Heatshield for a gas turbine engine

Abstract

A heat shield for a gas turbine engine has a main body having a first and second surface, the first surface exposed to a hot working gas, a plurality of walls upstanding from the second surface and an impingement plate. The impingement plate is on top of at least one wall and forms a chamber and has an array of impingement holes. At least one pair of divider walls are formed within the chamber and extend between the impingement plate and the second surface. The first divider wall extends from a first wall towards a second wall, the second divider wall extends from the second wall towards the first wall. The first and second divider walls both extend such that there is no clear line of sight in a perpendicular direction to the first divider wall and/or second divider wall and are spaced apart with respect to the perpendicular direction.

Claims

1. A heat shield for a gas turbine engine, the heat shield comprising: a main body having a first surface and a second surface, the first surface being exposed to a hot working gas, a plurality of walls upstanding from the second surface, and an impingement plate, wherein the impingement plate is attached to at least one wall of the plurality of walls and forms a chamber with the second surface and the plurality of walls and wherein the impingement plate comprises an array of impingement holes, at least one pair of divider walls comprising a first divider wall and a second divider wall formed within the chamber and extending between the impingement plate and the second surface, wherein the first divider wall has a length, the length extends from a first wall of the plurality of walls towards a second wall, the second wall opposing the first wall, the second divider wall having a length that extends from the second wall towards the first wall, wherein the first divider wall and the second divider wall both extend such that each divider wall overlaps one another when viewed in a perpendicular direction to the length of first divider wall and/or the second divider wall, and wherein the first divider wall and the second divider wall are spaced apart with respect to the perpendicular direction, wherein a length of the overlap is greater than 0% and less than 80% of a length of the chamber in a direction of the length of the first divider wall and/or the second divider wall, and wherein the first divider wall and the second divider wall each extend a full distance from the impingement plate to the second surface such that there are no gaps between the impingement plate and the second surface along the length of the first divider wall as the first divider wall extends from the impingement plate to the second surface, and wherein there are no gaps between the impingement plate and the second surface along the length of the second divider wall as the second divider wall extends from the impingement plate to the second surface.

2. The heat shield as claimed in claim 1, wherein the first divider wall is attached to the impingement plate or the second surface and/or the second divider wall is attached to the impingement plate or the second surface.

3. The heat shield as claimed in claim 1, wherein the length of the first divider wall and the length of the second divider wall each extend between 10% and 90% of the length of the chamber in the direction of the length of the first divider wall and the second divider wall.

4. The heat shield as claimed in claim 1, wherein the length of the overlap is greater than 40% and less than 80% of the length of the chamber in the direction of the length of the first divider wall and/or the second divider wall.

5. The heat shield as claimed in claim 1, wherein the first divider wall and the second divider wall are parallel to one another.

6. The heat shield as claimed in claim 1, wherein the first divider wall and the second divider wall are angled at an angle (a) with respect to one another.

7. The heat shield as claimed in claim 1, wherein the heat shield comprises two pairs of divider walls.

8. The heat shield as claimed in claim 1, wherein one of the plurality of walls is an upstream wall having lateral ends and one of the first divider wall and the second divider wall extends from the upstream wall, wherein the at least one pair of divider walls is located within 30% of a length of the upstream wall from one or each of the lateral ends of the upstream wall.

9. The heat shield as claimed in claim 1, wherein the at least one pair of divider walls divide the chamber into at least a first lateral zone and a central zone, wherein the array of impingement holes has at least a first set of impingement holes opening into the first lateral zone and a second set of impingement holes opening into the central zone, wherein the first set of impingement holes has a different arrangement of impingement holes compared to the second set of impingement holes, the different arrangement of impingement holes provides a different cooling effect.

10. The heat shield as claimed in claim 9, wherein the plurality of walls has at least one lateral wall and wherein the first lateral zone is located immediately adjacent to the lateral wall and the first lateral zone occupies up to 25% of the second surface within the chamber.

11. The heat shield as claimed in claim 9, wherein a second pair of divider walls divides the chamber into a second lateral zone, wherein the array of impingement holes has a third set of impingement holes opening into the second lateral zone, and wherein the third set of impingement holes has a different arrangement of impingement holes compared to at least the second set of impingement holes, the different arrangement of impingement holes provides a different cooling effect.

