Abstract:
An integrated duct and baffle arrangement employing a hairpin transition area such that the construction is adapted to flex under thermal conditions.

Description:
TECHNICAL FIELD 
   The invention relates generally to gas turbine engines and, more particularly, to a new duct and baffle construction. 
   BACKGROUND OF THE ART 
   Interturbine ducts (ITD) are used for channelling hot combustion gases from a high pressure turbine stage to a low pressure turbine stage. The ITD is typically integrally cast with the stator vane set of the low pressure turbine stage. Lug and slot arrangements are typically used to connect the inner annular wall of the cast ITD to an inner baffle protecting the rear facing side of the high pressure turbine rotor. Such a lug and slot arrangement has been heretofore required to accommodate the thermal gradient between the cast ITD inner wall and the baffle. 
   Although the conventional lug and slot arrangement is efficient, it has been found that there is a need to provide a new and simpler ITD/baffle interface. 
   SUMMARY OF THE INVENTION 
   It is therefore an aim of the present invention to provide a new gas turbine engine duct and baffle arrangement. 
   In one aspect, the present invention provides an interturbine duct (ITD) adapted to direct hot combustion gases from a high pressure turbine stage to a low pressure turbine stage of a gas turbine engine, the ITD comprising inner and outer flow path containing walls adapted to contain the combustion gases therebetween, a high pressure turbine baffle integrated to the inner flow path containing wall, and a flexible hairpin transition area providing for relative flexural movement between the high pressure turbine baffle and the inner wall under thermal conditions. 
   In a second aspect, the present invention provides a gas turbine engine duct and baffle arrangement comprising a duct for channelling hot combustion gases, and a baffle integrally connected to the duct via a flexible hairpin transition area. 
   In a third aspect, the present invention provides a turbine section of a gas turbine engine, comprising high and low pressure turbine stages, an interturbine duct (ITD) channelling hot combustion gases from the high pressure turbine stage to the low pressure turbine stage, a high pressure turbine baffle integrated to a front end portion of the ITD duct via a flex joint. 
   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  is a cross-sectional side view of a gas turbine engine; 
       FIG. 2  is a cross-sectional side view of an interturbine duct with an integrated baffle forming part of the gas turbine engine shown in  FIG. 1  in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a gas turbine engine  10  of a type preferably 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, a 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. 
   As shown in  FIG. 2 , the turbine section  18  comprises a turbine casing  17  containing at least first and second turbine stages  20  and  22 , also referred to as high pressure turbine (HPT) and low pressure turbine (LPT) stages, respectively. Each turbine stage commonly comprises a shroud  23   H ,  23   L , a turbine rotor  24   H ,  24   L  that rotates about a centerline axis of the engine  10 , a plurality of turbine blades  25   H ,  25   L  extending from the rotor, and a stator vane ring  26   H ,  26   L  for directing the combustion gases to the rotor. The stator vane rings  26   H ,  26   L  typically comprises a series of circumferentially spaced-apart vanes  27   H ,  27   L  extending radially between inner and outer annular platforms or shrouds  29   H ,  29   L  and  31   H ,  31   L , respectively. The platforms  29 ,  31  and the vanes  27  are typically made from high-temperature resistant alloys and preferably integrally formed, such as by casting or forging, together as a one-piece component. 
   An interturbine duct (ITD)  28  extends between the turbine blade  25   H  of the first turbine stage  20  and the stator vane ring  26   L  of the second turbine stage  22  for channelling the combustion gases from the first turbine stage  20  to the second turbine stage  22 . As opposed to conventional interturbine ducts which are integrally cast/machined with the stationary vane ring  26   L  of the second turbine stage  22  (see U.S. Pat. No. 5,485,717, for example), the ITD  28  is preferably fabricated from sheet material, such as sheet metal, and brazed, welded or otherwise attached to the turbine vane ring  26   L . The sheet metal ITD  28  is advantageously much thinner than cast ducts and therefore much more lightweight. The person skilled in the art will appreciate that the use of sheet metal or other thin sheet material to fabricate an interturbine duct is not an obvious design choice due to the high temperatures and pressures to which interturbine ducts are exposed, and also due to the dynamic forces to which the ITD is exposed during operation. Provision for such realities is therefore desired, as will now be described. 
   The ITD  28  comprises concentric inner and outer annular walls  30  and  32  defining an annular flowpath  34  which is directly exposed to the hot combustion gases that flows theretrough in the direction indicated by arrow  36 . The inner and outer annular walls  30  and  32  are preferably a single wall of a thin-walled construction (e.g. sheet metal) and preferably have substantially the same wall thickness. According to an embodiment of the present invention, the inner and outer annular walls  30  and  32  are each fabricated from a thin sheet of metal (e.g. an Inconel alloy) rolled into a duct-like member. It is understood that ITD  28  could also be fabricated of other thin sheet materials adapted to withstand high temperatures. Fabricating the ITD in this manner gives much flexibility in design, and permits the ITD  28  to be integrated with the engine case  17  if desired. The annular walls  30 ,  32  extend continusously smoothly between their respective ends, without kinks, etc, and thus provide a simple, smooth and lightweight duct surface for conducting combustion gases between turbine stages. 
