Patent Publication Number: US-2023160315-A1

Title: Bore compartment seals for gas turbine engines

Description:
BACKGROUND 
     The subject matter disclosed herein generally relates to rotor systems of gas turbine engines and, more particularly, to seals for separating bore compartments in rotor systems of gas turbine engines. 
     In turbomachinery (e.g., gas turbine engines), compressor air is used to cool the turbine. The cooling air that is bled from the compressor flowpath and supplied to the rear bore compartment typically has high temperature and pressure. In contrast, the forward compressor bore compartment contains relatively lower temperature and pressure air. To maintain rotor durability and performance, it is advantageous to avoid mixing of the two air supplies. Such separation may require the forward and rear bore compartments to be sealed from one another. One current solution is to use a piston seal ring between a tie shaft and a rotor bore. However, relative motion and vibration at this interface of the piston seal ring has led to high wear rates and loss of sealing. Accordingly, improved compartment sealing will improve rotor durability and engine performance. 
     SUMMARY 
     According to some embodiment, rotor systems are provided. The rotor systems include an engine shaft, a forward hub, a rear hub, a rotor disk arranged between the forward hub and the rear hub, and a seal tube configured to define a forward hub compartment and a rear hub compartment, wherein the forward hub compartment is defined forward of the rotor disk and the rear hub compartment is defined aft of the rotor disk. The seal tube is connected at a forward end to at least one of the rotor disk and the engine shaft and at a rear end to at least one of the rear hub and the engine shaft, and the seal tube includes at least one axial compliance element configured to enable axial extension and compression of the seal tube in an axial direction along the engine shaft. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the rotor systems may include a second rotor disk arranged forward of the rotor disk within the forward hub compartment. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the rotor systems may include a second rotor disk arranged aft of the rotor disk within the rear hub compartment. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the rotor systems may include that the at least one axial compliance element comprises a plurality of axial compliance elements between the forward end and the rear end of the seal tube. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the rotor systems may include that at least two axial compliance elements of the plurality of axial compliance elements have different radial heights. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the rotor systems may include that at least two axial compliance elements of the plurality of axial compliance elements have different axial lengths. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the rotor systems may include that at least two axial compliance elements of the plurality of axial compliance elements have different geometric shapes. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the rotor systems may include that at least two axial compliance elements of the plurality of axial compliance elements have different material thicknesses. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the rotor systems may include that the at least one axial compliance element has a squared geometry. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the rotor systems may include that the at least one axial compliance element has a rounded geometry. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the rotor systems may include that the at least one axial compliance element has a triangular geometry. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the rotor systems may include that the forward hub, the rear hub, and the rotor disk form a portion of a compressor section of a gas turbine engine. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the rotor systems may include that the forward hub, the rear hub, and the rotor disk form a portion of a turbine section of a gas turbine engine. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the rotor systems may include a plurality of airfoils connected to the rotor disk. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the rotor systems may include that the seal tube connects at the forward end by one of radial snap fit, axial snap fit, welding, threaded connection, or interference fit. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the rotor systems may include that the seal tube connects at the rear end by one of radial snap fit, axial snap fit, welding, threaded connection, or interference fit. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the rotor systems may include that the engine shaft is a tie shaft and the seal tube connects to the tie shaft at at least one of the forward end and the rear end. 
     According to some embodiments, gas turbine engines are provided. The gas turbine engines include a combustor section, a turbine section, a compressor section, and an engine shaft. At least one of the turbine section and the compressor section includes a rotor system having a forward hub, a rear hub, a rotor disk arranged between the forward hub and the rear hub, and a seal tube configured to define a forward hub compartment and a rear hub compartment, wherein the forward hub compartment is defined forward of the rotor disk and the rear hub compartment is defined aft of the rotor disk. The seal tube is connected at a forward end to at least one of the rotor disk and the engine shaft and at a rear end to at least one of the rear hub and the engine shaft, and the seal tube includes at least one axial compliance element configured to enable axial extension. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the gas turbine engines may include that the at least one axial compliance element comprises a plurality of axial compliance elements between the forward end and the rear end of the seal tube. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the gas turbine engines may include that the at least one axial compliance element has one of a squared geometry, a rounded geometry, and a triangular geometry. 
