Patent Publication Number: US-11391176-B2

Title: Method and apparatus for supplying cooling air to a turbine

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to gas turbine engines and more specifically to cooling turbine sections of turbomachinery. 
     A gas turbine engine includes, in serial flow communication, a compressor, a combustor, and a turbine. The turbine is mechanically coupled to the compressor and together the three components define a turbomachinery core. The core is operable to generate a flow of hot, pressurized combustion gases. The core forms the basis for several aircraft engine types such as turbojets, turboprops, and turbofans. 
     In some conventional gas turbine engines, cooling of components such as the outer band of the high-pressure turbine is accomplished by conveying intermediate-stage compressor air to the areas to be cooled using pipes. A problem with conventional turbine engines is that the pipes add weight to the engine and occupy space which could be otherwise used. Therefore, there is a need for a structure that is configured to provide cooling air from the compressor to the turbine in a gas turbine engine without pipes. 
     BRIEF DESCRIPTION OF THE INVENTION 
     This need is addressed by a secondary bore bleed air circuit extending from the compressor rotor, through an axial air duct, between two turbine mid-seals, to a central plenum. 
     According to one aspect of the present invention, there is provided a gas turbine engine that includes a turbine interstage region. The turbine interstage region is configured to conduct bore bleed air outwardly. The interstage region includes a central plenum. The interstage region also includes a first mid-seal and a second mid-seal. The central plenum is fluidly connected to bore bleed air by a flow circuit that passes between the first mid-seal and the second mid-seal. 
     According to another aspect of the present invention there is provided a method for supplying cooling air to a turbine in a gas turbine engine. The method includes a step of conveying the cooling air radially outward along a circuit defined between a first disk and a second disk. 
    
    
     
       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 sectional view with partial cutaways of an interstage region between a forward disk and an aft disk within a high-pressure turbine of a gas turbine engine wherein the interstage region is configured to conduct flow radially outward from an air duct to a central plenum; 
         FIG. 2  is a sectional view with partial cutaways of a compressor and a high-pressure turbine in a gas turbine engine; 
         FIG. 3  is a schematic view of a conventional gas turbine engine; 
         FIG. 4  is a perspective view of a second stage nozzle vane; 
         FIG. 5  is a sectional view with partial cutaways of an interstage region of a gas turbine engine according to an alternative embodiment of the present invention; 
         FIG. 6  is a sectional view with partial cutaways of an interstage region of a gas turbine engine according to another alternative embodiment of the present invention; and 
         FIG. 7  is a sectional view of an interstage region that is configured to conduct flow radially outward from an air duct to a central plenum according to another alternative embodiment of the present invention. 
     
    
    
     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 sectional view of an annular interstage region  10  of a gas turbine engine  9 . The interstage region  10  includes elements that are bodies of revolution extending around an axis  2  of the engine  9  and multiple individual elements that are radially distributed around the axis  2 . The interstage region  10  is configured to define a cooling circuit P that is defined by a combination of bodies of revolution and radially distributed elements. Interstage region  10  is described below with reference to an exemplary section(s) in which portions of bodies of revolution and individual examples of radially distributed elements are shown. 
     The circuit P fluidly connects a plurality of inner core air ducts  15  that are configured to transfer bore bleed air with an outer band plenum  93  defined in an outer band  89 . In this manner, the outer band  89  of the gas turbine engine  9  can be cooled with gases that flow radially outward within the turbine engine  9 . Thus, piping conventionally used to conduct gases from a compressor section to the outer band is avoided. While the illustrated example is a high-bypass turbofan engine, the principles of the present invention are also applicable to other types of engines, such as low-bypass turbofans, turbojets, turboprops, etc. 
