Abstract:
Methods and apparatus are provided for an axi-centrifugal compressor in a gas turbine engine for a business aviation or rotorcraft propulsion unit. The compressor includes an axial compressor section operable to affect a first pressure ratio along the flow path between a compressor inlet and a first section exit, and a centrifugal compressor section operable to affect a second pressure ratio along the flow path between a second section inlet and the compressor exit. The pressure rise across the axial and centrifugal compressor section is configured to have a tuning factor is in a range between 2.8 and 4.5 and a loading factor in a range between 0.6 and 0.8.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure generally relates to a gas turbine propulsion system, and more particularly relates to an axi-centrifugal compressor in which a proportioned distribution of the pressure rise across the axial and centrifugal compressor sections is achieved. 
       BACKGROUND 
       [0002]    Gas turbine propulsion systems for aircraft must deliver high performance in a compact, lightweight configuration. This is particularly important in smaller jet propulsion systems typically used in regional and business aviation applications as well as in other turbofan, turboshaft, turboprop and rotorcraft applications. A well-known way to improve engine efficiency is to increase the overall pressure rise of the compressor. However, this is typically done by increasing the number of compressor stages, which increases the volume, weight, and cost of the engine; all of which reduce the value and competitive position of the engine. Larger commercial transport engines typically utilize “all-axial” high pressure compressors. Relative to the larger commercial transport engines, these relatively-smaller propulsion engines more frequently utilize “axi-centrifugal” compressors. Compressors of this type typically include one or more axial compressor stages followed by a centrifugal stage. The centrifugal compressor provides a second mechanism to increase overall pressure rise but often results in an increase of the diameter of the impeller in the centrifugal stage, resulting in increased weight and engine size. 
         [0003]    Moreover, axial and centrifugal compressors have different optimal operating regimes, different physical characteristics, and different sensitivities to geometric and/or operational variation. As a result, conventional gas turbine propulsion systems pursue a very high level of aerodynamic loading in the centrifugal compressor section, relative to the axial compressor section, in an attempt to benefit from the centrifugal stabilizing effect and performance robustness. Unfortunately, this may require a relatively large and heavy centrifugal compressor section to achieve the overall pressure rise across the compressor. 
         [0004]    Thus, there is a need for an axi-centrifugal compressor configuration, which strategically distributes the pressure rise between the axial and centrifugal compressor sections so that the compressor may deliver the overall pressure rise with fewer stages, lighter weight, and lower cost than previously available without compromising other critical performance characteristics of the engine such as compressor stability and durability. A compressor section of this type in a gas turbine propulsion system will meet the demands for a new level of performance for a smaller propulsion engine to robustly provide a high level of overall pressure rise with the highest efficiency and stability, while concurrently decreasing parts count, length, weight, and cost. 
       BRIEF SUMMARY 
       [0005]    This summary is provided to describe select concepts in a simplified form that are further described in the Detailed Description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
         [0006]    The present disclosure provides an axi-centrifugal compressor configuration having a proportioned pressure ratio distribution across the compressor. In accordance with the present disclosure, a relatively high level of pressure rise is provided on the axial portion of the compressor, which leads to a reduction in the diameter and weight of the centrifugal stage compared to conventional compressor designs. A high level of pressure rise is also provided on each individual axial stage to minimize the stage count, which leads to a shorter length, lighter weight, lower cost, and high performance compared to conventional compressor designs. Aerodynamic over-loading of the axial stages that would otherwise result in boundary layer separations, low efficiency, and poor compressor operability is minimized. 
         [0007]    An axi-centrifugal compressor is provided in a gas turbine propulsion system typically used in regional and business aviation applications as well as in other turbofan, turboshaft, turboprop and rotorcraft applications (e.g., less than 15 klbf Sea Level Take Off Thrust). The compressor is rotatably supported on a shaft assembly in a housing and operable to affect a pressure ratio along a flow path between a compressor inlet and a compressor exit. An axial compressor section having one or more axial stages is operable to affect a first pressure ratio (PR ax ) along the flow path between the compressor inlet and a first section exit. A centrifugal compressor section is operable to affect a second pressure ratio (PR c ) along the flow path between a second section inlet and the compressor exit. The first section exit associated with the axial compressor section is in fluid communication with the second section inlet associated with the centrifugal section along the flow path. The compressor has a tuning factor defined as PR ax /PR c  in the range between 2.8 and 4.5 and a loading factor defined as (PR ax ) 1/n /PR c  in the range between 0.6 and 0.8, where n is the number of stages in the axial compressor section. 
