U.S. patent application number 15/269540 was filed with the patent office on 2018-03-22 for flutter avoidance through control of texture and modulus of elasticity in adjacent fan blades.
The applicant listed for this patent is Rolls-Royce Corporation. Invention is credited to Michael G. Glavicic.
Application Number | 20180080450 15/269540 |
Document ID | / |
Family ID | 61618347 |
Filed Date | 2018-03-22 |
United States Patent
Application |
20180080450 |
Kind Code |
A1 |
Glavicic; Michael G. |
March 22, 2018 |
FLUTTER AVOIDANCE THROUGH CONTROL OF TEXTURE AND MODULUS OF
ELASTICITY IN ADJACENT FAN BLADES
Abstract
A fan for a turbofan engine includes a plurality of blades and a
disk, the plurality of blades constructed of an anisotropic
material, the anisotropic material is a plate, sheet, or forging. A
first blade type has a first crystallographic texture and a first
natural frequency, and a second blade type has a second
crystallographic texture and a second natural frequency. The first
natural frequency is at least 4% greater than the second natural
frequency, and the first blade type and the second blade type are
attached to the disk in an alternating pattern to provide a flutter
damping effect. The fan blades may be cut from a plate of the
anisotropic material along orthogonal directions or forged from
round bar oriented along a first direction and a second orthogonal
direction, respectively.
Inventors: |
Glavicic; Michael G.;
(Indianapolis, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Rolls-Royce Corporation |
Indianapolis |
IN |
US |
|
|
Family ID: |
61618347 |
Appl. No.: |
15/269540 |
Filed: |
September 19, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F04D 29/668 20130101;
F05D 2300/605 20130101; F04D 29/324 20130101; F04D 29/023 20130101;
F04D 29/666 20130101 |
International
Class: |
F04D 19/00 20060101
F04D019/00; F04D 29/38 20060101 F04D029/38; F04D 29/32 20060101
F04D029/32 |
Claims
1. A fan for a turbofan engine, comprising: a disk; and a plurality
of blades attached to the disk, the plurality of blades constructed
of an anisotropic material, the anisotropic material comprising a
plate, sheet, or forging; wherein a first blade type has a first
crystallographic texture and a first natural frequency, wherein a
second blade type has a second crystallographic texture and a
second natural frequency, wherein the first natural frequency is at
least 4% greater than the second natural frequency, and wherein the
first blade type and the second blade type are attached to the disk
in a pattern to provide a flutter damping effect.
2. The fan of claim 1, wherein the first blade type is cut from a
plate, sheet or forging of the anisotropic material along a
processing direction, and wherein the second blade type is cut from
the plate, sheet, or forging of the anisotropic material along a
transverse direction, wherein the transverse direction is
orthogonal to the processing direction.
3. The fan of claim 2, wherein the anisotropic material includes
one or more alloys that exhibit changes in elastic modulus as a
result of their crystallographic texture, the one or more alloys
comprising at least one of titanium, aluminum, iron, nickel, and
zinc.
4. The fan of claim 3, wherein the blades are welded to the disk by
linear friction welding.
5. The system of claim 2, wherein the anisotropic material is a
first titanium alloy and the disk is made of a second titanium
alloy.
6. The fan of claim 5, wherein the plurality of blades undergo
superplastic forming.
7. The fan of claim 6, wherein the first titanium alloy is
Ti-6Al-4V.
8. The fan of claim 7, wherein each of the plurality of blades
comprises multiple layers.
9. The fan of claim 8, wherein at least two of the layers are
inflated by injecting the blade with a gas.
10. A fan for a turbofan engine, comprising: a plurality of blades
constructed of an anisotropic material, the plurality of blades
attached to a disk; and wherein a first blade type is obtained from
a round bar of an anisotropic material and has a first
crystallographic texture and a first natural frequency, wherein a
second blade type has a second crystallographic texture and a
second natural frequency, wherein the first natural frequency is at
least 4% greater than the second natural frequency, and wherein the
first blade type and the second blade type are attached to the disk
in an arrangement to provide a flutter damping effect.
11. The fan of claim 10, wherein the first blade type is forged
from the round bar of the anisotropic material oriented in a first
direction, and wherein the second blade type is forged from the
round bar of the anisotropic material oriented in a second
direction, wherein the first direction is orthogonal to the second
direction.
