Double wall combustor tile arrangement

[0001] This invention relates to improvements to a combustor of a gas turbine engine and in particular to an arrangement of heat resistant tiles of a double wall of a combustor.

[0002] In a double walled combustor of a gas turbine engine it is known to provide an inner wall which comprises heat resistant tiles with pedestals, which extend toward the outer wall thereby improving heat removal by a cooling air flow between the walls. The tiles are secured to the outer wall by integral studs, which are intended to allow the tiles to expand and contract with the thermal cycle of the engine. However, it is known that the studs lock-up and prevent thermal movement of the tiles. This damages the tiles and leads to large gaps around the tiles and an undesirable increase in the amount of cooling air required.

[0003] One way of reducing combustion product emissions is to employ lean-burn combustion which limits the peak flame temperature and hence production of nitrides of oxygen (NOx). To achieve lean-burn most of a compressed air flow from a compressor of the engine has to be committed to fuel/air mixing modules, with little compressed air remaining for cooling the combustor walls.

[0004] Employing the most advanced double wall cooling designs possible for lean-burn combustors is hence very beneficial to NOx control to minimise the cooling air quantity required. This enables the fuel/air mixing modules to be run even leaner and hence further reduces NOx emissions.

[0005] It is therefore an object of the present invention to provide a combustor double wall arrangement, which minimises the amount of cooling fluid used and allows tiles to expand and contract with the thermal cycle of the engine.

[0006] According to the present invention a double wall structure for a combustor of a gas turbine engine comprising an inner wall and an outer wall, the inner wall comprising a plurality of main tiles, the main tiles are secured to the outer wall by a securing means, wherein the inner wall further comprises a plurality of edge tiles which are secured to the outer wall by securing means, each edge tile overlapping at least one edge of a main tile thereby further securing the main tiles to the outer wall.

[0007] Preferably, the edge of the main tile comprises a stepped edge having a leg portion extending from the main tile towards the outer wall and a foot portion extending from a distal end of the leg portion, the edge is arranged to space apart the main tile from the outer wall, the foot portion being in slideable contact with the outer wall and the overlapping edge tile.

[0008] Alternatively, the edge of the main tile comprises a leg portion extending from the main tile towards the outer wall, the edge is arranged to space apart the main tile from the outer wall, the leg portion having a distal end in slideable contact with the outer wall, the overlapping edge tile being in slideable contact with the edge.

[0009] Preferably, the securing means is a stud, the stud comprises a threaded plug and a nut, in use the threaded plug is secured to the tile and extends through a hole defined in the outer wall and onto which the nut is fastened.

[0010] Preferably, the threaded plug of the stud further comprises a thickened portion, the thickened portion is disposed between the tile and the outer wall and defines the space therebetween.

[0011] Preferably, the edge tile overlaps adjacent edges of adjacent main tiles, the edges being generally aligned with a principal axis of the engine.

[0012] Preferably, the edge tile overlaps adjacent edges of adjacent main tiles, the edges being generally circumferentially aligned with respect to a principal axis of the engine.

[0013] Preferably, the edge tile overlaps adjacent edges of adjacent main tiles, the edges being generally axially aligned with a principal axis of the engine and the edges being generally circumferentially aligned to the principal axis.

[0014] Preferably, the edge tile comprises a plurality of angled effusion cooling holes.

[0015] Alternatively, the edge tile comprises a plurality of pedestals, each pedestal extending from the edge tile toward the outer wall.

[0016] Preferably, the securing means is located generally centrally of the main tiles. Preferably, the securing means is tightly secured to the main tiles. Preferably, further securing means are provided, the further securing means are loosely secured to the main tiles.

[0017] Preferably, a gas turbine engine comprising a combustor wherein the combustor comprises a double wall structure as hereinbefore described.

[0018] Preferably, a method of assembling a double wall structure of a combustor of a gas turbine engine, the wall structure comprising an outer wall and an inner wall, the method comprising the steps of: securing a plurality of main tiles to the outer wall by a first securing means; securing a plurality of edge tiles to the outer wall by securing means so that each edge tile overlaps at least one edge of a main tile thereby further securing the main tile to the outer wall.

[0019] Embodiments of the invention will now be described by way of example only, with reference to the accompanying diagrammatic drawings, in which:

[0020] FIG. 1 is a sectional side view of a gas turbine engine incorporating a combustor in accordance with the present invention.

[0021] FIG. 2 shows a sectional side view of part of a combustor of the engine shown in FIG. 1;

[0022] FIG. 3 shows a prior art sectional side view of a part of a radially outer wall structure of a combustor showing a wall element;

[0023] FIG. 4 is a sectional side view A-A on FIG. 2 showing part of a radially outer wall structure of a combustor double wall element of a first embodiment of the present invention;

[0024] FIG. 5 is a sectional side view A-A on FIG. 2 showing part of a radially outer wall of a combustor double wall element of a second embodiment of the present invention.

[0025] With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 has a principal axis X-X. The engine 10 comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and an exhaust nozzle 19.

