Gas turbine engine

A fan containment system for fitment around an array of radially extending fan blades mounted on a hub in an axial gas turbine engine. The fan containment system comprises a fan case having an annular casing element for encircling an array of fan blades. An annular fan track liner is positioned substantially coaxial to the annular casing element, and one or more pockets are provided in a radially outer side of the fan track liner. One or more dampers for damping vibration of the fan track liner are positioned in each of the one or more pockets and are arranged so as to contact the annular casing element.

FIELD OF INVENTION

The invention relates to a fan containment system, a casing assembly, a fan and/or a gas turbine engine.

BACKGROUND

Turbofan gas turbine engines (which may be referred to simply as ‘turbofans’) are typically employed to power aircraft. Turbofans are particularly useful on commercial aircraft where fuel consumption is a primary concern. Typically a turbofan gas turbine engine will comprise an axial fan driven by an engine core. The engine core is generally made up of one or more turbines which drive respective compressors via coaxial shafts. The fan is usually driven directly off an additional lower pressure turbine in the engine core.

The fan comprises an array of radially extending fan blades mounted on a rotor and will usually provide, in current high bypass gas turbine engines, around seventy-five percent of the overall thrust generated by the gas turbine engine. The remaining portion of air from the fan is ingested by the engine core and is further compressed, combusted, accelerated and exhausted through a nozzle. The engine core exhaust mixes with the remaining portion of relatively high-volume, low-velocity air bypassing the engine core through a bypass duct.

To satisfy regulatory requirements, such engines are required to demonstrate that if part or all of a fan blade were to become detached from the remainder of the fan, that the detached parts are suitably captured within the engine containment system.

The fan is radially surrounded by a fan casing. It is known to provide the fan casing with a fan track liner and a containment system designed to contain any released blades or associated debris. Often, the fan track liner can form part of the fan containment system.

The fan track liner typically includes an annular layer of abradable material which surrounds the fan blades. During operation of the engine, the fan blades rotate freely within the fan track liner. At maximum speed the blades may cut a path into this abradable layer creating a seal against the fan casing and minimising air leakage around the blade tips during cruise. Further incursions can occur during gusts or take off rotations over time.

A conventional fan containment system or arrangement100is illustrated inFIG. 1and surrounds a fan comprising an array of radially extending fan blades40. Each fan blade40has a leading edge44, a trailing edge45and fan blade tip42. The fan containment arrangement100comprises a fan case150. The fan case150has a generally frustoconical or cylindrical annular casing element152and a hook154. The hook154is positioned axially forward of an array of radially extending fan blades40. A fan track liner156is mechanically fixed or directly bonded to the fan case150. The fan track liner156is provided as a structural intermediate to bridge a deliberate gap provided between the fan case150and the fan blade tip42.

The fan track liner156has, in circumferential layers, an attrition liner158(also referred to as an abradable liner or an abradable layer), an intermediate layer which in this example is a honeycomb layer160, and a septum162. The septum layer162acts as a bonding, separation, and load spreading layer between the attrition liner158and the honeycomb layer160. The honeycomb layer160may be an aluminium honeycomb. The tips42of the fan blades40are intended to pass as close as possible to the attrition liner158when rotating. The attrition liner158is therefore designed to be abraded away by the fan blade tips42during abnormal operational movements of the fan blade40and to just touch during the extreme of normal operation to ensure the gap between the rotating fan blade tips42and the fan track liner156is as small as possible without wearing a trench in the attrition liner158. During normal operations of the gas turbine engine, ordinary and expected movements of the fan blade40rotational envelope cause abrasion of the attrition liner158. This allows the best possible seal between the fan blades40and the fan track liner156and so improves the effectiveness of the fan in driving air through the engine.

The purpose of the hook154is to ensure that, in the event that a fan blade40detaches from the rotor of the fan12, the fan blade40will not be ejected through the front, or intake, of the gas turbine engine. During such a fan-blade-off event, the fan blade40is held by the hook154and a trailing blade (not shown) forces the held released blade rearwards where the released blade is contained. Thus the fan blade40is unable to cause damage to structures outside of the gas turbine engine casings.

