Numerical Investigation on the Effect of Hole Imperfection on Film Cooling Performance
Authors: Rezzag Taha 
Published: May 2021

Film cooling holes in turbine blades are manufactured using different techniques, such as electro discharge, electro chemical and laser percussion drilling. The laser percussion drilling is the fastest one, making it a very attractive technique to use. However, some of the metal that has been melted by the laser solidifies inside the hole creating clumps that can reach up to 25% of the hole diameter. In order to comprehend the technique’s influence on film cooling effectiveness, the hole imperfections produced by laser drilling has been modeled as a discrete inner half-torus located at a specific location inside the hole.
Film cooling thermal and hydrodynamic fields were predicted using various turbulence models combined with wall functions and the enhanced wall treatment. The k-omega SST model (for blowing ratios of 0.45 and 0.90) and realizable k-epsilon model combined with the enhanced wall treatment (for blowing ratio of 1.25) were chosen as results were in good agreement with the available experimental data from literature. The effect of imperfection position is studied at 4 different locations (1D, 2D, 3D and 4D) inside the hole measured from the hole leading edge, for three blowing ratios (0.45, 0.90 and 1.25) and a density ratio of 1. Effectiveness results for a blowing ratio of 0.45 reveal that the centerline effectiveness is improved as the imperfection is located farther from the hole exit. Compared to the perfect hole, the locations of 1D and 2D show a deterioration in the centerline effectiveness while the locations of 3D and 4D show an improvement from x/D=0 to 10. Similar trends for the 1D and 2D locations can be seen for a blowing ratio of 0.90 where the centerline effectiveness is deteriorated. Furthermore, for a blowing ratio of 1.25, all imperfection locations show that a better film cooling performance is obtained for x/D=0 to 4 compared to the perfect hole but then deteriorates slightly onwards.
The present investigation also evaluates the influence of hole inclination angle with a hole imperfection on film cooling performance. Three hole inclination angles were investigated: 35°, 45° and 55°. Centerline effectiveness plots reveal a maximum effectiveness deterioration of 89% for a blowing ratio of 0.90 in the vicinity of the hole exit. Dimensionless temperature contours show that the jet produced in the presence of an imperfection is much more compact causing the counter rotating vortex pair to be closer to each other.
The final investigation of the present work evaluates the influence of imperfection shape and size on film cooling performance. A circular and rectangular profile imperfections were investigated at obstruction sizes of 26.3%, 35% and 40%. Centerline effectiveness plots reveal a deterioration of 262.5%, 533.2% and 735.7% in effectiveness compared the perfect case at 26.3%, 35% and 40% obstructions respectively for a blowing ratio of 0.9 at a dimensionless distance of 10 downstream of the hole exit. Dimensionless temperature contour reveal that the lateral spreading of the coolant is more affected by imperfection shape at the location of x/D=2 where the circular shaped imperfection provides better laterally averaged effectiveness than the rectangular shaped imperfection especially of the 35% obstruction size.