     The engine  9  has a longitudinal center line or axis  2 . As used herein, the terms “axial” and “longitudinal” both refer to a direction parallel to the centerline axis  2 , while “radial” refers to a direction perpendicular to the axial direction, and “tangential” or “circumferential” refers to a direction mutually orthogonal to the axial and radial directions. As used herein: the terms “forward” or “front” refer to a location relatively upstream in an air flow passing through or around a component; the terms “aft” or “rear” refer to a location relatively downstream in an air flow passing through or around a component; the terms “inner” and “radially inward” refer to locations relatively closer to the axis; and the terms “outer” and “radially outward” refer to locations relatively further from the axis. The direction of this flow is shown by the arrow “F” in  FIG. 3 . These directional terms are used merely for convenience in description and do not require a particular orientation of the structures described thereby. 
     Referring now to  FIGS. 2 and 3 , the engine  9  includes a fan nacelle  1  that is disposed concentrically about and coaxially along the axis  2 . The fan nacelle  1  is configured to house an inner core  3  such that the inner core  3  and the fan nacelle  1  share the axis  2 . A fan  4  is positioned within the fan nacelle  1  such that it is forward of the inner core  3 . A booster  5 , a compressor  12 , a combustor  7 , a high-pressure turbine  19 , and a low-pressure turbine  8  are positioned within the inner core  3 . The fan  4 , the booster  5 , the compressor  12 , the combustor  7 , the high-pressure turbine  19 , and the low-pressure turbine  8  are arranged in serial flow relationship. 
     A shaft  13  extends between the compressor  12  and the high-pressure turbine  19  such that they are mechanically connected. As seen in  FIG. 2 , a chamber  14  is defined aft of the compressor  12  and forward of the high-pressure turbine  19 . 
     Referring now to  FIG. 1 , the interstage region  10  is generally defined by a first stage disk  20  and a second stage disk  30 . The first stage disk  20  and the second stage disk  30  are bodies of revolution. The first stage disk  20  and the second stage disk  30  in part define an annular inner chamber  25 . The plurality of air ducts  15 , each defined within an associated rotating tube, extends from the chamber  14 , as shown in  FIG. 2 , to an inner chamber  25  of the interstage region  10  such that the chamber  14  and the interstage region  10  are fluidly connected. For each of the air ducts  15 , an associated opening  18  is defined through a wall  17  that separates that air duct  15  from the inner chamber  25 . 
     The first stage disk  20  includes a first stage disk bore  22  and a first stage disk web  23  that extends to a rim  24 . A plurality of radially disposed first stage blades  26  extends outwardly from the rim  24 . An aft arm  28 , which is an annular ridge defined on the first stage disk web  23  about the axis  2 , extends from the web  23  aft of the first stage disk  20 . A forward mid-seal  29  is positioned at the aft edge of the aft arm  28 . The forward mid-seal  29 , as shown, is configured as a two-tooth labyrinth seal. The second stage disk  30  includes a bore  32 , a web  33  that extends radially outward from the disk bore  32 , and a rim  34 . The second stage disk rim  34  is configured to support a plurality of radially disposed second stage blades  36 . 
     A plurality of second stage nozzle vanes  72  are radially distributed outwardly of the central plenum  60  such that the second stage nozzle vanes  72  are aft of the first stage blades  26  and forward of the second stage blades  36 . The plurality of second stage nozzle vanes  72  are supported by an inner band  73 . A forward hanger  75  is defined on the inner band  73  and extends radially inward. An aft hanger  77  is defined on the inner band  73  and extends radially inward. 
     A forward stator plate  87  is a body of revolution that is attached to forward hanger  75 . The forward stator plate  87  extends radially inward from the forward hanger  75  to a forward honeycomb block  94  attached thereto. An aft stator plate  88  is a body of revolution that is attached to the aft hanger  77 . The aft stator plate  88  extends radially inward from the aft hanger  77  to an aft honeycomb block  95  attached thereto. The forward stator plate  87  extends closer to the axis  2  than does the aft stator plate  88 . 
     A mid-seal disk  40  is positioned between the first stage disk  20  and the second stage disk  30 . The mid-seal disk  40  is a body of revolution and includes a bore  42 , a web  44 , and a curvic  45 . The curvic  45  is configured to mechanically link the mid-seal disk  40  with the first stage disk  20  via the aft arm  28  of the first stage disk  20 . A plurality of curvic passageways  61  are defined through the curvic  45  between a radially inward side and a radially outward side of the curvic  45 . An annular aft seal  47  is defined on the mid-seal disk  40 . The seal  47 , as shown, is configured as a three-tooth labyrinth seal in  FIG. 1 . It should be appreciated that the seals  29  and  47  can be configured as other types of rotating seals. 
