Abstract:
A method for assembling a gas turbine engine that includes providing a low-pressure turbine inner rotor that includes a first plurality of turbine blade rows configured to rotate in a first direction, and rotatably coupling a low-pressure turbine outer rotor to the inner rotor, wherein the outer rotor includes a second plurality of turbine blade rows that are configured to rotate in a second direction that is opposite the first rotational direction of the inner rotor and such that at least one of the second plurality of turbine blade rows is coupled axially forward of the first plurality of turbine blade rows.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to aircraft gas turbine engines, and more specifically to counter-rotating gas turbine engines. 
   At least one known gas turbine engine includes, in serial flow arrangement, a forward fan assembly, an aft fan assembly, a high-pressure compressor for compressing air flowing through the engine, a combustor for mixing fuel with the compressed air such that the mixture may be ignited, and a high-pressure turbine. The high-pressure compressor, combustor and high-pressure turbine are sometimes collectively referred to as the core engine. In operation, the core engine generates combustion gases which are discharged downstream to a counter-rotating low-pressure turbine that extracts energy therefrom for powering the forward and aft fan assemblies. Within at least some known gas turbine engines, at least one turbine rotates in an opposite direction than the other rotating components within the engine 
   At least one known counter-rotating low-pressure turbine has an inlet radius that is larger than a radius of the high-pressure turbine discharge. The increase inlet radius accommodates additional stages within the low-pressure turbine. Specifically, at least one known counter-rotating low-pressure turbine includes an outer rotor having a first quantity of low-pressure stages that are rotatably coupled to the forward fan assembly, and an inner rotor having an equal number of stages that is rotatably coupled to the aft fan assembly. 
   During engine assembly, such known gas turbine engines are assembled such that the outer rotor is cantilevered from the turbine rear-frame. More specifically, the first quantity of stages of the outer rotor are each coupled together and to the rotating casing, and the outer rotor is then coupled to the turbine rear-frame using only the last stage of the outer rotor, such that only the last stage of the outer rotor supports the combined weight of the outer rotor rotating casing. Accordingly, to provide the necessary structural strength to such engines, the last stage of the outer rotor is generally much larger and heavier than the other stages of the outer rotor. As such, during operation, the performance penalties associated with the increased weight and size may tend to negate the benefits of utilizing a counter-rotating low-pressure turbine. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In one aspect, a method for assembling a gas turbine engine is provided. The method includes providing a low-pressure turbine inner rotor that includes a first plurality of turbine blade rows configured to rotate in a first direction, and rotatably coupling a low-pressure turbine outer rotor to the inner rotor, wherein the outer rotor includes a second plurality of turbine blade rows that are configured to rotate in a second direction that is opposite the first rotational direction of the inner rotor and such that at least one of the second plurality of turbine blade rows is coupled axially forward of the first plurality of turbine blade rows. 
   In another aspect, a counter-rotating rotor assembly is provided. The rotor assembly includes an inner rotor including a first plurality of rows of turbine blades configured to rotate in a first direction, and an outer rotor including a second plurality of rows of turbine blades configured to rotate in a second direction that is opposite the rotational direction of the inner rotor, the outer rotor coupled within the rotor assembly such that at least one of the second plurality of rows of turbine blades is coupled axially forward of the inner rotor first plurality of rows of turbine blades. 
   In a further aspect, a gas turbine engine is provided. The gas turbine engine includes an inner rotor including a first plurality of rows of turbine blades configured to rotate in a first direction, and an outer rotor including a second plurality of rows of turbine blades configured to rotate in a second direction that is opposite the rotational direction of the inner rotor, the outer rotor is coupled within the rotor assembly such that at least one of the second plurality of rows of turbine blades is coupled axially forward of the inner rotor first plurality of rows of turbine blades. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a portion of an exemplary gas turbine engine; 
       FIG. 2  is a schematic diagram of an exemplary counter-rotating low pressure turbine assembly that can be used with the gas turbine engine shown in  FIG. 1 . 
       FIG. 3  is a schematic diagram of an exemplary counter-rotating low pressure turbine assembly that can be used with the gas turbine engine shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a cross-sectional view of a portion of an exemplary gas turbine engine  10  that includes a forward fan assembly  12  and an aft fan assembly  14  disposed about a longitudinal centerline axis  16 . The terms “forward fan” and “aft fan” are used herein to indicate that one of the fans  12  is coupled axially upstream from the other fan  14 . In one embodiment, fan assemblies  12  and  14  are positioned at a forward end of gas turbine engine  10  as illustrated. In an alternative embodiment, fan assemblies  12  and  14  are positioned at an aft end of gas turbine engine  10 . Fan assemblies  12  and  14  each include a plurality of rows of fan blades  19  positioned within a nacelle  18 . Blades  19  are joined to respective rotor disks  21  that are rotatably coupled through a respective fan shaft  20  to forward fan assembly  12  and through a fan shaft  22  to aft fan assembly  14 . 
