CA2025244A1

CA2025244A1 - Bolt shield for rotating exhaust duct

Info

Publication number: CA2025244A1
Application number: CA002025244A
Authority: CA
Inventors: Edward Mattew Duran; Clifford Stephen Creevy
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1989-11-17
Filing date: 1990-09-13
Publication date: 1991-05-18
Also published as: DE4034818A1; IT1257002B; IT9022037A1; GB9025010D0; GB2238027A; JPH03175137A; FR2654776A1; IT9022037A0

Abstract

BOLT SHIELD FOR ROTATING EXHAUST DUCT
ABSTRACT OF THE DISCLOSURE

Disclosed is an apparatus for shielding an attachment between two members forming a boundary for a gas stream. The present invention reduces turbulence in the gas stream, thus reducing drag, and can be desirably used in relation to a centerbody of an aircraft gas turbine engine.

Description

~ 2~2~'~4~

~OLT SHIELD ~OR ROTATING EXHAUST DUCT
The present invention relates generally to apparatus for providing a smooth aerodynamic surface within a gas stream and particularly to apparatus for 5 providing a smooth aerodynamic surface in the exhaust gas flow path of an exhaust duct of a gas turbine engine. More particularly, the present invention relates to a device to shield an attachment between two members of an exhaust duct from the exhaust gas 10 airstream.
BACKGROUND OF THE INVENTION
Gas turbine engines typically include a gas generator that defines an annular flow path and comprises, in an axial flow relationship, a compressor 15 section for compressing air flowing along the flow ~;~ path: a combustor section in which the compressed air is mixed with fuel and ignited to produce a high ':
energy gas stream; and a turbine section that extracts energy from~the gas stream to drive the compressor ~-~ 20 section. When such engines are used on modern air-~ craft, it is known in the art to add a second turbine ;~ ~ section~ known as a power turbine, to extract further energy from the gas stream to drive a fan or pro-peller. It is further known in the art that the power 2S turbine can Include alternately counter Fotating rows ~.

.
. , 2~2~

--i!--of airfoils, each of which drive a separate plurality of unducted counter rotating fan blades positioned either on the forward or aft portion of the engine.
Gas turbines utilizing counter rotating 5 unducted fan blades may include an e~tended~ rotating exhaust centerbody having a substantially conic configuration. This centerbody is of substantial length and its manufacture as a single unit is structurally desirable. The centerbody must be 10 attached at its base to the engine from the outside, ; however, and its length makes the attachment difficult to achieve. Consequently, the centerbody must be manufactured in sections so that it can be attached to the engine. Utilizing conventionally bolted joints 15 between the various centerbody sections requires the use of access holes in the centerbody structure to reach the bolt heads and thereby affects the attachment process. Since the centerbody is exposed to the gas stream exiting the rear of the engine, 20 these access holes are a source of turbulence in the ~` exhaust stream, which leads to increased noise and to increased drag, thereby lowering the efficiency of the enyine.
OBJECTS OF THE INVENTI~N
~ 25 It is a principal object of the present -~ invention to provide new and improved apparatus that ~; is not subject to the foregoing disadvantages.
It is another object of the present invention to provide an exhaust centerbody that minimizes the 30 turbulence produced in an exhaust gas stream.
It is yet another object of the present ~; invention to provide a bolt shield that minimizes the area of access holes, thereby reducing the turbulence produced in the exhaust gas stream of a gas turbine , ~ .
35 engine.

~:
.
.: . , '. . !

", " , ~
' ~ . ,' '' ' ` :

:,, :' '' ' :, ',:

-~ 2~52~

SUMMARY OF THE INVENTION
The foregoing objects of the present invention are achieved by providing apparatus capable of providing an aerodynamically efficient surface in an 5 exhaust gas stream of a gas turbine engine by covering a connective joint between two members of a plurality of members that together form an exhaust duct centerbody. In a preferred embodiment the shield has a hollow, frusto-conical configuration and includes a 10 base edge having a flange capable of mating with a forward portion of the~exhaust duct centerbody and an apex edge having a plurality of holes capable of receiving bolts to attach the shield to the aft portion of the exhaust duct. The side of the shield 15 covers the connective joint thereby presenting an uninterrupted surface to the gas stream.
Additional features and objects of the present invention will become apparent from a reading of the following detailed specification in conjunction with 20 the drawings, both of which are intended to be indicative of rather than in any way limiting on the scope of the appended claims. Where appropriatet applicable reference numerals have been carried forward in the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure l illustrates a gas turbine engine including aft mounted counter rotating fans.
Figure 2 shows in partial cross section an exhaust duct and an exhaust centerbody including the ~ 30 present invention.
;~ Figure 3 depicts a prior art bolted joint and an access hole between the bolted members.
Figure 4 depicts in greater detail the ~` invention depicted in Figure 2.
Figure 5 illustrates in perspective the invention shown in Figure 4.
:~
:

, , . ,:

:

2~2~244 DETAILED DESCRIPTION OF THE INVENTION
Figure 1 illustrates a gas turbine engine 10 of a type where the present invention may find application. Engine 10 is of the type proposed to be 5 used on modern aircraft that are propelled by the push provided by separate pluralities of counter rotating unducted fans attached to the aft portion of the engine. Engine 10 comprises several major sections arranged substantially symmetrically about an engine ; 10 centerline 34, including a gas generator 15 and a propulsor 20. As is well known in the art and as such will not be shown here, the gas generator lS defines an annular gas flow path and includes in an axial flow relation a compressor, a combustor and a turbine.
15 Propulsor 20 includes a second or power turbine that drives a forward and an aft propan 21 and 23 respectively. Forward propfan 21 includes a plurality of individual blades 22; aft propfan 23 includes a plurality of individual blades 24.
Also depicted in Figure 1 is an exhaust center-body 26, which is attached to a frame of engine 10.
Centerbody 26 has a substantially conic configura-` tion and has an a~is 35 coaxial with centerline 34.
In some embodiments of engine 10, exhaust centerbody 25 26 may be attached at its base to a rotating frame of the engine and thus will itself rotate in conjunction with the rotation of one of the pair of counter rotating turbines forming the power turbine. In most of such cases the centerbody will be attached to the 30 rotating frame supporting the aft propfan.
~- Centerbody 26 is comprised of a plurality of centerbody sections. As shown in Figure 2, centerbody 26 includes three such sections, a forward centerbody ; section 28, a mid centerbody section 30, and an aft 35 centerbody section 32, which are fastened together to : ~ . . ' ,, ;
:: . :
..... .
- " .,, .

?.~2~2~

form a unitary structure. Thus, forward centerbody section 28 is bolted to mid centerbody section 30 at a joint 31 and mid centerbody section 30 is in turn bolted to aft centerbody section 32 at a joint 33.
While centerbody 26 is depicted as comprising three sections bolted together, it may be made up of only two or more than three sections fastened together in some manner known in the art other than by the use of bolts. Each centerbody section may in turn be made up of smaller individual frusto-conical shaped members that are welded to each other to form a unit section.
As indicated in Figure 2, exhaust gases exit engine 10 through an annular exhaust duct 38 after -~ passing through the power turbine. Exhaust duct 38 is defined by centerbody 26 and by an outer exhaust nozzle 36, which, like centerbody 26, may be attached to a rotating engine frame on some embodiments of engine 10. After exiting the exhaust duct, the gases continue to flow aft of the engine. Any surface irregularity in the surface of centerbody 26 can serve as the source of turbulence in the exhaust stream and lead to the problems previously mentioned.
An example of such a surface irregularity is shown in Figure 3, a prior art method of attaching two ; centerbody members to each other. Thus, Figure 3 depicts an upstream centerbody section 40 having a first radial flange 42 and a downstream centerbody section 41 having a second radial flange 43 joined to ; each other by a bolt--nut combination 44. Also shown in Figure 3 is an access hole 45 that provides an assembler with access to bolt head 46. The axis of the bolt--nut combination is substantially parallel to a~is 35 and, therefore, makes an acute angle with the surface of the upstream and downstream sections.
~ Thus, even though a cross section taken through bolt '~' ~' ' ~' ~ .

~2~2~
13DV~09380 head 46 perpendicularly to the axis of the bolt is substantially circular, the projection of that cross section onto the surface of the second member has an elliptical configuration 47 as shown in the projection 5 in Figure 3 and, therefore, a larger area of surface disruption than that given by a circular hole through the same surface. Assuming that the diameter of a tool used on bolt head 46 to attach the two members together is equal to a, then the area of the ]O elliptical figure, and consequently, the size of the accsss hole 45 as seen by the tool, is given by the formula:
Ae=-rab, where Ae= the area of the ellipse;
~1 a = the length of the minor a~is; and b = the length of the major axis.
~-l The value of b in turn is given by b = a/sin ~;
20 where = the angle between the plane of a tangent to the surface of the centerbody and a plane lying parallel to bolt head 46.
Since b > a, unless the figure is a circle, and since ~ 25 the area of a circular hole needed to access bolt head `~ 46 is equal to ~a2, the area of the elliptical cutout will always be greater than a circular cutout and will increase in size as ~ gets smaller, i.e., as the length of centerbody 26 increases. Furthermore, 30 because the surface of downstream centerbody section 41 is curved, the surface area of the material removed to form access hole 45 is somewhat larger than that shown by the formula given to determine Ae.
In other words, in order to access bolt head 46 35 such that the two members may be attached to each ~: . .:.
' , ..:
.

-: . .
... .. . .

2~2~æ4~

other, one prior art method and apparatus for attaching them would require the removal of an elliptical piece of material from the second member.
Not only does this result in a surface irregularity, 5 as shown above, such that disturbances are produced in the exhaust stream, but it also serves to weaken the structural integrity of the second member.
As shown in Figures 4 and 5, a bolt shield 50 according to the present invention, which is not 10 subject to the foregoing disabilities, is used to shield the attachment of a pair of adjacent centerbody sections, such as sections 30 and 32, from the gas stream. Bolt shield 50 has a generally frusto-conical configuration, as best seen in Figure 5, and includes 15 base and apex edges 52 and 54, respectively, and a side 56 comprising a substantially continuous sheet extending therebetween. Side 56 is defined in part by inner and outer side surfaces 58 and 59, respectively. Base edge 52 of shield 50 includes a 20 flange 80 extending from inner surface 58 substantially parallel to axis 35. Apex edge 54 includes a plurality of circumferentially disposed, radially directed through-holes 66, each of which is capable of receiving a low profile bolt 6a. Each low 25 profile bolt 68 engages a captured nut 69, thereby attaching the apex edge of shield 50 to aft centerbody section 32.
The base edge 52 of shield 50, including flange 80, is configured for mating with the downstream or 30 apex edge 48 of upstream centerbody section 30. Edge 48 includes a circumferential flange 49. Thus, flange 80 forms a radial interference fit 60 and an axial ~! interference fit 62 whereby shield 50 is retained in fired relation to upstream centerbody section 30.

; : ' .

`~ 2~2r~æ~

Flange 49 includes a chamfer 70, thereby preventing creation of potentially damaging stress at that location.
Before installing shield 50 downstream centerbody section 32 would first be attached to upstream centerbody section 30 by means of bolt captured nut combination 72. Shield 50 would then be slid over the complete centerbody 26 until flange 80 engaged flange 49 of upstream section 30. Bolts 68 would then be individuaIly attached to captured nuts-69, thereby firrnly anchoring shield 50 in relation to centerbody 26. Once properly positioned, the e~terior surface 59 of side 56 provides a substantially uninterrupted aerodynamic surface which covers the connective joint between upstream and downstream centerbody sections 30 and 32.
While access holes are still necessary to install bolts 68 they can of necessity be smaller and fewer in number since the bolts are not weight bearing. That is, bolts 68 function simply to retain shield 50 in position with respect to centerbody sections 30 and 32 and do not support any weight whereas, for example, bolt-captured nut combination 72 functions to support centerbody section 32 and of necessity must be structurally stronger.
In addition, hole 66 is smaller because it is .
directed more nearly perpendicular to side surface 59 than is access hole 45 in member 41 as shown in Figure 3. Thus, even assuming the diameter of a tool needed to install bolt 68 as shown in Figure 4 is equal to the diameter of a tool needed to install bolt 46 as shown in Figure 3, a smaller access hole may be used in the former situation. This is seen by comparing the angle r of Figure 3 with the angle d of Figure 4.
The angle r is smaller than the angle d. As .
:::
:~

, .: ''~

2~2~2~

_g_ previously noted, as the angle through the surface decreases, the size of the access hole increases.
Thus the area of cutout needed to access bolt 68 is smaller than the area of cutout needed to access bolt 5 46 of Figure 3. As a result, the disruption in the exhaust gas flow caused by access hole 66 is less than - would be caused by access hole 45. Reducing the disruption in the gas flow leads to reduced turbulence and consequently a reduction in drag and noise levels.
Another desirable feature of the present invention is that it provides an annular containment volume 90 defined principally by inner surface 58 and downstream section 32. Should any bolt-captured nut combination 72 come loose, they will be retained 15 within containment volume 90.
While the present invention has been described in relation to its use for covering a connective joint between mid and aft centerbody sections, it will be understood that it is equally functional ;n relation ~ 20 to any two sections of the centerbody. In addition, - the present inven~tion, while being described in its relation to forming, in part, the inner flow path boundary of the exhaust gas stream, can be used equally well as a shield covering a joint between two 25 members forming in part an outer boundary of a gas stream. The present invention also finds application in any situation where it is desirable to reduce the : `:
turbulence created in a gas stream by a connective joint between two members lying within the gas stream.
Having thus described the present invention, additional numerous changes, substitutions, modifications and alterations will now suggest themselves to those skilled in the art, all of which fall within the spirit and scope of the present :

.
~' ''"' ~` ' ; ' ' ' :
. . .

.

2 ~ ~

~ 13DV-09380 invention. Accordingly, it is intended that the invention be limited only by the scope of the appended claims.

