vane-to-vane interface and film-cooling holes. In addition, with the increase in energy demands and depletion of natural gas resources, alternate fuels such as coal derived synthetic gas are being used in gas turbines. Coal derived fuels, however, contain traces of ash and other contaminants that deposit on endwall surfaces, thereby altering its surface conditions. The purpose of this study was to investigate the effects of realistic endwall features and surface conditions on leading edge endwall cooling. Endwall designs like placing film-cooling holes in a trench, which provide an effective means of improving cooling were also studied at the leading edge. An infrared camera was used to obtain measurements of adiabatic effectiveness levels and a laser Doppler velocimeter was used for flowfield measurements. This study was done on a large scale, low-speed, recirculating wind tunnel operating at a Reynolds number of 2.1e+5 and an inlet mainstream turbulence level of 1%. Endwall measurements were taken for coolant flow through varying slot width at the combustor-vane interface. A constant coolant mass flow and a narrower combustor-turbine interface slot caused the coolant to exit uniformly whereas increasing the slot width had an opposite effect. Measurements were also taken with hole blockage and spallation, which showed a 10-25% decrease in the effectiveness levels whereas near hole deposition showed a 20% increase in effectiveness levels. A comparison of the cooling effectiveness due to placement of film-cooling holes in a trench was made to film-cooling holes not placed in a trench. Measurements indicated a superior performance of trenched holes to holes without a trench. Trenched holes showed a 60% increase in effectiveness levels due to decreased coolant jet separation. / Ph. D.

Power electronics is a constantly growing and demanding technical field. Consumer demand and developing technologies have made the improvement of power density a primary emphasis of research for this area. Power semiconductors present some of the major challenges for increasing system level power density due to high loss density and interconnection requirements. Advanced cooling schemes, such as double-sided, forced liquid convection or multi-phase flow, can be implemented with non-wire bond packaging to improve thermal management while maintaining proper electrical performance. Embedded power is one such packaging technology, which provides a compact structure for interface of power semiconductor to fluid flow. The objective of this work was to identify the potential of implementing embedded power packaging with double-sided forced liquid convection. Physics based, electro-thermal models were first used to predict the improvement in heat transfer of double-sided, forced liquid convection with embedded power packaging over single-sided liquid cooled wire bond based packaging. A liquid module test bed was designed and constructed based on the electro-thermal models, which could be interfaced with high power MOSFET based samples implementing various packaging technologies. Experiments were used to verify the model predictions and identify practical limitations of high flow rate, double-sided liquid cooling with embedded power. An improvement of 45% to 60% in total junction to case thermal resistance is shown for embedded power packaging with double-sided liquid cooling for water flow rates between 0.25 and 4.5 gal/min. / Master of Science

Increasing the combustion temperatures in a gas turbine engine to achieve higher efficiency and power output also results in high heat loads to turbine components downstream of the combustor. The challenge of adequately cooling the nozzle guide vane directly downstream of the combustor is compounded by a complex vortical secondary flow at the junction of the endwall and the airfoil. This flow tends to increase local heat transfer rates and sweep coolant away from component surfaces, as well as decrease the turbine aerodynamic efficiency. Past research has shown that a large fillet at the endwall-airfoil junction can reduce or eliminate the secondary flow. Also, leakage flow from the interface gap between the combustor and the turbine can provide some cooling to the endwall. This study examines the individual and combined effects of a large fillet and realistic combustor-turbine interface gap leakage flow for a nozzle guide vane. The first study focuses on the effect of leakage flow from the interface gap on the endwall upstream of the vane. The second study addresses the influence of large fillets at the endwall-airfoil junction, with and without upstream leakage flow. Both studies were performed in a large low-speed wind tunnel with the same vane geometry. Endwall shear stress measurements were obtained for various endwall-airfoil junction geometries without upstream leakage flow. Endwall heat transfer and cooling effectiveness were measured for various leakage flow rates and leakage gap widths, with a variety of endwall-airfoil junction geometries. Results from these studies indicate that the secondary flow has a large influence on the coverage area of the leakage coolant. Increased leakage flow rates resulted in better cooling effectiveness and coverage, but also higher heat transfer rates. The two fillet geometries tested affected coolant coverage by displacing coolant around the base of the fillet, which could result in undesirably high gradients in endwall temperature. The addition of a large fillet to the endwall-airfoil junction, however, reduced heat transfer, even when upstream leakage flow was present. / Master of Science

An experimental and numerical investigation was conducted to determine the film cooling effectiveness of a normal slot and angled slot under realistic engine Mach number conditions. Freestream Mach numbers of 0.65 and 1.3 were tested. For the normal slot, hot gas ingestion into the slot was observed at low blowing ratios (M < 0.25). At high blowing ratios (M > 0.6) the cooling film was observed to "lift off" from the surface. For the 30o angled slot, the data was found to collapse using the blowing ratio as a scaling parameter (x/Ms). Results from the current experiment were compared with the subsonic data published to confirm this test procedure. For the angled slot, at the supersonic freestream Mach number, the current experiment shows that at the same x/Ms, the film cooling effectiveness increases by as much as 25% as compared to the subsonic case. The results of the experiment also show that at the same x/Ms, the film cooling effectiveness of the angled slot is considerably higher than that of the normal slot, at both subsonic and supersonic Mach numbers. The flow physics for the slot tests considered here are also described with computational fluid dynamic (CFD) simulations in the subsonic and supersonic regimes. / Master of Science

M.S.

