Gas turbine engines are prevalent in the today’s aviation and power generation industries. The majority of commercial aircraft use a turbofan gas turbine engines. Gas turbines used for power generation can achieve thermodynamic efficiencies as high as 60% when coupled with a steam turbine as part of a combined cycle. The success of gas turbines is a direct result of a half century’s development of the technology necessary to create such efficient, powerful, and reliable machines. One key area of technical advancement is the turbine cooling system. In short, increasing the turbine inlet temperature leads to a rise in cycle efficiency. Before the development of modern turbine cooling schemes, this temperature was limited by the softening temperature of the metallic turbine components. The evolution of component cooling systems – in conjunction with metallurgical advancements and the introduction of Thermal Barrier Coatings (TBC) – allowed for gradual increases in power output and efficiency. Today, the walls of gas turbine combustors are protected by a cool film that bypassed combustion; the 1st (and often 2nd) stage turbine blades and vanes are cooled via internal convection, a combination of turbulent channel flow, pin fin arrays, and impingement cooling; and some coolant air is bled onto the external surface of the blade and the blade endwall to establish a protective film on the exposed geometry. Modern research continues to focus on the optimization of these cooling designs, and a better understanding of the physics behind fluid behavior. The current study focuses on one particular cooling design: an impingement-effusion cooling system. While a single entity, the cooling schemes used in this system can be separated into impingement cooling on the backside iv of the cooled component and full coverage film cooling on the exposed surface. The result of this combination is a very high level of cooling effectiveness. The goal of this study is to explore a wide range of geometrical parameters and their effect on the overall cooling performance. Several parameters are taken outside the ranges normally investigated by the available literature. New methods of data comparison and normalization are offered in order to create an objective comparison of different configurations. Particular attention is given to the total coolant spent per unit surface area cooled. This study is also unique as it is a multi-modal heat transfer study, unlike the majority of impingement-effusion investigations, which only evaluate impingement heat transfer. Through determination of impingement heat transfer, film cooling effectiveness, and film cooling heat transfer on the target wall, a simplified heat transfer model of the cooled component is developed to show the relative impact of each parameter on the overall cooling effectiveness. The use of Temperature Sensitive Paint (TSP) for data acquisition allows for high resolution local heat transfer and effectiveness results. This has a quantitative benefit, giving the ability to average as desired and/or compare local data, for example the lateral distribution of film cooling effectiveness. However, the qualitative benefit of viewing the contours of heat transfer coefficient under an impinging jet array or downstream of a film cooling jet is instrumental in drawing conclusions about the behavior of the flow. The local data provides, in essence, a flow visualization on the test surface and adds (quite literally) another dimension to the heat transfer results. Impingement arrays with local extraction of coolant via effusion are able to produce higher overall heat transfer, as no significant cross flow is present to deflect the impinging jets. Low jet-to-target-plate spacing produces the highest yet most non-uniform heat transfer v distribution; at high spacing the heat transfer rate is much less sensitive to impingement height. Arrays with high hole-to-hole spacing and high jet Reynold’s number are more effective (per mass of coolant used) than tightly spaced holes at low jet Reyonld’s number. On the effusion side, staggered hole arrangements provide significantly higher film cooling effectiveness than their in-line counterparts as the staggered arrangement minimizes jet interactions and promotes a more even lateral distribution of coolant. These full coverage film cooling geometries typically show increases in effectiveness with each row of injection. Some additional cases were show with 15 film cooling rows, and generally the adiabatic wall temperature was decreasing through the last row. In the recovery region, results were highly dependant on blowing ratio; injection of excess coolant into the boundary layer at high blowing ratio allowed for cooling effectiveness to penetrate well downstream of the end of the array. From a heat transfer standpoint, compound angle injection resulted in higher enhancement than purely inclined injection, but this negative effect was outweighed by the substantial increase in film cooling effectiveness with the compounded geometry. Overall, the additive film superposition method under-predicted full coverage film cooling effectiveness trends for staggered hole arrangements; however, with more accurate estimation (or measurement) of recovery region trends for a single row of holes, this method may produce an acceptable result.

