U.S. patent application number 13/492499 was filed with the patent office on 2012-12-20 for tip treatment for a rotor casing.
This patent application is currently assigned to ROLLS-ROYCE PLC. Invention is credited to Matthew D. CURREN, Eric P. RAVEY, Fabienne M. RAVEY.
Application Number | 20120321443 13/492499 |
Document ID | / |
Family ID | 44357748 |
Filed Date | 2012-12-20 |
United States Patent
Application |
20120321443 |
Kind Code |
A1 |
RAVEY; Eric P. ; et
al. |
December 20, 2012 |
TIP TREATMENT FOR A ROTOR CASING
Abstract
A tip treatment bar and associated rotor casing structure,
wherein the tip treatment bar includes a solid composite structure
having a plurality of layers arranged therein so as to provide the
bar with a directional stiffness characteristic. Such a layered
composite arrangement can be used to tailor the vibration
characteristics of the bar. The tip treatment bar and casing
structure may be suited for mounting relative to a fan or
compressor in a gas turbine engine.
Inventors: |
RAVEY; Eric P.; (Derby,
GB) ; RAVEY; Fabienne M.; (Derby, GB) ;
CURREN; Matthew D.; (Derby, GB) |
Assignee: |
ROLLS-ROYCE PLC
London
GB
|
Family ID: |
44357748 |
Appl. No.: |
13/492499 |
Filed: |
June 8, 2012 |
Current U.S.
Class: |
415/119 |
Current CPC
Class: |
F05D 2300/603 20130101;
F05D 2300/44 20130101; F04D 29/685 20130101; F04D 29/522 20130101;
F01D 21/045 20130101; F05D 2300/702 20130101; Y02T 50/672 20130101;
Y02T 50/60 20130101; F01D 25/24 20130101 |
Class at
Publication: |
415/119 |
International
Class: |
F04D 29/66 20060101
F04D029/66 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 15, 2011 |
GB |
1110008.8 |
Claims
1. A tip treatment bar for a rotor casing, the tip treatment bar
having a composite structure comprising a plurality of internal
layers arranged therein with each internal layer comprising a
composite material having a plurality of fibres arranged within a
matrix material, the bar having a longitudinal axis and fibres
within at least one of said internal layer are substantially
aligned in a direction which is angled to the longitudinal axis so
as to provide the bar with a directional stiffness
characteristic.
2. A tip treatment bar according to claim 1, wherein the internal
layers of the bar are spaced from the outer surface of the bar by a
coating layer.
3. A tip treatment bar according to claim 1, comprising first and
second internal layers, wherein the fibres in a first layer are
aligned in a direction which is different from the alignment of
fibres in the second layer.
4. A tip treatment bar according to claim 1, wherein the bar has a
longitudinal axis and the internal layers extend substantially in
the direction of said axis such that the layers are substantially
uniform in a longitudinal direction and the bar is non-uniform in
cross section.
5. A tip treatment bar according to claim 1, comprising an inner
body of material, wherein at least on internal layer is arranged
circumferentially around said inner body.
6. A tip treatment bar according to claim 5, comprising a plurality
of layers arranged concentrically about said inner body.
7. A tip treatment bar according to claim 5, wherein the inner body
of material comprises a composite material having a plurality of
layers therein.
8. A tip treatment bar according to claim 1, comprising an
outermost composite layer and a coating applied to a portion of
said outermost composite layer such that a portion of the outermost
composite layer is exposed on an external surface of the bar.
9. A tip treatment bar according to claim 1, wherein the bar has a
plurality of lateral edges extending between opposing sides of the
bar and wherein the lateral edges are recessed with respect to the
ends of the bar.
10. A rotor casing comprising a plurality of bars according to
claim 1 and a support structure arranged to maintain the bars in a
circumferentially spaced array with respect to an axis of rotation
of the rotor in use.
11. A rotor casing according to claim 10, wherein each bar is
spaced from the support structure by a damping member.
12. A rotor casing according to claim 10, wherein the support
structure comprises first and second annular support members
arranged about a rotational axis of the rotor in use, each support
member having an annular array of openings shaped to receive an end
of each bar such that the bars are supported at their ends within
the openings and extend between the first and second support
members.
13. A rotor casing according to claim 12, wherein the bars have a
reduced depth dimension at each end thereof such that the portion
of the bars extending between the support members has a radially
inner surface that is substantially flush with an adjacent radially
inner surface of the first and/or second support member when
assembled for use.
14. A rotor casing according to claim 10, wherein the support
structure comprises a composite material.
15. A gas turbine engine comprising a tip treatment bar according
to claim 1.
16. A tip treatment bar for a rotor casing, the tip treatment bar
having a composite structure comprising a plurality of internal
layers arranged therein with each internal layer comprising a
composite material having a plurality of fibres arranged within a
matrix material, wherein at least one internal layer is arranged
circumferentially around said inner body so as to provide the bar
with a directional stiffness characteristic.
