Abstract:
A gas turbine engine combustor has forward bulkhead extending between inboard and outboard walls and cooperating therewith to define a combustor interior volume or combustion chamber. At least one of the walls has an exterior shell and an interior shell including a number of panels. Each panel has interior and exterior surfaces and a perimeter having leading and trailing edges and first and second lateral edges. A number of cooling passageways have inlets on the panel exterior surface and outlets on the panel interior surface. The shell has a plurality of holes for directing air to a space between the shell and heat shield and adapted for preferentially directing said air toward leading edge portions of first stage vanes of a turbine section.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional application of Ser. No. 10/691,790, filed Oct. 23, 2003, now U.S. Pat. No. 7,363,763, and entitled COMBUSTOR, the disclosure of which is incorporated by reference herein in its entirety as if set forth at length. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to combustors, and more particularly to combustors for gas turbine engines. 
     Gas turbine engine combustors may take several forms. An exemplary class of combustors features an annular combustion chamber having forward/upstream inlets for fuel and air and aft/downstream outlet for directing combustion products to the turbine section of the engine. An exemplary combustor features inboard and outboard walls extending aft from a forward bulkhead in which swirlers are mounted and through which fuel nozzles/injectors are accommodated for the introduction of inlet air and fuel. Exemplary walls are double structured, having an interior heat shield and an exterior shell. The heat shield may be formed in segments, for example, with each wall featuring an array of segments two or three segments longitudinally and 8-12 segments circumferentially. To cool the heat shield segments, air is introduced through apertures in the segments from exterior to interior. The apertures may be angled with respect to longitudinal and circumferential directions to produce film cooling along the interior surface with additional desired dynamic properties. This cooling air may be introduced through a space between the heat shield panel and the shell and, in turn, may be introduced to that space through apertures in the shell. Exemplary heat shield constructions are shown in U.S. Pat. Nos. 5,435,139 and 5,758,503. Exemplary film cooling panel apertures are shown in U.S. Patent Application Publication 2002/0116929A1 (now U.S. Pat. No. 6,606,861) and U.S. patent application Ser. No. 10/147,571 (now U.S. Pat. No. 7,093,439), the disclosures of which are incorporated by reference as if set forth at length. 
     Exemplary combustors are operated in a rich-quench-lean (RQL) mode. In an exemplary RQL combustor, a portion of the fuel-air mixing and combustion occurs in an upstream portion of the combustor in which the fuel-air mixture is rich (i.e., the spatial average composition is greater than stoichiometric). In this portion of the combustor, the fuel from the nozzles mix with air from the swirlers and participative cooling air in the fore portion of the combustor. In an intermediate quench portion, additional air flow (“process air”) is introduced through orifices in the combustor walls to further mix with the fuel-air mixture and, over a short axial distance, transition the mixture to lean (i.e., less than stoichiometric) on a spatially averaged basis. This is often termed quenching of the reaction as, given typical fuel-air ratios, most of the energy in the fuel has been converted by reacting. In a downstream region, the mixture is lean and diluted to the design point overall fuel-air ratio as participative cooling further dilutes the mixture. An exemplary RQL combustor is shown in the aforementioned U.S. &#39;929 publication. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention involves a gas turbine engine combustor. A forward bulkhead extends between inboard and outboard walls and cooperates therewith to define a combustor interior volume or combustion chamber. At least one of the walls has an exterior shell and an interior shell including a number of panels. Each panel has interior and exterior surfaces and a perimeter having leading and trailing edges and first and second lateral edges. A number of cooling passageways have inlets on the panel exterior surface and outlets on the panel interior surface. A rail protrudes from the exterior surface and is recessed from the leading edge by 3-10 mm along a majority of the leading edge. 
     In various implementations, the rail may contact the shell. The first wall may be the outboard wall. The inboard wall may have a similar structure. The shell may have a number of apertures positioned to direct cooling air against the panel exterior surface between the leading edge and the rail. The apertures may be positioned to preferentially direct such cooling air along areas circumferentially aligned with fuel injectors. The rail may be recessed along the entire leading edge by at least 3.5 mm. There may be a gap between the exterior surface and the shell having a height of 1-3 mm. 
