U.S. patent application number 15/542636 was filed with the patent office on 2018-09-27 for auxetic structures with angled slots in engineered patterns for customized npr behavior and improved cooling performance.
The applicant listed for this patent is Christopher BOOTH-MORRISON, Mehran FARHANGI, Matthew Christopher INNES, President and Fellows of Harvard College, Megan SCHAENZER, Ali SHANIAN. Invention is credited to Katia Bertoldi, Christopher Booth-Morrison, Mehran Farhangi, Matthew Christopher Innes, Farhad Javid, Megan Schaenzer, Ali Shanian.
Application Number | 20180274783 15/542636 |
Document ID | / |
Family ID | 56356521 |
Filed Date | 2018-09-27 |
United States Patent
Application |
20180274783 |
Kind Code |
A1 |
Bertoldi; Katia ; et
al. |
September 27, 2018 |
Auxetic Structures With Angled Slots In Engineered Patterns For
Customized NPR Behavior And Improved Cooling Performance
Abstract
Auxetic structures, effusion-cooling auxetic sheets, systems and
devices with auxetic structures, and methods of using and methods
of making auxetic structures are disclosed. An auxetic structure is
disclosed which includes an elastically rigid body with opposing
top and bottom surfaces. First and second pluralities of elongated
apertures extend through the elastically rigid body from the top
surface to the bottom surface. The first plurality of elongated
apertures extends transversely with respect to the second plurality
of elongated apertures. The first and/or second pluralities of
elongated apertures are obliquely angled with the top surface of
the elastically rigid body. The elongated apertures are
cooperatively configured to provide a desired cooling performance
while exhibiting stress reduction through negative Poisson's Ratio
(NPR) behavior under macroscopic planar loading conditions. For
example, the auxetic structure may exhibit an effusion cooling
effectiveness of approximately 30-50 Eta and a Poisson's Ratio of
approximately -0.2 to -0.9%.
Inventors: |
Bertoldi; Katia;
(Somerville, MA) ; Booth-Morrison; Christopher;
(Otterburn Park, CA) ; Farhangi; Mehran;
(Montreal, CA) ; Innes; Matthew Christopher;
(North Lancaster, CA) ; Javid; Farhad;
(Somerville, MA) ; Schaenzer; Megan; (Montreal,
CA) ; Shanian; Ali; (Montreal, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BOOTH-MORRISON; Christopher
FARHANGI; Mehran
INNES; Matthew Christopher
SCHAENZER; Megan
SHANIAN; Ali
President and Fellows of Harvard College |
Otterburn Park
Montreal
North Lancaster
Montreal
Montreal
Cambridge |
MA |
CA
CA
CA
CA
CA
US |
|
|
Family ID: |
56356521 |
Appl. No.: |
15/542636 |
Filed: |
January 9, 2016 |
PCT Filed: |
January 9, 2016 |
PCT NO: |
PCT/US16/12769 |
371 Date: |
July 10, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62118826 |
Feb 20, 2015 |
|
|
|
62101840 |
Jan 9, 2015 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 19/03 20130101;
F23R 2900/03041 20130101; F23R 2900/00014 20130101; F23R 3/06
20130101; F23R 2900/00005 20130101; F23R 3/002 20130101 |
International
Class: |
F23R 3/00 20060101
F23R003/00; F23R 3/06 20060101 F23R003/06 |
Claims
1. An auxetic structure comprising: an elastically rigid body with
opposing top and bottom surfaces and first and second pluralities
of elongated apertures extending through the elastically rigid body
from the top surface to the bottom surface, the first plurality of
elongated apertures extending transversely with respect to the
second plurality of elongated apertures, at least the first
plurality of elongated apertures being obliquely angled with the
top surface of the elastically rigid body, wherein the first and
second pluralities of elongated apertures are cooperatively
configured to provide a desired cooling performance and exhibiting
a desired stress performance through negative Poisson's Ratio (NPR)
behavior under macroscopic planar loading conditions.
2. The void structure of claim 1, wherein both the first and second
pluralities of elongated apertures are obliquely angled with the
top surface of the elastically rigid body.
3. The void structure of claim 1, wherein each aperture of the
first plurality of elongated apertures is angled approximately
40-70 degrees with the top surface of the elastically rigid
body.
4. The void structure of claim 1, wherein the cooling performance
includes an effusion cooling effectiveness of approximately
30-50%.
5. The void structure of claim 1, wherein the NPR behavior includes
a Poisson's Ratio of about -0.2 to about -0.9%.
6. The void structure of claim 1, wherein the elongated apertures
are engineered with a predefined porosity, a predetermined pattern,
or a predetermined aspect ratio, or any combination thereof, to
achieve the NPR behavior.
7. The void structure of claim 1, wherein the elongated apertures
have a predetermined porosity of about 0.3 to about 9%.
8. The void structure of claim 1, wherein each of the elongated
apertures has an aspect ratio of approximately 5-40.
9. The void structure of claim 1, wherein the first or the second
plurality of elongated apertures, or both, each has an S-shaped
plan-view profile.
10. The void structure of claim 1, wherein the first or the second
plurality of elongated apertures, or both, each has an elliptical
plan-view profile.
11. The void structure of claim 1, wherein the first or the second
plurality of elongated apertures, or both, each has a Z-shaped
plan-view profile.
12. The void structure of claim 1, wherein the first or the second
plurality of elongated apertures, or both, each has a
barbell-shaped plan-view profile, the barbell-shaped plan-view
profile including a pair of spaced boreholes connected by an
elongated slot.
