Abstract:
A mechanical joint for a gas turbine engine includes:(a) an annular first component having an annular, radially-extending first flange; (b) an annular second component having an annular, radially-extending second flange abutting the first flange; (c) a plurality of generally radially-extending radial channels passing through at least one of the first and second flanges; (d) a plurality of generally axially-extending channels extending through the first flange and communicating with respective ones of the radial channels; and (e) a plurality of fasteners clamping the first and second flanges together.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to gas turbine engines and more particularly to apparatus and methods for thermal management of mechanical joints in such engines. 
     A gas turbine engine includes a turbomachinery core with a primary gas flowpath passing serially through a high pressure compressor, a combustor, and a high pressure turbine. The core is operable in a known manner to generate a primary gas flow. In a turbojet or turbofan engine, the core exhaust gas is directed through an exhaust nozzle to generate thrust. A turboshaft engine uses a low pressure or “work” turbine downstream of the core to extract energy from the primary flow to drive a shaft or other mechanical load. 
     It is generally desired to seal off the primary flowpath to prevent leakage of high-pressure, high-temperature gases, so as to avoid damage to temperature-sensitive components outside the primary flowpath, for example stationary structural members, and efficiency losses, both from direct leakage and from undesirable clearance changes caused by thermal loading. One of the important seals in a gas turbine engine is the compressor discharge pressure (“CDP”) seal. Typically this will be a noncontact-type seal which includes seal teeth mounted on a rotor, surrounded by a stationary abradable member. 
     Thermal growth response of the CDP seal is directly related to engine performance and fuel efficiency. There is a need is to slow the natural thermal response of the stationary CDP seal member in order to match it with the relatively slow response of the rotor. A slow-responding CDP seal will rub less and maintain a tighter seal, which improves performance. 
     Seal response has been addressed in the past with the use of low thermal growth alloys and heat shields. However, low thermal growth alloys typically have strength limitations at high temperatures. Heat shields typically are made of sheet metal and cover the expanse of the outer diameter of the CDP seal. 
     Furthermore, the hardware around the CDP seal often includes a joint having a number of flanges which are bolted together. Such joints can have a short low cycle fatigue (“LCF”) life caused by large radial thermal gradients between the center and the radial edges of the flanges. The LCF life can be improved by using a single integrated flange, but such a design is not always feasible because of space limitations. 
     BRIEF SUMMARY OF THE INVENTION 
     These and other shortcomings of the prior art are addressed by the present invention, which provides a multi-flange bolted joint which has controlled thermal response and uniform thermal stresses. 
     According to one aspect of the invention, a mechanical joint for a gas turbine engine includes:(a) an annular first component having an annular, radially-extending first flange; (b) an annular second component having an annular, radially-extending second flange abutting the first flange; (c) a plurality of generally radially-extending radial channels passing through at least one of the first and second flanges; (d) a plurality of generally axially-extending channels extending through the first flange and communicating with respective ones of the radial channels; and (e) a plurality of fasteners clamping the first and second flanges together. 
     According to another aspect of the invention, a joint structure for a gas turbine engine includes: (a) an annular nozzle support coupled to a stationary turbine nozzle, including a radially-extending nozzle support flange, the nozzle support flange having a plurality of generally radially-extending first grooves formed therein; (b) an annular diffuser including a plurality of streamlined struts and a diffuser arm carrying a radially-extending diffuser flange abutting the nozzle support flange, the diffuser flange including a plurality of generally radially-extending second grooves formed therein, wherein the first and second grooves are circumferentially aligned so as to cooperatively define radial channels; (c) an annular outlet guide vane structure including a plurality of airfoil-shaped vanes and an outlet guide vane arm carrying a radially-extending outlet guide vane flange which abuts the diffuser flange, wherein the diffuser arm and the outlet guide vane arm cooperatively define an open volume therebetween; (d) a plurality of generally axially-extending channels extending through the diffuser and outlet guide flanges, and communicating with respective ones of the radial channels and the open volume; and (e) a plurality of fasteners clamping the nozzle support flange, diffuser flange, and outlet guide vane flange together. 
