Aerofoil blade for an axial flow turbomachine

1. An aerofoil blade, comprising:

a radially inner platform region;

a radially outer tip region;

an axially forward leading edge; and

an axially rearward trailing edge which is straight between the platform region and the tip region and oriented radially of an annular path;

a pressure surface which is convex in a radial direction between the radially inner platform region and the radially outer tip region; and

a suction surface which is concave in the radial direction between the radially inner platform region and the radially outer tip region;

wherein an axial width (W) of the aerofoil blade, being an axial distance between the leading edge and the straight trailing edge, varies parabolically between maximum axial widths (W max ) at the platform and tip regions, respectively and a minimum axial width (W min ) at a position between the platform region and the tip region;

wherein a K value, being equal to a ratio of a throat dimension (t) to a pitch dimension (p), varies between minimum values (K min platform ) and (K min tip ) at the platform region and the tip region respectively, and a maximum value (K max ) at a position between the platform region and the tip region;

wherein the minimum K value at the platform region (K min platform ) is substantially equal to the minimum K value at the tip region (K min tip ).

2. An aerofoil blade according to claim 1 , comprising:

a variable aerofoil section between the platform region and the tip region to provide parabolic variation in the axial width (W) of the aerofoil blade.

3. An aerofoil blade according to claim 2 , wherein the aerofoil sections between the platform region and the tip region have a same setting angle (β).

4. An aerofoil blade according to claim 1 , wherein the minimum axial width (W min ) of the aerofoil blade occurs at a position mid-height between the platform region and the tip region.

5. An aerofoil blade according to claim 4 , wherein parabolic variation in the axial width (W) of the aerofoil blade is symmetrical about the position mid-height between the platform region and the tip region.

6. An aerofoil blade according to claim 1 , wherein the maximum K value (K max ) occurs at a position mid-height between the platform region and the tip region.

7. An aerofoil blade according to claim 1 , wherein variation of the K value is parabolic and substantially symmetrical about a position mid-height between the platform region and the tip region.

8. An aerofoil blade according to claim 1 , wherein the aerofoil blade is a fixed blade of a turbine stage, the turbine stage, in combination, comprising:

a ring of the fixed blades.

9. An aerofoil blade according to claim 1 , wherein the aerofoil blade is in combination with a ring of similar blades arranged in an axial flow turbomachine having an annular path for working fluid.

10. An aerofoil blade according to claim 1 , wherein maximum axial widths (W max ) at the platform and tip regions are substantially the same.

11. An aerofoil blade according to claim 1 , wherein the airfoil blade is a fixed blade.

12. An axial flow turbomachine, including a ring of aerofoil blades, each aerofoil blade comprising:

a radially inner platform region;

a radially outer tip region;

an axially forward leading edge;

an axially rearward trailing edge which is straight between the platform region and the tip region and oriented radially of an annular path;

a pressure surface which is convex in a radial direction between the radially inner platform region and the radially outer tip region; and

a suction surface which is concave in the radial direction between the radially inner platform region and the radially outer tip region;

wherein an axial width (W) of the aerofoil blade, being an axial distance between the leading edge and the straight trailing edge, varies parabolically between maximum axial widths (W max ) at the platform and tip regions, respectively and a minimum axial width (W min ) at a position between the platform region and the tip region;

wherein a K value, being equal to a ratio of a throat dimension (t) to a pitch dimension (p), varies between minimum values (K min platform ) and (K min tip ) at the platform region and the tip region respectively, and a maximum value (K max ) at a position between the platform region and the tip region;

wherein the minimum K value at the platform region (K min platform ) is substantially equal to the minimum K value at the tip region (K min tip ).

13. An axial flow turbomachine according to claim 12 , each aerofoil blade comprises:

a variable aerofoil section between the platform region and the tip region to provide parabolic variation in the axial width (W) of the aerofoil blade.

14. An axial flow turbomachine according to claim 13 , wherein the aerofoil sections between the platform region and the tip region have a same setting angle (β).

15. An axial flow turbomachine according to claim 12 , wherein the minimum axial width (W min ) of the aerofoil blades occur at a position mid-height between the platform region and the tip region.

16. An axial flow turbomachine according to claim 15 , wherein parabolic variation in the axial width (W) of the aerofoil blades is symmetrical about the position mid-height between the platform region and the tip region.

17. An axial flow turbomachine according to claim 12 , the maximum K value (K max ) occurs at a position mid-height between the platform region and the tip region.

18. An axial flow turbomachine according to claim 12 , wherein variation of the K value is parabolic and substantially symmetrical about a position mid-height between the platform region and the tip region.