[0026] The gas turbine engine 10 works in the conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows, a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor 13 compresses the airflow directed into it before delivering that air to the high-pressure compressor 14 where further compression takes place.

[0027] The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low-pressure turbine 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by suitable interconnecting shafts (not referenced).

[0028] Referring to FIG. 2, the combustor 15 is constituted by an annular combustion chamber 20 having radially inner and outer wall structures 21 and 22 respectively. The combustor 15 is secured to a wall 23 by a plurality of pins 24 (only one of which is shown). Fuel is directed into the chamber 20 through a number of fuel nozzles 25 located at an upstream end 26 of the chamber 20. The fuel nozzles 25 are circumferentially spaced around the engine 10 and serve to spray fuel into air derived from the high-pressure compressor 14. The resultant fuel/air mixture is then combusted within the chamber 20.

[0029] The combustion process takes place within the chamber 20 and naturally generates a large amount of heat. It is necessary therefore, to arrange that the inner and outer wall structures 21 and 22 are capable of withstanding the heat.

[0030] The radially inner and outer wall structures 21 and 22 each comprise an outer wall 27 and an inner wall 28. The inner wall 28 is made up of a plurality of discrete wall elements in the form of tiles 29A and 29B.

[0031] Each of the tiles 29A, 29B has circumferentially extending edges 30 and 31, and the tiles are positioned adjacent each other, such that the edges 30 and 31 of adjacent tiles 29A, 29B overlap each other. Alternatively, the edges 30, 31 of adjacent tiles can abut each other. Each tile 29A, 29B comprises a base portion 32 which is spaced from the outer wall 27 to define therebetween a space 44 (see FIG. 3) for the flow of cooling fluid in the form of cooling air as will be explained below. Heat removal features in the form of pedestals 45 (see FIG. 3) are provided on the base portion 32 and extend into the space 44 towards the outer wall 27.

[0032] Conventionally, and as shown in the arrangement of FIG. 2, (first) securing means (35) are in the form of studs 35 comprising threaded plugs 34 and nuts 36. Each tile 28 has a plurality of threaded plugs 34 extending from the base portions 32 of the tiles 29A, 29B through apertures in the outer wall 27. Nuts 36 are screwed onto the plugs 34 to secure the tiles 29A, 29B to the outer wall 27.

[0033] Referring to FIG. 3, during engine operation, some of the air exhausted from the high-pressure compressor 14 is permitted to flow over the exterior surfaces of the chamber 20. The air provides the chamber 20 with cooling with some of this air being directed into the interior of the chamber 20 to assist in the combustion process. First and second rows of mixing ports 38, 39 are provided in the tiles 29B and are axially spaced from each other. The ports 38 correspond to apertures 40 in the outer wall 27, and the ports 39 correspond to apertures 41 in the outer wall 27.

[0034] Referring particularly to the tiles 29B, arrow A in FIG. 3 indicates air exiting via the open upstream edge 30 of the tile 29B and mixing with downstream air flowing from the upstream adjacent tile 29A, as indicated by arrow B. Arrow C indicates the resultant flow of air. Angled effusion holes 46 are provided centrally of the tile 29B between the ports 38 and 39. Arrow D indicates a flow of air exiting from the space 44 through the effusion cooling holes 46. Also, a flow of downstream air exits from the open downstream edge 31 of the tile 29B after mixing with upstream air flowing from the adjacent tile 29A, as indicated by arrow E. Air flows indicated by arrows C and E provide a film of cooling air over the interior surface of the tiles 29A and 29B thereby preventing overheating caused by the combustion of gases in the chamber 20.

[0035] During a normal operation cycle of the engine 10 the combustor 20 will be subject to varying amounts of combustion heat. This causes the tiles 29A and 29B to thermally expand relative to the outer wall 27. The studs 35 which allow the tiles 29A and 29B to slide usually accommodate these thermal expansions. However, it is known that some of the studs 35 prevent the tiles 29A and 29B from sliding particularly at extreme engine 10 operating conditions. This leads to high stresses in the tile 29A, 29B at elevated temperatures and subsequently premature failure thereof. Furthermore, if the nuts 36 are overtightened the tiles 29A, 29B will lock-up against the studs 35 as they thermally expand causing distortion, leading to fatigue and cracking. If the nuts 36 are loose then both frettage and cooling flow leakage around the tiles 29A, 29B edges 30, 31 will occur. These problems are more severe where advanced high-temperature alloys are used for the tiles 29A, 29B, because, although the alloys have superior oxidation properties they have inferior strength, when compared to conventional tile material.

[0036] To obviate these problems it would be easy to think that one solution would be to use a central stud 35 in each tile 29A, 29B thereby allowing the tile to be unconstrained at the (circumferential) edges 30, 31, as well as axially aligned edges (54, 61 in FIG. 4). However, it is likely that there would be significant and uncontrolled cooling fluid leakage around the (circumferential) edges 30, 31 and 54, 61 in FIG. 4, which in turn would lead to a reduced amount of coolant flow through the effusion cooling holes 46 and subsequent over-heating of the tiles 29A, 29B. An additional concern is that should the stud 35 fail the whole tile 29A, 29B would be released into the combustor 15 and passes downstream into the high-pressure turbine 16.

