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Blade Hub Leakage |
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Project Funded by: Pratt & Whitney |
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Since gas turbine blades are exposed to temperatures that typically exceed the melting temperature of the blades, numerous internal and external cooling schemes have been developed to prevent blade melting. These schemes typically use air that bypasses the combustion section as the cool air supply. Unfortunately, the complex geometry of rotary seals around turbine blades do not completely prevent the cooler air from leaking into the main passage. In addition to leakages through rotary seals, leakages also occur in gaps between the individual blades on a turbine disk. Figure 1 shows the three main leakage points around the blade platform: the front rim, the aft rim, and the featherseal. Reducing these leakages has been shown to increase turbine efficiency. Even though these leakage flows have been shown to cause slight efficiency losses, the leakage flows do provide cooling on the hub region of the turbine blade. This study aims to determine if there is any thermal advantage to leakage flows around the hub region. |
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Figure 1. Locations of the three main leakage features along the platform of turbine blades. |
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To measure the adiabatic effectiveness of leakage flows, a 11x scale linear blade cascade was tested in a low speed wind tunnel. Figure 2 shows an upstream view of the experimental test section. While the air temperature in the tunnel was heated to 50 degrees Celsius to model the hot gases, air at 22 degrees Celsius was supplied to the leakage features to model the cooler flow. Through infrared thermography, the temperature along the hub region was measured for various different leakage flow conditions. A sample of the experimental data taken along the hub region is show in Figure 3. Leakage flows from the front rim, aft rim, and featherseal can clearly be seen in the temperature distribution shown in Figure 3. In addition to thermal imaging, thermal rake measurements were taken along the leakage features to provide a three-dimensional view of the cooler air mixing with the hot main body passage. Through analysis of the leakage flow interactions with the complex main stream flows, the front rim leakage flow affected the interactions with the complex main stream flows, the front rim leakage flow affected the cooling of the platform more dramatically than featherseal or aft fin leakages. Additionally, the results indicated featherseal leakage should be minimized as there are no additional cooling benefits with increasing featherseal flow. |
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Figure 2. Front view of a four blade test section used to measure temperature along the platform. |
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Figure 3. Top view of temperature distribution along the platform caused by leakage flows. |