13. The void structure of claim 1, the first or the second
plurality of elongated apertures, or both, each has an I-shaped
plan-view profile, the I-shaped plan-view profile including a pair
of spaced semicircular slots connected by an elongated slot.
14. The void structure of claim 1, wherein the first and second
pluralities of elongated apertures are arranged in an array of rows
and columns.
15. The void structure of claim 14, wherein the rows are equally
spaced from each other and the columns are equally spaced from each
other.
16. The void structure of claim 1, wherein each of the elongated
apertures has a major axis perpendicular to a minor axis, the major
axes of the first plurality of elongated apertures being
substantially perpendicular to the major axes of the second
plurality of elongated apertures.
17. An effusion-cooling auxetic sheet structure comprising: a
metallic sheet with opposing top and bottom surfaces and first and
second pluralities of elongated apertures extending through the
metallic sheet from the top surface to the bottom surface, the
first plurality of elongated apertures having a first set of
geometric characteristics and a first pattern, the second plurality
of elongated apertures having a second set of geometric
characteristics and a second pattern, the first plurality of
elongated apertures being orthogonally oriented with respect to the
second plurality of elongated apertures, each of the elongated
apertures being obliquely angled with respect to the top surface of
the elastically rigid body, wherein the first geometric
characteristics and pattern of the first plurality of elongated
apertures are cooperatively configured with the second geometric
characteristics and pattern of the second plurality of elongated
apertures to provide minimum cooling performance behavior while
exhibiting negative Poisson's Ratio (NPR) behavior under
macroscopic planar loading conditions.
18. A method of manufacturing an auxetic structure, the method
comprising: providing an elastically rigid body with opposing top
and bottom surfaces; adding to the elastically rigid body a first
plurality of apertures extending through the elastically rigid body
from the top surface to the bottom surface, the first plurality of
apertures being arranged in rows and columns, each aperture of the
first plurality of elongated apertures being obliquely angled with
the top surface of the elastically rigid body; and adding to the
elastically rigid body a second plurality of apertures extending
through the elastically rigid body from the top surface to the
bottom surface, the second plurality of apertures being arranged in
rows and columns, wherein the first and second pluralities of
apertures are cooperatively configured to provide a cooling
performance while exhibiting stress reduction through negative
Poisson's Ratio (NPR) behavior under macroscopic planar loading
conditions.
19. The method of claim 18, wherein each aperture of the second
plurality of elongated apertures is obliquely angled with the top
surface of the elastically rigid body.
20. The method of claim 18, wherein each aperture of the first
plurality of elongated apertures is angled approximately 40-70
degrees with the top surface of the elastically rigid body.
21-33. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the right of priority to U.S.
Provisional Patent Application No. 62/118,826, filed on Feb. 20,
2015, and U.S. Provisional Patent Application No. 62/101,840, filed
on Jan. 9, 2015, both of which are incorporated herein by reference
in their respective entireties.
TECHNICAL FIELD
[0002] The present disclosure relates generally to porous materials
and cellular solids with tailored isotropic and anisotropic
Poisson's ratios. More particularly, aspects of this disclosure
relate to auxetic structures with engineered patterns that exhibit
negative Poisson's Ratio (NPR) behavior, as well as systems,
methods and devices using such structures.
BACKGROUND
[0003] When materials are compressed along a particular axis, they
are most commonly observed to expand in directions transverse to
the applied axial load. Conversely, most materials contract along a
particular axis when a tensile load is applied along an axis
transverse to the axis of contraction. The material property that
characterizes this behavior is known as the Poisson's Ratio, which
can be defined as the negative of the ratio of transverse/lateral
strain to axial/longitudinal strain under axial loading conditions.
The majority of materials are characterized by a positive Poisson's
Ratio, which is approximately 0.5 for rubber, approximately 0.3 for
aluminum, brass and steel, and approximately 0.2 for glass.
[0004] Materials with a negative Poisson's Ratio (NPR), on the
other hand, will contract (or expand) in the transverse direction
when compressed (or stretched) in the axial direction. Materials
that exhibit negative Poisson's Ratio behavior are oftentimes
referred to as "auxetic" materials. The results of many
investigations suggest that auxetic behavior involves an interplay
between the microstructure of the material and its deformation.
Examples of this are provided by the discovery that metals with a
cubic lattice, natural layered ceramics, ferroelectric
polycrystalline ceramics, and zeolites may all exhibit negative
Poisson's Ratio behavior. Moreover, several geometries and
mechanisms have been proposed to achieve negative values for the
Poisson's Ratio, including foams with reentrant structures,
hierarchical laminates, polymeric and metallic foams. Negative
Poisson's Ratio effects have also been demonstrated at the
micrometer scale using complex materials which were fabricated
using soft lithography and at the nanoscale with sheet assemblies
of carbon nanotubes.
