Patent Publication Number: US-10330121-B2

Title: Systems and methods for axial compressor with secondary flow

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to compressors, and more particularly relates to systems and methods for an axial compressor with a secondary fluid flow to improve at least one of a performance and a stability of the axial compressor. 
     BACKGROUND 
     Compressors can be used in a variety of applications, and for example, compressors, such as axial compressors, may be part of a gas turbine engine. Generally, compressors include multiple stages, where each stage includes a rotor and a stator. In multistage compressors, there may be a progressive reduction in stage pressure ratio, such that a rear stage develops a lower pressure ratio than a first stage. As the performance of the compressor can be defined by the maximum overall pressure ratio that can be achieved for a given mass flow, the lower pressure ratio in the rear stage may limit the performance and stability of the compressor. 
     Accordingly, it is desirable to provide systems and methods for an axial compressor with a secondary fluid flow to improve at least one of a performance and a stability of the axial compressor. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
     SUMMARY 
     According to various embodiments, a compressor is provided. The compressor comprises a first stage having a first rotor and a first stator and a second stage downstream from the first stage in a direction of a fluid flow. The compressor also comprises a secondary flow system that directs fluid from the second stage into the first stator to improve at least one of a performance and a stability of the compressor. 
     A method of improving at least one of a performance and a stability of an axial compressor is provided according to various embodiments. The axial compressor includes a first stage upstream from a second stage in a direction of a main fluid flow. In one embodiment, the method includes receiving a secondary fluid having a first static pressure; and directing the secondary fluid into a first stator of the first stage to disrupt a main fluid flow through the first stator, the main fluid flow through the first stator having a second static pressure that is different than the first static pressure. 
     Also provided according to various embodiments is an axial compressor. The axial compressor comprises a first stage having a first rotor and a first stator and a second stage having a second rotor and a second stator. The second stage is downstream from the first stage in a direction of an air flow. The axial compressor also comprises a secondary air flow system that directs air adjacent to the second stator into the first stator to disrupt the air flow through the first stator. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The exemplary embodiments will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
         FIG. 1  is a schematic partially cut-away illustration of a gas turbine engine that includes an axial compressor with a secondary fluid flow in accordance with various embodiments; 
         FIG. 2  is a schematic cross-sectional illustration of the gas turbine engine of  FIG. 1 , taken along line  2 - 2  of  FIG. 1 ; 
         FIG. 3  is a schematic meridional sectional view through a portion of the axial compressor of  FIG. 1 ; 
         FIG. 4  is a detail cross-sectional view of a portion of the axial compressor of  FIG. 1 , as indicated by line  4 - 4  in  FIG. 1 ; 
         FIG. 5  is a simplified view of the cross-section of  FIG. 4 ; 
         FIG. 5A  is a further cross-sectional view of  FIG. 5 , taken along line  5 A- 5 A of  FIG. 5 ; and 
         FIG. 6  is a flowchart illustrating an exemplary method for improving at least one of a performance and a stability of the axial compressor. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the application and uses. 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. In addition, those skilled in the art will appreciate that embodiments of the present disclosure may be practiced in conjunction with any type of compressor, and that the axial compressor described herein is merely one exemplary embodiment of the present disclosure. It should be noted that many alternative or additional functional relationships or physical connections may be present in an embodiment of the present disclosure. As used herein, the term module refers to any hardware, software, firmware, electronic control component, processing logic, and/or processor device, individually or in any combination, including without limitation: application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
     For the sake of brevity, conventional techniques related to signal processing, data transmission, signaling, control, and other functional aspects of the systems (and the individual operating components of the systems) may not be described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent example functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in an embodiment of the present disclosure. 
     With reference to  FIGS. 1 and 2 , an exemplary gas turbine engine  10  is shown, which includes a secondary air flow system according to various embodiments. It should be noted that while the secondary air flow system is discussed herein with regard to a gas turbine engine  10 , the secondary air flow system can be employed with any suitable engine, such as a turbojet engine, a scramjet engine, an auxiliary power unit (APU), etc. Thus, the following description is merely one exemplary use of the secondary air flow system. In this example, the exemplary gas turbine engine  10  includes a fan section  12 , a compressor section  14 , a combustion section  16 , a turbine section  18 , and an exhaust section  20 . As the fan section  12 , the combustion section  16 , the turbine section  18  and the exhaust section  20  can be substantially similar to a fan section, combustion section, turbine section and exhaust section associated with a conventional gas turbine engine, the fan section  12 , the combustion section  16 , the turbine section  18  and the exhaust section  20  will not be discussed in great detail herein. In addition, although the figures shown herein depict an example with certain arrangements of elements, additional intervening elements, devices, features, or components may be present in an actual embodiment. It should also be understood that  FIGS. 1 and 2  are merely illustrative and may not be drawn to scale. In addition, while the fluid discussed herein is described as air, it should be noted that the various teachings of present disclosure is not so limited, but rather, any suitable fluid can be employed. 