     The foregoing features and elements may be combined in various combinations without exclusivity, unless expressly indicated otherwise. These features and elements as well as the operation thereof will become more apparent in light of the following description and the accompanying drawings. It should be understood, however, that the following description and drawings are intended to be illustrative and explanatory in nature and non-limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter is particularly pointed out and distinctly claimed 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: 
         FIG.  1    is a schematic cross-sectional illustration of a gas turbine engine; 
         FIG.  2    is a schematic illustration of a portion of a turbine section of the gas turbine engine of  FIG.  1   ; 
         FIG.  3    is a schematic illustration of a rotor system of a gas turbine engine that may incorporate embodiments of the present disclosure; 
         FIG.  4 A  is a schematic illustration of a rotor system of a gas turbine engine in accordance with an embodiment of the present disclosure; 
         FIG.  4 B  is an enlarged illustration of a portion of the rotor system shown in  FIG.  4 A ; 
         FIG.  5    is a schematic illustration of a rotor system of a gas turbine engine in accordance with an embodiment of the present disclosure; 
         FIG.  6    is a schematic illustration of a rotor system of a gas turbine engine in accordance with an embodiment of the present disclosure; 
         FIG.  7    is a schematic illustration of a rotor system of a gas turbine engine in accordance with an embodiment of the present disclosure; 
         FIG.  8    is a schematic illustration of a rotor system of a gas turbine engine in accordance with an embodiment of the present disclosure; and 
         FIG.  9    is a schematic illustration of a rotor system of a gas turbine engine in accordance with an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     As shown and described herein, various features of the disclosure will be presented. Various embodiments may have the same or similar features and thus the same or similar features may be labeled with the same reference numeral, but preceded by a different first number indicating the figure to which the feature is shown. Although similar reference numbers may be used in a generic sense, various embodiments will be described and various features may include changes, alterations, modifications, etc. as will be appreciated by those of skill in the art, whether explicitly described or otherwise would be appreciated by those of skill in the art. 
     Detailed descriptions of one or more embodiments of the disclosed apparatus and/or methods are presented herein by way of exemplification and not limitation with reference to the Figures. 
       FIG.  1    schematically illustrates a gas turbine engine  20 . The gas turbine engine  20  is disclosed herein as a two-spool turbofan that generally incorporates a fan section  22 , a compressor section  24 , a combustor section  26  and a turbine section  28 . The fan section  22  drives air along a bypass flow path B in a bypass duct, while the compressor section  24  drives air along a core flow path C for compression and communication into the combustor section  26  then expansion through the turbine section  28 . With reference to  FIG.  1   , as used herein, “aft” refers to the direction associated with the tail (e.g., the back end) of an aircraft, or generally, to the direction of exhaust of the gas turbine engine (to the right in  FIG.  1   ). The term “forward” refers to the direction associated with the nose (e.g., the front end) of an aircraft, or generally, to the direction of flight or motion (to the left in  FIG.  1   ). An axial direction A is along an engine central longitudinal axis Ax (left and right on  FIG.  1   ). Further, radially inward refers to a negative radial direction relative to the engine axis Ax and radially outward refers to a positive radial direction (radial being up and down in the cross-section of the page of  FIG.  1   ). A circumferential direction C is a direction relative to the engine axis Ax (e.g., a direction of rotation of components of the engine; in  FIG.  1   , circumferential is a direction into and out of the page, when offset from the engine axis Ax). An A-R-C axis is shown throughout the drawings to illustrate the relative position of various components. 
     The exemplary engine  20  generally includes a low speed spool  30  and a high speed spool  32  mounted for rotation about the engine central longitudinal axis Ax relative to an engine static structure  36  via several bearing systems  38 . It should be understood that various bearing systems  38  at various locations may alternatively or additionally be provided, and the location of bearing systems  38  may be varied as appropriate to the application. 