A major challenge of developing faster and smaller microelectronic devices is that high flux of heat needs to be removed efficiently to prevent overheating of devices. The conventional way of heat removal using liquid reaches a limit due to low thermal conductivity and limited heat capacity of fluids. Adding solid nanoparticles into fluids has been proposed as a way to enhance thermal conductivity of fluids, but recent results show inconclusive anomalous enhancements in thermal conductivity. A possible way to improve heat transfer is to increase the heat capacity of liquid by adding phase change nanoparticles with large latent heat of fusion into the liquid. Such nanoparticles absorb heat during solid to liquid phase change. However, the colloidal suspension of bare phase change nanoparticles has limited use due to aggregation of molten nanoparticles, irreversible sticking on fluid channels, and dielectric property loss. This dissertation describes a new method to enhance the heat transfer property of a liquid by adding encapsulated phase change nanoparticles (nano-PCMs), which will absorb thermal energy during solid-liquid phase change and release heat during freeze. Specifically, silica encapsulated indium nanoparticles, and polymer encapsulated paraffin (wax) nanoparticles have been prepared using colloidal method, and dispersed into poly-α-olefin (PAO) and water for high temperature and low temperature applications, respectively. The shell, with a higher melting point than the core, can prevent leakage or agglomeration of molten cores, and preserve the dielectric properties of the base fluids. Compared to single phase fluids, heat transfer of nanoparticle-containing fluids have been significantly enhanced due to enhanced heat capacities. The structural integrity of encapsulation allows repeated uses of nanoparticles for many cycles. iv By forming porous semi crystalline silica shells obtained from water glass, supercooling has been greatly reduced due to low energy barrier of heterogeneous nucleation. Encapsulated phase change nanoparticles have also been added into exothermic reaction systems such as catalytic and polymerization reactions to effectively quench local hot spots, prevent thermal runaway, and change product distribution. Specifically, silica-encapsulated indium nanoparticles, and silica encapsulated paraffin (wax) nanoparticles have been used to absorb heat released in catalytic reaction, and to mitigate the gel effect during polymerization, respectively. The reaction rates do not raise significantly owing to thermal buffering using phase change nanoparticles at initial stage of thermal runaway. The effect of thermal buffering depends on latent heats of fusion of nanoparticles, and heat releasing kinetics of catalytic reactions and polymerizations. Micro/nanoparticles of phase change materials will open a new dimension for thermal management of exothermic reactions.

New heating techniques are required to better control heat transfer between heating tools and a composite towpreg in the automated fiber placement process. This dissertation suggests new heating techniques with liquid and rigid contact heat sources, and compares them with a widely used gas heat source for the fiber placement process. A thin towpreg composite model needs to be developed to describe the heat transfer. Subsequently, the response of the towpreg with each heat source was compared from manufacturing speed and energy efficiency viewpoints. The most promising heat source was developed for heat transfer modeling between a moving towpreg and dynamic heat source in the automated fiber placement. Through the heat transfer model, both the temperature controllability of the towpreg and manufacturing speed could be investigated. In addition, an accurate compaction process is needed in response to the growing demand for better composite processing. Since the errors in compaction mechanisms and robotic machinery in fiber placement have not been discussed in the literature, experimental investigation to address possible reasons for the variations in the compaction force was conducted with a compaction mechanism. A clearer understanding of the physical compaction process can lead to controllable process parameters for consistent ply compaction, such that the final parts quality can be enhanced. Even though this dissertation investigates the thin thermoset fiber placement process, the proposed approach could be universally applicable to other composite-fabrication processes. / Ph. D.

Heat transfer in cylinder spaces is important to the performance of many reciprocating energy conversion machines, such as gas compressors and Stirling machines. Work over the past 10 years has shown that heat transfer driven by oscillating pressure differs from steady state heat transfer, in magnitude arid phase. In-cylinder heat transfer under this oscillating condition can be out of phase with the temperature difference. For studies with closed piston-cylinder gas springs, this heat transfer phase shift has been successfully predicted with the use of a complex Nusselt number. Since a complex,number has both a magnitude and a phase, a complex Nusselt number can describe the phase shift between temperature difference and heat transfer. Quasi - steady heat transfer models, such as Newton's Law of Cooling, do not predict this phase shift. In this project, the problem of in-cylinder heat transfer with inflow and outflow was studied. The goal was to determine what the complex heat transfer coefficients were under these conditions. Because methods which measure the heat transfer directly, such as heat flux gauges, only give local results, past work has used pressure and volume measurements to calculate surface averaged values for the heat transfer. This becomes much more difficult to do with inflow and outflow because of the difficulty in accurately determining how much mass is in the cylinder at any given time. Two approaches were used to overcome this problem. They are the main substance of the work presented here. The actual experimental pressure and volume measurements were taken by Kafka (Virginia Tech Master's Thesis, 1994). / Master of Science

A computer model has been developed to analyze the flow of a dissociating gas through a porous metal matrix. The program predicts the transient temperature distributions through the coolant gas and matrix along with the pressure distribution and mass flow rate. The differential equations used in developing the program are documented in the literature of represent logical extensions of documented equations. The derivation of the finite difference equations is presented. Comparisons of experimental data with computer predictions are shown and indicate that the predictions fall within the experimental error in the data. A source listing of the computer program is contained in the Appendix.

