Abstract:
An improved gas turbine engine combustor with a liner and a dome connected to the liner trough small radius transition portions only, the dome having a plurality of fuel nozzles mounted therein and an interior directly exposed to a combustion region of the combustor, the dome including a plurality of effusion cooling holes provided non-perpendicularly to an entry surface of the holes, the dome being substantially planar.

Description:
TECHNICAL FIELD 
   The present invention relates generally to gas turbine engine combustors and, more particularly, to a low cost combustor configuration having improved performance. 
   BACKGROUND OF THE ART 
   Gas turbine combustors are the subject of continual improvement, to provide better cooling, better mixing, better fuel efficiency, better performance, etc. at a lower cost. Also, a new generation of very small gas turbine engines is emerging (i.e. a fan diameter of 20 inches or less, with about 2500 lbs. thrust or less), however larger designs cannot simply be scaled-down, since many physical parameters do not scale linearly, or at all, with size (droplet size, drag coefficients, manufacturing tolerances, etc.). There is, therefore, a continuing need for improvements in gas turbine combustor design. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention there is provided a gas turbine engine combustor comprising a liner defining an annular reverse-flow configuration, the liner extending from an annular upstream dome to a downstream exit, the liner reversing direction thereinbetween, the dome having a plurality of fuel nozzle mounted therein, the dome having an interior directly exposed to a combustion region of the combustor, the dome further including a plurality of effusion cooling holes provided non-perpendicularly to an entry surface of the holes, the effusion cooling holes adapted in use to cool the dome to relieve heat transferred from the combustion region, the dome being substantially planar. 
   In accordance with another aspect there is also provided a method a method of improving manufacturing accuracy of a heat shieldless annular reverse flow combustor, the method comprising the steps of providing a annular reverse flow combustor with an end dome adapted for receiving a fuel nozzle; maximizing a flat area of the end dome, the flat area disposed generally perpendicularly to a combustor axis; and drilling a plurality of effusion cooling holes in the flat area of the dome, to thereby improve the overall manufacturing tolerances of said drilling. 
   Further details of these and other aspects of the present invention will be apparent from the detailed description and Figures included below. 

   
     DESCRIPTION OF THE DRAWINGS 
     Reference is now made to the accompanying Figures depicting aspects of the present invention, in which: 
       FIG. 1  shows a schematic cross-section of a turbofan engine having an annular combustor; 
       FIG. 2  shows an enlarged view of the combustor of  FIG. 1 ; 
       FIG. 3  is a further enlarged view of  FIG. 2 ; and 
       FIG. 4  is a somewhat schematic cross-sectional view of a portion of a prior art combustor. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a gas turbine engine  10  preferably of a type provided for use in subsonic flight, generally comprising in serial flow communication a fan  12  through which ambient air is propelled, a multistage compressor  14  for pressurizing the air, an annular combustor  16  in which compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases which is then redirected by combustor  16  to a turbine section  18  for extracting energy from the combustion gases. 
   Referring to  FIG. 2 , the combustor  16  is housed in a plenum  20  defined partially by a gas generator case  22  and supplied with compressed air from compressor  14  by a diffuser  24 . Combustor  16  comprises generally a liner  26  composed of an outer liner  26 A and an inner liner  26 B defining a combustion chamber  32  therein. Combustor  16  preferably has a generally flat dome  34 , as will be described in more detail below. Outer liner  26 A includes a outer dome panel portion  34 A, a relatively small radius transition portion  36 A, a cylindrical body panel portion  38 A, long exit duct portion  40 A, while inner liner  26 B includes an inner dome panel portion  34 B, a relatively small radius transition portion  36 B, a cylindrical body panel portion  38 B, and a small exit duct portion  40 B. The exit ducts  40 A and  40 B together define a combustor exit  42  for communicating with turbine section  18 . The combustor liner  26  is preferably sheet metal. 
   Referring to  FIG. 3 , a plurality of effusion cooling holes  46  are provided in dome  34 , and a plurality of holes  48  in transition  36 . Dome  34  has no heat shield provided therein, and therefore holes  46  provide enough cooling to protect the dome end of the combustor. Effusion cooling holes  46  are angled at precise angles, and positioned at precise positions to provide the exact flow inside the combustor or operate it as efficiently as desired and for the desired maintenance interval before repair or replacement is required. Placement tolerances on the position of the holes, therefore, is typically less than 0.050″ while angular tolerances are a few degrees or less, the significance of which will be discussed further below. 
   Dome  34  includes a flat, planar area which is preferably optimized to be as large as possible, as will be discussed below. 
