Patent Document

BACKGROUND 
     In low-bypass ratio turbofan engines, a fan is used to produce thrust in two manners. First, the fan pushes primary air into the core of the gas turbine engine for supplying air to a combustion process used to push gas through an exhaust nozzle. Second, the fan pushes bypass air past the core of the gas turbine engine to directly produce thrust. The fan is typically located at the inlet of the gas turbine engine within a fan case. The fan case is connected to an intermediate case that includes ducting for dividing the output of the fan into primary and bypass airstreams. The bypass air is routed around to the rear of the gas turbine engine, while the primary air is routed from the low pressure fan into the high pressure compressor (HPC) of the gas turbine core. The HPC comprises a series of rotating blades and stationary vanes for incrementally increasing the pressure of the primary air. These blades and vanes, starting with the first-stage blades, are sequentially housed within a high pressure compressor (HPC) case aft duct, which is connected to the immediate downstream face of the intermediate case. Thus, the first-stage blades receive air routed from the intermediate case. In order to optimize the incidence of the primary air onto the first-stage blades, a set of inlet guide vanes (IGVs) is provided between the intermediate case and the HPC case aft duct. The outer diameter ends of IGVs include trunnions that are inserted into bores in the HPC case aft duct. The inner diameter ends of the IGVs include trunnions that are inserted into an inner diameter shroud. In order to prevent the inner diameter of the IGVs from moving during operation of the gas turbine engine, especially during a surge event, the inner diameter shroud is pinned to the intermediate case with a surge retainer. In order to increase engine efficiency, it is desirable to seal the airflow path between the IGVs and the first-stage blades, while simultaneously minimizing the cavity space between the IGVs and the first-stage blades. Thus, there is a need for an IGV inner diameter retention and sealing mechanism that reduces the cavity between the IGVs and the first blade. 
     SUMMARY 
     The present invention is directed toward an inner air seal carrier for use in a gas turbine engine having an inlet guide vane surge retainer. The inner air seal carrier comprises a body, a stationary sealing element and an outcropping. The machined body, which can be roll-formed or machined, secures around an inlet guide vane inner diameter shroud. The stationary sealing element is disposed on a radially inward face of the body for engaging with a rotatable sealing element of a compressor rotor. The outcropping is positioned on the radially inward face of the body forward of the stationary sealing element for engaging with the surge retainer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of a low-bypass ratio turbofan engine in which the inlet guide vane inner air seal surge retention system of the present invention may be used. 
         FIG. 2  shows a partial section view of the turbofan engine of  FIG. 1  in which the transition between an intermediate duct and a high pressure compressor case is shown. 
         FIG. 3  shows an inlet guide vane inner air seal surge retaining mechanism of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic diagram of a dual-spool, low-bypass ratio turbofan engine  10 , in which the advantages of the inlet guide vane inner air seal surge retention system of the present invention is particularly well illustrated. Although, in other embodiments the present invention is applicable to other types of gas turbine engines such as high-bypass ratio turbofans including geared turbofans. Engine  10  comprises a low pressure spool, comprising low pressure fan  12 , low pressure shaft  14  and low pressure turbine (LPT)  16 ; and a high-pressure spool, comprising high pressure compressor (HPC)  18 , high pressure shaft  20  and high pressure turbine (HPT)  22 . Engine  10  also includes combustor  24 , which is nested between HPC  18  and HPT  22 , and exhaust section  26 , which is used to accelerate exiting gases to produce thrust. The low pressure spool and the high pressure spool are each concentrically disposed around longitudinal engine centerline CL. Low pressure fan  12  includes one or more fan blade stages and, in various embodiments, includes a low pressure compressor section. Low pressure fan  12  is encased in fan case  27  and intermediate case  28 , which is connected with HPC case aft duct  30  and bypass duct  32  such that split flow-paths are each concentrically disposed around longitudinal engine centerline CL. Aft duct  30  typically comprises split upper and lower portions such that it is easily assembled around low pressure shaft  14 . Rotatable inlet guide vanes (IGVs)  34  are disposed between intermediate case  28  and HPC  18  to moderate airflows within engine  10  for improving engine performance. Inlet guide vanes  34  are secured at their inner diameters to intermediate case  28  with inner air seal surge retaining mechanism  36  of the present invention. 
