Note: Descriptions are shown in the official language in which they were submitted.
J~ 3
BACKGROUNI) C:)F TlHE_INVENTION
This invention relates to gas turbines and, more particularly,
to a concept for efficiently cooling ultrahigh temperature t-urbine rotor blades.
It is well understood that gas turbine engine shaft horsepower
and specific fuel consumption, which is the rate of fuel consumption per unit
of power output, can be improved by increasing turbine inlet temperatures.
However, current turbines are limited in inlet temperature by the physical
properties of their materials. To permit turbines to operate at gas stream
temperatures which are higher than the materials can normally tolerate, con-
siderable effort has been devoted to the development of sophisticated methods
of turbine cooling. In early turbine designsJ cooling of high temperature
components was limited t~ transferring heat to lower temperature parts by
conduction, and air cooling was limited to passing relatively cool air across
the face of the turbine rotor disk~.
In order to take advantage of the potential performance
improvements associated with even higher turbine inlet temperatures, modern
turbine cooling technolo~y utilizes hollow turbine nozzle vanes and blades to
permit operation at inlet gas temperatures in the 2(100 to 2300 F (1094 to
1260 C) range. Various techniques have been devised to air cool these hollow
blades and vanes, These incorporate three basic forms of air cooling, either
singly or in combination, depending on the level of gas temperatures encoun-
tered and the degree of sophistication permissi~le. These basic forms of air
cooling are known as convection, impingement and film cooling. U. S. Patent
Nos. 3, 700, 348 and 3, 715,170, assigned to the assignee of the present
invention, are excellent examples of advanced turbine air-cooling technology
incorporating these basic air-cooling forms.
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However, the benefits obtained from sophisticated air-cooling
techniques are at least partially offset by the extraction of the necessary
cooling air from the propulsive cycle. For example, probably the most
popular turbine coolant today is air which is bled off of the co~npressor portion
5 of the gas turbine engine and is routed to the hollow interior of the turbine
blades. Typically, the work which has been done on this air by the compressor
is partially lost to the cycle. Additionally, as the cooling air circulates
throughout the turbine blade it picl~s up heat Erom the metallic blades or vanes.
When this heated cooling air leaves the turbine blades, perhaps as a coolant
10 film, this heat energy is lost lo the cycle since the hot gases are normally
mixed with the exhaust gases and ejected from an engine nozzle. It would be
desirable, therefore? to have a cooling system wherein a medium other than
compressor bleed air is used and wherein the heat extracted by the cooling
medium is put back into the cycle in a useful and practical manner.
- 15 A partial solution to the foregolng problems has been the
suggestion of closed-loop cooling system~ for turbine blades which may or
may not also incorporate the concept o~ regeneration or recuperation to
; recover lost thermal energy. One such cooling arrangement which has been
proposed, for example, is that of U. S. Patent 2, 7~2, 000. In that patent, a
20 closed-system steam thermosiphon is used to cool the turbine blades, the
thermosiphon principle being that by~which`a coolant is caused to circulate
throughout the hollow bores of a turbine blade under the pump~ng~ action of
centrifugal force due to the difference in density between the heated coolant
(steam) exiting the blade and the coolant (steam or water) entering it. Each
25 blade is provided with its own thermosiphon which is associated with a cooler
or heat exchanger which, in turn, is cooled by a second cooling medium such
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as water or air, However, the difficulties in fabricating the æystem,
particularly the proposed fin-type heat exchangers, would preclude its practicalapplication in aircraft gas turbines. Furthermore, the system re~uires the use
of water (which is not normally available in an aircraft gas turbine engine
environment) or compressor bleecl air (with the same disadvantages discussed
above) Eor the secondary coo]ant.
Another arrangement utilizing the closed-loop thermosiphon
principle is that taught in U. S. Patent 2, 778, 601 wherein hollow turbine blades
are connected, via radially extending passages through the turbine disk, to a
common, manifolded, fluid reservoir comprising a hollow turbine shaft and a
coannular hollow feed tube. The disadvantages of this arrangement include the
necessity of providing a pair of coannular hollow members for a single turbine
stage. Since modern gas turbofan engines already incorporate as many as
three coannular sha-fts without closed-loop thermosiphon cooling, the complex-
ities oE adding additional shafting for this purpose would be forn~idableindeed.Also, since all of the blades are manifolded at a common supply, a leak in one
blade would r esult in a coolant loss in aLI blades. Still further, the disk
passages tend to degrade the disk structural integrity, an important considera-
tion in aircraft gas turbine engine design, and tend to increase its cost.
