Note: Descriptions are shown in the official language in which they were submitted.
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TURBINE AIRFOIL TRAILING EDGE COOLING SLOT
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
[0001] The present invention relates generally to gas
turbine engine turbine airfoil cooling and, more
specifically, to turbine airfoil trailing edge cooling
slots.
DESCRIPTION OF RELATED ART
[0002] In a gas turbine engine, air is pressurized in
a compressor and mixed with fuel in a combustor for
generating hot combustion gases. The hot gases are
channeled through various stages of a turbine which
extract energy therefrom for powering the compressor and
producing work, such as powering an upstream fan in a
typical aircraft turbofan engine application.
[0003] The turbine stages include stationary turbine
nozzles having a row of hollow vanes which channel the
combustion gases into a corresponding row of rotor blades
extending radially outwardly from a supporting rotor
disk. The vanes and blades have corresponding hollow
airfoils with corresponding cooling circuits therein.
[0004] The cooling air is typically compressor
discharge air which is diverted from the combustion
process and, therefore, decreases overall efficiency of
the engine. The amount of cooling air must be minimized
for maximizing the efficiency of the engine, but
sufficient cooling air must nevertheless be used for
adequately cooling the turbine airfoils for maximizing
their useful life during operation. Each airfoil
includes a generally concave pressure sidewall and, an
opposite, generally convex suction sidewall extending
longitudinally or radially outwardly along a span from an
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airfoil base to an airfoil tip and axially in a chordwise
direction between leading and trailing edges. For a
turbine blade, the airfoil span extends from a root at
the radially inner platform to a radially outer tip
spaced from a surrounding turbine shroud. For a turbine
vane, the airfoil extends from a root integral with a
radially inner band to a radially outer tip integral with
an outer band.
[0005] Each turbine airfoil also initially increases
in thickness aft of the leading edge and then decreases
in thickness to a relatively thin or sharp trailing edge
where the pressure and suction sidewalls join together.
The wider portion of the airfoil has sufficient internal
space for accommodating various forms of internal cooling
circuits and turbulators for enhancing heat transfer
cooling inside the airfoil, whereas, the relatively thin
trailing edge has correspondingly limited internal
cooling space.
[0006] Each airfoil typically includes various rows of
film cooling holes extending through the sidewalls
thereof which discharge the spent cooling air from the
internal circuits. The film cooling holes are typically
inclined in the aft direction toward the trailing edge
and create a thin film of cooling air over the external
surface of the airfoil that provides a thermally
insulating air blanket for additional protection against
the hot combustion gases which flow over the airfoil
surfaces during operation.
[0007] The thin trailing edge is typically protected
by a row of trailing edge cooling slots or a single
elongated slot which breach the pressure sidewall at a
breakout immediately upstream of the trailing edge for
discharging film cooling air thereover. Each trailing
edge cooling slot has an outlet aperture in the pressure
side which begins at a breakout and may or may not be
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bounded in the radial direction by exposed lands at aft
ends of axially extending partitions which define the
cooling slots.
[0008] The axial partitions may be integrally formed
with the pressure and suction sides of the airfoil and
themselves must be cooled by the air discharged through
the cooling slots defined thereby. The partitions
typically converge in the aft direction toward the
trailing edge so that the cooling slots diverge toward
the trailing edge with a shallow divergence angle that
promotes diffusion of the discharged cooling air with
little, if any, flow separation along the sides of the
partitions.
[0009] Aerodynamic and cooling performance of the
trailing edge cooling slots is directly related to the
specific configuration of the cooling slots and the
intervening partitions. The flow area of the cooling
slots regulates the flow of cooling air discharged
through the cooling slots and the geometry of the cooling
slots affects cooling performance thereof.
[0010] The divergence or diffusion angle of the
cooling slots can effect undesirable flow separation of
the discharged cooling air which would degrade
performance and cooling effectiveness of the discharged
air. This also increases losses that negatively impact
turbine efficiency. Portions of the thin trailing edge
directly under the individual cooling slots are
effectively cooled by the discharged cooling air, with
the discharged air also being distributed over the
intervening exposed lands at the aft end of the
partitions. The lands are solid portions of the pressure
sidewall integrally formed with the suction sidewall and
must rely for cooling on the air discharged from the
adjacent trailing edge cooling slots.