As can be seen fromFIG. 1, for the hook154to function effectively, a released fan blade40must penetrate the attrition liner158in order for its forward trajectory to intercept with the hook. If the attrition liner158is too hard then the released fan blade40may not sufficiently crush the fan track liner156.

However, the fan track liner156must also be stiff enough to withstand the rigours of normal operation without sustaining damage. This means the fan track liner156must be strong enough to withstand ice and other foreign object impacts without exhibiting damage for example. Thus there is a design conflict, where on one hand the fan track liner156must be hard enough to remain undamaged during normal operation, for example when subjected to ice impacts, and on the other hand allow the tip42of the fan blade40to penetrate the attrition liner158. It is a problem of balance in making the fan track liner156sufficiently hard enough to sustain foreign object impact, whilst at the same time, not be so hard as to alter the preferred hook-interception trajectory of a fan blade40released from the rotor. Ice that impacts the fan casing rearwards of the blade position is resisted by a reinforced rearward portion164of the fan track liner.

An alternative fan containment system is indicated generally at200inFIG. 2. The fan containment system200includes a fan track liner256that is connected to the fan casing250at both an axially forward position and an axially rearward position. At the axially forward position, the fan track liner is connected to the casing at hook254via a fastener266that is configured to fail at a predetermined load. In the event of a fan blade detaching from the remainder of the fan, the fan blade impacts the fan track liner256, the fastener266fails and the fan track liner pivots about a rearward point on the fan track liner. Such an arrangement is often referred to as a trap door arrangement. The trap door arrangement has been found to help balance the requirements for stiffness of the fan track liner with the requirements for resistance of operational impacts (e.g. ice impacts) ensuring a detached blade is held within the engine.

Often the fan track liner is formed from a plurality of arcuate fan track liner panels. Forming the fan track liner from a plurality of panels means that if a region of the fan track liner is damaged, only the affected panels need to be replaced. To ease removal of the fan track liner panels for such repair work, it is preferable for the fan track liner panels to be releasably connected to the fan casing, e.g. using bolts, instead of being adhered or bonded to the fan casing. However, when the fan track liner panels are releasably connected to the casing the fan track liner panels can vibrate during normal use, e.g. due to the pressure profile formed by passing blades during the operation of the fan. It is desirable to limit any such vibration to a minimal level. Excessive vibration can result in increased noise, increased blade to fan track liner clearance, and loss in engine performance. In very extreme cases, the vibration could lead to failure of the fan track liner panels or damaging interactions with the fan blades.

Several proposals for energy absorption during a fan blade off scenario are known, but these do not address the problem of damping vibration of the fan track liner during normal operation of a gas turbine engine.

SUMMARY OF INVENTION

In a first aspect the present invention provides a fan containment system for fitment around an array of radially extending fan blades mounted on a hub in an axial gas turbine engine. The fan containment system comprises a fan case having an annular casing element for encircling an array of fan blades. An annular fan track liner is positioned substantially coaxial to the annular casing element. One or more pockets are provided in a radially outer side of the fan track liner.

One or more resilient members for snubbing and/or damping vibration of the fan track liner may be positioned in each of the one or more pockets and may be arranged so as to contact the annular casing element.

The resilient members damp and/or snub vibration of the fan track liner during operation of the engine. Providing the resilient members in pockets helps to ensure that there is contact between the resilient members and the annular casing element even through there are differing manufacturing tolerances of the fan track liner and the casing element. This increased certainty of contact means that the force applied to the casing by the resilient members can be better controlled. Further, the pockets contribute to retention of the resilient members during operation of a gas turbine engine.

The one or more resilient members may be considered to be dampers and/or a snubbers. For example, one or more dampers for damping and/or snubbing vibration of the fan track liner may be positioned in each of the one or more pockets and may be arranged so as to contact the annular casing element. Additionally or alternatively, one or more snubbers for snubbing and/or damping vibration of the fan track liner may be positioned in each of the one or more pockets and are arranged so as to contact the annular casing element.

