U.S. patent application number 13/077812 was filed with the patent office on 2012-10-04 for stator-rotor assemblies with features for enhanced containment of gas flow, and related processes.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Corey Bourassa, Ronald Scott Bunker, Gregory Michael Laskowski, Gustavo Adolfo Ledezma.
Application Number | 20120251291 13/077812 |
Document ID | / |
Family ID | 45524384 |
Filed Date | 2012-10-04 |
United States Patent
Application |
20120251291 |
Kind Code |
A1 |
Ledezma; Gustavo Adolfo ; et
al. |
October 4, 2012 |
STATOR-ROTOR ASSEMBLIES WITH FEATURES FOR ENHANCED CONTAINMENT OF
GAS FLOW, AND RELATED PROCESSES
Abstract
A stator-rotor assembly is described, including at least one
interface region and a gap between a surface of the stator and a
surface of the rotor. The stator is a nozzle or vane that includes
circumferential endwalls. Each endwall includes at least one
leading edge and one trailing edge, relative to a hot gas flow
path. A trailing edge of at least one of the endwalls includes a
pattern of cavities that are capable of impeding the entry of hot
gas into a wheelspace region that adjoins the gap between the
stator and the rotor. The cavities can also be formed on various
sections of the rotor. The stator-rotor assembly can be
incorporated into various turbomachines, such as gas turbine
engines. Related processes are also described.
Inventors: |
Ledezma; Gustavo Adolfo;
(Delmar, NY) ; Bourassa; Corey; (Mechanicville,
NY) ; Bunker; Ronald Scott; (Waterford, NY) ;
Laskowski; Gregory Michael; (Saratoga Springs, NY) |
Assignee: |
GENERAL ELECTRIC COMPANY
Schenectady
NY
|
Family ID: |
45524384 |
Appl. No.: |
13/077812 |
Filed: |
March 31, 2011 |
Current U.S.
Class: |
415/1 ;
415/170.1 |
Current CPC
Class: |
F05D 2250/182 20130101;
F05D 2250/13 20130101; Y02T 50/60 20130101; F05D 2250/232 20130101;
F01D 5/082 20130101; Y02T 50/676 20130101; F05D 2250/70 20130101;
F01D 11/02 20130101 |
Class at
Publication: |
415/1 ;
415/170.1 |
International
Class: |
F01D 11/08 20060101
F01D011/08 |
Goverment Interests
[0001] This invention was made with Government support under
contract number DE-FC26-05NT42643, awarded by DOE. The Government
has certain rights in the invention.
Claims
1. A stator-rotor assembly, comprising at least one circumferential
endwall having a trailing edge that comprises a pattern of
cavities.
2. A stator-rotor assembly, comprising at least one interface
region between a surface of the stator and a surface of the rotor,
said surfaces being separated by a gap, wherein the stator is a
nozzle or vane that comprises inner and outer circumferential
endwalls; and each endwall includes at least one leading edge and
one trailing edge, relative to a hot gas flow path; and wherein a
trailing edge of the inner circumferential endwall comprises a
pattern of cavities that are capable of impeding the entry of hot
gas into a wheelspace region that adjoins the gap between the
stator and the rotor.
3. The assembly of claim 2, wherein the cavities have a curved
inner surface.
4. The assembly of claim 2, wherein the cavities have a depth that
is tapered along at least one dimension of the cavity.
5. The assembly of claim 2, wherein the cavities are in the shape
of a partial cone.
6. The assembly of claim 5, wherein each partial cone has a base
dimension, closest to the gap, that is wider than an opposite end
of the cone that is farthest from the gap.
7. The assembly of claim 2, wherein the pattern comprises an array
of uniformly-spaced cavities.
8. The assembly of claim 2, wherein the pattern comprises an array
of non-uniformly-spaced cavities.
9. The assembly of claim 2, wherein the cavities have an average
depth in the range of about 10% to about 80% of the depth of the
inner circumferential endwall.
10. The assembly of claim 2, wherein the cavities are situated
along the trailing edge, and are generally parallel to each
other.
11. The assembly of claim 10, wherein an edge of each cavity is in
contact with an edge of an adjacent cavity.
12. The assembly of claim 10, wherein each cavity is spaced from
adjacent cavities.
13. The assembly of claim 2, wherein the interface region between
the stator and rotor surfaces is a flow-restriction region that
limits the flow of gas from the hot flow path of the turbine
engine, through the gap, to a wheel-space region of the
stator-rotor assembly.
