Combustion chamber for a gas turbine engine

A combustion chamber for a gas turbine engine has a wall which is provided with rows of apertures. The apertures are arranged so that the axes of the apertures form an angle of between 25.degree. and 35.degree. with respect to the inner surface of the wall. The apertures have a first cylindircal portion and a second divergent portion to produce fan shaped apertures. An upstream portion of the wall has apertures arranged in axially spaced groups, each of which has three rows of apertures and a downstream portion of the wall has apertures arranged in axially spaced groups, each of which has two rows of apertures. The axes of adjacent apertures in each row are spaced apart by at least three times the diameter of the cylindrical portion. The apertures produce effective film cooling of the wall using less cooling air than conventional cooling rings. The apertures may be arranged locally to cope with hot spots.

FIELD OF THE INVENTION 
The present invention relates to combustion chambers for gas turbine 
engines, and is particularly concerned with cooling of the walls of the 
combustion chamber. 
BACKGROUND OF THE INVENTION 
One conventional method of cooling the walls of combustion chambers of gas 
turbine engines uses cooling rings which are positioned between and 
secured to axially spaced wall sections. These cooling rings are provided 
with a plurality of relatively large apertures arranged in a row, or a 
number of rows of relatively small apertures. These apertures direct a 
flow of cooling fluid onto the inner surface of the wall to form a film of 
cooling fluid which protects the wall from the high temperatures produced 
in the combustion chamber. However, such cooling rings are relatively 
wasteful of cooling fluid. 
A further problem with the cooling rings is that the thermal gradients 
produced across the cooling ring lead to cracking of the cooling ring and 
the large numbers of cooling apertures allows easy propagation of the 
crack and eventual failure of the cooling ring. 
A further conventional method of cooling the wall of combustion chambers of 
gas turbine engines uses walls which are formed from two or more laminae 
which are secured together to form internal passages therethrough for 
transpiration cooling of the wall by a cooling fluid. The cooling fluid is 
then directed through apertures out of the wall to from a cooling film of 
fluid on the inner surface of the wall. These arrangements are more 
efficient than the cooling rings using approximately a third of the 
cooling fluid, but the inner surface of the wall tends to become 
relatively hot because of ineffective film cooling due to the apertures 
being arranged normal to the inner surface and being spaced by relatively 
large distances. 
SUMMARY OF THE INVENTION 
The present invention seeks to provide a combustion chamber of a gas 
turbine with improved film cooling of the walls of the combustion chamber. 
Accordingly the present invention provides a combustion chamber for a gas 
turbine having at least one wall defining at least partially the 
combustion chamber, the wall having an inner surface and an outer surface, 
and additionally having at least one row of apertures extending 
therethrough for supplying cooling fluid onto the inner surface of the 
wall to form a cooling film of fluid on that surface, the axes of the 
apertures being arranged to form an angle of between 20.degree. and 
40.degree. with the inner surface of the wall, each aperture having a 
first portion and a second portion, the first portion being arranged to 
receive cooling fluid from cooling fluid flowing over the outer surface of 
the wall and to supply the cooling fluid to the second portion, the second 
portion being divergent and arranged to direct the cooling fluid over the 
inner surface of the wall to form the cooling film of fluid. 
The axes of the apertures may be arranged at an angle of between 25.degree. 
and 35.degree. with respect to the inner surface of the wall. 
The divergent portions of the apertures may be divergent at an angle of 
substantially 12.5.degree. with respect to the axes of the apertures. 
The first portion of the apertures may by cylindrical. 
The axes of the adjacent apertures in each row may be spaced apart by at 
least three times the diameter of the cylindrical portion of the 
apertures. 
The wall may have at least two rows of apertures, the apertures in each row 
being staggered with respect to the apertures in the adjacent row or rows. 
The adjacent rows of apertures may be spaced apart by at least two times 
the diameter of the cylindrical portion of the apertures. 
The cylindrical portion of the apertures may have a diameter of 
substantially 0.762 mm. 
The wall may be an upstream wall of the combustion chamber. 
The wall may be a tubular wall of a tubular combustion chamber, or may be 
an inner annular wall of an annular combustion chamber, or may be an outer 
annular wall of annular combustion chamber. 
An upstream portion of the wall may have the apertures arranged in axially 
spaced groups, each group having three rows of apertures. 
A downstream portion of the wall may have the apertures arranged in axially 
spaced groups, each group having two rows of apertures. 