17. A tip treatment bar according to claim 1, wherein the internal
layers of the bar are spaced from the outer surface of the bar by a
coating layer.
18. A tip treatment bar according to claim 16, comprising a
plurality of layers arranged concentrically about said inner
body.
19. A rotor casing comprising a plurality of bars according to
claim 16 and a support structure arranged to maintain the bars in a
circumferentially spaced array with respect to an axis of rotation
of the rotor in use.
20. A rotor casing according to claim 19, wherein each bar is
spaced from the support structure by a damping member.
Description
[0001] The present invention relates to a casing for a rotor and
more particularly to a casing structure comprising an array of
bars. The present invention also relates to bars for a rotor
casing.
[0002] Aerodynamic stall is a well known problem for bladed rotors,
such as fans or compressors. The criticality of operation of such
rotors typically determines the level of sophistication with which
the onset of stall conditions is investigated and also the
countermeasures put in place to avoid or mitigate the associated
risks. Gas turbine engines represent an example of machines for
which the aerodynamic performance of a fan or compressor is of
critical importance to both performance and safety.
[0003] International Patent Application PCT/GB94/00481 (published
as WO94/20759) describes a casing structure for a gas turbine
engine compressor, which comprises an annular cavity located about
the tip region of the compressor blades. Ribs are arranged about
the cavity to define circumferentially spaced slots there-between.
The effect of such a casing structure on the flow in the vicinity
of the blade tips is such as to delay the onset of stall and,
accordingly, such a structure is often referred to as an anti-stall
tip treatment system.
[0004] WO 94/20759 is concerned almost entirely with the
aerodynamic factors for delaying stall. However there are a number
of other important factors which impact on the design of a suitable
casing structure. Within gas turbine engine applications, for
example, the consideration of weight, strength and fatigue all
affect the possible implementation of such a casing structure.
[0005] Published UK Patent Applications GB 2 373 021, GB 2 363 167
and GB 2 362 432 each disclose advances to the basic system
disclosed in WO 94/20759 for the purpose of accommodating such
practical considerations. Those documents disclose various details
of tip treatment bars and associated mounting arrangements with a
view to reducing high cycle fatigue, whilst also allowing the bars
to yield in the event that they are struck by a blade or blade
fragment shed from the rotor.
[0006] However the inventors have identified that additional
considerations, over and above those discussed in the prior art,
can impact significantly on the function of such casing structures.
For example, the geometry of the bar and/or associated casing
structure can change during use, which can adversely affect the
clearance between the blade tips and the casing, amongst other
aerodynamic considerations. Furthermore the degree to which the
damping can be controlled according to the embodiments of the prior
art is not optimal.
[0007] Published UK Patent Application GB 2 373 024 discloses a
coating that may be applied to a tip treatment bar in order to
dissipate strain energy in use and thereby reduce the amplitude of
vibrations. Whilst this does provide a partial solution to the
problem of damping, it has been found that there is little scope to
control the response of the bar during operation using such a bar
construction.
[0008] Whilst any one of the above problems may be addressed in
isolation, it is to be noted that any proposed solution to one such
problem will also impact on another functional characteristic of
the casing or rotor. Accordingly the above-described partial
problems in fact relate to a mosaic of inter-related
characteristics of the system which must be carefully balanced in
order to arrive at an optimal solution.
[0009] It is an object of the present invention to provide a casing
structure for a rotor which mitigates or satisfies the
above-described problems to a greater degree.
[0010] According to the broad concept of the present invention,
there is provided a tip treatment bar for a rotor casing, the tip
treatment bar having a layered internal structure.
[0011] According to a first aspect of the invention, there is
provided a tip treatment bar for a rotor casing, the tip treatment
bar being of a substantially solid composite structure having a
plurality of laminae or layers arranged therein so as to provide
the bar with a directional stiffness characteristic.
[0012] The directional stiffness characteristic may be tailored to
tune the bar such that the natural frequencies of the bar fall
outside a normal operational frequency range of vibration, induced
by the rotor.
[0013] The bar has a plurality of internal layers. The internal
layers may be spaced from the outer surface of the bar by one or
more external layers or coatings. A composite structure of this
type may be considered to have a non-uniform core. Such a structure
differs from a bar formed of a single uniform core material which
has an outer coating layer applied thereto. The core of the
composite structure may be considered to account for a central
region which is anywhere between 50 and 95% of a cross sectional
dimension or area of the bar. According to the invention, that core
region may have a plurality of composite layers.
[0014] The aligned nature of the materials may be such that the
composite structure may be generally uniform in a first direction,
for example a longitudinal or axial direction, but non-uniform in a
second direction, which may be perpendicular to the first
direction, such as, for example, a transverse or radial
direction.