     The forward mid-seal  29  and the aft mid-seal  47  are configured to sealingly engage the forward honeycomb block  24  and the aft honeycomb block  95  respectively and are positioned closer to the axis than conventional misdeals are. Stated another way, the method-seal disk  40  is of lower diameter than if the forward mid-seal  29  and the aft mid-seal  47  are positioned outwardly closer to the nozzle and  72 . As a result, the potential leakage area across the forward to seal  29  in the aft seal  47  are lower than the potential leakage area in conventional seals. 
     An annular central plenum  60  is defined radially outward of the curvic  45 . The central plenum  60  is defined by an inner boundary element  62 , a forward boundary element  64 , an aft boundary element  66 , and an outer boundary element  68 . A plurality of radial diffuser vanes  54  is positioned within the central plenum  60  between the forward boundary element  64  and the aft boundary element  66 . The forward boundary element  64  is configured to separate the central plenum  60  from an annular forward chamber  56 . The aft boundary element  66  is configured to separate the central plenum  60  from an annular aft chamber  58 . A forward rotating plate  53  supports a plurality of impeller vanes  52  that are configured to prevent air from being pumped radially outward. 
     A transfer pipe  80  passes through at least one of the second stage nozzle vanes  72 . The transfer pipe  80  extends from a trumpet  82  that is positioned within the central plenum  60  at one end to a diffuser  84  at another end. The diffuser  84  is positioned within an annular outer band plenum  93  that is defined in part by the outer band wall  89 . A plurality of feed holes  86  are defined within the walls of beach transfer pipe  80 . The feed holes  86  are configured to conduct cooling gas into the associated vane  72  as will be discussed further below. According to the illustrated embodiment, a transfer pipe  80  is associated with all of the radially distributed second stage nozzle vanes  72 . 
     In the illustrated embodiment as shown in  FIG. 1 , the forward boundary element  64  is defined by the forward stator plate  87 . A forward heatshield  91  is positioned forward of the stator plate  87 . The aft boundary element  66  is defined by the aft stator plate  88 . A heatshield  92  is positioned aft of the stator plate  88 . It should be appreciated that the forward stator plate  87 , the forward heatshield  91 , the aft stator plate  88 , and the heatshield  92  are all bodies of revolution. 
     Flow circuits between the air duct  15  that extend outwardly through the interstage region  10  will now be described. A primary flow circuit P extends from the air duct  15  through the opening  18  and into the chamber  25 . Once in the chamber  25  the primary flow circuit P passes through the plurality of impeller vanes  52  and through the plurality of curvic passageways  61  defined through the curvic  45 . After passing through the curvic passageways  61 , the flow circuit P enters the central plenum  60 . 
     The flow circuit P exits the central plenum  60  by entering at least one of the pipes  80  after passing between the plurality of radial diffuser vanes  54 . The plurality of radial diffuser vanes  54  is positioned to direct the gas flow into the pipe(s)  80  while increasing the static pressure of the gas. Correspondingly, the trumpet portion  82  of pipes  80  is oriented to intake gas via flow circuit P to minimize pressure loss and capture some of the dynamic head of the gas. As shown in  FIG. 4 , the trumpet  82  of the pipe  80  in the illustrated embodiment is oriented at an angle of about 45° angle relative to the nozzle  72 . In addition, the trumpet  82  is turned to face an incoming flow, i.e. the highest total pressure at the trumpet  82  inlet. In other embodiments, the trumpet  82  can be oriented at different angles relative to the pipe  80 . 
     A forward secondary flow circuit S 1  is configured to conduct gas flow from the plenum  60  to the forward chamber  56  via the forward mid-seal  29 . The flow circuit S 1  continues radially outward away from the axis  2  to maintain a positive purge flow rate preventing high-temperature gases from entering the forward chamber  56 . 