     Inlet air A enters engine  10  and it is divided into streams of primary air A P  and secondary air A S  by flow divider  38  after it passes through fan  12 . Low pressure fan  12  is rotated by low pressure turbine  16  through shaft  14  to accelerate secondary air A S  (also known as bypass air) into bypass duct  32  and through exit guide vanes  40  within exhaust section  26 , thereby producing a portion of the thrust output of engine  10 . Primary air A P  (also known as gas path air) is also directed first into low pressure fan  12  and then routed to inlet guide vanes  34  in front of high pressure compressor (HPC)  18  by divider  38 . HPC  18  is rotated by HPT  22  through shaft  20 . Low pressure fan  12  and HPC  18  work together to incrementally step up the pressure of primary air A P  to provide compressed air to combustor section  24 . The compressed air is delivered to combustor section  24 , along with fuel through injectors  42 , such that a combustion process can be carried out to produce the high energy gases necessary to turn turbines  22  and  16 . Primary air A P  continues through gas turbine engine  10  whereby it is passed through exhaust nozzle  44  to produce thrust. 
     In order to improve the performance of engine  10 , it is desirable to increase the compression of primary air A P  and secondary air A S  as they flow through low pressure fan  12  and HPC  18 . Accordingly, engine  10  is provided with inlet guide vane  34  that redirects entering primary air A P  to optimize its incidence on the first stage blades within HPC  18 . The IGV also modulates the airflow through the HPC, thus reducing the occurrence of compressor surges. Compressor surges occur when an excessive increase in axial air pressure along the flow path causes flow instability or reversal within the HPC. Particularly, an axial air pressure increase causes the laminar gas-flow at the blades and vanes to become turbulent. The turbulent flow separates from the blades and vanes, detrimentally impacting compressor efficiency and causing high-pressure gases downstream to lurch or “surge” forward. Surges may fatigue various engine components such as the IGV. Engine performance is further enhanced by sealing the flow path, which volumetrically reduces the flow path cavity to increase compression efficiency. In order to seal the flow path around primary air A P , and to stabilize inlet guide vanes  34 , inlet guide vanes  34  are provided with inner air seal surge retaining mechanism  36 . 
       FIG. 2  shows inner air seal surge retaining mechanism  36  positioned between intermediate duct  28  and HPC case aft duct  30  of engine  10 . Primary air A P  is directed from within intermediate duct  28  to HPC  18  by divider  38 , while secondary air A S  is routed outside of HPC aft duct  30 , past HPC  18 . HPC  18  includes an array of first-stage blades and vanes, including first-stage blade  46  and first-stage vane  48 , that extend radially from engine centerline CL. First-stage blade  46  of HPC  18  rotates as it is driven by shaft  20  and HPT  22  to drive air past first-stage vane  48  to increase the pressure of primary air A P . IGV  34  and first-stage vane  48  are adjustable to control the flow incidence to first-stage blade  46 . 
     The outer diameter ends of IGV  34  and first-stage vane  48  include trunnions  50  and  52 , respectively, which are secured within bores in aft duct  30 . Trunnions  50  and  52  are connected to actuation mechanisms, such as a bell crank  53 , so that the pitch of the vanes can be adjusted to alter the airflow of primary air A P . The inner diameter end of first-stage vane  48  includes trunnion  54 , which is configured for rotation within split-ring inner diameter shroud  56 . Likewise, IGV  34  includes inner diameter trunnion  58 , which is configured for rotation in split-ring inner diameter shroud  60 . 
     Split-ring inner diameter shroud  60  and inner diameter shroud  56  stabilize the inner diameter ends of IGV  34  and vane  48 , respectively. Shrouds  60  and  56  also enable synchronized rotation of IGV  34  and vane  48  on trunnions  54  and  58 , respectively, by fixing the circumferential spacing of the vanes. Thus, inlet guide vane  34  and first-stage vane  48  are suspended from aft duct  30  such that they are cantilevered within the airflow of primary air A P . Typically, for compressor vanes no other inner diameter support is necessary. Compressor vanes, including first-stage vane  48 , are generally comprised of a high-strength material such as nickel and have a generally sturdy construction such that the combined radial strength, as provided by inner diameter shroud  56 , typically provides enough resistance to the bending stresses sustained during operation of engine  10 . Additionally, compressor vanes are generally short such that the bending stress imparted to them is small. However, for IGV  34 , which is generally longer than a compressor vane, additional inner diameter retention and support is typically required. 
     Inlet guide vane  34  is typically comprised of titanium rather than nickel since it is not subjected to as high of temperatures as vane  48  or other compressor vanes. Titanium is relatively less strong than nickel and is therefore more susceptible to bending stress. Furthermore, IGV  34  is subjected to oscillations due to the operation of engine  10  and, in particular, to surge events. Typically during operation of engine  10 , pressure builds up within HPC  18  such that IGV  34  is normally pushed forward within engine  10 . During surge events, however, flow direction within HPC  18  can instantaneously change and IGV  34  will bend back toward first-stage blade  46 , potentially resulting in contact with first-stage blade  46 . Thus, vane-angle of IGV  34  and first-stage vane  48  is actuated to control pressure within HPC  18  to alleviate surge conditions. Therefore, in addition to potentially large bending during surge events, IGV  34  is subjected to low-frequency bending cycles during normal engine operation as the vane-angle of IGV  34  and vane  48  are adjusted. In order to reduce the bending moment of IGV  34  during operation, and in particular during surge events, IGV  34  is restrained at its inner diameter end with inner air seal surge retaining mechanism  36 . 
