Combustion chamber for a gas turbine engine

Abstract: In a twin wall combustion chamber (1) for a gas turbine engine, the outer wall (2) has impingement holes (3) so that compressed air surrounding the chamber can pass through the holes to impinge on the inner wall (4), and the inner wall (4) has effusion holes (5) whereby air can effuse into the combustion chamber. The number of effusion holes (5) is greater than the number of impingement holes (3), the effusion holes (5) preferably being arranged in groups of seven, comprising six holes (5a) equi-spaced around a central seventh hole (5b), each group having an impingement hole (3) in a fixed positional relationship to the central hole, preferably downstream of it.

Description: FIELD OF THE INVENTION This invention relates to gas turbine engines, and in particular to cooling of combustion chamber walls in such engines. BACKGROUND TO THE INVENTION The combustion chambers in gas turbine engines are subject to very high temperatures in use, and as efforts are made to increase engine efficiency, higher operating temperatures become desirable. However, the ability of the combustion chamber walls to withstand higher temperatures becomes a limiting factor in engine development. New wall materials to withstand higher temperatures are constantly being developed, but there is usually some cost or functional penalty involved. As metal alloys become more exotic they tend to be more expensive, both in the materials required and in the complexity of manufacture. Ceramic materials, on the other hand, while being able to withstand high temperatures, tend to exhibit low mechanical strength. An alternative approach to the development of new materials is to improve the systems for cooling the walls in use. In one air cooling system, the combustion chamber is formed with twin walls spaced apart from each other by a small distance. Compressed air from the engine compressor surrounds the combustion chambers within the engine casing, and holes formed in the outer wall of the twin walls of the chamber allow air to impinge on the inner wall, creating a first cooling effect. Such holes are normally referred to as impingement holes. The air in the space between the walls is then admitted to the combustion chamber through a series of smaller holes, normally referred to as effusion holes, through the inner wall which are arranged to aid laminar flow of the cooling air in a film over the inner surface of the inner wall, cooling it and providing a protective layer from the combustion gases in the chamber. Examples of such cooling arrangements are disclosed in GB-A-2173891, US-A-5 758 504 and GB-A-2176274. This type of arrangement can have a significant effect in extending the operating life of a combustion chamber. It has now been found that by adopting a particular arrangement of effusion holes and associated impingement holes, the cooling effect can be enhanced. SUMMARY OF THE INVENTION According to the invention, there is provided a combustion chamber for a gas turbine engine, the combustion chamber having: upstream and downstream ends relative to the direction of combustion gas flow therethrough, an inner wall, an outer wall spaced apart from the inner wall thereby to define a cavity between the walls, the outer wall having a plurality of impingement cooling holes therethrough, whereby during operation of the engine compressed air surrounding the chamber can pass through the impingement holes to impinge on the inner wall, the inner wall having a plurality of effusion holes therethrough, whereby air can effuse from the cavity between the inner and outer walls into the combustion chamber, there being a greater number of effusion holes than impingement holes; wherein the effusion holes are arranged in groups, each group comprising a plurality of effusion holes substantially equally spaced apart from each other around a central effusion hole, each group of effusion holes having an impingement hole located in the outer wall such that air passing through the impingement hole impinges on the inner wall at a predetermined position relative to the central effusion hole within a boundary defined by the group of diffusion holes. Preferably, the effusion holes are arranged in groups of seven, comprising six effusion holes substantially equally spaced around a central seventh effusion hole. The predetermined position of the impingement hole relative to the central effusion hole is preferably such that air passing through the impingement hole impinges on the inner wall closer to the central effusion hole than to the other effusion holes and is in alignment with the central effusion hole along the direction of combustion gas flow in the chamber. Hence, each impingement hole may be located upstream or downstream of the central effusion hole in the group, but is more preferably arranged downstream of the central effusion hole such that the centreline of the impingement hole is spaced from the centreline of the central effusion hole by a distance at least equal to the diameter of the impingement hole. The groups are suitably arranged in rows extending circumferentially of the chamber. For convenience in manufacturing and to ensure uniform airflows, each group may be spaced from the next in the row by a distance substantially equal to the spacing between adjacent holes in a group and the groups in any one row may be displaced circumferentially from those in the or each adjacent row by a distance substantially equal to half the distance between the central holes in adjacent groups in a row. Furthermore, the longitudinal spacing between the rows may be such that the distance between two adjacent effusion holes which belong to different groups in adjacent rows is the same as the distance between two adjacent holes in the same group of effusion holes. In a preferred embodiment, additional effusion holes are provided centrally of each set of six holes defined between two adjacent groups in one row and the displaced adjacent group in the next row. The relative sizes and numbers of the impingement holes and the effusion holes are preferably such that during operation of the engine the pressure differential across the outer wall is at least twice the pressure differential across the inner wall; for example, approximately 70% of the total pressure drop across the outer and inner walls may occur across the outer wall and the remainder across the inner wall. It has been found that the combustion chamber wall temperature during operation of the engine is significantly lower using the arrangement of the invention than is achieved with known cooling arrangements. Benefits are gained from the enhanced film cooling not only in the combustion chamber can, but also into the transition duct which leads from the can into the turbine inlet. The enhanced