[0005] A significant challenge in the fabrication of auxetic
materials is that it usually involves embedding structures with
intricate geometries within a host matrix. As such, the
manufacturing process has been a bottleneck in the practical
development towards applications. A structure which forms the basis
of many auxetic materials is that of a cellular solid. Research
into the deformation of these materials is a relatively mature
field with primary emphasis on the role of buckling phenomena, on
load carrying capacity, and energy absorption under compressive
loading. Very recently, the results of a combined experimental and
numerical investigation demonstrated that mechanical instabilities
in 2D periodic porous structures can trigger dramatic
transformations of the original geometry. Specifically, uniaxial
loading of a square array of circular holes in an elastomeric
matrix is found to lead to a pattern of alternating mutually
orthogonal ellipses while the array is under load. This results
from an elastic instability above a critical value of the applied
strain. The geometric reorganization observed at the instability is
both reversible and repeatable and it occurs over a narrow range of
the applied load. Moreover, it has been shown that the pattern
transformation leads to unidirectional negative Poisson's Ratio
behavior for the 2D structure, i.e., it only occurs under
compression.
[0006] U.S. Pat. No. 5,233,828 ("828 patent") shows an example of
an engineered void structure--a combustor liner or "heat
shield"--utilized in high temperature applications. Combustor
liners are typically used in the combustion section of a gas
turbine. Combustor liners can also be used in the exhaust section
or in other sections or components of the gas turbine, such as the
turbine blades. In operation, combustors burn gas at intensely high
temperatures, such as around 3,000.degree. F. or higher. To prevent
this intense heat from damaging the combustor before it exits to a
turbine, the combustor liner is provided in the interior of the
combustor to insulate the surrounding engine. To minimize
temperature and pressure differentials across a combustor liner,
cooling feature have conventionally been provided, such as is shown
in the '828 patent, in the form of spaced cooling holes disposed in
a continuous pattern. As another example, U.S. Pat. No. 8,066,482
B2 presents an engineered structural member having
elliptically-shaped cooling holes to enhance the cooling of a
desired region of a gas turbine while reducing stress levels in and
around the cooling holes. European Patent No. EP 0971172 A1
likewise shows another example of a perforated liner used in a
combustion zone of a gas turbine. None of the above patent
documents, however, provide examples disclosed as exhibiting
auxetic behavior or being engineered to provide NPR effects.
[0007] U.S. Patent Application Pub. No. 2010/0009120 A1 discloses
various transformative periodic structures which include
elastomeric or elasto-plastic periodic solids that experience
transformation in the structural configuration upon application of
a critical macroscopic stress or strain. Said transformation alters
the geometric pattern, changing the spacing and the shape of the
features within the transformative periodic structure. Upon removal
of the critical macroscopic stress or strain, these elastomeric
periodic solids recover their original form. By way of comparison,
U.S. Patent Application Pub. No. 2011/0059291 A1 discloses
structured porous materials, where the porous structure provides a
tailored Poisson's ratio behavior. These porous structures consist
of a pattern of elliptical or elliptical-like voids in an
elastomeric sheet which is tailored, via the mechanics of the
deformation of the voids and the mechanics of the deformation of
the material, to provide a negative or a zero Poisson's ratio. All
of the foregoing patent documents are incorporated herein by
reference in their respective entireties and for all purposes.
SUMMARY
[0008] Aspects of the present disclosure are directed towards
auxetic structures with repeating patterns of elongated apertures
(also referred to herein as "voids" or "slots") that are engineered
to provide a desired negative Poisson's Ratio (NPR) behavior and
improved cooling performance. Unlike prior art NPR void shapes that
extend through the plane of the structure material, traversing the
thickness of the material in a direction normal to the material's
plane, NPR voids disclosed herein traverse the thickness of the
material at an angle that is oblique to the materials' plane. These
angled void configurations enhance the cooling performance of the
structure while retaining a low porosity and providing a desired
NPR behavior. Other aspects of the present disclosure are directed
to multi-functional NPR structures with angled air passages in the
hot section of a gas turbine. Additional aspects are directed
towards gas turbine combustors that are made with walls from a
material with engineered angled void features that provide
particular thermal, damping and/or acoustic functionalities. Such
functionalities include, for example, acoustic attenuation (or
noise damping), stress reduction (or load damping), and thermal
cooling (or heat damping).
[0009] According to aspects of the present disclosure, auxetic
structures with angled NPR slots are disclosed. In an example, an
auxetic structure includes an elastically rigid body, such as a
metallic sheet or other sufficiently elastic solid material, with
opposing top and bottom surfaces. First and second pluralities of
elongated apertures extend through the elastically rigid body from
the top surface to the bottom surface. The first plurality of
elongated apertures extends transversely (e.g., orthogonally) with
respect to the second plurality of elongated apertures. The first
and/or second pluralities of elongated apertures are obliquely
angled with the top and/or bottom surfaces of the elastically rigid
body. In an example, each slot traverses the thickness of a sheet
material at an angle that is oblique (e.g., approximately 40-70
degrees) to the material's plane. The elongated apertures are
cooperatively configured to provide a desired or minimum cooling
performance while exhibiting stress reduction through negative
Poisson's Ratio (NPR) behavior under macroscopic planar loading
conditions. By way of example, the elongated apertures are
engineered with a predefined porosity, a predetermined pattern,
and/or a predetermined aspect ratio to achieve the desired NPR
behavior. The auxetic structure may exhibit an effusion cooling
effectiveness of approximately 30-50%, a porosity of about 0.3 to
about 9%, and a Poisson's Ratio of approximately -0.2 to -0.9%.
Cooling effectiveness (Eta) can be defined as the difference of the
hot gas temperature to the wall temperature in the presence of a
cooling device divided by the difference of the hot gas temperature
to the temperature of the supplied cooling gas:
Eta=(T_hotgas-T_wall)/(T_hotgas-T_coolant).