     Another aspect of the invention involves a gas turbine engine combustor where at least one of the heat shield panels has a number of pins protruding from the exterior surface toward the shell and the shell has a number of holes for directing air to a space between the shell and the panel and adapted for preferentially directing the air toward leading edge portions of first stage vanes of a turbine section. Such panels may be the aft circumferential array of panels in the combustor. The holes may include a number of alternating first and second groups of holes having at least partial differences in at least one of size and distribution. The pins may contact the shell. The pins may be in a continuous uninterrupted array along the panel. The pins may be in a number of circumferential rows, each row being out of phase with any adjacent row. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description and claims below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal sectional view of a gas turbine combustor. 
         FIG. 1A  is an enlarged view of leading portion of an outboard wall of the combustor of  FIG. 1 . 
         FIG. 1B  is an enlarged view of trailing portion of the outboard wall of the combustor of  FIG. 1 . 
         FIG. 2  is an exterior view of a forward heat shield panel of the combustor of  FIG. 1 . 
         FIG. 3  is an exterior view of an aft heat shield panel of the combustor of  FIG. 1 . 
         FIG. 4  is an exterior view of a portion of a shell of the combustor of  FIG. 1 . 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  shows an exemplary combustor  20  positioned between compressor and turbine sections  22  and  24  of a gas turbine engine  26  having a central longitudinal axis or centerline  500  (spacing contracted). The exemplary combustor includes an annular combustion chamber  30  bounded by inner (inboard) and outer (outboard) walls  32  and  34  and a forward bulkhead  36  spanning between the walls. The bulkhead carries a circumferential array of swirlers  40  and associated fuel injectors  42 . The exemplary fuel injectors extend through the engine case  44  to convey fuel from an external source to the associated injector outlet  46  at the associated swirler  40 . The swirler outlet  48  thus serves as an upstream fuel/air inlet to the combustor. A number of sparkplugs (not shown) are positioned with their working ends along an upstream portion  54  of the combustion chamber  30  to initiate combustion of the fuel/air mixture. The combusting mixture is driven downstream within the combustor along a principal flowpath  504  through a downstream portion  56  to a combustor outlet  60  immediately ahead of a turbine fixed vane stage  62 . 
     The exemplary walls  32  and  34  are double structured, having respective outer shells  70  and  72  and inner heat shields. The exemplary heat shields are formed as multiple circumferential arrays (rings) of panels (e.g., inboard fore and aft panels  74  and  76  and outboard fore and aft panels  78  and  80 ). Exemplary panel and shell material are high temperature or refractory metal superalloys optionally coated with a thermal and/or environmental coating. Alternate materials include ceramics and ceramic matrix composites. Various known or other materials and manufacturing techniques may be utilized. In known fashion or otherwise, the panels may be secured to the associated shells such as by means of threaded studs  84  integrally formed with the panels and supporting major portions of the panels with major portions of their exterior surfaces facing and spaced apart from the interior surface of the associated shell. The exemplary shells and panels are foraminate, passing cooling air from annular chambers  90  and  92  respectively inboard and outboard of the walls  32  and  34  into the combustion chamber  30 . The exemplary panels may be configured so that the intact portions of their inboard surfaces are substantially frustoconical. Viewed in longitudinal section, these surfaces appear as straight lines at associated angles to the axis  500 . In the exemplary embodiment, the interior surface panel of inboard fore panel  74  is aftward/downstream diverging relative to the axis  500  at an angle θ 1 . The interior surface of the inboard aft panel  76  is similarly diverging at a greater angle θ 2 . The interior surface of the fore outboard panel  78  is aftward/downstream diverging at a very small angle θ 3 . The interior surface of the aft outboard panel  80  is very close to longitudinal, shown aftward/downstream converging at a small angle θ 4 . In the exemplary embodiment, the angles θ 1  and θ 3  are such that the cross-section of the chamber upstream portion  54  is approximately constant in terms of linear sectional dimension but aftward/downstream diverging along the central flowpath in terms of annular cross sectional area. The chamber downstream portion  56  is convergent, although at a much lesser rate. In the exemplary embodiment, the junctions between fore and aft panels substantially define a dividing area  510  between fore and aft combustion chamber portions  54  and  56 . Exemplary values for θ 1 , θ 2 , θ 3 , and θ 4  are: 11.894°, 29.074°, 11.894°, and 0.785°, respectively. 