   Gas turbine engine  10  also includes a core engine  24  that is downstream from fan assemblies  12  and  14 . Core engine  24  includes a high-pressure compressor (HPC)  26 , a combustor  28 , and a high-pressure turbine (HPT)  30  that is coupled to HPC  26  via a core rotor or shaft  32 . In operation, core engine  24  generates combustion gases that are channeled downstream to a counter-rotating low-pressure turbine  34  which extracts energy from the gases for powering fan assemblies  12  and  14  through their respective fan shafts  20  and  22 . 
     FIG. 2  is a schematic diagram of a straddle-mounted counter-rotating low-pressure turbine assembly  100  that may be used with a gas turbine engine similar to gas turbine engine  10  (shown in  FIG. 1 ). In the exemplary embodiment, low-pressure turbine  100  includes stationary outer casing  36  that is coupled to core engine  24  downstream from high-pressure turbine  30  (shown in  FIG. 1 ). Low-pressure turbine  100  includes a radially outer rotor  110  that is positioned radially inwardly of outer casing  36 . Outer rotor  110  has a generally frusto-conical shape and includes a plurality of circumferentially-spaced rotor blades  112  that extend radially inwardly. Blades  112  are arranged in axially-spaced blade rows or stages  114 . Although, the exemplary embodiment illustrates four stages  114 , it should be realized that outer rotor  110  may have any quantity of stages  114  without affecting the scope of the method and apparatus described herein. More specifically, outer rotor  110  includes M stages  114  of blades  112 . 
   Low-pressure turbine  100  also includes a radially inner rotor  120  that is aligned substantially coaxially with respect to, and radially inward of, outer rotor  110 . Inner rotor  120  includes a plurality of circumferentially-spaced rotor blades  122  that extend radially outwardly and are arranged in axially-spaced rows or stages  124 . Although, the exemplary embodiment illustrates three stages, it should be realized that inner rotor  120  may have any quantity of stages without affecting the scope of the method and apparatus described herein. More specifically, inner rotor  120  includes N stages  124  of blades  122 . In the exemplary embodiment, M=N+1. Accordingly, and in the exemplary embodiment, outer rotor  110  includes an even number of stages  114  and inner rotor  120  includes an odd number of stages  124  such that outer rotor  110  surrounds and/or straddles inner rotor  120 . 
   In the exemplary embodiment, inner rotor blades  122  extending from stages  124  are axially-interdigitated with outer rotor blades  112  extending from stages  114  such that inner rotor stages  124  extend between respective outer rotor stages  114 . Rotor blades  112  and  122  are therefore configured for counter-rotation of the rotors  110  and  120 . 
   In the exemplary embodiment, low-pressure turbine  100  also includes a rotor support assembly  130  that includes a stationary annular turbine rear-frame  132  that is aft of low-pressure turbine outer and inner blades  112  and  122 . A rotatable aft frame  134  is positioned aft of outer and inner blades  112  and  122  and upstream from turbine rear-frame  132 . Aft frame  134  is coupled to an aft end of outer rotor  110  for rotation therewith and to facilitate providing additional rigidity for supporting blades  112 . An annular turbine mid-frame  140  is upstream from outer and inner blades  112  and  122 . 
   Low-pressure turbine  100  also includes a first shaft  150  that is coupled between a forward end  152  of outer rotor  110  and a first shaft bearing  154  that is rotatably coupled to turbine mid-frame  140  via a structural member  156 . Specifically, first shaft  150  extends between forward end  152  and first shaft bearing  154  such that the weight of outer rotor  110  is distributed approximately equally about the circumference of gas turbine engine  10  at forward end  152 , via structural member  156 . 
   A second shaft  160  extends between inner rotor  120  and fan  14  such that inner rotor  120  is rotatably coupled to fan  14 . In the exemplary embodiment, second shaft  160  is positioned radially inward of first shaft  150 . A second shaft bearing  162  is coupled to second shaft  160  such that the weight of inner rotor  120  is distributed approximately equally about the circumference of gas turbine engine  10  at forward end  152 , via a structural member  164 . 
