Migration of Film-Coolant from Slot and Hole Ejection at a Turbine Vane Endwall

Kurzfassung

Thermodynamic and aerodynamic measurements at and near the endwall of turbine vanes were carried out in a linear cascade with a transonic flow field. The investigations were performed in the Windtunnel for Straight Cascades at DLR Göttingen at representative dimensionless engine conditions of Mach and Reynolds number, Ma_2is = 1.0 and Re₂ = 850000 respectively. The endwall film cooling configuration consisted of a slot in front of the vanes, film cooling holes inside the vane passages and a groove simulating the slit between two adjacent vane platforms, but there was no coolant leakage from the groove. Laser-Two-Focus velocimetry (L2F) was used to determine local velocities in the vicinity of the endwall. At a much larger number of locations compared to the velocity measurements the L2F-device was utilized as a seeding particle counter which enabled the determination of local coolant concentration. With these concentration measurements the migration of coolant from the different origins could be traced through the vane passage. By extrapolating the measured concentration values to the endwall adiabatic film cooling effectiveness could be obtained. The measurements at the present slot configuration were compared with previous ones where the slot position was closer to the vane entrance. Whereas the coolant ejection at the previous slot position produced a much more intense horse shoe vortex than without coolant, the new slot position causes no increase of secondary flow. This result proves the previous statement that positioning a coolant opening flush near the saddle point of the upstream endwall boundary layer stagnation region should be avoided. The new slot position improved film cooling effectiveness compared to the previous ones even with half the amount of coolant. By investigating the migration of film coolant from the holes inside the vane passage, ineffective holes could be identified and suggestions for improving the film coolant configuration could be given. At one location adiabatic film cooling effectiveness from these aerodynamic measurements could be compared with a thermodynamic measurement using infrared imaging.