Abstract:
A counter-rotating blade stage in lieu of a stator stage may compensate for relatively low rotational speed of a gas turbine engine spool. A first spool may have at least one compressor blade stage and at least one turbine blade stage. A combustor is located between the at least one compressor blade stage and the at least one turbine blade stage along a core flowpath. The at least one counter-rotating compressor blade stage is interspersed with the first spool at least one compressor blade stage. A transmission couples the at least one additional compressor blade stage to the first spool for counter-rotation about the engine axis.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to gas turbine engines. More particularly, the invention relates to low pressure compressor sections of multi-spool turbofan engines. 
         [0002]    A gas turbine engine may have one or more spools. Each spool may, for example, include the blades of associated compressor and turbine sections and a connecting shaft. Two- and three-spool engines are known. An exemplary two-spool engine included low and high speed spools. At cruise conditions, exemplary low spool speeds are 2000-7000 revolutions per minute (RPM) while exemplary high spool speeds are 9000-21000 RPM. The low spool includes low speed/pressure compressor and turbine (LPC and LPT) blades while the high spool includes high speed/pressure compressor and turbine (HPC and HPT) blades. A core flowpath through the engine may sequentially pass through the LPC, HPC, combustor, HPT, and LPT. 
         [0003]    In a turbofan engine, a fan drives air along a bypass flowpath. In many engines, the fan may be mounted as part of the low speed spool and may be partially integrated with the LPC. In designing an engine, there may be a mismatch between desirable fan speed and desirable low spool speed. Particularly in high bypass turbofan engines it is advantageous that the fan speed be less than the low spool speed. In an exemplary intermediate speed compromise, the fan may be smaller than otherwise desired and the LPC and LPT may have a greater number of blade stages than otherwise desired. As an alternative, the low spool may drive the fan through a reduction gearing system (e.g., an epicyclic system). This permits the LPC and LPT to operate at the relatively high speeds at which they are efficient while the much larger diameter fan operates at the relatively lower speeds at which it is efficient. 
       SUMMARY OF THE INVENTION 
       [0004]    One aspect of the invention involves compensating for low spool speed by providing a compressor with at least one counter-rotating blade stage in lieu of a stator stage. Thus, a first spool may have at least one compressor blade stage and at least one turbine blade stage. A combustor is located between the at least one compressor blade stage and the at least one turbine blade stage along a core flowpath. The at least one counter-rotating compressor blade stage is interspersed with the first spool at least one compressor blade stage. A transmission couples the at least one additional compressor blade stage to the first spool for counter-rotation about the engine axis. 
         [0005]    In various implementations, the engine may be a turbofan. At least one compressor blade stage and at least one turbine blade stage may, respectively, be of a low speed/pressure compressor (LPC) and a low speed/pressure turbine (LPT). The engine may further have a high speed/pressure compressor (HPC) and a high speed/pressure turbine (HPT). The HPC, HPT, and LPC may be conventional sections having blades and stator vanes but lacking counter-rotating blades. 
         [0006]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a partial sectional view of a turbofan engine according to the present invention. 
           [0008]      FIG. 2  is an enlarged view of a low pressure compressor section of the engine of  FIG. 1 . 
           [0009]      FIG. 3  is a view of an alternate low pressure compressor. 
       
    
    
       [0010]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0011]      FIG. 1  shows a turbofan engine  20  having a central longitudinal axis  500 . The engine has a core flowpath  502  and a bypass flowpath  504 . The engine has a forward/inlet/upstream end  22  and an aft/outlet/downstream end  24 . At upstream ends of the core and bypass flowpaths, the engine has a fan  26  comprising a circumferential array of blades  28  extending from inboard platforms  30  to outboard tips  32 . The exemplary tips are closely spaced apart from the inboard surface of a shroud  36 . The shroud may be held by a circumferential array of struts  38  extending from a structural case  40 . 
         [0012]    Proceeding downstream along the core flowpath  502 , the engine has a low pressure compressor (LPC) section  50 , a high pressure compressor (HPC) section  52 , a combustor section  54 , a high pressure turbine (HPT) section  56 , and a low pressure turbine (LPT) section  58 . The exemplary engine includes a low speed spool shaft  60 . An exemplary high speed spool may be formed as a rotor stack (e.g., without a center tie spool) or may have a shaft. In an exemplary implementation, each of the HPC, HPT, and LPT may comprise a plurality of interspersed stages of rotating blades and non-rotating stator vanes. In the engine  20 , however, the LPC  50  comprises two groups of interspersed counter-rotating blade stages. A first group may be formed on the low speed spool. The second group may effectively replace one or more LPC stator stages. 