         [0008]    A method is also provided for compressing a fluid along a flow path in a gas turbine propulsion system typically used in regional and business aviation applications as well as in other turbofan, turboshaft, turboprop and rotorcraft applications (e.g., less than 15 klbf Sea Level Take Off Thrust). The fluid is drawn in at a first inlet pressure along the flow path through a first inlet. The fluid is compressed along the flow path in an axial compressor section having one or more axial stages downstream from the first inlet to a first exit in the axial compressor section at a first outlet pressure. A first pressure ratio (PR ax ) is affected across the axial compressor section. The fluid is communicated from the first exit into a second inlet along the flow path at a first outlet pressure. The fluid is compressed along the flow path in a centrifugal compressor section from the second inlet to a second exit. A second pressure ratio is affected across the centrifugal compressor section. The pressure rise through the compressor is distributed between the axial and centrifugal compressor sections such that a tuning factor defined as PR ax /PR c  is in the range between 2.8 and 4.5 and a loading factor defined as (PR ax ) 1/n /PR c  is in the range between 0.6 and 0.8, where n is the number of stages in the axial compressor section. 
         [0009]    Furthermore, other desirable features and characteristics of the apparatus and method will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the preceding background. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The present disclosure will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
           [0011]      FIG. 1  is a simplified cross-sectional side view of a gas turbine engine according to an exemplary embodiment; and 
           [0012]      FIG. 2  is a partial cross-sectional view of an axi-centrifugal compressor rotatably supported on a shaft assembly suitable for use with the engine of  FIG. 1  in accordance with an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” Thus, any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. All of the embodiments described herein are exemplary embodiments provided to enable persons skilled in the art to make or use the invention and not to limit the scope of the invention which is defined by the claims. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary, or the following detailed description. 
         [0014]    Broadly, exemplary embodiments discussed herein include an axi-centrifugal compressor configuration having an proportioned pressure ratio distribution across the compressor. A relatively high pressure rise is provided on the axial portion of the compressor and across each stage in the axial compressor section. In addition, the aerodynamic over-loading in each axial stage is minimized. In this way, a proportioned pressure rise can be achieved in a small, light-weight and cost-effective manner. 
         [0015]    Reference now is made to the drawings in which  FIG. 1  shows a simplified, cross-sectional view of a gas turbine engine  100  according to an embodiment. The engine  100  may be disposed in an engine housing  110  and may include an intake section  115 , a compressor section  120 , a combustion section  130 , a turbine section  140 , and an exhaust section  150 . The compressor section  120 , turbine section  140  and exhaust section  150  are operably coupled to a shaft assembly  160  for rotation within the housing  110 . A fluid is drawn into the engine housing  110  through the intake section  115  and into the compressor section  120 . The compressor section  120  includes an axial compressor  122  which may include one or more axial stages and a centrifugal compressor  124  that increases the pressure of the fluid entering the engine  100 . Compression of the fluid may also result in heating of the fluid through the compressor section  120 . The compressor section  120  is in fluid communication with the combustion section  130  and directs the compressed fluid into a combustion chamber where the compressed fluid is mixed with fuel and combusted in the combustion chamber. Hot exhaust fluids are then directed into the turbine section  140 . The hot exhaust fluids expand through and rotate the turbine section  140  prior to being exhausted through the exhaust section  150  of the engine  100 . 
         [0016]    The turbine section  140  rotates to drive equipment in the engine  100  via rotors or spools concentrically disposed about an axis of rotation  170  within the shaft assembly  160 . Specifically, the turbine section  140  may include one or more rotors  142 ,  144  driven by the expanding exhaust fluids to rotate to the shaft assembly  160  and drive the compressor section  120  including the axial compressor section  122  and the centrifugal compressor section  124 . While  FIG. 1  depicts an exemplary configuration having an axi-centrifugal compressor core in a turboshaft engine, other embodiments may have alternate configurations. Thus, the exemplary embodiments discussed herein are not intended to be limited to a turboshaft engine, but rather may be readily adapted for use in other types of turbine engines including but not limited to turbofan and turboprop engines. 