[0037] Referring now to FIG. 4, the outer wall structure 22 comprises the inner wall 28 arranged in accordance with a first embodiment of the present invention. The inner wall 28 comprises main tiles 50 and edge tiles 52. The main tiles 50 are bolted to the outer wall 27 by a generally centrally located stud 35. The stud 35 is similar to that hereinbefore referred to and in use the threaded plug 34 is cast integrally with the tile 50, 52 and extends through a hole 33 defined in the outer wall 27 and onto which the nut 36 is fastened. Alternatively, the threaded plug 34 may be brazed to the tile 50, 52. This generally centrally located stud 35 tightly secures the main tile 50, as there is little or no relative movement at the centre of the tile 50.

[0038] Main tiles 50 are generally similar to the tiles 29A, 29B having pedestals 45 and effusion cooling holes 46, however, the tiles 50 further comprise stepped edges 54. The stepped edge 54 comprises a leg portion 58 and a foot portion 56. The leg portion 58 extends from the main tile 50 towards the outer wall 27 and at its distal end 55 the foot portion 56 extends away from and in generally the same plane as the main tile 50. The foot portion 56 is arranged to seal against the outer wall 27 and is in slideable contact therewith.

[0039] Where two circumferentially adjacent tiles 50 meet, an edge tile 52 is positioned to overlap the foot portion 58 of each adjacent tile 50. It is preferable for the edge tile 52 to be in slideable contact with the main tiles 50 so that the main tiles 29A, 29B are able to thermally expand and contract in their main plane. However, a small clearance may be provided between the foot portion 56 and both the outer wall 27 and the edge tile 52.

[0040] An expansion gap 48 is defined between the main tile 50 and the edge tile 52 to accommodate the thermal expansion of the main tile 50. Similarly, a second expansion gap 47 is defined between the stud 35 and the foot portion 56 to accommodate the thermal expansions.

[0041] A stud 35 has its threaded plug 34 cast integrally with the edge tile 52 and is secured by the nut 36 in conventional manner to the outer wall 27. The edge tile 52 may be provided with more than one stud 35 along its axial length. The outer wall 27 is provided with apertures 60 (not shown where the edge tile is) to allow cooling fluid into the space 44 between the tiles 52, 50 and the outer wall 27.

[0042] In addition, the edge tiles 52 represent only a small fraction of the total wall 22 area so a relatively large amount of cooling air may be used, compared to that supplied to the larger main tiles 29A, 29B. The edge tiles 52 can hence be operated at relatively cool temperatures, enabling minimal distortion thereof, therefore providing good location and slideable sealing engagement with the main tiles 29A, 29B. Therefore the edge tiles 52 may be made of a lower temperature capable and lower cost material than the larger main tiles 29A, 29B.

[0043] A further improvement of the arrangement of the present invention is the ease of assembly. The large main tiles 50 are assembled to the combustor outer wall 27 when cold and secured by their central stud 35 fixing, followed by fitting of the edge tiles 52. The conventional alternative approach of using edge-sealing strips (not shown, but commonly known in the art) that slide into slots on the tile edges present considerable assembly problems and even greater dismantling problems after service.

[0044] Importantly, should the main tile's 50 centre stud 35 fail, the tile 50 will remain secured to the outer wall 27 by the edge tiles and so cannot be released into the combustor 15. For safety reasons it may still be prudent to provide additional studs 35 near to the edges 30, 31, 54, 61 of the main tile 50, however these additional studs 35 would be relatively loosely fitted so as to not restrain the thermal growth of the main tiles 50.

[0045] Referring now to FIG. 5, a second embodiment of the present invention relates to the arrangement of the main tile 50 edges and the edge tile 52. The main tile 50 comprises an edge 61 and the leg portion 58, the leg portion 58 extending from the edge 61 toward the outer wall 27. The distal end of the leg portion is in slideable contact with the outer wall 27. The edge tile 52 is arranged to overlap the edges 61 of two adjacent main tiles 50, thereby securing the main tiles 50 to the outer wall 27. The edge tile 52 comprises a rim 64, which is in slideable contact with edges 61. The rim 64 comprises cooling holes 65. The threaded plug 34 further comprises a thickened portion 62 arranged to space apart the edge tile 52 and the outer wall 27.

[0046] It should be apparent to one skilled in the art that not only may the edge tiles 52 secure the main tiles 50 along their generally axially aligned edges 54, 61, but may also secure the main tiles 50 along their circumferential edges 30, 31.

[0047] Although the main tiles 50 have been described with reference to having pedestals 45, impingement cooling may alternatively cool the tiles 50. A configuration of an impingement-cooling tile (or wall element) is described and incorporated herein with reference to European Patent EP0576435 of the present Applicant. The edges of the wall elements described in EP0576435 are intended to be of a similar configuration as described herein. Furthermore and in accordance with the present invention the inner wall of EP0576435 is provided with edge tiles.

[0048] Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.