Abstract:
A method enables a gas turbine engine including a first rotor assembly and a second rotor assembly coupled in axial flow communication downstream from the first rotor assembly to be assembled. The method comprises coupling an upstream end of an extension duct to an outlet of the first rotor assembly, wherein the extension duct includes a plurality of panels coupled circumferentially, and coupling a downstream end of the extension duct to an inlet of the second rotor assembly using at least one fish mouth seal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to gas turbine engines, and more specifically to methods and apparatus for assembling gas turbine engines.  
           [0002]    At least some known gas turbine engines include a core engine having, in serial flow arrangement, a fan assembly and a high pressure compressor which compress airflow entering the engine, a combustor which burns a mixture of fuel and air, and low and high pressure rotary assemblies which each include a plurality of rotor blades that extract rotational energy from airflow exiting the combustor.  
           [0003]    An operating efficiency of known gas turbine engines is at least partially limited by combustor operating temperatures. To facilitate increased combustor temperatures, at least some known gas turbine engines use a smaller diameter core engine in comparison to a diameter of the low pressure turbine. More specifically, reducing a fan corrected tip speed and reducing fan pressure ratio facilitates increasing the engine efficiency.  
           [0004]    Generally an engine is designed as a compromise between performance, cost, and weight. Despite the thermodynamic benefits of operating at higher temperatures, the increased temperatures may also cause problems in designing a low pressure turbine that is operable with a high efficiency and a reasonable number of low pressure turbine stages. More specifically, as a result of the increased high pressure turbine operating temperature, a larger diameter low pressure turbine may be required to achieve a desired operating efficiency with a reasonable number of stages. However, known gas turbine engines are limited in the radius change between the exit of the high pressure rotary assembly and the low pressure turbine.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    In one aspect, a method for assembling a gas turbine engine including a first rotor assembly and a second rotor assembly coupled in axial flow communication downstream from the first rotor assembly is provided. The method comprises coupling an upstream end of an extension duct to an outlet of the first rotor assembly, wherein the extension duct includes a plurality of panels coupled circumferentially, and coupling a downstream end of the extension duct to an inlet of the second rotor assembly using at least one fish mouth seal.  
           [0006]    In another aspect of the invention, an annular turbine frame for a gas turbine engine is provided. The turbine frame comprises a plurality of panels coupled together to form an extension duct. The extension duct includes a radially outer panel portion and a radially inner panel portion. At least one of the radially outer and radially inner panel portions is coupled within the gas turbine engine by at least one fish mouth seal.  
           [0007]    In a further aspect, a gas turbine engine is provided. The gas turbine engine comprises a first rotor assembly, a second rotor assembly, and an extension duct. The second rotor assembly is downstream from the first rotor assembly, such that the second rotor assembly is coupled in axial flow communication with the first rotor assembly. The extension duct extends from an outlet of the first rotor assembly to an inlet of the second rotor assembly, and includes an upstream end, a downstream end, and a plurality of panels coupled together circumferentially therebetween. At least one of the extension duct upstream or downstream ends is coupled within the gas turbine engine by at least one fish mouth seal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is schematic illustration of a gas turbine engine;  
         [0009]    [0009]FIG. 2 is partial cross-sectional schematic view of a portion of the engine shown in FIG. 1; and  
         [0010]    [0010]FIG. 3 is an enlarged view of the engine shown in FIG. 1 and taken along area  3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    [0011]FIG. 1 is a schematic illustration of a gas turbine engine  10  including a fan assembly  12 , a high pressure compressor  14 , and a combustor  16 . Engine  10  also includes a high pressure turbine  18 , a low pressure turbine  20 , and a booster  22 . Fan assembly  12  includes an array of fan blades  24  extending radially outward from a rotor disc  26 . Engine  10  has an intake side  28  and an exhaust side  30 . In one embodiment, the gas turbine engine is a GE90 available from General Electric Company, Cincinnati, Ohio. Fan assembly  12  and turbine  20  are coupled by a first rotor shaft  31 , and compressor  14  and turbine  18  are coupled by a second rotor shaft  32 .  
         [0012]    In operation, air flows through fan assembly  12  and compressed air is supplied to high pressure compressor  14 . The highly compressed air is delivered to combustor  16 . Airflow (not shown in FIG. 1) from combustor  16  drives turbines  18  and  20 , and turbine  20  drives fan assembly  12  by way of shaft  31 .  
         [0013]    [0013]FIG. 2 is partial cross-sectional schematic view of an extension duct assembly  40  for use with engine  10 . FIG. 3 is an enlarged view of engine  10  taken along area  3  (shown in FIG. 2). Turbine  18  includes a plurality of stages  40 , and each stage includes a row of rotor blades  42  and a row of stationary vanes. In the exemplary embodiment, rotor blades  42  are supported by rotor disks  46 .  
         [0014]    A load-bearing annular turbine frame  48  extends downstream from turbine  18 . Frame  48  includes a radially outer structural member or casing  50  that extends circumferentially around turbine  18 , and a radially inner member or hub  52  that is coaxially aligned with respect to casing  50  about an axis of rotation of turbine engine  10 . Hub  52  is radially inward from casing  50  and a plurality of circumferentially spaced apart hollow struts  56  extend radially between casing  50  and hub  52 .  