         [0017]      FIG. 2  is a more detailed partial cross-sectional view of the compressor section  120  and a portion of the shaft assembly  160  of the engine  100  shown in  FIG. 1  in accordance with an exemplary embodiment. In  FIG. 2 , only half the cross-sectional view of the compressor section  120  is shown; the other half would be substantially rotationally symmetric about a centerline and axis of rotation  170 . Additionally, certain aspects of the engine  100  may not be shown in  FIG. 2 , or only schematically shown, for clarity in the relevant description of exemplary embodiments. The compressor section  120  defines a meanline flow path  180  (indicated by the long dash - short dash - short dash line) from a compressor inlet  182  through the axial compressor section  122  and the centrifugal compressor section  124  to a compressor exit  184 , which is in fluid communication with the combustion section  130  (not shown in  FIG. 2 ). As shown in  FIG. 2 , the axial compressor section  122  leads into the centrifugal compressor section  124  such that an exit  186  of the axial compressor section  122  is in fluid communication with an inlet  186  of the centrifugal compressor  124 . One skilled in the art will understand that  FIG. 2  illustrates a simplified cross-section through the compressor section  120 , and that other features may be included in the compressor section  120  along the flow path  180  as dictated by the specification and constraints associated with a particular intended use and without departing from the spirit and scope of the subject matter disclosed and claimed herein. 
         [0018]    The axial compressor section  122  progressively compresses fluids flowing generally axially (i.e., parallel to axis  170 ) along the flow path  180 . The axial compressor section  122  may include one or more axial compressor stages  122 . 1 ,  122 . 2 ,  122 . 3 . For example, as shown in  FIG. 2 , the axial compressor section  122  includes one or more stator assemblies  190 ,  192 ,  194 ,  196  and one or more blade assemblies  200 ,  202 ,  204 . The stator assemblies  190 ,  192 ,  194 ,  196  may include a plurality of stator vanes arranged in one or more vane rows which are stationary with respect to the engine housing  110  and function to diffuse and direct the fluid through the flow path  180 . The rotor assemblies  200 ,  202 ,  204  may include a plurality of rotor blades extending from a rotor hub  206  into the flow path  180  and configured in one or more blade rows which are rotatably driven on the shaft assembly  160 . As the rotor assemblies  200 ,  202 ,  204  rotate, the fluid flowing across each blade row are incrementally compressed along the flow path  180 . One skilled in the art will understand that the present disclosure is not limited to the specific number and arrangement of stator and rotor assemblies illustrated in  FIG. 2 , and that other configurations for the axial compressor section  122  are contemplated within the scope of the subject matter described and claimed herein. Modifications may include but are not limited to the use of multiple stators or double row stators within a given axial compressor stage, variable stator vanes, or struts. 
         [0019]    The centrifugal compressor section  124  compresses the fluid and directs the flow radially outward (i.e., in a direction which increases in a radial direction away from the axis  170 ) through an impeller assembly  210  driven on the shaft assembly  160 . The rotor assemblies  200 ,  202 ,  204  and the impeller assembly  210  shown in  FIG. 2  are coupled to a common drive shaft for co-rotation. However, one skilled in the art will understand that other drive shaft configurations may be used for operably coupling the axial compressor section  122  and the centrifugal compressor section  124  at various drive ratios. For example, one or more of the rotor assemblies  200 ,  202 ,  204  may be coupled for co-rotation, while the remaining rotor assemblies  200 ,  202 ,  204  and the impeller assembly  210  are not coupled for co-rotation. Alternately, one or more rotor assemblies  200 ,  202 ,  204  and the impeller assembly  210  may be coupled for co-rotation. Alternately, the shaft assembly  160  may include counter rotating shafts for the axial compressor section  124  and the centrifugal compressor assembly  126 , or for the various axial compressor stages  122 . 1 ,  122 . 2 ,  122 . 3 . 
         [0020]    As indicated above, the axial compressor section  122  includes a first stage  122 . 1  immediately downstream of the compressor inlet  182 , a second stage  122 . 2  downstream of the first stage  122 . 1 , and a third stage  122 . 3  downstream of the second stage  122 . 2 . Each of the axial compressor stages  122 . 1 ,  122 . 2 ,  122 . 3  contributes to a pressure rise from the compressor inlet  182  to the axial compressor exit  186 . The performance of the axial compressor section  122  can be characterized according to a first pressure rise (TPR A ) and a first pressure ratio (PR ax ) across the axial compressor section  122 , as well as the pressure ratio per axial stage  122 . 1 ,  122 . 2 ,  122 . 3  (PR/stage ax) as provided below: 
         [0000]    
       
