GB2447036A

GB2447036A - A containment casing

Info

Publication number: GB2447036A
Application number: GB0703828A
Authority: GB
Inventors: Ewan Fergus Thompson
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2007-02-28
Filing date: 2007-02-28
Publication date: 2008-09-03
Also published as: GB0703828D0

Abstract

Containment casings inhibit release of fragments as a result of fragmentation of a component such as a fan blade within a gas turbine engine. By providing an inner containment ring 32 suspended upon a cellular structure 33 whereby the ring 32 preferentially deflects and deforms into the cellular structure 33 rather than allow penetration of a fragment, that fragment is contained. The depth of the cellular structure 33 is such as to allow deflection of the ring 32 and so absorb energy and contain the fragment in use. Generally an outer casing 31 or other structural feature is provided to stabilize the cellular structure 33 to allow energy absorption. The outer casing 31 also provides a final barrier for containment of fragments in use.

Description

A CONTAINMENT CASING

The present invention relates to containment casings and more particularly to a containment casing for a gas turbine engine or other machine in which detachment or fragmentation of components must be contained to prevent injury or damage to other parts of the machinery and associated structures such as an aircraft fuselage.

Referring to Fig. 1, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustor 15, a turbine arrangement comprising a high pressure turbine 16, an intermediate pressure turbine 17 and a low pressure turbine 18, and an exhaust nozzle 19. The turbine blades and compressor blades are located such that their tips are adjacent portions of the engine casing 1 (a-d) through the flow path of the engine. The casings 1 or parts of the casing 1 provide containment should the blades fragment.

The gas turbine engine 10 operates in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.

The compressed air exhausted from the high pressure compressor 14 is directed into the combustor 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by suitable interconnecting shafts.

In view of the above it will be appreciated that within gas turbine engines there is a general concern with regard to situations known as "blade off" where blades are detached from the engine core and can unless contained penetrate through the engine casings and cause personal injury or damage to local structures. Typically prior containment casings have comprised solid or composite rings of material having sufficient strength to constrain expected impact forces as a result of blade or other disintegrations within an engine. It is also known to provide structures which comprise a lightweight cellular construction comprising a lightweight ring and an outer ring with a cellular structure between. The inner ring is designed to allow penetration by a blade fragment for absorption of energy by the cellular material sandwiched between the layers of casing. The outer casing is effectively designed to have sufficient strength to prevent penetration by a blade fragment penetrating the inner casing after energy absorption by the cellular material.

It will be appreciated in a number of machines potential dangers with regard to fragment penetration can occur. Thus with respect to high speed rotating fly wheels and shaft structures it will be understood that fragmentation can occur and casings are provided to prevent penetration within the expected percussive impact energy force ranges for such fragments.

Particularly with regard to gas turbine engines utilised in aircraft it will be understood that weight as well as simplicity of construction are important factors.

It will be understood that achieving containment systems in casings which are heavy in terms of the thickness of material used or require complex constructional techniques can be disadvantageous.

With regard to aspects of the present invention there is provided a containment casing for a gas turbine engine, the casing comprising a containment ring suspended upon a cellular structure, the containment ring presented to the cellular structure to be preferentially deflectable radially into the cellular structure under radial impact load.

Typically, the cellular structure is a honeycomb.

Generally, the cellular structure has a consistent circumferential depth. Alternatively, the cellular structure has a variable circumferential depth.

Typically, the cellular structure is formed from aluminium or titanium or steel or a composite material. It may also be formed of metallic or polymeric foam.

Possibly, the containment ring incorporates containment ribs or hooks to inhibit lateral deflection of fragments impinging upon the containment ring.

Generally, the containment ring is bonded to the cellular structure.

Normally, the casing incorporates an outer casing on an opposite side of the cellular structure to the containment ring. Normally, the outer casing is formed from a metal or composite. Generally, the outer casing stabilises the cellular structure.

Normally, the outer casing provides location mountings for the casing. Possibly, the outer casing provides fixings for cable and/or accessories.

Also in accordance with aspects of the present invention, there is provided a gas turbine engine comprising a containment casing as described above outboard of a plurality of fan blade tips.

The casing may be secured via the outer casing to the engine mountings.

An embodiment of aspects of the present invention will now be described by way of example only and with reference to the accompanying drawing in which Fig. 2 provides a schematic side cross-section of a containment casing in accordance with aspects of the present invention.