14. A stator-rotor assembly, comprising at least one interface
region between a surface of the stator and a surface of the rotor,
said surfaces being separated by a gap, wherein the rotor is a
bucket or blade that comprises inner and outer circumferential
endwalls; and each endwall includes at least one leading edge and
one trailing edge, relative to a hot gas flow path; and wherein a
trailing edge of the inner circumferential endwall comprises a
pattern of cavities that are capable of impeding the entry of hot
gas into a wheelspace region that adjoins the gap between the
stator and the rotor.
15. A gas turbine engine, comprising a stator-rotor assembly
according to claim 1.
16. A turbomachine, comprising at least one stator-rotor assembly,
wherein the stator-rotor assembly comprises at least one interface
region between a surface of the stator and a surface of the rotor,
said surfaces being separated by at least one gap, wherein the
stator is a nozzle or vane that comprises inner and outer
circumferential endwalls; and each endwall includes at least one
leading edge and one trailing edge, relative to a hot gas flow
path; and wherein a trailing edge of the inner circumferential
endwall of each nozzle or vane comprises a pattern of cavities that
are capable of impeding the entry of hot gas into a wheelspace
region that adjoins the gap between the stator and the rotor.
17. A method for restricting the flow of hot gas through a gap
between a stator and a rotor in a turbomachine, wherein the stator
is a nozzle or vane that comprises inner and outer circumferential
endwalls; and each endwall includes at least one leading edge and
one trailing edge, relative to a gas flow path; and wherein the
method comprises the step of forming a pattern of cavities on at
least a portion of the trailing edge of the inner endwall of the
stator component; wherein the cavities have a shape and size
sufficient to impede the entry of hot gas into a wheelspace area
that adjoins the gap between the stator and rotor.
18. The method of claim 17, wherein the cavities are formed by at
least one technique selected from the group consisting of milling
techniques, electro-discharge machining (EDM), electro-chemical
machining (ECM), and casting.
Description
BACKGROUND OF THE INVENTION
[0002] This invention relates generally to improvements in
turbomachines, such as turbine engines. More specifically, the
invention is directed to methods and articles for decreasing the
flow of gas (e.g., hot gas) into selected regions within the
stator-rotor assemblies of gas turbines.
[0003] The typical design of most turbine engines is well-known in
the art. They include a compressor for compressing air that is
mixed with fuel. The fuel-air mixture is ignited in an attached
combustor, to generate combustion gases. The hot, pressurized
gases, which in modern engines can be in the range of about 1100 to
2000.degree. C., are allowed to expand through a turbine nozzle,
which directs the flow to turn an attached, high-pressure turbine.
The turbine is usually coupled with a rotor shaft, to drive the
compressor. The core gases then exit the high pressure turbine,
providing energy downstream. The energy is in the form of
additional rotational energy extracted by attached, lower pressure
turbine stages, and/or in the form of thrust through an exhaust
nozzle.
[0004] In the typical scenario, thermal energy produced within the
combustor is converted into mechanical energy within the turbine,
by impinging the hot combustion gases onto one or more bladed rotor
assemblies. (Those versed in the art understand that the term
"blades" is usually part of the lexicon for aviation turbines,
while the term "buckets" is typically used when describing the same
type of component for land-based turbines). The rotor assembly
usually includes at least one row of circumferentially-spaced rotor
blades. Each rotor blade includes an airfoil that includes a
pressure side and a suction side. Each airfoil extends radially
outward from a rotor blade platform. Each rotor blade also includes
a dovetail that extends radially inward from a shank extending
between the platform and the dovetail. The dovetail is used to
mount the rotor blade within the rotor assembly to a rotor disk or
spool.
[0005] The rotor forms part of a stator-rotor assembly. The rows of
rotor blades on the rotor assembly and the rows of stator vanes on
the stator assembly extend alternately across an axially oriented
flowpath for "working" the combustion gases. The jets of hot
combustion gas leaving the vanes of the stator element act upon the
turbine blades, and cause the turbine wheel to rotate in a speed
range of about 3000-15,000 rpm, depending on the type of engine.
(Again, in terms of parallel terminology, the stator element, i.e.,
the element which remains stationary while the turbine rotates at
high speed, can also be referred to in the art as the "nozzle
assembly").
[0006] As depicted in the figures described below, the opening at
the interface between the stator element and the blades or buckets
can allow hot core gas to exit the hot gas path and enter the
wheel-space of the turbine engine. In order to limit this leakage
of hot gas, the blade structure typically includes axially
projecting angel wing seals. According to a typical design, the
angel wings cooperate with projecting segments or "discouragers"
which extend from the adjacent stator element, i.e., the nozzle.