     An aft secondary circuit S 2  is configured to conduct gas flow from the central plenum  60  into the aft chamber  58  via the aft mid-seal  47 . The flow circuit S 2  continues radially outward away from the axis  2  to maintain a positive purge flow rate preventing high temperature gases from passing inwardly into the aft chamber  58 . 
     The structure described above can be better understood through a description of the operation thereof based on a section of the interstage region  10 . Gases are generated such that the chamber  14  is at a pressure such that gas flow is generally radially outward from chamber  14  to the outer band plenum  93 . Thus, gases are conducted through the air duct  15  and into the inner chamber  25  along the flow circuit P. The plurality of impeller vanes  52  act to increase the total pressure of the gas of flow circuit P through the passages  61  defined in the curvic  45  into the central plenum  60 . 
     Pressure within the central plenum  60  acts to press forward on the forward stator plate  87  and aft on the aft stator plate  88 . Because the aft midseal  47  is located further radially outward than the forward midseal  29 , the aft stator plate is of less area than the forward stator plate  87 . As a result, the pressure within the central plenum  60  applies a net load forward against the larger forward plate  87 . The net result is that the aft axial load on the stator plates is reduced. Such a reduction in axial load allows for the stator plates to be of sufficient size for the forward midseal  29  and the aft midseal  47  to be positioned at the radially inward location. 
     The flow circuit P crosses through the plurality of radial diffuser vanes  54  which convert some of the dynamic head of the gas into an increase in static pressure. The diffuser  54  directs flow circuit P into the trumpet  82  of the pipe  80 . In this manner, gases traveling along the flow circuit P are directed into the pipe  80 . As the gases travel along flow circuit P through the pipe  80 , some portion of the gases therein exit the pipe  80  through the feed holes  86 . Gas that passes through the feed holes  86  enters a space defined within the vane  72  that is operable to cool the vane  72 . Gases within the vane  72  exit cooling holes or slots (not shown). The remainder of gases traveling along the flow circuit P pass through the pipe  80  and exit the diffuser end  84 . 
     It should be appreciated that not all of the gases of flow circuit P enter the transfer pipe  80 . In this regard, once in the central plenum  60  some of the gases separate from the flow circuit P to continue on the secondary circuits S 1  and S 2 . The secondary circuit S 1  extends through the mid-seal  29  and into the forward chamber  56  such that the forward chamber  56  and the central plenum  60  are fluidly connected. The engine  9  is configured such that the secondary cooling gas flow rate entering chamber  56  through mid-seal  29  is sufficient to prevent hot gases from traveling radially inward from the primary flowpath into the forward chamber  56 . The pressure within the central plenum  60  is greater than the pressure within the forward chamber  56 . 
     Circuit S 2  extends through the aft mid-seal  47  and into the aft chamber  58  such that the aft chamber  58  and the central plenum  60  are fluidly connected. The engine  9  is configured such that the secondary cooling gas flow rate entering chamber  58  through mid-seal  47  is sufficient to prevent hot primary flowpath gases from traveling radially inward into the aft chamber  58 . The gas pressure within the central plenum  60  is greater than the pressure within the aft chamber  58 . 
     Referring now to  FIGS. 5-7 , alternative embodiments are shown in those figures and described further below. Please note that each alternative embodiment is described using reference numbers in a given 100 series. Similar reference numbers in different 100 series refer to similar parts disclosed in the embodiment described above and/or another alternative embodiment. 
     In  FIG. 5  there is shown an alternative embodiment that includes a flow circuit P′ that extends from an inner chamber  125 , through a plenum  160 , into a pipe  180 , and into an outer band plenum  193 . Continuing to refer to  FIG. 5 , a first stage disk  120  having a plurality of first stage blades  126  extending from the outer end thereof is positioned forward of a second stage disk  130  that has a plurality of second stage blades  136  extending from an end thereof. 