     The fan section  12  includes a fan  22  mounted in a fan casing  24 . The fan  22  induces air from the surrounding environment into the engine and passes a fraction of this air toward the compressor section  14 . The compressor section  14  includes at least one compressor and, in this example, includes a low-pressure (LP) compressor  26  (may also be referred to as an intermediate-pressure (IP) compressor, a booster or T-stage) and a high-pressure (HP) compressor  28 . The LP compressor  26  raises the pressure of the air directed into it from the fan  22  and directs the compressed air into the HP compressor  28 . The LP compressor  26  and the HP compressor  28  may be axi-symmetrical about a longitudinal centerline axis C. The LP compressor  26  and the HP compressor  28  are mounted in a compressor casing  30  (hereinafter referred to as a shroud  30 ). 
     Still referring to  FIG. 2 , the combustion section  16  of gas turbine engine  10  includes a combustor  32  in which the high pressure air from the HP compressor  28  is mixed with fuel and combusted to generate a combustion mixture of air and fuel. The combustion mixture is then directed into the turbine section  18 . The turbine section  18  includes a number of turbines disposed in axial flow series.  FIG. 2  depicts a high pressure turbine  34 , an intermediate pressure turbine  36 , and a low pressure turbine  38 . While three turbines are depicted, it is to be understood that any number of turbines may be included according to design specifics. For example, a propulsion gas turbine engine may comprise only a high pressure turbine and a low pressure turbine. The combustion mixture from the combustion section  16  expands through each turbine  34 ,  36 ,  38 , causing them to rotate. As the turbines  34 ,  36 ,  38  rotate, each respectively drives equipment in the gas turbine engine  10  via concentrically disposed spools or shafts  40 ,  42 ,  44 . The combustion mixture is then exhausted through the exhaust section  20 . 
     With reference to  FIG. 3 , a schematic meridional sectional view through a portion of the HP compressor  28  is shown. In this example, the HP compressor  28  includes an axial compressor section  46  and a centrifugal compressor section  48 . The axial compressor section  46  includes one or more rotors  120 , one or more stators  122  and a secondary flow system or secondary air flow system  124  (schematically illustrated by reference numeral  124 ). The one or more rotors  120  and the one or more stators  122  are enclosed by the shroud  30  ( FIG. 2 ), and in one example, the secondary air flow system  124  can also be enclosed by the shroud  30 . The axial compressor section  46  can also include a strut  126  and an inlet guide vane system  128 . The centrifugal compressor section  48  can include an impeller  130 , a diffuser  132  and a deswirl section  134 . Since the strut  126 , inlet guide vane system  128 , impeller  130 , diffuser  132  and deswirl section  134  are generally known in the art, they will not be discussed in great detail herein. 
     With continued reference to  FIG. 3 , the axial compressor section  46  includes one or more compressor stages spaced in an axial direction along the longitudinal centerline axis C, with the one or more rotors  120  and the one or more stators  122  cooperating to define a stage. In one example, the axial compressor section  46  comprises a seven stage axial compressor. It should be noted, however, that the axial compressor section  46  can include any number of stages, and thus, the number of stages illustrated and described herein is merely exemplary. Furthermore, the secondary air flow system  124  can be employed with an axial compressor section  46  having any number of stages, and thus, it will be understood that the present teachings herein are not limited to an axial compressor section  46  having seven stages. 
     In this example, the one or more rotors  120  includes seven rotors  136 ,  137 ,  138 ,  139 ,  140 ,  141 ,  142  and the one or more stators  122  includes seven stators  144 ,  145 ,  146 ,  147 ,  148 ,  149 ,  150 . The seven rotors  136 - 142  and seven stators  144 - 150  cooperate to define seven stages of the axial compressor section  46 , with rotor  136  and stator  144  forming stage  1 , rotor  137  and stator  145  forming stage  2 , rotor  138  and stator  146  forming stage  3 , rotor  139  and stator  147  forming stage  4 , rotor  140  and stator  148  forming stage  5 , rotor  141  and stator  149  forming stage  6  and rotor  142  and stator  150  forming stage  7 . It should be noted that the number of rotors, number of stators and number of stages associated with the axial compressor section  46  is merely exemplary, as the axial compressor section  46  can include any number of rotors, stators and stages. In addition, it will be understood that the flow of air through the axial compressor section  46  is that viewed from the stator frame of reference. 
     With regard to  FIG. 4 , stage  6  and stage  7  of the axial compressor section  46  are shown in greater detail. As will be discussed in greater detail herein, in this example, the stage  6  and stage  7  flowfield of the axial compressor section  46  cooperate with the secondary air flow system  124 . It should be noted that while stage  6  and stage  7  are described and illustrated herein as cooperating with the secondary air flow system  124 , stage  1 , stage  2 , stage  3 , stage  4  and/or stage  5  can cooperate with the secondary air flow system  124 , if desired. Thus, the following description and the various teachings of the present disclosure are not limited to stage  6  and stage  7 . 
