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Browsing by Subject "Heat Transfer"

Now showing 1 - 6 of 6
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    Aero-Thermal Aspects of Endwall Cooling Flows in a Gas Turbine Nozzle Guide Vane
    (2019-01) Alqefl, Mahmood
    Gas turbine engines are still one of the favorite sources for shaft power. They are relatively clean and higher in efficiency than other common sources, and their working principle makes them especially practical for aircraft propulsion. Over the past decades, the desire for higher turbine inlet temperature has increased to achieve higher specific power and efficiency. Currently, the temperature of the gasses entering the turbine is typically higher than the melting point of its components. This leads to the need of effective turbine cooling. Endwalls are particularly challenging to cool due to the complex system of secondary flows near its surface that washes the protective film coolant into the mainstream. This three-dimensional aerodynamics is also a source for irreversibility. Due to the nature of film coolant injection, its interaction with endwall aerodynamics is coupled. The film coolant momentum affects the secondary flow formation, and the secondary flows affect the film coolant distribution over the endwall. To achieve better endwall designs and cooling schemes, better understanding of this interaction is needed. This thesis studies experimentally the aero-thermal interaction of cooling flows near the endwall of a first stage nozzle guide vane passage. The test section involves engine representative combustor-turbine interface geometry, combustor coolant, and endwall film cooling flows injected upstream of a linear cascade. The approach flow conditions represent flow exiting a low NOx combustor. The aero-thermal interaction is studied through detailed measurements of passage velocity fields, thermal fields and endwall adiabatic effectiveness for various film cooling mass flow to mainstream flow ratio (MFR). The contribution of combustor coolant towards cooling the endwall is also presented. The detailed measurements revealed a new dominant vortex in the passage that is opposing the passage vortex. This vortex dominates the coolant mixing and migration and completely changes our understanding of the system of secondary flows present in a film cooled nozzle guide vane passage.
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    Design of Medical Devices Involving Multi-disciplinary Processes and Based on Fundamental Physical Principles
    (2016-04) Krautbauer, Kevin
    This dissertation focuses on the optimal design of medical devices through the use of numerical simulation and the utilization of first principles of the participating phenomena. Through three broadly ranging case studies, the dissertation explores a wide variety of physical phenomena found within medical devices and in other applications. Pressure drop and sound generation are the primary focii of the leading case study which constitutes the first-ever analysis of the fluid mechanics of a therapeutic device for the treatment of cystic fibrosis. The treatment utilizes a time varying pressure that acts on the abdomen of the patient in order to break up masses of mucus. The second study is the first known effort to design peristaltic pumps using the principles of fluid-structure interaction. The time-dependent mechanics of peristaltic pumping were utilized to determine the deformations and pressures in the flexible-walled plastic tubing. The change of volume of the tubing serves to propel a liquid contained within the tube. Finally, the third study investigates the fluid mechanics and heat transfer mechanisms found in an enhanced-surface fluid warming device. The key analysis and design tools used throughout the aforementioned case studies of this dissertation are physical model formulation adapted to computational fluid dynamics (CFD), the theory of turbulence-based sound generation, Ogden’s hyperelastic model of polymeric materials, and the theory of heat transfer. The fluid flow phenomena dealt with in this work include three-dimensional, unsteady, laminar and turbulent flows. Heat transfer concepts utilized include conduction within both fluids and solids, advection within interacting parallel flow regions, and the theory of heat transfer enhancement. Each chapter contains multiple results pertaining to the device in question. These results serve to expand the reader’s knowledge of the underlying physical processes which control the function and effectiveness of the medical device.
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    Modeling hydrothermal inputs to cold-water streams in urban watersheds.
    (2011-05) Janke, Benjamin David
    This research investigated the impact of urban development on the temperature of cold-water streams, which are crucial to maintaining viable populations of biota that are unable to survive in warmer waters. Since the temperature of these streams is typically maintained by significant amounts of groundwater inflow and riparian shading, the land cover conversion associated with urban development - replacement of crops or natural land with buildings, roads, lawns, and parking lots - has a negative impact, as these land-use changes tend to increase the amount of impervious surface area and reduce the amount of natural shading provided by vegetation. As a result, surface runoff rates and temperatures from rainfall events are amplified, watershed infiltration is reduced, and stream temperature increases. A primary goal of the project was to produce a tool to assess the impact of proposed urban development on stream temperatures in a particular watershed. The research procedure focused primarily on understanding and developing models for the hydrologic and heat transfer processes within a watershed, with particular focus on rainfall-runoff. Specifically, two process-based models were developed: one for estimation of runoff flow and temperature from urban surfaces, and a second for estimation of groundwater input to a stream from observations of water quality. The runoff temperature model demonstrated that heat export by rainfall-runoff from a paved surface is determined by antecedent pavement temperature and rainfall intensity/duration, and that stream-wise gradients in runoff temperature are negligible. The model contributed to the development of a more comprehensive stormwater modeling tool (MINUHET) by justifying the simpler solution technique used by MINUHET's runoff model. MINUHET was shown to accurately simulate runoff flow rate and temperature at the outlet of a small urban watershed, particularly when hydrologic data is available for calibration. The roof surface temperature analysis provided evidence that the contribution of heat from rooftops is negligible relative to that of paved surfaces. Lastly, the use of temperature as a groundwater tracer was shown to be an effective and inexpensive method for determining groundwater input to a stream, provided that the limitations of the approach are borne in mind when applying the method.
