Gas Turbine Engine Rear Magnetic or Foil Bearing Cooling Using Exhaust Eductor

An object of the present invention is to provide an exhaust housing for a gas turbine engine having means for providing cooling air flow to a non-oil lubricated bearing mounted therein.

The present invention achieves this object by providing an exhaust housing having an eductor which includes an outer annular casing circumscribing an inner annular casing to define a flow path therebetween for receiving the exhaust gas from the turbine section of the engine. A plurality of struts are integral with both casings. The struts have cooling passages that extend from the exterior of the outer casing to the interior of the inner casing. A non-oil lubricated bearing is mounted within the interior of the inner casing for journaling a rotating shaft. During operation, the kinetic energy of the high velocity exhaust gas flowing between the casings and into a tailpipe, induces a flow of cooling air from the exterior of the outer casing, through the cooling passages, across the bearing, and then out through the tailpipe.

FIG. 1 is a cross sectional view of a gas turbine engine exhaust housing having a rotating shaft journalled on a magnetic bearing as contemplated by the present invention.

FIG. 2 is a cross sectional view of a gas turbine engine exhaust housing having a rotating shaft journalled on an air bearing as contemplated by the present invention.

FIGS. 1 and 2 show an aft portion 10 of a gas turbine engine which comprises a turbine section 20, an exhaust housing 30, and an annular tail pipe 60. The turbine section 20, which is only partially shown, includes a annular turbine casing 22 circumscribing a rotating tie shaft 24 to define a flow path 26 therebetween. The axial centerline of the shaft 24 is coincident with the engine centerline 12. A last stage turbine blade assembly 28 is mounted to the shaft 24 and is disposed in the flow path 26. The tie shaft 24 is overhung in that it extends axially beyond the turbine casing 22 into the exhaust housing 30.

The exhaust housing 30 includes an outer annular casing 32 flanged at both ends for mating with the turbine casing 22 at one end, and the tail pipe 60 at the other end. Disposed within the outer casing 32 is an inner annular casing 34 which defines a flow path 33 therebetween. In a manner familiar to those skilled in the art, the outer annular casing 32 and the inner annular casing 34 are arranged to form an eductor. A methodology for configuring the inner and outer casings 34, 32 to form an eductor can be found in Appendix A of F. A. TOMKINS & GARRETT TURBINE ENGINE COMPANY ENGINEERING STAFF, INSTALLATION HANDBOOK AIRBORNE AUXILIARY POWER UNITS (2nd ed. 1983), which is incorporated by reference. A plurality of circumferentially spaced struts 38 couple the outer casing 32 to the inner casing 34. Within the interior of the inner casing 34 is a cylindrical bearing carrier 36. The bearing carrier 36 is attached to an axial end 35 of the inner casing 34 so as to define a gap 37 therebetween. At its opposite end, the bearing carrier 36 is closed off by a bearing housing 45. The struts 38 have cooling passages 39 which place the gap 37 in fluid communication with the environment surrounding the engine 10, which is usually an aircraft compartment.

Referring to FIG. 1, a magnetic bearing 40 is mounted within the bearing carrier 36. The bearing 40 includes an annular stator 42 circumscribing an annular rotor 44 having two laminates 43 of a ferromagnetic material. The tie shaft 24 is journalled in the rotor 44 with axial tension provided by a tie nut 25. A self lubricating ball bearing 46 having a consumable graphite separator is mounted to a support member 48 that is integral with the bearing housing 45. The ball bearing 46 supports the rotor 44 in the event of a catastrophic failure. A seal 49 is disposed between the bearing carrier 36 and the annular rotor 44, to prevent the ingestion of hot gas and particles from the turbine section 20.

In an alternative embodiment, shown in FIG. 2, a conventional air or foil bearing 50 is mounted between the shaft 24 and the bearing carrier 36. The foil bearing 50 includes a foil carrier 52 carrying a plurality of overlapping foils 54. The foils 54 engage a journal 56 which is mounted to the shaft 24. In addition, the bearing carrier 36 has a plurality of air holes 58 that place the interior of the bearing 50 in fluid communication with the gap 37.

During operation of the preferred embodiment, the kinetic energy of the high velocity exhaust gas flowing through the flow path 33 and into the tailpipe 60 mixes with low velocity air flowing into the tailpipe 60 from the interior of the inner annular casing 34. Through this mixing, the kinetic energy of the exhaust gas is transferred to the low velocity air. The air is accelerated creating a drop in static pressure, that is a suction, that induces a flow of cooling air from the exterior of the outer casing 32, through the passages 39, into the gap 37, across the exterior surface of the bearing carrier 36, and then out to the tailpipe 60. The bearing house 45 seals the interior of the bearing carrier 36 from the gap 37, thereby preventing the eductor from inducing a flow of hot gas through the seal 49. Similarly, in the alternate embodiment, air flow is not only induced over the bearing carrier 36, but also through the holes 58 and foils 54. Because there is no bearing housing, the seal 49 must be configured to prevent the inducement of a hot gas flow by the eductor. To enhance the effectiveness of the eductor, mixing lobes can be added to the outer surface of the inner casing 34.

Thus, an exhaust housing for a gas turbine engine is provided having means for providing cooling air flow to non-oil lubricated bearings mounted therein.

Various modifications and alterations to the above described preferred and alternate embodiments of the exhaust housing will be apparent to those skilled in the art. For example, the cooling passages 39 can communicate with a manifold mounted on the exterior surface of the outer casing 32. Air can then be delivered to the manifold from any location in the aircraft. Accordingly, this description of the invention should be considered exemplary and not as limiting the scope and spirit of the invention as set forth in the following claims.