12. The heat shield as claimed in claim 9, wherein the different arrangement of impingement holes comprises any one at least one of a different density of impingement holes and a different cross-sectional area of impingement holes.

13. The heat shield as claimed in claim 9, wherein the heat shield has a centre line, wherein at least one divider wall of the at least one pair of divider walls is angled at an angle (α) relative to the centre line.

14. The heat shield as claimed in claim 1, wherein the heat shield is at least a part of at least one component of turbomachinery: a circumferential segment or a blade outer air seal (BOAS), a shroud of a turbine, a tile or a heat shield of a wall of a combustor, a platform or shroud of a blade or vane.

15. The heat shield as claimed in claim 3, wherein the length of the first divider wall and the length of the second divider wall each extend between 15% and 85% of the length of the chamber in the direction of the length of the first divider wall and the second divider wall.

16. The heat shield as claimed in claim 13, wherein the angle α is between +25° and −25°.

17. The heat shield as claimed in claim 13, wherein the angle α is between +15° and −15°.

18. The heat shield as claimed in claim 13, wherein the angle σ is between +15° and 0°.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above mentioned attributes and other features and advantages of this invention and the manner of attaining them will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein

(2) FIG. 1 shows part of a turbine engine in a sectional view and in which the present heat shield is incorporated,

(3) FIG. 2 is a view on the present heat shield looking radially inwardly and with dashed lines showing hidden features, the heat shield comprises an impingement plate having an array of impingement holes,

(4) FIG. 3 is a perspective view on the present heat shield looking radially inwardly and axially forwardly; the impingement plate has been removed,

(5) FIG. 4 is a cross-section A-A in FIG. 2 of the present heat shield,

(6) FIG. 5 is a view on a second embodiment of the present heat shield and looking radially inwardly and without the impingement plate.

DETAILED DESCRIPTION OF INVENTION

(7) FIG. 1 shows an example of a gas turbine engine 10 in a sectional view. The gas turbine engine 10 comprises, in flow series, an inlet 12, a compressor section 14, a combustor section 16 and a turbine section 18 which are generally arranged in flow series and generally about and along the direction of a longitudinal or rotational axis 20. The gas turbine engine 10 further comprises a shaft 22 which is rotatable about the rotational axis 20 and which extends longitudinally through the gas turbine engine 10. The shaft 22 drivingly connects the turbine section 18 to the compressor section 14.

(8) In operation of the gas turbine engine 10, air 24, which is taken in through the air inlet 12 is compressed by the compressor section 14 and delivered to the combustion section or burner section 16. The burner section 16 comprises a burner plenum 26, one or more combustion chambers 28 and at least one burner 30 fixed to each combustion chamber 28. The combustion chambers 28 and the burners 30 are located inside the burner plenum 26. The compressed air passing through the compressor 14 enters a diffuser 32 and is discharged from the diffuser 32 into the burner plenum 26 from where a portion of the air enters the burner 30 and is mixed with a gaseous and/or liquid fuel. The air/fuel mixture is then burned and the combustion gas 34 or working gas from the combustion is channelled through the combustion chamber 28 to the turbine section 18 via a transition duct 17.

(9) This exemplary gas turbine engine 10 has a cannular combustor section arrangement 16, which is constituted by an annular array of combustor cans 19 each having the burner 30 and the combustion chamber 28, the transition duct 17 has a generally circular inlet that interfaces with the combustor chamber 28 and an outlet in the form of an annular segment. An annular array of transition duct outlets form an annulus for channeling the combustion gases to the turbine 18. In other examples, the combustor section 16 may be an annular combustor as known in the art.

(10) The turbine section 18 comprises a number of blade carrying discs 36 attached to the shaft 22. In the present example, two discs 36 each carry an annular array of turbine blades 38. However, the number of blade carrying discs could be different, i.e. only one disc or more than two discs. In addition, guiding vanes 40, which are fixed to a stator 42 of the gas turbine engine 10, are disposed between the stages of annular arrays of turbine blades 38. Between the exit of the combustion chamber 28 and the leading turbine blades 38 inlet guiding vanes 44 are provided and turn the flow of working gas onto the turbine blades 38.

(11) The combustion gas from the combustion chamber 28 enters the turbine section 18 and drives the turbine blades 38 which in turn rotate the shaft 22. The guiding vanes 40, 44 serve to optimise the angle of the combustion or working gas on the turbine blades 38.

