Abstract:
A reverse flow combustor has an inlet end. A flowpath extends downstream from the inlet end through a turn. The turn directs the flowpath radially inward and reversing an axial flow direction. A large exit duct (LED) is along the turn. A small exit duct (SED) is along the turn and joined by a joint to a mounting structure to resist separation in a first axial direction. The joint comprises: a first surface on the SED facing partially radially inward; and a mounting feature engaging the first surface.

Description:
BACKGROUND 
     The disclosure relates to gas turbine engines. More particularly, the disclosure relates to attaching ceramic matrix composite (CMC) ducts in reverse flow combustors. 
     Ceramic matrix composite (CMC) materials have been proposed for various uses in high temperature regions of gas turbine engines. US Pregrant Publication 2010/0257864 of Prociw et al. (the disclosure of which is incorporated herein in its entirety as if set forth at length) discloses use in duct portions of an annular reverse flow combustor. The annular reverse flow combustor turns the flow by approximately 180 degrees from an upstream portion of the combustor to the inlet of the turbine section. Viewed in axial/radial section, an inlet dome exists at the upstream end of the combustor. Additionally, an outboard portion of the turn is formed by an annular wall known as large exit duct (LED) and an inboard portion of the turn is formed by an annular wall known as a small exit duct (SED). The LED and SED may be formed of CMC. The CMC may be secured to adjacent metallic support structure (e.g., engine case structure). The SED and LED are alternatively referred to via the same acronyms but different names with various combinations of “short” replacing “small”, “long” replacing “large”, and “entry” replacing “exit” (this last change representing the point of view of the turbine rather than the point of view of the upstream portion of the combustor). An outer air inlet ring is positioned between the LED and the OD of the inlet dome. An inner air inlet ring is positioned between the SED and the ID of the inlet dome. 
     Robustly and efficiently attaching a CMC to the metal presents engineering challenges. 
     SUMMARY 
     One aspect of the disclosure involves a reverse flow combustor having an inlet end. A flowpath extends downstream from the inlet end through a turn. The turn directs the flowpath radially inward and reversing an axial flow direction. A large exit duct (LED) is along the turn. A small exit duct (SED) is along the turn and joined by a joint to a mounting structure to resist separation in a first axial direction. The joint comprises: a first surface on the SED facing partially radially inward; and a mounting feature engaging the first surface. 
     In various implementations, the SED may comprise a thickened upstream region. The first surface may be a shoulder formed by the thickened upstream region. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially schematic axial sectional/cutaway view of a gas turbine engine. 
         FIG. 2  is an axial/radial sectional view of a combustor of the engine of  FIG. 1 . 
         FIG. 3  is a partial cutaway view of the combustor of  FIG. 2 . 
         FIG. 4  is a partial radially outward cutaway view of a leading edge of an SED of the combustor of  FIG. 2 . 
         FIG. 5  is a partial enlarged axial/radial sectional view of a second combustor. 
         FIG. 6  is an axial/radial sectional view of a third alternate combustor. 
         FIG. 7  is a partial exploded view of the combustor of  FIG. 6 . 
         FIG. 8  is a partial axial/radial sectional view of a fourth combustor. 
         FIG. 9  is a partial axial/radial sectional view of a fifth combustor. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a gas turbine engine  10  generally comprising in serial flow communication from upstream to downstream: a fan  12  through which ambient air is propelled; a multistage compressor  14  for pressurizing the air; an annular reverse flow combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases; and a turbine section  18  for extracting energy from the combustion gases. 
     The terms axial and radial as used herein are intended to be defined relative to the main central longitudinally extending engine axis  11  (centerline). Further, when referring to the combustor  16  herein, the terms upstream and downstream and intended to be defined relative to the general flow of air and hot combustion gases in the combustor, i.e. from a fuel nozzle end of the combustor where fuel and air are injected for ignition to a combustor exit where the combustion gases exit toward the downstream first turbine stage. 
     Referring to  FIG. 2 , the annular reverse flow combustor  16  comprises generally an inner combustor liner  17 , directly exposed to and facing the combustion chamber  23  defined therewithin. The inner liner  17  of the combustor  16  is thus exposed to the highest temperatures, being directly exposed to the combustion chamber  23 . As such, and as will be described in further detail below, the inner liner  17  is composed of at least one liner portion that is made of a non-metallic high temperature material such as a ceramic matrix composite (CMC) material. Such a CMC liner portion is much better able to withstand high temperatures with little or no cooling in comparison with standard metallic combustor liners. An air plenum  20 , which surrounds the combustor  16 , receives compressed air from the compressor section  14  of the gas turbine engine  10  (see  FIG. 1 ). This compressed air is fed into the combustion chamber  23 , however as will be described further below, exemplary CMC liner portions of the combustor  16  are substantially free of airflow passages (e.g., cooling holes) extending therethrough. This greatly simplifies their production, as no additional machining steps (such as drilling of cooling holes) are required once the CMC liner portions are formed. As such, the compressed air from the plenum  20  is, in at least this embodiment, fed into the combustion chamber  23  via air holes defined in metallic ring portions  32 ,  34  (e.g., high temperature nickel-based superalloys with thermal barrier coatings) of the combustor liner, as will be described further below. Metered air flow can also be fed into the combustion chamber via the fuel nozzles  30 . 