[0011] Notwithstanding, the small size of the these
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outlet lands and the substantial cooling performance of
the trailing edge cooling slots, the thin trailing edges
of turbine airfoils nevertheless, typically, limit the
life of those airfoils due to the high operating
temperature thereof in the hostile environment of a gas
turbine engine.
[0012] Accordingly, it is desired to provide a turbine
airfoil having improved trailing edge cooling and cooling
slots for improving airfoil durability and engine
performance. It is also desired to minimize the amount
of cooling flow used for trailing edge cooling in order
to maximize fuel efficiency of the turbine and the
engine.
SUMMARY OF THE INVENTION
[0013] A gas turbine engine turbine airfoil includes
widthwise spaced apart pressure and suction sidewalls
extending outwardly along a span from an airfoil base to
an airfoil tip and extending in a chordwise direction
between opposite leading and trailing edges. A spanwise
row of spanwise spaced apart trailing edge cooling holes
encased in the airfoil between the pressure and suction
sidewalls end at a single spanwise extending trailing
edge cooling slot extending chordally substantially to
the trailing edge. Each of the cooling holes includes in
downstream serial cooling flow relationship, a curved
inlet, a metering section with a constant area and
constant width flow cross section, and a spanwise
diverging section leading into the trailing edge cooling
slot. Axial partitions extend chordally between and
radially separate the cooling holes along the span. Aft
ends of the partitions include swept boat tails.
[0014] The boat tails may be swept with each of the
boat tails including a boat tail trailing edge having an
apex spanwise located between the pressure and suction
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sidewalls. The boat tail trailing edge sweeps aftwardly
or downstream from the apex. The boat tail trailing edge
sweeps from the apex spanwise or radially outwardly to
the pressure sidewall and inwardly to the suction
5 sidewall from the apex. The swept boat tails may further
include rounded cross sections through the aft ends of
the partitions between spanwise pairs of adjacent cooling
holes.
[0015] The pressure and suction sidewalls may include
pressure and suction sidewall surfaces respectively in
the hole and the pressure sidewall surface may be planar
through the entire metering and diverging sections. The
width may be constant through the metering and diverging
sections of the hole.
[0016] The airfoil may include a deck in the slot
extending chordwise or downstream from the diverging
sections of the cooling holes substantially to the
airfoil trailing edge and extending spanwise or radially
outwardly from a bottommost one to a topmost one of the
trailing edge cooling holes. Upper and lower deck
sidewalls spanwise bound the deck and extend from the
deck to an external surface of the pressure sidewall.
Fillets in slot corners between the upper and lower deck
sidewalls and the deck have fillet radii substantially
the same size as bottom corner radii of the flow cross
section of the diverging sections adjacent the bottom
corner radii.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The foregoing aspects and other features of the
invention are explained in the following description,
taken in connection with the accompanying drawings where:
[0018] FIG. 1 is a longitudinal, sectional view
illustration of an exemplary embodiment of turbine vane
and rotor blade airfoils having cooling holes culminating
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at a spanwise extending trailing edge cooling slot.
[0019] FIG. 2 is an enlarged view illustration of a
blade illustrated in FIG. 1.
[0020] FIG. 3 is a pressure side sectional view
illustration of the cooling holes with constant width
metering and diffusing sections leading into the trailing
edge cooling slot illustrated in FIG. 2.
[0021] FIG. 4 is a cross sectional schematical view
illustration of the cooling holes with constant width
metering and diffusing sections leading into the trailing
edge cooling slot taken through 4-4 in FIG. 3.
[0022] FIG. 5 is an upstream looking perspective view
illustration of the cooling holes and the trailing edge
cooling slot illustrated in FIG. 3.
[0023] FIG. 6 is an enlarged perspective view
illustration of boat tail aft ends of axial partitions
between the cooling holes illustrated in FIG. 5.
[0024] FIG. 7 is a cross sectional schematical view
illustration of an elongated flow cross section in the
constant width metering section taken through 7-7 in FIG.
3.
[0025] FIG. 8 is a cross sectional schematical view
illustration of an elongated flow cross section in the
diffusing section taken through 8-8 in FIG. 3.
[0026] FIG. 9 is a cross sectional schematical view of
a race track shaped flow cross section having four equal
radii.
[0027] FIG. 10 is a cross sectional schematical view
of an alternative race track shaped flow cross section
with a larger width to height ratio than the race track
shaped flow cross section illustrated in FIG. 9.
[0028] FIG. 11 is a cross sectional schematical view
of an alternative flow cross section with unequal top and
bottom corner radii.
