Effects of injection conditions and Mach number on unsteadiness arising within coolant jets over a pressure side vane surface

The thermal performance of a gas turbine airfoil with a cooled pressure side and a trailing edge cutback is investigated and discussed in relationship with the unsteady behavior of coolant injection. The focus is on the pressure side coolant injection through discrete holes. The cascade was tested at an exit Mach number of 0.2 and 0.6 for different coolant to mainstream mass flow ratios. Laser Doppler Velocimetry (LDV) and high speed flow visualizations were used to investigate the unsteady mixing process taking place between coolant and main flow downstream of the holes. This behavior was correlated with the film cooling effectiveness distributions over the vane surface. In the first row, hairpin clockwise rotating vortices and counterclockwise shear layer vortices were observed, depending on the coolant to mainstream velocity ratio. The second row turned out to be characterized by a lower velocity ratio and lower turbulent activity, consistently with the higher thermal protection observed also at high injection rates. The increase in mainstream Mach number kept the jets closer to the wall, resulting in lower turbulent characteristics and lower mixing, consistently with higher thermal protection.