12. The fan of claim 11, wherein the anisotropic material includes
one or more alloys that exhibit changes in elastic modulus as a
result of their crystallographic texture, the one or more alloys
comprising at least one of titanium, aluminum, iron, nickel, and
zinc.
13. The fan of claim 12, wherein the blades are welded to the disk
by linear friction welding.
14. The fan of claim 13, wherein the anisotropic material is a
first titanium alloy and the disk is made of a second titanium
alloy.
15. The fan of claim 14, wherein the first titanium alloy is
Ti-6Al-4V.
16. The fan of claim 15, wherein each of the plurality of blades
comprises at least two layers that are inflated by injecting a gas
between the at least two layers.
17. A method for producing a fan blade system for a turbofan
engine, the method comprising: providing a sheet of anisotropic
metal having a crystallographic texture, the sheet characterized by
a processing direction and a transverse direction orthogonal to the
processing direction; cutting a plurality of first fan blades from
the sheet along the processing direction; cutting a plurality of
second fan blades from the sheet along the transverse direction;
and mounting the first fan blades and second fan blades on a disk
in an arrangement to provide a flutter damping effect.
18. The method of claim 17, wherein the first fan blades have a
first natural frequency and the second fan blades have a second
natural frequency, and wherein the first natural frequency is at
least 4% greater than the second natural frequency.
19. The method of claim 17, wherein the anisotropic material is a
titanium alloy.
20. The method of claim 19, wherein the titanium alloy is
Ti-6Al-4V.
Description
FIELD OF DISCLOSURE
[0001] The present disclosure generally relates to fan blade
systems for gas turbine engines. More particularly, but not
exclusively, the present disclosure relates to configurations and
orientations of fan blade texture relative to low pressure fans of
turbofan engines.
BACKGROUND
[0002] Providing engine equipment to contend with potentially
disruptive phenomena, such as flutter, remains an area of interest.
Some fan blade systems employ various geometries that redirect
airflow or redistribute weight to reduce flutter. Specifically, fan
blade systems may include protruding portions that are directly
bonded to the fan blade. However, these options increase weight and
decrease efficiency. Overall, the existing systems to mitigate the
onset of fan blade flutter have various shortcomings relative to
certain applications. Accordingly, there remains a need for further
contributions in this area of technology.
SUMMARY
[0003] According to one aspect, a fan for a turbofan engine having
a plurality of blades constructed of an anisotropic material and a
disk is provided. The anisotropic material includes a rolled plate,
sheet or forged product of some kind. A first blade type has a
first crystallographic texture and a first natural frequency, and a
second blade type has a second crystallographic texture and a
second natural frequency. The first natural frequency is at least
4% greater than the second natural frequency, and the first blade
type and the second blade type are attached to the disk in an
alternating pattern to provide a flutter damping effect.
[0004] According to another aspect, a fan for a turbofan engine
having a plurality of blades constructed of an anisotropic material
attached to a disk is provided. A first blade type is obtained from
a round bar of an anisotropic material and has a first
crystallographic texture and a first natural frequency. A second
blade type has a second crystallographic texture and a second
natural frequency. The first natural frequency is at least 4%
greater than the second natural frequency, and the first blade type
and the second blade type are attached to the disk in an
alternating pattern to provide a flutter damping effect.
[0005] According to another aspect, a method for producing a fan
blade system for a turbofan engine includes providing a sheet of
anisotropic metal having a crystallographic texture, the sheet
characterized by a rolling direction and a transverse direction
orthogonal to the rolling direction. The method further includes
the steps of cutting a plurality of first fan blades from the sheet
along the rolling direction, cutting a plurality of second fan
blades from the sheet along the transverse direction, and mounting
the first fan blades and second fan blades on a disk in an
alternating arrangement in order to generate alternating blades
with different natural frequencies that differ by more than 4%.
[0006] Other aspects and advantages will become apparent upon
consideration of the following detailed description and the
attached drawings wherein like numerals designate like structures
throughout the specification.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 depicts a side sectional view of a turbofan engine
including a plurality of fan blades attached to a disk.
[0008] FIG. 2 depicts an isometric sectional view of fan blades
attached to a disk.