[0015] The internal layers may be aligned with a longitudinal axis
of the bar. Fibres within the at least one internal layer may be
arranged obliquely to the longitudinal axis of the bar.
[0016] According to one particular embodiment, the materials within
the bar may be arranged in a plurality of circumferential layers or
laminae. One or more layers may extend across a width of the bar.
The bar typically has opposing side walls, which may be
substantially parallel, and one or more layers may be arranged
obliquely to one or both of said side walls. One or more layers may
pass through the core of the bar. The bar may have an outer layer
comprising an erosion protection coating.
[0017] One or more of the internal layers within the bar may
comprise a plurality of aligned fibres or wires. The fibres within
one internal layer may be aligned differently or obliquely to
fibres within a further composite internal layer. The central body
or core material of the bar may itself be a composite material and
may comprise a plurality of layers. Alternatively, the central body
or core material may comprise a substantially solid uniform
material or else a cellular material.
[0018] Typically the composite structure of the bar comprises first
and second materials of different strengths and/or stiffnesses. The
stiffer or stronger material may be considered to be a reinforcing
material, whereas the less stiff or weaker material may be
considered to be a matrix material. In one embodiment the composite
structure comprises a resinous and/or polymer material, which may
comprise a thermosetting plastic. The composite structure may be
formed by a moulding process, such as, for example a resin transfer
moulding (RTM) process.
[0019] The composite makeup of the tip treatment bar according to
the present invention is advantageous since it allows improved
damping characteristics when compared to the hollow or uniformly
solid tip treatment bars of the prior art. The natural frequencies
of the tip treatment bar can be tailored in a manner that was not
hitherto possible. This is of particular benefit for rotor casing
components since such components are typically exposed to a range
of excitation during different operational conditions or states of
the rotor. For example, within a gas turbine engine, the nature of
the vibration will change between different aircraft flight phases
or envelopes. However the fluid dynamics of rotor systems typically
places tight geometrical constraints on components within the fluid
flow, such as tip treatment bars. That is to say, the fluid-washed
surface of such components must be first and foremost shaped for
their aerodynamic purpose. Therefore the composite nature of the
bar according to the invention provides a further degree of freedom
for tailoring the vibration response of the bar, substantially
without modifying its external geometry,
[0020] The natural damping achieved by an aligned composite layup
may, in general, reduce the vibration amplitude across a range of
excitation frequencies. In addition, the composite makeup may be
tailored to avoid resonance in particular excitation zones which
are known to be prevalent for particular types of rotor during
operation.
[0021] In one embodiment of the invention, the layered composite
structure may be arranged such that the natural frequency of the
tip treatment bar lies outside the range of excitation frequency
caused by the rotor during normal operation. Normal operation in
respect of a gas turbine engine may comprise substantially
steady-state or `cruise` conditions. Additionally or alternatively,
normal operation may comprise any or any combination of flight
phases of an aircraft, such as take-off, climb and/or descent.
Preferably harmonics may also be substantially avoided. Given the
range of operating conditions of gas turbine engines, it is
possible that a bar excitation frequency will cross an operational
frequency of the rotor. However the present invention allows the
bar to be tailored in such a manner that such excitation of the bar
may be transient in manner.
[0022] The control of the vibration response of tip treatment bars
in accordance with the invention can improve fatigue life.
Furthermore the composite structure can be lightweight. An
unforeseen benefit of the tip treatment bar composite structure for
use in gas turbine engine applications is that the aligned
composite structure beneficially accommodates the frangibility of
the bar structure, particularly in a direction transverse to the
composite material or laminae alignment. This frangibility is
important in aiming to satisfy the response of the tip treatment
bar to a `blade off` scenario, such that the bar will yield upon
impact with a blade or blade portion which is loosed at operational
rotor speeds.
[0023] A yet further unforeseen benefit of the composite bar
structure is that it can offer better control with regard to
thermal expansion. Thus the geometry of the bar under thermal
loading may be better predicted, which allows tighter control of
blade tip clearance. This can have a knock-on benefit to the
operation, for example, efficiency, of the rotor.
[0024] The composite structure may comprise multiple material
layers or lamina. One or more layers may be entirely contained
within an outer layer. A plurality of layers of the same or
different material makeup may be generally concentrically arranged.
The outer surface of the composite bar may comprise a plurality of
layers. The bar may have an external layup as well as an internal
layup. The bar may have a combination of internal and external
layers.
[0025] One or more internal or coating layers of the bar may
comprise a thermal barrier material.
[0026] According to a second aspect of the invention, there is
provided a rotor casing comprising a plurality of bar members and a
support structure arranged to maintain the bar members in a
circumferentially spaced array with respect to an axis of rotation
of the rotor in use, the bar members each having a composite
structure.