The angel wings and the discouragers overlap (or nearly overlap),
but do not touch each other, thus restricting gas flow.
[0007] A gap remains at the interface between adjacent regions of
the nozzle and turbine blade, e.g., between the adjacent angel
wing-discourager projections, when such a seal is used. The
presence of the gap is understandable, i.e., the clearance
necessary at the junction of stationary and rotating components.
However, the gap still provides a path which can allow hot core gas
to exit the hot gas path into the wheel-space area of the turbine
engine.
[0008] The leakage of hot gas by this pathway is disadvantageous
for a number of reasons. First, the loss of hot gas from the
working gas stream causes a resultant loss in energy available from
the turbine engine. Second, ingestion of the hot gas into turbine
wheel-spaces and other cavities can damage components which are not
designed for extended exposure to such temperatures, such as the
nozzle structure support and the rotor wheel.
[0009] Attempts have been made in the past to minimize the leakage
of hot gas from the working gas stream. These attempts have
sometimes involved the use of coolant air, i.e., "purge air", as
described in U.S. Pat. No. 5,224,822 (Lenehan et al). In a typical
design, the air can be diverted or "bled" from the compressor, and
used as high-pressure cooling air for the turbine cooling circuit.
Thus, the coolant air is part of a secondary flow circuit which can
be directed generally through the wheel-space cavity and other
inboard regions.
[0010] In one specific example, the coolant air can be vented to
the rotor/stator interface. In this manner, the coolant air can
function to maintain the temperature of certain engine components
under an acceptable limit. Moreover, the coolant air can serve an
additional, specific function when it is directed from the
wheel-space region into one of the gaps described previously. This
counter-flow of coolant air into the gap provides an additional
barrier to the undesirable flow of hot gas out of the gap and into
the wheel-space region.
[0011] While coolant air from the secondary flow circuit is very
beneficial for the reasons discussed above, there are drawbacks
associated with its use as well. For example, the extraction of air
from the compressor for high pressure cooling and cavity purge air
consumes work from the turbine, and can be quite costly in terms of
engine performance. Moreover, in some engine configurations, the
compressor system may fail to provide purge air at a sufficient
pressure during at least some engine power settings. Thus, hot
gases may still be ingested into the wheel-space cavity.
[0012] Another technique for minimizing the leakage of hot gas from
the working gas stream of a gas turbine is described in U.S. Pat.
No. 6,481,959 (Morris et al). This patent describes the use of a
supplemental air cooling system, to inhibit ingestion of hot gases
into various circumferential regions of the turbine disc cavity,
e.g., the gap and wheelspace regions. The system in Morris et al
includes a number of ingestion inhibiting dynamic jet orifices,
located on the underside of the trailing edges of a turbine
nozzle.
[0013] While the concept described in Morris et al may be suitable
in some situations, there are drawbacks associated with it as well.
For example, the air cooling system may require a diversion of air
from the compressor, and this can compromise engine performance, as
alluded to previously. Moreover, it appears that the air jets used
in the system must produce an airflow momentum greater than that of
the hot gas moving into the gap and wheelspace, so as to inhibit
such movement. Such a system would appear to require a complex
design, especially if the amount of cooling air needs to be
minimized.
[0014] In view of this discussion, it should be apparent that new,
relatively simple techniques for reducing the leakage of hot gases
from a hot gas flow path into undesirable regions within a turbine
engine or other type of turbomachine would be welcome in the art.
Moreover, reduction of the cooling and cavity purge-air flow which
is typically required to reduce the hot gas leakage would itself
have other important benefits. For example, higher core air flow
would be possible, thereby increasing the energy available in the
hot gas flow path.
[0015] Any innovations designed to accomplish these goals must
still adhere to the primary design requirements for a gas turbine
engine or other type of turbomachine. In general, overall engine
efficiency and integrity must be maintained. Any change made to the
engine, or to specific features within the engine, must not disturb
or adversely affect the overall hot gas and coolant air flow
fields. Moreover, the contemplated improvements should not involve
manufacturing steps or changes in those steps which are
time-consuming and uneconomical. Furthermore, the improvements
should be adaptable to varying designs in engine construction,
e.g., different types of stator-rotor assemblies.
