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Flow and Thermal Control for Airfoil-Endwall Junctions

A complex 3-D flow develops at the junction of an airfoil and the endwall in a modern gas turbine (such as an aircraft jet engine or an electrical power generation turbine), due to non-uniform flow entering the turbine from the combustor. The complex flow includes vortical structures that reduce the aerodynamic efficiency of the turbine and cause high heat transfer from the hot combustion gases to the turbine component, which can increase the component temperature and reduce the life expectancy of the part. Adding a large fillet to the junction of the airfoil and endwall has been shown to interfere with the development of the vortical structures and reduce heat transfer to the endwall. Also, in a typical engine, gaps exist between assembly components, such as between the combustor and the turbine. Leakage flow from these gaps can interact with the complex endwall flow and influence the endwall heat transfer.

For this project, we have measured endwall heat transfer coefficients for a gas turbine nozzle guide vane, with and without a fillet. Our results indicates that a fillet with a linear profile between the airfoil and the endwall changes the heat transfer coefficient distribution at the airfoil-endwall junction (Figure 1), and slightly lowers the area-averaged heat transfer coefficient. A novel technique using a nanometer–thickness oil film to measure the endwall shear stress magnitude and direction (oil film interferometry) showed that the linear profile fillet reduces the overturning effect at the exit of the nozzle guide vane passage, which is associated with the complex 3-D endwall flow (Figure 2). However, in the presence of combustor turbine gap leakage flow, a fillet with a linear profile tends to limit the coverage of the cool leakage flow by displacing it around the fillet. A fillet with an elliptical (curved) profile allows similar levels of leakage flow coverage as an endwall without a fillet (Figure 3).

Figure 1. Measurements of heat transfer for a gas turbine nozzle guide vane endwall, with (b) and without (a) a large linear fillet at the airfoil-endwall junction. Regions of low heat transfer are depicted by cooler colors (blue).

Figure 2. Endwall streamlines measured with the oil film interferometry method, for the unfilleted endwall (red streamlines) and an endwall with a linear fillet (black streamlines).

Other heat transfer and flow field measurements are planned in order to understand the interaction of the complex 3-D endwall flow with various fillet geometries and leakage interface flows. Design parameters such as fillet height along the airfoil, fillet extent along the endwall, fillet cross-sectional area, and cross-sectional shape (linear vs. elliptical) will be investigated for their relative importance in reducing the effect of the complex endwall flow on turbine heat transfer.

Figure 3. Measurements of the cooling effectiveness of leakage flow over the endwall from a combustor turbine interface gap, with (a) no fillet, (b) a linear fillet, and (c) an elliptical fillet with the same endwall footprint. Low cooling effectiveness values (red) correspond to high wall temperatures.