      
       TPR 
       A 
       =PE 
       1 
       −PI 
       1  
      
     
         [0000]    
       
      
       PR 
       ax 
       =PE 
       1 
       /PI 
       1  
      
     
         [0000]        PR /stage  ax =( PE   1   /PI   1 ) 1/n    
         [0021]    wherein:
       PI 1  is the pressure at the compressor inlet  182 ;   PE 1  is the pressure at the axial compressor exit  186 ; and   n is the number of axial stages.       
 
         [0025]    Likewise, the performance of the centrifugal compressor section  124  can be characterized according to a second pressure rise (TPR c ) and a second pressure ratio (PR c ) across the centrifugal compressor section  124  as provided below: 
         [0000]    
       
      
       TPR 
       c 
       =PE 
       2 
       −PI 
       2  
      
     
         [0000]    
       
      
       PR 
       c 
       =PE 
       2 
       /PI 
       2  
      
     
         [0026]    wherein:
       PI 2  is the pressure at the centrifugal compressor inlet  186 ; and   PE 2  is the pressure at the centrifugal exit  184 .       
 
         [0029]    As noted above, operation of the compressor section  120 , and particularly the contribution of the axial compressor section  122  and the centrifugal compressor section  124  are proportioned, while the aerodynamic over-loading of the axial stages  122 . 1 ,  122 . 2 ,  122 . 3  is minimized. Specifically, the compressor  120  has a tuning factor which satisfies the following condition: 
         [0000]    
       
         
           
             
               2.8 
               &lt; 
               
                 
                   PR 
                   ax 
                 
                 
                   PR 
                   c 
                 
               
             
             = 
             
               
                 
                   
                     PE 
                     1 
                   
                    
                   
                     / 
                   
                    
                   
                     PI 
                     1 
                   
                 
                 
                   
                     PE 
                     2 
                   
                    
                   
                     / 
                   
                    
                   
                     PI 
                     2 
                   
                 
               
               &lt; 
               4.5 
             
           
         
       
     
         [0000]    and a loading factor which satisfies the following condition: 
         [0000]    
       
         
           
             
               0.6 
               &lt; 
               
                 
                   PR 
                    
                   
                     / 
                   
                    
                   stage 
                    
                   
                       
                   
                    
                   ax 
                 
                 
                   P 
                   Rc 
                 
               
             
