Note: Descriptions are shown in the official language in which they were submitted.
2 0 ~ 3
13DV-99~2
PL~TFORN P~8EMB~Y FOR ATTAC~ING
ROTOR BI~DE8 TO a ROTOR DI8
BACK~;ROUND OF TH~ INVENTION
Field of the I~ventio~l
The present invention relates in general to turbine
rotors and particularly concerns the mounting of nonmetallic
rotor blades having airfoil shaped dovetails to a rotor disk
via a plurality of circumferentially spaced metal platform
members having rotor blade support surfaces corresponding to
the airfoil shaped dovetails of the rotor blades.
Description of Prior Developments
To improve the performance of turbines, new rotor blade
materials have been developed. Such materials include both
metals and nonmetallics. Nonmetallics, such as
carbon/carbon and ceramics are lighter than metal and
require little or no cooling. Unfortunately, most high
temperature nonmetallic materials like carbon/ carbon and
ceramics do not have the bending capabilities of metal.
The inability to withstand significant bending loads
presents a design problem insofar as the configuration of
nonmetallic rotor blades is concerned. More particularly,
rotor blades usually have a platform that forms the inner
flowpath of the gas stream. For example, as seen in Figures
1 and 2, a metal rotor blade 10 includes a platform 12 which
extends circumferentially outward in a cantilevered fashion
on each side of the airfoil root section 14 of airfoil 15.
When rotated during use, the platforms 12 are subjected to
centrifugal bending loads as well as bending loads from the
motive exhaust gases.
:
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~ etal platforms can be designed to withstand these
bending loads but nonmetallic platforms of materials like
carbon/carbon and ceramics have generally been considered
incapable of reliabl~ sustaining such loads. This has
5 resulted in the use of metallic materials for the platforms.
A previous attempt to solve the platform bending and loading
problem involved removing the nonmetallic platform from the
nonmetallic blade and replacing it with a metal platform.
As seen in Figures 3 through 6, a separate metal
10 platform 16 was created to replace the integral nonmetallic .
platform 12 previously formed homogeneously with prior rotor
blade designs of the type depicted in Figures 1 and 2. The
metal platform 16 was eguipped with forward and aft integral
legs 18, 20 with a dovetail 22 formed on each leg. The
dovetails 22 on each leg 18, 20 fit into the same disk
dovetail slot 24 (Figures 5 and 6) as the rotor blade 10.
The platform 16 included an airfoil shaped hole 26
sized larger than the blade airfoil root section 14 to
accommodate assembly of the platform 16 over the nonmetallic
airfoil 30. This oversizing was required because the blade
airfoil tip section 32 (Figure 5) is typically larger in
places than the root section 14.
The platform 16 was installed over the blade airfoil
tip 32 and lowered down to the airfoil root 14. Next, the
blade-platform assembly was inserted into and secured within
the disk dovetail slot 24 via blade dovetails 33 and
platform dovetails 22. Finally, as seen in Figure 5, the
forward then the aft blade seals and retainers 34, 36 were
installed on the rotor disk 38.
A significant problem associated with using the
separate metal platform 16 on the nonmetallic airfoil 30 of
the type noted above is the excessive loss of precious
cooling air 39 which spills out of the assembly clearance
gap 40 defined between the airfoil root section 14 and the
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airfoil shaped hole 26 in the platform 16. This leakage is
best seen in Figures 4 and 5. The cooling air 39 also leaks
out between adjacent platform edges 42 at the flowpath
surface 44 (Figures 5 & 6) and between the forward and aft
legs 18, 20.
Another problem encountered with the use of the
separate metal platform 16 is excessive bending experienced
by its unsupported central portion 45. That is, the
platform 16 bends at its center because it is only supported
by the forward and aft legs 18, 20.
Referring again to Figures 1 and 2, another area, other
than the blade platforms, where bending stress presents a
significant design problem is in the blade shank area 46
through which the airfoil root 14 transitions into a
straight dovetail neck 48. Critical high stress areas are
located at the leading and trailing edges 50, 52 where the
airfoil blade 15 extends circumferentially beyond the
straight dovetail neck 48 creating a large offset angle 54.
The larger the offset angle 54, the greater the bending load
in the shank area 46. Even with a small offset angle, the
resulting stress levels have been found unacceptable for
nonmetallic materials like carbon/carbon and ceramics.
In order to improve the shank bending proble~ and
loading probl~m associated with the design of Figures 1 and
2, two changes to the configuration of rotor blade 10 were
made as shown in Figures 7 and 8. First, a costly curved
dovetail 56 was introduced to help reduce th~ offset angle
54 in the shank area 46 adjacent the straight dovetail 58 of
Figure 1.
Next, the airfoil 15 was changed from a high camber
shape to a low chamber shape. This reduction in camber also
helped to reduce the offset angle 54 in the shank 46.
Unfortunately, by changing the airfoil 15 from a high camber
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profile to a low camber profile, a significant loss in
performance results.
Still another problem associated with the use of
nonmetallic rotor blades having curved dovetails and curved
dovetail necks 62 is the width of the disk dovetail post 60
(Fig. 6) which is, by necessity, extremely thin at the
trailing edge 52. This thin section experiences relatively
high stress levels during engine operation. Such stress can
result in reduced life of the rotor disk.