     Inner air seal surge retaining mechanism  36  provides a means for restraining axial movement of the inner diameter end of IGV  34  in the downstream or aft direction. Retaining mechanism  36  includes surge retainer  62  and carrier  64 . Inner air seal carrier  64  includes leading and trailing edge bent-flanges that slide into corresponding grooves on the leading and trailing edges of shrouds  60 , while surge retainer  62  comprises a spring-like member secured to intermediate case  28 . Surge retainer  62  engages carrier  64  to restrain downstream movement of the inner diameter end of IGV  34 . However, surge retainer  62  engages with carrier  64  so as to also permit sealing of the flow path along which primary air AP flows. 
     In order to increase the efficiency of HPC  18 , blade  46  is sealed at its inner and outer diameter ends. Blade  46  includes rotatable sealing elements  66  and  68  for engaging with stationary sealing elements  70  and  72  of IGV  34  and vane  48 , respectively. Aft duct  30  also includes stationary sealing element  74  for engaging with the outer diameter end of blade  48 . Blade  46  rotates between IGV  34  and vane  48  at high speeds, while IGV  34 , vane  48  and aft duct  30  remain stationary. In order to improve compression ratios of HPC  18  and to reduce the overall size of HPC  18 , it is desirable to reduce the distance between blade  46  and the stationary components surrounding it, while also preventing undesirable contact. Accordingly, aft duct  30  includes sealing element  74 , which comprises an abradable or sacrificial material such as honeycomb, that will yield upon contact of a rotating blade  46 . Thus, the outer diameter end of blade  46  can be held in close proximity with aft duct  30  to prevent leakage of primary air A P  around the tip of blade  46  without much risk of interference. Likewise, the inner diameter end of blade  46  is sealed by bringing rotating sealing elements into close proximity with stationary sealing elements  70  and  72 , respectively. Stationary sealing elements  70  and  72  also comprise abradable or sacrificial material such as honeycomb such that contact with rotating sealing element  66  or  68  is sustainable. Rotating sealing elements  66  and  68  comprise knife-edge surface or the like that upon rotational contact with stationary sealing elements  70  and  72  cut into or wear away the abradable honeycomb material. Thus, sealing elements  66  and  68  can be brought into close contact with sealing elements  70  and  72  to prevent escape of primary air A P  into the interior of engine  10 . Carrier  64  and stationary sealing member  70  of inner air seal surge retaining mechanism  36  thus permit the inner diameter end of IGV  34  to be stabilized to prevent damage caused by bending, yet also permit the inner diameter end of blade  46  to be sealed in a compact manner. Both retainer  62  and rotating seal member  66  engage carrier  64  from the innermost radial extent, or bottom, of carrier  64  such that blade  64  is brought into close proximity to IGV  34  to reduce the size of cavity C. 
       FIG. 3  shows inlet guide vane inner air seal surge retaining mechanism  36  restraining the inner diameter end of inlet guide vane  34 . Retaining mechanism  36  includes split-ring inner diameter shroud  60 , surge retainer  62 , carrier  64 , stationary sealing member  70 , mounting bolt  76 , shroud bolt  78  and shroud nut  80 . IGV  34  is suspended from HPC aft duct  30  ( FIG. 2 ) such that the inner diameter of IGV  34  is suspended within the flow path of primary air A P . Inner diameter trunnion  58  of IGV  34  is secured within split-ring inner diameter shroud  60 , which comprises forward shroud  60 A and aft shroud  60 B such that they can be secured to each half of aft duct  30 . Shroud bolt  78  and shroud nut  80  clamp forward shroud  60 A and aft shroud  60 B around inner diameter trunnion  58  such that the inner diameter end of IGV  34  is held in a fixed relationship to other IGVs of engine  10  within the air flow path. Carrier  64  is clamped around shroud  60  to prevent nut  80  from backing off of bolt  78 . Carrier  64  comprises a thin, sheet metal clip that can be deformed to fit around forward shroud  60 A and aft shroud  60 B to prevent nut  80  from disengaging bolt  78 . Aft shroud  60 B includes pocket  82  that permits nut  80  to be recessed within aft shroud  60 B allowing carrier  64  to easily fit around shroud  60 . Forward shroud  60 A includes notch  84  and aft shroud  60 B includes notch  86  that engage with flanges  88  and  90 , respectively, of carrier  64  to prevent carrier  64  from disengaging shroud  60  in the radial direction. Flange  88  abuts the leading edge of bolt  78  within notch  84 , while flange  90  engages notch  86  above nut  80 . Carrier  64  also includes jog  92  for engaging with surge retainer  62 , and stationary seal member  70  for engaging with rotating seal member  66 . Jog  92  is positioned on the forward portion of carrier  64 , while seal member  70  is positioned on an aft portion of carrier  64 . Surge retainer  62  is thus permitted to engage carrier  64  between jog  92  and seal member  70 . 