[0010] In accordance with other aspects of this disclosure,
effusion-cooling auxetic sheet structures are featured. In an
example, an effusion-cooling auxetic sheet structure is presented
which includes a metallic sheet with opposing top and bottom
surfaces. First and second pluralities of elongated apertures
extend through the metallic sheet from the top surface to the
bottom surface. The first plurality of elongated apertures has a
first set of geometric characteristics and is arranged in a first
pattern. Likewise, the second plurality of elongated apertures has
a second set of geometric characteristics and is arranged in a
second pattern. The elongated apertures of the first plurality are
orthogonally oriented with respect to the elongated apertures of
the second plurality. Each of the elongated apertures is obliquely
angled with respect to the top surface of the elastically rigid
body. The geometric characteristics and pattern of the first
plurality of elongated apertures are cooperatively configured with
the geometric characteristics and pattern of the second plurality
of elongated apertures to provide a desired or minimum cooling
performance while exhibiting negative Poisson's Ratio (NPR)
behavior under macroscopic planar loading conditions.
[0011] Other aspects of the present disclosure are directed to
methods of manufacturing and methods of using auxetic structures.
In an example, a method is presented for manufacturing an auxetic
structure. Said method includes: providing an elastically rigid
body with opposing top and bottom surfaces; adding to the
elastically rigid body a first plurality of apertures extending
through the elastically rigid body from the top surface to the
bottom surface, the first plurality of apertures being arranged in
rows and columns; and, adding to the elastically rigid body a
second plurality of apertures extending through the elastically
rigid body from the top surface to the bottom surface, the second
plurality of apertures being arranged in rows and columns. Each
aperture of the first and/or second pluralities of elongated
apertures is obliquely angled with the top surface of the
elastically rigid body. The first and second pluralities of
apertures are cooperatively configured to provide a desired or
minimum cooling performance while exhibiting a negative Poisson's
Ratio (NPR) behavior under macroscopic planar loading conditions.
By way of example, the elongated apertures are engineered with a
predefined porosity, a predetermined pattern, and/or a
predetermined aspect ratio to achieve the desired NPR behavior. The
auxetic structure may exhibit an effusion cooling effectiveness of
approximately 30-50% and a Poisson's Ratio of approximately -0.2 to
-0.9%. The elastically rigid body may take on various forms, such
as a metallic sheet or other sufficiently elastic solid
material.
[0012] The above summary is not intended to represent every
embodiment or every aspect of the present disclosure. Rather, the
foregoing summary merely provides an exemplification of some of the
novel aspects and features set forth herein. The above features and
advantages, and other features and advantages of the present
disclosure, which are considered to be inventive singly or in any
combination, will be readily apparent from the following detailed
description of representative embodiments and modes for carrying
out the present invention when taken in connection with the
accompanying drawings and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a graph of Nominal Strain vs. Poisson's Ratio
illustrating the Poisson's Ratio behavior of representative
structures with elongated through holes according to aspects of the
present disclosure.
[0014] FIGS. 2A-2C are illustrations of the representative
structures of FIG. 1 corresponding to specific data points from the
graph.
[0015] FIGS. 3A and 3B are side-view and perspective-view
illustrations, respectively, of an angled NPR S-slot according to
aspects of the present disclosure.
[0016] FIGS. 4A-4D are perspective-view illustrations of other
angled NPR slots in accordance with aspects of the present
disclosure.
[0017] FIGS. 5A and 5B are plan-view illustrations of an angled NPR
S-slot and an angled NPR Z-slot, respectively, with variable cap
rotation in accordance with aspects of the present disclosure.
[0018] FIGS. 6A-6D are plan-view illustrations of angled NPR
S-slots exhibiting a 0-degree angle, a 45-degree angle, a 55-degree
angle, and a 65-degree angle, respectively, in accordance with
aspects of the present disclosure.
[0019] FIGS. 7A-7C are graphical illustrations of the cooling
behaviors for non-NPR normal cooling holes, normal NPR cooling
slots, and angled NPR cooling slots, respectively, in accordance
with aspects of the present disclosure.
[0020] The present disclosure is susceptible to various
modifications and alternative forms, and some representative
embodiments have been shown by way of example in the drawings and
will be described in detail herein. It should be understood,
however, that the inventive aspects of this disclosure are not
limited to the particular forms illustrated in the drawings.
Rather, the disclosure is to cover all modifications, equivalents,
combinations and subcombinations, and alternatives falling within
the spirit and scope of the invention as defined by the appended
claims.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
[0021] This disclosure is susceptible of embodiment in many
different forms. There are shown in the drawings, and will herein
be described in detail, representative embodiments with the
understanding that the present disclosure is to be considered as an
exemplification of the principles of the present disclosure and is
not intended to limit the broad aspects of the disclosure to the
embodiments illustrated. To that extent, elements and limitations
that are disclosed, for example, in the Abstract, Summary, and
Detailed Description sections, but not explicitly set forth in the
claims, should not be incorporated into the claims, singly or
collectively, by implication, inference or otherwise. For purposes
of the present detailed description, unless specifically disclaimed
or logically prohibited: the singular includes the plural and vice
versa; and the words "including" or "comprising" or "having" means
"including without limitation." Moreover, words of approximation,
such as "about," "almost," "substantially," "approximately," and
the like, can be used herein in the sense of "at, near, or nearly
at," or "within 3-5% of," or "within acceptable manufacturing
tolerances," or any logical combination thereof, for example.
