Abstract:
Methods and apparatus for assembling a gas turbine engine are provided. The method includes coupling a first turbine shaft that includes m rows of turbine blades within the gas turbine such that the first turbine shaft is rotatable in a first direction, and coupling a second turbine shaft that includes n rows of turbine blades within the gas turbine such that the second turbine shaft is rotatable in a second direction wherein a torque split between the first and second turbine shafts is substantially proportional to the number of rows of turbine blades on each shaft relative to a total number of rows of blades on both shafts, and wherein m and n are selected to provide a torque split between the first turbine shaft and second turbine shaft of greater than about 1.2:1.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to gas turbine engines and, more particularly, to methods and apparatus for providing non-equal torque between counter-rotating gas turbine engine rotors that have one or more interdigitated rotors.  
         [0002]     At least some known gas turbine engines include a forward fan, a core engine, and a power turbine. The core engine includes at least one compressor, a combustor, a high-pressure turbine, and a low-pressure turbine coupled together in a serial flow relationship. More specifically, the compressor and high-pressure turbine are coupled through a high-pressure shaft to define a high-pressure rotor. The compressor compresses air entering the core engine that is then mixed with fuel and ignited to form a high energy gas stream. The gas stream flows through the high-pressure turbine, rotatably driving it and the high-pressure shaft that, in turn, rotatably drives the compressor.  
         [0003]     The gas stream is expanded as it flows through the low-pressure turbine. The low-pressure turbine rotatably drives the fan through a low-pressure shaft such that a low-pressure rotor is defined by the fan, the low-pressure shaft, and the low-pressure turbine. At least some known low pressure turbines include counter-rotating turbines that power counter-rotating fans and counter-rotating boosters and/or low pressure compressors.  
         [0004]     Improved engine efficiency and power output may depend on increased flexibility of design choices of each engine component. Specifically, the design of a low-pressure turbine may restrict available design choices for fans and/or boosters to be coupled to the low-pressure turbine. For example, at least some known low-pressure turbines include outlet guide vanes, which may be used to limit the tangential momentum of the combustion gas stream exiting the engine. Removing the outlet guide vanes decreases the overall engine weight, but may increase the detrimental effects of tangential momentum.  
         [0005]     Generally, operating known counter-rotating turbines when torque is split substantially equally between the forward and aft shaft shafts facilitates optimizing the efficiency of such turbines. However, improved engine performance may be achieved, for example, by operating the forward fan at a higher fan pressure ratio and/or higher rotational speed than the aft fan, providing boosters or flades (fan-on-blade) to the forward or aft fan shaft, or providing two rotors with an intervening stator on one shaft.  
         [0006]     Such operation, however, may result in a substantial non-equal torque demand between the counter-rotating rotors, such as, approximately 2:1 or greater. Such non-equal torque has not been attainable with known counter-rotating low-pressure rotor configurations.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0007]     In one embodiment, a method of assembling a gas turbine engine is provided. The method includes coupling a first turbine shaft that includes m rows of turbine blades within the gas turbine such that the first turbine shaft is rotatable in a first direction, and coupling a second turbine shaft that includes n rows of turbine blades within the gas turbine such that the second turbine shaft is rotatable in a second direction wherein a torque split between the first and second turbine shafts is substantially proportional to the number of rows of turbine blades on each shaft relative to a total number of rows of blades on both shafts, and wherein m and n are selected to provide a torque split between the first turbine shaft and second turbine shaft of greater than about 1.2:1.  
         [0008]     In another embodiment, a gas turbine engine turbine assembly is provided. The gas turbine engine turbine assembly includes a compressor; a high pressure turbine coupled to the compressor by a rotor shaft; and a low pressure turbine including at least one row of turbine stator blades spaced circumferentially apart and defining at least a portion of a flowpath extending through the low pressure turbine, the low pressure turbine further including a first rotor shaft coaxially aligned about second rotor shaft, the first rotor shaft including m first rows of turbine blades, the second rotor shaft including n second rows of turbine blades wherein m and n are different with respect to each other, the first rotor shaft rotatably coupled to a first compressor, the second rotor shaft rotatably coupled to a second compressor.  