By aspects of the present invention, a containment casing is provided with an objective of achieving a lighter and less complex structure than prior arrangements. As indicated such lightness and lack of complexity is particularly beneficial with regard to gas turbine engines utilised in aircraft where weight is a paramount concern.

A containment casing according to aspects of the present invention will typically comprise a dual wall containment system having an inner containment ring in the form of a collar and an outer casing with a cellular structure located between. Preferably, the outer casing effectively stabilises the cellular structure to allow as described below the inner containment ring to act in accordance with aspects of the present invention. If the cellular structure can be secured in another way it is possible to avoid use of the outer casing for stabilisation per se, but nevertheless in most circumstances provision of an outer casing as a safety feature as a final barrier for containment will be beneficial.

The outer casing is generally provided in an appropriate material in order to achieve a lightweight structure. In such circumstances the outer casing will generally take the form of a metal or composite structure extending circumferentially around a shaft or other component which is subject to possible fragmentation resulting in fragments with radial and tangential percussive force requiring containment within a casing in accordance with aspects of the present invention. The outer casing will typically provide all mounting loads. In such circumstances with regard to a gas turbine engine used in an aircraft the outer casing will receive all normal flight loads and provide connections between the containment casing in accordance to aspects of the present invention and other parts of the engine. In addition the outer casing will provide fixings to act as mounting points for wires, pipes and other accessories within a gas turbine engine and therefore provides the general chassis upon which the casing in accordance with aspects of the present invention will be secured. It is desirable that Lhe inner containment ring in accordance with aspects of the present invention in association with the cellular structure will reduce impact energy of fragments and therefore generally that presented to the outer casing. Thus, if the outer casing is to act as a final perimeter of containment that outer casing need not have such an extreme energy absorption capability and therefore can be formed from a lighter material or thinner more robust material type.

In accordance with aspects of the present invention an inner containment ring is supported and suspended upon a cellular material structure. This cellular structure will typically be of a honeycomb type and formed with an objective of achieving as low a weight coefficient for desired operational effect as required. Metallic or polymeric foam or sponge could also be used. Generally as indicated above the cellular structure will be stabilised by bonding to the outer casing or other structures.

In accordance with aspects of the present invention the inner containment ring will be formed from a high toughness material. In such circumstances the inner containment ring will be formed from a steel or titanium alloy or a composite fibre-metal laminate. The inner containment ring should be sufficiently tough that it will not be penetrated in preference to displacement and distortion into the cellular structure, that is to say the containment ring will be displaced or distorted rather then be punctured by percussive fragments engaging it. Normally the containment ring will comprise a flat sleeve of consistent thickness but, where required, may have a variable thickness. The containment ring may also comprise a roll of adjacent rings in a stack with a cover layer to inhibit penetration if required.

Within a gas turbine engine the containment ring is typically positioned outboard of fan blade tips such that should those fan blades disintegrate they will percussively engage the containment ring causing it to be displaced into the cellular structure.

In accordance with aspects of the present invention the inner containment ring as indicated will be tough and suspended or supported upon the cellular structure. In such circumstances upon percussive fragment impingement the containment ring will deflect radially into the cellular structure rather than be penetrated by those high energy fragments. In such circumstances the ring is radially displaced and deflected as indicated such that the cellular structure is crushed. The containment ring may also be deformed again absorbing energy from the impacting fragments.

Typically although suspended and supported upon the cellular structure the containment ring will also be bonded to the cellular structure.

In order to prevent lateral deflection of fragments typically the containment casing will incorporate ribs or hooks to stop forward motion of released blade fragments.

Fig. 2 shows a containment casing 30 in accordance with aspects of the present invention as a schematic cross-sectional illustration of one side. It will be appreciated typically the casing 30 would be centred around an axis and provide a cylinder or box cross-section for containment of percussive fragments in use. The casing 30 comprises an outer casing 31 and an inner casing 32 with a cellular structure 33 between. Typically an acoustic lining 34 is also provided to reduce acoustic noise levels particularly in a gas turbine engine.

The outer casing 31 may also incorporate ribs 35 to increase its strength and stiffness.

The cellular structure 33 is generally honeycombed and incorporates cells and flanges which may be crushed by radial deflection and distortion of the containment ring 32 in use.