     The inner liner  17  extends from an upstream end  21  of the combustor  16  (where a plurality of fuel nozzles  30 , which communicate with the combustion chamber  23  to inject fuel therein, are located) to a downstream end (relative to gas flow in the combustion chamber) defining the combustor exit  27 . The inner liner  17  is, in at least one embodiment, comprised of the main liner portions, namely a dome portion (inlet dome)  24  at the upstream end (inlet end)  21  of the combustor, and a long exit duct portion  26  and a short exit duct portion  28  which together form the combustor exit  27  at their respective downstream ends. Each of the dome portion  24 , long exit duct portion  26  and short exit duct portion  28 , that are made of the CMC material and which make up a substantial part of the inner liner  17 , have a substantially hemi-toroidal shape and constitute an independently formed shell. 
       FIG. 2  shows a rich burn and quick quench combustor where the three CMC components  24 ,  26 ,  28  form the inner liner of combustor. The disclosure is primarily concerned with the attachment of CMC SED  28 . 
     Although ceramic materials have excellent high temperature strength, their coefficients of thermal expansion (CTE) are much lower than those of metals such as the rings  32  and  34 . Thermal stress arising from the mismatch of CTEs poses a challenge to the insertion of CMC combustor liner components into gas turbine engines. Exemplary joints thus allow relative movement between the CMC and its metal support structure(s), without introducing damaging thermal stresses. 
     The nature of the dome  24  and the LED  26  make them relatively easy to compliantly mount. In axial/radial section their exterior surfaces (away from the hot gas of the combustor interior) are generally convex. It is thus easy to compliantly compressively hold them in place. For example, the exemplary dome and LED are contained within respective shells  40  and  50  with compliant mounting members  42  and  52  respectively engaging the exterior surfaces  44  and  54  of the dome and SED. The exemplary shells  40  and  50  are metallic shells mounted to adjacent structure. The exemplary spring members  42  are half leaf spring tabs secured to the interior surface of the shell  40 . The exemplary spring members  52  are more complex assemblies of pistons and coil springs with piston heads engaging the LED exterior surface  54 . 
     The exemplary dome further includes an interior surface  45 , an outboard rim  46 , and an inboard rim  47 . The exemplary liner section  40  also includes an outboard rim  48  and an inboard rim  49 . The exemplary outboard rim  48  is secured to a mating surface of the outer air inlet ring (outer ring)  34  (e.g., via welding) and the exemplary inboard rim  49  is secured to the inner air inlet ring (inner ring)  32  such as via welding. 
     Similarly, the LED has an interior surface  53 , upstream rim  55  and a downstream rim  56 . The liner  50  includes an upstream portion (e.g., a rim)  57  and a downstream portion (e.g., a flange)  58 . The exemplary rim  57  is secured to the outer ring  34  (e.g., via welding). The exemplary flange  58  is secured to a corresponding flange  60  of the platform ring (inner ring)  61  of an exit vane ring  62 . The exemplary exit vane ring  62  includes a circumferential array of airfoils  63  extending from the platform  61  to a shroud ring (outer ring)  64 . 
     The SED extends from an upstream rim  80  to a downstream rim  82  and has a generally convex interior surface  84  and a generally concave exterior surface  86 . The LED downstream rim  56  and SED downstream rim  82  are proximate respective upstream rims  88  and  90  of the vane inner ring  61  and outer ring  64 . The first blade stage of the first turbine section is downstream of the vane ring  62  with the blade airfoils  66  shown extending radially outward from a disk  68 . 
     For mounting of the SED, a leading/upstream portion/region  100  of the SED is shown directed radially inwardly toward the upstream rim  80  (e.g., off-axial by an angle θ 1 ). The exemplary SED is of generally constant thickness (e.g., subject to variations in local thickness associated with the imposed curvature of the cross-section of the SED in the vicinity of up to 20%). The inward direction of this portion  100  thus creates associated approximately frustoconical surface portions  102  and  104  of the surfaces  84  and  86  along the region  100 . The surface portion  104  thus faces partially radially inward. The surface portion  104  may, thus, be engaged by an associated mounting feature to resist axial separation in a first axial direction  106  (forward in the exemplary engine wherein combustor inlet flow is generally forward). Movement in a second direction  107  opposite  106  is resisted by engagement of the surface portion  102  with a corresponding angled downstream surface  108  of the ring  32  (e.g., also at θ 1 ). Exemplary θ 1  are 20-60°, more narrowly, 30-50° or 35-45°). The SED may be retained against outward radial movement/displacement by engagement of the surface portion  102  with the downstream surface  108  and/or by hoop stress in the CMC. For example, alternative implementations may lack the surface  108  and thus rely entirely upon hoop stress to retain the SED against outward radial movement. An exemplary SED is formed of CMCs such as silicon carbide reinforced silicon carbide (SiC/SiC) or silicon (Si) melt infiltrated SiC/SiC (MI SiC/SiC). The CMC may be a substrate atop which there are one or more protective coating layers or adhered/secured to which there are additional structures. It may be formed with a sock weave fiber reinforcement including continuous hoop fibers. 