U. S. Patent 2, 849, 210 teaches a turbine using the closed-loop
thermosiphon principle wherein the hollow interiors are fluidly connected to an
annular condensing chamber near the turbine disk bore by a plurality of tubes
extending down one side of the disk between a manifolded vaporizing chamber
associated with the blades and the manifolded condensing chamber, However,
the loss of coolant from one blade would result in a loss of cooling capability
for all blades. Also, recommended coolants are water, sodium, sulphur,
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potassiurn, mercury and fluorocarbons, none of which are normally available
in Kas turhine enKines. Thus, additional weight would be added merely or the
coolant, and it must be remembered that weight is another important consid-
eration in aircraft gas turbine engine design. In an alternative embodirnent,
the patent teaches that the external coolant may be fuel or some of the
compressed air which is fed to the engine cornbustor, in which instances at
least a portion of the heat extracted from the turbine buckets would be returnedto the gas turbine engine as usable energy. However, cooling a turbine blade
by routing fuel through its hollow interior presents a potential fire hazard, and
the use Or compressor discharge air has the inherent disadvantages described
above which the closed-loop system is intended to avoid.
Yet another cooling arrangement, for example, is that proposed
in U. S. Patent 2, 883,151, In that patent the hollow turbine blades are again
cooled by the closed-loop s$eam thermosiphon principle, with the rotor blade
interiors communicating with longitudinally extending, coannular passages
within the rotatable turbine shaft via radially extending passages through the
turbine disc, The thermosiphon, in turn, is cooled by fuel circulating through
another pair of coannular passages in a stationary stub shaft within the rotating
shaft, Heat transfer is by convection between the rotating and stationary sha~ts.
- 20 The difficulty of fabricating double coannular shafts is not insignificant, Other
disadvantages which have been mentioned with regard to the previous systems
include the manifolding of all blades to a common coolant supply, the proximity
of fuel to the rotating hot turbine, and the degradation of integrity due to the- radially extending bores therein.
One more arrangement, for example, is that of U, S. Patent
3, 756, 020, wherein the turbine of a regenerative engine is cooled by a
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closed-system thermosiphon which passes through heat exchangers. These,
in turn, are cooled by either fuel or air which are then routed to the combustor.
However, the individual blade cooling circuits pass through the disc and are
manifolded at the heat exchanger, thereby presenting disadvantages enumerated
above.
It will therefore be appreciated that although the concepts of
turbine cooling by the thermosiphon principle and regenerative engines are not
new per se, a need exists for applying these concepts to an aircraft gas turbineengine in an efficient and reliakle manner.
S~MMARY OF THE INVENTION
Accordingly, it is the prirnary object of the present invention to
provide an aircraft gas turbine engine in which the turbine rotor blades are
cooled to withstand the high temperature gases of cornbustion,
It is another object of the present invention to provide a turbine
wherein the rotor blades are individually cooled by the closed-system thermo-
siphon principle.
It is another object of the present invention to provide an air-
craft gas turbine engine wherein the heat is extracted from the rotor blades
-~ and returned to the engine power cycle as useful energy.
It is still another object of the present invention to provide a
cooled turbine rotor wherein the cooling system does not disrupt the structural
integrity of the turbine disc or rotor.
It is still another object of the present invention to provide a
cooled turbine rotor wherein the hazards of fire due to the coolant medium
are minimized.
It is yet another object of the present invention to provide a
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cooled turbine rotor wherein reliance upon heat transfer between relatively
moving components is eliminated.
Yet another object of the present invention is to provide an
aircraft gas turbine engine with a cooled turbine rotor wherein the turbine
cooling system is located at the minimum possible radius so that it does not
adversely affect the starting balance of the rotor.
These, and other objects and advantages, will be more clearly
understood from the following detailed descriptions, the dr~wings and specific
examples, all of which are intended to be typical of, rather than in any way
10 limiting on, the scope of the present invention.
Briefly stated, the above objectives are attained in an aircraft
gas turbine engine by providing a turbine wherein the rotor disc bears a
plurality of hollow turbine blades. These blades are each coolFd internally by
the circulation of steam in a closed-loop, single-phase thermosiphon system.