[0022] Aspects of the present disclosure are directed towards
auxetic structures which include repeating patterns of angled slots
that provide negative Poisson's Ratio (NPR) behavior when
macroscopically loaded. Poisson's Ratio (or "Poisson coefficient")
can be generally typified as the ratio of transverse contraction
strain to longitudinal extension strain in a stretched object.
Poisson's Ratio is typically positive for most materials, including
many alloys, polymers, polymer foams and cellular solids, which
become thinner in cross section when stretched. The auxetic
structures disclosed herein exhibit a negative Poisson's Ratio
behavior.
[0023] According to aspects of the disclosed concepts, when an
auxetic structure is compressed along one axis (e.g., in the
Y-direction), coaxial strain results in a moment around the center
of each cell because of the way the adjacent apertures are
arranged. This, in turn, causes the cells to rotate. Each cell
rotates in a direction opposite to that of its immediate neighbors.
This rotation results in a reduction in the transverse axis
(X-direction) distance between horizontally adjacent cells. In
other words, compressing the structure in the Y-direction causes it
to contract in the X-direction. Conversely, tension in the
Y-direction results in expansion in the X-direction. At the scale
of the entire structure, this mimics the behavior of an auxetic
material. But many of the structures disclosed herein are composed
of conventional materials. Thus, the unadulterated material itself
may have a positive Poisson's Ratio, but by modifying the structure
with the introduction of the angled-slot patterns disclosed herein,
the structure behaves as having a negative Poisson's Ratio.
[0024] FIG. 1 is a graph of Poisson's Ratio (PR) against Nominal
Strain illustrating the Poisson's Ratio behavior of three
representative void structures shown in FIGS. 2A-2C. The chart of
FIG. 1 shows the Poisson's Ratio of each test piece under load. At
a certain level of deformation, the "instantaneous" PR can be
determined and plotted against a parameter (e.g., nominal strain)
representing the level of deformation. When a designer has a
desired NPR for an intended application, the level of deformation
corresponding to that PR can be determined and the geometry of the
holes at that condition determined. This hole shape pattern can
then be machined (manufactured) on an unstressed part to achieve a
component with the desired PR.
[0025] As seen in FIGS. 2B and 2C, the NPR aperture patterns can
consist of horizontally and vertically oriented, elongated holes
(also referred to as "apertures" or "voids" or "slots"), shown as
elliptical through slots. These elongated holes are arranged on
horizontal and vertical lines (e.g., rows and columns of a square
array in FIG. 2B) in a way that the vertical lines are equally
spaced and the horizontal in both dimensions lines are equally
spaced (also .DELTA.x=.DELTA.y). The center of each slot is on the
crossing point of two of the lines. Horizontally oriented and
vertically oriented slots alternate on the vertical and horizontal
lines such that any vertically oriented slot is surrounded by
horizontally oriented slots (and vice versa), while the next
vertically oriented slots are found on both diagonals. These voids
can also act as cooling and/or damping holes and, due to their
arrangement, also as stress reduction features. One or more of the
slots shown herein can be replaced by elongated NPR protrusions or
semispherical NPR dimples.
[0026] Also disclosed are gas turbine combustors that are made with
one or more walls from a material with any of the specific auxetic
structure configurations disclosed herein. In some embodiments, the
angled slots are generated in a metal body directly in a
stress-free state such that the apertures are equivalent in shape
to collapsed void shapes found in rubber under external load in
order to get NPR behavior in the metal body without collapsing the
metallic structure in manufacturing. Various manufacturing routes
can be used to replicate the void patterns in the metallic
component. The manufacturing does not necessarily contain buckling
as one of the process steps. The auxetic structures disclosed
herein are not limited to the combustor wall; rather, these
features can be incorporated into other sections of a turbine
(e.g., a blade, a vane, etc.).
[0027] In a conventional combustor wall, holes used for cooling air
flow and damping also act as stress risers. In some of the
disclosed embodiments, as the wall material at a hot spot presses
against its surrounding material, e.g., in a vertical direction,
the negative Poisson's Ratio will make the wall material contract
in the horizontal direction, and vice versa. This behavior will
reduce the stresses at the hotspot significantly. This effect is
stronger than just the impact of the reduced stiffness. Stress at
hot spot gets reduced, for example, by 50% which, in turn, leads to
an increase in stress fatigue life by several orders of magnitude.
The stress reduction by the NPR behavior does not increase the air
consumption of the combustor wall. The longer life could be used as
such or the wall material could be replaced by a cheaper one in
order to reduce raw material costs.
[0028] It has also been demonstrated that the replacement of
circular combustor cooling holes with a fraction of elongated and
angled air passages of 2-3% reduces thermo-mechanical stress by a
factor of at least five, while maintaining the cooling and damping
performance. For example, elliptical cooling holes in the combustor
have been predicted to result in a five-fold decrease in the worst
principal stress. Inducing NPR behavior, thus, adds further
functionality to the cooling holes of the combustor in that the NPR
behavior generates a five-fold reduction in worst principal stress
as compared to traditional cooling holes. In stress fatigue of a
combustor-specific superalloy, halving the component stress
increases the fatigue life by more than an order of magnitude. In
some embodiments, the superalloy may be a nickel-based superalloy,
such as Inconel (e.g. IN100, IN600, IN713), Waspaloy, Rene alloys
(e.g. Rene 41, Rene 80, Rene 95, Rene N5), Haynes alloys, Incoloy,
MP98T, TMS alloys, and CMSX (e.g. CMSX-4) single crystal
alloys.