   Low-pressure turbine  100  also includes a third shaft  170  that rotatably couples fan  12 , outer rotor  110 , and turbine rear-frame  132  together. More specifically, low-pressure turbine  100  includes a third shaft differential bearing  172  coupled between second shaft  160  and third shaft  170 , and a third bearing  174  coupled between third shaft  170  and turbine rear-frame  132 . Specifically, third shaft  170  extends between fan  12  and turbine rear-frame  132  such that the weight of outer rotor  110  at an aft end  176  is distributed approximately equally about the circumference of gas turbine engine  10  at aft end  176 , via bearing  174  and turbine rear-frame  132 . In one embodiment, at least one of first bearing  154 , second bearing  162 , third differential bearing  172 , and third bearing  174  is a foil bearing. In another embodiment, at least one of first bearing  154 , second bearing  162 , third differential bearing  172 , and third bearing  174  is at least one of a roller bearing or a ball bearing. 
   In the exemplary embodiment, during engine operation, a radial force generated during rotation of outer rotor  110  is transmitted to bearings  154  and  174 . Specifically, as low-pressure turbine  100  rotates, bearings  154  and  174  contact turbine mid-frame  140  and turbine rear-frame  132  respectively to facilitate reducing radial movement of outer rotor  110 . Since each respective bearing  154  and  174  is coupled to outer casing  36  through turbine mid-frame  140  and turbine rear-frame  132 , outer rotor  110  is maintained in a relatively constant radial position with respect to outer casing  36 . More specifically, utilizing straddle-mounted low-pressure turbine  100  that includes an odd number of turbine stages  114  and  124  collectively, that are supported at both ends by bearings  154  and  174  respectively, facilitates eliminating the at least one known differential bearing that is coupled between the concentric low-pressure shafts when an even number of total stages are used in at least one known counter-rotating low-pressure turbine. Moreover, utilizing straddle-mounted low-pressure turbine  100  facilitates reducing the weight of gas turbine engine  10  by eliminating a large over-turning moment generated by a known low-pressure turbine. 
     FIG. 3  is a schematic diagram of a straddle-mounted counter-rotating low-pressure turbine assembly  200  that may be used with a gas turbine engine similar to gas turbine engine  10  (shown in  FIG. 1 ). In the exemplary embodiment, low-pressure turbine  200  includes stationary outer casing  36  that is coupled to core engine  24  downstream from high-pressure turbine  30  (shown in  FIG. 1 ). Low-pressure turbine assembly  200  includes radially outer rotor  110  that is positioned radially inwardly of outer casing  36 . Outer rotor  110  has a generally frusto-conical shape and includes plurality of circumferentially-spaced rotor blades  112  that extend radially inwardly. Blades  112  are arranged in axially-spaced blade rows or stages  114 . Although, the exemplary embodiment illustrates four stages  114 , it should be realized that outer rotor  110  may have any quantity of stages  114  without affecting the scope of the method and apparatus described herein. More specifically, outer rotor  110  includes M stages  114  of blades  112 . 
   Low-pressure turbine  200  also includes radially inner rotor  120  that is aligned substantially coaxially with respect to, and radially inward of, outer rotor  110 . Inner rotor  120  includes plurality of circumferentially-spaced rotor blades  122  that extend radially outwardly and are arranged in axially-spaced rows or stages  124 . Although, the exemplary embodiment illustrates three stages, it should be realized that inner rotor  120  may have any quantity of stages without affecting the scope of the method and apparatus described herein. More specifically, inner rotor  120  includes N stages  124  of blades  122 . In the exemplary embodiment, M=N+1. Accordingly, and in the exemplary embodiment, outer rotor  110  includes an even number of stages  114  and inner rotor  120  includes an odd number of stages  124  such that outer rotor  110  surrounds and/or straddles inner rotor  120 . 
   In the exemplary embodiment, inner rotor blades  122  extending from stages  124  are axially-interdigitated with outer rotor blades  112  extending from stages  114  such that inner rotor stages  124  extend between respective outer rotor stages  114 . The blades  112  and  122  are therefore configured for counter-rotation of the rotors  110  and  120 . 
   In the exemplary embodiment, low-pressure turbine  200  also includes rotor support assembly  130  that includes stationary annular turbine rear-frame  132  that is aft of low-pressure turbine outer and inner blades  112  and  122 . Rotatable aft frame  134  is positioned aft of outer and inner blades  112  and  122  and upstream from turbine rear-frame  132 . Frame  134  is coupled to an aft end of outer rotor  110  for rotation therewith and to facilitate providing additional rigidity for supporting blades  112 . Annular turbine mid-frame  140  is upstream from outer and inner blades  112  and  122 . 