[0009] FIG. 2A depicts an isometric sectional view of fan blades
attached to a disk.
[0010] FIG. 3 depicts a schematic of compressor performance having
operating regions known as flutter boundaries.
[0011] FIG. 4 depicts a schematic showing angular variation of the
Young's Modulus of various anisotropic metals.
[0012] FIG. 5 depicts a perspective view of a rolled plate having
hexagonal close packed crystal units subjected to unidirectional
rolling.
[0013] FIG. 6 depicts a plan view of a plate subjected to
unidirectional rolling with specimens oriented relative to the
rolling direction.
[0014] FIG. 7 depicts the principal directions of a beam to
illustrate the anisotropic mechanical properties of titanium.
[0015] FIG. 8 depicts an isometric view of a simple cantilever beam
to illustrate the concept of natural frequency.
[0016] FIG. 8A depicts an isometric view of the simple cantilever
beam of FIG. 8 in motion to illustrate the concept of natural
frequency.
[0017] FIG. 8B depicts a side sectional view of the simple
cantilever beam of FIG. 8 along line 8B-8B.
[0018] FIG. 9 depicts a graphical representation of the effects of
fan blade mistuning.
DETAILED DESCRIPTION
[0019] For the purposes of promoting an understanding of the
principles of the disclosure, reference will now be made to the
embodiments illustrated in the drawings and specific language will
be used to describe the same. It will nevertheless be understood
that no limitation of the scope of the disclosure is thereby
intended. Any alterations and further modifications in the
described embodiments, and any further applications of the
principles of the disclosure as described herein are contemplated
as would normally occur to one skilled in the art to which the
disclosure relates.
[0020] Turbofan engine systems have numerous performance
requirements to consider including: fuel efficiency, component
strength, useful life, fan bade off (FBO) containment (which may
entail debris of various size and energy), noise emission, and
power output. Turbofan engine systems include a fan system
comprising a plurality of fan blades, a disk, and a barrel. The fan
blades may be made of a metal, such as titanium, or an alloy of
various metals. Such alloys include Ti-6Al-4V (Ti-64) and
Ti-6Al-2Sn-4Zr-2Mo-0.15Si (Ti-6242). The disk may be made from the
same material as the blades, or a different metal altogether. The
design constraints for disks and blades are somewhat different. For
example, high tensile strength and low cycle fatigue resistance are
most relevant for disk materials, and high cycle fatigue and creep
resistance are the main desired characteristics for blades. For
example the disk may be made of Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) or
Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti-17). The barrel may be metallic, such as
aluminum, or composite, and the containment blanket is typically
made of dry fabric wrap comprising an aramid fiber such as
Kevlar.TM..
[0021] During a flutter event, the fan blades must have sufficient
capability to withstand the structural loads that exist. Hollow fan
blades are optimized to be light and strong and may have
significant cost and weight advantages over solid fan blade
systems. Those benefits would be significantly reduced if the fan
blade size was increased or if additional features were added to
reduce the onset of flutter or to withstand flutter loads. Certain
fan blade systems in service have metallic plates that are
mechanically attached to one or more portions of the airfoil, to
mistune fan blades. Those features increase cost and weight, and
may reduce the overall efficiency of a fan blade system.
[0022] Referring to FIG. 1, a turbofan engine 50 is illustrated
having a fan blade 52, a compressor section 54, a combustor 56, and
a turbine section 58, which together can be used to produce a
useful power. Air enters the turbofan engine 50, is compressed
through action of the compressor 54, mixed with a fuel, and
combusted in the combustor 56. The turbine 58 is arranged to
receive a flow from the combustor 56 and extract useful work from
the flow. The turbofan engine 50 may have a disk 60 attached to the
fan blades 52 that transfers power from the shaft to the blades
which force air into the turbine section. Further, the present
disclosure contemplates use in other applications that may not be
aircraft related such as industrial fan applications, power
generation, pumping sets, naval propulsion, weapon systems,
security systems, perimeter defense/security systems, and the like
known to one of ordinary skill in the art.