BRIEF DESCRIPTION OF THE INVENTION
[0016] One embodiment of the invention is directed to a
stator-rotor assembly, comprising at least one circumferential
endwall having a trailing edge that comprises a pattern of
cavities. In some embodiments, the stator-rotor assembly comprises
at least one interface region between a surface of the stator and a
surface of the rotor. The surfaces are separated by a gap. The
stator is a nozzle or vane that comprises inner and outer
circumferential endwalls; and each endwall includes at least one
leading edge and one trailing edge, relative to a hot gas flow
path. A trailing edge of the inner circumferential endwall
comprises a pattern of cavities that are capable of impeding the
entry of hot gas into a wheelspace region that adjoins the gap
between the stator and the rotor.
[0017] Another embodiment relates to a turbomachine. The
turbomachine comprises at least one stator-rotor assembly, having
an interface region with surfaces being separated by at least one
gap, as described above. The trailing edge on at least one endwall
comprises a pattern of cavities that are capable of impeding the
entry of hot gas into a wheelspace region, as described above and
detailed further below.
[0018] An additional embodiment is directed to a method for
restricting the flow of hot gas through a gap between a stator and
a rotor in a turbomachine. As described herein, the stator is a
nozzle or vane that comprises inner and outer circumferential
endwalls; and each endwall includes at least one leading edge and
one trailing edge, relative to a gas flow path. The method
comprises the step of forming a pattern of cavities on at least a
portion of the trailing edge of the inner endwall of the stator
component. The cavities have a shape and size sufficient to impede
the entry of hot gas into a wheelspace area that adjoins the gap
between the stator and rotor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic illustration of a cross-section of a
portion of a gas turbine.
[0020] FIG. 2 is an enlarged view of the cross-sectional turbine
portion of FIG. 1.
[0021] FIG. 3 is another enlarged view of the cross-sectional
turbine portion of FIG. 1.
[0022] FIG. 4 is a perspective of the endwall region of one section
of a stator-rotor assembly, according to the prior art.
[0023] FIG. 5 is another perspective of the endwall region of a
section of a stator-rotor assembly, according to embodiments of
this invention.
[0024] FIG. 6 is a partial, side-elevation view of an endwall
surface which includes a cavity.
[0025] FIG. 7 is a top-view perspective of a cavity within an
endwall surface.
[0026] FIG. 8 is another partial, side-elevational view of an
endwall surface which includes a cavity.
[0027] FIG. 9 is another top-view perspective of a cavity within an
endwall surface, according to some embodiments of the
invention.
[0028] FIG. 10 is a top-view perspective of a cavity within an
endwall surface, according to other embodiments of the
invention.
[0029] FIG. 11 is a top-view perspective of a cavity within an
endwall surface, according to additional embodiments of the
invention.
[0030] FIG. 12 is a perspective of a modified endwall region of one
section of a stator-rotor assembly, according to some embodiments
of the invention.
[0031] FIG. 13 is another perspective of a modified endwall region,
according to inventive embodiments.
[0032] FIG. 14 is a depiction of a computer-assisted model for
numerical predictions of hot gas flow, purge flow, and flow
interactions, in a region of a stator-rotor assembly.
DETAILED DESCRIPTION OF THE INVENTION
[0033] FIG. 1 is a schematic illustration of a section of a gas
turbine engine, generally designated with numeral 10. The engine
includes axially-spaced rotor wheels 12 and spacers 14, joined to
each other by a plurality of circumferentially spaced, axially
extending bolts 16. The turbine includes various stages having
nozzles, for example, first-stage nozzle 18 and second-stage nozzle
20, comprised of a plurality of circumferentially spaced stator
blades. Between the nozzles and rotating with the rotor are a
plurality of rotor blades or buckets, the first and second-stage
rotor blades 22 and 24, respectively, being illustrated.
[0034] Each rotor blade, e.g., blade 22, includes an airfoil 23
mounted on a shank 25, which includes a platform 26. (Some of the
other detailed features of the rotor blades are not specifically
illustrated here, but can be found in various sources, e.g., U.S.
Pat. No. 6,506,016 (Wang), which is incorporated herein by
reference). Shank 25 includes a dovetail 27, for connection with
corresponding dovetail slots formed on rotor wheel 12.
[0035] Blade or bucket 22 includes axially projecting angel wings
33, 34, 50 and 90 (sometimes called "angel wing seals"), as
depicted in FIG. 1. The angel wings are typically integrally cast
with the blade. As described previously, they are generally in
opposing position to "lands" or discouragers 36 and 64, which
protrude from the adjacent nozzles 20 and 18, respectively. As one
example, discourager 64 is shown in an opposing, overlapping
position, relative to angel wing 90. The hot gas path in a turbine
of this type is generally indicated by arrow 38. It should be
understood that surfaces and other features described in these
figures are sometimes referenced in terms of the direction of hot
gas flow. For example, the "leading" edge of a feature usually
refers to the region that comes into initial contact with the hot
gas, while the "trailing" edge refers to a downstream region.