15 Each blade is provided with an individual steam circuit which includes a
passageway uniquely attached to the outer faces of the turbine rotor disk, each
closed-loop passageway passing through a rotating heat exchanger which, in
one embodiment, is borne by an engine lubrication duct, The heat absorbed
from the blades by the steam is extracted by the engine lubricating oil which is20 routed from within the duct and through the heat exchanger into heat exchangerelationship with the thermosiphon loop passageways. The lubricating oil is
that which provides the usual engine lubrication function so that an additional
coolant need not be carried by the aircraft. Subsequently, this oil is cooled byengine fuel or the fan bypass stream airflow (in a gas turbofan engine) in a
25 stationary heat exchanger relatively remote from the turbine. The use of the
fuel as the final heat sink produces a regenerative engine in that at least a
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portion of the heat removed from the turbine is reintroduced intothe engine
cycle as heated engine fuel, The effects of leakage are minimized since each
blade has associated therewith its own indi~idual closed-loop thermosiphon
system and fuel is not used as the intermediate coolant.
In the event that a leak should develop in a blade or in a steam
circuit which would eventually jeopardize the structural integrity of the blade,a secondary coolant system is provided. In the secondary coolant system,
compressor bleed air is routed to the hollow interior of each blade, but
compressor bleed airflow therethrough is normally precluded by a plug filling
the entrance to the hollow blade interior. If the blade temperature should rise
due to the unlikely loss of steam thermosiphon cooling effectiveness, the plug,
having a lower melting temperature than the blade, would melt and cooling air
would be forced into the blade hollow interior. Similar plugs through the blade
airfoil portion would also melt to permit the coolant air to exit from the bladeinterior, thereby providing a through-flow of cooling air which, though probablyinsufficient to maintain an indefinite blade life capability, would sui~ice to
sustain an aircraEt mission until rnaintenance could be performed. The turbine
rotor assembly which comprises the blades, disk, steam thermosiphon pas~age-
ways and heat exchanger is designed in such a manner that it can be installed
in and removed from the engine without rupturing the sealed steam thermo-
siphon system,
BBIEF DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims particularly
poiNting out and distinctly claiming the subject matter which is regarded as
part of the present invention, it is believed that the invention will be more
fully understood from the following description of the preferred embodiments
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which are given by way of example with the accompanying drawings in which:
Figure 1 is a partial cross-sectional view of an aircraft gas
turbojet engine incorporating the subject invention and illustrating schemati-
cally the relationship of various engine systems;
Figure 2 is a partial cross-sectional view, similar to Figure 1,
depicting the adaptation of the subject invention to an aircraft gas turbofan
engine of the dual~spool variety;
Figure 3 is an enlarged cross-sectional view of the turbine
portion of the engine of Figure 1 depicting the closed-loop thermosiphon
cooling system of the present invention in greater detail;
Figure 4 is an enlarged cutaway view of the internal cooling
circuitry of a turbine blade of Figure 3;
Figure S illustrates an enlarged sector of the turbine as viewed
along line 5-5 of :Eiigure 3, and depicts in more particularity the routing of the
coolant passageways along the sides of the turbine disk;
Figure 6 is a view, similar to Figure 3, illustrating an embodi-
ment of the subject invention incorporating a secondary coollng system;
. Figure 7 is a view, siInilar to Figure 4, showing the turbine";
blade internal cooling circuitry in the embodiment of Figure 6;
Figure 8 is an enlarged isometric view of a means for attaching
the thermosiphon cooling passageways to the sides of the turbine disk of Figure
3;
Figure 9 is an exploded view of the attaching means of Figure 8;
Figure 10 is a cross-sectional view of the attaching means
taken along line 10-10 of Figure 8;
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Figure ll is a plan view, partially in cross-section, depicting
the attaching means of Figure 8 in greater detail;
Figure 12 is a view, similar to Figure 8, of an alternative
embodiment of the attaching means; and
Figure 13 is an exploded view of the attaching means of Figure
12.
DESCRIPTION OF THE PREFERRED E MBODIMENT
Referring to the drawings wherein like numerals correspond to
like elements throughout, attention is first directed to Figure l wherein an
aircraft gas turbojet engine depicted generally at 20 and embodying the present
invention is diagrammatically shown. While it is recognized that turbojet
engines are, by now, well known in the art, a brief description of the operationof the engine will enhance appreciation of the interrelationship of the various
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components in light of the ir.vention soon to be described. Basically, the
engine may be considered as comprising an axial flow compressor 22 having a
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rotor 24 bearing a plurality of rotatable blade rows 26 (rotors) and a compres-
sor caslng 28 bearing a~ plurality of stationary blade rows 30 ~stators) interposed
in alternating relationship with the rotor blade rows 26. Air enters compressor
. .
inlet 32, is compressed by the axial flow compressor 22, and then is discharged
to a combustor 34 where it is mixed with fuel, supplied by means such as
aircraft fuel tank 35 and an engine fuel control 36 of a type well known in the
art and responsive to pilot throttle inputs, and burned to provide high energy
combustion gases which drive a turbine rotor 38. Turbine rotor 38 comprises
a turbine clisk 40 bearing a plurality of turbine rol:or blades 42, only one of
which is shown for clarity, and drives, in turn, the compressor rotor 24
- through interconnecting shaft 43 in the usual manner of a gas turbine engine.