[0027] Each bar member may comprise a substantially solid composite
structure having a plurality of materials arranged therein so as to
provide the bar with a directional stiffness characteristic. Each
bar member may comprise a plurality of layers of different material
therein. Each bar may be in accordance with the first aspect of the
present invention and may comprise any of the optional features
described in relation to the first aspect.
[0028] In one embodiment each bar is spaced or isolated from the
support structure by a damping member. An individual damping member
may be provided for each bar. The damping member may comprise a
cuff member arranged about the bar and/or interposed between the
bar and the support structure. The damping member may serve
substantially to isolate the bar from the support structure.
[0029] The, or each, damping member may comprise a rubber material.
The, or each, damping member may comprise a composite material
arranged to provide the damping member with anisotropic stiffness
characteristics. Damping means of this type can be tailored such
that elastomeric damping members behave effectively in particular
modeshapes using the anisotropic effect of the filler material.
This can help further optimise natural frequencies of the bar and
support structure when, for example, an unexpected mass is added to
the bars such as coating for erosion protection.
[0030] The support structure may comprise one or more annular
support members, which may be arranged about a rotational axis of
the rotor. The support structure may comprise a plurality of
openings, each arranged to receive a bar. The support structure may
comprise a pair of spaced support members, such that the bars
extend there-between. The openings in each support member may be
shaped to receive an end of the bar members.
[0031] The directional stiffnesses of the bars and mounting system
in combination may enable the amplitude of vibration to be
controlled and/or the natural frequency of the tip treatment system
as a whole to be tailored to substantially avoid resonance during
rotor operation.
[0032] According to a further aspect of the invention, there is
provided a gas turbine engine comprising a tip treatment bar
according to the first aspect and/or a rotor casing according to
the second aspect.
[0033] The term `directional stiffness` as used herein may be
considered to comprise, for example, axial, radial, torsional or
through-thickness stiffness.
[0034] Practicable embodiments of the invention are described below
in further detail by way of example with reference to the
accompanying drawings, of which:
[0035] FIG. 1 is a partial axial sectional view of a 25 fan stage
in a gas turbine engine;
[0036] FIG. 2 is a three-dimensional view of tip treatment bars
mounted in a section of an annular support structure suitable for
use in the engine of FIG. 1;
[0037] FIG. 3 is a three-dimensional view of a single isolated tip
treatment bar and corresponding section of the support
structure;
[0038] FIG. 4 is a cross section through the tip treatment bar of
FIG. 3;
[0039] FIG. 5 is a three-dimensional view of damping members
applied to a tip treatment bar; and,
[0040] FIG. 6 is a three-dimensional view of a further embodiment
of a single isolated tip treatment bar.
[0041] FIG. 1 shows a fan casing 2 of a gas turbine engine. A fan,
shown here by a single blade 4, is mounted for rotation in the
casing 2. The fan takes the form of a fan blade assembly, having a
plurality of assembled fan blades, which is mounted within the
engine to a shaft, by which the fan is driven in rotation. Guide
vanes 6 and 8 may be provided upstream and downstream,
respectively, of the fan 4. The casing 2 includes a
circumferentially extending chamber 10, which may be considered to
comprise an annular void or recess.
[0042] The chamber 10 communicates with the main or global gas flow
through the fan (represented by an arrow 12) through an array of
slots 14 (see FIG. 2) defined between tip treatment bars 16
disposed around the casing. The function of the chamber 10 in
delaying the onset of stalling of the blades 4 is disclosed in
International Patent Publication W094/20759.
[0043] The tip treatment bars 16 are supported by annular end
supports 18 to provide a tip treatment ring 20 (shown in FIG. 2),
which is fitted within the casing 2 and extends around the fan 4.
The bars are circumferentially spaced in a regular manner about the
fan axis. The bars extend in a direction which is generally
parallel with the rotational axis of the fan (i.e. they are spaced
relative to the direction of travel of the rotor blade tips during
operation), although they may be angled slightly relative
thereto.
[0044] Vibration is induced in the bars 16 in operation of the
engine at a frequency determined by the passage of the blades 4.
This vibration can lead to fatigue failure of the bars 16. The
vibrating bars 16 deflect in a generally circumferential direction
as indicated diagrammatically in FIG. 2 by an arrow 21, and
consequently fatigue failure tends to be initiated by cracking at
the slot ends.
[0045] In the embodiments discussed below, the bars 16 are formed
separately from the end support 18. The bars are generally
quadrilateral in section and may take the form of a parallelogram.
Such a profile is typically required to fulfill the aerodynamic
requirements of the bars. The internal make-up of the bars is
non-uniform as will be discussed below. More particularly, the
internal make-up of each bar may be uniform in a first direction
through the bar (for example, along its length) but may be
non-uniform in a second direction, which is perpendicular to the
first direction (for example, through a width or depth of the bar).