[0036] FIG. 2 is an enlarged view of a portion of the engine
depicted in FIG. 1, with emphasis on the general region featuring
first stage nozzle (stator) 18 and first stage rotor blade 22. (The
region can be referred to as the "stator-rotor assembly",
designated as element 21 in the figure). Nozzle 18 includes
discourager 58, and radial face 60, along with lower discourager
face 62.
[0037] It is evident from FIG. 2 that some of the portions of
nozzle 18 and blade 22 face each other in an interface region 92.
The facing surfaces are separated by at least one gap (two gaps are
shown here, as described below). Thus, upper gap 76 generally lies
between lower discourager face 62 and angel wing tip 74. Lower gap
77 generally lies between lower surface 69 of discourager 64 and
the tip 91 of angel wing 90. In this instance, gaps 76 and 77
generally define buffer cavity 80, and provide a pathway between
axial gap 78 and the "inboard" regions of the turbine engine, e.g.,
wheel-space region 82.
[0038] The term "interface region" is used herein to describe the
general area of restricted dimension which includes gaps 76 and 77,
along with the surrounding portions of nozzle 18 and blade 22. For
the purpose of general illustration, interface region 92 in FIG. 2
is shown as being bounded by dashed boundary lines 94 and 96. The
precise boundary for the interface region will vary in part with
the particular design of the stator-rotor assembly. One exemplary
manner in which to define a typical interface region would depend
on the length (viewed as "height" in FIG. 2) of rotor blade 22.
Thus, if the height of blade 22 within hot gas path 38 is
designated as "H", the interface region (upper boundary line 94)
can be estimated as extending from platform 26 up to about 10% of
height H. In terms of the "inboard" region of the stator-rotor
assembly (i.e., for lower boundary line 96), the interface region
can be estimated to extend that same length (about 10% of H) below
the lowest portion of the most inboard discourager, i.e., lower
angel wing 90. (Boundary line 96 would thus also always extend
across wheel space region 82 to include the lowest discourager on
the stator, i.e., discourager 64 in FIG. 2). The interface region
can often be referred to as a "flow-restriction" region.
[0039] In accordance with normal engine operation, combustion gas
being directed into the engine along hot gas path 38 flows aftward
through stator-rotor assembly 21, continuing through other
stator-rotor assemblies in the engine. (Technically, the combustion
gas should be referred to as "post-combustion" at this stage.
Moreover, it should be understood that the "hot gas" is often a
mixture of gases. While the mixture is usually dominated by
post-combustion gases, it may also include various coolant
injections and coolant flow, e.g. from nozzle 18 and/or from
coolant air stream 98, discussed below). As the hot gas stream
enters axial gap 78, a portion of the gas stream (dashed arrow 37)
may escape through upper gap 76 and flow into buffer cavity 80. (In
some extreme situations which would be very unusual, the hot gas
could continue to move through lower gap 77 and enter wheel-space
region 82). As mentioned above, coolant air, indicated by arrow 98,
is usually bled from the compressor (not shown), and directed from
the inboard region of the engine (e.g., wheel-space 82) into buffer
cavity 80, to counteract the leakage of hot gas. The deficiencies
which sometimes are present in such a gas flowpath system were
described previously.
[0040] FIG. 3 is an enlarged view of the endwall region 100,
featured in FIGS. 1 and 2. Surface 102 can be considered a
"platform" of first stage nozzle 18, but is referenced herein as
the top surface of inner circumferential endwall 104. The trailing
edge portion 106 of surface 102 usually terminates and forms a
relatively sharp, trailing edge 108 with radial face 60, as shown
in the figure. (As those skilled in the art understand, this region
may be provided with one or more protective coatings).
[0041] FIG. 4 is another view of an endwall region very similar to
that shown in FIG. 3, and representing a portion of a conventional
stator-rotor assembly. FIG. 4 is taken from a perspective that
depicts the upper surface region 102 and 106 of endwall 104. The
figure also depicts the relatively sharp, trailing edge 108, formed
by the junction of surface 106 and radial face 60, extending along
a width dimension 110 of nozzle 18. It appears that this sharp
trailing edge promotes the formation of a shear layer, resulting
from the interaction of high-speed flow in the hot gas path with
relatively low-speed flow in the wheelspace. The shear layer can be
unstable, leading to large flow oscillations into and out of the
wheelspace region. (It should be understood that, in some
embodiments of this invention, the cavities may also be placed on
the circumferential endwall(s) of rotating components, e.g., in a
location that is also sometimes referred to as the "blade
platform". These components may also be considered to be part of
the stator-rotor assemblies in turbomachines).
