Seal accommodating thermal expansion between adjacent casings in gas turbine engine

A casing around a turbine and a casing around discharge nozzles have a concentrically arranged shell portion. The seal contains internal pressure while accommodating eccentric, expansion and axial travel. Arcuate seal segments have one leg sealing against a radial surface extending from the inner shell and the other leg against the outer shell. A linkage guides travel of the segments.

TECHNICAL FIELD 
The invention relates to gas seals and in particular to seals between 
non-rotating members in a gas turbine engine which experience relative 
displacement. 
BACKGROUND OF THE INVENTION 
A gas turbine engine as used in an aircraft requires a compressor, a gas 
turbine and an exhaust duct. The exhaust duct includes discharge nozzles 
which may be controlled for thrust direction. 
All the forces placed on the engine must be transmitted to the airframe. 
The thrust controlling nozzles place loads similar to, and in the same 
general location as airframe empennage loads. It is therefore advantageous 
to integrate the nozzle structural members with the airframe structural 
members, so as to eliminate the need for redundant structures, with the 
attendant weights savings. To obtain maximum benefit, these combined loads 
must be divorced from the loads generated by the gas turbine itself If 
this is not done, these vectoring and engine loads become statically 
indeterminate, and introduce bending into the engine, which is detrimental 
to engine life and performance, negating any weight savings due to 
integrated structure. Differential expansion, both radially and axially, 
must be accepted and differential movement, including eccentric movement, 
must be tolerated between the nozzle structure and the upstream structure. 
The gas pressure in the order of 3 to 4 atmospheres must also be sealed 
between the gas turbine exhaust and the nozzle. 
Attempts to permit this movement with appropriate sealing using bellows 
have been a problem because of a structural instability known as 
squirming, which occurs when a bellows is made sufficiently thin to avoid 
transferring loads between the nozzle and engine It is desirable to seal 
such moderate pressures with minimal leakage between the components where 
relative displacements of up to 4 centimeters in any direction are 
anticipated. 
SUMMARY OF THE INVENTION 
The gas turbine engine has an upstream casing surrounding the compressor 
and turbine as well as a downstream casing operating as an exhaust duct to 
convey the gases to the discharge nozzle. These two casings are to be 
independently supported and therefore experience relative movement. 
The downstream casing has a cylindrical extension which is coextensive with 
and concentrically surrounding a cylindrical portion of the upstream 
casing. At this coextensive location there is thereby formed an outer 
shell and an inner shell with an annular space between these shells. The 
annular space is exposed on one side to the pressure inside the upstream 
and downstream casings, more particularly the gas pressure upstream of the 
exhaust nozzle. On the second side the annular space is exposed to 
external or ambient pressure. It is at this location where the gas seal is 
required to achieve minimum leakage while permitting relative movement of 
the two components. 
A circumferential radially extending seal surface extends from the inner 
shell toward the outer shell with the surface facing toward the internal 
pressure side. A plurality of arcuate seal segments are circumferentially 
arranged within this annular space. Each seal segment includes and 
L-shaped section with one leg in sealing contact with the radially 
extending seal surface, and the other leg in sealing contact with the 
internal surface of the outer shell. Locating means secure each seal 
segment loosely adjacent to the seal surface, with the internal pressure 
pressing the segments into contact to achieve the sealing. 
The locating means comprises a plurality of links secured to the inner 
shell with the links extending at an angle with respect to the radial 
direction. They are pinned both to the inner shell and to the segments.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Gas turbine engine 10 has an upstream casing 12 supported generally on 
supports 14, 15, and 16. The downstream casing 18 includes exhaust nozzle 
20 and is supported generally at support locations 22, 23, and 24. 
The downstream casing has a cylindrical extension 26 coextensive with and 
surrounding a cylindrical portion 28 of the upstream casing. The extension 
26 is designated as outer shell 26 which surrounds shell 28 forming an 
annular space 30 between the shells. This space is exposed on one side to 
internal pressure 32 while the other side is exposed to the external 
ambient pressure 34. Seal 36 is located in this annular space. 
Referring to FIG. 2, a circumferential radially extending seal surface 38 
is secured to the inner shell 28. The seal surface faces toward the 
internal pressure side. A plurality of arcuate seal segments 40 are 
circumferentially arranged within the annular space. Each seal segment 
includes an L-shaped section with one leg 44 in sealing contact with the 
radially extending seal surface 38. The other leg 46 is in sealing contact 
with the outer shell 26. 
Locating means 48 functions to keep the seal segments 40 loosely adjacent 
the seal surface 38. Referring also to FIG. 3, a link 50 for each seal 
segment is connected to the inner shell by a pin connection 52. It is 
connected to each seal segment 40 by pin connection 54. These links extend 
at an angle with respect to the radial direction, this angle being in the 
same clockwise direction for each link. 
Should the inner shell expand with respect to the outer shell the links 
will pivot, and as viewed in FIG. 3, the seal segments shift to the left 
in a clockwise direction. Should the inner shell move eccentrically down 
as shown in FIG. 3, the lower segments seen in FIG. 3 will move in a 
clockwise direction while the segments at the top would move in a 
counter-clockwise direction. Sufficient clearance between the segments 
must be established to permit this potential differential movement of the 
various segments. 
FIGS. 4 and 5 illustrate one of the segments in detail. It can be seen that 
leg 46 is arcuately shaped to fit against the internal surface of shell 26 
including however, a recess 55 suitable for accepting a reduced thickness 
portion 56 of an adjacent segment. In a similar manner, leg 44 has a 
recess 57 and a corresponding reduced thickness portion 58 at the other 
end sized to fit within recess 57. Bracket 59 is a portion of the pin 
connection 54. The reduced thickness portions 56 and 58 are sized with 
respect to the recesses 55 and 57 to substantially fill the recess except 
for the clearance at the end required for differential movement in the 
closing direction. Only minor leakage at the ends of this interface will 
occur. 
FIGS. 6 and 7 illustrate a modification of the seal segments to further 
minimize leakage. End portions 56 and 58 are not only truncated in 
thickness but is also truncated in height and width. Recesses 55 and 57 
are similarly reduced in height and width to snugly accept the portions 56 
and 58. In this manner a seal is effected along surfaces 60 and 62 of the 
installed segments, thereby further reducing seal leakage. 
The seal arrangement accommodates substantial eccentricity, as well as 
axial travel and differential expansion.