Abstract:
A film damper for a gas turbine engine includes an annular inner member and an annular outer member located radially outboard of the annular inner member, the annular outer member and the annular inner member defining a damper annulus therebetween. A fluid supply passage delivers a flow of fluid into the damper annulus from the annular outer member, and a backflow prevention device is located at the fluid supply passage to prevent backflow of the flow of fluid from the damper annulus into the fluid supply passage.

Description:
BACKGROUND 
       [0001]    This disclosure relates to gas turbine engines, and more particularly to squeeze film dampers utilized in gas turbine engines. 
         [0002]    Squeeze film dampers are widely used in gas turbine engines to minimize rotor deflections. The damper is typically contained in a bearing compartment filled with air. A damper film is supplied by oil at an elevated pressure. The oil within the squeeze film damper provides resistance to rotor whirl as it is pushed around the damper annulus. Eventually the oil flows through the end seals of the damper into the bearing compartment where the oil is scavenged and recirculated through an engine lubrication system. 
         [0003]    The squeeze film damper generates a force on the rotor by squeezing a film of oil between two cylindrical cross-sectional regions, the outer region fixed to supports and considered rigid, and the inner region whirling with the rotor. A pressure field is developed as the rotor whirls, resolving the net force acting on the rotor with components aligned with the eccentricity and components parallel and perpendicular to the eccentricity, which enables the forces to be expressed in terms of a squeeze film damper generated stiffness and damping constant, respectively. There is a resultant region of positive pressure with respect to the circumferential mean of the pressure within the damper, and also a region in which the pressure is reduced to below a mean pressure. 
         [0004]    In the typical system, the mean pressure, or “steady” pressure, within the damper is typically set by the characteristics of the oil supply system and the leakage of the oil through the seals. As the seals approach ideal seals, i.e. no leakage, the mean pressure approaches the supply pressure. The unsteady part of the pressure, or dynamic pressure amplitude, builds with whirl amplitude. The larger the whirl, the larger the dynamic pressure amplitude becomes. This idealized model works well conceptually until the zero-to-peak amplitude of the dynamic pressure exceeds that of the steady pressure. At this point, the simplified model begins to predict negative pressure over certain regions of the circumference. While negative pressures are conceptually possible in some situations, the prediction of negative pressures typically implies that the oil will either cavitate, or the seals on the ends of the damper will back-flow air from within the bearing compartment into the squeeze film region, or some combination of both. Once air, or typically any gas, is entrained within the squeeze film damper, the effectiveness of the damper, and the ability of analytical models to predict the performance of the damper, becomes compromised. 
         [0005]    From a design perspective, it is typically preferable to design squeeze film dampers with sufficient mean pressure such that the dynamic pressure does not result in regions of negative pressure. However, in some cases, it is impractical to provide sufficient supply pressure to achieve the desired damping and stiffness characteristics from the damper. 
       SUMMARY 
       [0006]    In one embodiment, a film damper for a gas turbine engine includes an annular inner member and an annular outer member located radially outboard of the annular inner member, the annular outer member and the annular inner member defining a damper annulus therebetween. A fluid supply passage delivers a flow of fluid into the damper annulus from the annular outer member, and a backflow prevention device is located at the fluid supply passage to prevent backflow of the flow of fluid from the damper annulus into the fluid supply passage. 
         [0007]    Additionally or alternatively, in this or other embodiments the fluid supply passage includes a circumferentially extending fluid plenum disposed at the annular outer member. 
         [0008]    Additionally or alternatively, in this or other embodiments the backflow prevention device is a plurality of reed valves. 
         [0009]    Additionally or alternatively, in this or other embodiments the plurality of reed valves are located between a circumferential supply plenum and the damper annulus. 
         [0010]    Additionally or alternatively, in this or other embodiments one or more sealing elements extend between the annular outer member and the annular inner member. 
         [0011]    Additionally or alternatively, in this or other embodiments the one or more sealing elements are one or more O-rings and/or one or more piston rings. 
         [0012]    Additionally or alternatively, in this or other embodiments a fill port is fluidly coupled to the fluid supply passage. The fill port has a first throat portion and a second portion extending from the first throat portion increasing an effective radial clearance between the fill port and the fluid supply passage. 
         [0013]    In another embodiment, a shaft arrangement for a gas turbine engine includes a shaft located at and rotatable about an engine axis and a film damper located at the shaft to reduce or attenuate vibration of the shaft. The film damper includes an annular inner member surrounding the shaft and an annular outer member located radially outboard of the annular inner member, the annular outer member and the annular inner member defining a damper annulus therebetween. A fluid supply passage delivers a flow of fluid into the damper annulus from the annular outer member, and a backflow prevention device is located at the fluid supply passage to prevent backflow of the flow of fluid from the damper annulus into the fluid supply passage. 
