Bifeed serpentine cooled blade

1. A turbine blade comprising:

forward and aft serpentine cooling circuits, terminating in corresponding forward and aft impingement channels, with each circuit having two metered inlets for distributing more inlet flow to first passes thereof than to last passes thereof;

a hollow airfoil having partitions joining opposite pressure and suction sides and spaced chordally between opposite leading and trailing edges to define said serpentine circuits, and cooperating impingement channels;

a dovetail integrally joined to said airfoil at a platform, and including four inlet channels separately joined to said serpentine circuits; and

a metering plate joined to said dovetail to cover said four inlet channels and distribute more flow to said first passes than to said last passes;

wherein:

an tip airfoil tip of said hollow airfoil comprises a tip floor recessed therein to directly cover said forward and aft serpentine circuits;

said four inlet channels have larger flow areas than said serpentine circuits; and

said metering plate includes first, second, third, and fourth aperture inlets being smaller in flow area than said four inlet channels to correspondingly meter said cooling flow therethrough.

2. A blade according to claim 1 wherein said inlet channels comprise:

a first inlet channel extending through said dovetail and diverging to join said forward serpentine circuit at said last pass thereof;

a second inlet channel extending through said dovetail and converging to join said forward serpentine circuit at said first pass thereof;

a third inlet channel extending through said dovetail and converging to join said aft serpentine circuit at said first pass thereof; and

a fourth inlet channel extending through said dovetail and diverging to join said aft serpentine circuit at said last pass thereof.

3. A blade according to claim 2 wherein:

said forward serpentine circuit includes said forward first pass commencing at said platform in flow communication with said second inlet channel and terminating at said tip floor in a forward first flow bend joining a second pass which in turn terminates at said platform in a forward second flow bend joined to said first inlet channel and said forward last pass; and

said aft serpentine circuit includes said aft first pass commencing at said platform in flow communication with said third inlet channel and terminating at said tip floor in an aft first flow bend joining an aft second pass which in turn terminates at said platform in an aft second flow bend joined to said fourth inlet channel and said aft last pass.

4. A blade according to claim 3 wherein said first and fourth inlet channels diverge at said forward and aft second bends, respectively.

5. A blade according to claim 3 wherein:

said first inlet channel at said forward second and last passes has a flow area greater than either pass; and

said fourth inlet channel at said aft second and last passes has a flow area greater than either pass.

6. A blade according to claim 3 wherein:

said partition between said forward impingement channel and said forward last pass is perforate behind said leading edge for impingement cooled thereof;

said partition between said aft impingement channel and said aft last pass is perforate before said trailing edge for impingement cooling thereof;

said partition between said forward second and last passes is imperforate except for a forward bleed hole therebetween near said tip floor; and said partition between said aft second and last passes is imperforate except for an aft bleed hole therebetween near said tip floor.

7. A blade according to claim 3 wherein said forward bleed hole is disposed below said forward first bend to additionally cool said airfoil leading edge near said tip, and said aft bleed hole is disposed at said aft first bend to additionally' cool said airfoil trailing edge near said tip.

8. A method of cooling said blade according to claim 3 in a gas turbine engine comprising:

channeling a total flowrate of said cooling flow to said blade; and

metering said total flowrate to divert some of said flow from said second and third inlet channels instead to said first and fourth inlet channels to increase operating temperature of said airfoil over the midchord region thereof while decreasing temperature of said airfoil along said leading and trailing edges as combustion gases flow over said airfoil.

9. A turbine blade comprising:

an airfoil extending in chord between leading and trailing edges and extending in span from a root at an integral platform joined to a dovetail to an opposite tip;

a first serpentine cooling circuit extending through said airfoil forward from the midchord thereof toward said leading edge, and terminating in a forward impingement cooling circuit;

a second serpentine cooling circuit extending through said airfoil aft from said first serpentine circuit toward said trailing edge, and terminating in an aft impingement cooling channel;

a first inlet channel extending through said dovetail to join said first serpentine circuit at a forward last pass thereof;

a second inlet channel extending through said dovetail to join said first serpentine circuit at a forward first pass thereof;

a third inlet channel extending through said dovetail to join said second serpentine circuit at an aft first pass thereof;

a fourth inlet channel extending through said dovetail to join said second serpentine circuit at an aft last pass thereof; and

a metering plate joined to said dovetail to cover said four inlet channels and distribute more cooling flow to said second and third inlet channels than to said first and fourth inlet channels, respectively;

wherein:

said airfoil tip comprises a tip floor recessed therein to directly cover said first and second serpentine circuits;

said four inlet channels have larger flow areas than said serpentine circuits; and

said metering plate includes first, second, third, and fourth aperture inlets being smaller in flow area than said four inlet channels to correspondingly meter said cooling flow therethrough.

10. A blade according to claim 9 further comprising a plurality of partitions joining together opposite pressure and suction sides of said airfoil, and spaced chordally apart to define corresponding channels of said two serpentine circuits terminating at said impingement channels and fed by said inlet channels extending through said dovetail.

11. A blade according to claim 10 wherein:

said first serpentine circuit includes said forward first pass commencing at said platform in flow communication with said second inlet channel and terminating at said tip floor in a forward first flow bend joining a second pass which in turn terminates at said platform in a forward second flow bend joined to said first inlet channel and said forward last pass; and

said second serpentine circuit includes said aft first pass commencing at said platform in flow communication with said third inlet channel and terminating at said tip floor in an aft first flow bend joining an aft second pass which in turn terminates at said platform in an aft second flow bend joined to said fourth inlet channel and said aft last pass.

12. A blade according to claim 11 wherein:

said second and third inlet channels converge at said forward and aft first passes, respectively; and

said first and fourth inlet channels diverge at said forward and aft second bends, respectively.

13. A blade according to claim 12 wherein:

said first inlet channel at said forward second and last passes has a flow area greater than either pass; and

said fourth inlet channel at said aft second and last passes has a flow area greater than either pass.

14. A blade according to claim 11 wherein:

said partition between said forward impingement channel and said forward last pass is perforate behind said leading edge for impingement cooled thereof;

said partition between said aft impingement channel and said aft last pass is perforate before said trailing edge for impingement cooling thereof;

said partition between said forward second and last passes includes a forward bleed hole therebetween near said tip floor; and

said partition between said aft second and last passes includes an aft bleed hole therebetween near said tip floor.

15. A blade according to claim 14 wherein said forward bleed hole is disposed below said forward first bend to additionally cool said airfoil leading edge near said tip, and said aft bleed hole is disposed at said aft first bend to additionally cool said airfoil trailing edge near said tip.

16. A blade according to claim 14 wherein said partition between said aft impingement channel and said aft last pass includes impingement holes inclined laterally therethrough for impingement cooling the back of said airfoil pressure side before said trailing edge.

17. A method of cooling said blade according to claim 11 in a gas turbine engine comprising:

channeling a total flowrate of said cooling flow to said blade; and

metering said total flowrate to divert some of said flow from said second and third inlet channels instead to said first and fourth inlet channels to increase operating temperature of said airfoil over the midchord region thereof while decreasing temperature of said airfoil along said leading and trailing edges as combustion gases flow over said airfoil.

18. A method of cooling said blade according to claim 11 in a gas turbine engine comprising:

channeling a total flowrate of said cooling flow to said blade;

operating said blade in said engine with a crack in said tip causing some of said cooling flow to leak from one of said serpentine circuits; and

metering a sufficient portion of said total flowrate through said first and fourth inlet channels and into said serpentine circuits to effectively offset loss of cooling flow from said tip leak.