[0029] FIG. 12 is a cross sectional schematical view
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of another alternative flow cross section with in
elongated and fully curved and includes curved quarter
sides.
DETAILED DESCRIPTION OF THE INVENTION
[0030] Illustrated in FIG. 1 is an exemplary gas
turbine engine high pressure turbine stage 10
circumscribed about an engine centerline axis 8 and
positioned between a combustor 20 and a low pressure
turbine (LPT) 24. The combustor 20 mixes fuel with
pressurized air for generating hot combustion gases 19
which flows downstream D through the turbines.
[0031] The high pressure turbine stage 10 includes a
turbine nozzle 28 upstream of a high pressure turbine
(HPT) 22 through which the hot combustion gases 19 are
discharged into from the combustor 20. The exemplary
embodiment of the high pressure turbine 22 illustrated
herein includes at least one row of circumferentially
spaced apart high pressure turbine blades 32. Each of
the turbine blades 32 includes a turbine airfoil 12
integrally formed with a platform 14 and an axial entry
dovetail 16 used to mount the turbine blade on a
perimeter of a supporting rotor disk 17.
[0032] Referring to FIG. 2, the airfoil 12 extends
radially outwardly along a span S from an airfoil base 34
on the blade platform 14 to an airfoil tip 36. During
operation, the hot combustion gases 19 are generated in
the engine and flow downstream D over the turbine airfoil
12 which extracts energy therefrom for rotating the disk
supporting the blade for powering the compressor (not
shown). A portion of pressurized air 18 is suitably
cooled and directed to the blade for cooling thereof
during operation.
[0033] The airfoil 12 includes widthwise spaced apart
generally concave pressure and convex suction sidewalls
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42, 44. The pressure and suction sidewalls 42, 44 extend
longitudinally or radially outwardly along the span S
from the airfoil base 34 to the airfoil tip 36. The
sidewalls also extend axially in a chordwise direction C
between opposite airfoil leading and trailing edges LE,
TE. The airfoil 12 is hollow with the pressure and
suction sidewalls 42, 44 being spaced widthwise or
laterally apart between the airfoil leading and trailing
edges LE, TE to define an internal cooling cavity or
circuit 54 therein for circulating pressurized cooling
air or coolant flow 52 during operation. The pressurized
cooling air or coolant flow 52 is from the portion of
pressurized air 18 diverted from the compressor.
[0034] The turbine airfoil 12 increases in width W or
widthwise from the airfoil leading edge LE to a maximum
width aft therefrom and then converges to a relatively
thin or sharp airfoil trailing edge TE. The size of the
internal cooling circuit 54 therefore varies with the
width W of the airfoil, and is relatively thin
immediately forward of the trailing edge TE where the two
sidewalls integrally join together and form a thin
trailing edge portion 56 of the airfoil 12. A spanwise
extending trailing edge cooling slot 66 is provided at or
near this thin trailing edge portion 56 of the airfoil 12
to cool it.
[0035] Referring to FIGS. 3 and 5, a spanwise row 38
of spanwise spaced apart trailing edge cooling holes 30
encased or buried and formed in the airfoil 12 between
the pressure and suction sidewalls 42, 44 end at the
trailing edge cooling slot 66. The trailing edge cooling
slot 66 extends chordally substantially to the trailing
edge TE. The trailing edge cooling holes 30 are disposed
along the span S of the trailing edge TE in flow
communication with the internal cooling circuit 54 for
discharging the coolant flow 52 therefrom during
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operation. A floor or deck 130 of the slot 66 extends
chordwise or downstream from the diverging sections 102
of the cooling holes 30 substantially to the airfoil
trailing edge TE. The deck 130 extends spanwise or
radially outwardly from a bottommost one 132 to a topmost
one 134 of the trailing edge cooling holes 30. The deck
130 is spanwise bound by upper and lower deck sidewalls
136, 138 which extend from the deck 130 to an external
surface 43 of the pressure sidewall 42. A slot surface
60 extends widthwise between the upper and lower deck
sidewalls 136, 138 along the deck 130. Fillets 62 in
slot corners 64 between the upper and lower deck
sidewalls 136, 138 and the deck 130 have fillet radii RF
that may be substantially the same size as bottom corner
radii RT of the flow cross section 74 of the diverging
sections 102 adjacent the bottom corner radii RT
(illustrated in FIGS. 7-10). The fillet radii RF helps
with castability of the trailing edge cooling slot 66.