This may otherwise be expressed as the bar makeup being non-uniform
in section.
[0046] In the embodiment described below, the end supports 18 may
be made from a carbon fibre/bismaleimide composite material, which
enables the tip treatment ring to be light in weight while being
capable of withstanding the is relatively high temperatures (in
excess of 150 to 200.degree. C.) encountered in operation. However
alternative composite materials may be used, subject to the
practical requirements of, for example, weight, strength and
temperature resistance.
[0047] As can be seen in FIGS. 2, 3 and 6 the supports 18 comprise
front 18a and rear 18b annular supports, which are located in
respective upstream and downstream positions in use relative to the
global flow direction 12 through the fan. The front and rear
supports may be considered to constitute annular rails. Each of the
front and rear supports has a plurality of openings 19, each
opening being shaped to receive a corresponding end of a bar 16.
The openings 19 are regularly spaced around the annular body of the
supports. The openings 19 in the front and rear supports are
aligned such that the bars extend there-between in regularly spaced
array once assembled.
[0048] The openings generally take the form of radially extending
slots so as to accommodate the shape of the bars, which, as shown
in the embodiment of FIG. 2, have a depth dimension (in a radial
direction in use) which is larger than the width of the bars (in a
circumferential direction in use). The front and/or rear supports
may have a radially inner 22 and/or outer 24 rim formation. Such
rim formations may be oriented and/or dimensioned so as to allow
for the desired degree of frangibility of the support members, for
example in response to impact by a portion of a fan blade in
use.
[0049] Turning now to FIG. 3, there is shown a general view of
solid composite bar mounted within the openings 19 in the front 18a
and rear 18b support rails. It can be seen that the bar comprises a
solid composite structure in that it comprises a plurality of
layers which make up the sectional area of the bar.
[0050] In this example, it can be seen in the sectional view of
FIG. 4, that the bar 16 is formed of an inner body of material 26
surrounded by a plurality of circumferential layers 28-32 arranged
around the inner body 26. Those layers are each of uniform depth
along the length of the bar (i.e. in a direction between the
opposing ends of the bar mounted in the front and rear supports
respectively). The depth of each of those layers is also
substantially uniform in a direction around the inner material
26.
[0051] In FIG. 4, it can also be seen that the section of the bar
takes the form of a non-perpendicular parallelogram, such that the
upper and lower surfaces of the bar are obliquely arranged or
angled relative to the side walls. Accordingly, at least a portion
of the internal layers, such as the transverse region of the layer
that extends between the side walls, is also obliquely arranged
relative to the side walls.
[0052] The inner body 26 is formed of a composite material, such as
a fibre-reinforced material. The matrix material may be, for
example, a maleimide such as bismaleimide (BMI) or an epoxy resin.
The inner body is itself formed of a plurality of composite layers
27, which are generally parallel in alignment. The fibres in each
of those layers are typically aligned in a common direction, which
is shown as being oblique to the side walls of the bar. In
alternative embodiments, the central body 27 could be formed of a
solid, uniform or cellular material, such as a conventional polymer
material.
[0053] Each of the layers 28 to 32 may comprise the same or a
different material makeup to the inner body. Adjacent layers within
the bar structure may also be different. However in this embodiment
the layer 28 adjacent to the inner body 26 comprises substantially
the same material makeup to the inner body. The composite layer 28
is oriented differently to the layers 27 in the inner body 26. The
layer 28 is a circumferential layer which encloses the inner body
26. The fibres in layer 28 are also oriented differently to those
of layers 27 and may be arranged, for example, substantially
parallel with the side walls of the bar or else at an angle thereto
which differs from that of the layers 27.
[0054] The options for each of the material layers 30 and/or 32 are
the same as those described above for layer 28. Those layers may
have the same or a different material makeup and/or fibre
arrangement from layer 28 or body 26. For example, one or more
layers may comprise the same composite material as the inner body,
or else another layer, but it may be oriented such that the fibres
therein lie in a generally different direction to the inner body or
other layer.
[0055] In another embodiment, one or more internal layers 26-32 may
comprise a non-composite material, such as a suitable polymer
material. The materials used are subject to the preferred
requirement that they are mouldable materials.
[0056] The outer layer 34 in this embodiment is a coating layer,
which comprises a material suitable to provide an erosion
protection coating. In this example, the outer layer material 34
comprises a fluoroelastomer, although any other suitably chemically
and thermally stable polymer, or other, material could be used.