     With regard to  FIG. 4 , the rotors  141 - 142  each include a disk  154  and a plurality of blades  156 . The disk  154  of each of the rotors  141 - 142  are coupled to the shaft  44  associated with the gas turbine engine  10  ( FIG. 2 ). The shaft  44  rotates each of the rotors  141 - 142  at a desired speed. In this example, the disk  154  is annular and is coupled to the shaft  44  about a bore  160  defined along a central axis of the disk  154 . The disks  154  are sized and shaped to cooperate with fore and aft bearings as is generally known, to couple the respective rotor  141 - 142  to the shaft  44  for rotation. The disk  154  of each of the rotors  141 - 142  also defines a perimeter or circumference  162 . In this example, the blades  156  are coupled to the circumference  162  of the disk  154 . Generally, the blades  156  are formed or cast with the disk  154 , however, the blades  156  can be coupled to the disk  154  through a suitable technique, such as welding, or the individual blades  156  can be inserted into and retained in slots defined in the disk  154 . 
     The blades  156  are coupled to the disk  154  of each of the rotors  141 - 142  along the circumference  162  to turn and accelerate a fluid in the stator frame of reference, such as air, as the fluid moves through or past the blades  156 . It should be noted that this particular arrangement of the blades  156  on each of the rotors  141 - 142  is merely exemplary, as the rotors  141 - 142  can have any desired number and arrangement of blades  156  to turn and accelerate the fluid as desired. Further, it should be noted that the blades  156  accelerate the fluid from a stationary frame of reference or a stator frame of reference. The blades  156  of each of the rotors  141 - 142  extend outwardly, radially or in a direction away from the central axis of the rotors  141 - 142  towards a respective one of a sixth stage shroud housing  164  and a seventh stage shroud housing  166 . Thus, the sixth stage shroud housing  164  and the seventh stage shroud housing  166  can enclose a respective stage of the axial compressor section  46 . For example, the sixth stage shroud housing  164  can enclose the rotor  141  and the stator  149  (stage  6 ), and the seventh stage shroud housing  166  can enclose the rotor  142  and the stator  150  (stage  7 ). As will be discussed in greater detail below, at least the sixth stage shroud housing  164  cooperates with the secondary air flow system  124 . 
     With continued reference to  FIG. 4 , the sixth stage shroud housing  164  includes a rotor portion  168  and a stator portion  170 . In one example, the rotor portion  168  includes a mating extension  172  to couple the sixth stage shroud housing  164  to a corresponding extension  174  of the shroud  30 . The rotor portion  168  extends generally in an axial direction relative to the centerline C of the gas turbine engine  10  and substantially perpendicular to an axis of the blades  156 . The rotor portion  168  generally extends from an area adjacent to the extension  174  of the shroud  30  to an area adjacent to the stator  149 , and serves to substantially enclose the rotor  141 . 
     The stator portion  170  is coupled to the rotor portion  168  and to the stator  149 . In one example, the rotor portion  168  can be integrally formed with the stator portion  170 ; however, the rotor portion  168  and the stator portion  170  can comprise discrete components coupled together via a suitable technique, such as welding, mechanical fasteners, etc., if desired. The stator portion  170  substantially extends from the rotor portion  168  to a terminal end  176 . Generally, the terminal end  176  of the stator portion  170  lies in the same plane as an end  178  of the stator  149 . In this example, the terminal end  176  of the stator portion  170  is spaced a distance apart or away from the seventh stage shroud housing  166 , however, the sixth stage shroud housing  164  and seventh stage shroud housing  166  can be coupled together, if desired. 
     The stator portion  170  defines a plenum  180 . The plenum  180  is in communication with the secondary air flow system  124 , as will be discussed further herein. In one example, the plenum  180  includes a first side  182 , a second side  184  and a third side  186 , which cooperate to define a chamber over the stator  149 . It should be noted that the shape and number of sides associated with the plenum  180  is merely exemplary, as the plenum  180  can have any desired shape to facilitate a secondary air flow through the stator  149 . In addition, it should be noted that the use of the plenum  180  is merely exemplary. For example, a secondary air flow can be introduced into the stator  149  via any suitable technique, such as the use of a strut, tube or a pipe that directs a secondary air flow into the stator  149 . Thus, the secondary air flow need not be directed into one or more interior passages  191  of the stator  149 , as discussed further herein. Further, the secondary air flow need not be directed into the stator  149 . Rather, the secondary air flow can be directed in front of the stator  149 , in a direction substantially perpendicular to the main gas path air flow M to disrupt the flow of air through the stator  149 . 
