US20180080450A1

US20180080450A1 - Flutter avoidance through control of texture and modulus of elasticity in adjacent fan blades

Info

Publication number: US20180080450A1
Application number: US15/269,540
Authority: US
Inventors: Michael G. Glavicic
Original assignee: Rolls Royce Corp
Current assignee: Rolls Royce Corp
Priority date: 2016-09-19
Filing date: 2016-09-19
Publication date: 2018-03-22

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.

Description

FIELD OF DISCLOSURE

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

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

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.
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.
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%.
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

FIG. 1 depicts a side sectional view of a turbofan engine including a plurality of fan blades attached to a disk.

FIG. 2 depicts an isometric sectional view of fan blades attached to a disk.

FIG. 2A depicts an isometric sectional view of fan blades attached to a disk.

FIG. 3 depicts a schematic of compressor performance having operating regions known as flutter boundaries.

FIG. 4 depicts a schematic showing angular variation of the Young's Modulus of various anisotropic metals.

FIG. 5 depicts a perspective view of a rolled plate having hexagonal close packed crystal units subjected to unidirectional rolling.

FIG. 6 depicts a plan view of a plate subjected to unidirectional rolling with specimens oriented relative to the rolling direction.

FIG. 7 depicts the principal directions of a beam to illustrate the anisotropic mechanical properties of titanium.

FIG. 8 depicts an isometric view of a simple cantilever beam to illustrate the concept of natural frequency.

FIG. 8A depicts an isometric view of the simple cantilever beam of FIG. 8 in motion to illustrate the concept of natural frequency.

FIG. 8B depicts a side sectional view of the simple cantilever beam of FIG. 8 along line 8B-8B.

FIG. 9 depicts a graphical representation of the effects of fan blade mistuning.

DETAILED DESCRIPTION

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.
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™.
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.
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.
Referring to FIG. 2 and FIG. 2A, side sectional views of two fan blade systems 60 a, 60 b show (1) a mechanical blade-disk attachment in which the fan blade 52 a has a dovetail 62 that is retained by the disk 60 a (analogous to a tongue and groove) and (2) a blisk arrangement in which the fan blade 52 b is attached to the disk 60 b (to form a blisk) by a weld 64 rather than a dovetail 62 or other root geometry that extends into a disk 60 a.
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.
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.
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.
Referring to FIG. 4, the angular modulus behavior of E for 0°<θ<90° for the group of hexagonal close-packed metals comprising: hafnium (Hf), titanium (Ti), zirconium (Zr), and scandium (Sc).
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 θ is zero and N coincides with the [0001] direction) and a maximum (in some cases) on the prismatic planes where θ is 90°. In most cases, E tends to exhibit a minimum value between 0°<θ<90°. In the case of Ti, the behavior of E exhibits a maximum when θ is zero and a minimum when θ is 90°. This illustrates the anisotropic nature of various HCP materials with regard to modulus of elasticity.
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.
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 (˜104 GPa).
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.
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.
Referring to FIG. 7, a beam 77 is shown to illustrate the hexagonal close packed 79 crystal arrangement having a texture.
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.
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 80 a is shown having a fixed end 82 a, attached to a support 81 a, and a free end 84 a. FIG. 8A illustrates a beam 80 b having a fixed end 82 b, attached to a support 81 b. The beam 80 b is shown oscillating at the free end 84 b to illustrate the concept of natural frequency. FIG. 8B shows a side sectional view of the simple cantilever beam 80 a and support 81 a of FIG. 8 along line 8B-8B. A closed form equation for natural frequencies of a cantilever beam are:
$\begin{matrix} ω_{nf} = α_{n}^{} \sqrt{\frac{El}{{mL}^{4}}} & (1) \end{matrix}$
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:
$\begin{matrix} \frac{ω_{nf}^{LD}}{ω_{nf}^{TD}} = \sqrt{\frac{E (LD)}{E (TD)} =} \sqrt{\frac{107}{134}} = 0.8935 & (2) \\ ω_{nf}^{LD} = 0.8935 ω_{nf}^{TD} & (3) \end{matrix}$
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.
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.
In order to mistune the fan blisk 60 b, the fan blades 52 b 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).
For example, odd fan blades are cut along the processing or rolling direction (corresponding to 0° in FIG. 6) and even blades are cut along the transverse direction (90° in FIG. 6).
Referring to FIG. 9, a graphical representation of the effects of fan blade mistuning (represented by application factor) according to a simulation is shown.
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).
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.
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.
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.
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.
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.
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.
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.
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 ˜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.
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.
The embodiment(s) detailed above may be combined, in full or in part, with any alternative embodiment(s) described.

INDUSTRIAL APPLICABILITY

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.
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.
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.

Claims

We claim:

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.