   A plurality of air-guided fuel nozzles  50 , having supports  52  and supplied with fuel from internal manifold  54 , communicate with the combustion chamber  32  through nozzle openings  56  to deliver a fuel-air mixture  58  to the chamber  32 . As depicted in  FIG. 2 , the fuel-air mixture is delivered in a cone-shaped spray pattern, and therefore referred to in this application as fuel spray cone  58 . 
   In use, referring again to  FIGS. 2 and 3 , high-speed compressed air enters plenum  20  from diffuser  24 . The air circulates around combustor  16 , as will be discussed in more detail below, and eventually enters combustion chamber  32 , inter alia, through a plurality of effusion cooling holes  46  in dome  34 , and holes  48  in transition  36 . Once inside the combustor  16 , the air is mixed with fuel and ignited for combustion. Combustion gases are then exhausted through exit  42  to turbine section  18 . 
   Effusion cooling of dome  34  is achieved by directing air though angled holes  46  in a combustor liner. Holes  46  in dome panel  34  are angled outwardly away from nozzle  50 , while holes  48  in transition portions  36 A,B are provided generally parallelly to body panel portion  38 A,B to direct cooling air in a louver-like fashion along the interior of body panel portions  38 A,B to cool them. 
   The combustor  16  is preferably provided in sheet metal, and may be made by any suitable method. Holes  46  are preferably drilled in the sheet metal, such as by laser drilling. It will be appreciated that some holes  46  are provided relatively close to body panels  38 A,B, and necessarily are so to provide good film cooling of the outer portions of dome  34 . 
   Referring to the prior art depicted in  FIG. 4 , while drilling of combustor holes an be controlled with great precision, such precision adds to the cost of the part. As well, the positional and angular manufacturing tolerances provided may result in some over-drilling of holes  46  (represented by the stippled arrow) which can result in damage to the liner, or may result in holes which are not entirely drilled-through (represented by the solid arrow). Holes may also be mislocated, resulted in hot spots, etc. As gas turbine engine size decreases, manufacturing tolerances of course do not scale linearly (if at all) and, hence, such manufacturing tolerance issue become increasingly critical to combustor design. 
   Referring again to  FIG. 3 , the inventors have recognized that the manufacturing tolerances which must be provided when hole-drilling on non-planar combustor walls is greater than is required for a planar surface. Accordingly, therefore, providing combustor  16  with small radius transition portions  36 A,B and a flat dome permits drilling to completed more precisely, more easily and with minimal risk of damaging the adjacent body panels. As mentioned, this is because manufacturing tolerances for drilled holes provided on curved or conical surfaces are much larger than the comparable tolerances for drilling on a flat, planar surface. Thereby, maximizing the flat area of the combustor dome, the present invention provides an increase area over which cooling holes may be more accurately provided. This is especially critical in heat shield-less combustor designs (i.e. in which the liner has no inner heat shield, but rather the dome is directly exposed to the combustion chamber), since the cooling of the dome therefore become critical, and the cooling pattern must be precisely provided therein. By improving the manufacturing tolerances of the combustor dome, the chance of holes not completely drilled-through, or drilling damage occurring on a liner surface downstream of the drilled hole (i.e. caused by the laser or drilling mechanism hitting the liner after completing the hole) are advantageously reduced. Thus, by making the dome end flat, holes may be drilled much closed to the “corners” (i.e. the intersection between the dome and the side walls), with reduced risk of accidentally damaging the liner side walls downstream of the hole (i.e. by over-drilling). The invention therefore, is particularly applicable to very small turbofan gas engines, having a fan size of 24 inches or less, and more preferably, 20 inches or less, in which engines the annular combustor height, shown at H in  FIG. 2 , may be 4 inches or less. Although a flat dome, depending on its configuration, may present dynamic or buckling issues in larger-sized configurations, the very small size of a combustor for a very small gas turbine engine will in part reduce this tendency. The curved transition portions also provide some strength, as compared to a perpendicular corner. This aspect of the invention is thus particularly suited for use in very small gas turbine engines. In contrast, conventional annular reverse-flow combustors have curved domes to provide stability against dynamic forces and buckling. However, as mentioned, this typical combustor shape presents interference and tolerance issues, particularly when providing a heat shield-less combustor dome. 
   Advantageously, in very small combustor designs, a flat-domed combustor also permits the enclosed volume of the combustor to be maximized within a minimum envelope. 
   The above description is meant to be exemplary only, and one skilled in the art will recognize that further changes may be made to the embodiments described without departing from the scope of the invention disclosed. Modifications will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.