     Surge retainer  62  is secured to intermediate duct  28  with a circular pattern of bolts  76 , or some other such fastener. Surge retainer  62  includes radial extension arm  94 , axial extension arm  96  and axial retention hook  98 . Radial extension arm  94  comprises an elongate extension that permits retainer  62  to extend radially from the connection at bolt  62  to carrier  64 . Axial extension arm  96  permits retainer  62  to extend axially from intermediate case  28  to carrier  64 . Axial retention hook  98  extends radially from axial extension arm  96  to engage with jog  92  to prevent axial movement of the inner diameter end of IGV  34 . Surge retainer  62  is comprised of a continuous circular structure such that it abuts intermediate case  28  continuously around engine centerline CL. However, in other embodiments, retainer  62  may comprise a split-ring configuration, or may comprise a crenellated or scalloped structure for weight reduction. 
     Axial extension arm  96  and axial retention hook  98  are shaped to match the profile of jog  92 . In the embodiment shown, jog  92  comprises a rectangular-like projection or corrugation in carrier  64 , and axial retention hook  98  comprises a similarly shaped flange. However, in other embodiments jog  92  can have other shapes. In still other embodiments, jog  92  comprises a projection, protrusion or other such outcropping attached to carrier  64 . In any embodiment, axial retention hook  98  engages a downstream or aft facing portion of jog  96  to prevent movement of IGV  34  in the downstream direction. Retainer  62  is also configured to prevent forward or upstream movement of IGV  34 . Radial extension arm  94  and axial extension arm  96  are shaped and configured such that they provide a spring-like biasing force against jog  92  after assembly of inlet guide vane inner air seal surge retaining mechanism  36 . For example, radial extension arm  94  lays flush with intermediate case  28  such that intermediate case  28  provides bending resistance to and stiffens retainer  62 . Thus, the force of axial extension arm  96  against jog  92  prevents forward movement of IGV  34  and, in other embodiments can be used to pin carrier  64  against intermediate duct  28 . Thus, in the various embodiments, retainer  96  is not rigidly affixed to carrier  64  such that IGV  34  is not rigidly restrained, but is permitted some degree of movement in the axial direction. 
     Additionally, axial retention hook  98  engages jog  92  without interfering with rotating seal member  66  of blade  48 . Stationary seal member  70  is placed on carrier  64  away from jog  92  to permit axial retention hook  98  to access carrier  64  between jog  92  and seal member  70 . Seal member  70  is placed toward the trailing edge of carrier  64  such that seal member  66  does not need to extend far beyond blade  48 . Seal member  70  is also wide enough such that any small movements of IGV  34  due to surge or other engine events do not disrupt the seal between seal member  70  and seal member  66 . Additionally, carrier  64  and seal member  70  do not extend beyond the trailing edge of IGV  34  such that blade  48  can be brought into close proximity to IGV  34 , thus reducing the cavity size C between IGV  34  and first-stage blade  48 . Specifically, seal member  70  and jog  92  are positioned underneath IGV  34  on the innermost diameter surface of carrier  64 . In the embodiment shown, stationary seal member  70  and rotating seal member  66  comprise a knife-edge seal/honeycomb material interface. However, in other embodiments, other sealing arrangements such as brush seals may be used. In still other embodiments, stationary seal member  70  can be configured as a knife-edge seal, and rotational seal member  66  can be configured as an abradable material. 
     Inlet guide vane inner air seal surge retaining mechanism  36  provides a lightweight and inexpensive means for securing the inner diameter end of IGV  34  in a sealed manner. Surge retainer  62  and carrier  64  comprise thin, sheet metal structures making the raw materials necessary for construction inexpensive and easily repairable or replaceable. In other embodiments, surge retainer  62  and carrier  64  are machined from a ring structure. Additionally, retainer  62  and carrier  64  are easily manufactured in that the sheet metal is readily shaped or bended to form the components. Furthermore, seal member  70  is readily brazed to carrier  64 . 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Technology Category: 2