Turbulent coolant dispersion in the wake of a turbine vane trailing edge [electronic resource]
Keywords: Mechanical Engineering.

Magnetic resonance-based velocity (MRV) and concentration (MRC) measurements were performed to measure the time-averaged, three-dimensional, three-component velocity and scalar concentration fields in a double passage vane cascade representative of a high pressure turbine vane from a gas turbine engine. The understanding and prediction of the highly three-dimensional flow and heat transfer in modern gas turbine engines is a problem that has not been solved over many years of turbomachinery research. Turbine vanes and blades are both internally and externally cooled to withstand the hot gas environment. The external film cooling is generally fed by discrete holes on the vane surface, except for at the trailing edge, which is cooled by slots that are cut into the pressure side of the vane. Hot streaks from the combustor and cool streaks from the vane film cooling impose strong inlet temperature variations on the turbine blades, which can lead to local hot or cold spots, high thermal stresses, and fatigue failures. Furthermore, the complex three dimensional flows around the vane may act to concentrate cool or hot fluid exiting the vane row. Experiments were performed to show the validity of the application of the scalar transport analogy to the study of turbulent thermal energy transport using turbulent passive scalar transport studies. These experiments were conducted in a three-dimensional mixing layer in the wake of a blunt splitter plate built into two identical test sections. One test section was magnetic resonance-compatible and used water as the working fluid and the other was adapted for high subsonic Mach number air flows and allowed physical access for a thermocouple probe to take temperature profiles. In the water-based MRV/MRC experiments, the mainstream flow was water and the secondary flow was a copper sulfate solution. In the air experiments, the main flow was room temperature air and the secondary flow was heated. The energy separation effect due to coherent vortex structures in the compressible flow experiments affected the measured temperature profile because of the small difference in stagnation temperature between the two flows. This effect is expected to be negligible in the high temperature difference flows found in real engine conditions. This effect is easily corrected in the temperature profiles extracted from this experiment. The agreement between the corrected temperature and the concentration data was found to be excellent, validating the application of MRC for quantitative measurement of thermal transport in turbomachinery components via the scalar transport analogy. The MRV/MRC experimental technique was applied to the study of turbulent dispersion of coolant injected through trailing edge cooling slots, with the focus on dispersion in the vane wake. A new high concentration MRC technique was developed to provide accurate measurements in the far wake of the turbine vane. Three component velocity data showed the development of the passage vortex, a key element of the vane secondary flows. This mean flow structure is the dominant mechanism for turbulent mixing near the cascade endwalls. However, strong variations in coolant concentration remained in the wake downstream of the center span region. Asymmetric dispersion in this region indicated that longitudinal vortices shed from the coolant injection structures played a dominant role in the wake spreading. A separate experiment was performed to evaluate the behavior of the dispersion of combustor hot streaks in the turbine vane cascade. The velocity and concentration distributions were evaluated using the MRV/MRC experimental technique. Streamtubes and concentration isosurfaces reveal that the streaks spread slowly as they pass through the cascade. This suggests that turbulence suppression by strong acceleration plays a significant role in maintaining the streaks. It is important to note that coherent hot streaks still exist at the exit of the test section in the far wake of the vane. The concluding message from these experiments is that the temperature distribution of the gases impacting the blades downstream of the turbine vanes remains significantly non-uniform and that accurate prediction of the temperature distribution downstream of the vanes is critical for advanced turbine cooling design.