     In this example, the first side  182  of the plenum  180  defines at least one conduit or tube  188 , which is in communication with a portion of the secondary air flow system  124  to receive air from the secondary air flow system  124 . In one example, the first side  182  can include two to four tubes  188  spaced apart along a perimeter or circumference of the first side  182 , however, it will be understood that the first side  182  can include any number of tubes  188 , such as a single tube  188 , in communication with the secondary air flow system  124 . In addition, it should be noted that while the tube  188  is illustrated herein as being defined near a middle of the first side  182 , the tube  188  can be defined through the second side  184 , if desired. Thus, the location of the tube  188  relative to the plenum  180  illustrated herein is merely exemplary. 
     The first side  182  is coupled to the second side  184  and the third side  186 . The second side  184  is adjacent to the rotor portion  168  and is coupled to the third side  186 . The third side  186  defines one or more openings  190  through which air from the plenum  180  can flow into one or more interior passages  191  in the stator  149 . In one example, the one or more openings  190  are substantially cylindrical, however, the one or more openings  190  can have any desired geometrical shape, such as rectangular, etc. Generally, the third side  186  can define about one opening  190  to about a number of openings  190  equal to a number of interior passages  191  defined in the stator  149  around a perimeter or a circumference of the third side  186  to enable air from the plenum  180  to enter the one or more interior passages  191  of the stator  149 . It should be noted that the number of openings  190  is merely exemplary, as the third side  188  can have any number of openings  190  based on the desired secondary air flow into the stator  149 . The third side  188  can be coupled to the stator  149 . 
     The seventh stage shroud housing  166  includes a rotor portion  192  and a stator portion  194 . In one example, the rotor portion  192  includes a mating extension  196  to couple the seventh stage shroud housing  166  to the corresponding extension  174  of the shroud  30 . The rotor portion  192  extends generally in an axial direction relative to the centerline C of the gas turbine engine  10  and substantially perpendicular to an axis of the blades  156 . The rotor portion  192  generally extends from an area adjacent to the extension  174  of the shroud  30  to an area adjacent to the stator  150 , and serves to substantially enclose the rotor  142 . 
     The stator portion  194  is coupled to the rotor portion  192  and to the stator  150 . In one example, the rotor portion  192  can be integrally formed with the stator portion  194 ; however, the rotor portion  192  and the stator portion  194  can comprise discrete components coupled together via a suitable technique, such as welding, mechanical fasteners, etc. The stator portion  194  substantially extends from the rotor portion  192  to a terminal end  197 . In this example, the terminal end  197  of the stator portion  194  extends outwardly or along an axis substantially transverse to a longitudinal axis of the stator portion  194 . 
     With continued reference to  FIG. 4 , the stator  149  is positioned between the rotor  141  and the rotor  142 , and is coupled to the stator portion  170  of the sixth stage shroud housing  164 . Generally, the stator  149  is positioned between the rotor  141  and the rotor  142  such that a first gap  198  is defined between the stator  149  and the rotor  141  and a second gap  200  is defined between the stator  149  and the rotor  142 . It should be noted that the first gap  198  between rotor  141  and the stator  149  need not be the same size or dimension as the second gap  200  between the rotor  142  and the stator  149 . The first gap  198  facilitates the movement of the rotor  141  relative to the stator  149 , and the second gap  200  facilitates the movement of the rotor  142  relative to the stator  149 . As will be discussed, the first gap  198  also enables a secondary air flow through the stator  149  to exit into a main gas path air flow M ( FIG. 3 ). 
     The stator  149  is fixed or stationary relative to the rotors  141 - 142 , and does not move or rotate with the shaft  44 . The stator  149  includes a hub  202 , one or more vanes  204  and in this example, the stator  149  is positioned above a rotating seal  206 . In one example, the hub  202  and the one or more vanes  204  can be integrally formed together, via a suitable casting process, but one or more of the hub  202  and the one or more vanes  204  can be formed as discrete components and coupled together through a suitable technique, such as welding, for example. The hub  202  can be substantially annular, and can comprise a ring. The hub  202  includes a perimeter or circumference  208 , and one or more openings  210  can be defined through the circumference  208 . 
     As will be discussed, the one or more openings  210  enable air from the secondary air flow system  124  to flow through one or more interior passages  191  in the stator  149  and into a hub cavity  213  defined between the hub  202  and the rotating seal  206 . It should be noted that the hub cavity  213  need not be defined by a rotating seal, and that a hub cavity can be defined by the hub  202  itself. Thus, the use of the rotating seal  206  is merely exemplary. Generally, the interior passages  191  in the stator  149  are defined through one or more of the vanes  204 . Stated another way, one or more of the vanes  204  of the stator  149  defines an interior passage  191 . In one example, the interior passage  191  extends from an end  204   a  of the vane  204  adjacent to the opening  190  to an end  204   b  of the vane  204  adjacent to the rotating seal  206 . It should be noted that while a single interior passage  191  is illustrated herein, the stator  149  can include any number of interior passages  191 , from one to about the number of vanes  204  associated with the stator  149 . Furthermore, the number of interior passages  191  need not be equal to the number of openings  190 , if desired. 