[0036] The trailing edge cooling holes 30 are
illustrated in more particularity in FIG. 3. Each
cooling hole 30 includes, in downstream serial cooling
flow relationship, a downstream converging or bellmouth
shaped curved inlet 70, a constant area and constant
width flow cross section metering section 100, and a
spanwise diverging section 102 which leads into the
trailing edge cooling slot 66 and supplies the slot with
cooling air or coolant flow 52. Referring to FIGS. 4-6,
the trailing edge cooling slot 66 begins at a breakout 58
located at downstream ends 69 of the diverging sections
102. The diverging sections 102 of the cooling holes 30
lead into the trailing edge cooling slot 66 which
breaches the external surface 43 of the pressure sidewall
42 at a breakout lip 49 spaced forward or upstream from
the trailing edge TE.
[0037] Referring to FIGS. 3 and 5, the cooling holes
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30 are separated radially along the span S from each
other by corresponding axial partitions 68 which extend
downstream toward the trailing edge TE. The curved inlet
70 is illustrated herein as downstream converging or,
5 more particularly, a bellmouth inlet. The inlet 70 is
defined at and between forward ends 72 of the partitions
68. The partitions 68 include semi-circular forward ends
72 having diameters 73 that define the bellmouth inlet
70. Each of the cooling holes 30 includes spanwise
10 spaced apart upper and lower hole surfaces 46, 48 along a
corresponding adjacent pair of upper and lower ones 25,
26 of the axial partitions 68. A spanwise height H of
the hole 30 is defined between the upper and lower hole
surfaces 46, 48 of the upper and lower ones 25, 26 of the
axial partitions 68 as illustrated in FIG. 3. The
metering section 100, the diverging section 102, and the
trailing edge cooling slot 66 have downstream extending
first, second, and third lengths Li, L2, and L3
respectively as illustrated in FIG. 3.
[0038] Referring
to FIGS. 3, 5, and 6, aft ends 86 of
the partitions 68 have aerodynamically-shaped swept boat
tails 88 design and shaped to reduce aerodynamic losses
due to flow separation wakes at the aft ends 86. The
swept boat tails 88 are also designed to facilitate flow
spreading past the slot breakout 58 at the downstream end
69 of the diverging section 102. Each of the swept boat
tails 88 include a boat tail trailing edge 90 extending
spanwise between the pressure and suction sidewalls 42,
44 and having an apex 92 spanwise located between the
breakout lip 49 or the pressure sidewall 42 and the
suction sidewall 44 as illustrated in FIG. 6. The boat
tail trailing edge 90 sweeps aftwardly or downstream from
the apex 92. The boat tail trailing edge 90 sweeps from
the apex 92 spanwise or radially outwardly to the
breakout lip 49 or the pressure sidewall 42 and inwardly
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to the suction sidewall 44 from the apex 92. Each of the
swept boat tails 88 includes rounded cross sections 96
through the aft ends 86 of the partitions 68 between
spanwise pairs 94 of adjacent cooling holes 30. The boat
tail trailing edges 90 provide additional strength at the
breakout lip 49 and merges the flows of the different
cooling holes 30 at the floor or deck 130 upstream of the
breakout 58 which is an exit of the cooling holes 30.
[0039] The cooling holes 30, the trailing edge cooling
slot 66, and the swept boat tails 88 are designed to
provide a spanwise deck 130 film effectiveness over the
entire slot deck 130 all the way downstream or aft to the
terminus of the deck 130 the airfoil trailing edge TE,
even at significantly reduced cooling flow. Airfoil
cooling design studies have shown a potential cooling
flow reduction of about 10 percent of stage 1 blade flow.
The study also indicated at the same time, trailing edge
temperatures are still 80 to 90 degrees F lower that more
conventional slot designs, so further flow reductions are
possible. This is a significant benefit to engine
performance.
[0040] Referring to FIGS. 3-5, a hole width W of the
hole 30 is defined between pressure and suction sidewall
surfaces 39, 40 of the pressure and suction sidewalls 42,
44 respectively as illustrated in FIG. 4. The trailing
edge cooling slot 66 and the deck 130 are open and
exposed to the hot combustion gases 19 that pass through
the high pressure turbine 22. The deck 130 extends for
the entire third length L3 along the suction sidewall 44.
[0041] The adjacent pair of upper and lower ones 25,
26 of the axial partitions 68 and the pressure and
suction sidewalls 42, 44 spanwise bound the hole 30.
