Abstract:
A compressor includes: a compressor spool rotatable about an axis carrying axially-spaced-apart blade rows of compressor blades; a casing surrounding the compressor blades, the casing carrying a liner assembly defining a boundary of a primary compressor flowpath; and a plurality of axially-spaced-apart stator rows of stator vanes carried by the liner assembly, the stator rows alternating axially with the blade rows. At least some of the stator rows are variable stator rows, the stator vanes of which are mounted on trunnions passing through the casing, and are pivotable relative to the casing. An actuator arm is coupled to each trunnions, outside the casing. At least one first bleed slot passes through the liner structure between axially adjacent first and second ones of the variable stator rows; and a first flow path defined by the casing communicates with the first bleed slot and with the exterior of the casing.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to thermodynamics in gas turbine engines and more particularly to apparatus for extracting bleed air in such engines. 
     A gas turbine engine includes a turbomachinery core having a high pressure compressor, combustor, and high pressure turbine in serial flow relationship. The core is operable in a known manner to generate a primary flow of propulsive gas. A typical turbofan engine adds a low pressure turbine driven by the core exhaust gases which in turn drives a fan rotor through a shaft to generate a bypass flow of propulsive gas. In the case of a high bypass engine this provides the majority of the total engine thrust. 
     A typical axial flow high pressure compressor in such an engine includes a number of stages. Each stage has a row of rotating airfoils or blades and row of stationary airfoils or vanes. The vanes serve to turn the airflow exiting an upstream row of blades before it enters the downstream row of blades. It is known to construct one or more rows of vanes so that their angle of incidence can be changed in operation. These are referred to as variable stator vanes or simply “VSVs”. The VSVs enable throttling of flow through the compressor so that it can operate efficiently at different flow rates, without the losses incurred by other mechanisms such as bleed valves. Because of high overall pressure ratios and stage count in many compressors, there will often be many stages of VSVs. 
     It is known to extract high-pressure compressed air from the high pressure compressor. This referred to as “bleed air” and may be used for purposes such as engine or aircraft anti-icing, boundary layer control devices, aircraft environmental control systems and the like. For optimal engine performance, bleed should occur at the stage that provides the minimum source pressure the user requires. However, in the prior art, sources have been limited to stages aft of the last VSV stage, because of the structural difficulty of extracting air from the VSV stages. Thus, the only conveniently available bleed source is at an undesirably high pressure. 
     Accordingly, there is a need for a compressor which allows air to be bled from the VSV stages. 
     BRIEF DESCRIPTION OF THE INVENTION 
     This need is addressed by the present invention, which provides a compressor bleed apparatus which bleeds air between VSV stages and provides a path for the air to be extracted clear of structure external to the compressor casing. 
     According to one aspect of the invention a compressor apparatus includes: a compressor spool mounted for rotation about a centerline axis and carrying a plurality of axially-spaced-apart blade rows, each blade row including an annular array of airfoil-shaped compressor blades; a casing surrounding the compressor blades, the casing carrying a liner assembly which defines a boundary of a primary gas flowpath through the compressor; a plurality of axially-spaced-apart stator rows carried by the liner assembly, each stator row including an annular array of airfoil-shaped stator vanes, wherein the stator rows alternate axially with the blade rows, wherein at least some axially adjacent ones of the stator rows are variable stator rows, the stator vanes of which are mounted on trunnions passing through the casing, so as to be pivotable relative to the casing; an actuator arm coupled to each of the trunnions, outside the casing; at least one first bleed slot passing through the liner structure between axially adjacent first and second ones of the variable stator rows; and a first flow path defined by the casing, communicating with the at least one first bleed slot and with the exterior of the casing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
         FIG. 1  is a half cross-sectional view of a high pressure compressor of a gas turbine engine, constructed in accordance with an aspect of the present invention; 
         FIG. 2  is an enlarged view of a portion of  FIG. 1 ; 
         FIG. 3  is a half cross-sectional view of an alternative high pressure compressor of a gas turbine engine, constructed in accordance with an aspect of the present invention; 
         FIG. 4  is a half cross-sectional view of another alternative high pressure compressor of a gas turbine engine, constructed in accordance with an aspect of the present invention; 
         FIG. 5  is an enlarged view of a portion of  FIG. 4 , showing an optional duct and sleeve; 
         FIG. 6  is a perspective view of a portion of the compressor of  FIG. 4 ; and 
         FIG. 7  is a cross-sectional view taken along lines  7 - 7  of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  depicts a portion of a high pressure compressor  10 , which is part of a gas turbine engine as described above. The compressor  10  includes an axially-elongated annular spool  12  mounted for rotation about a centerline axis “A”. The spool  12  may be built up from several smaller components. In accordance it includes one or more drum portions  14  and several annular disks  16  which all rotate together as a unit. The spool  12  is depicted in half-section but it will be understood that it is a body of revolution. Several blade rows are carried at the outer periphery of the spool  12 . Each blade row comprises an annular array of airfoil-shaped compressor blades  18  which extend radially outward from the spool  12 . An annular liner assembly  20  closely surrounds the compressor blades  18  and defines the radially outer boundary of a primary gas flowpath through the compressor  10 . The liner assembly  20  is built up from a plurality of smaller components, some of which will be described in more detail below. An annular casing  22  surrounds the liner assembly  20  and provides structural support to it. Several stator rows are carried by the liner assembly  20 . Each stator row comprises an annular array of airfoil-shaped stator vanes  24  which extend radially inward from the liner assembly  20 . The stator rows alternate with the blade rows in the axial direction. Each blade row and the axially downstream stator row constitute a “stage” of the compressor  10 . In operation, the compressor  10  draws in air (from the left side of the figure) and compresses it as it pumps it axially downstream, towards the right side of the figure. Each stage contributes an incremental pressure rise to the air, with the highest pressure being at the exit of the last stage. 