[0057] The inner body 26 and layers 28 to 32 may be considered to
form a solid core of the bar. This construction has a plurality of
internal layers and can be distinguished from the application of a
coating layer to the exterior of the bar only. The ply of the
internal composite layers may be of suitable thickness and may be
in the region of 0.05 to 0.5 mm for example. In this example, a ply
of approximately 0.1 mm was used. However the thickness of the
different internal layers could also be different as necessary to
optimize the bar for use.
[0058] As shown in FIGS. 3 and 4, a plurality of layers of the
material may be exposed in one or more outer walls of the bar. In
this embodiment, the outer material layer 34 of the bar does not
completely surround the adjacent layer 32. In particular the outer
layer covers three faces of the bar, shown as the three sides of
the section view of the bar in FIG. 4. The adjacent layer 32 is
thus exposed on one face of the bar. In this embodiment the layer
32 is exposed on a radially outwardly facing face of the bar when
mounted for use in the front and rear supports 18. In this regard
the bar can be described as having not only an internal composite
layup but also an external layup, since a plurality of layers are
exposed in the outer surface of the bar,
[0059] It has been found by the inventors that any or any
combination of such internal and/or external layup can be used to
significant benefit in providing the bar with a directional
structural characteristic, such that the bar is orthotropic. Such
directional properties are not only due to the fibre alignment of
the inner body material but also due to the bar layup such that the
bar can be considered to be `multiply orthotropic`. Having a solid
tip treatment bar made from composite in this manner is
advantageous as it provides directional stiffness which can be used
to tune natural frequencies of the bar. Thus the material and layup
can be tailored for a known vibration frequencies that occur during
operation of the rotor to avoid zones of resonance during in
service. Additionally or alternatively the amplitude of vibration
of the bar itself can be reduced or minimized over a wide range of
operational frequencies of the rotor (for example relating to
different flight envelopes of an aircraft), thereby increasing the
fatigue life of the tip treatment bar.
[0060] The damped response of a bar as described above is
advantageous over and above the general inherent vibration damping
benefits that occur due to use of a composite material per se.
[0061] Also it is of notable benefit that the above advantages can
be achieved whilst maintaining a suitable frangibility of the bar
(i.e. in a generally radial direction in use) to meet the fan blade
containment requirements. This is due, at least in part, to the
relatively brittle failure mechanism of composites compared to
metals during impulse or impact events such as partial fan blade
release. The containment design generally requires that a loosed
fan blade part must be contained to avoid any hazardous events and
that the blade fragment(s) are broken up through surrounding parts
such as front and/or rear support rail and tip treatment bars.
[0062] The strength of any of the layers can be tailored as
required, for example by improving inner and/or outer ply failure
strengths.
[0063] A bar according to any aspect or embodiment of the invention
may be formed using a moulding process. It has been determined that
a wide variety of internal and external composite layup can be
achieved using a resin transfer moulding process. This process
moulds preforms, through resin injection, into the desired shape of
the mould. The mould is in vacuum before injection and the pressure
of injection wets the fibres in the mould. The nature of the
preform defines the type of fibre in the mould which can hold its
shape during the injection. Starting by bonding the inner body 26
as one or more infill blocks of the composite bar, layers 28 to 32
of the bar are wrapped and bonded around the infill blocks which
can be located in the mould before it is closed. In this regard,
the layers 28-32 can be co-formed or formed using a common moulding
process. The inner body 26 may also be co-formed but in this
embodiment it is preferable for practical reasons to form the
layered inner body 26 separately.
[0064] The erosion coating 34 is bonded onto the bars, in this
embodiment by a secondary bonding. Bismaleimide (BMI) or epoxy
resins offer suitable thermal characteristics and low viscosity
such that they can be resin transfer moulded. Resin Transfer
Moulding is a cost effective method of manufacturing a solid
multi-layer bar of organic matrix composites because it can be at
least semi-automated. The coating 34 may be considered to provide a
sheath about the core bar material.
[0065] A long length of the bar makeup can be produced using the
methods described above and then cut into individual bars along its
length.
[0066] In any or all of the above described embodiments, the bars
are provided with damping members arranged interposed between the
bars and the support structure 18. The damping members 36 are shown
in FIG. 5 and comprise a damping material arranged around at least
an end portion of the bar, typically at each end thereof. The
damping material surrounds the outer periphery of the bar in the
form of a collar or cuff formation, which may be referred to in the
art as forming `damping boots`.
[0067] The damping members 36 space each end of each tip treatment
bar 16 from the respective support 18. For this purpose, the
openings 19 in each support 18a or 18b have generally the same
shape as the cross-section of the tip treatment bars 16, but is
substantially larger in dimension so as to accommodate the volume
of the damping members therein. The space between each tip
treatment bar 16 and the wall of the corresponding opening is
filled by the damping material.