(12) The turbine section 18 drives the compressor section 14. The compressor section 14 comprises an axial series of vane stages 46 and rotor blade stages 48. The rotor blade stages 48 comprise a rotor disc supporting an annular array of blades. The compressor section 14 also comprises a casing 50 that surrounds the rotor stages and supports the vane stages 46. The guide vane stages include an annular array of radially extending vanes that are mounted to the casing 50. The vanes are provided to present gas flow at an optimal angle for the blades at a given engine operational point. Some of the guide vane stages have variable vanes, where the angle of the vanes, about their own longitudinal axis, can be adjusted for angle according to air flow characteristics that can occur at different engine operations conditions.

(13) The casing 50 defines a radially outer surface 52 of the passage 56 of the compressor 14. A radially inner surface 54 of the passage 56 is at least partly defined by a rotor drum 53 of the rotor which is partly defined by the annular array of blades 48.

(14) The turbine section 18 further comprises a casing 58 and an annular array of heat shields 60 mounted to the casing 58 and partly defining a working gas path through the turbine section. The heat shields 60 are mounted radially outwardly of the rotor blades 38. In other gas turbine engines, the heat shields 60 may be mounted between annular arrays of rotor blades 38 and/or may be mounted on the radially inner casing 56.

(15) The present invention is described with reference to the above exemplary turbine engine having a single shaft or spool connecting a single, multi-stage compressor and a single, one or more stage turbine. However, it should be appreciated that the present invention is equally applicable to two or three shaft engines and which can be used for industrial, aero or marine applications.

(16) The terms upstream and downstream refer to the flow direction of the airflow and/or working gas flow through the engine unless otherwise stated. The terms forward and rearward refer to the general flow of gas through the engine. The terms axial, radial and circumferential are made with reference to the rotational axis 20 of the engine.

(17) The term ‘heat shield’ is used to denote not only a heat shield as described herein, but also components such as —a circumferential segment or a blade outer air seal (BOAS) or a shroud of a turbine system 18 of the gas turbine engine 10, —a tile or a heat shield of a wall 54 of the combustor system 16 of a gas turbine engine 10, —a platform or shroud of a blade or vane 38, 44 of the gas turbine engine 10. When applied to a blade or vane either or both the radially inner and radially outer platform or shroud may incorporate the present heat shield configuration. The heat shield is described below with reference to a radially outer circumferential segment of a turbine that defines part of the working gas washed surface. Where the heat shield is applied to a radially inner platform or other component the terms radially inner and radially outer may be transposed.

(18) The present heat shield 60 will now be described with reference to FIGS. 2 to 5.

(19) Referring to FIGS. 2, 3 and 4, the heat shield 60 is a circumferential segment of an annular array of circumferential segments that form part of the gas washed outer surface of the gas path through the turbine section 18. The heat shield 60 is located radially outwardly of rotating blades 38 and forms a tip gap therebetween.

(20) The heat shield 60 has a main body 61, a leading edge 62, a trailing edge 64 and to the left and to the right lateral edges 66, 68 respectively. When installed in a gas turbine engine immediately and circumferentially adjacent heat shields 60 may abut or be in close proximity to one another such that one left lateral edge 66 is facing one right lateral edge 68 and a gap may exist therebetween. The heat shield 60 has a first surface or gas washed surface 70, which is also a radially inner surface and that partly defines the radially outer gas washed surface of the gas path in the turbine section 18. The gas washed surface 70 may also be referred to as the hot side, that being subject to the hot working gases flowing through the gas path. The heat shield 60 has a second surface or cold side or surface 72 which is a radially outer surface relative to the hot gas flow.

(21) The heat shield 60 is mounted to the casing 58 by a front hook or hanger 74 and a rear hook or hanger 76. The front hook 74 and the rear hook 76 engage with corresponding features on the casing 50. Other or additional securing means for securing the heat shield to the casing 50 or other supporting structure may be provided as known in the art.

(22) The heat shield 60 has a centre-line 21 which when viewed radially inwardly towards the rotational axis 20 of the gas turbine 10 is parallel to the rotational axis 20. The heat shield 60 is generally symmetrical about its centre-line 21. The heat shield 60 is generally arcuate when viewed in FIG. 4 (along axis 21) and its curvature is that of part of the circumferential surface of the array of heat shields 60 that forms the gas washed surface of the turbine section 18.

