Abstract:
A tile is provided for lining the hot side of a wall of a combustor. The tile has a tile body with one or more bosses protruding from the cold side thereof. The or each boss extends, in use, through the wall of the combustor and has a threaded recess formed therein for threadingly connecting with a bolt which is inserted into the recess from the cold side of the combustor wall. The bolt fastens the tile to the combustor wall.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is entitled to the benefit of British Patent Application No. GB 0913580.7, filed on Aug. 5, 2010. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a tile for lining the hot side of a wall of a combustor. 
       BACKGROUND OF THE INVENTION 
       [0003]    The temperature of the gases released by the combustion process in a gas turbine engine may peak above 2100° C. and average 1500° C. This is much higher than the melting point of combustion chamber materials. Thus many gas turbine engine combustors employ ceramic or ceramic-coated tiles to line the combustor wall. 
         [0004]    The individual tiles are attached to the combustor wall (e.g. the annular “cold skin” which forms the sides of the combustor) and cooling air passes through holes in the wall to impinge on the cold side of the tiles. The air then moves through a series of pedestals projecting from the cold side of the tiles to improve the convective heat transfer coefficient. 
         [0005]      FIG. 1  shows a cut-away section through a combustor and illustrates a conventional fastening arrangement for a lining tile. The tile  1  has a threaded fixing stud  3 , which extends from the cold side of the tile body  5  and passes through a hole  7  in the combustor wall  9 . A washer  11  is placed over the stud and a nut  13  screws onto the stud to pull the tile towards the combustor wall, pedestals  15  on the cold side of the tile body determining the final spacing between the tile body and the wall. 
         [0006]    Alternative tile fixing arrangements are proposed in U.S. Pat. No. 4,085,580 and U.S. Pat. No. 5,079,915. 
         [0007]    To produce higher engine efficiencies and reduce emissions, there is a tendency for engine cycles to increase in temperature. The higher combustor temperatures, which this requires can require different cooling arrangements. In particular, pedestal cooling may be superseded by impingement effusion tile cooling arrangements. In such arrangements, the tile body has an array of effusion cooling through-holes, at a low angle (e.g. about)20° to the surface. Cooling air effusing from the holes forms a protective cooling layer on the hot side of the tiles. 
         [0008]    The cooling air effusion holes may be formed by laser machining of the tile body. However, this presents a problem in that laser machining is a line of sight process. As illustrated in the diagram of  FIG. 2 , which is a schematic cross-section through a tile, the fixing stud  3  obstructs laser drilling of effusion holes  17  in an area of the tile body  5  around the stud because of interference of the stud with the laser tool and/or the path (dashed, single headed arrow lines) of the laser beam. Thus the result can be a significant area (the extent of which is indicated by the double headed arrow line) around each stud that is devoid of cooling holes. Alternatively, approach vectors may have to be defined for the laser that avoid interference with the studs to produce holes in the vicinity of the studs. This requires extra programming and extra machining time, and may produce sub-optimal X, Y positions of the holes on the surface of the tile and/or sub-optimal angles of the holes to the surface. 
       SUMMARY OF THE INVENTION 
       [0009]    Accordingly, a first aspect of the present invention provides a tile for lining a wall of a combustor the wall having a hot side intended to face a combustion volume and a corresponding cold side intended to face away from the combustion volume, the tile having a tile body with one or more bosses protruding from a cold side thereof, the or each boss extending, in use, through the wall of the combustor, wherein the boss has a threaded recess formed therein for threadingly connecting with a bolt which is inserted into the recess from the cold side of the combustor wall, whereby the bolt fastens the tile to the combustor wall. 
         [0010]    Such a boss can have a significantly reduced height relative to a conventional fixing stud, and thus, in a tile with laser machined effusion holes, the area around the boss which can be problematic to machine may be much reduced. Although the boss is therefore particularly advantageous when used in relation to a tile with laser machined effusion holes, it may also be used on tiles with other types of cooling features, such as cooling pedestals. 