   Low-pressure turbine  200  also includes a first shaft  250  that is coupled between forward end  152  of outer rotor  110  and fan  14 . More specifically, first shaft  250  is rotatably coupled to turbine mid-frame  140  via a structural member  252  and a first shaft bearing  254 . First shaft  250  extends between outer rotor  110  and fan  14  such that fan  14  is rotationally coupled to outer rotor  110  and such that the weight of outer rotor  110  is distributed approximately equally about the circumference of gas turbine engine  10  at forward end  152 , via structural member  252 . 
   A second shaft  260  extends between inner rotor  120  and fan  12  such that inner rotor  120  is rotatably coupled to fan  12 . In the exemplary embodiment, second shaft  260  is positioned radially inward of first shaft  250 . A shaft bearing  262  is coupled to second shaft  260  such that the weight of inner rotor  120  is distributed approximately equally about the circumference of gas turbine engine  10  at forward end  152 , via a structural member  252 . A shaft bearing  264  is coupled to second shaft  260  such that the weight of inner rotor  120  is distributed approximately equally about the circumference of gas turbine engine  10  at aft end  176 , via a structural member  266 . More specifically, second shaft  260  is supported at forward end  152  by turbine mid-frame  140  and supported at aft end  176  by turbine rear-frame  132 . 
   Low-pressure turbine  200  also includes a third shaft  270  that rotatably couples outer rotor  110  to turbine rear-frame  132 . More specifically, third shaft  270  extends outer rotor aft end  176  to turbine rear-frame  132  such that the weight of outer rotor  110  at an aft end  176  is distributed approximately equally about the circumference of gas turbine engine  10  at aft end  176 , via bearing  264  and turbine rear-frame  132 . In one embodiment, at least one of bearings  254 ,  262 , and/or  264  is a differential foil bearing. In another embodiment, at least one of bearings  254 ,  262 , and/or  264  is at least one of a differential roller bearing or a differential ball bearing. 
   In the exemplary embodiment, during engine operation, a radial force generated during rotation of outer rotor  110  is transmitted to bearings  254  and  264 . Specifically, as low-pressure turbine  200  rotates, bearings  254  and  264  contact turbine mid-frame  140  and turbine rear-frame  132  respectively to facilitate reducing radial movement of outer rotor  110 . Since each respective bearing  254  and  264  is coupled to outer casing  36  through turbine mid-frame  140  and turbine rear-frame  132 , outer rotor  110  is maintained in a relatively constant radial position with respect to outer casing  36 . More specifically, utilizing straddle-mounted low-pressure turbine  200  that includes an odd number of turbine stages  114  and  124  collectively, that are supported at both ends by bearings  254  and  264  respectively, facilitates eliminating the at least one known differential bearing that is coupled between the concentric low-pressure shafts when an even number of total stages are used in at least one known counter-rotating low-pressure turbine. Moreover, utilizing straddle-mounted low-pressure turbine  200  facilitates reducing the weight of gas turbine engine  10  by eliminating a large over turning moment generated by a known low-pressure turbine that includes an outer rotor having an even number of stages. 
   The exemplary embodiments described above illustrate a counter-rotating low-pressure turbine having an outer rotor that includes an even number of stages and an inner rotor that includes an odd number of stages such that the outer rotor straddles the inner rotor. Since, the outer rotor straddles the inner rotor, the outer rotor is configurable to couple to either the forward or aft fan assembly. Utilizing a straddle-mounted counter-rotating low-pressure turbine facilitates reducing the weight of the gas turbine engine by eliminating the large over turning moment of a conventional low-pressure turbine that includes an outer rotor having an even number of stages. Moreover, the straddle-mounted turbines described herein facilitate handling a blade out event in which a large turbine unbalance may result in the outer rotating casing while also improving gas turbine engine performance by providing increased tip clearance control between the outer rotor and the casing. Moreover, the straddle-mounted turbines described herein facilitate reducing the weight of the turbine rear-frame by distributing the weight of the outer rotor between the turbine mid-frame and turbine rear-frame. 
   Exemplary embodiments of straddle-mounted counter-rotating low-pressure turbines are described above in detail. The components are not limited to the specific embodiments described herein, but rather, components of each system may be utilized independently and separately from other components described herein. Straddle-mounted turbines can also be used in combination with other known gas turbine engines. 
   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.