[0023] Referring to FIG. 2 and FIG. 2A, side sectional views of two
fan blade systems 60a, 60b show (1) a mechanical blade-disk
attachment in which the fan blade 52a has a dovetail 62 that is
retained by the disk 60a (analogous to a tongue and groove) and (2)
a blisk arrangement in which the fan blade 52b is attached to the
disk 60b (to form a blisk) by a weld 64 rather than a dovetail 62
or other root geometry that extends into a disk 60a.
[0024] Referring to FIG. 3, a schematic of compressor performance
shows flutter boundaries that result in flutter. Flutter is an
aero-structural self-excited vibration that leads to undesired
instability and is common with fan blades. Some important forms of
flutter include stall flutter, unstalled flutter, supersonic
unstalled flutter, supersonic stalled flutter, trans-sonic stalled
flutter, and choke flutter.
[0025] The crystallographic texture of a material is a statistical
measure of what proportion of the macroscopic material is aligned
to specific crystallographic directions. In Ti alloys the elastic
modulus varies as a function of direction within the hexagonal
closed packed directions (FIG. 4). The formation of a
crystallographic texture as a result of the thermo-mechanical
processing of the titanium alloy can change the effective elastic
modulus that a macroscopic component will exhibit depending upon
the thermos-mechanical processing path followed. For example,
texture can be controlled by controlling the direction of
processing such as by rolling a sheet of titanium, or forging the
anisotropic material in a way that causes the material to undergo
strain that results in a desired crystallographic texture.
[0026] Many physical, chemical and mechanical properties of
crystals depend on their crystalline orientations and it follows
that directionality or anisotropy of these properties will result
wherever a texture exists in polycrystalline materials. Some of the
important examples are elastic modulus (E), Poison's ratio,
strength, ductility, toughness, magnetic permeability and the
energy of magnetization. These types of anisotropy apply to
materials of cubic as well as lower crystal symmetry. In hexagonal
metals, other properties such as thermal expansion and electrical
conductivity may also show directionality.
[0027] Referring to FIG. 4, the angular modulus behavior of E for
0.degree.<.theta.<90.degree. for the group of hexagonal
close-packed metals comprising: hafnium (Hf), titanium (Ti),
zirconium (Zr), and scandium (Sc).
[0028] As a general observation, with the exception of titanium,
E-behavior in FIG. 4 tends to exhibit a maximum on the (0001) basal
plane (i.e. when .theta. is zero and N coincides with the [0001]
direction) and a maximum (in some cases) on the prismatic planes
where .theta. is 90.degree.. In most cases, E tends to exhibit a
minimum value between 0.degree.<.theta.<90.degree.. In the
case of Ti, the behavior of E exhibits a maximum when .theta. is
zero and a minimum when .theta. is 90.degree.. This illustrates the
anisotropic nature of various HCP materials with regard to modulus
of elasticity.
[0029] It is possible to make some general comments on the effects
of crystallographic texture on elastic anisotropy of HCP
polycrystals. First, the metals with polar diagrams which most
approach circularity with an anisotropy factor close to unity
should experience smaller directional variations in the resulting
elastic moduli, E and G, as result of metal processing. These
include Mg and Y. In contrast, metals with significant departures
from circularity, and anisotropy factors much less or greater than
unity, are likely to experience considerable directional variations
in their elastic moduli as a result of processing. In particular,
these include Zn and Cd. Important metals such as Be, Ti Zr and Co
are likely to experience some variations in their polycrystal
moduli, but not to the same extent as Zn and Cd.
[0030] If a strong texture is present it is possible to anticipate
some elastic anisotropy effects. Extruded rods of hexagonal metals
such as pure Ti often exhibit a cylindrical symmetry fiber texture
where the basal plane poles (i.e. [0001]) of the grains are
perpendicular to the extrusion axis. Consequently the tensile
modulus along the extrusion axis should approach that of the
modulus normal to the prismatic planes of the monocrystal
(.about.104 GPa).
[0031] Referring to FIG. 5, an example orientation of the HCP
crystal units 68 in a Ti-6Al-4V plate 66 subjected to
unidirectional rolling in a rolling (processing) direction 70 is
shown. As stated above, rolling a plate of titanium alloy will
impart a texture, as the basal planes align with the transverse
direction 72, and increase the anisotropic nature of the material
as illustrated further below.
