Aerofoil

1. An aerofoil component of a gas turbine engine, the component having an aerofoil portion which spans, in use, a working gas annulus of the engine, the aerofoil portion comprising:

a pressure side outer wall and a suction side outer wall which respectively define the external pressure side and suction side aerofoil surfaces of the aerofoil portion, each outer wall extending from the leading edge to the trailing edge of the aerofoil portion;

one or more main passages which extend in the annulus-spanning direction of the aerofoil portion and which receive, in use, a first flow of coolant therethrough;

one or more suction wall passages which extend in the annulus-spanning direction of the aerofoil portion and which receive, in use, a second flow of coolant therethrough, each suction wall passage being bounded on opposing first sides by the suction side outer wall and an inner wall of the aerofoil portion, the inner wall separating the suction wall passages from the main passages; and

a plurality of dividing walls which extend between the suction side outer wall and the inner wall, each suction wall passage being bounded on opposing second sides by a pair of the dividing walls, one of the pair of the dividing walls being closer to the leading edge and the other of the pair of the dividing walls being closer to the trailing edge,

wherein the dividing walls have fillets to smooth the transitions from the dividing walls to the inner wall and the suction side outer wall, the fillets being shaped, such that on transverse cross-sections to the annulus-spanning direction of the aerofoil portion, the radii of curvature of the fillets are equal, within ±25%, to the thickness of the suction side outer wall, and

wherein the length of the surface of the inner wall forming a boundary between the inner wall and the suction wall passage, on the transverse cross-section is increased relative to the length of the surface of the suction side outer wall forming a boundary between the suction side outer wall and the suction wall passage, and the inner wall curves into the suction wall passages to give each suction wall passage a kidney-bowl shape on the transverse cross-sections.

2. The aerofoil component according to claim 1 , wherein said opposing second sides of the suction wall passages are substantially semi-circular.

3. The aerofoil component of claim 2 , wherein the inner wall curves into each suction wall passage such that, on the transverse cross-sections, the inner wall forms a protrusion into the suction wall passage, the protrusion turning through at least 90° of arc.

4. The aerofoil component of claim 1 , wherein the inner wall is thinner than the suction side outer wall.

5. The aerofoil component of claim 1 , wherein, on the transverse cross-sections and in respect of each suction wall passage, the inner wall decreases in thickness from locations adjacent the fillets of the respective dividing walls to a central region of the inner wall.

6. The aerofoil component of claim 1 , wherein the minimum thicknesses of the dividing walls are equal to twice the thickness of the suction side outer wall, within ±25%.

7. The aerofoil component of claim 1 , wherein the suction side outer wall has a plurality of effusion holes for passing coolant from the suction wall passages to the suction side aerofoil surface.

8. The aerofoil component of claim 1 , wherein each suction wall passage has heat transfer augmentation formations provided by the suction side outer wall and/or the inner wall, the heat transfer augmentation features causing the coolant flow to separate from and reattach to the suction side outer wall and/or the inner wall on which the transfer augmentation formations are provided.

9. The aerofoil component of claim 1 , wherein the inner wall has a plurality of through-holes for producing impingement jets impinging on the suction side outer wall, the impingement jets being formed from coolant passing through the through-holes from the main passages into the suction wall passages.

10. The aerofoil component of claim 1 , wherein each suction wall passage further has a plurality of pedestals extending across the passage to connect the inner wall to the suction side outer wall.

11. A gas turbine engine having an aerofoil component of a gas turbine engine, the component having an aerofoil portion which spans, in use, a working gas annulus of the engine, the aerofoil portion comprising:

a pressure side outer wall and a suction side outer wall which respectively define the external pressure side and suction side aerofoil surfaces of the aerofoil portion, each outer wall extending from the leading edge to the trailing edge of the aerofoil portion;

one or more main passages which extend in the annulus-spanning direction of the aerofoil portion and which receive, in use, a first flow of coolant therethrough;

one or more suction wall passages which extend in the annulus-spanning direction of the aerofoil portion and which receive, in use, a second flow of coolant therethrough, each suction wall passage being bounded on opposing first sides by the suction side outer wall and an inner wall of the aerofoil portion, the inner wall separating the suction wall passages from the main passages; and

a plurality of dividing walls which extend between the suction side outer wall and the inner wall, each suction wall passage being bounded on opposing second sides by a pair of the dividing walls, one of the pair of the dividing walls being closer to the leading edge and the other of the pair of the dividing walls being closer to the trailing edge,

wherein the dividing walls have fillets to smooth the transitions from the dividing walls to the inner wall and the suction side outer wall, the fillets being shaped, such that on transverse cross-sections to the annulus-spanning direction of the aerofoil portion, the radii of curvature of the fillets are equal to the thickness of the suction side outer wall, within ±25%,

wherein the length of the surface of the inner wall forming a boundary between the inner wall and the suction wall passage, on the transverse cross-section is increased relative to the length of the surface of the suction side outer wall forming a boundary between the suction side outer wall and the suction wall passage, and the inner wall curves into the suction wall passages to give each suction wall passage a kidney-bowl shape on the transverse cross-sections.