     An aft arm  128  extends from the first disk  120  toward a forward arm  138  that extends from the second stage disk  130 . A curvic  145  is defined at the junction of the aft arm  128  and the forward arm  138 . Multiple passageways  161  are defined through the curvic  145  such that the chamber  125  is fluidly connected to the plenum  160 . The curvic  145  is configured to mechanically link second stage disk  130  with the first stage disk  120 . 
     An aft mid-seal disk  140  is positioned between the first stage disk  120  and a second stage disk  130 . An aft seal  147  is defined on the aft mid-seal disk  140 . A forward mid-seal disk  141  is positioned between the first stage disk  120  and the aft mid-seal disk  140 . A forward seal  129  is defined on the forward mid-seal disk  141 . It should be appreciated that the seals  129  and  147  can be configured as other types of rotating seals than shown. 
     The plenum  160  is defined radially outward of the curvic  145 . The plenum  160  is defined by an inner boundary element  162 , a forward boundary element  164 , an aft boundary element  166 , and an outer boundary element  168 . The forward boundary element  164  is configured to separate the plenum  160  from a forward chamber  156 . The aft boundary element  166  is configured to separate the plenum  160  from an aft chamber  158 . 
     A second stage nozzle vane  172  is positioned radially outward of the plenum  160 . The transfer pipe  180  is positioned such that it passes through the second stage nozzle  172 . The transfer pipe  180  extends from a trumpet  182  that is positioned within the plenum  160  at one end to a diffuser  184  at another end. The diffuser  184  is positioned within a plenum  193  outward of the outer band and defined by an outer wall  189  and the nozzle vane  172 . A plurality of feed holes  186  are defined within the walls of the transfer pipe  180 . 
     In the alternative embodiment as shown in  FIG. 5 , the forward boundary element  164  is defined by the forward mid-seal disk  141 . The aft boundary element  166  is defined by the aft mid-seal disk  140 . 
     The primary flow circuit P′ extends from chamber  125  through the passageways  161  defined in the curvic  145  and into the plenum  160 . The flow circuit P′ exits the plenum  160  by entering the pipe  180 . A forward secondary circuit S 1 ′ is configured to conduct gas flow from the plenum  160  and into the forward chamber  156  via the forward mid-seal  129 . The circuit S 1 ′ continues outwardly away from the axis  2  to maintain a positive purge flow rate sufficient to prevent high-temperature gases from entering the forward chamber  156 . An aft secondary circuit S 2 ′ is configured to conduct gas flow from the plenum  160  into the aft chamber  158  via the aft mid-seal  147 . The flow circuit S 2 ′ continues outwardly away from the axis  2  to maintain a positive purge flow rate sufficient to prevent high temperature gases from passing inwardly into the aft chamber  158 . 
     In  FIG. 6  there is shown another alternative embodiment that includes a flowpath P that extends from an inner chamber  225 , through a plenum  260 , into a pipe  280 , and into an outer band plenum  293 . Continuing to refer to  FIG. 6 , a first stage disk  220  having a plurality of first stage blades  226  extending from the outer end thereof is positioned forward of a second stage disk  230 . The disk  230  has a plurality of second stage blades  236  extending from an end thereof. 
     An aft arm  228  extends from the first disk  220  toward a forward arm  238  that extends from the second stage disk  230 . A curvic  245  is defined at the junction of the aft arm  228  and the forward arm  238 . Multiple passages  261  are defined through the curvic  245  such that the chamber  225  is fluidly connected to the plenum  260 . The curvic  245  is configured to mechanically link second stage disk  230  with the first stage disk  220 . 
     A forward seal  229  is attached to the first stage disk  220 . An aft seal  247  is attached to the second stage disk  230 . 
     The plenum  260  is defined radially outward of the curvic  245 . The plenum  260  is defined by an inner boundary element  262 , a forward boundary element  264 , an aft boundary element  266 , and an outer boundary element  268 . The forward boundary element  264  is configured to separate the plenum  260  from a forward chamber  256 . The aft boundary element  266  is configured to separate the plenum  260  from an aft chamber  258 . 