     The air from the secondary air flow system  124  flows through the interior passages  191 , into a hub cavity  213 , or the area defined between the hub  202  and the rotating seal  206 . In one example, the one or more openings  210  are substantially cylindrical, however, the one or more openings  210  can have any desired geometrical shape, such as rectangular, etc. Generally, the one or more openings  210  are defined through the circumference  208  such that a respective one of the openings  210  is aligned with a respective one of the interior passages  191  to ensure air flow through the hub  202  into the hub cavity  213 . Generally, the circumference  208  can define about one to about a number of openings  210  about equal to the number of vanes  204  to enable air from the stator  149  to enter the hub cavity  213 . It should be noted that the number of openings  210  is merely exemplary, as the circumference  208  can have any number of openings  210  based on the desired air flow through the stator  149 . Furthermore, as discussed previously, the secondary air flow can be introduced into the hub  202  of the stator  149  via any suitable technique, and thus, the secondary air flow need not be directed into one or more vanes  204  of the stator  149 . 
     The vanes  204  are coupled to the circumference  208  of the hub  202  and the stator portion  170  of the sixth stage shroud housing  164  at a first end  149   b  of the stator  149 . It should be noted that while the stator  149  is described herein as being coupled to the sixth stage shroud housing  164  at the first end  149   b , the stator  149  can be coupled to the axial compressor section  46  so as to be fixed via any suitable technique. The vanes  204  are coupled to the hub  202  of the stator  149  along the circumference  208 . The vanes  204  increase the static pressure of the air and direct or guide the air as the air moves through the vanes  204 . It should be noted that this particular arrangement of the vanes  204  on the stator  149  is merely exemplary, as the stator  149  can have any desired number and arrangement of vanes  204  to increase the static pressure of the air and direct or guide the air as desired. As discussed, one or more of the vanes  204  can include the interior passage  191 . The interior passage  191  permits a secondary air flow through the stator  149 , as will be discussed in greater detail herein. 
     The rotating seal  206  can be coupled to the disk  154  of the rotor  141  adjacent to the circumference  162  of the rotor  141 . It should be noted that the coupling of the rotating seal  206  to the rotor  141  is merely exemplary. In one example, the rotating seal  206  is coupled to the rotor  141  so as to be disposed a distance D away from the hub  202  of the stator  149  or from a second end  149   c  of the stator  149 . With reference to  FIGS. 4 and 5 , the rotating seal  206  serves to reduce a leakage of air around the stator  149 . The rotating seal  206  also redirects and controls the amount of the air from an exit of the stator  149  toward a front or first side  149   a  of the stator  149 . In this regard, in one example, the rotating seal  206  includes at least one seal  212 . In this example, the rotating seal  206  includes three seals  212 , which serve to substantially restrict a flow of air towards the rotor  142 . Stated another way, the seals  212  substantially control the amount of the air flow from the stator  149  towards the first side  149   a  of the stator  149  to reduce fluid leakage around the hub  202  of the stator  149 . 
     With continued reference to  FIG. 4 , the stator  150  is positioned adjacent to the rotor  142 , and is coupled to the stator portion  194  of the seventh stage shroud housing  166 . Generally, the stator  150  is positioned adjacent to the rotor  142  such that a third gap  214  is defined between the stator  150  and the rotor  142 . The third gap  214  allows the movement of the rotor  142  relative to the stator  150 . The stator  150  is fixed or stationary relative to the rotor  142 , and does not move or rotate with the shaft  44 . The stator  150  includes a hub  216  and one or more vanes  218 . In one example, the hub  216  and the one or more vanes  218  can be integrally formed together, via a suitable casting process, but one or more of the hub  216  and the one or more vanes  218  can be formed as discrete components and coupled together through a suitable technique, such as welding, for example. 
     The hub  216  can be substantially annular, and can comprise a ring. The hub  216  includes a perimeter or circumference  222 . The vanes  218  are coupled to the circumference  222  of the hub  216  and the stator portion  194  of the seventh stage shroud housing  166 . It should be noted that while the stator  150  is described herein as being coupled to the seventh stage shroud housing  166 , the stator  150  can be coupled to the axial compressor section  46  so as to be fixed or stationary relative to the rotor  142  via any suitable technique. The vanes  218  are coupled to the hub  216  of the stator  150  along the circumference  222 . The vanes  218  increase the static pressure of the air and direct or guide the air as the air moves through the vanes  218 . It should be noted that this particular arrangement of the vanes  218  on the stator  150  is merely exemplary, as the stator  150  can have any desired number and arrangement of vanes  218  to increase the static pressure of the air and direct or guide the air as desired. 
