Gas turbine engine rear magnetic or foil bearing cooling using exhaust eductor

An exhaust housing having an eductor is provided. The eductor 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 an engine. A plurality of struts are integral with both casings. The struts have 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 cooling airflow from the exterior of the exhaust section, through the passages, across the bearing, and then out to the tailpipe.

SUMMARY OF THE INVENTION 
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.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
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.