         [0009]     In yet another embodiment, a gas turbine engine assembly including a counter-rotatable low-pressure turbine is provided. The gas turbine engine assembly includes a low pressure turbine flowpath, a first forward fan shaft including a forward fan coupled to a compressor end of the first forward fan shaft and m first low pressure turbine blade rows extending into the low pressure turbine flowpath, a second aft fan shaft coaxially aligned about a longitudinal axis with the first forward fan shaft, the second aft fan shaft including n second low pressure turbine blade rows extending into the low pressure turbine flowpath wherein m and n are positive integers representing a number of blade rows, and wherein m and n are different with respect to each other. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic illustration of an exemplary high bypass, counter-rotating turbofan gas turbine engine;  
         [0011]      FIG. 2  is an enlarged cross-sectional view of a prior art counter-rotating low pressure turbine assembly that may be used with a gas turbine engine similar to the gas turbine engine shown in  FIG. 1 ;  
         [0012]      FIG. 3  is a schematic diagram of the prior art counter-rotating low pressure turbine assembly shown in  FIG. 2 ;  
         [0013]      FIG. 4  is a schematic diagram of an exemplary counter-rotating low pressure turbine assembly having the same total number of low pressure turbine blade rows as the turbine shown in  FIG. 3 ;  
         [0014]      FIG. 5  is a schematic diagram of another exemplary counter-rotating low pressure turbine assembly; and  
         [0015]      FIG. 6  is a schematic diagram of another exemplary counter-rotating low-pressure turbine assembly. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]      FIG. 1  is a schematic illustration of an exemplary high bypass, counter-rotating turbofan gas turbine engine  10  having in serial flow communication an inlet  12  for receiving ambient air  14 , a counter-rotating fan assembly  15  including a forward fan  16  and an aft fan  17 , a compressor  18 , a combustor  20 , a high pressure turbine  22 , and a counter-rotating low pressure turbine assembly  23  that includes a forward fan low pressure turbine  24  and an aft fan low pressure turbine  25 . High pressure turbine  22  is coupled to compressor  18  by a high pressure shaft  26 , forward fan low pressure turbine  24  is coupled to forward fan  16  through a forward fan shaft  27 , and aft fan low pressure turbine  25  is coupled to aft fan  17  through an aft fan shaft  28 . In the exemplary embodiment, forward fan shaft  27  rotates about an axis of symmetry  32  extending from an upstream side  34  of engine  10  aft to a downstream side  36  of engine  10  coaxially with and radially inwardly from aft fan shaft  28 .  
         [0017]     In operation, air flows through counter-rotating fans  16  and  17  and a portion of the compressed air is supplied to high-pressure compressor  18 . The rest of the compressed air is bypassed around high-pressure compressor  18 . Highly compressed air is delivered to combustor  20 . Combustion gases  38  from combustor  20  propel turbines  22 ,  24 , and  25 . High-pressure turbine  22  rotates high pressure shaft  26  and high-pressure compressor  18 , while low pressure turbine  24  rotates forward fan shaft  27  and forward fan  16  about axis  32  and low-pressure turbine  25  rotates aft fan shaft  28 , and aft fan  17  about axis  32 .  
         [0018]      FIG. 2  is an enlarged cross-sectional view of a prior art counter-rotating low pressure turbine assembly  23  that may be used with a gas turbine engine similar to gas turbine engine  10  (shown in  FIG. 1 ). Low-pressure turbine assembly  23  includes a low-pressure turbine flowpath  200 . In the exemplary embodiment, inner forward fan shaft  27  and outer aft fan shaft  28  include counter-rotatable interdigitated low pressure inner and outer shaft turbines (shown generally at  24  and  25  in  FIG. 2 ), having low pressure inner and outer shaft turbine rotors  202  and  204 , respectively. Low-pressure inner and outer shaft turbine rotors  202  and  204  include low-pressure first and second turbine blade rows  206  and  208 , respectively, extending across low-pressure turbine flowpath  200 . Counter-rotatable low pressure inner and outer spools  210  and  212  include low pressure inner and outer shaft turbine rotors  202  and  204  drivingly coupled to forward and aft fan blade rows  16  and  17  by low pressure inner and outer shafts  27  and  28 , respectively.  