, ~,~
::

,`

,~, .

~:~
;

' .

~' ~'.. ' ' ,,.

: '. ;
'.' ~ '. :

Claims

1. Apparatus for reducing aerodynamic disruptions arising from a connective joint between at least a first member and a second member lying in a gas stream, said apparatus comprising a first edge for attaching to said first member, a second edge for attaching to said second member, and a continuous surface extending between said edges, said continuous surface shielding said joint from said gas stream.

2. The apparatus of claim 1 wherein said apparatus has a substantially frusto-conical configuration and wherein said continuous surface comprises the side surface of said frusto-conical apparatus, said side surface including interior and exterior surfaces.

3. The apparatus of claim 2 wherein said first edge includes a circumferential flange extending from said interior side surface substantially parallel thereto and said first member is configured for mating with said first edge.

4. The apparatus of claim 2 wherein said second mating edge includes a plurality of circumferentially disposed, radially directed holes for receiving fasteners for attaching said second mating edge to said second member.

5. The apparatus of claim 2 wherein said first member and said second member are connected to each other by means for fastening and said apparatus and said members jointly define a containment volume for containing loose fastening means.

6. Apparatus for reducing aerodynamic disruptions arising from a joint between an upstream section and a downstream section of an exhaust centerbody of a gas turbine engine, said exhaust centerbody lying in an exhaust gas stream and having an axis defining an axial direction and a radial direction normal thereto, said apparatus providing a substantially continuous aerodynamic surface substantially free of aerodynamic interruptions and comprising a first mating edge for with said upstream section, a second mating edge for mating with said downstream section, and a side surface extending between said edges.

7. The apparatus of claim 6 wherein said apparatus has a substantially frusto-conical configuration and wherein said side surface of said frusto-conical apparatus includes an interior and an exterior surface.

8. The apparatus of claim 7 wherein said first edge includes a flange extending from said interior surface substantially parallel thereto and said upstream section is configured for mating with said first mating edge.

9. The apparatus of claim 7 wherein said second mating edge includes a plurality of circumferentially disposed, radially directed holes for receiving fasteners for attaching said second mating edge to the second member.

10. The apparatus of claim 7 wherein said first member and said second member are connected to each other by means for fastening and said apparatus and said members jointly define a containment volume for containing loose fastening means.

11. A shield for reducing aerodynamic disruptions arising from a joint between first and second members forming a boundary of a gas flow path and for providing a substantially continuous aerodynamic surface substantially free of aerodynamic disruptions over said joint, each of said members having a substantially frusto-conical configuration defined in part by a base edge, an apex edge, and a side surface extending therebetween, said apex edge of said first member being attached to said base edge of said second member, said shield having a frusto-conical configuration defined in part by a substantially circular base edge, a substantially circular, coaxial apex edge, and a side surface extending therebetween, wherein said shield base edge is attached to said first member and said shield apex edge is attached to said second member.

12. The shield of claim 11 wherein said gas flow path has a substantially annular configuration defined in part by said boundary.

13. The shield of claim 12 wherein said shield apex edge includes a plurality of circumferentially spaced, radially directed through holes for receiving bolts to attach said shield apex edge to said second member.

14. The shield of claim 12 wherein said shield base edge includes a flange extending from said side surface and said apex edge of said first member is configured for mating with said shield base edge.

15. The shield of claim 14 wherein said first and second members are connected to each other by means for fastening and said shield and said members jointly define a containment volume for containing said fastening means.

16. A rotating exhaust duct for a gas turbine engine, said exhaust duct comprising a substantially annular exhaust gas flow path defined by nesting inner and outer shells each having a substantially frusto-conical configuration, said inner shell extending beyond the end of the outer shell and comprising a plurality of circumferentially split sections attached to each other, wherein said exhaust duct further includes at least one shield for reducing aerodynamic disruptions in the exhaust gas stream arising from a joint between an adjacent pair of said sections.

17. The exhaust duct of claim 16 wherein said shield has a substantially frusto-conical configuration and is defined by a substantially circular base edge, a substantially circular, coaxial apex edge, and a continuous side surface extending therebetween.

18. The exhaust duct of claim 17 wherein said base edge includes a flange extending therefrom substantially parallel to said shield side surface for attachment to an upstream section of said inner shell.

19. The exhaust duct of claim 17 wherein said apex edge includes a plurality of circumferentially spaced, radially directed through holes for receiving fasteners for attaching said shield apex edge to a downstream section of said inner shell.

20. The exhaust duct of claim 19 wherein said adjacent pair of sections are connected to each other by means for fastening and said shield and said sections jointly define a containment volume for containing loose fastening means.

21. The invention as defined in any of the preceding claims including any further features of novelty disclosed.