[0029] It has been shown that optimized porosity offers increased
cooling function. As used herein, "porosity" can be defined to mean
the surface area of the apertures, AA, divided by the surface area
of the structure, AS, or Porosity=AA/AS. It may be desirable, in
some embodiments, that the porosity of a given void structure be
approximately 0.3-9.0% or, in some embodiments, approximately 1-4%
or, in some embodiments, approximately 2%. By comparison, many
prior art arrangements require a porosity of 40-50%.
[0030] There may be a predetermined optimal aspect ratio for the
elongated apertures to provide a desired NPR behavior. As used
herein, "aspect ratio" of the apertures can be defined to mean the
length divided by the width of the apertures, or the length of the
major axis divided by the length of the minor axis of the
apertures. It may be desirable, in some embodiments, that the
aspect ratio of the apertures be approximately 5-40 or, in some
embodiments, approximately 20-30. An optimal NPR may comprise, for
example, a PR of about -0.2 to about -0.9 or, for some embodiments,
about -0.5. Aspects of the disclosed concepts can be demonstrated
on structural patterns created with a pattern lengthscale at the
millimeter, and are equally applicable to structures possessing the
same periodic patterns at a smaller lengthscale (e.g., micrometer,
submicrometer, and nanometer lengthscales) or larger lengthscales
so far as the unit cells fit in the structure.
[0031] Turning next to FIGS. 3-6, there are shown various examples
of angled-slot auxetic structures which exhibit desired NPR
behaviors and enhanced cooling performance in accordance with the
present disclosure. FIGS. 3A and 3B, for example, illustrate an
auxetic structure, designated generally at 300, which utilizes an
alternating pattern of elongated asymmetrical slots. The foregoing
slots are elongated in that each has a major axis (e.g., a length)
that is larger than and perpendicular to a minor axis (e.g., a
width). As shown, the auxetic structure 300 comprises an
elastically rigid body 310, which may be in the form of a metallic
sheet or other solid material with adequate elasticity to return
substantially or completely to its original form once macroscopic
loading conditions are sufficiently reduced or eliminated.
Elastically rigid body 310 has a first (top) surface 314 in
opposing spaced relation to a second (bottom) surface 316.
Fabricated into the elastically rigid body 310 is a first plurality
of S-shaped through slots (also referred to herein as "apertures"
or "voids" or "slots"), represented herein by slot 312, which
extend through the body 310 from the top surface 314 to the bottom
surface 316. A second plurality of S-shaped through
slots/apertures, represented herein by slots 318, also extends
through the elastically rigid body 310 from the top surface 314 to
the bottom surface 316. The pattern of elongated apertures present
in the elastically rigid body 310 may be similar in arrangement to
what is seen in FIGS. 2B and 2C.
[0032] S-shaped through slots 312, 318 are arranged in an array or
matrix of rows and columns, with the first plurality of elongated
apertures 312 extending transversely with respect to the second
plurality of elongated apertures 318. Note that hidden lines
indicating the internal structural configuration of slots 318 have
been omitted from FIGS. 3A and 3B for clarity to better show the
internal structural configuration of slots 312. For at least some
embodiments, the rows are equally spaced from each other and,
likewise, the columns are equally spaced from each other. According
to the illustrated embodiment of FIGS. 3A and 3B, for example, each
row and each column comprises vertically oriented S-shaped through
slots 312 interleaved with horizontally oriented S-shaped through
slots 318. In effect, each vertically oriented through slot 312 is
neighbored on four sides by horizontally oriented through slots
318, while each horizontally oriented through slot 318 is
neighbored on four sides by vertically oriented through slots 312.
With this arrangement, the minor axes of the first plurality of
S-shaped through slots 312 are parallel to the rows of the array,
whereas the minor axes of the second plurality of S-shaped through
slots 318 are parallel to the columns of the array. Thus, the major
axes of the through slots 318, which are parallel to the rows of
the array, are perpendicular to the major axes of the through slots
312, which are parallel to the columns of the array. It is also
envisioned that other patterns and arrangements for achieving
stress reduction through NPR behavior are within the scope and
spirit of the present disclosure.
[0033] The illustrated pattern of elongated, angled slots provides
a specific porosity (e.g., a porosity of about 0.3 to about 9.0%)
and a desired cooling performance (e.g., an effusion cooling
effectiveness of approximately 30-50%) while exhibiting a desired
negative Poisson's Ratio behavior (e.g., a PR of about -0.2 to
about -0.9) under macroscopic planar loading conditions (e.g., when
tension or compression is applied in the plane of the sheet). When
the auxetic structure 300 is stretched, for example via tensile
force F.sub.T along a vertical axis Y, axial strain in the vertical
direction results in a moment around the center of each cell, which
causes the cells to rotate. A cell may consist of two laterally
adjacent vertical slots aligned with two vertically adjacent
horizontal slots to form a square-shaped unit. Each cell rotates in
a direction opposite to that of its immediate neighboring cells.
This rotation increases the X-direction distance between
horizontally adjacent cells such that stretching the structure in
the Y-direction causes it to stretch in the X-direction. The first
plurality of S-shaped through slots 312 have (first) engineered
geometric characteristics, including a predefined geometry and a
predefined aspect ratio, while the second plurality of S-shaped
through slots 318 have (second) engineered geometric
characteristics, including a predefined geometry and a predefined
aspect ratio, that are cooperatively configured with (third)
engineered geometric characteristics of the aperture pattern,
including NPR-slot density and cell arrangement, to achieve a
desired NPR behavior under macroscopic loading conditions.