The present invention will be more fully described by way of example with 
reference to the accompanying drawings, in which:

DETAILED DESCRIPTION OF THE INVENTION 
A turbofan gas turbine engine 10 is shown in FIG. 1, and this comprises in 
axial flow series an inlet 12, a fan section 14, a compressor section 16, 
a combustor section 18, a turbine section 20 and an exhaust nozzle 22. The 
operation of the turbofan gas turbine engine 10 is quite conventional in 
that air flows into the inlet 12 and is given an initial compression by 
the fan section 14. This air is divided into two portions. The first 
portion of air is passed through the fan duct (not shown) to the fan 
nozzle (not shown). The second portion of air supplied to the compressor 
section 16 where the air is further compressed before being supplied to 
the combustor section 18. Fuel is burnt in the air supplied to the 
combustor section 18 to produce hot gases which flow through and drive the 
turbine section 20 before passing through the exhaust nozzle 22 to 
atmosphere. The turbine section 20 is arranged to drive the fan section 14 
and compressor section 16 via shafts (not shown). 
The combustor section 18 is shown more clearly in FIGS. 2 to 10. The 
combustor section comprises an outer casing 24 and an annular combustion 
chamber 26 enclosed by the casing 24. The annular combustion chamber 26 is 
defined by an annular upstream wall 28, an annular outer wall 30 and an 
annular inner wall 32. An annular outer passage 25 for the flow of cooling 
air is formed between the casing 24 and the annular outer wall 30, and an 
inner passage 27 for the flow of cooling is formed within the annular 
inner wall 32. 
The annular upstream wall 28 is provided with a plurality of 
equi-circumferentially spaced apertures 36, and a fuel injector 34 is 
positioned coaxially in each of the apertures 36. The annular upstream 
wall 28 comprises an upstream wall member 37 and a downstream wall member 
38 with a chamber 39 formed therebetween. The upstream wall member 37 has 
a plurality of apertures (not shown) for supplying air to the chamber 39. 
The downstream wall member 38 shown in FIG. 9 and 10 is formed from a 
plurality of arcuate segments 54 each of which has a central aperture 40 
formed substantially in its centre to receive a fuel injector 34. Each 
segment 54 is secured to the upstream wall member 37 by a number of bolts 
64 and nuts (not shown). 
The segments 54 of the downstream wall member 38 have an inner surface 56 
and an outer surface 58, and the segments 54 are provided with a plurality 
of rows of apertures 60 extending therethrough which supply cooling air 
from the chamber 39 onto the inner surface 56 of the segments 64 to form a 
cooling film of air. The rows of apertures 60 extend radially with respect 
to the axis of the annular combustion chamber 26. The apertures 60 are 
arranged so that their axes form an angle of between 20.degree. and 
40.degree. with the inner surface 56 of the segments 54. The apertures 60, 
have first portions which are cylindrical, and second portions which are 
divergent. The cylindrical portions supply cooling air from the chamber 39 
to the divergent portions, and the divergent portions direct the cooling 
air over the inner surface 56 of the segments 54 to form a cooling film of 
air. The divergent portions of the apertures diverge at an angle, in this 
example, of 12.5.degree. with respect to the axes of the apertures. The 
axes of the adjacent apertures 60 in each row are spaced apart by three 
times the diameter of the cylindrical portion of the aperture. 
It is to be noted that the rows of apertures 60 are arranged in groups of 
three rows, each group of rows of apertures being angularly spaced from 
the next group. The apertures in each row are staggered with respect to 
the apertures in the adjacent row or rows in that group. 
The adjacent rows of apertures in each group are spaced apart by at least 
two times the diameter of the cylindrical portion of the apertures. 
There are two groups of three rows of apertures 60 on one circumferential 
half of the segment 54, and another two groups of three rows of apertures 
60 on the other circumferential half of the segment 54, these groups of 
apertures 60 are arranged to direct the cooling air in a circumferential 
direction towards the central aperture 40. 