     The low speed spool  30  generally includes an inner shaft  40  that interconnects a fan  42 , a low pressure compressor  44  and a low pressure turbine  46 . The inner shaft  40  is connected to the fan  42  through a speed change mechanism, which in exemplary gas turbine engine  20  is illustrated as a geared architecture  48  to drive the fan  42  at a lower speed than the low speed spool  30 . The high speed spool  32  includes an outer shaft  50  that interconnects a high pressure compressor  52  and high pressure turbine  54 . A combustor  56  is arranged in exemplary gas turbine  20  between the high pressure compressor  52  and the high pressure turbine  54 . An engine static structure  36  is arranged generally between the high pressure turbine  54  and the low pressure turbine  46 . The engine static structure  36  further supports bearing systems  38  in the turbine section  28 . The inner shaft  40  and the outer shaft  50  are concentric and rotate via bearing systems  38  about the engine central longitudinal axis Ax which is collinear with their longitudinal axes. 
     The core airflow is compressed by the low pressure compressor  44  then the high pressure compressor  52 , mixed and burned with fuel in the combustor  56 , then expanded over the high pressure turbine  54  and low pressure turbine  46 . The turbines  46 ,  54  rotationally drive the respective low speed spool  30  and high speed spool  32  in response to the expansion. It will be appreciated that each of the positions of the fan section  22 , compressor section  24 , combustor section  26 , turbine section  28 , and fan drive gear system  48  may be varied. For example, gear system  48  may be located aft of combustor section  26  or even aft of turbine section  28 , and fan section  22  may be positioned forward or aft of the location of gear system  48 . 
     The engine  20  in one example is a high-bypass geared aircraft engine. In a further example, the engine  20  bypass ratio is greater than about six (6), with an example embodiment being greater than about ten (10), the geared architecture  48  is an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3 and the low pressure turbine  46  has a pressure ratio that is greater than about five. In one disclosed embodiment, the engine  20  bypass ratio is greater than about ten (10:1), the fan diameter is significantly larger than that of the low pressure compressor  44 , and the low pressure turbine  46  has a pressure ratio that is greater than about five 5:1. Low pressure turbine  46  pressure ratio is pressure measured prior to inlet of low pressure turbine  46  as related to the pressure at the outlet of the low pressure turbine  46  prior to an exhaust nozzle. 
     The geared architecture  48  may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3:1. It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine and that the present disclosure is applicable to other gas turbine engines including direct drive turbofans. 
     A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The fan section  22  of the engine  20  is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet (10,688 meters). The flight condition of 0.8 Mach and 35,000 ft (10,688 meters), with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (‘TSFC’)”—is the industry standard parameter of lbm of fuel being burned divided by lbf of thrust the engine produces at that minimum point. “Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.45. “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram ° R)/(518.7 ° R)] 0.5 . The “Low corrected fan tip speed” as disclosed herein according to one non-limiting embodiment is less than about 1150 ft/second (350.5 m/sec). 
     Although the gas turbine engine  20  is depicted as a turbofan, it should be understood that the concepts described herein are not limited to use with the described configuration, as the teachings may be applied to other types of engines such as, but not limited to, turbojets, turboshafts, etc. 
     The turbine section  28  is housed within a case  80 , which may have multiple parts (e.g., turbine case, diffuser case, etc.). In various locations, components, such as seals, may be positioned between airfoils  60 ,  62  and the case  80 . For example, as shown in  FIG.  2   , blade outer air seals  82  (hereafter “BOAS”) are located radially outward from the blade  60 . As will be appreciated by those of skill in the art, the BOAS  82  may include BOAS supports that are configured to fixedly connect or attach the BOAS  82  to the case  80  (e.g., the BOAS supports may be located between the BOAS  82  and the case  80 ). As shown in  FIG.  2   , the case  80  includes a plurality of case hooks  84  that engage with BOAS hooks  86  to secure the BOAS  82  between the case  80  and a tip of the airfoil  60 . 