cooling extends the life of the combustion chamber can and its transition duct, especially when combustion temperatures are increased to improve combustion efficiency. Brief Description of the Drawings In the drawings, which illustrate exemplary embodiments of the invention: Figure 1 is a diagrammatic sectional view of a combustion chamber; Figure 2 is an enlarged partial view of the wall of the combustion chamber within box A in Figure 1; Figure 3 is an enlarged plan diagram showing the arrangement of cooling holes in a single group of such holes; Figure 4 is a view similar to Figure 3 but on a reduced scale and showing the relationship between adjacent groups of cooling holes in accordance with one embodiment of the invention; and Figure 5 is a corresponding view to that of Figure 4, but showing an alternative embodiment of the invention. Detailed Description of the Illustrated Embodiments Referring first to Figure 1, the combustion chamber can 1 has a conventional inlet or upstream end 10 for fuel and combustion air, and a discharge or downstream end 12, the flow of the combustion air and combustion gases through the chamber being indicated by arrows B and D respectively. Downstream of the inlet end 10 the can is generally cylindrical about its longitudinal axis L-L and has twin walls 2, 4 spaced apart by a small distance in conventional manner to provide a cooling air space cavity 13 between them. The structure of the twin walls may be seen more clearly from Figure 2, with the outer wall 2 being provided with impingement holes 3 therethrough, while the inner wall 4 has effusion holes 5 therethrough. Although the impingement holes are shown in Figure 2 as being normal to the longitudinal axis L-L of the can, they may advantageously be angled towards the downstream direction, say at an angle of 30° to the axis L-L, to assist the creation of a boundary layer laminar flow or cooling film over the inner surface of the inner wall 4. The effusion holes are conveniently formed by laser drilling. It will be seen that the impingement holes are arranged such that during operation of the engine, compressed air C from the space within the engine casing surrounding the combustion chamber 1 flows into the cavity 13 between the walls 2 and 4 and impinges directly on the hot inner wall 4 at a position offset from the positions of the effusion holes 5 so that an initial cooling effect on inner wall 4 is achieved by the impingement. As more clearly illustrated in Figure 3, the effusion holes 5 are arranged in polygonal groups, each group comprising a number of effusion holes 5a substantially equally spaced apart from each other around a central effusion hole 5b. Each group of effusion holes is associated with a respective impingement hole 3 which is located in the outer wall 2 such that air passing through the impingement hole impinges on the inner wall 4 at a predetermined position 14 relative to the central effusion hole. This centre of impingement 14 is within the polygonal boundary defined by the diffusion holes 5a. In the preferred embodiment of the invention, air passing through the impingement holes 3 impinges on the inner wall 4 closer to the central effusion hole 5b than to the other effusion holes 5a, the centre of impingement 14 being in alignment with the central effusion hole 5b along the direction D of combustion gas flow in the chamber, and preferably downstream of hole 5b. We have found that the best results are obtained if the effusion holes 5 are arranged in the inner wall 4 in groups of seven as shown, with each of six holes 5a defining with the next adjacent hole an equal side of a hexagon, the seventh effusion hole 5b being at the centre of the hexagon. In this best mode of working the invention, the impingement hole 3 in the outer wall 2 associated with the group is positioned downstream of the central effusion hole 5b such that the horizontal distance d between the centreline of the central hole 5b and the centreline of the impingement hole 3 is at least equal to the diameter of the impingement hole. It will be seen that the impingement holes 3 have a significantly greater diameter than the effusion holes, although the number of effusion holes is substantially greater than the number of impingement holes. The relative sizes and numbers of the two types of hole are designed to ensure that the pressure differential across the outer wall 2 is at least twice the pressure differential across the inner wall 4. Preferably, approximately 70% of the pressure drop across the two walls occurs across the outer wall and the remainder across the inner wall. One exemplary arrangement of the groups of effusion holes is shown in Figure 4. The groups G 1 , G 2 , etc., each consisting of seven effusion holes 5a and 5b and the associated impingement hole 3, are arranged in parallel rows R 1 , R 2 , etc., extending circumferentially around the can. Regarding layout of the groups within each row, each group G 1 is spaced from the next group G 2 in the row by a distance S, which as shown is also the spacing between adjacent holes in a group along each side of the hexagon in which they are arranged. Regarding the relationship of the rows to each other, the groups in one row R 1 are offset circumferentially from those in the next adjacent row R 2 by half the distance X between the adjacent central holes 5b 1 , 5b 2 . Furthermore, the longitudinal spacing between the rows is such that the distance between two adjacent effusion holes which belong to different groups in adjacent rows is the same as the distance between two adjacent holes in the same group. Hence, considering effusion hole 5a 1 in group G 1 of row R 1 and an adjacent effusion hole 5a 2 of another group in the adjacent row R 2 , the distance between them is S. In an alternative arrangement of groups shown in Figure 5, additional effusion holes 5c have been added to fill the spaces between the groups in the arrangement shown in Figure 4. This arrangement increases further the uniformity of coolant gas distribution through the inner wall, further enhancing the cooling film over the inner surface of the inner wall 4. While we have found groups of seven effusion holes to be optimum, as shown in Figures 3 to 5, we do not exclude the possibility that in some circumstances, it may be desirable to have a higher or lower number of effusion holes in each group. The exact number would be established by reference to model tests (virtual or hardware) to take account of differing standards of combustor and differing combustion conditions. Furthermore, although reference has been made to the holes 5a being equally spaced around central hole 5b, it would of course be possible to vary the exact spacing and positioning of the holes slightly without departing from the scope of the invention as defined by the claims.

