Abstract:
A gas turbine engine combustor has inboard and outboard walls. A forward bulkhead extends between the walls and cooperates therewith to define a combustor interior volume. In longitudinal section, a first portion of the combustor interior volume converges from fore to aft and a second portion, aft of the first portion converges from fore to aft more gradually than the first portion.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     (1) Field of the Invention  
         [0002]     This invention relates to combustors, and more particularly to combustors for gas turbine engines.  
         [0003]     (2) Description of the Related Art  
         [0004]     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 and Ser. No. 10/147,571, the disclosures of which are incorporated by reference as if set forth at length.  
         [0005]     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  
       [0006]     One aspect of the invention involves a gas turbine engine combustor having inboard and outboard walls. A forward bulkhead extends between the walls and cooperates therewith to define a combustor interior volume. In longitudinal section, a first portion of the combustor interior volume converges from fore to aft and a second portion, aft of the first portion converges from fore to aft more gradually than the first portion.  
         [0007]     In various implementations, the first portion may represent at least 25% of the interior volume and the second portion may represent at least 35% of the interior volume. The first portion may represent at least 35% of the interior volume and the second portion may represent at least 50% of the interior volume. The first and second portions, in combination, may represent at least 80 or 90% of the interior volume. The inboard wall may have a second portion aft of a first portion and at a longitudinal interior angle thereto of between 180° and 210°. The outboard wall may have a second portion aft of a first portion and at a longitudinal interior angle thereto of between 180° and 210°. These angles may be between 185° and 205°. The walls may each have an exterior shell and an interior multi-panel heat shield. In longitudinal section, the inboard and outboard walls may consist essentially of a number of straight sections.  
         [0008]     The details of one or more embodiments of the invention are set forth in the accompanying drawing and the description and claims below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0009]      FIG. 1  is a longitudinal sectional view of a gas turbine engine combustor.  
         [0010]      FIG. 2  is a longitudinal sectional view of a second gas turbine engine combustor.  
         [0011]      FIG. 3  is a view of an inboard wall of the second combustor of  FIG. 2 , with outer wall and bulkhead removed to permit viewing. 
     
    
       [0012]     Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0013]     The figure 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 . 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 diffuser 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 a principal fuel/air inlet to the combustor. One or more sparkplugs  50  are positioned with their working ends  52  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 .  
         [0014]     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 for thermal/environmental performance. 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 integrally formed with the panels and supporting major portions of the panels with their exterior surfaces facing and spaced apart from the interior surface of the associated shell. The exemplary shells and panels are foraminate, with holes (not shown) (e.g., as in U.S. patent application Ser. No. 10/147,571) 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  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 lesser angle θ 2 . The interior surface of the fore outboard panel  78  is aft/downstream converging at a very small angle θ 3 . The interior surface of the aft outboard panel  80  is aftward/downstream diverging at an 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 aftward/downstream converging along the central flowpath both in terms of linear sectional dimension and annular cross sectional area. The chamber downstream portion  56  is similarly convergent, although at a much smaller rate. The converging upstream portion serves to induce higher bulk velocities and reduce residence time at rich conditions. The convergence also promotes a small separation between inner and outer walls in the central region of the combustor. The small separation facilitates effective introduction of process air. The process air for mixing with the fuel-air mixture from the primary zone may be introduced in the vicinity of the transition between upstream and downstream portions  54  and  56  or in the downstream lean zone. Additionally, by keeping the combustor outer wall relatively close to the engine centerline, heat shield surface area and mass may be reduced relative to other combustor configurations. This reduction serves to limit the amount of cooling required and thus the amount of cooling air required. The air which otherwise would be required for cooling may, alternatively, then be introduced upstream (e.g., at the swirler) so as to participate in the combustion process to achieve a desired combustion profile and emissions performance. Air which might otherwise be used for film cooling can also be delivered downstream of the swirler (e.g., via the process air holes) to achieve a desired combustion profile. In the exemplary embodiment, the longitudinal interior (within the combustion chamber  30 ) angle between the interior surfaces of the inboard wall panels is shown as θ 1  and that of the outboard wall panels is shown as θ O . In the exemplary embodiment, both these angles are somewhat greater than 180°. 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 . An exemplary range of θ I  and θ O  are 180°-210°. A tighter lower bound is 185° and tighter upper bounds are 200° and 205°.  
         [0015]     The combustor may be operated in an RQL mode. A given optimization of parameters may seek to balance results in terms of capacity, efficiency, output parameters (e.g., temperature distribution), and, notably, emissions control based upon factors including the dimensions and the identified angles as well as the amount and distribution of air introduced through the swirlers and panels. In exemplary implementations, the largest portion of air flow through the combustor will be process air introduced through the panels, typically a majority (e.g., 40-70%). Coolant air (e.g., film cooling air passing through the heat shield panels) may be the next largest amount (e.g., 15-35%) with the remainder being introduced along with the fuel at the swirler. These conditions/proportions, as well as the combustion profile/performance will vary about such ranges based upon the operating condition of the engine. For example, at relatively low power operating conditions, a very high proportion of the combustion (e.g., in the vicinity of 95%) will occur in the rich primary and quench zones, with a significant portion upstream of the dividing area  510 . At a higher-power condition, this amount may be less, approximately evenly split between rich and lean zones. By way of example, an annular boundary  520  slightly upstream of the dividing area  510  shows the approximate boundary between rich and transition regions, with the exemplary process dilution air being introduced through a circular array of relatively large coaligned apertures in the heat shield panels and shells near the upstream (leading) edges of the downstream heat shield panels. A downstream boundary  522  similarly separates the transition and lean zones. The locations of the boundaries  520  and  522  will depend upon the location and dimensions of the apertures and upon operating conditions.  
         [0016]      FIG. 2  shows an alternate combustor  120  which differs from the combustor  20  principally in that the walls and their associated panels are dimensioned so that the transition between upstream and downstream chamber portions  154  and  156  is located further upstream. The different arrangement may be dictated by the different envelope offered by the associated engine, including one or more factors of: diffuser geometry; relative position of compressor outlet/exit and turbine inlet; igniter position/orientation, and the like. Thus any particular embodiment may have a somewhat differing arrangement of primary, quench, and lean zone volumes and characteristics.  FIG. 3  shows the fore and aft panels  174  and  176  of the inboard wall  132 . Each aft panel  176  is shown as having a circumferential array of alternating large and small apertures  190  and  192  positioned relatively forward along such panel. These apertures provide for introduction of the process air to the combustion chamber. 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.  
         [0017]     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.