Bell-shaped fan cooling holes for turbine airfoil

1. A turbine airfoil having an external surface defining a curvature, comprising:

(a) a leading edge and an axially spaced-apart trailing edge, the leading edge having an axially-extending aerodynamic external surface curvature;

(b) a root and a tip spaced-apart along a radially-extending span axis;

(c) a pressure sidewall and a laterally-spaced-apart suction sidewall;

(d) a cooling circuit positioned between the pressure sidewall and the suction sidewall for channeling a fluid flow for cooling the airfoil;

(e) a plurality of cooling holes formed in the external surface of the airfoil in fluid communication with the cooling circuit, at least some of the plurality of cooling holes comprising:

(i) a metering section downstream of the cooling circuit;

(ii) a diffuser section downstream of the metering section and upstream of the external surface of the airfoil, the diffuser section having at least a bottom wall and opposed side walls defining an exit opening on the external surface for exiting a cooling film, the bottom wall having a downstream diverging, non-planar surface extending between the opposed side walls that defines a convex curvature in the direction of the span axis at the exit opening.

2. A turbine airfoil according to claim 1 , wherein the metering section has a uniform cross-section normal to the direction of fluid flow through the metering section.

3. A turbine airfoil according to claim 1 , wherein the metering section has a uniform cross-section normal to the direction of fluid flow through the metering section and selected from the group consisting of circular, oval and rectangular.

4. A turbine airfoil according to claim 1 , wherein the cooling holes are formed in the suction sidewall of the airfoil.

5. A turbine airfoil according to claim 1 , wherein the exit opening of the diffuser section defines a radially-extending vertical height dimension and a laterally-extending width dimension.

6. A turbine airfoil according to claim 1 , wherein the bottom wall of the diffuser section defines an angle of between 0 and 30 degrees relative to the metering section.

7. A turbine airfoil according to claim 1 , wherein the surface area coverage of the leading edge by the cooling holes is about 35 percent.

8. A turbine airfoil according to claim 1 , wherein the area of each exit opening is between about 0.12 mm 2 and about 0.65 mm 2 .

9. A turbine airfoil according to claim 1 , wherein the area of the exit opening is about 0.45 mm 2 .

10. A turbine airfoil according to claim 1 , wherein the airfoil comprises a turbine blade.

11. A turbine airfoil having an external surface defining a curvature, comprising:

(a) a leading edge and an axially spaced-apart trailing edge, the leading edge having an axially-extending aerodynamic external surface curvature;

(b) a root and a tip spaced-apart along a radially-extending span axis;

(c) a pressure sidewall and a laterally-spaced-apart suction sidewall;

(d) a cooling circuit positioned between the pressure sidewall and the suction sidewall for channeling a fluid flow for cooling the airfoil;

(e) a plurality of cooling holes formed in the external surface of the airfoil in fluid communication with the cooling circuit, each of the plurality of cooling holes comprising:

(i) a metering section downstream of the cooling circuit;

(ii) a diffuser section downstream of the metering section and upstream of the external surface of the airfoil, the diffuser section having at least a bottom wall and opposed side walls defining an exit opening on the external surface for exiting a cooling film; and

(f) wherein the bottom wall of the diffuser section defines a non-planar, convex, curved surface extending between the side walls in the direction of the span axis at the exit opening for achieving a flow of the cooling film from the cooling circuit with enhanced spread, enhanced attachment and less turbulence to the external surface of the airfoil local to each respective cooling hole.

12. A turbine airfoil according to claim 11 , wherein the airfoil comprises a turbine blade.

13. A turbine airfoil according to claim 11 , wherein the curvature of the side walls of the diffuser section is between 10 and 30 degrees of arc of a circle defined by the side walls, and the curvature of the bottom wall of the diffuser section is between 10 and 30 degrees of arc of a circle defined by the bottom wall.

14. A turbine airfoil according to claim 13 , wherein the circle defined by the side walls has a radius of 5 mm, and the circle defined by the bottom wall has a radius of 7.1 mm.

15. A turbine airfoil having an external surface defining a curvature, comprising:

(a) a leading edge and an axially spaced-apart trailing edge, the leading edge having an axially-extending aerodynamic external surface curvature;

(b) a root and a tip spaced-apart along a radially-extending span axis;

(c) a pressure sidewall and a laterally-spaced-apart suction sidewall;

(d) a cooling circuit positioned between the pressure sidewall and the suction sidewall for channeling a fluid flow for cooling the airfoil;

(e) a plurality of cooling holes formed in the external surface of the airfoil in fluid communication with the cooling circuit, each of the plurality of cooling holes comprising:

(i) a metering section downstream of the cooling circuit;

(ii) a diffuser section downstream of the metering section and upstream of the external surface of the airfoil, the diffuser section having at least a bottom wall and opposed side walls defining an exit opening on the external surface for exiting a cooling film; and

(f) wherein the bottom wall of the diffuser section has a downstream diverging, convex, curved, non-planar configuration extending between the side walls in the direction of the span axis at the exit opening, and wherein the curvature of the bottom wall is between 10 and 30 degrees of arc of a circle defined by the bottom wall.

16. A turbine airfoil according to claim 15 , wherein the curvature of the bottom wall is 20 degrees of arc of a circle defined by the bottom wall.