This study describes a detailed investigation on the effects that upstream step misalignment and upstream purge film cooling have on the endwall heat transfer for nozzle guide vanes in a land based power generation gas turbine at transonic conditions. Endwall Nusselt Number and adiabatic film cooling effectiveness distributions were experimentally calculated and compared with qualitative data gathered via oil paint flow visualization which also depicts endwall flow physics. Tests were conducted in a transonic linear cascade blowdown facility. Data were gathered at an exit Mach number of 0.85 with a freestream turbulence intensity of 16% at a Re = 1.5 x 106 based on axial chord. Varied upstream purge blowing ratios and a no blowing case were tested for 3 different upstream step geometries, one of which was the baseline (no step). The other two geometries are a backward step geometry and a forward step geometry, which comprised of a span-wise upstream step of +4.86% span and -4.86% span respectively.
Experimentation shows that the addition of upstream purge film cooling increases the Nusselt Number at injection upwards of 50% but lowers it in the throat of the passage by approximately 20%. The addition of a backward facing step induces more turbulent mixing between the coolant and mainstream flows, thus reducing film effectiveness coverage and increasing Nusselt number by nearly 40% in the passage throat. In contrast, the presence of a forward step creates a more stable boundary layer for the coolant flow, thus aiding to help keep the film attached to the endwall at higher blowing ratios. Increasing the blowing ratio increases film cooling effectiveness and endwall coverage up to a certain point, beyond which, the high momentum of the coolant results in poor cooling performance due to jet liftoff. Near endwall streamlines without purge cooling generated by Li et al. [1] for the same geometries were compared to the experimental data. It was shown that even with the addition of upstream purge cooling, the near endwall streamlines as they moved downstream matched strikingly well with the experimental data. This discovery indicates that while the coolant flow will likely affect the flow streamlines three dimensionally, they are minimally effected by the coolant flow near the endwall as the flow moves downstream. / Master of Science / Gas turbine engines are commonly used for power production by burning natural gas. This leads to exceedingly hot temperatures through several stages of the engine. These temperatures often exceed the melting points of the metal components, especially in the region immediately following the combustion zone. Relatively cooler air from the compressor stage of the engine is used to cool these hot regions using sophisticated cooling schemes (external/internal cooling). The performance of these schemes can be severely influenced by unintentional but unavoidable geometric discrepancies caused by non-uniform thermal expansion and manufacturing tolerances of the engine components.
This study investigates the impact of these geometric variations (specifically: combustor line/nozzle guide vane platform misalignment) on a commonly employed external cooling scheme (purge cooling) where the cooler air creates a protective layer between the metal and the hot gases. The geometric variation is found to make significant impact to the performance of the cooling scheme. The misalignment in one direction is found to be detrimental to the purge cooling effectiveness, while the other geometric misalignment helps the cooling scheme. In addition, increasing the amount of cooling does not necessarily mean better cooling because the increased amount of coolant can jet off of the surface before it can protect it from the hot gas. Quantitative results explaining the effects geometric misalignment and purge cooling are presented in the research herein.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/83237 |
| Date | 16 May 2018 |
| Creators | Luehr, Luke Emerson |
| Contributors | Mechanical Engineering, Ng, Wing Fai, Lowe, K. Todd, Ekkad, Srinath, Diller, Thomas E. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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