Pressure-Side Bleed Film Cooling: Part I — Steady Framework for Experimental and Computational Results

Abstract

This study combines both experiments and computations to investigate pressure-side bleed on the trailing edge of a turbine blade. Realistic engine conditions are considered with a lip thickness to slot height ratio of 0.9 and mainstream Mach numbers of 0.7 at the coolant injection point expanding to sonic conditions at the exit plane of the test section. The purpose of this study is to understand the complex physics of pressure-side bleed, in particular, the unusual behavior that occurs with increasing blowing ratio. Experimentally, it is shown that as the blowing ratio increases, the film cooling effectiveness at a point near the end of the test section increases for blowing ratios less than 0.8, while decreasing over the range of blowing ratios from 1.0 through 1.25. For blowing ratios higher than 1.25, effectiveness increases. This phenomenon has been repeated experimentally for many years without being fully understood. Parts I and II of this paper describe the mechanism responsible for the unusual experimental results. This mechanism is unsteady vortex shedding. Experimental results are from a row of jets with the use of foreign gas injection that simulates the engine conditions that would be seen by the pressure side of an airfoil with pressure-side bleed. These results consist of the pressure distribution due to the nozzle and the effectiveness along the test surface downstream of the injection site. The computational model is designed to replicate the experimental setup. High-quality grids, high-order discretization schemes, and an advanced turbulence model are employed to ensure that the computational results can be used to explain the complex physics of transonic pressure-side bleed film cooling. The grid consists of 2.2 million cells and a high-quality, unstructured, multi-topology, super-block mesh with the resolution of the viscous sub-layer and y+ < 1 on all surfaces. The simulations are fully converged and grid-independent. Effects of blowing ratio are examined, with blowing ratio ranging from 0.5 to 2.0 and a density ratio of 1.52. The geometry consists of not only the transonic mainstream flow and the jet, but also the creeping plenum flow. As a result of the significant lip thickness to slot height ratio, it is shown that unsteady effects are the dominant mechanism in the physics of pressure-side bleed film cooling.