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A stationary row of turbine nozzle vanes 44 orient the flow into the rotating
turbine blades. A propulsive force, to the left in Figure 1, is thus obtained bythe discharge of combustion gases from a representative nozzle 46 defined, in
part, by plug 48 and circumscribing shroud 50,
The engine of Figure 1 is further provided with a lubrication
system including an oil tank or xeservoir 52 and a pump 54 which supplies oil
` under presswre through passage means such as conduit 56 and nozz]es 58 to
lubricate rear engine bearing 60, and also through conduit 64 and nozzles 66 to
lubricate forward engine bearing 68. A portion of the oil is also pumped
through passage 70 and nozzle 72 to the interior 74 of a rotatable oil duct 76
which is connected to turbine rotor 38 through frustoconical shaft 78 and is
rotatable therewith, Duct 76 is slightly conical so as to act as a pump to
deliver the oil into forward bearing sump 80 from which it is pumped through
pump 82 back to the oll tank via conduit 84. The purpose for oil duct 76 will
soon become apparent as the description of the invention becomes developed
herein. However, it will be appreciated by those skilled in the art that some
aircraft gas turbine engines are routinely designed with a center oil duct
similar to duct 76 which serves as a conduit to carry and distrlbute the lubri-
cant to various bearings. An example is that shown in U. S. Patent 3, 248, B80,
which is also assigned to the assignee of the present inventio~ Therefore,
duct 76 or its equivalent rnay already be present in a gas turbine engine or maybe added to the lubrication system, as here, for the purpose of the present
invention.
Attention is now directed to Figures 3 and 4 wherein the turbine
rotor portion of the engine of Figure 1 is shown in greater detail. Turbine
blades 42, borne about the périmeter of rotatable turbine disk 40, are provided
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with an aerodynamically contoured airfoil portion 86 and a base or root portion
88 for attachn;lent to the disk, as by the well-known dovetail method (see Figure
5) The relatively thin exterior wa]ls of the airfoil portion define a substantially
hollow interior in the form of serpentine passages 90. Connected to the forward
5 and aft ends of the dovetail root 88 of each blade 42 are relatively thin-walled
tubes 92 and 94, respectively, which communicate with the hollow interior of
the blade through a pair of access openings; inlet 95 and exit 97. Tubes 92 and
94 are connected together by a U shaped tube 96 to form a closed-loop passage-
way for each blade. Thus, the number of such closed-loop passageways equals
10 the number of turbine blades in the turbine rotor,
- Associated with oil duct 76, and rotatable therewith, is a heat
exchanger 98 through which each closed-loop passageway, and more particular-
ly, each tube 96, passes, This heat exchanger is annular in design, comprising
an inner extremity defined by a portion of rotatable oil duct 76 and an outer
concentric wall 100, with bulkheads 102 and 104 extending therebetween to
deflne an irlterior cavity 118, The bulkheads are OI sufficient thickness to
preclude leakage around tubes 96 whether the joints 106 (where the tube6 pierce
the bulkheads) are brazed or swaged. Note that these joints 106 are exposed
to facilitate visual inspection, Furthermore, bulkheads 102 and 104 could be
of double-wall construction to a~sure an even better seal around tubes 96,
Affixed to the outer wall 100 of the heat exchanger are two or more radial
flanges 1û8, 110 which support the exposed portions of tubes 96~ Wear collars
112 are fastened to tubes 96 where they are captured by flanges 10~, 110 in
order to prevent fretting of the tubes.