         [0019]     Inner forward fan shaft  27  and outer aft fan shaft  28  are at least partially rotatably aligned co-axially with and radially inward of high-pressure shaft  26 . In the exemplary embodiment, low pressure inner and outer shaft turbine rotors  202  and  204  each include four rows of low pressure first and second turbine blade rows  208  and  206 , respectively.  
         [0020]     In the exemplary embodiment, low pressure first and second turbine blade rows  206  and  208  are alternately interdigitated. In alternative embodiments, low pressure first and second turbine blade rows  206  and  208  may be only partially interdigitated or may be completely tandem.  
         [0021]     An aftmost or fourth row  218  of first low pressure turbine blade rows  206  is coupled to a rotating frame  220  that supports a radially outer turbine ring assembly  222  and is rotatably supported by a center frame  224  and a turbine aft frame  226 . Radially outer turbine ring assembly  222  includes three separate turbine rotor rings  228  from which the first three first low-pressure turbine blade rows  206  are supported respectively. Turbine rotor rings  228  are coupled together by bolted connections  230 . Low-pressure outer shaft turbine rotor  204  includes four low-pressure first turbine disks  232  that support low-pressure turbine blade rows  208 .  
         [0022]      FIG. 3  is a schematic diagram of prior art counter-rotating low-pressure turbine assembly  23  (shown in  FIG. 2 ). Items shown in  FIG. 2  that are also shown in  FIG. 3  are called out in  FIG. 3  using the same reference numbers used in  FIG. 2 . Accordingly, counter-rotating low pressure turbine assembly  23  includes inner forward fan shaft  27  and outer aft fan shaft  28  coupled to four low pressure first turbine blade rows  206  and four low pressure second turbine blade rows  208  that are alternately interdigitated. In the exemplary embodiment, there are no intervening stators within turbine assembly  23 .  
         [0023]     Blade rows  206  are mounted outward from flowpath  200  and may utilize conventional disk construction. Blade rows  208  are mounted inward from a rotating outer casing (not shown). Each of inner forward fan shaft  27  and outer aft fan shaft  28  include the same number of blade rows  206  and blade rows  208 , for example four rows, which is described as a four by four configuration. In alternative embodiments, a blade row may be added to forward fan shaft  27  such that a five by four, forward-to-aft configuration is formed. Similarly, a blade row may be eliminated from aft fan shaft  28 , which would result in a four by three configuration. Accordingly, an odd by even configuration from an even by even configuration or an even by odd configuration from an odd by odd configuration may be formed within the same basic low pressure turbine assembly architecture.  
         [0024]     For the above described configurations, because a net torque acting on forward fan shaft  27  and aft fan shaft  28  equals the change in angular momentum between the gas entering the most forward row of blade row  208  and the gas exiting blade row  206 , and assuming an equal change of tangential momentum for each blade row, a torque split between forward fan shaft  27  and aft fan shaft  28  may be determined to be 50/50, 56/44 and 57/43. Torque split or the ratio of torque supplied by each shaft relative to the total torque supplied may be determine using m/(m+n) and n/(m+n), where m and n are the number of blade rows coupled to each respective turbine shaft. As is apparent, the torque split between the shafts is limited to a relatively narrow range of possibilities because m and n can differ by only zero or one.  
         [0025]     In operation, counter-rotating fans with widely different torque requirements and/or additional boosters coupled to one fan shaft may not be able to be implemented optimally due to the torque split limitation of the counter-rotating turbines. Such narrow range of design possibilities may limit the efficiency and/or power boost possibilities due to the limited torque split available in prior art low-pressure turbines.  
