This study examines the unsteady transonic pressure-side bleed film cooling on the trailing edge of a turbine blade and resolves the key mechanism responsible for the unusual relationship between film cooling effectiveness and increasing blowing ratio. This study is meant to show that unsteadiness is the key mechanism causing the unexpected results seen in the experiments. It is believed that this unsteadiness is highly dependent on the ratio of the lip thickness to slot height and the shedding frequencies of the passage and coolant vortices, which depend on blowing ratio. For low blowing ratio, hot passage flow has the dominant vortices. For high blowing ratio, coolant flow has the dominant vortices. For intermediate blowing ratio, the vortices have the potential to interact and cause the unusual behavior seen in pressure-side bleed film cooling. On the basis of these observations, experiments were repeated with pressure probes used to acquire the shedding frequencies at the effectiveness measurement location, which showed that unsteadiness was indeed present. Realistic engine conditions are considered with lip thickness to slot height ratio of 0.9 and mainstream Mach numbers of 0.7 at the coolant injection point and expanding to sonic conditions at the exit plane of the test section. Numerical results are from a 2-D mid-plane cut of the original geometry and a full-pitch 3-D model. Computations use high quality grids, high order discretization schemes, and an advanced turbulence model. The 3-D grid consists of 4.4 million cells and a high quality, unstructured, multi-topology mesh with resolution of the viscous sublayer and y+ < 1 on all surfaces. The simulations are fully converged, time accurate, and grid-independent. A novel methodology is used to introduce unsteadiness into the simulations. Effects of blowing ratio are examined, where blowing ratio is equal to 1.0 for 3-D and ranges from 0.3 to 1.5 for 2-D with a density ratio of 1.52. By performing an unsteady simulation, the unusual relationship between the effectiveness and blowing ratio is demonstrated in an unsteady framework.

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