     A second stage nozzle vane  272  is positioned radially outward of the plenum  260 . The transfer pipe  280  is positioned such that it passes through the second stage nozzle  272 . The transfer pipe  280  extends from a trumpet  282  that is positioned within the plenum  260  at one end to a diffuser  284  at another end. The diffuser  284  is positioned within a plenum  293  outward of the primary flowpath outer band and is defined by an outer wall  289  and the nozzle vane  272 . A plurality of feed holes  286  are defined within the walls of the transfer pipe  280 . 
     In the embodiment shown in  FIG. 6 , the inner boundary  262  is defined by aft arm  228  and forward arm  238 . The forward boundary element  264  is defined by the first stage disk  220  and the seal  229 . The aft boundary element  266  is defined by second stage disk  230  and the seal  247 . The outer boundary element is defined by the nozzle vane  272 . The cooling flow circuit P″ extends from chamber  225  through the passageways  261  defined in the curvic  245  and into the plenum  260 . 
     The flow circuit P″ exits the plenum  260  by entering the pipe  280 . A forward secondary circuit S 1 ″ is configured to conduct gas flow from the plenum  260  and into the forward chamber  256  via the forward mid-seal  229 . The flow circuit S 1 ″ continues outwardly away from the axis  2  to maintain a positive purge flow rate sufficient to prevent high-temperature gases from entering the forward chamber  256 . An aft secondary circuit S 2 ″ is configured to conduct gas flow from the plenum  260  into the aft chamber  258  via the aft mid-seal  247 . The circuit S 2 ″ continues outwardly away from the axis  2  to maintain a positive purge flow rate sufficient to prevent high temperature gases from passing inwardly into the aft chamber  258 . 
     Referring now to  FIG. 7 , it shows another alternative embodiment that includes an interstage region  310 . The interstage region  310  includes a central plenum  360  that is not fluidly connected to an outer band by a tube. The interstage region  310  is generally defined by a first stage disk  320  and a second stage disk  330 . The first stage disk  320  and the second stage disk  330  are bodies of revolution. The first stage disk  320  and the second stage disk  330  in part define an annular inner chamber  325 . A plurality of air ducts  315 , each defined within an associated rotating tube, extends from the chamber  314  as shown in  FIG. 2  to an inner chamber  325  of the interstage region  310  such that the chamber  314  and the interstage region  310  are fluidly connected. For each of the air ducts  315 , an associated opening  318  is defined through a wall  317  that separates that air duct  315  from the inner chamber  325 . 
     The first stage disk  320  includes a first stage disk bore  322  and a first stage disk web  323  that extends to a rim  324 . A plurality of radially disposed first stage blades  326  extends outwardly from the rim  324 . An aft arm  328 , which is an annular ridge defined on the first stage disk web  323  about the axis  2 , extends from the web  323  aft of the first stage disk  320 . A forward mid-seal  329  is positioned at the aft edge of the aft arm  328 . The forward mid-seal  329 , as shown, is configured as a two-tooth labyrinth seal. The second stage disk  330  includes a bore  332 , a web  333  that extends radially outward from the disk bore  332 , and a rim  334 . The second stage disk rim  334  is configured to support a plurality of radially disposed second stage blades  336 . 
     A plurality of second stage nozzle vanes  372  are radially distributed outward of the central plenum  360  such that the second stage nozzle vanes  372  are aft of the first stage blades  326  and forward of the second stage blades  336 . The plurality of second stage nozzle vanes  372  are supported by an inner band  373 . A forward hanger  375  is defined on the inner band  373  and extends radially inward. An aft hanger  377  is defined on the inner band  373  and extends radially inward. 
     A forward stator plate  387  is a body of revolution that is attached to forward hanger  375 . The forward stator plate  387  extends radially inward from the forward hanger  375  to a forward honeycomb block  394  attached thereto. An aft stator plate  388  is a body of revolution that is attached to the aft hanger  377 . The aft stator plate  388  extends radially inward from the aft hanger  377  to an aft honeycomb block  395  attached thereto. The forward stator plate  387  extends closer to the axis  2  than does the aft stator plate  388 . 