[0032] Referring to FIG. 6, the mechanical properties of a plate of
rolled titanium alloy 74 may be tested using a series of specimens
78 that are cut from the plate 74 at various angles with respect to
the processing direction which is the rolling direction 76.
[0033] Referring to FIG. 7, a beam 77 is shown to illustrate the
hexagonal close packed 79 crystal arrangement having a texture.
[0034] The orientation dependence of mechanical properties
(anisotropy) is clear from the experimental data below. The
mechanical properties were measured for a titanium plate
illustrated in FIG. 7. In the longitudinal direction, tensile
strength is 1027 MPa, yield strength is 952 MPa, and the elastic
modulus is 107 GPa. In the transverse direction tensile strength is
1358 MPa, yield strength is 1200 MPa, and the elastic modulus is
134 GPa. In the short transverse direction tensile strength is 938
MPa, yield strength is 924 MPa, and the elastic modulus is 104
GPa.
[0035] The natural frequencies of objects are related to the
elastic modulus of the material the object is constructed of and
the physical geometry of the object. Referring to FIG. 8, a simple
cantilever beam 80a is shown having a fixed end 82a, attached to a
support 81a, and a free end 84a. FIG. 8A illustrates a beam 80b
having a fixed end 82b, attached to a support 81b. The beam 80b is
shown oscillating at the free end 84b to illustrate the concept of
natural frequency. FIG. 8B shows a side sectional view of the
simple cantilever beam 80a and support 81a of FIG. 8 along line
8B-8B. A closed form equation for natural frequencies of a
cantilever beam are:
.omega. nf = .alpha. n 2 El mL 4 ( 1 ) ##EQU00001##
[0036] The natural frequencies of two cantilever beams, one
extracted from a rolled titanium plate in the longitudinal
direction, the other extracted in the transverse directions can be
compared as follows:
.omega. nf LD .omega. nf TD = E ( LD ) E ( TD ) = 107 134 = 0.8935
( 2 ) .omega. nf LD = 0.8935 .omega. nf TD ( 3 ) ##EQU00002##
[0037] Hence the natural frequency in the transverse direction is
11.9% larger than the natural frequency in the longitudinal
direction. Control of the modulus of elasticity of the material
used to make the fan blade allows for control the natural frequency
of the individual fan blades.
[0038] The rolled or super-plastically formed sheet material is
processed by cutting out fan blade shaped layers of titanium that
are sandwiched together to form a fan blade type structure. The
layers of titanium may be heated and inflated to form a hollow fan
blade using a gas.
[0039] In order to mistune the fan blisk 60b, the fan blades 52b
are arranged in a way that alternating blades (odd) have a natural
frequency of the first magnitude, and even blades have a natural
frequency of the second magnitude (11.9% greater than the first
magnitude).
[0040] For example, odd fan blades are cut along the processing or
rolling direction (corresponding to 0.degree. in FIG. 6) and even
blades are cut along the transverse direction (90.degree. in FIG.
6).
[0041] Referring to FIG. 9, a graphical representation of the
effects of fan blade mistuning (represented by application factor)
according to a simulation is shown.
[0042] Amplification factor is shown as a value between 0 and 700.
The graph 100 shows the effects of mistuning using blades having a
change in natural frequency represented by %. The graph 100 shows
areas of low amplification factor (102, 104, 106, 108), areas of
medium amplification factor (110, 112, 114, 116, 118), and areas of
high amplification factor (120, 122, 124, 126).
[0043] If blades are arranged in an alternating fashion, only a 4%
change in the natural frequency of each blade would be required to
achieve acceptable level of mistuning.
[0044] To provide a flutter dampening effect two types of blades
may be installed in a number of different arrangements such as an
alternating pattern (ex: 1, 2, 1, 2, etc.), a grouped pattern (ex:
1, 1, 2, 2, 1, 1, 2, 2, etc.), or a random pattern (ex: 1, 1, 2, 1,
2, 2, 2, 1, 1, 2, etc.). To provide a mistuning effect, the
preferred amplification factor of the fan blade should preferably
be less than 200.
[0045] Forging is another way to achieve fan blades with dissimilar
texture. The forging process used to forge, solution heat treat,
and cool adjacent blades of the same alloy in a finished component
may be used to provide the mistuning needed in a fan blade design.