     In the illustrated example, only some of the stages of the compressor  10  are shown. The stages forward and aft of those illustrated are not of consequence to the invention. The stages which are shown are labeled sequentially “S1” through “S7”. These numbers are used solely for the sake of easy reference and do not necessarily correspond to the actual number of the stages in the complete compressor  10 . The four stages S1 through S4 shown on the left side of the figure (towards an inlet end of the compressor  10 ) incorporate variable stator vanes or simply “VSVs”; the stator vanes  24  of these stages are constructed so that their angle of incidence can be changed in operation (i.e. these stator vanes  24  can be pivoted about the radial axes shown in dashed lines). The remaining stages to the right side of the figure (towards an exit end of the compressor) do not incorporate VSVs. It is noted that the principles of the present invention are generally applicable to any axial compressor having two or more axially adjacent stages of VSVs, without regard to the total number of stages, or how many stages include VSVs. The VSVs enable throttling of flow through the compressor  10  in a known manner, so that it can operate efficiently at both high and low mass flow rates. The stator vane  24  of each stage S1 through S4 has a corresponding trunnion (generically referred to as “ 26 ” and labeled  26 A through  26 D, respectively) that extends radially outward through the liner assembly  20  and the casing  22 . An actuator arm (generically referred to as “ 28 ” and labeled  28 A through  28 D, respectively) is attached to the distal end of each trunnion  26 A- 26 D. All of the actuator arms  28 A- 28 D for an individual stage are coupled together by a ring  30  (generically referred to as “ 30 ” and labeled  30 A through  30 D, respectively). Rotation of the rings  30 A- 30 D about the engine&#39;s longitudinal axis A thus causes all of the actuator arms  28  coupled to that specific ring  30 A- 30 D to move in unison, in turn pivoting all of the trunnions  26 A- 26 D with their attached stator vanes  24  in unison. 
     In this particular example a rear bleed slot  32 , which may comprise an annular array of individual slots, penetrates the liner assembly  20  aft of the stage S6. The rear bleed slot  32  communicates with a rear plenum  36  defined between the liner assembly  20  and the casing  22 . One or more rear extraction ports  38  in the casing  22  provide a location to extract the air from the rear plenum  36 . In use the rear extraction port  38  would be coupled to appropriate pipework or ducting external to the casing  22  (not shown). 
     A middle bleed slot  40 , which may comprise an annular array of individual slots, penetrates the liner assembly  20  aft of the stage S3. The middle bleed slot  40  communicates with a middle plenum  42  defined between the liner assembly  20  and the casing  22  and isolated from the rear plenum  36 . One or more middle extraction ports  44  in the casing  22  provide a location to extract the air from the middle plenum  42 . In use this extraction port  44  would be coupled to appropriate pipework or ducting external to the casing  22  (not shown). 
     A forward bleed slot  46 , which may comprise an annular array of individual slots, penetrates the liner assembly  20  aft of the stage S1. The forward bleed slot  46  communicates with a forward plenum  48  defined between the liner assembly  20  and the casing  22  and isolated from the rear and middle plenums  36  and  42 . One or more forward extraction ports  50  in the casing  22  provide a place to extract the air from the forward plenum  48 . In use this extraction port  50  would be coupled to appropriate pipework or ducting external to the casing  22  (not shown). 