     The blades  60  are mounted, attached to, or otherwise part of respective rotor disks  88 . Between the rotor disks  88  are cavities  90 . The cavities  90  are defined on forward and aft (axially) sides of the rotor disks  88 . Cooling air is supplied to the rotor disks  88  within the cavities  90  for the purpose of cooling the rotor disks  88 , other components of the system, and/or for supplying cooling air into and around the airfoils  60 ,  62  and components thereof. 
     Typically, in gas turbine engines and other turbomachinery, compressor air can be used to cool the turbine. In some such configurations, cooling air that is bled from a compressor flow path travels through a rear bore compartment on the way to the turbine, having high temperatures and pressures. However, the forward bore compartment contains relatively lower temperature and pressure air which is being routed to cool other engine components. To maintain rotor durability and performance it is crucial to avoid mixing of the two air supplies. This requires the forward and rear bore compartments to be sealed from one another. 
     For example, turning now to  FIG.  3   , a schematic illustration of a rotor system  300  of a gas turbine engine is shown. The rotor system  300  may be part of an engine as shown and described above. The rotor system  300  may be a compressor section or a turbine section of a gas turbine engine, for example. As shown, the rotor system  300  is formed of a group of rotor disks  302   a - d  which each include respective airfoils  304   a - d  connected thereto. The airfoils  304   a - d  may be connected to the rotor disks  302   a - d  through known means, such as mounting configurations, integrally formed blades, platform attachments, or the like. In this illustrative partial view, the rotor system  300  includes four rotor disks  302   a - d  with associated airfoils  304   a - d . However, it will be appreciated that rotor systems that incorporate embodiments of the present disclosure may include any number of rotor disks and associated blades. Each rotor disk  302   a - d  includes a respective rotor bore  306   a - d . The rotor disks  302   a - d  may be arranged about a engine shaft  308  (e.g., tie shaft). Forward of a forward-most rotor disk  302   a  is a forward hub  310  and aft of an aft-most rotor disk  302   d  is a rear hub  312 . The forward hub  310  and the rear hub  312  may each connect to the engine shaft  308 . 
     Between adjacent rotor disks  302   a - 302   d  are defined rotor cavities  314 . Further, a forward cavity  316  is defined between the forward-most rotor disk  302   a  and the forward hub  310 . Similarly, a rear cavity  318  is defined between the aft-most rotor disk  302   d  and the rear hub  312 . The cavities  314 ,  316 ,  318  are configured to receive air to provide cooling to the rotor disks  302   a - d  and/or for cooling other components of the rotor system  300  and/or other (e.g., downstream) components and/or sections of the turbine engine. 
     The cavities  314 ,  316 ,  318  may be fluidly connected such that cooling flow can pass from a forward end toward an aft end of the series of cavities  314 ,  316 ,  318 . It may be advantageous to separate portions of the cavities  314 ,  316 ,  318 . For example, a forward bore compartment  320  and a rear bore compartment  322  may be defined and fluidly separate from each other, with each bore compartment  320 ,  322  defined by one or more of the cavities  314 ,  316 ,  318 . In this illustrative embodiment, the forward bore compartment  320  is defined by the forward cavity  316  and the rotor cavities  314  and the rear bore compartment  322  is formed of the rear cavity  318 . The fluid separation of the forward bore compartment  320  from the rear bore compartment  322  may be achieved using a sealing element  324 . The sealing element  324  may be, in some configurations, a piston seal ring that engages and seals between the engine shaft  308  and the aft most rotor bore  306   c  that defines the forward bore compartment  320 . In some configurations, the rear bore compartment  318  may include one or more of the rotor cavities  314 , with the sealing element  324  being arranged between the engine shaft  308  and a more forward rotor disk. For example, the sealing element  324  may be arranged between the second-to-aft-most rotor disk  306   c,  and thus the aft-most rotor disk  306   d  may be arranged within the rear bore compartment  322 . 