To demonstrate this, consider two blade types forged in the
following ways.
[0046] The dies used in the forging process of the first blade type
are designed so that the radial direction in the blade sees a
strain that is effectively the same as the longitudinal direction
(rolling direction) of a rolled plate. Hence the radial direction
of the first blade type will have an elastic modulus of 107 GPa in
this case.
[0047] The dies used in the forging process for the second blade
type are designed so that the radial direction of the blade sees a
strain that is effectively the same as the transverse direction in
the rolled plate. Hence the radial direction of the second blade
type will have an elastic modulus of 134 GPa in this case.
[0048] If these two blades are the inertial welded next to one
another on a blisk the two blades will have an 11% change in their
natural frequencies. Based upon FIG. 9, the alternating arrangement
of these blades will provide an aerodynamic damping effect
(mistuning) that is appreciable and does not require any changes in
the inertial welding process used in the fabrication of the
blisk.
[0049] If the blade is superplastic formed (SPF) in which material
flow is a result of grain boundary sliding and not plastic
deformation, the crystallographic texture of the input stock
material only needs to be controlled or processed in a manner to
get the desired crystallographic texture. During superplastic
forging the intensity of the crystallographic texture in the sheet
will reduce in intensity but remains essentially the same
texture.
[0050] The anisotropy in the crystallographic texture or elastic
modulus following SPF will diminish but persists following the SPF
process. In order to utilize this anisotropy in tuning a pair of
blades, all one would do is control the crystallographic of the
sheet stock material and directions of the input sheet material
used during the superplastic forming process (i.e. the sheet will
have a pronounced crystallographic texture due to rolling along a
processing direction--such as shown in FIG. 5 having a Young's
modulus of 107 GPa in one direction and 134 in another orthogonal
direction. On even number blades the sheet stock is oriented with
the rolling direction at 12 o'clock. These blades will have an
elastic modulus of 107 GPa. On odd numbered blades the rolling
direction is aligned at 3 o'clock (orthogonal to the even blades).
These blades will have an elastic modulus of 134 GPa. Hence one
would then expect a .about.11% difference in the fundamental
frequencies of the blades. It is important to note that the texture
intensity diminishes during the SPF process, as this anisotropy is
what provides the anisotropy that drives the change in elastic
modulus and hence fundamental frequencies of the blades.
[0051] Alloys that recrystallize which may be treated to control
crystallographic texture, modulus and natural frequency include
titanium alloys, nickel alloys, aluminum alloys, zinc alloys, iron
alloys, cadmium alloys, beryllium alloys, zirconium alloys, and
cobalt alloys.
[0052] The embodiment(s) detailed above may be combined, in full or
in part, with any alternative embodiment(s) described.
INDUSTRIAL APPLICABILITY
[0053] Important advantages of a fan blade system comprising a
plurality of blades comprising an anisotropic material that
includes at least two blade types having different natural
frequencies includes improved resistance to flutter (mistuning),
reduction in the weight of the blade (when compared to other
anti-flutter solutions), and increased life of the fan blade
system.
[0054] The use of the terms "a" and "an" and "the" and similar
references in the context of describing the invention (especially
in the context of the following claims) are to be construed to
cover both the singular and the plural, unless otherwise indicated
herein or clearly contradicted by context. Recitation of ranges of
values herein are merely intended to serve as a shorthand method of
referring individually to each separate value falling within the
range, unless otherwise indicated herein, and each separate value
is incorporated into the specification as if it were individually
recited herein. All methods described herein can be performed in
any suitable order unless otherwise indicated herein or otherwise
clearly contradicted by context. The use of any and all examples,
or exemplary language (e.g., "such as") provided herein, is
intended merely to better illuminate the disclosure and does not
pose a limitation on the scope of the disclosure unless otherwise
claimed. No language in the specification should be construed as
indicating any non-claimed element as essential to the practice of
the disclosure.
[0055] Numerous modifications to the present disclosure will be
apparent to those skilled in the art in view of the foregoing
description. Various embodiments of this disclosure are described
herein, including the best mode known to the inventors for carrying
out the disclosure. It should be understood that the illustrated
embodiments are exemplary only, and should not be taken as limiting
the scope of the disclosure.
* * * * *