Referring to FIGS. 7 and 8, the cooling hole 30 has a
generally spanwise elongated flow cross section 74 and
the spanwise height H is substantially greater than the
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hole width W. The cooling hole 30 has a height to width
ratio H/W in a range of about 2:1 to 10:1 (see FIGS. 7-
12). The pressure and suction sidewall surfaces 39, 40
of the pressure and suction sidewalls 42, 44 respectively
widthwise bound the hole 30.
[0042] The embodiment of the cooling hole 30
illustrated in FIG. 4 has a fixed or constant width W
through the cooling hole 30 and the pressure and suction
sidewall surfaces 39, 40 are parallel through the entire
first and second lengths Li, L2 of the cooling hole 30.
The pressure sidewall surface 39 is flat or planar
through the entire metering and diverging sections 100,
102 and their corresponding first and second lengths Li,
L2 of the cooling hole 30. In this embodiment of the
cooling hole 30, the suction sidewall surface 40 is flat
or planar through the entire metering and diverging
sections 100, 102 and their corresponding first and
second lengths Li, L2 of the cooling hole 30. The deck
130 is coplanar with suction sidewall surface 40 in the
hole 30. The inlet 70, the metering section 100, and the
diverging section 102 have the same hole width W or are
of constant width W in the embodiment of the trailing
edge cooling holes 30 illustrated in FIG. 3 and
schematically illustrated in solid line in FIG. 4. The
diverging section 102 diverges in a spanwise direction.
[0043] The cooling holes 30 and trailing edge cooling
slot 66 are cast in cooling features. Casting these
features provides good strength, low manufacturing costs,
and durability for the airfoil and blades and vanes. The
race track shaped flow cross section 74 with the
rectangular section 75 between spanwise or radially
spaced apart rounded or semi-circular inner and outer end
sections 82, 84 provides good cooling flow
characteristics which reduces the amount of the coolant
flow 52 needed to cool the airfoils. The bottom corner
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radii RT contribute to good cooling, castability, and
strength of these cooling features.
[0044] Four exemplary shapes suitable for the flow
cross section 74 are illustrated in FIGS. 9-12. The race
track shaped flow cross section 74 illustrated in FIG. 9
is spanwise elongated, has four equal corner radii R, and
has a width to height ratio W/H in a range of 0.25-0.50.
The race track shaped flow cross section 74 illustrated
in FIG. 10 is spanwise elongated, has four equal corner
radii R, and has a width to height ratio W/H in a range
of 0.15-0.50. The race track shaped flow cross section
74 illustrated in FIG. 11 is similar to the one
illustrated in FIG. 9 but has unequal top and bottom
corner radii RT, RB. An exemplary range of a corner
ratio RB/RT is 1-3. The race track shaped flow cross
section 74 illustrated in FIG. 12 is spanwise elongated
and fully curved and includes curved quarter sides 78
that may be elliptical, parabolic, or polynomial blends.
[0045] The spanwise elongated metering section 100
with the constant width W is sized to control the
quantity of coolant flow 52 to benefit the engine cycle.
The spanwise elongated metering section 100 and
diverging section 102 expand the flow coverage at the
breakout 58, evenly distributes coolant flow 52 in the
trailing edge cooling slot 66. The constant width W
metering section 100 upstream of the diverging section
102 of the hole 30 helps keep the coolant flow 52 fully
attached in the diverging section 102.
[0046] The constant width and separately the planar
pressure sidewall surface 39 of the cooling hole 30 helps
keep a coolant velocity of the coolant flow 52 and a gas
velocity of the hot combustion gases along the external
surface 43 of the pressure sidewall 42 at the breakout
about equal to minimize aero losses which could result in
a negative effect on turbine efficiency. These two
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features also help keep the coolant flow 52 flow attached
in the slot 66.
[0047] The present invention has been described in an
illustrative manner. It is to be understood that the
terminology which has been used is intended to be in the
nature of words of description rather than of limitation.
While there have been described herein, what are
considered to be preferred and exemplary embodiments of
the present invention, other modifications of the
invention shall be apparent to those skilled in the art
from the teachings herein and, it is, therefore, desired
to be secured in the appended claims all such
modifications as fall within the true spirit and scope of
the invention.
[0048] Accordingly, what is desired to be secured by
Letters Patent of the United States is the invention as
defined and differentiated in the following claims:
