EP0922837B1

EP0922837B1 - Fan case liner

Info

Publication number: EP0922837B1
Application number: EP19980309221
Authority: EP
Inventors: Keven G. Van Duyn
Original assignee: United Technologies Corp
Current assignee: RTX Corp
Priority date: 1997-11-11
Filing date: 1998-11-11
Publication date: 2005-08-10
Anticipated expiration: 2018-11-11
Also published as: DE69831124T2; EP0922837A2; DE69831124D1; EP0922837A3; JPH11200813A

Description

The present invention relates to gas turbine engines, and more particularly, to providing a hardened liner in the fan case of the engine to minimize damage to the fan case in the event of a fan blade loss.
A gas turbine engine, such as a turbofan engine for an aircraft, includes a fan section, a compression section, a combustion section, and a turbine section. An axis of the engine is centrally disposed within the engine, and extends longitudinally through these sections. A primary flow path for working medium gases extends axially through the engine. A secondary flow path for working medium gases extends parallel to and radially outward of the primary flow path.
During operation, the fan draws air into the engine. The fan raises the pressure of the air drawn along the secondary flow path, thus producing useful thrust. The air drawn along the primary flow path into the compressor section is compressed. The compressed air is channelled to the combustor section, where fuel is added to the compressed air, and the air-fuel mixture is burned. The products of combustion are discharged to the turbine section. The turbine section extracts work from these products to power the fan and compressor. Any energy from the products of combustion not needed to drive the fan and compressor contributes to useful thrust.
The fan section includes a rotor assembly and a stator assembly. The rotor assembly of the fan includes a rotor disk and a plurality of outwardly extending rotor blades. Each rotor blade includes an airfoil portion, a root portion, and a tip portion. The airfoil portion extends through the flow path and interacts with the working medium gases to transfer energy between the rotor blade and working medium gases. The stator assembly includes a fan containment case assembly, which circumscribes the rotor assembly in close proximity to the tips of the rotor blades. The fan containment case assembly includes a fan case which provides a support structure, a plurality of fabric wraps disposed radially outwardly of the fan case, a plurality of circumferentially adjacent acoustic panels and a plurality of circumferentially adjacent rub strips disposed radially inwardly of the fan case.
Conventional fan cases are typically a solid metal casing which forms a rigid structure to support the fabric wraps. The plurality of rub strips are formed from a relatively compliant material. In the event that the tip of a fan blade makes contact with the rub strips, the compliance of the rub strips minimizes the risks of damage to the fan blade.
There are two specific clearances between the fan blade tips and the fan containment case assembly which are of importance. The first one is characterized as a performance clearance and is defined as the clearance between the blade tips and the soft rub strip in the inner surface of the fan case. The second clearance is characterized as an effective structural clearance and is defined as the clearance between the blade tips and a hard metallic surface in the fan case. The present invention is concerned with this structural clearance, as opposed to the performance clearance.
Severe rotor imbalance can occur in an engine, particularly after a fan blade breaks off from the rotor assembly. One cause of fan blade loss is impact with foreign objects, such as birds, hailstones or other objects which, on occasion, are ingested into the engine. The detached fan blade is thrown outwardly and passes through the fan case, but is typically caught by the fabric wraps in the fan containment case assembly. Blade loss produces an imbalance in the rotor and causes the rotor shaft to deflect radially outwardly.
Deflection of the rotor away from its longitudinal axis may also lead to additional damage to the rotor assembly. The more the rotor deflects, the greater is the radial load on the rotor bearing supports.
The fan case structure stops the deflection of the rotor assembly. The damage to the rotor assembly is reduced by decreasing the fan tip-to-case clearance as the shaft deflection is limited by the proximal portion of the fan case assembly. Minimizing the amount of radial deflection of the shaft minimizes the likelihood of damage occurring in the shaft, the rotor bearings and the bearing support structures.
Heretofore, the deflection of the shaft was restrained by the fan blades embedding themselves in the fan case. In doing so, the fan blades rapidly cut away and destroy the fan case because the blades are usually of a harder material than the fan case. As the fan blades continue to rotate, they couple with the static fan case structure and transmit the kinetic energy of the rotor shaft to the case, causing twisting of and damage to the case. Due to the coupling of the rotor shaft with the fan case, high torque loads are transmitted to the fan case. This torque loading of the fan case during a fan blade loss event, results in tremendous loads being imposed upon the related engine mounts and engine case structure.