         [0014]    Additionally or alternatively, in this or other embodiments the fluid supply passage includes a circumferentially extending fluid plenum located at the annular outer member. 
         [0015]    Additionally or alternatively, in this or other embodiments the backflow prevention device is a plurality of reed valves. 
         [0016]    Additionally or alternatively, in this or other embodiments the plurality of reed valves are located between a circumferential supply plenum and the damper annulus. 
         [0017]    Additionally or alternatively, in this or other embodiments one or more sealing elements extend between the annular outer member and the annular inner member. 
         [0018]    Additionally or alternatively, in this or other embodiments the one or more sealing elements are one or more O-rings and/or one or more piston rings. 
         [0019]    Additionally or alternatively, in this or other embodiments a fill port is fluidly coupled to the fluid supply passage, the fill port having a first throat portion and a second portion extending from the first throat portion increasing an effective radial clearance between the fill port and the fluid supply passage. 
         [0020]    Additionally or alternatively, in this or other embodiments a rolling element bearing is located radially between the annular inner member and the shaft. 
         [0021]    Additionally or alternatively, in this or other embodiments the shaft is one of a high speed shaft or a low speed shaft of the gas turbine engine. 
         [0022]    In yet another embodiment, a gas turbine engine includes a shaft located at and rotatable about an engine axis, and an oil film damper located at the shaft to reduce or attenuate vibration of the shaft. The oil film damper includes an annular inner member surrounding the shaft, and an annular outer member located radially outboard of the annular inner member, the annular outer member and the annular inner member defining a damper annulus therebetween. A fluid supply passage delivers a flow of fluid into the damper annulus from the annular outer member, and a backflow prevention device located at the fluid supply passage to prevent backflow of the flow of fluid from the damper annulus into the fluid supply passage. 
         [0023]    Additionally or alternatively, in this or other embodiments the fluid supply passage includes a circumferentially extending fluid plenum located at the annular outer member. 
         [0024]    Additionally or alternatively, in this or other embodiments the backflow prevention device is a plurality of reed valves. 
         [0025]    Additionally or alternatively, in this or other embodiments a fill port is fluidly coupled to the fluid supply passage, the fill port having a first throat portion and a second portion extending from the first throat portion increasing an effective radial clearance between the fill port and the fluid supply passage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The subject matter which is regarded as the present disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the present disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0027]      FIG. 1  is a cross-sectional view of an embodiment of a gas turbine engine; 
           [0028]      FIG. 2  is a cross-sectional view of an embodiment of a squeeze film damper arrangement for a gas turbine engine; 
           [0029]      FIG. 3  is a schematic view of an embodiment of a damper arrangement having a circumferential supply plenum; 
           [0030]      FIG. 4  is schematic view of an embodiment of a damper arrangement including a plurality of check valves; 
           [0031]      FIG. 5  is a schematic view of an embodiment of a damper arrangement including an array of reed valves; and 
           [0032]      FIG. 6  is a schematic view of an embodiment of a damper arrangement having a fill port with increased radial clearance to a supply plenum. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]      FIG. 1  is a schematic illustration of a gas turbine engine  10 . The gas turbine engine generally has includes fan section  12 , a low pressure compressor  14 , a high pressure compressor  16 , a combustor  18 , a high pressure turbine  20  and a low pressure turbine  22 . The gas turbine engine  10  is circumferentially disposed about an engine centerline X. During operation, air is pulled into the gas turbine engine  10  by the fan section  12 , pressurized by the compressors  14 ,  16 , mixed with fuel and burned in the combustor  18 . Hot combustion gases generated within the combustor  18  flow through high and low pressure turbines  20 ,  22 , which extract energy from the hot combustion gases. 
         [0034]    In a two-spool configuration, the high pressure turbine  20  utilizes the extracted energy from the hot combustion gases to power the high pressure compressor  16  through a high speed shaft  24 , and the low pressure turbine  22  utilizes the energy extracted from the hot combustion gases to power the low pressure compressor  14  and the fan section  12  through a low speed shaft  26 . The present disclosure, however, is not limited to the two-spool configuration described and may be utilized with other configurations, such as single-spool or three-spool configurations, or gear-driven fan configurations. 
         [0035]    Gas turbine engine  10  is in the form of a high bypass ratio turbine engine mounted within a nacelle or fan casing  28  which surrounds an engine casing  30  housing an engine core  32 . A significant amount of air pressurized by the fan section  12  bypasses the engine core  32  for the generation of propulsive thrust. The airflow entering the fan section  12  may bypass the engine core  32  via a fan bypass passage  34  extending between the fan casing  28  and the engine casing  30  for receiving and communicating a discharge flow F 1 . The high bypass flow arrangement provides a significant amount of thrust for powering an aircraft. 