Gas turbine engines for propulsion operate under harsh conditions including gas temperatures that exceed the melting point of the metal, high mechanical stresses, and particulate ingestion such as sand. To maintain a low and uniform metal temperature to extend the life of a turbine component, a complex scheme of internal convective cooling and external film-cooling is required. Gas turbine engines operated in sandy or dusty environments can ingest a large quantity of sand into the mainstream and, more importantly, into the cooling system. Sand ingested into the coolant system has the potential to reduce or block off the flow intended to cool the turbine blades or vanes. If the source of coolant air to a critical region of a turbine blade were partially blocked, it would result in a substantial reduction in component life. This study includes establishing a methodology for testing sand ingestion characteristics on a simulated turbine component with film-cooling holes at room temperature and engine temperatures. The study evaluates a simple array of laser drilled film-cooling holes, similar to a showerhead on the leading edge of an airfoil. The blocking characteristics of this design indicate that increasing the airflow or decreasing the sand amount results in a decreased blockage. It was also determined that as the metal temperature increases, the blockage from a given amount of sand increases. The methodology used in the primary portion of this thesis was modified to test sand ingestion characteristics on actual turbine blades with film-cooling holes at room temperature and engine temperatures. The study evaluated the blockage performance of several different turbine blades including the F-100-229-full, F-100-229-TE, and the F-119 with a new trailing edge cooling methodology know as a microcircuit. It was shown that increasing the airflow or pressure ratio, or decreasing the sand amount would result in decreased blockage. It was also shown that over a certain metal and coolant temperature, the blockage is significantly worsened. However, it was also shown on the F-119 turbine blade that below a given metal temperature, there is no impact of metal or coolant temperature on sand blockage. / Master of Science

Modern gas turbine engines operate in environments where particle ingestion, especially sand ingestion, can affect the cooling of various turbine parts. The most critical areas are in the combustor and the first stage components of the turbine. Gas temperatures in these areas are the highest compared to other areas and exceed the melting points of the constituent metals. To extend the life of hot section components, internal convective cooling and external film-cooling are required. This study examined the effects of sand ingestion on various cooling geometries. The first part investigated impingement and film-cooling implemented in a double-walled cooling geometry for the purpose of reducing sand size and thereby reducing blockage due to sand ingestion. The second part analyzed the cooling performance of actual turbine blade outer air seals injected with sand. Results from these studies showed areas of impingement that promote particle fragmentation are advantageous in reducing particle size and reducing blockage due to particle ingestion. Blockage was significantly increased based on the percentage of large particles present in the sand samples. Increasing the pressure ratio and decreasing the sand amount were also shown to reduce blockage. / Master of Science

This study is an investigation of the film cooling effectiveness and heat transfer coefficient of a two-dimensional turbine rotor blade in a linear transonic cascade. Experiments were performed in Virginia Tech's Transonic Cascade Wind Tunnel with an exit Mach number 0f 1.2 and an exit Reynolds numbers of 5x106 to simulate real engine flow conditions. The freestream and coolant flows were maintained at a total temperature ratio of 2(+,-)0.4 and a total pressure ratio of 1.04. The freestream turbulence was approximately 1%. There are six rows of staggered, discrete cooling holes on and near the leading edge of the blade in a showerhead configuration. Cooled air was used as the coolant. Experiments were performed with and without film cooling on the surface of the blade. The heat transfer coefficient was found to increase with the addition of film cooling an average of 14% overall and to a maximum of 26% at the first gauge location. The average film cooling effectiveness along the chord-wise direction of the blade is 25%. Trends were found in both the uncooled and the film-cooled experiments that suggest either a transition from a laminar to a turbulent film regime or the existence of three-dimensionality in the flow-field over the gauges. / Master of Science

Test data of forced draft cooling towers is all too meager, and that available is, in many instances, incomplete. It is the opinion of the authors that putting cooling tower design on a rational basis can be brought about only by two methods. Either complete and intensive study of all existing towers or a thorough study of models, similar to the research conducted on airplanes in wind tunnels and on boat hulls in towing tanks, is necessary. Models used should be so constructed that quantity, condition, and velocity of air; quantity and condition of water; and type and arrangement of filling may be controlled. It is by the second method that the authors believe rational tower design may best be brought about. / Master of Science