Gas turbine engine end-wall component

1. An end-wall component of a mainstream gas annulus of a gas turbine engine, the gas turbine engine comprising an annular arrangement of vanes and the mainstream gas annulus including a main flow direction from upstream of the annular arrangement of vanes to downstream the annular arrangement of vanes, the end-wall component comprising a cooling arrangement including ballistic cooling holes through which, in use, a dilution cooling air is jetted into the mainstream gas annulus upstream of the annular arrangement of vanes to reduce the mainstream gas temperature adjacent the end-wall,

wherein the ballistic cooling holes are arranged in one or more circumferentially extending rows,

wherein an axial position of the ballistic cooling holes in the or each row varies so as to have axial variation, and

wherein the axial variation is sinusoidal with circumferential periodicity which matches the periodic distribution of the vanes with circumferential mid-vane peaks which extend downstream so as to provide a sinusoidal coolie flow distribution from the ballistic cooling holes.

2. The end-wall component as claimed in claim 1 , wherein the end wall component is a radially inner platform of a nozzle guide vane and the axial variation includes upstream and downstream peaks relative to the flow direction, wherein the downstream peaks of an inner platform lie along a gas flow line of a stagnation region local to a leading edge of the vanes.

3. The end-wall component as claimed in claim 1 , wherein the ballistic cooling holes are arranged in two axially spaced rows so as to provide an upstream row and a downstream row, wherein at least a portion of one of the rows has a portion adjacent a stagnation region of the vane.

4. The end-wall component as claimed in claim 3 , wherein either or both of the upstream and downstream rows have axial variation in relation to the flow direction.

5. The end-wall component as claimed in claim 3 , wherein either or both of the upstream and downstream rows are intermittent so as to have circumferentially extending portions of two or more ballistic cooling holes interspersed with circumferential portions having no ballistic cooling holes.

6. The end-wall component as claimed in claim 5 , wherein the portion with no ballistic cooling holes is aligned with a circumferential mid-vane portion.

7. The end-wall component as claimed in claim 1 , wherein the ballistic cooling holes have a diameter of between 1.3 mm and 2.8 mm.

8. The end-wall component as claimed in claim 1 , wherein the ballistic cooling holes have a trajectory which is inclined to the main rotational axis of the engine at an angle of between 45 and 65 degrees.

9. The end-wall component as claimed in claim 3 , wherein the downstream holes are inclined at a shallower angle to the end wall component surface than the upstream holes.

10. The end-wall component as claimed in claim 3 , wherein either or both of the upstream and downstream rows of ballistic cooling holes have a half-wave sinusoidal configuration, wherein the half-wave sinusoidal portion extends in a downstream direction towards a circumferential mid-vane portion.

11. The end-wall component as claimed in claim 1 , wherein one or more of the ballistic cooling holes has elliptical or racetrack-shaped transverse cross-sections relative to the direction of flow through the holes, and the long axis of the transverse cross-section at the exit of each cooling hole to the mainstream gas annulus is aligned with the direction of flow of the mainstream gas over the exit to within ±20°.

12. The end-wall component as claimed in claim 1 , wherein a first portion of the ballistic cooling holes have a first diameter, and a second portion of ballistic cooling holes have a second diameter which is different to the first diameter.

13. The end-wall component as claimed in claim 1 , further comprising a plurality of film cooling holes located between adjacent vanes.

14. A nozzle guide vane comprising the end wall component according to claim 1 .