         [0011]    The tile may have any one or any combination of the following optional features. 
         [0012]    Typically, the tile body has a generally rectangular or square shape, although it may be curved to e.g. fit to an annular combustor wall. There may be a boss at each corner of such a tile. 
         [0013]    Typically, the combustor is a combustor of a gas turbine engine. 
         [0014]    Typically, a plurality of cooling air effusion holes, e.g. angled at 20° or less to the hot side surface of the tile body, traverse the tile body. 
         [0015]    Preferably, the tile body has one or more spacing projections (such as one or more rails extending along the perimeter of the tile body) protruding from the cold side thereof, the spacing projections engaging with the hot side of the combustor wall when the tile is fastened thereto to determine a spacing of the tile body from the combustor wall. Such spacing projections, being discrete from the or each boss, avoid the need for the boss to set the spacing of the tile body from the combustor wall. Cooling air can flow into the gap between the tile body and the combustor wall and thence to e.g. the effusion holes. 
         [0016]    The boss can be made as long as is necessary to accommodate in the recess a desired number of thread turns. However, as an alternative to increasing the height of the boss to accommodate more thread turns, optionally the recess may be extended into the tile body. 
         [0017]    The or each boss may have a respective spacer which, in use, interposes between a head of the bolt and the boss, the spacer engaging at one side with the head of the bolt and at the opposite side with the cold side of the combustor wall. The spacer can thus determine a spacing between the head of the bolt and the cold side of the combustor wall. 
         [0018]    The spacer may contain one or more cooling air passages through which cooling air is sent to the hot side of the combustor wall e.g. to a gap between the tile body and the combustor wall. 
         [0019]    Optionally, one or more channels may extend through the or each boss to carry cooling air to the foot of the recess. Typically, the or each channel fluidly connects to one or more effusion holes extending from the foot of the recess to the hot side of the tile body. In this way, cooling air may introduced to the region of the tile body, which in a tile with a conventional fixing stud is difficult to provide with effusion holes. 
         [0020]    Conveniently, the cooling air carried by the one or more channels through the boss is received from cooling air passage(s) of the spacer. 
         [0021]    A second aspect of the present invention provides a combustor having one or more walls lined with tiles according to the previous aspect (the tile optionally having any one or any combination of the referred to above). For example, the lined wall(s) can be an annular side wall of the combustor, and/or a heat shield front wall of the combustor. 
         [0022]    Where the terms “hot side” and “cold side” and “hot surface” and “cold surface” are used these relate both to their intended and actual use. Accordingly, where the combustor or tile is not in use it is entirely feasible for the hot and cold sides or surfaces to be at the same ambient temperature. However, when the combustor or tile is in use the hot surface will be the surface facing towards the combustion volume and the cold surface will be an opposing surface facing away from the combustion volume. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  shows a cut-away section through a combustor, illustrating a conventional fastening arrangement for a lining tile; 
           [0024]      FIG. 2  shows a schematic cross-section through a tile having a conventional fixing stud; 
           [0025]      FIG. 3  shows a schematic perspective view of a tile according to a first embodiment of the present invention and the corresponding portion of combustor wall to which the tile is fixed; 
           [0026]      FIG. 4  shows a schematic cross-section through one of the fixing arrangements which are provided at the corner regions of the rectangular tile body of the tile of  FIG. 3 ; and 
           [0027]      FIG. 5  shows a schematic cross-section through a fixing arrangement of a tile according to a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]      FIG. 3  shows a schematic perspective view of a tile  101  according to a first embodiment of the present invention and the corresponding portion of a combustor wall  109  (i.e. cold skin) to which the tile is fixed. For clarity, the combustor wall is made transparent. Further, the tile and portion of combustor wall are as flat objects, although in reality they would curve around the annulus of the wall. The tile has a rectangular tile body  105 , and  FIG. 4  shows a schematic cross-section through one of the fixing arrangements, which are provided at the corner regions of the body. An array of cooling air effusion holes  117  traverses the tile body, but in  FIGS. 3 and 4  only a small number of these holes are shown. 