[0068] Each damping member may be formed as a separate component
before assembly with its respective tip treatment bar 16 and the
end supports 18. Alternatively, the boots may be formed by moulding
the damping material in situ between the tip treatment bar 16 and
the end support 18, in a potting process. The boots are bonded to
the respective bars 16 and end supports 18 by means of 15
a-suitable adhesive, such as a silicone adhesive as is available
under the name SILCOSET 152. The damping material itself could be a
silicone elastomer, such as the material available under the name
SILASTIC J.
[0069] By introducing a damping means in the form of an elastomeric
intermediary between the bar and the front and rear rails, the
natural frequencies of vibration of the vanes, when installed, are
conventionally reduced. However the inventors have determined that
this effect can be disadvantageous as the bar's natural frequencies
of vibration may then be of such a value that they interact with
the engine order forcing frequencies, which can result in greater
bar vibration amplitudes. The inventors have determined that
providing a relatively stiff material, such as, for example, aramid
particles or layers or carbon nanotubes, inside the damping members
can improve the performance of the system. Providing the damping
member in the form of a composite material itself, with inherent
stiffness and self damping properties, enables the mounting system
stiffness to be modified in at least one direction without
penalising damping characteristics.
[0070] The damping members may comprise an internal multi-layer
composite.
[0071] Controlling the bonding between the damping members and the
bar and/or support members is also an important consideration for
controlling natural frequencies of the mounting system. The use of
an adhesive would typically require the adhesive to be relatively
compliant compared to the damping material itself and, as such, the
adhesive can impact significantly the resultant stiffness
calculation for the damping member arrangement. Even a relatively
thin adhesive thickness, if applied over the entire bonding
surface, will reduce the resultant stiffness considerably.
[0072] In view of the above, grooves may be provided in the inner
and/or outer surfaces of the damping members for receiving an
adhesive that bonds the damping member to the tip treatment bars
and front/rear rails, in which the bar is mounted. Using such a
grooved elastomeric damping member, the bonding can be achieved
such that the adhesive is only present in the grooves of the
elastomeric collar. In this regard the groove walls (i.e.
protrusions) in the damping member surface can be formed so as to
be an interference fit with the corresponding opposing surfaces of
the bar and/or support member openings 19, which results in a
squeezing effect on assembly, to urge the adhesive into the troughs
of the grooves, rather than the outermost surface of the groove
walls.
[0073] This configuration, and method of forming the assembly, has
the effect that there is little or no adhesive at the outermost
protruding portions of the damping member. As such, the mounting
stiffness is dominated by the path between the bars and the
front/rear support members via the damping material itself, such
that the adhesive has very little effect on this load path. It has
been found that, by varying the dimensions of the grooves, the
stiffness of the mounting can be tuned to a desired frequency. In
addition, if a specific modeshape can be excited by an up or
downstream rotor/engine order, the natural frequencies could be
simply reduced by increasing slightly glue line thickness and/or
removal of support member or damping material.
[0074] Along with the above discussed methods of controlling the
vibration response of the tip treatment bar assembly, such features
can further provide for a system in which damping is enhanced.
Accordingly a further definition of the invention may comprise a
solid multi-layer composite bar configuration with enhanced damping
members for mounting the tip treatment bar to a support member.
Such enhanced damping members may comprise a stiffened composite
material and/or grooves to allow improved control of adhesive
dispersion between the damping member and bar or support.
[0075] Such an arrangement may provide a suitable thermal
resistance and/or thermal mismatch between components to meet
required system performance. The composite damping material and/or
grooves may further facilitate the directional stiffness of the
mounting system to improve design durability, for example by
minimizing or avoiding zones of resonance during normal
operation.
[0076] Turning now to FIG. 6, there is shown a further embodiment
of the invention in which the geometry and internal layered
structure of the bar has been modified. All other feature of those
embodiments may be as described above and will not be repeated here
for conciseness.
[0077] In the embodiment of FIG. 6, it can be seen that the bar 100
has an internal layer or body 102 which passes through the core of
the bar. The layer 102 is sandwiched between further layers 104 and
106 on either side thereof. The layers 104 and 106 may comprise the
side walls of a generally circumferential layer of the type
described above.
[0078] In the embodiment of FIG. 6, the bar 100 has a central layer
102 which is generally parallel with the bar side walls and layers
104 and 106. The bar 100 is thus generally symmetrical about a mid
plane thereof.
[0079] The bar 100 has upper 108 and lower 110 layers which extend
between the bar side walls. The central layer 102 in this
embodiment is an internal layer in that it is surrounded in section
by the other layers 104 to 110. The layers 104 to 110 in this
embodiment may be formed as a single circumferential layer. However
the upper and lower edges of the bar have been cut away at each end
of the bar so as to provide lateral recesses or grooves 112. One or
more internal layers of the bar are thus exposed at those grooves
112 as shown. Modification of the outer shape of the bar in this
manner is useful in mounting the bar, such that the damping members
can be located or formed in the grooves 112. In this manner, the
bar can be mounted in the support rails 18 such that the radially
inner edge of the bar is substantially flush with the inner edge of
the rotor casing. This helps to provide a minimum tip clearance
between the bars and the fan blades 4. In this regard, the grooves
112 at the radially inner edge of the bar 100 may be deeper than
the grooves 112 at the radially outer edge thereof to accommodate
the thickness of both the damping members and also the casing
structure.