       FIG. 2  illustrates in more detail the structure used to bleed air between the stages S1 and S2. An annular shroud  52  surrounds the compressor blade  18  as described above. The shroud  52  may be made up of a plurality of segments arranged in a ring to form a complete 360 degree assembly. The shroud segments may include forward and aft rails  54  and  56  to mount them in slots in the adjacent portions of the liner assembly  20 . The forward bleed slots  46  described above are formed in the shroud  52  and communicate with the forward plenum  48 . In this particular example the forward bleed slots  46  are disposed between the forward rail  54  and the generally tapered-cylindrical central portion  58  of the shroud  52 . A typical construction would have the liner assembly  20  comprised of a ring of segments and the casing  22  formed in two sections bolted together at a split line flange  60 . To avoid leakage at the joints between these components, a duct  62  may be positioned in the third plenum  48 . As an example, it may have a forward wall  64  and an aft wall  66  which define a flowpath between the shroud  52  and the extraction port  50 . The duct  62  may be made from two or more arcuate segments assembled into a complete annular shape. 
     The forward plenum  48  lies axially between two stages having VSVs. In order to provide adequate space to bleed air from the stage S1 and extract that air from the forward plenum  48 , the operating hardware of the VSVs is positioned differently than in prior art practice. Specifically, the actuator arms  28 B of the stage S1 extend axially forward, while the actuator arms  28 C of the stage S2 extend axially rearward. As used herein, the term “axially” refers to directions parallel to the longitudinal axis A shown in  FIG. 1 . This creates an open void “V” outboard of the casing  22 , denoted by dashed lines in  FIG. 2 , not present in prior art configurations. The open void V permits the connection of external pipes or ductwork (not shown) to the extraction port  50 . 
     In operation, air can be bled from the stages S1, S3, and S6, providing air flows at three discrete pressures. As much of the bleed air as possible would be extracted at the lowest possible pressure (i.e. the most forward stage possible) in order to minimize the impact on efficiency and specific fuel consumption (“SFC”). In contrast with prior art bleed arrangements, air may be extracted at a desired pressure despite the fact that such pressure is found at the location of the VSV stages. 
     A similar air bleed configuration may be implemented in a compressor  110  in which the casing and liner are integrated into a single wall. For example,  FIG. 3  illustrates a portion of a compressor having a spool  112 , compressor blades  118 , and stator vanes  124 . An annular casing  122  surrounds the compressor blades  118  and serves both as a mount for the stator vanes  124  and as a shroud for the compressor blades  118 . In effect, it comprises a casing and a liner assembly as described above, in one integral unit. Some of the stator vanes  124  are variable-angle (or “VSVs”) and include trunnions  126 A- 126 D coupled to actuator arms  128 A- 128 D and rings  130 A- 130 D, respectively. For illustrative purposes the stator vanes  124  of two axially-adjacent stages will be described. One stage labeled S1′ includes a trunnion  126 A, an actuator arm  128 A, and a ring  130 A. The stage S2′ immediately downstream of the stage S1′ includes a trunnion  126 B, an actuator arm  128 B, and a ring  130 B. The actuator arm  128 A extends axially forward and the actuator arm  128 B extends axially rearward, creating a void “V′” shown by dashed lines. A bleed slot  146  is formed through the casing  122  and communicates with a plenum  148 . Because of the presence of the void V′, the plenum  148  can in turn be coupled to appropriate piping or ductwork (not shown). 
       FIG. 4  illustrates an alternative configuration for bleeding air from a compressor. The figures shows a portion of a high pressure compressor  210 , which is part of a gas turbine engine as described above and is similar in overall construction to the compressor  10  and components which are identical to the compressor  10  will be described in abbreviated fashion. The compressor  210  includes an annular spool  212  with blade rows of compressor blades  218 . An annular liner assembly  220  closely surrounds the compressor blades  218  and defines the radially outer boundary of a primary gas flowpath through the compressor  210 . The liner assembly  220  is built up from a plurality of smaller components, some of which will be described in more detail below. An annular casing  222  surrounds the liner assembly  220  and has several stator rows of stator vanes  224 . 
     In the illustrated example, only some of the stages of the compressor  210 , labeled “S1″” through “S6″” are shown. As noted above, these numbers are used solely for the sake of easy reference and do not necessarily correspond to the actual number of the stages in the complete compressor  210 . The first three stages shown (i.e. S1″-S3″) incorporate variable stator vanes as described above. The vane  224  of each stage S1″ through S3″ has a corresponding trunnion that extends radially outward through the liner assembly  220  and the casing  222 . The trunnions of stages S2″ and S3″ are labeled  226 B and  226 C, respectively. The trunnions of stage S1″ are not shown. The actuating hardware for the trunnions is not shown. 