[0042] With continuing reference to FIG. 4 (and FIG. 5, discussed
below), at least a portion of the trailing edge 108 is provided
with a pattern of cavities. The cavities are capable of impeding
the entry of hot gas into the wheelspace region 82 that adjoins the
gap between the stator 18 and the rotor blade 22. Although the
inventors do not wish to be bound by any particular theory for this
phenomenon, it appears that the modified edge surface resulting
from the presence of the cavities inhibits shear layer instability
that would otherwise form above the trench cavity, aft of endwall
104 (FIG. 4). In this manner, it appears that the large flow
oscillations can be broken up into smaller (and weaker) shear
layers that have less of an influence on the gas flow into the
wheelspace region. The inhibition of such instability appears to be
especially significant in this particular region, where the flow of
hot gas at high speeds over the endwall occurs at a relatively high
angle, rather than occurring orthogonal to the gap 76.
[0043] As used herein, the term "cavity" is meant to embrace a
variety of depressions, indentations, channels, grooves, dimples,
pits, or any other type of discrete sinkhole. In some preferred
embodiments, each cavity has a curved inner surface. As described
below, the cavity may have a depth that is tapered along at least
one dimension.
[0044] FIG. 5 depicts a trailing edge 120, similar to that of edge
portion 106 in FIG. 4. Trailing edge 120 includes a pattern of
cavities 122 within endwall surface 130, each in the shape of a
partial cone. The depth and degree-of-curvature of each cone (which
may also be characterized as a "groove") can vary significantly, as
discussed below. In some preferred embodiments, the depth of cavity
122 is tapered. For example, the tapering can extend from a
relatively wide opening 124, at the trailing or "aft" section of
endwall 126, closest to the gap, to a relatively narrow top portion
128, that is usually flush with surface 130. The top portion is
upstream of opening 124 (relative to gas flow 38), and extends
farther along the surface 130 of endwall 126. The cavities are
usually (though not always) parallel to each other, relative to the
length-dimension discussed below. Moreover, in most embodiments,
the cavities can be thought of as elongate, relative to the
direction of hot gas flow.
[0045] FIG. 6 is a cross-sectional illustration of one of the
cavities 122 (FIG. 5), but is meant to provide guidance regarding
the possible depth for a variety of cavities of different shapes.
In FIG. 6, the depth "D" represents the depth of the cavity within
endwall 126. The depth can vary considerably, Factors which are
relevant to selection of optimum depth include the type and speed
of gas flow over the cavities (in one or more streams); the degree
to which gas flow should be restricted or otherwise disrupted; the
shape and size of the stator and/or rotor surfaces on which the
cavities are located; the manner in which the cavities are to be
formed; and the size of the local stator-rotor gap region.
Typically (though not always), the absolute depth of the cavity
will be greater for nozzles/stators in a land-based turbine, as
compared to those in an aviation turbine.
[0046] The cavity depth D is usually in the range of about 10% to
about 80% of the endwall depth ("EWD") shown in FIG. 5. In some
preferred embodiments, the depth is about 20% to about 50% of the
endwall depth, while in other preferred embodiments, the depth is
about 30% to about 70% of the endwall depth ("EWD"). The width of
the cavity, at its widest dimension, is usually about 50% to about
200% of its depth; and in some instances, about 100% to about 150%
of its depth.
[0047] In some instances, a typical range of cavity depth for a
land-based turbine would be in the range of about 1 mm to about 3
mm. In the case of an aviation turbine, the range may usually be in
the range of about 0.2 mm to about 1 mm. Those skilled in the art
will be able to select the most appropriate cavity depth for a
given situation, based on the factors mentioned above, as well as
fluid flow studies, discharge coefficient tests, computational
fluid dynamics predictions, and the like. A typical range for
cavity width is about 1 mm to about 10 mm.