     A drawback to such sealing elements is that they may form imperfect seals, thus allowing for bleed between the forward and rear bore compartments. For example, relative motion and vibration at the location of the sealing element may lead to high wear rates of the seals, causing degradation and/or failure thereof. Upon degradation or failure, the two previously fluidly separate compartments will become fluidly connected. This fluid connection can cause a rise in temperature in the forward bore compartment due to the relatively hot/high pressure air from the rear bore compartment flowing into the forward bore compartment. This in turn can result in lower efficiency and/or undesired operating temperatures. 
     In view of this, and in view of other considerations, embodiments of the present disclosure are directed to improved sealing configurations for turbine sections and/or compressor sections (i.e., rotor system) of gas turbine engines. In accordance with some embodiments, an axially compliant sealing tube is arranged to connect between two rotor stages via interface retention features. In one non-limiting embodiment, axial and radial snaps can be employed at the interface between the sealing tube and a part of the rotor system. However, it will be appreciated that a variety of other interface retention features can be used without departing from the scope of the present disclosure. For example, and without limitation, the connection between the sealing tube and the rotor system can include snap fits (radial and/or axial), snap rings, welding, threaded connections, interference fits, etc., and/or combinations thereof. 
     The seal tube, in accordance with embodiments of the present disclosure, is an axially compliant structure that provides sealing between bore compartments while allowing or accommodating relative motion, vibrations, thermal expansion, etc. In some embodiments, the seal tubes described herein may include one or more undulation features that allow the tube to act as a soft spring in an axial direction. The size, shape, and quantity of the undulations can be used to tune a spring rate of the seal tube based on system requirements. A low spring rate of the seal tube can allow the seal tube to be compressed during assembly and operation with relatively low force. The assembly compression may be sufficient to keep the seal tube interface retention features seated on the rotors (or other seating structures) during operation when pressure, speed, and temperature loads cause the rotors and/or hubs of the rotor system to deflect away from one another. Such compression/expansion of the seal element is beneficial because it can reduce sliding motion on the interfaces, which in turn can reduce wear. The reduction in wear may improve rotor durability and reduce binding load caused by such wear. In addition to the seal element itself providing a fluid barrier, interface retention features that hold the seal tube in place can act as seals which prevent mixing of the forward and rear bore compartment air. 
     turning now to  FIGS.  4 A- 4 B , schematic illustrations of a rotor system  400  of a gas turbine engine in accordance with an embodiment of the present disclosure are shown. The rotor system  400  may be part of an engine as shown and described above. The rotor system  400  may be a compressor section or a turbine section of a gas turbine engine, for example. As shown, the rotor system  400  is formed of a group of rotor disks  402   a - d  which each include respective airfoils  404   a - d  connected thereto. The airfoils  404   a - d  may be connected to the rotor disks  402   a - d  through known means, such as mounting configurations, integrally formed blades, platform attachments, or the like. In this illustrative partial view, the rotor system  400  includes four rotor disks  402   a - d  with associated airfoils  404   a - d . However, it will be appreciated that rotor systems that incorporate embodiments of the present disclosure may include any number of rotor disks and associated blades. Each rotor disk  402   a - d  includes a respective rotor bore  406   a - d . The rotor disks  402   a - d  may be arranged about a engine shaft  408 . Forward of a forward-most rotor disk  402   a  is a forward hub  410  and aft of an aft-most rotor disk  402   d  is a rear hub  412 . The forward hub  410  and the rear hub  412  may each connect to the engine shaft  408 . 
     Between adjacent rotor disks  402   a - 402   d  are defined rotor cavities  414 . Further, a forward cavity  416  is defined between the forward-most rotor disk  402   a  and the forward hub  410 . Similarly, a rear cavity  418  is defined between the aft-most rotor disk  402   d  and the rear hub  412 . The cavities  414 ,  416 ,  418  are configured to receive air to provide cooling to the rotor disks  402   a - d  and/or for cooling other components of the rotor system  400  and/or other (e.g., downstream) components and/or sections of the turbine engine. 
     The cavities  414 ,  416 ,  418  may be fluidly connected such that cooling flow can pass from a forward end toward an aft end of the series of cavities  414 ,  416 ,  418 . It may be advantageous to separate portions of the cavities  414 ,  416 ,  418 . For example, a forward bore compartment  420  and a rear bore compartment  422  may be defined and fluidly separate from each other, with each bore compartment  420 ,  422  defined by one or more of the cavities  414 ,  416 ,  418 . In this illustrative embodiment, the forward bore compartment  420  is defined by the forward cavity  416  and the rotor cavities  414  and the rear bore compartment  422  is formed of the rear cavity  418  and one of the rotor cavities  414  (e.g., forward of the aft-most rotor disk  402   d ). 
     In this illustrative embodiment, a seal tube  424  is arranged to fluidly separate the forward bore compartment  420  and the rear bore compartment  422 . The seal tube  424  is a hoop structure or basket structure that is arranged about the engine shaft  408  during assembly of the rotor system  400 . The seal tube  424  may include one or more axial compliance elements  426  along an axial length thereof. The axial compliance elements  426  allow for or enable axial extension and compression of the seal tube  424  without unseating the seal tube  424  from engagement with the rotor system  400 . As shown in  FIG.  4 B , a forward end  428  of the seal tube  424  engages with a portion  430  of a rotor hub  406   c.  Similarly, a rear end  432  of the seal tube  424  engages with a portion  434  of the rear hub  412 . As such, the seal tube  424  can provide a fluid barrier between the forward bore compartment  420  and the rear bore compartment  422 . Although illustratively shown as a single, unitary structure, in other embodiments, the seal tube  424  may be formed from two or more sub-rings or sub-tubes that are sealingly joined together to form the structure of the seal tube  424 . 
     As shown in  FIG.  4 B , the rear bore compartment  422  extends in an axial forward direction from the rear hub  412  to the second-aft-most rotor disk  402   c.  As such, the aft-most rotor disk  402   d  is thus wholly positioned within the rear bore compartment  422 . Additionally, as shown, the rear hub  412  may include a rear bore  436 , as will be appreciated by those of skill in the art, which is also arranged within the rear bore compartment  422 . 
     In this illustrative embodiment, the seal tube  424  is configured with three axial compliance elements  426 . The axial compliance elements  426 , in this embodiment, are convolutions or undulations of material of the seal tube  424 . The axial compliance elements  426  may take any desired shape, size, and form to achieve a desired amount of axial compliance (e.g., compression and expansion in an axial direction along the engine, such as along the engine shaft  408 ). For example, alternative geometric shapes, changes or variations in material thickness, radial height, axial length, etc. may all be properties of the axial compliance elements  426  that may be adjusted or changed to achieve a desired axial compliance and maintaining a fluid seal between the bore compartments  420 ,  422 . 
     As shown in  FIGS.  4 A- 4 B , one of the axial compliance elements  426  may be arranged radially inward from a bore (e.g., rear bore  436 ). In such configurations, a gap  438  may be defined between the bore and the material of the seal tube  424 . The rear bore  436  and/or the axial compliance element  426  are arranged to form the gap  438  and prevent contact between the two elements during operation. 