The resilient member may be made from a viscoelastic material. As will be understood by the person skilled in the art, a viscoelastic material exhibits both viscous and elastic characteristics when undergoing deformation. Example viscoelastic materials include natural rubber, synthetic rubber or elastomers.

A second aspect of the invention provides a fan containment system for fitment around an array of radially extending fan blades mounted on a hub in an axial gas turbine engine. The fan containment system comprises a fan case having an annular casing element for encircling an array of fan blades. An annular fan track liner is positioned substantially coaxial to the annular casing element and has one or more pockets formed therein. One or more viscoelastic dampers for damping vibration of the fan track liner are positioned in each of the one or more pockets.

The following are optional features of the first and/or the second aspect. As will be understood by the person skilled in the art the optional features may be used in combination with one or more of the other disclosed optional features. Features applicable to the resilient members are equally applicable to the viscoelastic dampers.

The pockets may extend only partially through the fan track liner. In this way, the pockets avoid interference with a gas washed surface of the fan track liner.

A gap may be provided between the fan track liner and the annular casing element.

The dampers may be dimensioned relative to the pockets and the gap between the fan track liner and the annular casing element, such that the damper contacts the casing element even when the respective pocket is located in a region having a maximum gap (compared to the rest of the gap) between the fan track liner and the casing element.

The resilient member may be shaped for optimal load transfer to the casing element. For example, the area of the resilient member in contact with the casing may be smaller than the area of the resilient member in contact with the fan track liner (e.g. the area of the base of the resilient member may be larger than the area of the resilient member in contact with the fan track liner).

The resilient member may be conical or frusto-conical in shape.

An end of the resilient member adjacent the casing element may be castellated. For example, the resilient member may be cylindrical with a castellated end.

Provision of a conical member and/or a castellated member contributes to providing a member that always applies a force to the annular casing element even if the damper is located in a region having a relatively large gap between the fan track liner and the annular casing element (compared to the rest of the fan track liner).

The resilient members may be positioned at locations corresponding to anti-node points of expected operational modes of vibration. Expected modes of vibration can be calculated using known testing and/or modelling techniques. Positioning the members at anti-node points can help to further reduce vibration of the fan track liner.

The resilient members may be constructed so as to compress into the pockets so that there is substantially no protrusion of the resilient members from the fan track liner at a pre-determined load. Such an arrangement means that under large loading conditions, e.g. bird or ice impact, the resilient members can compress (e.g. compress fully) into the pockets so as to allow any high loading impacts to be reacted by the fan track liner.

The fan track liner may comprise a separation layer. The fan track liner may comprise an intermediate layer positioned on a radially inner side of the separation layer. The fan track liner may comprise an abradable layer positioned on a radially inner side of the separation layer and/or the intermediate layer. A septum layer may be provided between the intermediate layer and the abradable layer.

The fan track liner may comprise a further intermediate layer connected to a radially outer side of the separation layer. In exemplary embodiments, the separation layer may be tray.

The further intermediate layer may comprise the one or more pockets. For example, the pockets may extend through the entire or near entire thickness of the intermediate layer, e.g. the pockets may terminate at the separation layer.

Provision of the further intermediate layer and provision of the pockets in said further intermediate layer is a preferred method of ensuring the dampers do not impede on the operation of the fan track liner in the event that a fan blade (or part of a fan blade) is released from a hub or in the event that the fan track liner is impacted by ice or a bird.

The further intermediate layer may comprise a honeycomb structure (e.g. an aluminium (or alloy thereof) honeycomb structure).

The fan track liner may comprise a plurality of arcuate fan track liner panels positioned coaxially so as to define the annular fan track liner.

The fan case may comprise a hook projecting in a generally radially inward direction from the annular casing element and positioned axially forward of an array of fan blades when the fan containment system is fitted around said fan blades.

A third aspect of the invention provides a gas turbine engine comprising the fan containment system according to the first or second aspect.

A fourth aspect of the invention provides a gas turbine engine comprising:a casing;a component positioned radially outward of the casing;a bracket connecting the component to the casing;an intermediate layer (e.g. honeycomb layer) positioned between the component and the casing, andone or more dampers;wherein one or more pockets are formed in the intermediate layer and one of the one or more dampers is positioned in each of the one or more pockets.

