Particle-Image Velocimetry Measurements of Film Cooling in an Adverse Pressure Gradient Flow

The turbulent flow field of a film cooling flow is investigated using the particle-image velocimetry (PIV) technique. Cooling jets are injected from a multi-row hole configuration into a turbulent boundary layer flow of a flat plate in the presence of a zero and an adverse pressure gradient. The investigations focus on full-coverage film cooling. Therefore, the film cooling configuration consists of three staggered rows of holes with a lateral spacing of p/D = 3 and a streamwise row distance of l/D = 6. The inclined cooling holes feature a fan-shaped exit geometry with lateral and streamwise expansions. Jets of air and CO₂ are injected separately at different blowing ratios into a boundary layer to examine the effects of the density ratio between coolant and mainstream on the mixing behavior and consequently, the cooling efficiency. For the zero pressure gradient case the measurement results indicate the different nature of the mixing process between the jets and the crossflow after the first, second, and third row. The mainstream velocity distributions evidence the growth of the boundary layer thickness at increasing row number. The interaction between the undisturbed boundary layer and first two rows leads to maximum values of turbulent kinetic energy. The presence of an adverse pressure gradient in the mainstream clearly intensifies the growth of the boundary layer thickness and increases the velocity fluctuations in the upper mixing zone. The measurements considering an increased density ratio show higher turbulence intensities in the shear zone between the jets and the main flow leading to a more pronounced mixing in this area. The results of the experimental measurements are used to validate numerical findings from a large-eddy simulation. This comparison shows a very good agreement for mean velocity distributions and velocity fluctuations.