Quantitative Visualization of Full-Coverage Discrete-Hole Film Cooling

Abstract

Water tunnel experiments were carried out to study full-coverage discrete-hole film cooling for geometries applicable to gas turbine combustor liners. The cooling holes were spaced at a pitch to diameter ratio of 6.5 and had an injection angle of 20° to the cooled surface. The mainstream flow direction was in line with the cooling holes. Blowing ratios from 0.5 to 5.7 were studied, which is a range typical of combustor liners. A unique multiple plane PLIF (planar laser-induced fluorescence) technique was used to measure time-averaged film cooling concentration at various heights above the surface to be cooled. Three-dimensional data sets were generated to quantitatively visualize cooling-jet film coverage, structure, and interaction. Film coverage as close as 0.25 mm (0.010 in.) from the surface was measured, thereby yielding data that approach adiabatic film effectiveness. Two sets of film cooling experiments were conducted. One set used a model with a relatively small array of 2.54 mm (0.100 in.) holes, meant to be a large-scale model of hole sizes encountered in combustor liners. The second set used a large array of 0.51 mm (0.020 in.) nominal diameter laser-drilled holes, manufactured in the same manner as combustor liner cooling holes. The results show that near-wall film coverage is minimum for blowing ratios from 1.7 to 3.3. At blowing ratios less than 1.7 and greater than 3.3, the film coverage was improved. Jet structure and interaction was also observed. In particular, jet separation behavior and coalescence were visualized, and both were generally a function of blowing ratio.