The containment ring 32 as indicated is formed from a tough but relatively flexible material such that when suspended and located upon the cellular structure 33 it is preferentially displaced in the direction of arrow head A when subject to radial percussive loading as a result of impingement and impact by fragments. The containment ring 32 may incorporate a hook or rib 36 to inhibit forward movement of deflected impingement fragments in engagement with the containment ring 32. The containment ring 32 is designed to spread impact loads so generally is a flat sleeve. However, the ring 32 may be formed by bands of material aligned to form the ring then these bands may be individually or collectively formed to displace into the cellular material. It is also possible to incorporate ribs into the casing.

As can be seen the containment ring 32 is generally located above positions where impingement by fragments is most likely. Generally, the thickness and depth of the cellular structure 33 is consistent, but dependent upon operational requirements and with a view to achieving optimised weight for performance it will be understood that the thickness of cellular material may be reduced at circumferential locations about the periphery of the casing such that the containment ring 32 thickness or toughness as well as the depth of the cellular structure is matched by the potential for fragment engagement.

During a fragmental engagement event it will be understood that the fragment such as a fan blade fragment in a gas turbine engine will impinge upon the containment ring 32. As the ring 32 is only lightly constrained and suspended upon the cellular structure 33, it will be appreciated that the ring 32 can move relatively freely in comparison with a prior more robust and secured containment ring. In such circumstances the ring 32 will preferentially deflect arid distort into the cellular structure 33. Such movement, that is to say deflection and distortion, is advantageous in that the effect is to distribute loading over a broader area. It is known that localised impact in a small region of a containment ring can cause failure of the casing in that the fragment will penetrate arid rupture the inner containment then pass through the casing. This is the reason why relatively heavy thick containment rings have been commonplace in the past. By providing a lightly constrained, but tough containment ring it will be appreciated that the containment ring will move by deflection and distortion at least initially into the cellular structure 33. This will spread impact loading over a large area of the casing 30 and so limit stressing in the materials from which the casing 30 are formed. In short the suspended ring 32 remains intact and will crush parts of the cellular structure over a broader impact front in comparison with prior arrangements. Furthermore, where the containment ring 32 is bonded to the cellular structure it. will be appreciated that on the radial impact side the cellular structure 33 will be crushed and deformed whilst on an opposite side the bonding between the ring 32 and the structure will be brought into tension acting as a brake upon ring displacement and so again providing for energy absorption. Any absorption as indicated is achieved through crushing of the cellular structure 33 as well as deflection of the ring 32 and, as indicated where bonding is utilised, straining of the bonding on the opposite side to impact with the cellular structure.

Aspects of the present invention allow provision of a containment casing as a containment system in which matching of the material types, whether that be a metal such as steel or titanium or a composite, with expected impact loadings. By utilising the effect of a tough suspended containment ring 32 and energy absorption it will be appreciated that generally thinner containment ring 32 dimensions can be more reassuringly utilised with the cellular structure depth 33 similarly adjusted for expected necessary deflection under radial impact loads of a containment ring 32. It will be appreciated if there is more confidence with regard to the containment ring 32 operation and supporting cellular structure 33 in retaining and preventing release of fragments it is then possible to similarly be more confident in utilising a thinner dimensioned outer casing 31 as a final containment barrier for fragment release.

In view of the above it will be appreciated that generally in accordance with aspects of the present invention it is possible to achieve a lower weight and lower complexity containment casing structure in accordance with aspects of the present invention.

Although as indicated containment casings in accordance with aspects of the present invention can be formed other than with regard to cylindrical structures it will be understood that generally in order to provide benefits in accordance with aspects of the present invention the containment ring should be substantially circular. In such circumstances the fragment impact loads will be translated to the ring whereas for example a flat containment ring surface is more likely to be penetrated by a fragment impact load rather than achieve preferred displacement and deflection of the containment ring into the cellular structure. Aspects of the present invention are particularly applicable where a misslie or other high impact object or fragment requires containment within a circular or cylindrical casing. Aspects of the present invention have particular advantages with regard to gas turbine engines where weight penalties may be a significant factor.