         [0013]    In the example of  FIG. 2 , a counter-rotating compressor spool  70  has blade stages  72 ,  74 , and  76  interspersed with blade stages  78  and  80  of the low speed spool. In the particular example of  FIG. 2 , from upstream-to-downstream, these are arranged as  72 ,  78 ,  74 ,  80 , and  76 . The counter-rotation of the spool  70  may be driven by a transmission  90 . The exemplary transmission  90  is an epicyclic transmission having a central externally-toothed sun gear  92  mounted to the shaft  60 . A circumferential array of externally-toothed idler gears  94  are engaged to the gear  92 . The exemplary gears  94  are carried on journals  96  carried by a carrier ring  98 . The exemplary carrier ring  98  is fixedly mounted relative to an engine static structure  100 . The static structure  100  is coupled to the shaft  60  via multiple bearing systems  102  and  104  to permit rotation of the shaft  60 . 
         [0014]    The transmission  90  further includes an internally-toothed ring gear  105  encircling and engaged to the gears  94 . The exemplary ring gear  105  is supported relative to the static structure by one or more bearing systems  106  and  108 . The exemplary transmission  90  causes a counter-rotation of the spool  70  relative to the low speed spool. In the example of  FIG. 2 , the fan blades  28  are mounted via a hub  120  to the shaft  60 . At an outboard rear end of the hub  120 , a blade platform ring  122  is secured (e.g., via a bolt circle  124 ). The platform ring  122  extends to an aft end  126 . 
         [0015]    An outboard surface  128  of the platform ring  122  locally forms an inboard boundary of the core flowpath  502 . The blades of stages  78  and  80  extend from inboard ends fixed to (e.g., unitarily cast/machined with or mounted to) the platform ring  122  to free outboard tips. In the example of  FIG. 2 , the blades of the downstreammost stage  76  of the spool  70  are mounted to an outboard end of a support  130 . The outboard ends of the blades of the stage  76  are secured relative to a shroud ring  132  (e.g., unitarily cast/machined with or mounted to). The inboard surface  133  of the shroud ring  132  forms a local outboard boundary of the core flowpath  502 . The exemplary shroud ring  132  has an aft/downstream end  134  adjacent the blade stage  76  and extends forward to an upstream end  136 . The outboard ends of the blades of the stages  72  and  74  are mounted to the shroud ring  132 . These blades have free inboard ends adjacent the platform ring  122  outboard surface  128 . The support  130  is affixed to the ring gear  105  to drive rotation of the blades of stage  76  and, through the shroud ring  132 , the blades of stages  72  and  74 . 
         [0016]    In an exemplary implementation, a ratio of the rotational speed of the spool  70  to that of the low spool is between −0.4:1 and −0.8:1, more narrowly, between −0.6:1 and −0.7:1. A speed ratio of the high spool relative to the low spool may be condition dependent. An exemplary ratio of high spool speed to low spool speed at steady-state cruise conditions is between 1.5:1 and 4.5:1, more narrowly, 2.5:1 and 4:1. 
         [0017]    In a reengineering situation relative to a baseline conventional turbofan engine, the number of LPC stages may be reduced. This may help create a more longitudinally compact engine. Engine weight may also be reduced as may part count. Reliability may potentially be increased. Additionally, the pressure ratio of the engine could be increased to increase thrust while maintaining the baseline engine length. 
         [0018]      FIG. 3  shows an alternate LPC  200  which may be otherwise similar to the LPC  50  but has a different transmission  202 . The transmission  202  may be otherwise similar to the transmission  90 . However, it has irrotatably connected pairs of idler gears  204  and  206 . The gears  204  engage the gear  92  whereas the gears  206  engage the ring gear  105 . The exemplary gears  206  are of larger diameter than their associated gears  204  so as to increase the rotational speed of spool  70  beyond the speed capability of a similarly dimensioned transmission  90 . An exemplary ratio of the rotational speed of the spool  70  to that of the low spool is between −0.7:1 and −2:1, more narrowly, between −1:1 and −1.25:1. 
         [0019]    An exemplary engine family could be provided wherein several otherwise similar or identical engines could have different gear ratios to provide different pressure ratios (and thus thrusts). In this family, increased pressure ratio and thrust would be associated with increased magnitude of the speed of the spool  70 . For example, amongst the family members, the aerodynamics of the LPC (e.g., blade count, blade size, and airfoil shape) could be preserved. Although the LPT could be similarly preserved, the changes in pressure ratio would tend to favor providing some corresponding LPT changes. 
         [0020]    Among alternative variations are geared turbofans wherein the counter-rotating spool is driven directly or indirectly by the fan transmission. 
         [0021]    In another exemplary reengineering situation, further changes may be made to the HPC and HPT. For example, by maintaining stage count or even adding stages to the LPC, the HPC may be unloaded. This facilitates reduction in the number of HPC stages and the associated HPC part count and cost. This would be appropriate in an extensive reengineering or a clean sheet engine design due to difficulties in removing stages from an existing compressor. 
         [0022]    One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when implemented as a reengineering of an existing engine configuration, details of the existing configuration may influence details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.