(23) The heat shield 60 has lateral walls 78, 80 and hook walls 82, 84. The hook walls 82, 84 are part of the front hook 74 and rear hook 76 respectively. The lateral walls 78, 80 and the hook walls 82, 84 are referred to generally as ‘walls’, thus the heat shield 60 has a plurality of walls and which are upstanding from the second surface 72.

(24) The heat shield 60 further comprises an impingement plate 86. The impingement plate 86 comprises an array of impingement holes 90. In this exemplary embodiment, the impingement plate 86 is located on the cold side 72 of the heat shield 60 or radially outwardly of the heat shield 60. The impingement plate 86 is generally situated and sized to cover over most of the second surface 72, bounded by the walls 78, 80, 82, 84 a distance such that impingement jets of cooling fluid impinge on the second surface 72 in an optimal manner. The impingement plate 86 is attached to and advantageously is located on top of at least one wall 78, 80, 82, 84 and which is advantageously one or both of the lateral walls 78 and 80. The impingement plate 86, the walls 78, 80, 74, 76 and the second surface 72 form a chamber 88. The impingement plate 86 is braised or welded on to the walls 78, 80, 82, 84 of the heat shield 60 although other arrangements to attach or methods of attaching are possible. Ideally, the impingement plate 86 is sealed against the walls 78, 80, 82, 84 to prevent egress of the coolant from the chamber and which could adversely affect the pressure of the coolant and therefore where it is desired to flow.

(25) The heat shield 60 further comprises at least one pair of divider walls 92, 94 comprising a first divider wall 92 and a second divider wall 94 formed within the chamber 88 and extending between the impingement plate 86 and the second surface 72. As seen in FIGS. 2, 3 and 4 there are two pairs of divider walls and the two pairs are symmetrically disposed about the centre-line 21 of the heat shield 60. In other examples, the two pairs of divider walls 92, 94 and any additional pairs of divider walls may be non-symmetrical. The divider walls 92, 94 are integrally formed or manufactured with the heat shield 60 by casting, additive manufacturing or other technique.

(26) Alternatively, the divider walls 92, 94 are integrally formed or manufactured with the impingement plate 86 by casting, additive manufacturing or other technique. Integrally forming at least one of the divider walls 92, 94 with the impingement plate 86 would allow simple and be easy modifications to the configuration of the divider walls to be tested to find the best configuration, in other words impingement plates 86 with different divider wall configurations may be manufactured cheaply yet applied to the same base design of heat shield. Further, where the heat shield is used in gas turbine engines with different power ratings, impingement plates having different divider wall configurations can be easily implemented. In addition to the different divider wall configurations the impingement hole sizes and/or locations and/or densities can be easily adjusted for different applications including engine upgrades where combustion gas temperatures are higher.

(27) The first and/or second divider walls 92, 94 extend the full distance from the impingement plate 86 to the second surface or vice versa such that there are no gaps between the first and/or second divider walls 92, 94 and the impingement plate 86 and/or the second surface 72. Therefore, it is intended that there are no gaps for coolant over the divider walls 92, 94 and instead only around the free ends of the divider walls 92, 94.

(28) The first divider wall 92 is located laterally or circumferentially outside the second divider wall 94, that is to say it is located further away from the centre-line 21 than the second divider wall 94 or nearer the lateral edges 66, 68 than the second divider wall 94. The first divider wall 92 has a length that extends from a first wall 74 of the plurality of walls towards a second wall 76, but does not touch the second wall 76. The second wall 76 opposes the first wall 74 across the chamber 88. The second divider wall 94 has a length that extends from the second wall 76 towards the first wall 74. In this example, the first wall is part of the front hook 74 and the second wall is part of the rear hook 76. It should be noted that the first wall and the second wall are not required to be part of a hook feature, but in this example it is convenient that they are.