       FIGS. 1A and 2  show further details of the exemplary fore outboard panel  78  (the fore inboard panel  74  being generally similarly formed). The panel has a main body portion  100  having interior (hot-facing the combustion chamber) and exterior (cold-facing away from the combustion chamber) surfaces  102  and  104  ( FIG. 1A ). The body is circumscribed by a perimeter having leading and trailing portions  106  and  108  and connecting lateral portions  110  and  112  ( FIG. 2 ). A rail system extends from the exterior surface  104  and includes a first portion  114  recessed from the leading edge by a distance D ( FIG. 1A ). The distal rim portions of the rail system contacts the shell interior surface so that the portions of the rail system have a height H coincident with the separation between major portions of the panel exterior surface and shell interior surface. Exemplary values for D and H are 3.8 mm and 1.7 mm. The rail system further includes a second portion  116  along the trailing edge, lateral perimeter portions  118  and  120  along the lateral edges  110  and  112 , and intermediate longitudinal rails  122 ,  124 , and  126 . The rail system also includes portions  130  and  131  surrounding combustion process air (mixing/dilution) apertures or orifices  132  and  133  which provide direct communication through aligned corresponding apertures in the associated shell to introduce air from the associated chamber  92  or  90  into the combustion chamber to lean the combustion gases. In the illustrated embodiment, the first orifices  132  are larger than the second orifices  133 . These orifices circumferentially alternate along the panel. The respective large and small orifices of the inboard panels are exactly out of phase with those of the outboard panels. Accordingly, a large orifice of one panel will be circumferentially aligned with a small orifice of the other. This creates intermeshing air streams which further enhances mixing within the combustor. The panels further include arrays of film cooling holes  140  extending between the surfaces  104  and  102  ( FIG. 1A ). In the illustrated embodiment, air is passed through holes  142  in the shell  72  to impingement cooling spaces  144  between the shell interior surface  146  and the panel body exterior surface  104 . These holes  142  may be positioned and oriented to direct streams of air against intact portions of the surface  104  to provide impingement cooling of such surfaces. After such direction, the gas passes through the holes  140  which are angled so that their discharge provides a desired film cooling of the surface  102 . The shell  72  further includes a group of holes  150  positioned between the leading edge  106  and rail portion  114 . These holes are positioned so that their discharge impacts the surface  104  ahead of the rail  114  and flows forward, wrapping around the leading edge  106  and then aftward between the surface  102  and an adjacent portion  160  of a heat shield panel  162  on the bulkhead. The holes  150  serve to initiate film cooling along the panel interior surface and are discussed in further detail below. 
       FIGS. 1B and 3  show further details of the aft outboard heat shield panel  80  (the aft inboard panel may be similar). Many details may be similar to those of the fore panel and, therefore, are not discussed in as great length. For purposes of identification, the panel  80  has a body  180  with interior and exterior surfaces  182  and  184  and leading, trailing, and two lateral edges  186 ,  188 ,  190 , and  192 . The exemplary panel has a rail system with portions along the leading and lateral edges, intermediate longitudinal portions, and a portion  200  forwardly recessed from the trailing edge  188  by a distance D 2 . The rail system may have a similar height as with the fore panel. Exemplary values for D 2  are 12.4 mm for an outboard panel and 8.7 mm for an inboard panel in the exemplary configuration. The choice of D 2  will be based on how close the last row of cooling holes can be placed to the combustor exit. This, in turn, is largely determined by combustor exit sealing geometry and the nature of the drilling tool to be used. In the exemplary embodiment, such consideration places the holes farther forward of the aft panel trailing edge along the outboard wall than along the inboard wall, thus the diameter difference. Between the portion  200  and the trailing edge  188 , an array of pins  202  extend from the exterior surface  184  toward and contacting the shell interior surface. In the exemplary embodiment, the pin array extends aft from approximately a midpoint of this region. In the exemplary embodiment, the pin array is substantially uninterrupted and includes multiple rows (e.g., four) with the pins of each row being offset (e.g., exactly out of phase) from the pins of the adjacent rows. A chamber  204  is formed between the pin array and the rail portion  200 . This space is fed with air from the chamber  92  through holes  206  extending at an angle θ 5  to the local shell surface. An exemplary θ 5  is 46.955°. 