No description available.

No description available.

The use of shallow fluidized beds for heat exchange has been suggested because they give high bed-to-surface heat transfer rate and require very low bed pressure. However, in comparison with research on deep fluidized beds, only relatively few studies have been devoted to heat transfer in shallow beds, and results from the available literature are often inconsistent. This study represents an integrated research on the hydrodynamics and bed-to-surface heat transfer in shallow beds. The results from this study provide the quantitative basis for the design and efficient operation of shallow fluidized-bed heat-recovery systems. Based upon their physical appearance, shallow fluidized beds have been categorized into nine different types. A "phase diagram" (plot of superficial gas velocity versus static bed height) can be used to delineate the ranges of fluidization variables within which each type of shallow beds will be seen. Pressure-drop data in gas flowing upward through a shallow bed reflect pressure recovery in jets formed immediately above a gas distributor at the bottom of the bed. Pressure-recovery data provide an effective means of distinguishing a shallow bed from a deep one, and suggest that the power consumption across a fluidized bed can be reduced dramatically by dividing a single deep bed into many multi-staged shallow beds. A computerized light probe has been developed for measurements of particle volume-fraction distribution and its statical fluctuation (standard deviation). These data have been shown to quantitatively define: (1) different types of shallow beds; (2) relative magnitude of solid mixing; (3) bed surface and bed height; and (4) jet penetration depth. Based upon observations of the hydrodynamic behavior of shallow fluidized beds, three regions can be identified for heat-transfer applications: a jet-affected region at the bottom, a free-board region at the top, and, sandwiched between theses, a homogeneous region. Only heat-transfer data in the homogeneous region are sufficiently well-behaved to be subjected to quantitative correlation in terms of fluidization variables. For relatively coarse particles (Geldart's Group B particles) the vigor of solid mixing can be the most important factor in affecting the heat-transfer performance. Bed voidage and static electricity effects are found to be important for smaller and/or lighter particles (i.e., Geldart's Group A particles). / Ph. D.

Freeze-drying (lyophilization) is a drying process which is used to remove water from heat sensitive products, usually for the purpose of preservation. By removing water, the product becomes more stable at room temperature. This is a common process in the pharmaceutical industry because freeze-drying offers the advantage of drying at low temperatures and producing very low residual moisture contents. Often the materials dried in this manner are heat sensitive and require the highest possible quality. However, freeze-drying is a very slow process, often requiring 24 to 48 hours. During the process, vacuum pumps and refrigeration systems run continuously, making freeze-drying a very expensive process. The goal of this project was to show that volumetric heating can be used in pharmaceutical freeze-drying and that this mode of heating offers some advantages. There were two approaches taken to the work, one experimental and one analytical. The experimental approach was broken into two phases, one focused on comparing microwave and conventional freeze-drying and the other focused on demonstrating the advantages of volumetric heating. In the analytical approach, a mathematical model was used to confirm the trends observed in phase II of the experimental work. Experiments were conducted in a conventional laboratory freeze-dryer and the drying rate results were compared to the results obtained with an experimental microwave freeze-drying apparatus. Experiments were also conducted with the vaccine strain <i>A. pleuropneumoniae</i>. A viability study was conducted, comparing the viability loss caused by each process. The viability study showed a slightly higher viability loss for the microwave process. A comparison of drying curves showed that the microwave process resulted in a slight improvement in primary drying time: 2.5 hours for the microwave process compared to 3 hours for the conventional process. There was a significant difference in overall drying times: 4 hours for the microwave process compared to 11 hours for the conventional process. This result was caused by a lower residual moisture content at the start of secondary drying and a higher secondary drying temperature for the microwave process. Experiments were also conducted to show that using lower chamber pressure results in higher drying rates. This is not the case in a conventional freeze-dryer since heating is dependent on the chamber pressure in the low pressure environment of freeze-drying. Thus, an advantage of volumetric heating was demonstrated. The results show that a modest increase in pressure, from 0.05 to 0.3 Torr, caused a one third reduction in primary drying time. The mathematical model developed in the analytical work relied on the D'Arcy equation to describe the flow of vapor in the porous dried layer. The results of the model confirm trends seen in the measured temperature and weight profiles. Analyzing the effect of varying the chamber pressures shows that lowering the pressure in the range of 1 to 0.01 Torr results in a significant increase in drying rate giving as much as a two thirds reduction in drying time for the case studied. A model incorporating mass transport equations derived from the dusty gas model was also presented. This model offers the benefit of a more accurate prediction of mass transport through the porous dried layer. NOTE: (09/2008) An updated copy of this ETD was added after there were patron reports of problems with the file. / Ph. D.