         [0029]    Each fixing arrangement comprises a boss  119  protruding from the cold side of the tile body  105  to extend through a hole  129  in the combustor wall  109 . The boss has a threaded recess  121  into which is screwed a bolt  123  from the cold side of the combustor wall. A ring-shaped spacer  125  encircles the boss where it projects from the cold side of the combustor wall. The bolt passes through the central hole  125   a  in the spacer with the head  123   a  of the bolt pushing down on the top of the spacer to urge it against the combustor wall and thereby determine the spacing between the head  123   a  of the bolt and the wall. The head of the bolt and the spacer, which may have a curved surface to match the curved combustor wall, seal respectively the central hole in the spacer and the hole in the combustor wall. A rail  127  extends around the perimeter of the cold side of the tile body and is pulled by the bolt against the hot side of the combustor wall to determine the spacing between the tile body from the combustor wall. Together, the spacer and the rail determine the extent to which the bolt penetrates into the recess. In  FIG. 3  the bolt and fixing spacer of only one of the fixing arrangements are shown. 
         [0030]    Advantageously, because the distance marked with the doubled headed arrowed line in  FIG. 4  can be used to threadingly connect the bolt  123  to the boss  119 , the distance by which the boss projects from the cold side of the combustor wall  109  is significantly reduced relative to the equivalent projection distance of the fixing stud  3  of the conventional tile  5 , shown in  FIGS. 1 and 2 , in which all mating threads are situated outside the combustor wall  9 . This facilitates laser machining of the effusion holes  117  of the tile  105  of the first embodiment at a close proximity to the boss, reducing the area around the boss which is devoid of effusion holes or for which different laser tool approach vectors have to be programmed, and ultimately improving the cooling performance of the tile. 
         [0031]    The projection distance of the boss can be further reduced if the recess  121  extends into the tile body  105 , allowing the mating threads to be lowered in the recess. 
         [0032]    The spacer  125  has a plurality of radially extending passages  131 , which carry cooling air from the cold side of the combustor wall  109  to a central cavity  133  formed in the spacer. From here, the cooling air passes through the hole  129  in the combustor wall  109 , into the gap maintained between the combustor wall and the tile body  105 , and then out through the effusion holes  117  to provide a protective cooling layer on the hot side of the tile body. 
         [0033]      FIG. 5  shows a schematic cross-section through a fixing arrangement of a tile according to a second embodiment of the present invention. Features of the tile of the second embodiment, which are the same as features of the tile of the first embodiment shown in  FIGS. 3 and 4  share the same reference numbers. 
         [0034]    The tile  105  of the second embodiment has enhancements to improve the cooling of the tile in the vicinity of the boss  119 . More specifically, one or more channels  135  run along the side of the recess  121 . Cooling air conveyed by the radially extending passages  131  in the spacer  125  is thus carried, via the central cavity  133  of the spacer, to the channel or channels  135  and thence to the foot of the recess. From here, an effusion hole  137  extends across the tile body bringing cooling air to a position directly underneath the boss  119 . In an alternative version of the tile, instead of having a channel or channels  135  running along the side of the recess  121 , the tile may have one or more channels  139  extending between the recess and the radially outer surface of the boss, e.g. to exit in the hole  129  of the combustor wall  109  (as shown in  FIG. 5 ) or in the central cavity of the spacer. 
         [0035]    The channel or channels  135  running along the side of the recess  121  could be formed during casting of the tile  101 . The channel or channels  139  extending between the recess  121  and the radially outer surface of the boss  119  could be formed by post-casting machining. 
         [0036]    Although the tiles  101  of the first and second embodiments have been described in respect of an annular combustor cold skin, the invention could also be applied to tiles lining the heat shield (front wall) of a combustor. 
         [0037]    While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting.