Abstract:
An assembly comprises a shaft, and a support structure surrounding the shaft; and a magnetic seal system comprising an annular seal assembly including a ring sealingly mounted to a shaft to rotate therewith and slidingly axially displaceable along the shaft, and an annular seal supported by the ring. An annular magnet assembly is configured to be non-rotatingly supported adjacent to and surrounding the shaft, the annular magnet assembly configured and positioned relative to the ring to exert a sufficient attracting force on the ring to biasingly displace the ring axially along the shaft into sealing contact with the magnet. A cooling fluid feeding conduit, a cooling fluid exhaust conduit distinct from the cooling fluid feeding conduit are provided. An annular cavity is defined at least partially by or in a radially outer surface of the annular magnet, the annular cavity being in fluid communication with the cooling fluid feeding conduit, and with the cooling fluid exhaust conduit, for circulation of cooling fluid in the annular cavity.

Description:
TECHNICAL FIELD 
     The application relates generally to magnetic seals of the type used to seal a rotating shaft and, more particularly, to cooling of such magnetic seals. 
     BACKGROUND OF THE ART 
     Magnetic seals are sometimes used for non-contact sealing in rotating systems like gas turbine engines. The high speeds at which these engines run, however, often requires cooling of the seals. Although oil can be used to cool the hot seal surfaces, this can also cause additional oil leakage through the sealing interface, decreasing the overall effectiveness of the seal. Room for improvement exists. 
     SUMMARY 
     In one aspect, there is provided a magnetic seal system comprising: an annular seal assembly including a ring, the ring configured to be sealingly mounted to a shaft to rotate therewith and slidingly axially displaceable along the shaft, and an annular seal supported by the ring; an annular magnet assembly configured to be non-rotatingly supported adjacent to and surrounding the shaft, the annular magnet assembly configured and positioned relative to the ring to exert a sufficient attracting force on the ring to biasingly displace the ring axially along the shaft into sealing contact with the magnet, the magnet at least partially defining an annular cavity at least partially by or in a radially outer surface of the annular magnet, the annular cavity being in fluid communication with a cooling fluid inlet and a cooling fluid exhaust distinct from the inlet. 
     In a second aspect, there is provided an assembly comprising: a shaft; a support structure surrounding the shaft; and a magnetic seal system comprising an annular seal assembly including a ring sealingly mounted to a shaft to rotate therewith and slidingly axially displaceable along the shaft, and an annular seal supported by the ring; an annular magnet assembly non-rotatingly supported adjacent to and surrounding the shaft, the annular magnet assembly positioned relative to the ring to exert a sufficient attracting force on the ring to biasingly displace the ring axially along the shaft into sealing contact with the magnet, a cooling fluid feeding conduit, a cooling fluid exhaust conduit distinct from the cooling fluid feeding conduit, and an annular cavity defined at least partially by or in a radially outer surface of the annular magnet, the annular cavity being in fluid communication with the cooling fluid feeding conduit, and with the cooling fluid exhaust conduit, for circulation of cooling fluid in the annular cavity. 
     In a third aspect, there is provided a method for cooling an annular magnet of a magnetic seal assembly sealing a gap between a shaft and a support structure, the method comprising: feeding cooling fluid to an annular cavity surrounding the annular magnet via a first conduit; circulating the cooling fluid in the annular cavity such that the cooling fluid flows directly against a radially outer surface of the annular magnet; and exhausting the cooling fluid via a second conduit distinct from the first conduit. 
     Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures, in which: 
         FIG. 1  is a schematic cross-sectional view of a turbofan gas turbine engine; 
         FIG. 2  is a schematic sectional view of a magnetic seal system in accordance with an embodiment of the present disclosure; 
         FIG. 3  is a schematic sectional view of a magnetic seal system in accordance with another embodiment of the present disclosure; and 
         FIG. 4  is a schematic sectional view of a magnetic seal system in accordance with yet another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a turbofan gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan  12  through which ambient air is propelled, a multistage compressor  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. An accessory gearbox  19  may be driven by either one of the compressor  14  and the turbine section  18 . 
     Referring to  FIG. 2 , a magnetic seal system in accordance with the present disclosure is generally shown at  20 , for instance of the type used in the accessory gearbox  19  of the gas turbine engine  10 . It is also contemplated to use the magnetic seal system  20  in other applications as well. For example, the magnetic seal system  20  can be used as an output shaft seal on a turboshaft and turboprop engines, as well as a bearing cavity seal on engine mainshafts. The magnetic seal system  20  is used to seal a space A between a shaft  30  and a support structure  40  (i.e., a structure of the apparatus using the magnetic seal system  20 , a housing thereof, etc), to block fluid passage through the space A. In the illustrated embodiment, the space A is an annular space. 
     The magnetic seal system  20  comprises an annular seal assembly including a mating ring  21 , also known as seal runner. The mating ring  21  typically consists of a structurally rigid material, such as a metal, with a ferromagnetic content. The mating ring  21  is mounted to the shaft  30  to rotate therewith by friction action of a seal  23 , such that the mating ring  21  may move in translation along the shaft. In other words, the mating ring  21  is secured to the shaft  23  to rotate with it, but a sufficient force can be used to displace the mating ring  21  along the shaft  30 . Accordingly, the mating ring  21  is axially displaceable along the shaft  30 , in axial direction X. One or more seals  23  may be provided to seal off an interface between the mating ring  21  and the shaft  30 . For instance, the seal  23  may be an O-ring, a gasket, etc, made of a material capable of withstanding the pressures and temperatures in the apparatus. Moreover, the material must be resistant to the nature of ambient fluids (e.g., oil). 
     The mating ring  21  defines a shoulder  25  that is configured to receive thereon an annular seal  26 , part of the annular seal assembly. The annular seal  26  forms part of the dynamic seal interface of the magnetic seal system  20 , as it will rotate with the mating ring  21  and the shaft  30 , while rubbing against an annular magnet  27 , part of an annular magnet assembly. The annular seal  26  is therefore connected to the mating ring  21  to rotate with it. In the illustrated embodiment, the shoulder  25  provides a pair of abutment surfaces for the annular seal  26 , to strengthen the connection between the annular seal  26  and the mating ring  21  and cause concurrent rotation. Other arrangements are contemplated, such as an annular groove in the mating ring  21  to accommodate a portion of the annular seal  26 . The annular seal  26  is made of a material that will wear off gradually, while forming a contact surface conforming to the component it will rub against, to create the dynamic seal interface. For example, the annular seal  26  is made of carbon, or equivalent. 
     The annular magnet  27  is connected to the support structure  40 , in manners described hereinafter. The annular magnet  27  is sized to surround the shaft  30 , yet not contact it. The annular magnet  27  exerts an attracting force, such that the mating ring  21  is drawn toward the annular magnet  27 . As it is movable on the shaft  30  in direction X, the mating ring  21  presses the annular seal  26  against one of the lateral surfaces  27 A of the annular magnet  27 . The annular contact interface between the annular seal  26  and the lateral surface  27 A of the magnet  27  is therefore the dynamic seal interface, blocking fluid from passing through the space A. 
     The annular magnet  27  also has a radially outer surface  27 B. The surface  27 B is said to be a radially outer due to its positional relation to the shaft  30 . The radially outer surface  27 B shares edges  27 C with the lateral surfaces  27 A, and is the surface between these edges  27 C. As shown in  FIG. 2 , the radially outer surface  27 B may have a hollow annular cavity  28  defined therein. The hollow annular cavity  28  defines an empty space therein. The annular cavity  28  may form a full annulus, or may have an axial wall therein to define a C as opposed to a full annulus. The annular cavity  28  may be machined or cast into the annular magnet  27 . Annular seal cavities  29  may be provided on opposed sides of the annular cavity  28 , each being defined to receive at least one seal  29 A therein. For instance, the seals  29 A may each be an O-ring, a gasket, etc, made of a material capable of withstanding the pressures and temperatures in the apparatus, and resistant to the nature of ambient fluids, such as oil. Axial or corner seals could be used as alternatives to the annular seal cavities  29 . It is contemplated to construct the annular magnet  27  and the support structure  40  is such a way that the interface therebetween does not require additional seals such as the seals  29 A. 
     Still referring to  FIG. 2 , the support structure  40  is shown defining an abutment shoulder  41  against which the annular magnet  27  may be abutted. A locking ring  42  may block the annular magnet  27  in the axial direction X, for the annular magnet  27  to be held captive in the manner shown in  FIG. 2 . Alternatives to the locking ring  42  are considered, such as a threaded lock sleeve, additional structure, etc, to hold the annular magnet  27  captive in the support structure  40 . 
     The support structure  40  may also have an annular cavity  43  machined therein, and axially aligned with the annular magnet  27 . In the illustrated embodiment, a common annular cavity is defined by the combination of annular cavities  28  and  43 , each forming an annular cavity portion. This common annular cavity is connected to a source of cooling fluid, via a cooling fluid feeding conduit  45  and an outlet  45 A thereof. The common annular cavity is also connected to a cooling fluid exhaust conduit  46  via an inlet  46 A thereof, distinct from the feeding conduit  45 , for the exhaust of the cooling fluid from the annular cavity. As a result, cooling fluid may circulate in the annular cavity by this arrangement of distinct conduits  45  and  46 , for the cooling fluid to absorb heat of the annular magnet  27 . Hence, the cooling fluid is in direct contact with the annular magnet  27 . 
     While a common annular cavity consisting of the combination of the annular cavities  28  and  43  (which hence form cavity portions), the annular cavity may consist of a single one of the annular cavities  28  and  43 . If the arrangement is without the annular cavity  43 , as in  FIG. 3 , the feeding conduit  45  would feed the cooling fluid directly into the annular cavity  28  via its outlet  45 A. Alternatively, if the arrangement is without the annular cavity  28 , as in  FIG. 4 , the feeding conduit  45  would feed the cooling fluid directly into the annular cavity  43  via its outlet  45 A, but the cooling fluid would still come into contact with the radially outer surface  27 B, as the annular cavity  43  is open to the radially outer surface  27 B. It is observed that the arrangement featuring the annular cavity  28 , with or without the annular cavity  43 , offers greater heat exchange surface with the material of the annular magnet  27 , in comparison to an arrangement without the annular cavity  28 . 
     Hence, the annular cavity, as in any one of  FIGS. 2, 3 and 4 , is entirely circumscribed or defined by the annular magnet  27  and the support structure  40 , and does not rely on other members such as seals to define its boundaries. Seals  29 A may help prevent leaks, but do not define the annular cavity. Hence, the annular cavity is said to provide internal cooling, as the system preserves the cooling fluid in a closed cavity, the cooling fluid not being misted out to the environment of space A. The annular cavity is bound by the rigid walls of the annular magnet  27  and of the support structure  40 . 
     In an embodiment, the outlet of the feeding conduit  45  and the inlet of the exhaust conduit  46  are generally diametrically opposed, to ensure a suitable surrounding flow of cooling fluid around the annular magnet  27 . The expression “generally” is used to indicate that the conduits  45  and  46  may be offset by a few degrees from being substantially diametrically opposed. If the annular cavity is C-shaped, the outlet of the feeding conduit  45  and the inlet of the exhaust conduit  46  are at opposed ends of the C. 
     The feeding conduit  45  and the exhaust conduit  46  may be machined or fabricated directly in the support structure  40 . Alternatively, the feeding conduit  45  and the exhaust conduit  45  may be separate tubes, pipes and/or conduits extending to and from the annular cavity. 
     The cooling fluid may be cooling air or cooling oil, supplied by cooling fluid source P 1  connected to the feeding conduit  45 . The exhaust conduit  46  may be connected to a scavenge cavity, a tank, or any other component collecting the cooling fluid, generally shown at P 2 . In an embodiment, the pressure at P 1  is greater than that at P 2 , to induce a flow of the cooling oil from P 1  to P 2 . Hence, the pressure differential between P 2  and P 1  is negative during use. 
     The magnetic seal system  20  may therefore operate a cooling method that follows. Cooling fluid is fed to the annular cavity surrounding the annular magnet  27  via a first conduit, the feeding conduit  45 . The cooling fluid is circulated in the annular cavity  28  and/or  43  such that the cooling fluid flows directly against a radially outer surface of the annular magnet  27 . The cooling fluid is exhausted via a second conduit, the exhaust conduit  46 , distinct from the first conduit  45 . The feeding and exhausting of the cooling fluid may comprise creating a negative pressure differential between the second conduit  46  and the first conduit  45 . The feeding and exhausting of the cooling fluid may also comprise inletting the cooling fluid in the annular cavity  28  and/or  43  at a location generally diametrically opposed to that of outletting the cooling fluid. 
     The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. For example, the magnetic seal system  20  may be found in reduction gearboxes, or to seal a space between a shaft and surrounding structure in other environments. The annular magnet  27 , although shown as an integral monolithic magnet, may be a non-magnet ring, supporting a plurality of discrete magnets, provided such discrete magnets produce sufficient attracting forces to displace the annular seal assembly as the annular seal  26  wears. Although not shown, anti-rotation features may be provided (lugs, keys) to ensure that the annular magnet  27  is fixed relative to the support structure  40 , and to ensure that the mating ring  21  rotates with the shaft  30  while being axially displaceable thereon. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.