Numerical analysis for film cooling characteristics of trenched hole under the effects of rib-disturbed feed flow and curved surface

Film holes are extensively utilized to protect the blade external surface by ejecting coolant, forming a protective film, and separating the hot gas and blade surface. Trenched holes, caused by the turbine blade coated with thermal barrier, greatly feature better film cooling performance than traditional cylinder holes. In this study, the effects of a rib-disturbed feed flow on the film cooling performance of trenched holes are studied through the numerical method. Two typical rib attacking angles, 45° and 135°, are compared with the blowing ratio increasing from 0.5 to 2.0. The effects of the curved surface (convex and concave) are also included. Numerical results prove that film effectiveness with the coolant fed by the rib-disturbed internal flow is sensitive to the blowing ratio. The rib-turbulated cooling flow entering the film hole is featured with different swirling states; therefore, the interaction between the mainstream and the cooling air of varied swirling state leads to different film coverage and effectiveness. Overall, 135° ribs induce better adiabatic film cooling effectiveness than 45° ribs, with a maximum improvement 34.9% at M = 0.5. Film effectiveness on the convex surface is better than that on flat and concave surfaces. Area-averaged η on convex and concave surfaces is, respectively, 4.7% higher and 6.2% lower than that on the flat surface. Normal pressure gradient established on the convex surface contributes to reducing the turbulence intensity and improving the film lateral coverage.