     With reference to  FIG. 3 , the secondary air flow system  124  directs air from a higher static pressure stage of the axial compressor section  46  into lower static pressure stage of the axial compressor section  46 . In this regard, the static pressure of the air in the axial compressor section  46  increases with each stage of the axial compressor section  46  (i.e. the static air pressure increases as the air flows downstream). Thus, the air in stage  2  has a higher static pressure than the air in stage  1 , the air in stage  3  has a higher static pressure than the air in stage  2  and stage  1 , the air in stage  4  has a higher static pressure than the air in stage  3 - 1 , the air in stage  5  has a higher static pressure than the air in stages  4 - 1 , the air in stage  6  has a higher static pressure than the air in stages  5 - 1  and the air in stage  7  has a higher static pressure than the air in stages  6 - 1 . By injecting higher static pressure air into a lower static pressure air flow at the hub of the respective stator  144 - 149 , the hub air flow in the lower static pressure stator  144 - 149  is disrupted, which causes the main gas path air flow M or the air flowing through the stator  144 - 149  from an upstream rotor  136 - 141  to be directed towards the terminal ends or tips of the respective blades of the respective rotor  138 - 142  of the adjacent stage. In this example, the secondary air flow system  124  will be described herein as directing higher static pressure air from stage  7  into the stator  149  of lower static pressure stage  6 . It should be understood that this particular example of the secondary air flow system  124  is merely exemplary, as the teachings of the secondary air flow system  124  can be applied or used to direct downstream air to any desired upstream stator  144 - 149  to disrupt or destabilize the flow of air through the hub of the respective upstream stator  144 - 149 . 
     For example, the secondary air flow system  124  can direct air from stage  7  into the stator  149  of stage  6 , the stator  148  of stage  5 , the stator  147  of stage  4 , the stator  146  of stage  3 , the stator  145  of stage  2  and/or the stator  144  of stage  1 . The secondary air flow system  124  can also direct air from stage  6  into the stators  148  of stage  5 , the stator  147  of stage  4 , the stator  146  of stage  3 , the stator  145  of stage  2  and/or the stator  144  of stage  1 . Further, the secondary air flow system  124  can direct air from stage  5  to the stator  147  of stage  4 , the stator  146  of stage  3 , the stator  145  of stage  2  and/or the stator  144  of stage  1 . Similarly, the secondary air flow system  124  can direct air from stage  4  to the stator  146  of stage  3 , the stator  145  of stage  2  and/or the stator  144  of stage  1 . The secondary air flow system  124  can also direct air from stage  3  to the stator  145  of stage  2  and/or the stator  144  of stage  1 . The secondary air flow system  124  can also direct air from stage  2  to the stator  144  of stage  1 . Thus, the following description is merely an exemplary embodiment for the secondary air flow system  124 . Moreover, while a single secondary air flow system  124  is described herein as directing fluid from a single high static pressure stage to a single low static pressure stage, the secondary air flow system  124  can direct air from a single high static pressure stage to multiple low static pressure stages. Thus, the secondary air flow system  124  is not limited to directing downstream fluid from a stage of the axial compressor section  46  to a single stage of the axial compressor section  46  upstream. Furthermore, the secondary air flow system  124  is not limited to directing air from a downstream stage to an adjacent upstream stage. Rather, the secondary air flow system  124  can direct higher static pressure air to any lower static pressure air stator  144 ,  145 ,  146 ,  147 ,  148 ,  149 . 
     Furthermore, the secondary air flow system  124  need not direct air from a stage of the axial compressor section  46  to an upstream stage of the axial compressor section  46 . Rather, with reference to  FIG. 5 , the secondary air flow system  124  can comprise a remote or external source  234  of higher static pressure air, which can be injected into a respective one of the stators  144 - 148 . The external source  234  is illustrated schematically in  FIGS. 4 and 5  as being outside of the shroud  30 , and thus, remote from the HP compressor section  28 . It will be understood, however, that the external source  234  can comprise a source of air external to the gas turbine engine  10  itself, and thus, the location of the external source  234  in  FIGS. 4 and 5  is merely exemplary. The external source  234  can be in communication with the tube  188  through any suitable device, such as a tube, strut, etc. to introduce the higher static pressure air into the plenum  180 . 
