Study on the physics of film-cooling effectiveness enhancement by the converging-expanding hole

Numerical simulation was carried out to investigate the flow fields for three types of shaped hole,namely,cylindrical hole,fan-shaped hole and converging-expanding hole,to reveal how the converging-expanding hole enhance the cooling effectiveness.Realizable k-ε turbulence model with enhanced wall function was used.Results show that the jet from cylindrical hole produces strong streamwise coupling vortices due to its strong and concentrated normal momentum.The fan-shaped hole improves the cooling effectiveness by weakening the jet's normal momentum and the coupling vortices.The converging-expanding hole reduces flow separation in the hole and produces a jet with small streamwise thickness and large lateral width as well as large lateral velocity magnitude.These factors make its coupling vortices have an effect to push the jet to the wall,producing the best coverage and the best cooling effectiveness.The average cooling effectiveness of converging-expanding hole is larger than cylindrical hole's and fan-shaped hole's by about 110%and 15%respectively(M=0.5),or 560%and 60%(M=2).