     The exemplary mounting feature comprises a circumferential array of radially outwardly-projecting distal tabs  110  of a metallic clamp ring  112 . The clamp ring is pulled axially in the direction  107  via an annular array of hook bolt assemblies  114 . Exemplary hook bolt assemblies  114  are mounted to the dome shell  40 . Exemplary hook bolt assemblies include a fixed base (support)  120  mounted to an inboard portion of the dome shell. A threaded shaft  122  extends through an aperture in the base  120  and is engaged by a nut  124  which may be turned (tightened) to draw the shaft at least partially axially in the direction  107 . The shaft is coupled to a hook  126  (see also,  FIG. 3 ) which engages a corresponding aperture  127  in the ring  112  to allow tightening of the nut to draw the ring in the direction  107 . The combination of flexing of the tabs  110  with the angle of the region  100  and face  108  allows for differential thermal expansion with sliding engagement between the ring face  102  and the face  108 . The clamp load can be controlled by the stiffness of the tabs  110 , metal ring  112 , and hook bolt supports  120 . 
     In the exemplary mounting configuration, the gripping of the portion  100  is the only mounting of the SED with the downstream rim  82  being slightly spaced apart from adjacent structures. 
     Rotational registration and retention of the SED to the ring  32  may also be provided. Exemplary rotational registration and retention means comprises a circumferential series of recesses  140  ( FIG. 4 ) in the rim  80  and region  100 . These recesses  140  cooperate with protruding portions  142  of the ring  32  (e.g., protruding from the main frustoconical portion of the surface  108 ). The exemplary recesses are through-recesses extending all the way between the surfaces  102  and  104 . In alternative implementations, the recesses  140  and protruding portions  142  may be reversed with recesses appearing in the ring and protruding portions appearing on the SED. 
       FIG. 5  shows an otherwise similar system with hooks penetrating the ring from outboard to inboard (in distinction to inboard-to-outboard). 
       FIGS. 6 and 7  show mounting features comprising circumferential straps  200 . Each strap extends from a first circumferential end  202  ( FIG. 7 ) to a second circumferential end  204 . The exemplary straps are fastened to the inner ring  32  and capture the SED. The exemplary implementation is based upon the SED and ring configuration of the  FIG. 2  embodiment with each strap fastened between two adjacent ones of the protrusions  142  (e.g., via screws  210  extending into threaded bores  212  in the protrusions  142 ). Each exemplary strap  200  thus has a first surface  220  and a second surface  222 . The first surface  220  engages the associated protrusions  142  and is held spaced-apart from the remainder of the surface  108  so that intact portions of the region  100  between the recesses  140  in the SED are captured between the surface  220  and the surface  108 . Springs such as Bellville washers  230  can be introduced with the bolts to maintain a constant clamp load. In the exemplary implementation, there are 2-10 such straps, more narrowly, an exemplary exactly two such straps. 
       FIG. 8  shows an alternative configuration wherein a leading portion  300  of the SED  301  is relatively thickened compared with a remaining portion  302  (e.g., along the portion  300  the thickness T is at least 150%, more narrowly, 150-250% or 175-225% the thickness along the portion  302 ). The leading portion extends generally axially to a leading/upstream rim  303 . At a junction between the thickened portion  300  and the remainder, a portion  310  of the exterior surface transitions and thus is directed partially radially inward and partially in the direction  106  (e.g., at an angle θ 2  which may be the same size as θ 1 ). An annular resilient member  312  is captured between this surface and a corresponding surface portion  314  of a liner  316 . The liner extends from an upstream rim/end  318  which is secured to the inner ring  306 . The surface portion  314  faces partially radially outward and partially opposite the direction  106  to allow capturing of the member  312 . An exemplary member  312  is a metallic generally C-sectioned sheetmetal member such as is used as a seal. The exemplary member  312  is a U seal or an Omega seal which compresses to transmit force in both the radial and axial directions. Other types of springs such as canted coil springs can also be employed. 
     The SED  301  may be installed by a process comprising: 1) sliding the U seal  312  onto the metal baffle plate  316 ; 2) cooling the assembly of the seal  312  and plate  316  to thermally contract them (e.g., to −40 C); 3) heating the CMC SED  301  to expand it (e.g., to 1000 C); 4) sliding/inserting the cooled assembly of seal  312  and plate  316  into the heated CMC SED  301 ; and 5) welding the baffle plate  316  to inner air inlet ring  306 . Thus, during the inserting, the SED is at a hotter-than-ambient temperature and the assembly is at a cooler-than-ambient temperature 
       FIG. 9  shows an alternate configuration of a similar SED with a resilient member  400  replacing the member  312 . 
     One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, when implemented in the remanufacture of the baseline engine or the reengineering of a baseline engine configuration, details of the baseline configuration may influence details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.