         [0036]    The engine casing  30  generally includes an inlet case  36 , a low pressure compressor case  38 , and an intermediate case  40 . The inlet case  36  guides air to the low pressure compressor case  38 , and via a splitter  42  also directs air through the fan bypass passage  34 . 
         [0037]    Referring now to  FIG. 2 , illustrated is an embodiment of a shaft of the gas turbine engine  10 , for example high speed shaft  24 . While the present description is in the context of the high speed shaft  24 , the present disclosure may be readily utilized with low speed shaft  26 . The high speed shaft  24  is supported by a bearing, for example, a rolling element bearing  44  located at a bearing housing  46 . The rolling element bearing  44  is, in turn, supported by a rotationally fixed damper housing  48 . The damper housing  48  includes an inboard element  50  to which a bearing outer race  52  of the rolling element bearing  44  is secured. An outboard element  54  of the damper housing  48  is located radially outboard of the inboard element  50  with two O-ring seals  56  or other seal elements such as piston ring seals to define a damper annulus  58  between the O-ring seals  56  and between the inboard element  50  and the outboard element  54 . The damper annulus  58  extends circumferentially about the engine centerline X. 
         [0038]    The damper annulus  58  is supplied with a flow of oil  60  from one or more oil supply ports  62  located at the outboard element  54 . The flow of oil  60  fills the damper annulus  58  to dampen vibration of the high speed shaft  24 . The flow of oil  60  eventually seeps through or around the O-ring seals  56  and into the bearing housing  46 , where it is scavenged. 
         [0039]    In some embodiments, as shown in  FIG. 3 , the outboard element  54  includes a circumferential supply plenum  64  (shown best in  FIG. 4 ), which in some embodiments is defined by a circumferential groove  66  in the outboard element  54 , extending from the one or more oil supply ports  62 . The circumferential supply plenum  64  aids in ensuring that there is an ample supply of oil around the circumference of the damper annulus  58 . 
         [0040]    In a typical squeeze film damper arrangement, certain operating conditions such as whirl of the high speed shaft  24  can lead to localized back flow of the flow of oil  60  at certain circumferential locations of the supply plenum  64 . Referring now to  FIG. 4 , locally-reacting, fill-only check valves  66  may be positioned between the supply plenum  64  and the damper annulus  58 . In some embodiments, multiple check valves  66  are distributed around the circumference of the damper annulus  58 . The check valves  66  are fill-only, such that the flow of oil  60  can proceed from the supply plenum  64  into the damper annulus  58 , but the flow of oil  60  is prevented from flowing in the opposite direction, from the damper annulus  58  into the supply plenum  64 . The net result is an increase in net mass flow of the flow of oil  60  into the damper annulus  58  and an ability to better support a circumferential pressure gradient within the damper annulus  58 . In some embodiments, as shown in  FIG. 5 , rather than check valves  66 , a plurality of reed valves  68  may be positioned between the supply plenum  64  and the damper annulus  58  to prevent backflow of the flow of oil  60  from the damper annulus  58  into the supply plenum  64 . 
         [0041]    Check valves  66  and/or reed valves  68  are particularly effective for squeeze film dampers operating with  1 ) “low” supply pressure and  2 ) “low” supply impedance. Low supply pressure implies that the intended, idealized amplitude of the circumferential pressure distribution approaches an offset between supplied pressure and damper annulus  58  pressure. Low supply impedance means that the pressure of the inlet oil supply is not significantly influenced by the mass flow supplied to the damper annulus  58 , for example, if the oil supplied to the damper annulus  58  is supplied by “teeing off” a small fraction of a larger oil flow into the bearing housing  46 . 
         [0042]    To further reduce impedance of the flow of oil  60 , as shown in  FIG. 6 , a fill port  70  extends from a relatively narrow throat portion  72  circumferentially outwardly to a relatively wider outlet portion  74  of the fill port  70  at the supply plenum  64  to increase an effective radial clearance  76  at the fill port  70 . This reduces flow losses at an entrance to the supply plenum  64  from the fill port  70 . The increased effective radial clearance  76  may also be accomplished, utilizing radial grooves in the fill port  70  or enlarging the fill port  70  diameter near the supply plenum  64  interface with the fill port  70 . This clearance relief should extend a sufficient distance from the fill port  70  to sufficiently reduce the in-flow impedance. 
         [0043]    Increasing oil supply pressure is well known to improve damper effectiveness. However, increasing oil supply pressure at low rotational speeds is often impractical. The present disclosure provides to increase the flow of oil  60  pressure within the damper annulus  58 , without increasing supply pressure. 
         [0044]    While the present disclosure has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the present disclosure is not limited to such disclosed embodiments. Rather, the present disclosure can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the scope of the present disclosure. Additionally, while various embodiments of the present disclosure have been described, it is to be understood that aspects of the present disclosure may include only some of the described embodiments. Accordingly, the present disclosure is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.