Aerothermodynamics of Impingement and Film Cooling in a Gas Turbine Blade
Underwood, Sean Christopher
University of Kansas
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The service life of gas turbine engine turbine blades depends on the blade’s material, service temperature and total stress. In high-performance gas turbines, film cooling is widely used to reduce the blade service temperature. Often impingement cooling is also employed to target the stagnation point heat transfer for internally-cooled gas turbine blades. A novel thermal wind tunnel was designed to study the combined effect of the impingement and film cooling on blunt airfoils. The hot exhaust plume of a micro-jet is used as the source of high-temperature gas flow in the thermal wind tunnel. An ejector nozzle was designed and integrated with the hot jet to provide a thermally controlled test section environment in the research facility. Measurements of freestream parameters such as gas speed, turbulence intensity and gas temperature were made. An airfoil that utilizes leading-edge (internal) impingement as well as film cooling holes on its suction surface was designed and fabricated. A cooling sleeve is used inside the airfoil to guide the impingement jets on the leading edge and to supply the coolant to the film holes. The surface temperature distribution is measured by an array of eight thermocouples flush-mounted on the airfoil surface downstream of the film holes. The initial ranges of blowing parameters (Mb) investigated were between 5 and 6. Numerical simulation using a commercially available Reynolds-Averaged Navier-Stokes (RANS) software was used and validated by the experimental measurements. The numerical simulations for the airfoil consisted of two thermal wall boundary conditions, the adiabatic and conjugate heat transfer (CHT) models. The adiabatic model focuses on the effect of film cooling on an adiabatic wall. The conjugate heat transfer model represents the solid and fluid heat transfer exchange, conduction and convection. Verification and validation was completed to ensure accurate aerothermodynamic simulations. The experimental and numerical data showed a close comparison for the suction surface temperatures and cooling effectiveness. A broader range of characteristic parameters (blowing parameter, turbulence intensity (Tu) and density ratio) were studied to show their impact on film cooling effectiveness parameter. The effects from the blowing parameter are reported for different Mb of 0.53 to 5.95 with two turbulent intensities, 5% and 20%. The adiabatic film effectiveness parameter showed two unique trends: low Mb with low Tu or high Mb with high Tu both exhibited improved film cooling effectiveness. Jet detachment is also detected at Mb ~ 1.5 for the current film cooling set up. The study of turbulence intensity effects was completed in the range of 5% to 25 % for two density ratios of 1.65 and 1.99. The turbulence intensity study showed that higher Tu caused the adiabatic film effectiveness to decrease by an average 18%. The density ratio (DR) in the film cooling is studied to explore the real turbine environment. The velocity ratio and turbulence intensity is held at a constant of 0.64 and 20%, respectively, for a range of the density ratio: 1.49 to 1.99. The results show that coolant density would cause the adiabatic film effectiveness to increase an average of 12% from the baseline (DR: 1.65) to the representative engine condition (DR: 1.99).
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