Thus, the challenge for modern gas turbine engines, during fan blade loss events, is the limiting of the rotor shaft deflection while minimizing the torque loading of the fan case from the rotor shaft kinetics.
US-A-5,403,148 discloses a gas turbiine engine having the faetures of the preamble of claim 1.
According to the present invention, there is provided a gas turbine engine as claimed in claim 1.
Thus a fan case in a gas turbine engine includes a liner of hardened material attached thereto wherein during a fan blade loss condition, the blade tips skid on the hardened liner and reduce the destructive cutting away of the fan case. This liner of hardened material provides a skid-surface for the blades to circumferentially glide on and precludes or reduces embedding of the blades in the fan case and thus minimizes torque loading of the fan case. Further, the fan case structure of the present invention limits the deflection of the rotor shaft during a fan blade loss event. The liner of hardened material comprises a plurality of shingles.
A primary advantage of the present invention is the minimization of damage to the fan case, thus resulting in a durable fan case in the event of a fan blade loss. The hardened fan case liner of the present invention reduces the destructive cutting away of the fan case by the fan blades. A further advantage of the fan case of the present invention is its ability to provide an appropriate restraining structure to the deflection of the rotor shaft during a fan blade loss event. In addition, the hardened liner reduces frictional forces and therefore, the torque transmitted from the rotor to the engine cases. Another advantage is the ease and cost of manufacturing and incorporating into the fan case the liner of the present invention. The simplicity of the structure of the liner and the use of economic materials, allows for cost effective manufacturing processes. Further, fan cases of the prior art can be retrofitted to include the present invention in a cost effective manner.
Some preferred embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which:
FIG. 1 is a perspective view of a typical axial flow, turbofan gas turbine engine.
FIG. 2 is a perspective view of the rotor assembly of the prior art showing a released fan blade.
FIG. 3 is a cross-sectional schematic representation of the fan containment case assembly including a fan case taken along the lines 3-3 of FIG. 2.
FIG. 4 is a schematic representation of the fan case liner embodying the present invention under operating conditions.
FIG. 5 is a schematic representation of an alternate embodiment of the fan case liner of the present invention.
Referring to FIG. 1, an axial flow, turbofan gas turbine engine 10 comprises a fan section 14, a compressor section 16, a combustor section 18 and a turbine section 20. An axis of the engine Ar is centrally disposed within the engine and extends longitudinally through these sections. A primary flow path 22 for working medium gases extends longitudinally along the axis Ar. The secondary flow path 24 for working medium gases extends parallel to and radially outward of the primary flow path 22.
The fan section 14 includes a stator assembly 27 and a rotor assembly 28. The stator assembly has a fan containment case assembly 30 which forms the outer wall of the secondary flow path 24. The rotor assembly 28 includes a rotor disk 32 and a plurality of rotor blades 34. Each rotor blade 34 extends outwardly from the rotor disk 32 across the working medium flow paths 22 and 24 into proximity with the fan containment case assembly 30. Each rotor blade 34 has a root portion 36, an opposed tip 38, and a midspan portion 40 extending therebetween.
Referring to FIG. 3, a fan case liner 42 is disposed in the fan containment case assembly 30. The fan containment case assembly circumscribes the rotor assembly 28 in close proximity to the tips 38 of the rotor blades 34. The containment case assembly 30 includes a liner 42, a plurality of circumferentially adjacent rub strips 44 and a plurality of circumferentially adjacent acoustic panels 46 disposed radially inwardly of a support structure or a fan case 48. A plurality of fabric wraps 50 are disposed radially outwardly of the fan case. The fan case is typically a solid metal casing which forms a rigid structure to support the fabric wraps. The term "fabric" 50 includes, but is not limited to, tape, woven material or the like, and restrains a fan blade in the event of a fan blade loss. The rub strips 44 are formed from a relatively compliant material. The rub strips 44 permit the fan blades 34 to be in close proximity to the fan case to minimize the amount of air that flows around the fan blades, thus reducing fluid flow leakage around the fan blades to improve fan performance. In the event that the tip 38 of a fan blade 34 makes contact with the rub strips 44, the compliance of the rub strips minimizes the risk of damage to the fan blade 34. The fan case liner 42, is made from hardened material such as from alloys of stainless steel or nickel. The nickel alloy Inconel 718, or stainless steel alloys, such as AISI 321 or AISI 347, are examples of alloys that can be used to manufacture the liner. The liner is thus manufactured from material that is harder than the fan blade tip material which is typically titanium. For ease of installation, the liner could be manufactured as arcuate segments which can then be bonded to the fan case. This arrangement, using a one-piece liner does not fall within the scope of the present invention, but its description has been retained to assist the reader in understanding the invention.
