Experiments on Film Cooling of Gas Turbine Vane Passage Surfaces: The Effects of Various Distributions of Combustor Coolant and Endwall Injection Coolant

Abstract

The efficiency of gas turbines is known to increase with the exit temperature of the combustor gases. However, this temperature is limited by the melting point of various equipment downstream of the combustor. To increase this limit, coolants injected at different locations form low-temperature films on the surfaces of these regions to avoid melting and thermal stress damage. This injection significantly changes the flow field in the vane passage. Therefore, there is a need to study the aerodynamic and thermal effects of combustor, transition duct and passage coolant injection, which can assist the designers of gas turbines to employ better cooling schemes with minimal use of coolant, thus increasing the efficiency and durability of turbines. The study presented in this thesis discusses experimental tests performed to understand the coolant effectiveness for cooling the endwall and vane surfaces of a nozzle guide vane cascade. The test section contains an engine representative combustor-turbine interface along with a contoured endwall. High turbulence intensity as well as high Reynolds numbers are achieved in the facility to closely simulate engine conditions. In addition to recording the surface effectiveness values, in-field thermal and aerodynamic measurements were taken. It has been previously discovered that the effusion and louver coolants, used to cool the combustor section, can also be credited in cooling the endwall and the vane surfaces. Therefore, this study is helpful to understand the effects of changing the mass flow ratios of different coolants injected upstream of the passage. Especially, louver coolant injection in the combustor transition duct region, due to its location and injection angle, is suspected to provide significant passage endwall cooling. In-field measurements provide insight to the coolant transport through the passage and its interaction with the mainstream. As the net combustor coolant momentum is higher than the film coolant momentum, the changes in the flow field due to its injection are more significant and are sustained to the end of the passage, giving more streamwise coverage than with a conventional film cooling setup. The film coolant mass flow ratio is varied in this study to see how the interaction of different coolants changes with changes of their injected momentum. The measurements reveal that the upstream coolant flow helps in keeping the film coolant attached to the endwall for most of the passage and also keeping the film thickness fairly uniform in the pitchwise direction. An increase in louver coolant mass flow rate shows higher endwall cooling effectiveness. The velocity contours show that a dominant vortex is present due to combustor coolant injection and that the passage vortex was, although present, diminished due to this other vortex. While changing the louver coolant mass flow rate did not change the intensity of these vortices, changes in the film coolant mass flow rate increased the intensity of the dominant vortex near the pressure surface. Also, the presence of this new vortex helps in film cooling the pressure surface, a region where, conventionally, the least amount of coolant coverage is recorded.