[0034] Each slot of the first and/or second pluralities of
elongated S-shaped through slots 312, 318 can be obliquely angled
with respect to the top surface 314 or bottom surface 316, or both,
of the auxetic structure's 300 elastically rigid body 310. In an
example, slot 312 is shown in FIG. 3A traversing the entire
thickness of the material at an angle that is oblique to the
material's horizontal plane. For at least some embodiments, each
aperture has an angle .PHI. of approximately 20-80 degrees or, in
some embodiments, approximately 40-70 degrees with the top and
bottom surfaces 314, 316 of the auxetic structure's body 310. These
macroscopically patterned NPR voids--S-shaped angled slots (FIGS.
3A, 3B, 4A and 5A) or, equivalently, I-shaped angled slots (FIG.
4B), barbell-shaped angled slots (FIG. 4C), elliptical angled slots
(FIG. 4D), Z-shaped angled slots (FIG. 5B), C-shaped angled slots,
etc.--serve as effusion cooling holes which allow a cooling fluid
FL to traverse one surface of the auxetic structure, pass through
the body at an inclination angle .alpha., as shown in FIG. 3A, and
traverse the opposing surface of the auxetic structure. This
configuration enhances film cooling performance as compared to
traditional cooling slots/holes that are normal to the thickness of
the body and, thus, more restrictive of cooling fluid flow.
Inclination angle .alpha. can be defined as the angle between the
injection vector and its projection on the material plane. This
inclination angle can be varied in a 360.degree. rotational angle
of freedom to achieve numerous desired combinations of auxetic
behavior and film cooling performance. Cooling effectiveness (Eta)
can be typified as a non-dimensional value that quantitatively
represents how effectively a fluid flowing over a porous surface
protects that surface from a high temperature mainstream flow.
Cooling effectiveness can be defined as the difference of the hot
gas temperature to the wall temperature in the presence of a
cooling device divided by the difference of the hot gas temperature
to the temperature of the supplied cooling gas:
Eta=(T_hotgas-T_wall)/(T_hotgas-T_coolant).
[0035] Patterned angled NPR-slot features, such as those disclosed
in FIGS. 3-6, have been shown to cool significantly better than
conventional right-angled (normal) circular holes and cooling slots
as the internal surface area of the slots is larger than that of
normal circular holes or slots. Adiabatic film cooling
effectiveness is also increased compared to traditional normal
cooling holes and slots, for example, due to a more even
distribution of cooling air over the surface and reduced coolant
jet penetration into the mainstream flow. This can be seen when
comparing the cooling behaviors for representative non-NPR normal
cooling holes (Eta=17%), normal NPR cooling slots (Eta=36%), and
angled NPR S-slots (Eta=44%) of FIGS. 7A, 7B and 7C, respectively.
Angled NPR-slot film can benefit from the Coanda Effect, which
causes the coolant jet to better adhere to the wall, rather than
lifting off and penetrating the mainstream flow. This helps to
decrease the inclination angle, which in turn decreases coolant jet
penetration and increases cooling performance of NPR slots. From an
aerodynamic perspective, the reduced penetration of the coolant jet
of angled NPR slots decreases aerodynamic losses due to film
cooling compared with normal coolant slot flow. The inclination
angle can be varied to achieve a desired combination of auxetic
behavior and film cooling performance.
[0036] It has been determined that having inclined cooling slots
help to provide better film cooling effectiveness coverage in
comparison to normal cooling holes with internal walls that are
perpendicular to cooling flow. In addition, early investigation
demonstrates that coolant ejection from an angled NPR slot is more
efficient than ejection from normal cooling holes because the
mixing process is less intensive for the closed film ejected from
the slot. While the high thermal stresses encountered on gas
turbine blades and vanes typically do not allow for the use of
highly elongated slots, angled NPR slots help to reduce or
otherwise eliminate high thermal stresses on turbine blades/vanes
while enhancing film cooling performance. For at least some
embodiments, it is generally desirable to minimize surface porosity
and the amount of coolant used in a turbine engine; normal NPR
slots can be replaced with a smaller number of angled NPR slots to
minimize porosity. In this case, cooling flow consumption will be
reduced while the film cooling performance of the effusion slots is
maintained.
[0037] As an exemplary implementation of the disclosed features,
one can consider a combustor liner with sheet metal walls in which
conventional round effusion holes or normal effusion slots are
replaced with a pattern of angled S-shaped NPR slots forming an
auxetic structure. Cooling air fed through these angled S-shaped
slots removes heat from the structure and produces an even
distribution of cooling air over the surface. These angled slots,
which have an increased internal surface area, enhance film cooling
performance and improve mechanical response. Moreover, angled NPR
slots are capable of sustaining higher flame temperatures, and help
impart to the sheet a much longer life compared to conventional
sheet metal walls with normal effusion holes.