     In addition, it should be understood that the secondary air flow system  124  can include a valve  230  to control the flow of the air through the tube  188 . Generally, the valve  230  can comprise any suitable mechanical or electro-mechanical device that is movable between an opened position to allow the flow of air through the tube  188  and a closed position to prevent the flow of air through the tube  188 , and various positions there between, if desired, as known to those skilled in the art. In one example, the valve  230  can be disposed in the tube  188 , however, the valve  230  can be positioned at any desired location to control the flow of air into the plenum  180 . Further, the valve  230  can be in communication with a control module  232 , which is illustrated schematically in  FIGS. 4 and 5 . The control module  232  can be associated with or part of an engine control module for the gas turbine engine  10 , and thus, it should be noted that the location of the control module  232  in  FIGS. 4 and 5  is merely exemplary. Based on the receipt of sensor data measured and observed by one or more sensors associated with the axial compressor section  46  and/or the gas turbine engine  10 , input from other modules associated with the gas turbine engine  10  or upon the receipt of user input, the control module  232  can output the one or more control signals to the valve  230  to move the valve  230  between the opened position and the closed position. Thus, the secondary air flow system  124  can be controlled via the control module  232  and the valve  230  based on the requirements of the gas turbine engine  10 . It should be noted that the use of the valve  230  is merely exemplary, as the secondary air flow system  124  can be a passive system or can always be in operation (i.e. not controlled by a valve  230 ) so long as downstream higher static pressure air is available for use by the secondary air flow system  124 . 
     In the example of  FIG. 4 , the secondary air flow system  124  directs fluid into the stator  148  to disrupt the hub flow of air through the stator  148 , which in turn causes the air to flow towards an outboard region, a terminal end or tip  156   a  of the blades  156  of the rotor  142 , thereby decreasing the pressure gradient at the tip  156   a  of the rotor  142  and improving the range of the rotor  142  to stall. In this example, the secondary air flow system  124  includes a plenum  224 . It should be noted that the use of the plenum  224  is merely exemplary, as the secondary air flow system  124  can include any suitable passage or conduit for directing a secondary air flow into the tube  188 . The plenum  224  is defined by the rotor portion  192  and the stator portion  194  of the seventh stage shroud housing  166 , and a portion of the shroud  30 . For ease of understanding, the plenum  224  is illustrated in  FIG. 4  in broken lines, however, it will be understood that the plenum  224  is defined by the structure of the seventh stage shroud housing  166  and a portion of the shroud  30 . The plenum  224  is disposed adjacent to the stator  150  to receive a portion of the air exiting the stator  150 , which enters into the plenum  224  at a portion of the plenum  224  generally identified as  228 . 
     In this example, as air enters the axial compressor section  46  from the fan section  12  ( FIG. 2 ), with reference to  FIG. 3 , the air flows through the inlet guide vane system  128  and is turned and accelerated by the rotor  136  in the stator frame of reference. The air exiting the rotor  136  enters the stator  144 , and the stator  144  increases the static pressure of the air and directs the air into the rotor  137 . From the rotor  137 , the stator  145  further increases the static pressure of the air and directs the air into the rotor  138 . The rotor  138  further turns and accelerates the air, and the air enters the stator  146 . The stator  146  further increases the static pressure of the air, which is guided into the rotor  139 . The rotor  139  further turns and accelerates the air, and the air enters the stator  147 . The stator  147  increases the static pressure of the air, which is guided into the rotor  140 . From the rotor  140 , the air flows into the stator  148 . The stator  148  increases the static pressure of the air and guides the air into the rotor  141 . 
     With reference to  FIGS. 4 and 5 , the air turned and accelerated by the rotor  141  enters the stator  149  in a direction substantially perpendicular to a longitudinal axis L of the vanes  204 . Provided that air is available downstream, air enters the plenum  224  of the secondary air flow system  124  and flows through the plenum  224  to the plenum  180 . As the air exiting the stator  150  has a high static pressure, the air naturally flows into the plenum  224  without requiring additional features, such as a pump or flow guides, for example. The air from the plenum  180  exits the one or more openings  190  into the stator  149 , flows through the interior passages  191  and exits into the hub cavity  213  via the one or more openings  210  in the hub  202 . Thus, the secondary air flow system  124  directs higher static pressure air into the hub  202  of the stator  149 . From the hub cavity  213 , the air flows through the first gap  198  ( FIG. 4 ), and back into the stator  149  flowfield near the first side  149   a  of the stator  149  where the flow of the main gas path air flow M is intentionally disrupted. 
     With reference to  FIGS. 5 and 5A , a simplified view of  FIG. 4  is shown. In  FIGS. 5 and 5A , the rotors  141 - 142  have been removed to more clearly show the secondary air flow path through the secondary air flow system  124  into the hub  202  of the stator  149 . As shown in  FIGS. 5 and 5A , the air from the plenum  180  flows down through the stator  149 , substantially parallel to the longitudinal axis L of the stator  149 , and exits into the hub cavity  213  via the one or more openings  210 . From the hub cavity  213 , the air flows through the first gap  198  ( FIG. 4 ), and back into the stator  149  flowfield near the first side  149   a  of the stator  149  where the flow of the main gas path air flow M is intentionally disrupted. 