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    Nano-scale Heat Transfer in Nanostructures: Toward Understanding and Engineering Thermal Transport
    (2017-05) Ma, Jihong
    Heat transfer is vital throughout research and industry. This thesis focuses on heat transfer in nanostructures and amorphous materials, in which the arrangement of atoms is crucial for the effectiveness of heat transport. Defects and mechanical deformations in a material which cause displacement or reconfiguration of atoms relative to that material’s “normal” or “pristine” condition can dramatically influence its heat transport efficiency. Since the 1950’s, there has been little progress in understanding the defects–thermal transport property relationship. Using novel numerical techniques and large-scale computations performed on modern supercomputers, I have studied heat transport in nanomaterials containing various defects and mechanical deformations. From the properties of atomic vibrations in my simulations, the effects these deformations have on heat transport can be deduced. Three research projects are presented in this thesis. The study of heat transport in screw-dislocated nanowires with low thermal conductivities in their bulk form represents the knowledge base needed for engineering thermal transport in advanced thermoelectric and electronic materials. This research also suggests a new potential route to lower thermal conductivity, which could promote thermoelectricity. The study of high-temperature coating composite materials helps with the understanding of the role played by composition and the structural characterization, which is difficult to be approached by experiments. The method applied in studying the composition-structure-property relationship of amorphous Silicon-Boron-Nitride networks could also be used in the investigation of various other similar composite materials. Such studies can further provide guidance in designing ultra-high-temperature ceramics, including space shuttle thermal protection system materials and high-temperature-resistance coating. The understanding of the impact of bending and collapsing on thermal transport along carbon nanotubes is important as carbon nanotubes are excellent materials candidates in a variety of applications, including thermal interface materials, thermal switches and composite materials. The atomistic study of carbon nanotubes can also provide crucial guidance in multi-scale study of the materials to enable large-scale thermal behavior prediction.
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    Optimizing borehole Heat exchanger spacing to maximize advective heat transfer
    (2013-09) Meester, Jennifer
    This generalized study provides first order insights into the effect of heat advection on ground source heat pump (GSHP) system operation and the optimization of spacing between borehole heat exchangers (BHE) using groundwater flow and heat transport models. In these systems, there is a threshold Péclet number, the ratio of heat advection rate to heat conduction rate, beyond which the efficiency of heat transfer between the BHEs and the aquifer is significantly increased, thus lowering the temperature drop of the circulating fluid in the BHE and increasing the overall efficiency of the BHE system. This threshold Péclet number depends on the groundwater flow rate and effective thermal diffusivity (among other factors) of the system and, for the given conditions, is approximately 2 with a 1% change in BHE outlet temperature and approximately 11 with a 5% change. In GSHP systems with standardized spacings between BHEs, groundwater heat advection can cause negative thermal interactions between heat exchangers, which can be eliminated and in some situations replaced with positive thermal interactions by optimizing the spacing between BHEs. Above the threshold Péclet number, there is specified spacing between heat exchangers that will allow for the utilization of the previous season's heat injection or extraction, a half year transport distance. For the GSHP system simulated in this study, the BHE spacings for optimization are 6.65 m and 13.8 at groundwater flow velocities of 2.5 x 10-5 m/s and 5 x 10-5 m/s, respectively. It may also be possible to space the heat exchangers at a distance that captures heat after a year and a half of transport (for systems with only slight heat advection dominance), but more simulations are necessary to investigate the results of such a strategy.
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    Surfactant Effects On Pool Boiling Of Dilute Emulsions On A Horizontal Surface
    (2020-05) Proper, John
    Previous research has demonstrated that heat transfer of water may be enhanced by either the addition of surfactants or through emulsifying the water with a small volume of volatile disperse phase. However, the combination of surfactant and emulsion has not yet been thoroughly investigated. To accomplish this, experiments of boiling heat transfer were conducted with dilute FC-72 in water emulsions and Tween-20 surfactant. Boiling occurred over a flat upward-facing aluminum nitride heater in a vessel nearly 1 L in volume. FC72-in-water emulsions with volume fractions of 0.1, 0.5, and 1.0 % were tested with concentrations of 0, 10, 60, and 100 ppm Tween-20 surfactant. Turbulent mixing induced by pumping through a check valve caused the fluid to emulsify. Size distributions of oil droplets in the emulsions were measured via laser diffraction. The resulting boiling curves for emulsion without surfactant confirmed previously observed trends, with an increase in heat transfer occurring near 80 °C. Boiling with Tween-20 surfactant did not show any increase in heat transfer, in both aqueous surfactant systems and emulsions with surfactant. Laser diffraction imaging showed that emulsions with different volume fractions can have different particle size distributions even though the emulsification process was the same. Addition of surfactant to the emulsion tended to cluster the droplet diameter distribution around a mean of 2 µm. The similarity of boiling behavior between mixtures of water and Tween-20 and emulsions with Tween-20 suggests that the boiling behavior is characterized mostly by interface chemistry unique to Tween-20, rather than a secondary effect of droplet size distribution. However, there is considerable variety in surfactants, and there is still value in testing the effect of other surfactants on the boiling behavior of dilute oil-in-water solutions.