As discu8sed earlier, oil duct 76 is conical in order to pump
oil therethrough by centrifugal pumping action. An inwardly extending annular
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dam 114 diverts at least a portion of the oil through introducing means such as
a plurality of holes 116, only one of which is shown for clarity, and into the
interior chamher 118 of heat exchanger 98. Inside the heat exchanger are
provided labyrinth means comprising, for example, a series of radial walls
120 which direct the oil overJ under and around tubes 96 to absorb heat there-
from in a manner soon to be described. An alternative method of providing
oil circulation would be to pro~ide an Archimedean screw runningthrough the
interior of the heat exchanger for its entire length. In the embodiment of
Figure 3, the radial walls 120 provide further support for tubes 96 in the
t0 rotating environment. Means for discharging oil from the heat exchanger 96
comprises, for example, a second plurality of holes 122, again only one of
- ~ which being shown for clarity. Oil thus re-enters the interior of oil duct 76
as indicated by the arrows in Figure 3. To permit a constant flow of oil throughthe heat exchanger regardless of minor changes in engine attitude (for example,
when an aircraft is in a climb or dive) the oil exit holes 122 are located at a
higher radius than the oil inlet holes 116~
The present invention proposes means for cooling the turbine
blades primarily by the thermosiphon principle with steam as a primary cooling
.
medium, although it will become apparent that other coolants such as liquid
metals (potassium and sodium~ might be equally suitable in certain applications.The closed-loop system passageways comprising tubes 92, 94 and 96 and blades
42 may be partially filled with the coolant through any access port (not shown)
which is subsequently plugged to conserve the sealed system. This means
for cooling the blades operates as follows: if water is used as the coolant, it
will turn to steam as the turbine rotor is operated at elevated temperatures.
: Since the steam density will vary with temperature, the steam which has been
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cooled by the heat exchanger 98 will be impelled radially outwardly through
tubes 92 into the serpentine blade passages 90. As this steam circulates, it
will pick up heat convected through the walls of the blade airfoil portion 86 and
have its density reduced. This will cause the steam to travel radially inwardly
through tubes 94 into the heat exchanger wherein the process is continuously
repeated. The heat picked up by the steam as it passes through the blades is
removed by the lubricating oil passing through the rotating heat exchanger in
heat exchange relationship with tubes 96. The lubricating oil is subsequently
directed, via means such as conduit 84, through a second heat exchanger shown
in Figure 1 at 124 wherein the oil and fuel to be burned in combustor 34 are
maintained in heat exchange relationship with conduits 123, 125 comprising
means for communicating between the fuel supply and the heat exchanger.
Thus, at least a portlon of the heat absorbed by the turbine blades is reintro-
duced back into the power cycle as heated fuel. The present concept of utilizingtwo heat exchangers, steam-oil heat exchang~er 98 (means for cooling the
steam) and oil-fuel heat exchanger 124 (means ~or cooling the oil), has an
important advantage over prior art regenerative cooling schemes in that the
fuel is maintained at a substantial distance from the hot turbine rotor section,thereby reducing the risk of serious fire in the event of leakage. Furthermore,
the steam which circulates through the blades presents no fire hazard whatso-
ever,
It i~ important that the heat exchanger be firmly connected
tangentially, axially and radially to the turbine disc 40 so as to assure that the
turbine rotor disk, shaft 43 and oil duct 76 rotate as a system and to preclude
steam tubes 92, 94 and 96 from bending, stretching or twisting 1mnecessarily.
To that end, means such as interlock 126 are provided between the turbine disk
-13 -
40 and the heat exchanger 98 to prevent the heat exchanger from rotating with
respect to the disk. Additionally, positioning means such as, for example,
a lip and rabbet 128 on structural member 130, which extends inwardly from
turbine shaft 43, positions the heat exchanger radially and axially with respectto the turbine disk and shaft.
Referring now to Figures 3, 5 and 8 - Il, there is depicted a
scheme for mechanically attaching tubes 92 and 94 to the outer faces 132, 134,
respectively, of turbine disk 40 so as to prevent the heat generated by the
steam Irom affecting the structural integrity of the disk. Note that these tubespass outside of the disk and not through it so as to further enhance disk struc-tural integrity. Furthermore, the method of mechanical attachment now to be
described permits easy assembly and replacement of tubes 92, 94 and blades
40.
Means for retaining, such as a plurality of radially spaced rings
136, are formed integrally with disk 40 and extend generally axially from the
faces thereof. These rings serve to support and position tubes 92, 94 axially,
:~ radially and tangentially with respect to the disk. As is best shown in Figures
8 - 11 where a portion of one such ring 136 is shown in greater particularity,
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each ring is provided with a plura].ity of circumferentially spaced D-shaped
slo~s 138, equal in number to tubes 92 or 94 on the respective disk face and
contoured to receive tube wear collars 140 which have been previously placed
around the tubes 92, 94 and, possibly, affixed thereto as by brazing. Alter-
natively, resilient wear tubes could be provided where appropriate. The wear
collars are provided with upper and lower lips 142, 144, respectively, which
; 25 sandwich ring 136 and which serve to locate the tubes radially. These collars
also protect the tubes from wear due to friction caused by etlgine vibration
., .