         [0015]    Frame  48  also includes a plurality of conventional fairings  60 , each of which surrounds a respective strut  56  to facilitate shielding each strut from combustion gases flowing through turbine engine  10 . More specifically, each strut  56  includes a radially outer end  62  and an opposite radially inner end  64 . In the exemplary embodiment, each strut radially inner end  64  is coupled to hub  52  with a bolted connection  66 . In an alternative embodiment, strut inner ends  64  are coupled by welding to hub  52 . In a further alternative embodiment, strut inner ends  64  are integrally formed with hub  52 . A plurality of collars  70  surround, and are integrally formed with, each strut radially outer end  64 , to removably couple each strut outer ends  64  to casing  50  such that loads induced to hub  52  are transmitted into casing  50  through collars  70 .  
         [0016]    An extension duct  100  extends downstream from turbine frame  48 . Specifically, extension duct  100  includes a plurality of panels  102  coupled together circumferentially such that a flow passageway  103  is defined through extension duct  100 . In the exemplary embodiment, twelve panels  102  are coupled together circumferentially. More specifically, panels  102  define a radially outer panel portion  104  and a radially inner panel portion  106  that is spaced radially inwardly from panel portion  104  such that flow passageway  103  is defined therebetween. Panel portions  104  and  106  extend axially between an upstream end  110  of extension duct  100  and a downstream end  112  of extension duct  100 . A doubler panel  120  is coupled against extension duct radially inner panel portion  106  to provide structural support to extension duct  100 .  
         [0017]    Extension duct  100  radially outer panel portion  104  is coupled to low pressure turbine casing  122  at aft end  112  by a bolted connection  124 , such that extension duct aft end  112  is positioned adjacent a leading edge  125  of a low pressure turbine nozzle outer band  126 . Radially outer panel portion  104  is also coupled to high pressure turbine casing  50  by a bolted connection  128 , such that extension duct upstream end  110  is downstream of fairings  60 . A doubler panel  121  is also coupled against extension duct radially outer panel portion  104  to provide structural support to extension duct  100 .  
         [0018]    Radially inner panel portion  106  is retained in position at aft end  112  by at least one fish mouth seal  140 . Specifically, seals  140  facilitate retaining extension duct aft end  112  in position relative to a leading edge  142  of a low pressure turbine nozzle inner band  144 . During operation, seals  140  permit extension duct  100  to shift radially to accommodate thermal expansion and/or thermal stresses induced therein, such that sealing of extension duct  100  with respect to low pressure turbine nozzle inner band  144  is facilitated. Seals  140  also facilitate reducing wear and maintenance costs to frame  48 . Radially inner panel portion  106  is retained in position at upstream end  110  by a fastener  150  that extends through a lapped joint  162  formed such that a portion  164  of doubler panel  120  is positioned against a radially inner surface  166  of an aft fairing  60 .  
         [0019]    Extension duct  100  extends axially between high pressure turbine  18  and low pressure turbine  20  such that fluid flowing axially therethrough therein is also channeled radially outwardly by extension duct  100 . More specifically, at upstream end  110 , extension duct radially outer panel portion  104  defines an inlet radius R 1  that is smaller than an outlet radius R 2  defined by radially outer panel portion  104  at extension duct aft end  112 . Accordingly, extension duct  100  accommodates an increased radius change in the flowpath between high pressure turbine  18  and low pressure turbine  20 . In one embodiment, a ratio of outlet radius R 2  to inlet radius R 1  is approximately equal to 1.75. In an alternative embodiment, a ratio of outlet radius R 2  to inlet radius R 1  is more or less than approximately 1.75. More specifically, extension duct  100  accommodates an increased radius change that is larger than radius changes employed by known gas turbine engines. The increased outlet radius R 2  facilitates low pressure turbine  20  operating with an increased operating efficiency in comparison to other known low pressure turbines that have an increased number of turbine stages. In the exemplary embodiment, low pressure turbine  20  is a counter-rotating turbine. Alternatively, low pressure turbine  20  is a conventionally rotating turbine.  
         [0020]    During operation, extension duct  100  accommodates an increased radius change between high and low pressure turbines  18  and  20 , respectively, in comparison to other known gas turbine engines. More specifically, the increased radius change enables low pressure turbine  20  to operate at the same, or an increased, operating efficiency as other known low pressure turbines that include more turbine stages. As a result, extension duct  100  facilitates increasing operating efficiencies of engine  10 , without the weight sacrifice associated with the additional low pressure turbine stages. Furthermore, fish mouth seals  140  accommodate thermal expansion and thermal stresses that may be induced to extension duct  100  while facilitating external sealing of flow passageway  103 .  
         [0021]    The above-described airframe is cost-effective and highly reliable. The frame includes a load-bearing portion coupled to an extension duct. The extension duct accommodates an increased and desired radius change in the flowpath between the high and low pressure turbines. The increased radius change enables the gas turbine engine to operate with a larger diameter low pressure turbine that has an operating efficiency that is typically achieved with an increased number of turbine stages in at least some known low pressure turbines. Furthermore, the extension duct is movable radially to accommodate thermal expansion differences between the high and low pressure turbines. As a result, the extension duct overcomes known manufacturing gas turbine engine radius change limits in a cost-effective and reliable manner.  
         [0022]    Exemplary embodiments of turbine frames are described above in detail. The frames are not limited to the specific embodiments described herein, but rather, components of each assembly may be utilized independently and separately from other components described herein. Each extension duct component can also be used in combination with other turbine frame components. Furthermore, each extension duct component may also be used with other gas turbine engine configurations.  
         [0023]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.