Claims: A combustion chamber (1) for a gas turbine engine, the combustion chamber having: upstream and downstream ends (10, 12) relative to the direction of combustion gas flow (D) therethrough, an inner wall (4), an outer wall (2) spaced apart from the inner wall thereby to define a cavity (13) between the walls, the outer wall (2) having a plurality of impingement cooling holes (3) therethrough, whereby during operation of the engine compressed air (C) surrounding the chamber (1) can pass through the impingement holes (3) to impinge on the inner wall (4), the inner wall having a plurality of effusion holes (5) therethrough, whereby air can effuse from the cavity (13) between the inner and outer walls into the combustion chamber, there being a greater number of effusion holes than impingement holes; characterised in that the effusion holes (5) are arranged in groups, each group comprising a plurality of effusion holes (5a) substantially equally spaced apart from each other around a central effusion hole (5b), each group of effusion holes (5) having an impingement hole (3) located in the outer wall such that air can pass through the impingement hole to impinge on the inner wall (4) at a predetermined position (14) relative to the central effusion hole (5b) within a boundary defined by the group of diffusion holes. A combustion chamber according to claim 1, wherein the effusion holes are arranged in groups of seven, comprising six effusion holes substantially equally spaced around a central seventh effusion hole. A combustion chamber according to claim 1 or claim 2, wherein the predetermined position of the impingement hole (3) relative to the central effusion hole (5b) is such that air can pass through the impingement hole to impinge on the inner wall (4) closer to the central effusion hole than to the other effusion holes (5a). A combustion chamber according to any preceding claim, wherein the predetermined position of the impingement hole (3) relative to the central effusion hole (5b) is such that air can pass through the impingement hole to impinge on the inner wall (4) in alignment with the central effusion hole along the direction of combustion gas flow (D) in the chamber. A combustion chamber according to claim 4, wherein the predetermined position of the impingement hole relative to the central effusion hole is such that air can pass through the impingement hole to impinge on the inner wall downstream of the central effusion hole. A combustion chamber according to any preceding claim, wherein the respective centre lines of the impingement hole and the central effusion hole are spaced apart by a distance ( d ) at least equal to the diameter of the impingement hole. A combustion chamber according to any preceding claim, wherein the groups of effusion holes are arranged in rows extending circumferentially of the chamber. A combustion chamber according to claim 7, wherein each group is spaced from an adjacent group in the row by a distance substantially equal to the spacing between adjacent holes in a group. A combustion chamber according to claim 7 or claim 8, wherein each row is spaced from the adjacent rows by a distance substantially equal to the spacing between adjacent holes in a group. A combustion chamber according to any one of claims 7 - 9, wherein the groups in any one row are displaced circumferentially from those in the or each adjacent row by a distance substantially equal to half the separation between the central holes in adjacent groups in a row. A combustion chamber according to claim 10, wherein additional effusion holes are provided centrally of each set of six holes defined between two adjacent groups in one row and the displaced adjacent group in the next row. A combustion chamber according to any preceding claim, wherein the relative sizes and numbers of the impingement holes and the effusion holes are such that during operation of the engine the pressure differential across the outer wall is at least twice the pressure differential across the inner wall. A combustion chamber according to claim 12, in which approximately 70% of the total pressure drop across the outer and inner walls occurs across the outer wall and the remainder occurs across the inner wall. A gas turbine engine containing at least one combustion chamber in accordance with any preceding claim.

Inventors: ALKABIE HISHAM SALMAN, MCMILLAN ROBIN THOMAS DAVID, ALKABIE, HISHAM SALMAN, MCMILLAN, ROBIN THOMAS DAVID
Applicants: ALSTOM POWER UK LTD, ALSTOM POWER UK LTD.
Application year: 2000
Main classification: F23R3
IPC codes: F23R3, F02C7