Abstract:
A rotating seal for a gas turbine engine includes: (a) an annular seal body; (b) a sealing component carried by the seal body which is adapted to form one-half of a rotating seal interface; and (c) an impeller carried by the seal body which comprises a plurality of radially-inwardly-extending impeller blades.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to gas turbine engine bearing sumps and more particularly to control of oil flow in bearing sumps. 
         [0002]    A gas turbine engine includes one or more shafts which are mounted for rotation in several bearings, usually of the rolling-element type. The bearings are enclosed in enclosures called “sumps” which are pressurized and provided with an oil flow for lubrication and cooling. In most cases one of the boundaries of the sump will be a dynamic seal between a rotating component of the engine and the engine&#39;s stationary structure. 
         [0003]    Many dynamic seals, such as carbon seals, require secondary seals to prevent oil leakage past the primary sealing surface. A device called a “windback” comprising a helical thread and mating rotating surface is frequently used. The windage caused by the rotating surface pushes the oil mist away from the interface, causing any oil accumulated within the helical thread to be driven through the thread groove back into the sealed cavity. The axial component of windage generated by the air shearing acts as a driving force to keep oil mist away. The tangential component of windage pushes oil collected at the bottom of helical thread back into sealed cavity. Windage is a secondary effect of shaft rotation and its effectiveness strongly depends on shaft speed and the radial gap between rotating and stationary parts. 
         [0004]    In a prior art windback, the grooves between the teeth are at the same diameter; there are no axial or tangential angles to facilitate oil drainage. The pitch of the thread is relatively small compared to the diameter, therefore, the axial windage effect is limited. Furthermore, oil collected at the thread root has to travel through the total length of the thread circumference. Oil collected must overcome gravity to return back to oil-wetted cavity if the shaft axis is horizontal. Under conditions where the windage is not adequate to drive oil completely around circumference of the thread and back to the oil-wetted cavity, oil leakage might occur. Windback effectiveness is usually difficult to predict. If oil/air mist passes the secondary seal, performance of the primary seal is jeopardized. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    These and other shortcomings of the prior art are addressed by the present invention, which provides a rotating seal incorporating an impeller which moves oil mist away from a seal interface using centrifugal force. 
         [0006]    According to one aspect, a rotating seal for a gas turbine engine includes: (a) an annular seal body; (b) a sealing component carried by the seal body which is adapted to form one-half of a rotating seal interface; and (c) an impeller carried by the seal body which comprises a plurality of radially-inwardly-extending impeller blades. 
         [0007]    According to another aspect of the invention, a bearing assembly for a gas turbine includes: (a) a rolling element bearing enclosed in a wet cavity; (b) a stationary component forming a portion of a boundary between the wet cavity and a dry cavity; (c) a rotating component disposed adjacent the stationary component and forming a portion of the boundary between the wet cavity and the dry cavity, wherein the stationary and rotating components cooperate to define a rotating seal interface between the wet and dry cavities; and (d) an impeller carried by the rotating component which comprises a plurality of radially-extending impeller blades adapted to move oil away from the seal interface towards the wet cavity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0009]      FIG. 1  is a half-sectional view of a gas turbine engine incorporating a rotating oil seal constructed according to an aspect of the present invention; 
           [0010]      FIG. 2  is an enlarged view of a bearing compartment of the gas turbine engine of  FIG. 1 ; 
           [0011]      FIG. 3  is perspective cross-sectional view of a rotating seal shown in  FIG. 2 ; 
           [0012]      FIG. 4  is an enlarged view of a portion of  FIG. 3 ; 
           [0013]      FIG. 5  is another perspective sectional view of the impeller of  FIG. 3 ; and 
           [0014]      FIG. 6  is an enlarged view of a portion of the interior of the impeller shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  depicts a gas turbine engine  10 . The engine  10  has a longitudinal axis  11  and includes a fan  12 , a low pressure compressor or “booster”  14  and a low pressure turbine (“LPT”)  16  collectively referred to as a “low pressure system”. The LPT  16  drives the fan  12  and booster  14  through an inner shaft  18 , also referred to as an “LP shaft”. The engine  10  also includes a high pressure compressor (“HPC”)  20 , a combustor  22 , and a high pressure turbine (“HPT”)  24 , collectively referred to as a “gas generator” or “core”. The HPT  24  drives the HPC  20  through an outer shaft  26 , also referred to as an “HP shaft”. Together, the high and low pressure systems are operable in a known manner to generate a primary or core flow as well as a fan flow or bypass flow. While the illustrated engine  10  is a high-bypass turbofan engine, the principles described herein are equally applicable to turboprop, turbojet, and turboshaft engines, as well as turbine engines used for other vehicles or in stationary applications. 
         [0016]    The inner and outer shafts  18  and  26  are mounted for rotation in several rolling-element bearings. The bearings are located in enclosed portions of the engine  10  referred to as “sumps”.  FIG. 2  shows an aft sump  28  of the engine  10  in more detail. The aft end  30  of the outer shaft  26  is carried by a bearing  32  which is referred to as the “#4R bearing”, denoting its location and type. The outer race  34  of the bearing  32  is attached to a static annular frame member  36  of the engine  10 . The frame member  36  has a main body portion  38  that extends in a generally radial direction. A stationary seal arm  40  extends axially aft from the main body portion  38 . The distal end of the stationary seal arm  40  includes a number of annular seal teeth  42  which extend radially outwards, and at the extreme end, an annular sealing surface  44 . 