The outer annular wall 30 shown in FIGS. 3 to 8 has an inner surface 44 and 
an outer surface 46, and has a plurality of rows of apertures 48. The 
apertures 48 extend through the outer annular wall 30 to supply cooling 
air from the outer annular passage 25 onto the inner surface 44 of the 
outer annular wall 30 to form a cooling film of air. The rows of apertures 
48 extend circumferentially with respect to the axis of the annular 
combustion chamber 26. The apertures 48 are arranged so that their axes 
from an angle of between 20.degree. and 40.degree. with respect to the 
inner surface of the outer annular wall 30. The apertures 48 have first 
portions 50 which are cylindrical, and second portions 52 which are 
divergent. The cylindrical portions 50 supply cooling air flowing over the 
outer surface 46 of the outer annular wall 30 in the outer annular passage 
25 to the divergent portions 52, and the divergent portions 52 direct the 
cooling air in a downstream direction over the inner surface 44 of the 
outer annular wall 30 to form a cooling film of air. The divergent 
portions 52 of the apertures 48 diverge at an angle .alpha.=12.5.degree. 
with respect to the axes of the apertures 48. The axes of the adjacent 
apertures 48 in each row are spaced apart by a distance S, the distance S 
is three times the diameter d of the cylindrical portion 50 of the 
apertures 48. The divergent portions 52 of the apertures 48 diverge in a 
circumferential direction to produce a fan shaped aperture. 
It is to be noted that the rows of apertures 48 are arranged in groups of 
three rows over an upstream portion 31 of the outer annular wall 30, and 
are arranged in groups of two rows over a downstream portion 33 of the 
outer annular wall 30. Each group of three rows of apertures in the 
upstream portion 31, or each group of two rows of apertures in the 
downstream portion 33 is axially spaced from the next group. The apertures 
48 in each row are staggered with respect to the apertures 48 in the 
adjacent row or rows in that group. 
The adjacent rows of apertures 48 in each group are spaced apart by at 
least two times the diameter d of the cylindrical portion 50 of the 
apertures 48. 
Preferably the apertures 48 are arranged so that their axes form an angle 
of between 25.degree. and 35.degree. with respect to the inner surface 44 
of the outer annular wall 30. 
The cylindrical portions 50 of the apertures 48 in this example have a 
diameter d of 0.762 mm, and the apertures are formed by laser drilling or 
other suitable method. 
The spacing S, or pitch, between the apertures is the most important 
dimension, and this is related to the angle of divergence of the 
apertures. The spacing S between the apertures increases with the angle of 
divergence of the apertures. In this example the angle .alpha. of 
divergence of the apertures is 12.5.degree.and the spacing S is three 
times the diameter d. Apertures having angles .alpha. of greater than 
12.5.degree. will have a spacing S greater than three times the diameter 
d. 
The apertures are inclined with respect to the inner surface of the 
upstream wall or annular outer wall so that the cooling air flowing 
through the apertures forms a cooling film of air on the inner surface of 
the upstream wall or annular outer wall. Apertures arranged at 90.degree. 
to the inner surface of the walls do not form cooling films of air because 
the cooling air does not flow over the inner surface of the wall. 
The apertures are divergent to improve the effectiveness of the cooling 
film of air by reducing the velocity of the air, causing the cooling air 
to spread out and merge with the cooling air from adjacent apertures in 
each row, and to ensure the cooling film remains on the inner surface of 
the walls. 
However, with the single row of apertures although the effectiveness of 
cooling is improved, there is some entrainment of hot gases, produced in 
the combustion process, between the cooling film of air and the inner 
surface of the walls. 
The use of several closely spaced rows of apertures arranged as a group is 
particularly beneficial, because the cooling film of air discharged over 
the inner surface of the wall by the first row of apertures acts as a 
barrier to inhibit the entrainment of hot gases between the cooling film 
produced by the second row of apertures and the inner surface of the wall, 
and likewise the cooling films of air discharged over the inner surface of 
the wall by the second row of apertures acts as a further barrier to 
inhibit the entrainment of the hot gases between the cooling film produced 
by the third row of apertures and the inner surface of the wall. The use 
of several closely spaced rows of apertures produces a thicker cooling 
film of air which prevents the hot gases contacting the inner surface of 
the walls. 
The use of walls with cooling apertures as described is more effective than 
the prior art cooling ring, because it uses a smaller amount of air to 
cool the same area, the invention uses approximately two thirds of the 
quantity of cooling air used by the prior art cooling ring. 
The annular inner wall may also be provided with rows of apertures 
similarly arranged to the rows of apertures in the annular outer wall. 
The invention although it has been described with reference to an annular 
combustion chamber may equally well be applied to tubular combustion 
chambers, or other arrangement of combustion chamber. 
The rows of cooling apertures are simple to produce and they may be 
arranged at any location axial and/or circumferential to cope with local 
hot spots, ie local arrangements of rows of cooling apertures may 
positioned to provide film cooling for areas of the combustion chamber 
which are normally overheated. 
The divergent portions of the adjacent apertures in each row are arranged 
such that the divergent portions do not merge together ie there is a space 
separating the divergent portions of the adjacent apertures in each row.