     The size and distribution of the holes  150  and  206  of  FIGS. 1A and 1B , respectively, may be selected to achieve desired cooling properties.  FIG. 4  shows an exterior surface  210  of the outboard shell  72 . The view shows process air apertures  212  and  213 , respectively, coextensive with the associated apertures  132  and  133  of  FIG. 2  and circular and elongate mounting apertures  214  and  215  accommodating the panel mounting studs  84  (shown with nuts removed for purposes of illustration). The circular apertures  214  serve to register the central pair of studs of each associated panel while the elongate nature of the holes  215  accommodate lateral pairs to permit local circumferential relative movement upon thermal expansion/contraction of elements.  FIG. 4  shows exemplary single-row arrays of the holes  150  and  206 . The row of holes  150  is divided into two alternating groups of holes  220  and  222 . The holes of these exemplary groups are of substantially equal cross-section. However, the on-center spacing of the first group is smaller (e.g., 30%-70%) to provide an associated region of enhanced flow. Each of these enhanced flow regions is aligned between an associated pair of the fuel injectors/swirlers to provide enhanced cooling to counter the concentration of heat generated immediately downstream of overlapping spray zones such injector/swirlers. In the exemplary embodiment, the number of holes in the first group is smaller than that in the second group and the circumferential span of the first group is much smaller than that of the second (e.g., less than 30% and, more narrowly, less than 20%). Exemplary diameters for these holes are 0.6 mm. Exemplary on-center spacing of the first and second groups is 1.8 mm and 3.5 mm. Other permutations of spacing, size, shape, and the like may be utilized as may be variations of such parameters within groups. 
     The row of holes  206  is divided into groups  230  and  232 , respectively, providing more and less concentrated cooling. Each enhanced flow group  230  is associated with a corresponding vane  234  of the stage  62 . The positioning of this group along with the associated angle θ 5  ( FIG. 1B ) relative to the shell interior surface may be used to substantially counteract a bow wave of the vane  234 . The bow wave or “horseshoe vortex” results from the interaction of the combustor output with the vane  234 . As flow from the combustor approaches the vane leading edge  238  (which may be coincident or nearly coincident with the forward extremity  236 ) it stagnates at the leading edge to form a localized region of high static pressure. This stagnation creates high spatial pressure gradients and complex three-dimensional flows particularly in the region where the vane airfoil meets the (outboard) endwall  240  ( FIG. 1 ) and (inboard) platform  242  ( FIG. 1 ). The three-dimensional flows at the vane leading edge tend to wrap around leading edge in a U-shape with one leg along the pressure side and one leg along the suction side thus the term “horseshoe vortex”. The pressure gradients make it difficult to cool the endwalls/platforms and adjacent portions of the vane airfoil, as the cooling flow will tend to be directed toward regions of lower static pressure. Additionally, the three-dimensional flows/gradients may drive hot combustor gases back toward the combustor walls. 
     In the exemplary embodiment, the number, shape, and angling of the holes/passageways  206  helps to direct and meter the flow (subject to having sufficient numbers and size of pins) to provide desired cooling performance while having sufficient velocity and mass flow to counter the bow wave yet not having so great a mass flow so as to constitute an excessive inefficiency. The exemplary group  230  is positioned ahead of the forwardmost extremity  236  of the vane airfoil, shifted slightly toward the pressure side thereof. In the exemplary embodiment, the circumferential spacing of vanes  234  is much smaller than that of the fuel injectors and, accordingly, the circumferential length of the pairs of hole groups are correspondingly smaller. Thus, for example, the circumferential span of the groups  230  and  232  may be nearly equal. Flow concentration is achieved, in the exemplary embodiment, by having larger cross-section holes in the group  230  as well as having a smaller on-center spacing in that group. Exemplary diameter and on-center spacing for the holes of the groups  230  are 1.0 mm and 5.9 mm for an outboard panel and 1.0 mm and 5.1 mm for an inboard panel. Exemplary diameter and on-center spacing for the holes of the groups  232  are 1.4 mm and 3.1 mm for an outboard panel and 1.3 mm and 3.3 mm for an inboard panel. An exemplary circumferential span of the first group is between 60 and 150% that of the second, more narrowly, 80 and 120%. 
     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when applied as a reengineering of an existing combustor, details of the existing combustor will influence details of the particular implementation. Accordingly, other embodiments are within the scope of the following claims.