         [0026]      FIG. 4  is a schematic diagram of an exemplary counter-rotating low pressure turbine assembly having the same total number of low pressure turbine blade rows  206  and  208  as turbine  23  (shown in  FIG. 3 ). However, in the exemplary embodiment, forward fan shaft  27  is configured with five low-pressure first turbine blade rows  206  and aft fan shaft  28  is configured with three low-pressure second turbine blade rows  208 . Two stator blade rows  402  have been added to this configuration. In the exemplary embodiment, stator blade rows  402  extend into flow path  200  from an inner non-rotating portion of the turbine assembly. Determining torque split, as described above for this configuration yields a torque split of 5/8:3/8, or 63/37. The torque split is significantly greater for this configuration than for the prior art turbine described above. Low pressure turbine blade rows  206  are interdigitated with low pressure turbine blade rows  208  and stator blade rows  402   
         [0027]      FIG. 5  is a schematic diagram of another exemplary counter-rotating low-pressure turbine assembly. Low-pressure turbine blade rows  206  and  208 , two stator blade rows  402  are shown interdigitated similarly as turbine  23  shown in  FIG. 4 . However, in this exemplary embodiment, stator blade rows  402  extend into flow path  200  from an outer non-rotating portion of the turbine assembly. Accordingly, outer shaft turbine rotor  204  includes both an inner rotating portion  502  and an outer rotating portion  504 . Determining torque split, as described above for this configuration yields a torque split of 5/8:3/8, or 63/37. In an alternative embodiment, forward fan shaft  27  is configured with six low pressure first turbine blade rows  206  and aft fan shaft  28  is configured with two low pressure second turbine blade rows  208  and may include four stator blade rows  402 . In this alternative embodiment, the torque split is 75/25. Such a torque split is useful to facilitate an engine design wherein fans and/or boosters of widely varying operating parameters may be used to optimize the operation of engine  10 .  
         [0028]      FIG. 6  is a schematic diagram of another exemplary counter-rotating low-pressure turbine assembly  600 . Low pressure turbine blade rows  206  and  208 , and two stator blade rows  402  that extend into flow path  200  from an outer non-rotating portion of the turbine assembly are shown interdigitated similarly as turbine  500  shown in  FIG. 5 . In the exemplary embodiment, forward fan shaft  27  is configured with three low pressure first turbine blade rows  208  interdigitated with two stator blade rows  402 , and aft fan shaft  28  is configured with five low pressure second turbine blade rows  208 . Determining torque split, as described above for this configuration yields a torque split of 3/8:5/8, or 37/63. In an alternative embodiment, forward fan shaft  27  is configured with two low pressure first turbine blade rows  206  and aft fan shaft  28  is configured with six low pressure second turbine blade rows  208  and may include four stator blade rows  402 . In this alternative embodiment, the torque split is 25/75. In a further alternative embodiment, stator blade rows  402  may extend into flowpath  200  from an inner non-rotating portion of turbine  600  and aft fan shaft  28  is split such that aft fan shaft  28  includes an inner shaft portion and an outer shaft portion.  
         [0029]     The exemplary embodiments described above illustrate a counter-rotating low pressure turbine having eight total rotating blades by way of illustration only and is not limiting as configurations of other numbers for low pressure turbine blade rows  206  and  208  may be applied to achieve a desired turbine torque split to accommodate a torque demand of wide variety of counter-rotating compressor arrangements.  
         [0030]     The above-described methods and apparatus are cost-effective and highly reliable methods and apparatus for providing a torque split between counter-rotating rotors in a low pressure turbine of a gas turbine engine that is greater than twenty percent. The methods provide for satisfying a plurality of compressor torque demands to facilitate optimal engine designs. Accordingly, the nonequal torque split methods and apparatus facilitates assembly, operation, and maintenance of machines, and in particular gas turbine engines, in a cost-effective and reliable manner.  
         [0031]     Exemplary embodiments of blade mapping method and system components 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. Each blade mapping system component can also be used in combination with other blade mapping system components.  
         [0032]     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.