(29) The first divider wall 92 and second divider wall 94 both extend such that there is no clear line of sight in a perpendicular direction, indicated by arrow 96, to the first divider wall 92 and/or second divider wall 94. Thus in one extreme example (FIG. 5) a free end 93 of the first divider wall 92 is exactly level with a free end 95 of the second divider wall 94 when viewed along the perpendicular arrow 96. However, in FIGS. 2 and 3 the first divider wall 92 and the second divider wall 94 overlap one another with respect to the direction of the length of the first divider wall 92 or the second divider wall 94. In the example shown in FIGS. 2 and 3 the first divider wall 92 and the second divider wall 94 extend 85% of the chamber's 88 dimension measured along the direction of the length of the first and/or second divider walls 92, 94. Alternatively, the chamber's 88 dimension could be measured in the direction of the centre-line 21. In general, the presently described divider walls may be implemented in other heat shields 60 where the first divider wall 92 and the second divider wall 94 each extend between 10% and 90% across the chamber 88, although advantageously between 15% and 85%, of the chamber in the direction along the length of the first and/or second divider walls 92, 94 although the divider walls must not have a clear line of sight between their ends 93, 95 when viewed perpendicular to one or other of the divider walls 92, 94. Indeed, the first divider wall 92 and second divider 94 wall both extend such that they overlap one another when viewed in a perpendicular direction 96 to at least one of the first divider wall 92 or second divider wall 94. The overlap is greater than 0% and less than 80% of the chamber's extent in the direction of the length of the first and/or second divider walls 92, 94. Advantageously, the overlap is greater than 40% and less than 80% of the chamber's extent in the direction of the length of the first and/or second divider walls 92, 94.

(30) The first divider wall 92 and the second divider wall 94 are parallel to one another as shown in FIGS. 2, 3, 4 and 5, although the first divider wall 92 and the second divider wall 94 may be angled a with respect to one another and as shown in FIG. 5 by the second divider wall 94′ being shown in dashed lines. The first divider wall 92 and second divider wall 94 are spaced apart a distance 97 with respect to the perpendicular direction relative to one of the divider walls. The distance 97 is the minimum distance between the first divider wall 92 and the second divider wall 94. In FIG. 5 the angled second divider wall 94′ has its free end 95 a minimum distance 97 away from the first divider wall 92. As shown in FIG. 5, the minimum distance 97 is 10% of the distance 98 between the opposing or facing surfaces of the first wall of the front hook 74 and the wall of the second wall of the rear hook 76. In other embodiments the minimum distance may be between and including 5% and 15% of the distance 98.

(31) The or each pair of divider walls 92, 94 are located within 30% of the (circumferential) length of the upstream wall 74 from a respective lateral edge or end 66, 68. In other words one pair of divider walls 92, 94 is located within 30% of the (circumferential) length of the upstream wall 74 from the left hand lateral end 66 and the other pair of divider walls 92, 94 is located within 30% of the (circumferential) length of the upstream wall 74 from the right hand lateral end 68.

(32) The first divider wall 92 of at least one pair of divider walls is angled a relative to the centre line 21. In FIG. 2 a line 21′ is parallel to the centre-line 21 and a centre-line 99 of the first divider wall 92 is shown with the angle α. In the exemplary embodiment of FIG. 2 the angle α is 25° for the first divider wall 92 on the left hand side of the heat shield 60. In other words and with respect to the direction of flow of the working gas 34, the first divider wall 92 is angled away from the centre-line 21. For the right hand side divider wall 92 the angle α is also 25° and here the first divider is also angled away from the centre-line 21 with respect to the direction of the working gas flow 34. However, each pair of divider walls and particularly the first divider wall 92 may be angled anywhere between and including α+25° to −25° from the centre-line 21′. A negative angle indicating that the first divider wall 92 is angled towards the centre-line 21. Advantageously the angle α is between +15° and −15° and most advantageously α is between +15° and 0°.

(33) Referring to FIGS. 2, 3, 4 and 5, the heat shield 60 comprises an array of cooling holes 100, shown as dashed lines, which extend from the chamber 88 to the lateral sides 66, 68 and the downstream side or trailing edge 64. The chamber 88 is divided into three main zones or sub-chambers, two lateral zones 102, 104 and a central zone 106. The two lateral zones 102, 104 are each located either side of the central zone 106 and are laterally outside of the central zone 106 with respect to the centre-line 21. The cooling holes 100A, 100B that extend from the lateral zones 102, 104 respectively mostly extend to the lateral sides 66, 68 respectively and the cooling holes 100C that extend from the central chamber 106 extend to the trailing edge 64 of the heat shield 60.