   The outer annular wall  32  extends from an upstream edge  35 , having annular flange  37  adjacent HPT shroud  23   H , the flange extending radially away (relative to the engine axis) from ITD  28 , to a downstream end flange  38 , the flange having an S-bend back to accommodated platform  31   L  smoothly, to minimize flow disruptions in path  34 . The annular end flange portion  38  is preferably brazed to the radially outward-facing surface  39  of the outer platform  31   L . The outer annular wall  32  is not supported at its upstream end (i.e. at flange  37 ) and, thus, it is cantilevered from the stator vane set  26  of the second turbine stage  22 . The flange  37  is configured and disposed such that it impedes the escape of hot gas from the primary gas path  34  to the cavity surrounding ITD  28 , which advantageously helps improve turbine blade tip clearance by assisting in keeping casing  17  and other components as cool as possible. Meanwhile, the cantilevered design of the leading edge  35  permits the leading edge to remain free of and unattached from the turbine support case  17 , thereby avoiding interference and/or deformation associated with mismatched thermal expansions of these two parts, which beneficially improves the life of the ITD. The flange  37 , therefore, also plays an important strengthening role to permit the cantilevered design to work in a sheet metal configuration. 
   The inner annular wall  30  is mounted to the stator vane set  26  of the second turbine stage  22  separately from the outer annular wall  32 . The inner annular wall  30  has a downstream end flange  40 , which is preferably cylindrical to thereby facilitate brazing of the flange to a front radially inwardly facing surface of the inner platform  29   L  of the stator vane set  26   L  of the second turbine set  22 . The provision of the cylindrical flange  40  permits easy manufacture within tight tolerances (cyclinders can generally be more accurately formed (i.e. within tighter tolerances) than other flange shapes), which thereby facilitates a high quality braze joint with the vane platform. 
   The inner annular wall  30  is integrated at a front end thereof with a baffle  42  just rearward of the rotor  24   H  of the first turbine stage  20 . The baffle  42  provides flow restriction to protect the rear face of the rotor  24   H  from the hot combustion gases. The integration of the baffle  42  to the ITD inner annular wall  30  is preferably achieved through a “hairpin” or U-shaped transition which provides the required flexibility to accommodate thermal growth resulting from the high thermal gradient between the ITD inner wall  30  and the baffle  42 . 
   The upstream end portion of the inner annular wall  30  is preferably bent outward at a first 90 degrees bend to provide a radially inwardly extending annular web portion  44 , the radial inner end portion of which is bent slightly axially rearward to merge into the inclined annular baffle  42 . A forward-facing C-seal  45  is provided forwardly facing on web  44 , to provide the double function of impeding the escape of hot gas from the primary gas path  34  and to strengthen and stiffen web  44  against dynamic forces, etc. The inner annular wall  30 , the web  44  and the baffle  42  form a one-piece hairpin-shaped member with first and second flexibly interconnected diverging segments (i.e. the ITD inner annular wall  30  and the baffle  42 ). In operation, the angle defined between the ITD inner annular wall  30  and the baffle  42  will open and close as a function of the thermal gradient therebetween. There is no need for any traditional lug-and-slot arrangement to accept the thermal gradient between the baffle  42  and the ITD inner wall  30 . The hairpin configuration is cheaper than the traditional lug and slot arrangement because it does not necessitate any machining and assembly. The baffle  42  is integral to the ITD  28  while still allowing relative movement to occur therebetween during gas turbine engine operation. Since ITD  28  is provided as a single sheet of metal, sufficient cooling must be provided to ensure the ITD has a satisfactory life. For this reason, a plurality of cooling holes  60  is provided in web  44  for appropriate communication with an upstream secondary air source (not shown). Cooling holes  60  are adapted to feed secondary air, which would typically be received from a compressor bleed source (not shown) and perhaps passed to holes  60  via an HPT secondary cooling feed system (not shown) therethrough, and directed initially along inner duct  30  for cooling thereof. This cooling helps the single-skin sheet metal ITD to have an acceptable operational life. The U-shaped bent portion of the hairpin-shaped member is subject to higher stress than the rectilinear portion of ITD inner wall  30  and is thus preferably made of thicker sheet material. The first and second sheets are preferably welded together at  46 . However, it is understood that the hairpin-shaped member could be made from a single sheet of material. 
   The baffle  42  carries at a radial inner end thereof a carbon seal  48  which cooperate with a corresponding sealing member  50  mounted to the rotor  24 . The carbon seal  48  and the sealing member  50  provide a stator/rotor sealing interface. Using the baffle  42  as a support for the carbon seal is advantageous in that it simplifies the assembly and reduces the number of parts. 
   The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without department from the scope of the invention disclosed. For example, the ITD  28  could be supported in various ways within the engine casing  17 . Also, if the stator vane set  27  is segmented, the inner and outer sheet wall of the ITD  28  could be circumferentially segmented. It is also understood that various flex joint or elbows could be used at the transition between the ITD inner wall  30  and the baffle  42 . Finally, it is understood that the above-described integrated duct and baffle arrangement could have other applications. Still other modifications which fall within the scope of the present invention 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.