Large-Eddy Simulations of trailing-edge cutback film cooling at low blowing ratio

For high-pressure turbine blades, an efficient cooling of the trailing edge can be achieved by ejecting a film into the flow over a cutback on the pressure-side of the blade. Here, results of well-resolved Large-Eddy Simulations (LES) are reported that match an existing experimental setup. A low blowing ratio M=0.5 was chosen and compared to results for an engine-typical value of 1.1. LES and experiments agreed reasonably well for mean and r.m.s. velocity profiles and adiabatic film-cooling effectiveness η_aw. By imposing different flow conditions in the coolant channel, the LES data show that the flow regime of the coolant at ejection has a significant impact on the performance of the resulting cooling film for both blowing ratios. There are two surprising results: (1) a counter-intuitive behavior causing an also experimentally observed increase in η_aw for a reduced blowing ratio. (2) For M=0.5, a turbulent coolant sustains a higher cooling effectiveness farther downstream compared to a laminar coolant, whereas, for M=1.1, the opposite is observed. It is shown that both phenomena are related to a change in type and strength of the dominant coherent structures that are formed behind the cutback lip and convected downstream along the trailing edge.