(34) Lateral gaps that extend axially exists between circumferentially adjacent heat shields 60 and a circumferential gap exists between the trailing edge 64 and other immediately downstream adjacent structure. These gaps can allow ingestion of hot working gases which is not desirable and would otherwise lead to thermal degradation of the heat shield 60. Gas pressure in the lateral gaps can often be higher than the gas pressure in the circumferential gap. To prevent ingestion of hot gases into these gaps coolant is supplied to the gaps via the cooling holes 100A, B, C. In addition, the coolant passing through the cooling holes 100A, B, C also cools the material of the heat shield 60. Often there are different requirements for sealing the gaps and cooling of the lateral sides and trailing edge regions. For the examples in FIGS. 2, 3, 4, and 5 the lateral regions and sides 66, 68 are subject to a greater coolant requirement than the trailing edge 64. Thus it is desirable to have greater mass flow of coolant per unit of area or length flowing through the cooling holes 100 that extend to the lateral edges 66, 68 than to those that extend to the trailing edge 64 from the central zone 106. The greater coolant requirement to the lateral sides is achieved by virtue of a higher density of cooling holes 100 that extend to the lateral edges 66, 68 than the trailing edge 64. However, in other embodiments the greater coolant requirement to the lateral sides 66, 68 may be achieved by virtue of large diameter cooling holes or higher pressure coolant feeding into the cooling holes 100A, 100B from the lateral chambers 102, 104 than the pressure in the central chamber 106. Furthermore, any implementation may comprise any one or more of higher density cooling holes, larger cooling hole diameter and higher pressure feed to the cooling holes 100A, 100B that extend to the lateral edges 66, 68 than the cooling holes 100C.

(35) In addition to the cooling holes 100, further cooling of the heat shield 60 is achieved by impingement cooling jets formed by the coolant passing through the impingement plate's 86 impingement holes 90. A coolant supply provides pressurised coolant radially outwardly of the impingement plate 86. Coolant passes through the impingement holes 90 and impinges on the second surface 72 of the main body of the heat shield 60, thereby removing heat from the material of the heat shield 60. The impingement holes 90 are arranged to provide sufficient cooling to parts of the second surface 72 so that a more constant temperature gradient is achieved over the heat shield 60. A more constant temperature gradient reduces thermal stresses and increases the life of the component. In addition, the impingement cooling will reduce the absolute temperature of the component thereby reducing oxidation and therefore thermal degradation.

(36) As described earlier the chamber 88 is divided into three zones, lateral zones 102, 104 and central zone 106. The density of the impingement holes 90A, 90B feeding coolant directly into lateral zones 102, 104 are greater than the density of impingement holes 90C that feed directly into the central zones 106. As mentioned above the impingement holes 90 that feed coolant directly into the lateral chambers 102, 104 may be greater in diameter instead or as well as having a greater density that the impingement holes 90C. The coolant flow, which is directed through the impingement cooling holes 90C into the central zone or chamber 106, may increase the pressure in the lateral zones or chambers 102, 104 by overflowing with coolant the trailing edge holes 100 in the lateral zones or chambers 102, 104. The reduced mass flow of coolant flowing through the impinging holes 90A, 90B which feed directly into the lateral chambers 102,104 decreases the pressure drops along the lateral holes 100A, 100B and will increase the pressure of the coolant. In this way the lateral holes 100A, 100B are suitably pressurised to prevent hot gas ingestion. In particular the coolant flow through the impingement holes 90A, 90B in the lateral chambers 102, 104 is constrained in the lateral chamber 102, 104 and is directed to flow through the lateral holes 100 by increasing the mass flow in the lateral chambers 102,104.

(37) The lateral zones 102, 104 and the central zones 106 are separated by the pairs of divider walls 92, 94. As mentioned previously the divider walls 92, 94 of each pair are spaced apart a distance 97 to allow an amount of coolant to flow from one zone to another, thus there is some distribution of pressure which is advantageous particularly during transient operating conditions to better balance the cooling requirements with the working gas temperatures variations.

(38) Impingement holes 90 may be provided between the divider walls 92, 94 to allow an impingement jet to impingement on the surface 72 between the divider walls 92, 94. The amount of coolant allowed to flow through these impingement holes may be designed further reduce or minimise the temperature gradient across the heat shield 60 and to assist in pressurizing the lateral and or central chambers accordingly.