[0080] The bar in FIG. 6 also has a coating layer 114 on a
plurality of surfaces thereof, of the type described above. In this
embodiment, the coating layer covers the side walls and a base wall
of the bar 108 but not the radially outer wall, at which the layer
108 is exposed. Accordingly, such a layered structure has both an
internal and external layup as described above. Any of the features
of FIG. 6 may be substituted for any of the features described in
relation to FIGS. 1 to 5 above wherever it is practicable to do
so.
[0081] It is envisaged that further technical advantages may be
achieved according to further embodiments of the invention by
providing one or more of the bar layers with additional integral
features as described below. Integrated features may include macro,
meso, micro or nano scale damping material. For example, carbon
nanotubes could be added into the reinforced structural resin
system to further improve damping characteristic. Typically,
approximately 1% nanotubes within the composite structure can
increase structural damping by up to approximately 50% in a cost
effective manner.
[0082] Also, different types of integrated features may be provided
for different technical reasons. For example, the multi-layer
makeup of the bar may accommodate one or more wire members arranged
along the length of the bar between the front and rear support
rails. Such wires may be arranged in, and aligned with, one or more
of the layers. Such wires may offer greater tensile strength than
the remainder of the bar, such that they work in conjunction with
the frangibility of the bar to help fragment a blade or blade
portion if it strikes the tip treatment bar assembly. That is to
say, the wire(s) could act in the manner akin to so-called `cheese
wire` which serve to break up a blade portion upon impact
therewith.
[0083] In another embodiment, one or more layers of the bar could
accommodate optical fibres linking components in an engine
instrumentation and/or engine health monitoring system. Such
optical fibres could be embedded into the solid composite bar for
engine test and service support.
[0084] In view of the above description, it is proposed that
embodiments of the invention provide means to optimise natural
frequencies of tip treatment bars, which is especially important
for bars of the type shown in FIGS. 2, 3, and 6, the geometry and
weight of which are highly constrained by aerodynamic performance
requirements and the engine configuration. The response of the bars
can be both tuned and damped in this manner. Also the
support/mounting system for the bars can be designed to participate
in the tuning of natural frequencies and overall damping by means
of directional stiffness characteristics. Thus, the mechanical
behavior of the bar and support system is such as to prevent
critical primary bar mode excitation with specific engine orders
for the running range.
[0085] Optional modifications to the above described embodiments
are discussed below within the context of the invention.
[0086] In a further embodiment of the invention, one bar end can be
fixed whilst the other end can be loosely constrained within a
support member opening, for example to allow a constrained degree
of motion in a direction which is parallel with the rotor axis.
Alternatively, both ends can be flexible to allow the bars to move
into the front and rear rail slots. Such options may be important
to accommodate relatively high variation of coefficients of thermal
expansion between components. The damping members may move into the
support rail and flex to take into account thermal growth whilst
solid composite bar configuration improves thermal resistance of
tip treatment bars.
[0087] The tip clearance of the fan blade to the tip treatment bars
is also required to be controlled for any engine transient or
steady-state condition. The tip treatment bar and support system of
the present invention has been found to allow suitable control of
tip clearance in the normal operating temperature range of
160-200.degree. C.
[0088] Any internal layer or coating of the tip treatment bars or
support structure may comprise a thermal barrier material to
enhance thermal resistance. The internal composite bar may comprise
a high temperature capability resin system and/or a thermal barrier
material, for example as an additional coating or layer or filler
material therein.
[0089] Additionally or alternatively, the support members may
comprise composite materials, for example, to improve frangibility.
Any of the composite materials described above may provide damping
enhancements by means of carbon nanotubes within the composite
structure.
[0090] The invention can be as described above, where natural
frequency reduction is enhanced by locally adding mass to any, or
any combination of, the composite inner body or layers, the bar
coating, bonding materials or the damping member.
[0091] The invention is particularly suited for tip treatment bars
in a gas turbine engine. However the invention can be used in any
rotor configuration where like or similar issues arise, such as
within compressors, fans, pumps or the like. The invention is
considered particularly beneficial when both a general level of
damping is required and also where it is important to be able to
control the natural frequencies of vibration of the mounted
component. The invention may be suited for use with vanes at the
low or intermediate pressure compressors in a gas turbine engine or
other organic matrix composite components with similar
requirements.
* * * * *