.:
and thermal cycling. Once the collars 140 are installed in their respective
I)-shaped slots 138, means for locking, such as split locking ring 146, is
inserted within a retaining groove 148 machined along the bottom edge of ring
136, preferably prior to machining of the slots 138. Locking ring 146
completes the capture of collar 19;0 and, in particular, constrains its axial
movement. Radial movement of the locking ring is limited, in turn, by the
base of slot 148.
As is best shown in Figure 5, tubes 94 (and, similarly, tubes
92 on the other disk face) are formed with a series of S-shaped bends between
each disk ring 136 and between the radially outermost ring 136 and blade roots
88. These bends provide flexibility to the tubes and prevent the formation of
cracks therein due to the centrifugal force field and thermocycling. I'h1s
flexibility is also desirable from an assembly standpoint due to the tolerance
stackup of the blades, disk, tubes, wear collars and heat exchanger,
Figures 12 and 13 depict an alternative embodiment of the
mechanical attachment of tubes 92 and 94 to their respective disk faces.
Therein, a slightly modified collar 150 is split along its longitudinal axis into
~ two halves which may be placed about the tubes prior to insertion into slots
`~ 138 of Figures 8 - 11. In all other respects, the function and assembly would
be the same as that described above. However, the split collar affords more
flexibility in assembly in that the tubes may now be bent prior to adding the
collar instead of vice versa. Alternatively, rings 136 could be replaced with
a plurality of U-shaped retainers 152 located in a ring, the number of which in
each row being equal to the number of tubes as hereinbefore, Again, D-shaped
slots would receive the collars of a type denominated as either 140 or 150 and
the retainers would be attached to the disc faces as by bolts 153 IFigure 13)
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through the base 154 OI each U-shaped retainer. The split retainer ring 146
feature of Figures 8 - 11 could be retained as illustrated in Figure 13 or else
individual retaining bands 156 could be installed within the slots of each
retainer 154, the ends of the bands being then crimped back 180 and tack-
welded together as at 158. Additionally, a resilient liner 155 could be added
to collar 150 for darnping vibration between the tubing and the collar. While
the mechanical attachments of Figures ~ - 13 have been illustrated with respect
to their application to a rotating disk, it is clear that these attachments woùld
be equally appropriate for attaching any tubing to any structure, stationary or
1 0 rotating.
Referring again to the total turbine rotor cooling system as is
best shown in Figure 3, it becomes readily apparent that there are only four
steam tube joints associated with each individual closed steam circuit and that
all four are exposed for easy visual inspectioTl. In particular, these joints are
referred to as 160 between tubes 92 and 96, 162 between tube 92 and the inlet
; side of the root 88 of blade 42, 164 between the outlet side of the blade and
tube 94, and 166 between tubes 94 and 96. C>E course, if more or less tube
; segments are employed, the number of joints will increase accordingly. All
joints are brazed or welded without subjecting the disk 40 to a furnace braze
cycle which might materially reduce the high strength properties of the disk.
;:,
In fabricating the cooled turbine rotor, all but two o~ the joints
are made prior to the installation of the disk. In particular, heat exchanger
98 with tubes 96, tubes 92 and blade 42 are assembled together and brazed as
one unit. A dummy disk could be used as a fixture to hold these various
components in their correct relative alignment during the brazing cycle.
Thereafter, the joints would be pressure checked and the disk inserted axially
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into place. Then the other half of the tubes 94 would be installed and brazed
using an induction coil at joints 164 and 166. After all of the steam joints are ~-
made and each blade's circuit pressure checked, a small amount of water (or
other selected coolant) is added through an opening, not shown, in each circuit
5 which i9 subsequently plugged to preserve the integrity of each sealed circuit.
To complete the turbine rotor assembly, shafts 43 and 78 are bolted to the
disk 40 as at 168 and 170, respectively (Figure 3). After balancing, the
assembly is ready for engine installation. Since the disk, tubes and heat
exchanger are enclosed by shafts 43 and 78, windage losses from the tubes
10 will not present a problem.