[0038] Shown in FIGS. 4A-4D are perspective-view illustrations of
other auxetic structures, designated generally at 400A, 400B, 400C
and 400D, respectively, with angled NPR slots in accordance with
aspects of the present disclosure. Although differing in
appearance, the auxetic structures 400A-400D may include any of the
features, options, and alternatives described herein with respect
to the other auxetic structures. In the same vein, unless
explicitly disclaimed or logically prohibited, any of the auxetic
structures disclosed herein can share features, options and
alternatives with the other disclosed embodiments. Auxetic
structures 400A-400D each comprises an elastically rigid body 410A,
410B, 410C and 410D, respectively, fabricated with a plurality of
elongated and angled apertures 412A, 412B, 412C and 412D,
respectively, arranged in a pattern to provide a desired cooling
performance while exhibiting a predetermined NPR behavior under
macroscopic planar loading conditions. In FIG. 4A, elongated
apertures 412A have an S-shaped plan-view profile, whereas the
elongated apertures 412B in FIG. 4B have an I-shaped plan-view
profile, which includes a pair of spaced semicircular slots
connected by an elongated linear slot. By comparison, elongated
apertures 412D in FIG. 4D have an elliptical plan-view profile,
whereas the elongated apertures 412C in FIG. 4C have a
barbell-shaped plan-view profile, which includes a pair of spaced,
rounded boreholes connected by an elongated linear slot. Any of the
foregoing angled NPR slots can be manufactured by laser cutting,
for example, by laying out a linear pattern of NPR slots along the
inclination angle to the surface.
[0039] With continuing reference to FIGS. 4A-4D, the profile of the
angled NPR slots that appears on the outer (top) surface can be
designed as a projection of a standard shape--e.g., a standard "S"
414A, a standard "I" 414B with rounded arms, a standard barbell
414C with circular ends, and a standard ellipse 414D. Optionally,
the profile of the angled NPR slots that appears on the outer (top)
surface can be highly distorted from the original image depending,
for example, on the desired angle and/or orientation of the slot.
FIGS. 6A-6D illustrate slot distortion on an outer surface of a
tubular auxetic structure: FIG. 6A illustrating normal NPR S-slots
exhibiting a 0-degree angle; FIG. 6B illustrating angled NPR
S-slots exhibiting a 45-degree angle; FIG. 6C illustrating angled
NPR S-slots exhibiting a 55-degree angle; and FIG. 6D illustrating
angled NPR S-slots exhibiting a 65-degree angle.
[0040] A new NPR slot shape, for instance, Z-shaped slots 512A
(FIG. 5A) and S-shaped slots (FIG. 5B), can be developed by
reducing cap length 511A and 511B and/or cap height 513A and 513B
to provide a horizontal projection similar to an existing or
"standard" S-shape/Z-shape. The size and shape of the caps can be
varied to achieve a desired combination of auxetic behavior and
film cooling performance. Film cooling performance of angled
effusion S-shaped slots or, equivalently, Z-shaped slots can be
improved by producing a longer cooling thermal layer above the hot
surface. A longer cooling thermal layer can be created by
increasing the lateral area of the slots normal to the free
mainstream fluid by rotating the S-shaped slot cap in the
counter-clockwise direction (or clockwise direction for Z-shaped
slot caps). This cap rotation angle 515A and 515B can be varied to
achieve a desired combination of auxetic behavior and film cooling
performance. By rotating the caps of the S-shaped slots in the
counter-clockwise direction, the maximum mechanical stress at the
top of the caps will be reduced and the film cooling performance of
the effusion slots will be improved due to the increased coverage
of the cooling thermal layer above the hot surface.
[0041] Aspects of this disclosure are also directed to methods of
manufacturing and methods of using auxetic structures. By way of
example, a method is presented for manufacturing an auxetic
structure, such as the auxetic structures described above with
respect to FIGS. 3-6. The method includes, as an inclusive yet
non-exclusive set of acts: providing an elastically rigid body,
such as the elastically rigid body 310 of FIGS. 3A and 3B, with
opposing top and bottom surfaces; adding to the elastically rigid
body a first plurality of apertures, such as the elongated S-shaped
slots 312 of FIGS. 3A and 3B, extending through the elastically
rigid body from the top surface to the bottom surface; and, adding
to the elastically rigid body a second plurality of apertures, such
as the elongated S-shaped slots 318 of FIGS. 3A and 3B, extending
through the elastically rigid body from the top surface to the
bottom surface. The first and second pluralities of apertures are
arranged in rows and columns. Each aperture of the first and/or
second plurality is obliquely angled with the top surface of the
elastically rigid body. The first and second pluralities of
apertures are cooperatively configured to provide a predefined
cooling performance while exhibiting a predetermined negative
Poisson's Ratio (NPR) behavior under macroscopic planar loading
conditions. By way of example, the elongated apertures are
engineered with a predefined porosity, a predetermined pattern,
and/or a predetermined aspect ratio to achieve the desired NPR
behavior. The auxetic structure may exhibit an effusion cooling
effectiveness of approximately 30-50% and a Poisson's Ratio of
approximately -0.2 to -0.9%. The elastically rigid body may take on
various forms, such as a metallic sheet or other sufficiently
elastic solid material.
[0042] In some embodiments, the method includes at least those
steps enumerated above and illustrated in the drawings. It is also
within the scope and spirit of the present invention to omit steps,
include additional steps, and/or modify the order presented above.
It should be further noted that the foregoing method can be
representative of a single sequence for designing and fabricating
an auxetic structure. However, it is expected that the method will
be practiced in a systematic and repetitive manner.
[0043] The present invention is not limited to the precise
construction and compositions disclosed herein. Rather, any and all
modifications, changes, combinations, permutations and variations
apparent from the foregoing descriptions are within the scope and
spirit of the invention as defined in the appended claims.
Moreover, the present concepts expressly include any and all
combinations and subcombinations of the preceding elements and
aspects.
* * * * *