     According to another aspect of the invention, a method is provided for reducing thermal gradients in a mechanical joint for a gas turbine engine of the type including two or more annular components each having radially-extending flanges which are clamped together with a plurality of fasteners, wherein exterior surfaces of the annular components are exposed in operation to high-temperature gases. The method includes passing high-temperature gases from the exterior through central portions of the clamped flanges. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
         FIG. 1  is a cross-sectional view of a portion of a gas turbine engine constructed in accordance with an aspect of the present invention; 
         FIG. 2  is a perspective sectional view of a bolted diffuser-to-stator joint constructed in accordance with the present invention; 
         FIG. 3  is a perspective view of a portion of a bolt shield for use with the joint shown in  FIG. 2 ; 
         FIG. 4  is an exploded perspective view of a portion of the joint shown in  FIG. 2 ; 
         FIG. 5  is a front elevational view of the joint shown in  FIG. 4 ; 
         FIG. 6  is a perspective view of an alternative OGV flange; 
         FIG. 7  is a perspective sectional view of a bolted diffuser-to-stator joint incorporating the alternative OGV flange shown in  FIG. 6 ; 
         FIG. 8  is a front perspective view of a joint incorporating an alternative flow directing feature; 
         FIG. 9  is an aft perspective view of the joint of  FIG. 8 ; 
         FIG. 10  is a cross-sectional view of alternative OGV structure; and 
         FIG. 11  is a cross-sectional view of another alternative OGV structure incorporating a heat shield. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  depicts a portion of a gas turbine engine including, generally, a high pressure compressor  10 , an outlet guide vane (OGV) structure  12 , a diffuser  14 , a combustor  16 , a turbine nozzle  18 , and a high pressure turbine rotor  20 . In the illustrated example, the engine is a high-bypass turbofan engine. However, the principles described herein are equally applicable to turboprop, turbojet, and turbofan engines, as well as turbine engines used for other vehicles or in stationary applications. 
     The compressor  10  includes a plurality of alternating stages of rotating blades  22  and stationary nozzles  24 . Only the final stages of the compressor  10  are shown. The blades  22  are carried on a spool  26  which includes a shaft arm  28  that extends axially aft and radially inward. This is coupled to a turbine shaft  30  which is part of a turbine rotor disk  32 . A seal disk  34  is clamped in the joint between the shaft arm  28  and the turbine shaft, and carries a plurality of annular seal teeth  36 . 
     The OGV structure  12  includes an array of airfoil-shaped vanes  38  located just downstream of the compressor  10 . The vanes  38  are disposed between annular inner and outer platforms  40  and  42 . An annular arm  44  includes a forward portion  46  extending generally axially aft and radially inward from the inner platform  40 , a center portion  48  which is generally cylindrical, and an aft portion  50  which extends axially aft from the center portion  48 . 
     A stationary annular seal member, referred to as a CDP seal  52 , is carried on the inboard surface of the center portion  48 . The CDP seal  52  is made from a compliant material of a known type, for example an abradable compound, a honeycomb or other cellular structure, or a metallic brush seal. The CDP seal  52  surrounds the seal teeth  36  leaving a small radial gap. An annular OGV flange  54  extends radially outward at the aft end of the aft portion  50 . 
     The diffuser  14  includes an array of streamlined struts  56  located just downstream of the OGV vanes  38 . The struts  56  are disposed between annular inner and outer platforms  58  and  60 . An annular arm  62  extends generally axially aft and radially inward from the inner platform  58 . An annular diffuser flange  64  extends axially inward at the aft end of the arm  62 . 
     An annular inner nozzle support  66  extends axially forward from the turbine nozzle  18 . An annular nozzle support flange  68  extends axially inward at the forward end of the inner nozzle support  66 . Optionally, the nozzle support flange  68  may have an array of scallops  70  or other negative features formed therein (best seen in  FIG. 2 ). These reduce the mass and thus the thermal inertia of the nozzle support flange  68 . This in turn increase the thermal response of the inner nozzle support  66  and reduces thermal gradients therein during operation. 