     A rear bleed slot  232 , which may comprise an annular array of individual slots, penetrates the liner assembly  220  aft of the stage S5″. The rear bleed slot  232  communicates with a rear plenum  236  defined between the liner assembly  220  and the casing  222 . One or more rear extraction ports  238  in the casing  222  provide a location to extract the air from the rear plenum  236 . In use the rear extraction port  238  would be coupled to appropriate pipework or ducting external to the casing  222  (not shown). 
     A middle bleed slot  240 , which may comprise an annular array of individual slots, penetrates the liner assembly  220  aft of the stage S2″. The middle bleed slot  240  communicates with a middle plenum  242  defined between the liner assembly  220  and the casing  222  and isolated from the rear plenum  236 . One or more middle extraction ports  244  in the casing  222  provide a location to extract the air from the middle plenum  242 . In use the middle extraction port  244  would be coupled to appropriate pipework or ducting external to the casing  222  (not shown). 
     A forward bleed slot  246 , which may comprise an annular array of individual slots, penetrates the liner assembly  220  aft of the stage S1″. The forward bleed slot  246  communicates with a forward plenum  248  defined between the liner assembly  220  and the casing  222  and isolated from the rear and middle plenums  236  and  242 . One or more forward extraction ports  250  in the casing  222  provide a place to extract the air from the forward plenum  248 . In use this forward extraction port  250  would be coupled to appropriate pipework or ducting external to the casing  222  (not shown). 
       FIG. 5  illustrates in more detail the structure used to bleed air from the stage S1. An annular shroud  252  surrounds the compressor blades  218  as described above. The shroud  252  may be made up of a plurality of segments arranged in a ring to form a complete 360-degree assembly. The shroud segments may include forward and aft rails  254  and  256  to mount them in slots in the surrounding portions of the liner assembly  220 . The forward bleed slots  246  described above are formed in the shroud  252  and communicate with the forward plenum  248 . In this particular example the forward bleed slots  246  are disposed between the forward rail  254  and the generally tapered-cylindrical central portion  258  of the shroud  252 . 
     The bushings  260  which receive the trunnions  226 C pass through an annular wall-like boss  262  which is part of the casing  222 . In order to pass bleed air across the stage S2, the boss  262  is penetrated at several locations around its periphery by apertures  264 . The apertures  264  with the trunnions extending across them can be seen in  FIG. 6 . Optionally, the trunnions  226 C may have an axially-elongated noncircular shape which is smaller is a circumferential direction than an axial direction, as seen in  FIG. 7 , in order to increase the lateral space between adjacent trunnions  226  and thereby permit more flow through the apertures  264 . Optionally, to prevent leakage of bleed air between the trunnions  226  and the casing  222 , hollow sleeves  265  may be positioned surrounding the trunnions  226 , extending radially across the apertures  264 . 
     Various means may be used to avoid leakage through the forward plenum  248 . As noted above, a typical construction would have the liner assembly  220  comprised of a ring of segments and the casing  222  formed in two sections bolted together at a split line flange  259 . To avoid leakage at the joints between these components, annular front and rear ducts  266  and  268  may be positioned in the front plenum  248 . The front duct  266  comprises an arcuate outer wall  270  and an inner wall  272  with an L-shaped cross-section. Together the inner and outer walls  270  and  272  define a flowpath between the shroud  252  and the front face  274  of the boss  262 . The rear duct  268  comprises an outer wall  276  with a generally U-shaped cross-section and an inner wall  278  with a generally linear cross-section extending aft and radially outward at an angle. Together the inner and outer walls  278  and  276  define a flowpath between the aft face  280  of the boss  262  and the inner surface  282  of the casing  222 . Both the front and rear ducts  266  and  268  may be made from two or more arcuate segments assembled into a complete annular shape. 
     The bleed configurations described above can be combined and/or adapted as need for any desired bleed location. A particular compressor may have one or multiple bleed locations within either the VSV stages or the non-VSV stages. While locating the bleed port aft of the variable stages maintains compressor length and minimizes VSV complexity, bleeding directly out of the cavity reduces bleed system/leakage losses. In contrast with prior art bleed arrangements, air may be extracted at a desired pressure without regards to the axial location within the compressor. Analysis indicates that the bleed arrangements described here can result in a significant reduction in engine SFC. 
     The foregoing has described a bleed arrangement for a gas turbine engine compressor. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.