[0048] FIG. 7 represents another perspective of cavity 122, along a
plane generally aligned with the path of hot gas 38. The figure is
meant to provide guidance, regarding the desired length "L" of the
cavity, extending from cavity opening 124 to the "upstream" end 132
of the cavity. In general, the cavity will have a length that is in
the range of about 50% to about 200% of endwall length ("EWL") 134,
as shown in the figure. In some specific embodiments, the length
will be about 25% to about 100% of the endwall length. In some
instances, a length for the cavity will be about 5 mm to about 20
mm, in the case of a land-based turbine; and about 2.5 mm to about
10 mm, in the case of an aviation turbine. Those skilled in the art
will be able to select the most appropriate length for the cavity,
based on the factors noted above for cavity depth. Usually, the
upstream end 132 of the cavity is flush with endwall surface
136.
[0049] As mentioned above, the cavities may be present in a variety
of shapes, as shown, for example, in FIG. 8. The figure is a
cross-sectional view similar to that of FIG. 6. In FIG. 8, cavity
140 has a relatively flat bottom surface 142. The sidewalls 144 can
be slanted, as shown, and the slanting angle can vary as well. (The
depth of the cavity can also vary, as in the embodiment of FIG. 6).
Furthermore, surface edges 146, shown as being relatively sharp,
could alternatively be somewhat rounded. The factors noted above
will influence the particular shape.
[0050] Additional, non-limiting examples of cavity shapes can be
provided. For example, FIG. 9 depicts a cavity 150, in which the
interior region 152 can generally be in the shape of a square or
rectangle. Alternatively, the interior region can be "bowl"-shaped,
with a curved bottom surface 154. In each instance, the cavity
terminates at a wall 156, which may be slanted or perpendicular. In
other words, the cavity's depth does not decrease (extending from
edge 157) to a point flush with endwall surface 158, as in other
embodiments.
[0051] The cavity 160 depicted in FIG. 10 can be in the shape of a
circular groove. As in the other embodiments, the depth, length,
and width of the cavity can vary considerably. Moreover, interior
end portion 162 can taper up to surface 164; or can terminate in a
wall, as in FIG. 9. The end portion is shown as being
triangle-shaped, but other shapes are possible as well. In some
preferred embodiments for this shape, the width "w" remains
constant, back to end portion 162. (This is in contrast to the
conical shape depicted in FIG. 5, wherein the width may
progressively narrow, away from the endwall edge).
[0052] The cavity 170 depicted in FIG. 11 can have a
substantially-square, "diffuser" shape, with an end portion 172
flush with surface 174. The interior side surfaces 176 of the
cavity can be in the form of corner fillets, as shown in the
figure. Again; the overall dimensions can vary, as in the other
embodiments.
[0053] In terms of cavity location, reference can be made to FIG.
4, wherein the cavities have not yet been incorporated. For
embodiments of the present invention, the cavities could be
incorporated along the entire circumferential length 110 of the
trailing edge. Alternatively, the cavities may be incorporated into
only a portion of the edge, or in selected regions along the edge.
The most appropriate arrangement of cavities can be determined
without undue effort, experimentally and/or by modeling. Factors
and techniques set forth above, such as flow coefficient tests,
will provide guidance in this respect. FIG. 12 provides an
illustration of a pattern of one type of cavities 180, incorporated
along the entire dimension of the trailing edge 182 of an endwall
184.
[0054] Moreover, the cavities may be in contact with each other,
e.g., where the edge of each cavity is in contact with an edge of
an adjacent cavity. Alternatively, the cavities may be spaced from
each other, depending on their shape, as well as the other factors
noted herein. The degree of spacing may thus vary according to many
of those same factors, including, of course, the shape of the
cavity. As a non-limiting illustration with reference to FIG. 5,
the spacing "a" between cavities may have a length that is about
10% to about 100% of the width "b" of a typical cavity. The cavity
spacing may be uniform or non-uniform.
[0055] Moreover, the actual border region between cavities need not
be a relatively flat ridge, i.e., resulting in a very discrete
border between each cavity. Reference is made to FIG. 13, which is
an enlarged view of the cavities depicted in FIG. 12. In this
embodiment, the sidewall 190 between each cavity 192 (i.e., at or
near the trailing edge) blends into the sidewall of an adjacent
cavity. This "continuous" feature may be advantageous in certain
embodiments, as compared to the more discrete cavity-boundary
depicted in FIG. 5.
[0056] While many of the primary embodiments are directed to the
use of cavities for endwall sections on stationary portions of the
stator-rotor assembly, the cavities may also be employed on some of
the rotating components. Various types of rotors may be modified in
this manner, e.g., unshrouded rotors; and those that have an
attached shroud. In each instance, the cavities, as described
previously, can be very useful for modifying the inner endwalls of
such components.