     A mid-seal disk  340  is positioned between the first stage disk  320  and the second stage disk  330 . The mid-seal disk  340  is a body of revolution and includes a bore  342 , a web  344 , and a curvic  345 . The curvic  345  is configured to mechanically link the mid-seal disk  340  with the first stage disk  320  via the aft arm  328  of the first stage disk  320 . A plurality of curvic passageways  361  are defined through the curvic  345  between a radially inward side and a radially outward side of the curvic  345 . An annular aft seal  347  is defined on the mid-seal disk  340 . The seal  347 , as shown, is configured as a three-tooth labyrinth seal in  FIG. 1 . It should be appreciated that the seals  329  and  347  can be configured as other types of rotating seals. 
     The forward mid-seal  329  and the aft mid-seal  347  are configured to sealingly engage the forward honeycomb block  324  and the aft honeycomb block  395  respectively and are positioned closer to the axis than conventional misdeals are. Stated another way, the method-seal disk  340  is of lower diameter than if the forward mid-seal  329  and the aft mid-seal  347  are positioned outwardly closer to the nozzle and  372 . As a result, the potential leakage area across the forward to seal  329  in the aft seal  347  is lower than the potential leakage area in conventional seals. 
     An annular central plenum  360  is defined radially outward of the curvic  345 . The central plenum  360  is defined by an inner boundary element  362 , a forward boundary element  364 , an aft boundary element  366 , and an outer boundary element  368 . A plurality of radial diffuser vanes  354  is positioned within the central plenum  360  between the forward boundary element  364  and the aft boundary element  366 . The forward boundary element  364  is configured to separate the central plenum  360  from an annular forward chamber  356 . The aft boundary element  366  is configured to separate the central plenum  360  from an annular aft chamber  358 . A forward rotating plate  353  supports a plurality of impeller vanes  352  that are configured to prevent air from being pumped radially outward. 
     In the illustrated embodiment as shown in  FIG. 1 , the forward boundary element  364  is defined by the forward stator plate  387 . A forward heatshield  391  is positioned forward of the stator plate  387 . The aft boundary element  366  is defined by the aft stator plate  388 . A heatshield  392  is positioned aft of the stator plate  388 . It should be appreciated that the forward stator plate  387 , the forward heatshield  391 , the aft stator plate  388 , and the heatshield  392  are all bodies of revolution. 
     Flow circuits between the air duct  315  that extend outwardly through the interstage region  310  will now be described. A primary flow circuit P′″ extends from the air duct  315  through the opening  318  and into the chamber  325 . Once in the chamber  325  the primary flow circuit P′″ passes through the plurality of impeller vanes  352  and through the plurality of curvic passageways  361  defined through the curvic  345 . After passing through the curvic passageways  361 , the flow circuit P′″ enters the central plenum  360 . The flow circuit P′″ exits the central plenum  360  through various split line leakage along the inner band  373 . 
     A forward secondary flow circuit S 1 ′″ is configured to conduct gas flow from the plenum  360  to the forward chamber  356  via the forward mid-seal  329 . The flow circuit S 1  continues radially outward away from the axis  2  to maintain a positive purge flow rate preventing high-temperature gases from entering the forward chamber  356 . 
     An aft secondary circuit S 2 ′″ is configured to conduct gas flow from the central plenum  360  into the aft chamber  358  via the aft mid-seal  347 . The flow circuit S 2 ′″ continues radially outward away from the axis  2  to maintain a positive purge flow rate preventing high temperature gases from passing inwardly into the aft chamber  358 . 
     The gas turbine engine having a secondary cooling flow circuit defined within an interstage region from an air duct defined through the bore to the outer bands of the high-pressure turbine section of the engine has advantages over the prior art. In particular, the engine described above does not require piping from intermediate (non-compressor-discharge) stages within the compressor such as stage  7  to the outer band of the high-pressure turbine. In this way the engine described above weighs less and has more space available for structure other than piping in conventional engines, while still benefiting from the use of lower-stage compressor bleed gas (non-compressor-discharge). 
     The foregoing has described a structure and a method for directing gases outwardly from the core passageway to cool the outer band of a gas turbine engine without additional piping. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
     Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.