     With reference to  FIG. 4 , from the first side  149   a  of the stator  149 , the air is directed through the stator  149  into the rotor  142  and is displaced outward towards the outboard region and the tips  156   a  of the blades  156 . The rotor  142  turns and accelerates the air, which enters the stator  150 . The stator  150  further increases the static pressure of the air, and directs the air into the impeller  130  ( FIG. 3 ). A portion of the air from the stator  150  also enters the plenum  224  at  228 . 
     The secondary air flow system  124  decreases the pressure gradient acting on the outboard region and the tips  156   a  of the blades  156  of the rotor  142  by disrupting the air flow at the hub  202  of the stator  149  and moving the air flow in the stator  149  towards the outboard region and the tips  156   a  of the blades  156 . By disrupting the hub air flow through the stator  149 , the margin to stall of the rotor  142  is improved. In one example, the margin to stall of the rotor  142  is increased by about 3.0 percent (%) based on an increased flow of 1.0 percent (%) through the stator  149  from the secondary air flow system  124 . The increased margin to stall of the rotor  142  raises the pressure ratio that can be achieved for a given mass flow at stage  7  of the axial compressor section  46 , thereby improving at least one of the performance and the stability of the axial compressor section  46 . 
     Thus, according to various embodiments, with reference to  FIG. 6  and continuing reference to  FIGS. 1-6 , a method for improving at least one of the performance and the stability of the axial compressor section  46  is provided. It should be noted that as used herein, the term “stability” means the stall margin or stall line of the compressor. Thus, the method described and illustrated herein improves the stall margin or stall line of the axial compressor section  46 . In one example, the method starts at  300 . At  302 , the method receives a secondary fluid, such as air, having a first static pressure. For example, the air is from a downstream stage, such as stage  2 , stage  3 , stage  4 , stage  5 , stage  6  or stage  7  of the axial compressor section  46  and has a higher static pressure. At  304 , the method directs the secondary fluid, such as air, into the stator  144 ,  145 ,  146 ,  147 ,  148 ,  149  associated with an upstream stage (i.e. stage  1 , stage  2 , stage  3 , stage  4 , stage  5 , stage  6 ) to disrupt a main fluid flow through the stator  144 ,  145 ,  146 ,  147 ,  148 ,  149  in which the main fluid flow through the stator  144 ,  145 ,  146 ,  147 ,  148 ,  149  has a second static pressure, which is different than the first static pressure. For example, the main fluid flow through the upstream stator  144 ,  145 ,  146 ,  147 ,  148 ,  149  has a second static pressure that is less than the secondary fluid received downstream at the first static pressure. In one example, the method directs the fluid, such as air, into the stator  144 ,  145 ,  146 ,  147 ,  148 ,  149  associated with an upstream stage (i.e. stage  1 , stage  2 , stage  3 , stage  4 , stage  5 , stage  6 ) to disrupt a main gas path air flow M through a hub of the stator  144 ,  145 ,  146 ,  147 ,  148 ,  149 . The method can direct the secondary fluid into the stator  149  at any suitable position or location to disrupt the main gas path air flow M through the stator  149 , such as by directing secondary fluid into the stator  149  near the first side  149   a  of the stator  149 , near the first end  149   b  of the stator  149  or through the interior passages  191 , through the hub  202  into the hub cavity  213 . Thus, directing the secondary fluid into the stator  149  does not necessarily require the secondary fluid flow directly into the stator  149 , but the secondary fluid flow can be directed at the first side  149   a  of the stator  149  such that the secondary fluid flow disrupts the main gas path air flow M through the stator  149 . By disrupting the main gas path air flow M through the upstream stator  144 ,  145 ,  146 ,  147 ,  148 ,  149  the performance and/or the stability of the axial compressor section  46  is improved. The method ends at  306 . 
     It should be noted that while the secondary air flow system  124  has been described and illustrated herein for improving the performance and/or the stability of the axial compressor section  46 , the present teachings of this disclosure can be applied to other portions of the gas turbine engine  10  to improve a performance and/or a stability. For example, with reference to  FIG. 2 , a secondary air flow of downstream air, such as air from the HP compressor  28 , can be directed upstream into the fan  22 . The secondary air flow can be introduced into the fan  22  via any suitable technique, such as a bore, tube, strut, etc. As a further example, with continued reference to  FIG. 2 , a secondary air flow of downstream air, such as air from the HP compressor  28 , can be directed upstream into the LP compressor  26 . The secondary air flow can be introduced into the LP compressor  26  via any suitable technique, such as a bore, tube, strut, etc. 
     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 claim language. 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 language of the claim. The process steps may be interchanged in any order without departing from the scope of the invention as long as such an interchange does not contradict the claim language and is not logically nonsensical. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, 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 disclosure in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the disclosure as set forth in the appended claims and the legal equivalents thereof.