             = 
             
               
                 
                   
                     ( 
                     
                       
                         PE 
                         1 
                       
                        
                       
                         / 
                       
                        
                       
                         PI 
                         1 
                       
                     
                     ) 
                   
                   
                     1 
                     n 
                   
                 
                 
                   
                     PE 
                     2 
                   
                    
                   
                     / 
                   
                    
                   
                     PI 
                     2 
                   
                 
               
               &lt; 
               0.8 
             
           
         
       
     
         [0000]    While a tuning factor in the range between 2.8 and 4.5 provides advantages described herein, additional advantages may be gained for a tuning factor in the range between 3.5 and 4.0. Likewise, while a loading factor in the range between 0.6 and 0.8 provide advantages described herein, additional advantages may be gained for a loading factor in the range between 0.65 and 0.75. 
         [0030]    To satisfy these conditions, a proportioned distribution of pressure rise across the axial stage compressor  122  and the centrifugal compressor  124 , as well as across axial stage  122 . 1 ,  122 . 2 ,  122 . 3  (i.e., stage matching) must be achieved. Moreover, the span-wise gradient of pressure rise across each axial stage should be configured to maximize the overall pressure rise attained. While a basic axial compressor section  122  has been illustrated and described herein, one skilled in the art will understand that additional compressor elements may be included to ensure that the axi-centrifugal compressor  120  satisfies the tuning and loading factors over the range of expected operating conditions. For example, the use of stability enhancing devices such as rotor tip casing treatments; variable stagger inlet guide vanes and stators; boundary layer separation control devices such as fluidic actuators (suction and blowing), vortex generators and plasma actuators placed on the end walls and/or airfoils to control destabilizing boundary layer separations; and airflow bleed (in or out) may be used to modify the stage matching at different operating conditions. 
         [0031]    A compressor section  120  satisfying both of these conditions effectively distributes the pressure rise among the axial compressor stage  122  and the centrifugal compressor section  124  so that the compressor section  120  may achieve the desired overall pressure rise with fewer stages, lighter weight and lower cost as compared to conventional compressor sections. In particular, a compressor section  120  with a tuning factor within a range between 2.8 and 4.5 and a loading factor within a range between 0.6 and 0.8 achieves a relatively high pressure rise across the axial compressor section by providing a high level of pressure rise on each individual axial stage, while avoiding detrimental aerodynamic over-loading of any axial stage. The result is a compact and efficient compressor section in which the axial compressor section is shorter, lighter, lower cost and higher performance, and the centrifugal compressor section is smaller in diameter and lower in weight. 
         [0032]    Unless otherwise explicitly indicated, the term “pressure” as used herein is intended to mean a total pressure at a given location, for example the total pressure at an inlet or exit of either the axial compressor section or the centrifugal compressor section. “Total pressure” refers to the sum of static pressure and dynamic pressure as expressed by Bernoulli&#39;s principle. Any contribution attributable to gravitational head may also be included in the “total pressure.” Accordingly, the terms “pressure rise” and “pressure ratio” are also considered in terms of total pressures, unless explicitly indicated otherwise. 
         [0033]    In this document, relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Numerical ordinals such as “first,” “second,” “third,” etc. simply denote different singles of a plurality and do not imply any order or sequence unless specifically defined by the context in which it is used. The sequence of the text in any of the claims does not imply that process steps must be performed in a temporal or logical order according to such sequence unless it is specifically defined by the context in which it is used. The process steps may be interchanged in any order without departing from the scope of the invention, provided an interchange in order does not contradict the claim language and is not logically nonsensical. 
         [0034]    Furthermore, depending on the context, words such as “connect” or “coupled to” used in describing a relationship between different elements do not imply that a direct physical connection must be made between these elements. For example, two elements may be connected to each other physically, electronically, logically, or in any other manner, through one or more additional elements. 
         [0035]    While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.