[0057] Another embodiment of the present invention is directed to a
turbomachine, which includes at least one stator-rotor assembly,
such as those described above. Gas turbine engines (e.g.,
turbojets, turboprops, land-based power generating turbines, and
marine propulsion turbine engines), represent examples of a
turbomachine. Other types are known in the art as well.
Non-limiting examples include a wide variety of pumps and
compressors, which also happen to incorporate a stator-rotor
assembly through which fluids (gas or liquid) flow. In many of
these other turbomachine designs, new techniques for reducing the
leakage of fluid from a flow path into other regions of the machine
would be of considerable interest. Thus, the stator-rotor
assemblies in any of these turbomachines could include patterns of
cavities as described in this disclosure.
[0058] Still another embodiment of this invention is directed to a
method for restricting the flow of gas (e.g., hot gas) through a
gap between a stator and rotor in a turbomachine. The method
includes the step of forming a pattern of cavities on at least a
portion of the trailing edge of the inner endwall of the stator
component. (The stator can be either a nozzle or vane, depending on
the intended use of the stator-rotor assembly). The cavities have a
shape and size sufficient to impede the entry of hot gas into a
wheelspace area that adjoins the gap between the stator and rotor.
(Those familiar with various types of turbomachines understand that
a large number of stator-rotor assemblies are typically present,
and each may include the modified endwalls described herein). The
method can also be used to form cavities on selected regions of a
rotating component in the assembly.
[0059] The cavities can be formed by a variety of methods.
Non-limiting examples include machining methods, such as various
milling techniques. Other machining processes which are possible
include electro-discharge machining (EDM) and electro-chemical
machining (ECM). In some cases, the cavities could be formed during
casting of the particular component, e.g., the investment-casting
of a turbine rotor or nozzle. As one example, an investment mold
surface could be provided with a selected pattern of positive
features, e.g., "mounds", domes, pyramids, pins, or any other type
of protrusions or turbulation. (Some of the methods for providing
these features to various surfaces are described in U.S. patent
application Ser. No. 10/841,366 (R. Bunker et al; issued as U.S.
Pat. No. 7,302,990), which is incorporated herein by reference).
The shape of the positive features would be determined by the
desired shape of the cavities, which would be inverse to the
positive feature. Thus, after removal of the mold, the part would
include the selected pattern of cavities. Those skilled in the art
will be able to readily determine the most appropriate technique
(or combination of techniques) for forming the cavities on a given
surface.
EXAMPLES
[0060] Computer models were generated to simulate the interaction
between a hot gas flow path and a coolant purge flow. The models
were based on the wheelspace cavity region of a stator-rotor
assembly, for upstream vanes. (This region is similar to the
general region in FIG. 2 that is collectively indicated by
wheel-space region 82, buffer cavity 80, upper gap 76, and axial
gap 78). The computer models were based in part on a transient
stator-rotor sliding mesh simulation on a periodic domain, using an
unsteady Reynolds Averaged Navier-Stokes (URANS) turbulence
model.
[0061] The hot gas flow path was traced, using a passive scalar.
The passive scalar simulated a tracing gas seeded at the flow path
inlet. A scalar concentration "C=1" represented 100% hot gas; while
"C=0" represented 100% coolant purge flow.
[0062] FIG. 14 is a depiction of the computer-assisted model for
numerical predictions of hot gas flow, purge flow, and flow
interactions, in the selected region of a stator-rotor assembly.
The "baseline" illustration on the left represents a stator-rotor
endwall that has not been modified. The illustration on the right
represents the modified endwall according to embodiments of this
invention, having a series of uniformly-spaced cavities similar to
those depicted in FIG. 5.
[0063] With continuing reference to FIG. 14, the lighter,
gray-scaled contour levels in the buffer cavity represent high
levels of hot gas. The boundary between the lighter-gray contours
and the solid black region provides an illustration of how a
significantly-smaller region inside the buffer cavity is exposed to
hot gases, when the modified endwall is employed, as compared to
the situation with no modification. The data for each figure are
based on a constant amount of cavity purge flow. The data show that
the level of hot gas ingress into the buffer cavity decreases
significantly when the endwall is modified, as shown by the scalar
concentration levels in the two illustrations of FIG. 14.
[0064] This invention has been described by way of specific
embodiments and examples. However, it should be understood that
various modifications, adaptations, and alternatives may occur to
one skilled in the art, without departing from the spirit and scope
of the claimed inventive concept. All of the patents, articles, and
texts mentioned above are incorporated herein by reference.
* * * * *