     The OGV flange  54 , diffuser flange  64 , and the nozzle support flange  68  are clamped together at a joint  72  and secured by a plurality of fasteners such as the illustrated bolts  74 , which pass through aligned bolt holes in the joined components. This joint  72  is shown in detail in  FIG. 2 . The joint  72  is provided with means for bleeding compressor discharge air to the volume between the CDP seal  52  and the diffuser  14  while minimizing temperature gradients within the coupled flanges. In the illustrated example this is done by providing a bleed flowpath which passes through the center of the joint  72 . The nozzle support flange  68  has an array of generally radially-oriented grooves  76  formed in its forward face. These are aligned with corresponding grooves  78  formed in an aft face of the diffuser flange  64  to cooperatively define radial channels  80 . Depending on the particular application, it may be possible to form a single radial channel through one of the flanges. 
     The exposed ends of the bolts  74  are surrounded with an annular windage shield  82 , which is shown in more detail in  FIG. 3 . It is an arcuate member (it may be continuous or segmented) having a generally “L”-shaped cross section with an axially-extending leg  84  and a radially-extending leg  86 . A number of pockets  81  with bolt holes  83  are positioned around the circumference of the windage shield  82 . The axially-extending leg  84  has a slot  85  formed in its forward edge adjacent each of the pockets  81  and extending a portion of the distance between two pockets  81 . The radially-extending leg  86  has a slot  87  formed in its radially outer edge between each of the pockets  81 , which extends substantially the whole distance between adjacent pockets  81 . When installed, as seen in  FIG. 2 , a small radial gap is present between the outer end of the radially-extending leg  86  and the radially inner surface of the inner nozzle support  66 , and a small axial gap is present between the forward end of the axially-extending leg  84  and the nozzle support flange  68 . 
     Referring back to  FIG. 2 , a plurality of generally axially-extending holes  88  are formed through the diffuser flange  64  and intersect the radial channels  80 . A plurality of apertures  90  are also formed through the OGV flange  54 , and are aligned with the holes  88 . Together, the holes  88  and the apertures  90  define axial channels  92 . The axial and radial channels  92  and  80  define the complete flowpath through the joint  72 . 
     Optionally, the joint  72  may include some means for flow deflection, i.e. blocking flow from the apertures  90  from passing directly forward and impinging on the CDP seal  52 . In the illustrated example, this is accomplished by incorporating a shielding structure in the bolts  74 . Each of the bolts  74  has a shank and an enlarged head  96 . The end of the shank opposite the head  96  is threaded in a known manner. The head  96  generally includes at least one anti-rotation feature that interfaces with the OGV arm  44  to prevent rotation when a nut  98  is installed on the bolt  74 . In the illustrated example the head  96  includes an opposed pair of flat side faces  100  for this purpose. The shielding structure comprises one or more laterally-extending features which block flow through the apertures  90 . In this example the head  96  is generally a rectangular solid, and the portions of the head  96  intermediate the flat side faces  100  incorporates one or more laterally-extending tabs  104 . 
     The bolts  74  may be sized such that the tabs  104  of adjacent bolts  74  define a small gap therebetween. Alternatively, depending on the specific application, the shielding structures of adjacent bolts  74  may be configured to overlap each other. For example, in  FIGS. 4 and 5 , bolts  74 A and  74 B employ the tabbed configuration described above. The heads  96 A of alternate ones of the bolts  74 A are slightly thicker than the heads  96 B of the adjacent bolts  74 B. As a result, when installed, the tabs  104 A of bolts  74 A are positioned axially forward of the tabs  104 B of bolts  74 B. This permits the tabs  104 A and  104 B to overlap in a lateral direction, blocking flow from the apertures  90  from passing axially forward, as seen most clearly in  FIG. 5 . 
     In operation, both the inboard and outboard surfaces of the joint  72  are exposed to CDP air at high temperature, for example about 700° C. (1300° F.). CDP air also enters the radial channels  80 , passes through them into the axial channels  92 , and out into the space between the OGV structure  12  and the diffuser  14 . As the air passes through the channels  80  and  92 , it heats the inner portions of the OGV flange  54 , diffuser flange  64 , and the nozzle support flange  68 , minimizing any temperature gradients through the radial thickness of those components. The air exits the apertures  90  at a location substantially aft of the CDP seal  52 , avoiding direct impingement and thus reducing the thermal response of the CDP seal  52 . Impingement on the CDP seal  52  is further avoided because the air exiting the apertures  90  is deflected in a radially outboard direction by the bolt heads  96 , as depicted by the arrow labeled “A” in  FIG. 1 . 