     Although  FIGS.  4 A- 4 B  illustrate a single configuration, those of skill in the art will appreciate that variations thereon are possible without departing from the scope of the present disclosure. For example, the seal tube  424  includes three axial compliance elements  426 , although in other embodiments, as few as a single axial compliance element may be employed, or in other embodiments more than three may be used. Further, although shown with a substantially curved-square undulation geometry, the geometry of such undulations may be varied, such as curved, rounded, triangular, polygonal, etc. Further, the material thickness of the seal tube  424  may be uniform from the forward end  428  to the rear end  432 . However, in other embodiments, the material thickness of the seal tube may be varied along the axial length thereof. For example, increased material thickness may be provided at the forward and rear ends, with a lower material thickness at the axial compliance elements. The opposite may be true, with a greater thickness at the axial compliance elements and less thickness at the ends that join to the components of the rotor system. Additionally, in the illustrative embodiment of  FIGS.  4 A- 4 B , the seal tube  424  is attached to a rotor bore  406   c  and the rear hub  412 . In other embodiments, the seal tube may be attached to (at either or both ends) the engine shaft  408 , or to other appropriate structures. Such mounting will still be intended to form the rear bore compartment  422 , regardless of the specific mounting configuration. 
     Turning now to  FIG.  5   , a schematic illustration of a portion of a rotor system  500  having a seal tube  502  in accordance with an embodiment of the present disclosure is shown. The seal tube  502  is arranged to provide a fluid seal between a forward bore compartment and a rear bore compartment, similar to that described above. In this embodiment, the seal tube  502  includes a rounded axial compliance element  504 , a squared axial compliance element  506 , and a triangular axial compliance element  508 . In this embodiment, the different geometries are illustrated in the single seal tube  502 , thus having multiple different geometric axial compliance elements. However, in other embodiments, each axial compliance element of a seal tube may have the same geometry as all other axial compliance elements, although a mixture, as illustrated may also be employed. 
     Turning now to  FIG.  6   , a schematic illustration of a portion of a rotor system  600  having a seal tube  602  in accordance with an embodiment of the present disclosure is shown. The seal tube  602  is arranged to provide a fluid seal between a forward bore compartment and a rear bore compartment, similar to that described above. In this embodiment, the seal tube  602  includes multiple axial compliance elements having different material thicknesses. For example, one axial compliance element has an axial component  604  with an increased material thickness and another axial compliance element has a radial component  606  with an increased material thickness.  FIG.  6    also illustrated the seal tube  602  attached to a engine shaft  608  at a rear end  610  of the seal tube  602 . Further, in this illustrative configuration, a forward end  612  of the seal tube  602  connects in a different manner than that illustratively shown in the other embodiments. This may be achieved, in part, due to the seal tube  602  having an increased material thickness portion  614  at the forward end  612  of the seal tube  602 . 
     Turning now to  FIG.  7   , a schematic illustration of a portion of a rotor system  700  having a seal tube  702  in accordance with an embodiment of the present disclosure is shown. The seal tube  702  is arranged to provide a fluid seal between a forward bore compartment and a rear bore compartment, similar to that described above. In this embodiment, the seal tube  702  includes a single axial compliance element  704 . In this embodiment, the axial compliance element  704  has an increased radial height  706  and an increased axial length  708 , as compared to the other illustrative embodiments. It will be appreciated that both the radial height  706  and the axial length  708  may be changed to achieve a desired axial compliance for the seal tube  702 . Further, multiple axial compliance elements may be used even with greater height and/or length axial compliance elements, and thus such increased dimension configuration is not limited to situations where only a single axial compliance element is present. 
     Turning now to  FIG.  8   , a schematic illustration of a portion of a rotor system  800  having a seal tube  802  in accordance with an embodiment of the present disclosure is shown. The seal tube  802  is arranged to provide a fluid seal between a forward bore compartment  804  and a rear bore compartment  806 , similar to that described above. In this embodiment, the seal tube  802  includes multiple axial compliance elements  808  and extends for a greater axial length than prior illustrated configurations. As shown, the seal tube  802  extends forward farther and thus the rear bore compartment  806  includes two rotor disks  810  in their entirety and the rear or aft side of another rotor disk  812 . Further, as shown, the axial compliance elements  808  are not restricted to the location along the aft-most portion thereof. Rather, as shown, one or more axial compliance elements  808  may be arranged between forward rotor disks  810  within the rear bore compartment  806 . 
