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Endwall Film-Cooling and Heat Transfer |
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Project Funded by: Department of Energy (University Turbine Systems Research) |
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Industrial Partners: Pratt & Whitney |
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Over the past several years, research in gas turbine development has been towards increasing the turbine inlet temperatures, which in turn increases the work output and thermal efficiency. Though engine performance is dependent on the turbine inlet temperatures, it is also highly dependent on the airfoil life. One means of preventing component burnout in the turbine is to effectively use film-cooling. Over sustained operational hours, airfoil roughness and film-cooling hole blockage on the airfoil and endwall, which are due to combinations of particle deposition and repeated TBC (thermal barrier coating) applications, can lead to reduced engine performance. Our studies typically involve measuring the film-cooling performance for real airfoil surface conditions. These studies are being done on a large scale linear vane cascade in a wind tunnel facility as shown in Figure 1. Since the geometry is being an actual first vane, curvature and pressure gradient effects are inherently present. The focus of our tests is on the endwall portion of a first stage nozzle guide vane, which often limits the life of the part. |
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Figure 1. Illustration of Penn State ExCCL wind tunnel facility. |
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Initial studies on the endwall of the vane were done to compare predictions and measurements of film-cooling along a smooth surface with no hole blockage. Film-cooling holes are present on the component to provide a thin sheath of coolant air, thereby reducing the component temperature and hence enhancing its life. Film-cooling hole placement has traditionally been based upon designer experience. The placement of these holes is difficult because the trajectory of the jets is not intuitive and also the jet trajectory is highly dependent on the local blowing ratio for the cooling holes. Tests were carried out for different cooling patterns and mechanisms. Results for two of the cooling hole patterns are shown in Figure 2. These results indicated that pattern #1 provided a more uniform coverage as compared with pattern #2. |
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One of the important findings from the above work was a lack of endwall film-cooling along the region where the two turbine vanes mate as shown in pattern #2. The gap between the two mating vane endwalls is sometimes referred to as a gutter. Exiting this gutter are varying amounts of leakage depending on the type of seal and engine conditions. Though this leakage does provide cooling, a simple continuous slot (gutter) is unable to control the location and trajectory of the coolant flow. This low momentum flow is likely to increase the strength of the secondary flows, which would in turn affect the nearby endwall film-cooling performance. A test section was built to simulate the above slot (gutter) and direct the leakage flow at the vane-to-vane interface to further reduce endwall temperatures. Tests were conducted to study the cooling due to the leakage flow through this slot. Tests were also conducted to study the increase in the strength of the secondary flows due to the low momentum leakage flow.
After completion of the work with smooth surfaces, tests were carried out to measure the above parameters on realistic surface conditions. The in-service turbine vane usually has a varied amount of surface roughness and cooling hole blockage. The focus of this study was to evaluate the film-cooling performance on the vane and endwall after sustained operational hours. Moreover, we studied how the blockage of the cooling holes can impact the cooling performance. The results from these tests help engine manufacturers to better design an industrial turbine by increasing the component life. |
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Figure 2. Contours of adiabatic effectiveness for baseline film-cooling at different cooling rates. (a) Pattern #1, (b) Pattern #2 |