     Reduction of thermal gradients in the joint  72  is also improved by selective configuration of the windage shield  82 . Ordinarily in prior art usage a fastener windage shield would cover the underlying bolts  74  as completely as possible to minimize frictional heating of the air flow. However, the incorporation of the slots  85  and  87  permits some heated air flow to circulate through the interior of the windage shield  82  in axial and tangential directions, and to contact the nozzle support flange  68 , before exiting the windage shield  82 , as shown by the arrows “B” and “C” in  FIG. 2 . As noted above, the nozzle support flange  68  may have an array of scallops  70  or other negative features formed therein, which reduce its thermal inertia and improve its response to the flow through the radial gap. 
     Depending on the specific application, it may be necessary or desirable to increase the exit flow area from the OGV flange  54 . As an example of how this may be accomplished,  FIG. 6  shows a portion of an OGV structure  112  having an OGV flange  154  similar in construction to the OGV flange  54  described above. The OGV flange  154  incorporates generally radially-aligned slots  190  which extend to an outboard edge of the OGV flange  154 .  FIG. 7  shows the OGV flange  154  clamped to a diffuser flange  64  and a nozzle support flange  68  with bolts  74 . The inboard ends of the slots  190  communicate with the holes  88  in the diffuser flange  68 . The slots  190  provide additional flow area past the heads  96  of the bolts  74 , as compared to the configuration shown in  FIG. 2 . 
       FIGS. 8 and 9  depict an alternative configuration for flow deflection within the joint. This configuration utilizes an OGV structure  412  which is generally similar to the OGV structure  112  described above. It includes an OGV flange  454  having a plurality of generally radially-aligned slots  490 . Bolts  474 , similar to bolts  74 , are received in bolt holes in the OGV flange  454 . Each of the bolts  474  has a threaded shank and an enlarged head  496 , and in this example the head  496  is generally a rectangular solid. The lateral surfaces of the head  496  have a peripheral groove  498  formed therein. When the joint is assembled, a clip  500  is disposed between each of the bolt heads  496  and is received in the grooves  498 . Only one clip is shown in  FIGS. 8 and 9 . 
     The clip  500  is formed from flat stock, for example sheet metal, and is generally “J”-shaped, with a pair of fingers  502  located at its outer end. The fingers  502  engage recesses  504  formed in the OGV flange  454  at the edges of the slots  490 . When installed, the clips  504  form a barrier to prevent air flow in the axially forward direction, but allow air flow radially outward through the space between the fingers  502 . 
     In addition to, or as an alternative to the flow deflection features described above, response of the CDP seal  52  can be controlled by addition of mass so as to increase its thermal inertia. For example,  FIG. 10  illustrates an alternative OGV structure  212  in which the material thickness in the center portion  248  of the arm  244  is substantially increased. For example, the material thickness may be at least about twice the thickness as that of the OGV structure  12  depicted in  FIG. 1 . 
       FIG. 11  illustrates another alternative OGV structure  312  incorporating an annular heat shield  314  which surrounds a center portion  348  of an arm  344 . The heat shield  314  may be constructed of sheet metal or a similar material and closely conforms to the various bends in the arm  344 . The heat shield  314  may be secured with a plurality of fasteners such as the illustrated bolts  316 . As is the case with the increased mass arm  244 , the heat shield  314  may be used in addition to or as an alternative to the flow deflection features described above. 
     The joint configuration described above has several benefits over prior art designs. This configuration allows the use of cast material instead of forged for the inner nozzle support  66 . Furthermore the design will improve specific fuel consumption (SFC) due to reduced CDP seal clearance. Additional benefits may be gained due to the more circumferentially uniform flow as the space between the OGV structure  12  and the diffuser  14  is fed by a large number of holes (for example,  68  holes) instead of the small number of holes used in prior art designs. Another advantage is the dual-purposing of the bolts  74  to deflect impingement on the CDP seal  51 , potentially eliminating the need for additional hardware to slow seal response, such as a heat shield. 
     The foregoing has described a bolted joint arrangement for a gas turbine engine. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.