     Turning now to  FIG.  9   , a schematic illustration of a portion of a rotor system  900  in accordance with an embodiment of the present disclosure is shown. In this embodiment, the rotor system  900  includes two seal tubes  902 ,  904  arranged to provide a fluid seal between various bore compartments of the rotor system  900 . As shown, a forward seal tube  902  is arranged to connect between a forward hub  906  and a forward rotor  908  to define a forward bore compartment  910 . Similarly, a rear seal tube  904  is arranged to connect between a rear hub  912  and a rear rotor  914  to define a rear bore compartment  916 . In this configuration, the rotor system  900  is part of an engine that does not include a tie shaft, as described above, although a low spool shaft  918  is shown that connects between a turbine and a fan of the engine system. In this configuration, the rotors  908 ,  914  and other rotors of the rotor system may be part of a welded rotor drum or rotor stack, as will be appreciated by those of skill in the art. The rotors  908 ,  914 , the hubs  906 ,  912 , and other rotors of the rotor system  900  may be connected by one or more rotor connectors  920 . The rotor connectors  920  may be welds, bolted-flange connections, or other connection devices or mechanisms, as will be appreciated by those of skill in the art. 
     In this embodiment, because there is no tie shaft, the seal tubes  902 ,  904  directly connect between the respective hubs  906 ,  912  and the respective rotors  908 ,  914 . In some embodiments, the forward rotor  908  and the rear rotor  914  may be the same rotor, with the forward seal tube  902  attaching to a forward face of the rotor and the rear seal tube  904  attaching to a rear face of the rotor. In other embodiments, a seal tube may be connected between two rotors (e.g., not attached to the forward or rear hub). For example, with reference to  FIG.  9   , a seal tube could be attached to a rear face of the forward rotor  908  and a forward face of the rear rotor  914  and define a compartment therebetween. 
     It will be appreciated that the features illustrated and described in the embodiments of  FIGS.  4 A- 9    may be mixed and matched to achieve a desired axial compliance for the seal tubes of the present disclosure. Different geometries, material thicknesses, numbers of axial compliance elements, connection points, etc. may be employed to achieve a desired sealing and axial compliance to maintain the seal and fluid separation during use and operation. 
     Advantageously, embodiments of the present disclosure are directed to seal tube configurations having built in undulations or axial compliance features that improve axial flexibility to maintain fluid separation of bore compartment cavities. The seal tubes described herein may reduce sliding at the seal tube interfaces with the mounting structures (e.g., tie shaft, rotor hub, etc.) which will reduce wear and improve rotor durability. Improved sealing may be achieved. Such sealing, as provided by the seal tubes described herein, even if not a perfect seal, may provide for more uniform sealing and thus more uniform leakage therethrough. Accordingly, improved control of any leakage from one bore compartment to another may be achieved. 
     In prior piston seal ring configurations used to separate the forward and rear bore compartments, the tolerances and end gap(s) may allow for non-uniform leakage and thermal conditioning at the seal interface. Configurations that use non-compliant sealing tubes and/or piston seal rings have the potential to transmit large non-uniform binding loads. The non-uniform leakage and/or binding loads caused by interface wear or part and assembly tolerances can result in centerline shift and high vibration. However, advantageously, in accordance with embodiments of the present disclosure, the seal tubes having retention features at the rotor interfaces can eliminates the potential for non-uniform leakage and improves engine reliability. Further, the axial compliance of the seal tubes described herein can reduce the magnitude of non-uniform axial loads, thus reducing vibration and improving reliability. 
     The use of the terms “a”, “an”, “the”, and similar references in the context of description (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or specifically contradicted by context. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the particular quantity). All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. As used herein, the terms “about” and “substantially” are intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. For example, the terms may include a range of ±8%, or 5%, or 2% of a given value or other percentage change as will be appreciated by those of skill in the art for the particular measurement and/or dimensions referred to herein. It should be appreciated that relative positional terms such as “forward,” “aft,” “upper,” “lower,” “above,” “below,” and the like are with reference to normal operational attitude and should not be considered otherwise limiting. 
     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, combinations, sub-combinations, 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.