The dampers may perform the function of damping and/or snubbing.

The component may be a raft assembly. In the present application, a raft assembly is referred to as a substantially rigid composite panel in which electrical conductors are embedded, the electrical conductors may form part of an electrical harness for a gas turbine engine.

DETAILED DESCRIPTION

With reference toFIG. 3a bypass gas turbine engine is indicated at10. The engine10comprises, in axial flow series, an air intake duct11, fan12, a bypass duct13, an intermediate pressure compressor14, a high pressure compressor16, a combustor18, a high pressure turbine20, an intermediate pressure turbine22, a low pressure turbine24and an exhaust nozzle25. The fan12, compressors14,16and turbines18,20,22all rotate about the major axis of the gas turbine engine10and so define the axial direction of the gas turbine engine.

Air is drawn through the air intake duct11by the fan12where it is accelerated. A significant portion of the airflow is discharged through the bypass duct13generating a corresponding portion of the engine thrust. The remainder is drawn through the intermediate pressure compressor14into what is termed the core of the engine10where the air is compressed. A further stage of compression takes place in the high pressure compressor16before the air is mixed with fuel and burned in the combustor18. The resulting hot working fluid is discharged through the high pressure turbine20, the intermediate pressure turbine22and the low pressure turbine24in series where work is extracted from the working fluid. The work extracted drives the intake fan12, the intermediate pressure compressor14and the high pressure compressor16via shafts26,28,30. The working fluid, which has reduced in pressure and temperature, is then expelled through the exhaust nozzle25generating the remainder of the engine thrust.

The intake fan12comprises an array of radially extending fan blades40that are mounted to the shaft26. The shaft26may be considered a hub at the position where the fan blades40are mounted.FIG. 3shows that the fan12is surrounded by a fan containment system300that also forms one wall or a part of the bypass duct13.

In the present application a forward direction (indicated by arrow F inFIG. 3) and a rearward direction (indicated by arrow R inFIG. 3) are defined in terms of axial airflow through the engine10.

Referring now toFIG. 4, the fan containment system300is shown in more detail. The fan containment system300comprises a fan case350. The fan case350includes an annular casing element352that, in use, encircles the fan blades40of the gas turbine engine (indicated at10inFIG. 3). The fan case350further includes a hook354that projects from the annular casing element in a generally radially inward direction. The hook354is positioned, in use, axially forward of the fan blades and the hook is arranged so as to extend axially inwardly, such that if a fan blade (or part of a fan blade) detaches from the rotor the hook354prevents the fan blade from exiting the engine through the air intake duct (indicated at11inFIG. 3).

In the present embodiment, the hook354is substantially L-shaped and has a radial component extending radially inwards from the annular casing element352and an axial component extending axially rearward towards the fan blades40from the radial component.

A fan track liner356is connected to the fan case350at the hook354via a connector that is configured to permit movement of the fan track liner relative to the hook when a pre-determined force is applied to the fan track liner. In the present embodiment, the connector includes a plurality of circumferentially spaced fasteners366designed to shear/fracture at a predetermined load such that movement of the fan track liner radially outwards towards the annular casing element352is permitted when a load exerted on the fan track liner exceeds a predetermined level (in alternative embodiments an alternative fastening mechanism may be used e.g. a crushable collar or a sprung fastener).

A forward portion of the fan track liner356is spaced radially inward from the annular casing element352so that a voidal region380is formed between the forward portion of the fan track liner356and the casing element352.

In the present embodiment, the fan track liner356is formed from a plurality of arcuate panels positioned substantially coaxial so as to define the annular fan track liner.

A standoff379protrudes radially inwardly from the casing element352. The standoff is positioned axially between a forward end of the fan track liner356and a rearward end of the fan track liner. Each fan track liner panel is connected to the standoff via a fastener381, e.g. a bolt. The fastener381is covered by material of the fan track liner so that the fan track liner panels have a substantially smooth gas washed surface. The standoff may be a series of L-shaped protrusions or a continuous L-shaped protrusion extending circumferentially around a radially inner surface of the casing element.