Referring to FIG. 4, a segmented fan case liner of an engine embodying the present invention is disposed radially outwardly of the rub strip 44 in the fan containment case assembly 30. Each segment 52 or shingle is offset from its adjacent shingle, yet there is an overlap region 54, shown clearly in FIG. 5, between adjacent shingles. As shown in FIG. 5, the fan case liner 42 is attached to the fan case 48 by either rivets 56, or adhesives as shown in FIG. 4. The rivets 56 are located in the overlap region 54 between adjacent shingles.
In the event of a fan blade loss during engine operation, the detached blade is thrown radially outwardly. It typically will pass through the fan case 48 and will be caught by the fabric wraps 50 in the fan containment case assembly 30.
The blade loss produces an imbalance in the rotor and causes the rotor to deflect radially outwardly in close proximity to the fan case. The separation between the fan blades and the inner surface of the fan case is minimized in modern engines to decrease the radial deflection of the rotor assembly. Due to the rotor deflection and the reduced clearance between the fan blades and the fan case, the fan blade tips rapidly cut away the compliant rub strip 44 in the innermost surface of the fan containment case assembly. The thin fan case liner, made from shingles of hardened materials such as steel or nickel alloys, provides a skid surface for the relatively softer blades. The fan blades move circumferentially along on the skid-surface of the liner. The embedding of the blades in the fan case is eliminated or reduced; and as a result, the unwanted torque loading of the case is reduced. Without the hardened liner, the fan blades would continue to cut away and firmly embed in the fan case. The present invention, thus provides for a system that allows for limiting rotor deflection during a fan blade loss event and provides a skid-plate function which eliminates or reduces the generation of additional torque loading on the case.
The shinglesprovide a skid-surface for the fan blades to circumferentially rotate upon. However, by being segmented, the damage to the liner after a fan blade loss event is limited to the loss of one or more adjacent shingles. The remaining shingles continue to provide an effective skid-surface for the fan blades to glide on.
A primary advantage of the present invention fan case liner is the minimization of damage to the fan case thus, resulting in a durable fan case in the event of a fan blade loss. The liner reduces the destructive cutting away of the fan case by the fan blades. A further advantage of the present invention fan case is its ability to provide an appropriate restraining structure to the deflection of the rotor shaft during a fan blade loss event. In addition, the liner reduces frictional forces, and as a result, reduces torque loads transmitted from the fan rotor to the case. Another advantage is the ease and cost of manufacturing and incorporating the hardened fan case liner of the present invention. The simplicity of the structure of the liner and the use of economical materials, allows for cost effective manufacturing processes. Further, current, prior art fan cases can be retrofitted to include the fan case liner in a cost effective manner. By incorporating the present invention liner, current engines limit damage to the fan containment case assembly and to the rotor shaft.
Although the invention has been shown and described with respect to detailed embodiments thereof, it should be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the scope of the claims.

Claims

A gas turbine engine (10) disposed about a longitudinal axis (A_r) said engine comprising:

a rotor (28) including a fan, the fan having blades (34) ; and

a stator (27) including a fan case (30) disposed radially outward of the fan, and further comprising
a segmented hardened liner (42) having an interior surface, disposed in the fan case so as to circumscribe the fan blades such that, in use, in the event of a fan blade loss condition, the liner reduces the damage to the fan case by allowing the fan blades to skid along the interior surface thereof; characterised in that said segmented, hardened liner (42) comprises a plurality of shingles (52) circumferentially disposed in the fan case (30), said shingles being offset from adjacent shingles and having an overlap region (54) between the adjacent shingles.
A gas turbine engine as claimed in claim 1 wherein the liner material is an alloy of stainless steel or nickel.
A gas turbine engine as claimed in claim 1 or 2 wherein said shingles are attached to the fan case (48) by rivets (56) located in the overlap region (54) between adjacent shingles.
A gas turbine engine as claimed in claim 1 or 2 wherein said shingles are attached to the fan case (48) by adhesive.