Keywords

Fluid dynamics, Gas turbines -- Cooling, Heat -- Transmission

Abstract

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.

Notes

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Graduation Date

2011

Semester

Fall

Advisor

Kapat, Jayanta

Degree

Master of Science in Mechanical Engineering (M.S.M.E.)

College

College of Engineering and Computer Science

Department

Mechanical and Aerospace Engineering

Degree Program

Mechanical Engineering; Thermo-Fluids Track

Format

application/pdf

Identifier

CFE0004140

URL

http://purl.fcla.edu/fcla/etd/CFE0004140

Language

English

Release Date

December 2011

Length of Campus-only Access

None

Access Status

Masters Thesis (Open Access)

Subjects

Dissertations, Academic -- Engineering and Computer Science, Engineering and Computer Science -- Dissertations, Academic

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