A rearward end of the fan track liner356is connected to a support member382. The support member382protrudes radially inwards from the annular casing element352. In the present embodiment, the support member382is formed of a series of circumferentially spaced L-shaped protrusions, but in alternative embodiments the support member may extend fully around the annular casing element (i.e. with no interruptions/spacing). In the present embodiment, the fan track liner356is connected to the support using a plurality of fasteners383. The connection and manufacturing tolerances of the annular casing to the support member is such that any step between the fan track liner and adjacent panel (e.g. acoustic panel) will be out-of-flow (i.e. stepped radially outward) so as to improve aerodynamics.

In the event of a fan blade (or part of a fan blade) being released from the remainder of the fan, the fan blade will impact the fan track liner and the fastener of the impacted fan track liner panel or fan track liner panels will fail. The impacted fan track liner panel will then pivot, at least initially, about the standoff379to make room for the fan blade to impact the hook354for containment.

The construction of the fan track liner356will now be described in more detail. The fan track liner356includes a tray378to which an intermediate layer360is connected (e.g. bonded) to a radially inner surface of the tray. An attrition layer (or abradable layer)358is positioned, in use, proximal to the fan blades40. A septum layer362provides an interface between the attrition layer and the intermediate layer360, forming part of the bond between the two. The septum layer362also separates the attrition layer and the intermediate layer and distributes any applied load between the attrition layer and the intermediate layer. The tray378is connected to the hook354via the fastener366so as to connect the fan track liner356to the fan case350.

A further intermediate layer (which may also be referred to as a backing layer or a filler layer)384is bonded to a radially outer surface of the tray378. The further intermediate layer384is provided at an axially rearward end of the fan track liner. More specifically, the further intermediate layer is spaced rearwardly from the standoff and extends to a rearward end of the fan track liner. The further intermediate layer384extends radially outwardly from the tray towards the annular casing element, with a gap provided between the further intermediate layer and the annular casing element. However, as will be appreciated by the person skilled in the art, due to manufacturing tolerances, the radial length of the gap may vary as a significant proportion of the intended (small) nominal radial length.

In the present embodiment the intermediate layer and the further intermediate layer are formed from an aluminium honeycomb structure. However, in alternative embodiments the honeycomb structure may be made from any other suitable material, e.g. an alternative metallic material, or the intermediate layer may be formed by a material such as suitable foam.

The septum layer, attrition layer and tray may be made from any suitable material, but by way of example only, the septum layer may be formed from a carbon fibre or glass reinforced polymer; the attrition layer may be formed of an epoxy resin that is curable at room temperature; and the tray may be formed of a carbon fibre or glass reinforced polymer.

Pockets386are formed in the fan track liner panel. In the present embodiment the pockets extend through the depth of the further intermediate layer384and terminate at the tray378. The pockets may be considered counter bores provided in the intermediate layer. In the present embodiment the pockets or counter bores have a circular cross section, but the pockets may be provided with any suitable cross section. The pockets are spaced at regular or irregular intervals circumferentially around the fan track liner356. In the present embodiment two rows of pockets are provided, one axially rearward of the other. However, it will be appreciated that the pockets may not be provided in rows and may instead be staggered axially in location around the fan track liner. In alternative embodiments an alternative pocket arrangement may be provided, for example, the pockets386may be positioned so as to correspond to anti-node points of vibration modes known to occur during the operation of the gas turbine engine.

A resilient member that acts as both a damper and a snubber is positioned in each pocket386. In the present embodiment, each component is a conical damper388.

The damper extends out of the pocket and contacts the annular casing element352. The conical shape of the damper means that the damper has a firm base on the fan track liner356and a smaller contact with the annular casing element352. As will be appreciated by the person skilled in the art, the annular casing element will usually be made to higher tolerance levels than the fan track liner, which means that the gap between the fan track liner and the casing element will vary. The small contact area between the conical damper and the annular casing element means that the damper can be sized to project radially outward from the outer intermediate layer of the liner384and are designed to deform on contact with the annular casing element352when the fasteners381and383are secured. Thus the resilient members provide firm contact between the fan track liner356and the annular casing352between the fastening points381and383in a way that overcomes the variation of the gap between the fan track liner356and the annular casing352due to manufacturing tolerances.