-~ Replacement of a turbine blade is a relatively simple matter.
First, the turbine rotor 38 is rernoved from the engine and shafts 43 and 78
disconnected therefrom. Tube 92 may be cut as at location 171 and tube 94
may be cut just below joint 164. Then the d~efective blade can be slid forward
out of its dovetail slot 173 (see Figure 5~ and any residual coolant removed
.
from the circuit. A replacement blade is then inserted into the dovetail slot
and the tubes 92 and 94 reconnected using braze couplings. Coolant would
then be added as with the original blade, and the turbine rotor balanced and
reinstalled.
Figure 2 illustrates how a somewhat similar steam thermosiphon
system could be installed in an aircraft gas turbine engine of the two-spool
turbofan variety, as opposed to the single-spool turbojet engine of Figure 1.
As is well understood in the art, the hot gases of Fombustion, after driving
- turbine rotor 38 (which in the terminology of turbofan engines is referred to as
2S the high pressure turbine rotor), pass to and drive a second, low p~essur-e
turbine 172 which is typically of the multistage variety and which drives, in
. .
turn, a front-mounted fan (not shown~ through frustoconical shaft 174 and an
inner drive shaft 176. This inner drive shaft is hollow, defining to its interior
a lubricating oil ~coolant) passage 178. A slightly modified heat exchanger 180
concentric with shaft 176 is connected to disc 40 and shaft 43 as discussed
above, and each closed-loop blade coolant passageway similarly passes
through this modified heat exc:hanger. A conical oil duct 182 is attached to theforward end of the heat exchanger as at bolted connection 184. Lubricating
oil enters passage 178 through oil nozzle 186. A plurality of apertures 188 in
drive shaft 176 permits the oil to flow into the annular space 190 between drive.~ ' ':
shaft 176 and the inner wall 192 of heat exchanger 180. Dam 194, at least
partially spanning passage 178, diverts the oil into apertures 188. Windback
seals 196, 198 on the inner diameter of the heat exchanger discourage the oil
frorn running anywhere other than through a second plurality of apertures 200
in wall 192 and into the heat exchanger. Thus, apertures 188, 200 and annular
space 190 constitute one example of means for communicating between the
coolant passage 178 and heat exchanger 180. However, even if oil did work
past the windback seals, it would merely run to the forward or aft engine
sumps, not shown in Figure 2. The oil discharges from the heat exchanger
through means commumcating between heat exchanger 180 and duct 182 com-
prising, for example, a series of apertures 202 in forward bulkhead 204,
Inside of oil duct 182 and spaced between shaft 176 and duct 182 is an inter-
mediate duct 206 carried by duct 182 through spacer webs 208 and defining a
coolant annulus 207 in fluid communication with heat exchanger 180. Duct 206
prevents oil fromdripping onto drive shaft 176 when the engine rotation stops.
Thus, a system is presented for a gas turbofan engine wherein the heat
exchanger, rotating at turbine disc 40 rotational speed is provided with a
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lubricant îrom an inner shaft 176 rotating at the low pressure turbine
rotational speed. Thus, the closed-loop thermosiphon is equally applicable,
with modification, to gas turbine engines of both the turbojet and turbofan
variety~
Also depicted schematically in Figure 2 is an alternative
embodiment of the secondary heat exchanger. Whereas Figure 1 teaches the
use oE an oil-fuel heat exchanger to recover at least a portion of the heat
removed from the blades and to return it to the engine cycle as heated fuel,
thereby functioning as a regenerative engine, it may be satisfactory in some
applications to eliminate the regenerative feature, thereby simplifying the
system. A gas turbofan engine is particularly well suited to using a radiator,
.: .
shown schematically at 210 and located in the fan bypass duct (not shown), to
remove the heat from the oil by placing it in heat exchange relationship with
the fan bypass flow stream, In this embodiment, however, the heat would be
lost from the cycle. Thus, it is preferable to use the regenerative system of
Figure 1 wherever practical.
Returning briefly to Figures l and 3, there is ~hown associated
with compressor rotor 24 a radial inflour compressor stage 212 which is used
to extract bleed air from the compressor. Some o this bleed air circulates
inside shafts 43 and 78, obtaining access to the turbine disc through apertures
214 in structural member 130, This bleed air maintains the blade roots and
the disk at a reduced temperature level and provides additional cooling for the
individual steam thermosiphon circuits.