         [0017]    The aft end  46  of the inner shaft  18  extends aft of the outer shaft  26  and is mounted for rotation in a rear frame structure  48  of the engine by a rolling element bearing  50 . The inner shaft  18  has a disk  52  extending generally radially outward from it. The disk  52  extends between the inner shaft  18  and the LP turbine  16  (see  FIG. 1 ) and transmits torque between the LP turbine  16  and the inner shaft  18 . 
         [0018]    A rotating seal  54  extends axially forward from the disk  52 . The rotating seal  54  has a generally frustoconical body with forward and aft ends  56  and  58 , and its axis of rotation coincides with that of the engine  10 . The forward end  56  of the rotating seal  54  includes a radially inward-facing seal pocket  60  which may contain a compliant seal material  62  of a known type such as abradable phenolic resin, a metallic honeycomb structure, a carbon seal, or a brush seal. Just aft of the seal pocket  60  is an impeller  64  which is described in more detail below. An annular, generally conical inner seal arm  66  extends axially forward from a point aft of the impeller  64 . As seen in cross-section, the forward end  56  of the rotating seal  54  and the inner seal arm  66  overlap the stationary seal arm  40  in the axial direction. 
         [0019]    The forward end of the rotating seal  54  overlaps the aft end of the stationary seal arm  40  in the axial direction, and the seal pocket  60  is aligned with the seal teeth  42  in the axial direction, so that they cooperatively form a rotating, non-contact seal interface  68 . It is noted that the structure of the sealing components could be reversed; e.g. the rotating seal  54  could include radially-extending seal teeth while the stationary seal arm  40  could include a seal pocket. The impeller  64  is positioned adjacent the annular sealing surface  44  of the stationary seal arm  40 . 
         [0020]    Collectively, the outer shaft  26 , the inner shaft  18 , the disk  52 , the stationary seal arm  40 , and the rotating seal  54  define a “wet” cavity or “oiled” cavity  70 . In operation, the bearing  32  is supplied with oil from a jet, supply line, or orifice in a known manner to provide lubrication and cooling. The interaction of the oil supply and the bearing  32  creates a mist of oil within the wet cavity  70 . Because the wet cavity  70  is pressurized, air flow tends to transport the oil mist along a leakage path past the seal interface  68 , as depicted by the arrow marked “L” in  FIG. 2 . This condition is worsened at low engine operating speeds when the air pressure in the “dry” cavity  72  adjacent the seal interface  68  is relatively low. This leakage causes oil loss which is undesirable from a cost, safety, and pollution standpoint. The function of the impeller  64  is to reduce or prevent this leakage. 
         [0021]      FIGS. 3-6  illustrate the rotating seal  54  in more detail. For illustrative clarity, the inner seal arm  66  is not shown in  FIGS. 3-6 . The impeller  64  comprises a ring of impeller blades  74  separated by grooves  76 . The impeller blades  74  are oriented at an angle “A” to the rotational axis of the rotating seal  54  (see  FIG. 6 ), and at an angle “B” in the measured from the radial direction, as seen in  FIG. 4  (i.e. they are tangentially “leaned”). The angle of the impeller blades  74  can be optimized to ensure adequate axial driving force to keep air/oil mixture away from the sealing interface  68  at all operating conditions, in other words, at all speeds of the rotating seal  54  and at all expected air pressure gradients across the seal interface  68 . In the illustrated example, angle A is about 45 degrees and angle B is about 20 degrees If desired, the impeller blades  74  may be given an airfoil cross-sectional shape. The grooves  76  between the impeller blades  74  form a series of radially diverging spiral-shaped pathways. Referring to  FIG. 4 , the radial depth “D 1 ” of the grooves  76  at the aft edges of the impeller blades  74 , is greater than the depth “D 2 ” of the grooves  76  the forward edges of the impeller blades  74 . The dimensions D 1  and D 2  may also be conceptualized as the radial span of the impeller blades  74 . With this axially diverging channel configuration, oil collected at the root  78  of the impeller blades  74  will be driven by centrifugal force and channeled aft towards the wet cavity  70 . 
         [0022]    In comparison to a prior art windback seal, the centrifugal force, as a driving force, is much stronger than windage generated by air shearing. It is also much stronger than gravity effects on the oil which might resist oil drainage. Furthermore, because each of the grooves  76  is open at the aft end, much more open area for oil drainage is provided as compared to a windback. The impeller  64  thus allows oil to drain much easier than the traditional windback. Comparative computational fluid dynamics (CFD) analysis have shown substantially lower oil leakage flow with the impeller  64  of the present invention. 
         [0023]    While the invention has described with respect to a particular bearing and seal arrangement, it is noted that the impeller  64  may be used in any sump or location in the engine where it is desirable prevent oil leakage. 
         [0024]    The foregoing has described an oil seal with a dynamic impeller for a gas turbine engine. 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, the invention being defined by the claims.