The damper is made from a viscoelastic material. In this way the damper displays both elastic and viscous properties so as to damp vibration of the fan track liner during operation of the engine.

The viscoelastic material and/or the shape of the damper is also selected such that under major loads, for example ice or bird impact, the dampers can compress into the pockets so as to allow the load to be reacted by the full area of the further intermediate layer that will contact the casing element in such a high loading event. It will be appreciated by the person skilled in the art that the construction of the fan track liner will be selected so as to allow such movement without sustaining permanent damage.

Advantageously, the described damper arrangement reduces vibration of the fan track liner during operation of the gas turbine engine. In the present embodiment, this advantageously means that the fan track liner can be formed from a plurality of panels without suffering from unacceptable vibration levels.

As well as damping the vibration, the conical damper388also performs a snubbing function. That is, the conical damper388resists movement in one direction (i.e. towards the annular casing element352) and so the natural response of the fan track liner is snubbed. For example, a “bowstring” vibration mode of the fan track liner between its bolted fixing points to the casing is limited or prevented because half of the vibration cycle in which the panel would otherwise deflect towards the casing is snubbed by the presence of the damper cones.

Positioning of the dampers within the pockets means that the dampers are fully enclosed by the pockets and the casing element, so the risk of potential failure by non-retention of the dampers is mitigated.

It will be appreciated by one skilled in the art that, where technical features have been described in association with one embodiment, this does not preclude the combination or replacement with features from other embodiments where this is appropriate. Furthermore, equivalent modifications and variations will be apparent to those skilled in the art from this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting.

For example, the damper has been described as a conical damper, but the damper may take any suitable shape. For example, the damper may be cylindrical in shape and/or may have one end that is castellated.

In the described embodiment the damper is made from a viscoelastic material, but in alternative embodiments any other damper displaying both elastic and viscous properties may be used.

The fan containment system described has a trap door arrangement, but the dampers may be used with other types of fan containment systems. For example, a gas turbine engine having composite fan blades may not have a hook or a trap door arrangement because the majority of a released blade is likely to break up on impact with the fan track liner, but provision of dampers in a similar way as described can still be beneficial.

The position of the further intermediate layer has been described as axially rearward of the standoff, but the further intermediate layer may be positioned at any point along the tray of the fan track liner, provided that it does not restrict operation of the fan containment system, e.g. in a trap door arrangement a gap between the fan track liner and the annular casing element should be provided near to the hook to give the fan track liner room to move towards the annular casing element in a fan blade off event.

In the present embodiment, pockets are provided in the further intermediate layer and terminate at the tray, but in alternative embodiments the pockets may extend further into the fan track liner than the tray or the tray may include depressions to accommodate a portion of the dampers. In further alternative embodiments, no intermediate layer may be provided and instead the pockets may be provided in one or more of the other layers of the fan track liner, and/or the tray may include depressions to accommodate the dampers.

In the present embodiment, a tray is provided between the intermediate layer and the further intermediate layer, but in alternative embodiments the tray may be replaced by any suitable separation or septum layer. In further alternative embodiments, any suitable number of septum layers may be provided. The pockets may extend to any one of the septum layers. In further alternative embodiments the pockets may be alternative depths extending to different septum layers and containing different sized conical dampers for tuning to different modes of vibration.

An alternative application of the honeycomb and damper arrangement described above is for use in damping a raft bracket390, or some other type of bracket connected to a casing (e.g. fan casing) of the gas turbine engine. For example, the raft390may be connected to a radially outer surface of an annular casing of the gas turbine engine via two or more brackets. An aluminium honeycomb layer may be provided between the casing and the raft390. Holes or pockets may be formed in the honeycomb layer and conical dampers positioned therewithin. In this way, the conical dampers will damp vibration of the raft390relative to the casing. The raft390may also be referred to as a substantially rigid composite panel in which electrical conductors are embedded, the electrical conductors may form part of an electrical harness for a gas turbine engine.