Attention is now directed to Figure 7 wherein one embodiment
of a unique secondary cooling system for the closed-loop thermosiphon cooled
turbine is diagrammatically shown which utilizes this air bled from compressor
-19-
rotor 24. This secondary cooling scheme is used to provide
partial turbine blade cooling in the likely event of a steam
thermosiphon circuit failure.
Figure 7 shows a slightly modified turbine
blade 216 which incorporates as its primary method of
cooling the closed-circuit steam thermosiphon principle
of Figures 3 and 4~ Steam enters the hollow, serpentine
interior 218 of the blade through tube 92 and exits via
~ tube 94 as described above. However, the blade interior
; 10 is also in potential fluid communication with a secondary
cooling supply through bore 22 in the blade root. The
secondary coolant supply comprises compressor bleed air
which is drawn up into the blades through a conventional
rim entry system ~not shown) wherein each blade bore 220
is located pr~ximate the turbine disk outer rim. Such
a system is typified by U. S. Patent 3,891,348 dated
June 24, 1975 which is assigned to the assignee of the
present invention. This secondary coolant air is prevented
; from entering the interior of the turbine blades under
normal operating conditions by filling means such as a
melt-out plug 226 which blocks bore passage 220. Plug
226 is ~abricated of a material having a melting point less
than that of the blade primary structure. ~lso associated
with the blade are coolant discharge means, one form of
which being casting holes 228 in the tip portion of the
blade, the holes also being provided with filler means in
the form of melt-out plugs 230. ~ike plugs 22~, plugs 230
have a melting point temperature less than that of the
primary turbine blade material. Under normal ~perating
conditions, the blade would be cooled by the steam thermosiphon
principle. In the event of failure of the primary steam
cooling system which causes the temperature of a blade to
20 -
rise, plugs 230 would melt~ As the temperature increased
even furthex, inner plug 226 would melt and secondary
cooling air would rush into the blade interior and out
through holes 228. While this secondary cooling system
may not be sufficient to hold the blade temperatures to
levels which would ensure long life, it would preserve
the blade long enough to return an aircraft to a mainten-
ance facility where repairs to the primary cooland system
could be effected.
In an alternative embodiment shown in Figure
6, the secondary cooling air could be drawn up through
radially extending bores within slightly modified turbine
disk 224. The fabrication and operation of such a bore
entry turbine disk is described ully in U.S. Patents
3,588,277 dated June 28, 1971; 2,742,706 dated July 3, 1973 -
and 3,982,852 dated September 28, 1976 and which are
assigned to the assignee of the present invention.
Essentially, bleed air is pumped radially outwardly through
turbine disk bores 222 which communicate with blade bores
2220. Yet another embodiment would be to cast a small
secondary air-cooling circuit in the blade along with,
but fluidly divorced from, the primary steam circuit. In
all of these secondary cooling schemes it is recognized
that the use of secondary cooling air is at the expense
of engine cycle performance and efficiency. However,
the dual-cooling approach offers the advantage of
increased reliability.
It has now become apparent that the turbine
blade cooling system described herein offers many advantages
over prior art systems~ For example, the steam-oil heat
exchanger has been designed and placed in the engine in
such a manner that it does not change the configuration
.
~ 21 -
i
:
or design of nearby hardware since it has been placed in
an area which was otherwise vacant. Thus, it does not
increase the engine length or diameter. Because it is
located at the lowest possible diameter, the design is
compact, lightweight, void of high stresses and has
virtually no balance problems since all the blade coolant
circuits have equal amounts of coolant. Other liquid-
cooled turbines have suffered vibration problems because
each blade or circuit was fed from a common source such
as from a distributor or manifold. Additionally, the
present invention employs conventional turbine disks and
a common, safe, on-board, secondary coolant (oil) is
used. The more volatile fuel coolants are divorced from
the turbine rotor structure. The system employs conventional
manufacturing techniques with relatively low-cost, long-
life turbine blades. Minimum reliance is placed on
precious compressor bleed air. However, means are provided
to use this air in a secondary capacity as a back-up
coolant system~ The low diameter clesign is ideal f~r
bringing this secondary cooling air in and out of the blades.
Also, the system can be the basis for a regenerative engine
with resultant decreases in engine specific fuel consumption.
- Finally, each blade is provided with an individual cooling
circuit such that failure of one circuit would not necessarily
jeopardize the entire turbine.
It should become obvious to one skilled in
the art that certain changes can be made to the above-
described invention without departing from the broad
inventive concepts thereof. It is intended that the
appended claims cover all such variations in the present
invention's broader inventive concepts.
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