The effect of global warming due to increased level of greenhouse gas emissions from coal fired thermal power plants and crisis of reliable energy resources has profoundly increased the importance of natural gas based power generation as a major alternative in the last few decades. Although gas turbine propulsion system had been primarily developed and technological advancements over the years had focused on application in civil and military aviation industry, use of gas turbine engines for land based power generation has emerged as the most promising candidate due to higher thermal efficiency, abundance of natural gas resources, development in generation of hydrogen rich synthetic fuel (Syngas) using advanced gasification technology for further improved emission levels and strict enforcement in emission regulations on installation of new coal based power plants. The fundamental thermodynamic principle behind gas turbine engines is Brayton cycle and higher thermal efficiency is achieved through maximizing the Turbine Inlet Temperature (TIT). Modern gas turbine engines operate well beyond the melting point of the turbine component materials to meet the enhanced efficiency requirements especially in the initial high pressure stages (HPT) after the combustor exit. Application of thermal barrier coatings (TBC) provides the first line of defense to the hot gas path components against direct exposure to high temperature gases. However, a major portion of the heat load to the airfoil and passage is reduced through injection of secondary air from high pressure compressor at the expense of a penalty on engine performance. External film cooling comprises a significant part of the entire convective cooling scheme. This can be achieved injecting coolant air through film holes on airfoil and endwall passages or utilizing the high pressure air required to seal the gaps and interfaces due to turbine assembly features. The major objective is to maximize heat transfer performance and film coverage on the surface with minimum coolant usage. Endwall contouring on the other hand provides an effective means of minimizing heat load on the platform through efficient control of secondary flow vortices. Complex vortices form due to the interaction between the incoming boundary layer and endwall-airfoil junction at the leading edge which entrain the hot gases towards the endwall, thus increasing surface heat transfer along its trajectory. A properly designed endwall profile can weaken the effects of secondary flow thereby improving the aerodynamic and associated heat transfer performance. This dissertation aims to investigate heat transfer characteristics of a non-axisymmetric contoured endwall design compared to a baseline planar endwall geometry in presence of three major endwall cooling features – upstream purge flow, discrete hole film cooling and mateface gap leakage under transonic operating conditions. The preliminary design objective of the contoured endwall geometry was to minimize stagnation and secondary aerodynamic losses. Upstream purge flow and mateface gap leakage is necessary to prevent ingestion to the turbine core whereas discrete hole cooling is largely necessary to provide film cooling primarily near leading edge region and mid-passage region. Different coolant to mainstream mass flow ratios (MFR) were investigated for all cooling features at design exit isentropic Mach number (0.88) and design incidence angle. The experiments were performed at Virginia Tech's quasi linear transonic blow down cascade facility. The airfoil span increases in the mainstream flow direction in order to match realistic inlet/exit airfoil surface Mach number distribution. A transient Infrared (IR) thermography technique was employed to measure the endwall surface temperature and a novel heat transfer data reduction method was developed for simultaneous calculation of heat transfer coefficient (HTC) and adiabatic cooling effectiveness (ETA), assuming a 1D semi-infinite transient conduction. An experimental study on endwall film cooling with endwall contouring at high exit Mach numbers is not available in literature. Results indicate significant benefits in heat transfer performance using the contoured endwall in presence of individual (upstream slot, discrete hole and mateface gap) and combined (upstream slot with mateface gap) cooling flow features. Major advantages of endwall contouring were observed through reduction in heat transfer coefficient and increase in coolant film coverage by weakening the effects of secondary flow and cross passage pressure differential. Net Heat Flux Reduction (NHFR) analysis was carried out combining the effect of heat transfer coefficient and film cooling effectiveness on both endwall geometries (contoured and baseline) where, the contoured endwall showed major improvement in heat load reduction near the suction side of the platform (upstream leakage only and combined upstream with mateface leakage) as well as further downstream of the film holes (discrete hole film cooling). Detailed interpretation of the heat transfer results along with near endwall flow physics has also been discussed. / Ph. D.