It will be understood that containment casings in accordance with aspects of the present invention may be used in other machines and structures, but typically with most benefit where weight is a particular problem in order to justify compromises with respect to geometry. Where weight is less important greater material thicknesses may be utilised without the manufacturing and constructional processes necessary to provide an inner containment ring suspended upon a cellular structure in accordance with aspects of the present invention. Generally casings in accordance with aspects of the present invention will be particularly advantageous where weight is critical in view of the containment benefits of using the cellular structure giving improved structural strength for weight.

Aspects of the present invention as indicated depend upon the containment ring 32 being tough in the sense that it will retain its integrity and will not be penetrated or ruptured by an impact in preference to at least initial impingement by an impact fragment. In such circumstances the depth of the cellular structure will be such that the ring will deflect and deform until the webs, plates or baffles of the cellular structure have collapsed and folded. In such circumstances a containment ring 32 with the folded collapsed cellular structure underneath bunches against the outer casing 31. In such circumstances the containment ring 32 crushed parts of the cellular structure 33 and the outer casing 31 will then provide essentially a solid thickness depth through which the reduced energy fragment must penetrate for release from the containment casing 30. In such circumstances the thicknesses of the ring 32 the cellular structure 33 and casing 31 will be designed in order to achieve the desired containment restraint of an impinging fragment. It will be understood that the cellular structure may be designed to preferentially collapse into a certain orientation between the containment ring 32 and the outer casing 31 for best effect with respect to impact fragment containment.

The containment ring 32 as indicated should preferentially deflect and deform into the cellular structure rather than be penetrated by an impacting fragment. In order to improve such penetration resistance it is possible that the containment ring 32 may itself incorporate a reinforcing web comprising a metallic rib or embedded fibres or weft and weave elements having interstices of sufficiently small diameter to restrain fragments. It will be understood that it may be possible to predict fragment dimensions for impingement to the containment ring and therefore design the reinforcing web to restrain such fragment sizes.

Modifications and alterations to aspects of the present invention will be appreciated by those skilled in the art. Thus, for example it is generally beneficial to provide a single containment ring 32 and cellular structure 33, but in some situations a stack of containment rings and cellular structures may be achieved such that the concentric containment rings will respectively deform and deflect radially under impact loads to achieve containment in accordance with aspects of the present invention.

Furthermore, such stacked or multilayered containment casings in accordance with aspects of the present invention may be arranged such that such stacking occurs only over segments of the circumference of the containment casing where fragment penetration through the casing would be most detrimental such as control surfaces of an aircraft.

Claims

Claims: - 1. A containment casing for a gas turbine engine, the casing

comprising a containment ring suspended upon a cellular structure, the containment ring presented to the cellular structure to be preferentially deflectable radially into the cellular structure under radial impact load.
2. A casing as claimed in claim 1 wherein the cellular structure comprises a honeycomb or a metallic or polymeric foam or sponge.
3. A casing as claimed in claim 1 or claim 2 wherein the cellular structure has a uniform circumferential depth about the containment ring.
4. A casing as claimed in claim 1 or 2 wherein the cellular structure has a variable circumferential depth about the containment ring.
5. A casing as claimed in the preceding claims wherein the cellular structure is formed from an aluminium or steel or titanium or composite material.
6. A casing as claimed in any preceding claim wherein the containment ring is formed from an aluminium or steel or titanium or composite material.
7. A casing as claimed in any preceding claim wherein the containment ring incorporates containment ribs or hooks.
8. A casing as claimed in any preceding claim wherein the containment ring is bonded to the cellular structure.
9. A casing as claimed in any preceding claim wherein the cellular structure is stabilised by a structural feature.
10. A casing as claimed in any preceding claim wherein the casing incorporates an outer casing upon which the cellular structure is secured.
11. A casing as claimed in claim 10 wherein the outer casing is formed of a metal or composite material.
12. A casing as claimed in claim 10 or claim 11 wherein the outer casing provides mountings for location of the casing.
13. A casing as claimed in any of claims 10 to 12 wherein the outer casing provides fixings for cable and/or wiring and/or piping and/or accessories.
14. A casing as claimed in any preceding claim wherein the casing comprises a plurality of containment rings suspended upon a cellular structure concentric about an axis of the containment casing.
15. A containment casing substantially as hereinbefore described with reference to the accompanying drawings.
16. A gas turbine engine comprising a containment casing as claimed in any preceding claim outboard of a plurality of fan blades of a gas turbine engine.
17. A gas turbine engine substantially as hereinbefore described with reference to the accompanying drawings.