A thin dovetail post is required because a
carbon/carbon or ceramic blade will only work satisfactorily
with a large single tang dovetail which is wider than
conventional multiple tang or "fir tree" dovetails.
Moreover, the nonmetallic airfoil 15 must transition into a
relatively large dovetail neck 62 which provides the
required support between the airfoil and the curved dovetail
56. If possible, the resulting thin dovetail post should be
avoided.
Accordingly, a need exists for a rotor blade mounting
assembly which avoids the problems associated with
conventional metallic blade platforms and which readily
accommodates the working stress levels present in modern gas
turbine engine rotor blades.
SUM~ARY OF THE INVENTION
The present invention has been developed to overcome
the problems and fulfill the needs noted above and therefore
has as an object the provision of a nonmetallic or ceramic
airfoil blade which includes an optimum high camber airfoil
contour and which avoids the use of homogeneously formed
platforms of the type supported by conventional offset blade
shank portions.
Another object of the invention is the provision of a
nonmetallic or ceramic airfoil blade having a virtually
shank-free configuration wherein the airfoil leads straight
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2~1983
and directly into a blade dovetail without kinks, doglegs or
o~fsets in the blade root and dovetail areas.
Another object of the invention is the provision of a
metal platform for mounting a non-metallic or ceramic
airfoil blade to a rotor disk in such a manner that leakage
of the blade cooling air between the blade and platform is
carefully controlled and such that impingement and/or film
cooling is applied to the platforms and blades only where
needed.
Still another object of the invention is the provision
of an airfoil blade platform which is supported around its
entire periphery so as to minimize undesirable platform
bending.
Yet another object of the invention is the provision of
an airfoil blade and platform assembly which allows for the
use of large, wide, low stress dovetail posts formed in the
rim of a rotor disk.
Another object of the invention is the provision of
nonmetallic or ceramic airfoil blade mounting platforms that
have straight dovetails which allow the use of straight
dovetail slots in a rotor disk. Such slots may be easily
broached or formed in the rotor disk with a wire EDM
apparatus.
~riefly, the present invention includes an airfoil
blade and platform assembly wherein the airfoil blades do
not connect directly to the disk by a dovetail fit or pinned
connection or the like. Specially designed air cooled metal
platforms are used to support nonmetallic or ceramic rotor
blades. The root end of the blade airfoil terminates
smoothly, without changing airfoil contour, into a specially
designed dovetail.
The platforms are contoured to accept and compliment
the blade airfoil and the special airfoil-shaped dovetail.
Adjacent platforms surround the blade airfoil root and
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2~1983
dovetail securing it axially, circumferentially and
radially. The platforms are mounted to the rotor disk via
dovetail interconnections and are held axially within the
disk by conventional blade seal/retainers.
Each platform includes a pressure chamber into which
cooling air is channeled to cool the platform by convection
and then by film cooling. Film cooling takes place as the
cooling air passes through metering holes in the gas stream
side of the platform or through holes strategically placed
to cool the platform, disk rim and blade root area to
acceptable temperatures.
The aforementioned objects, features and advantages of
the invention will, in part, be pointed out with
particularity, and will, in part, become obvious from the
following more detailed description of the invention, taken
in conjunction with the accompanying drawings, which form an
integral part thereof.
DRIEF DESCRIPTION OF THE DR~WIP~GS
In the drawings:
Figure 1 is an aft view of a prior art metal rotor
blade taken through line A-A of Figure 2;
Figure 2 is a partially sectioned top plan view of the
prior art rotor blade of Figure 1 showing a straight
dovetail neck in phantom;
Figure 3 is a perspective view of a prior art metal
platform designed for use with nonmetallic rotor blades;
Figure ~ is a partially sectioned top plan view taken
through line B-B of Figure 5 showing the metal platform of
Figure 3 mounted around a non-metallic rotor blade airfoil
according to the prior art,
Figure 5 is a fragmental side elevation view of the
metal platform of Figure 3 mounted to a non-metallic rotor
blade airfoil which, along with the metal platform, is
mounted to a rotor disk of a gas turbine engine;
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20~1983
Figure 6 is a fragmçntal view of the trailing edye of
the rotor disk rim and the metal platform dovetails of
Figure 5 with the air~oils and aft blade seal and retainer
of Figure 5 removed for clarity;
Figure 7 is an aft view of the trailing edge of a prior
art nonmetallic rotor blade taken along line C-C of Figure
8;
Figure 8 is a top plan view of the rotor blade of
Figure 7 showing a curved dovetail neck in phantom;
Figure 9 is a side elevation view taken along line D-D
of Figure 11 of a non-metallic or ceramic rotor blade
mounted to a rotor disk via metallic platforms designed in
accordance with the present invention;
Figure 10 is a fragmental view of the forward face of
the assembly of Figure 9 taken along line E-E thereof;
Figure 11 is a top plan view of several rotor blades
mounted to the rotor disk of Figure 9 and taken along line
F-F thereof;
Figure 12 is a sectional view taken along line G-G of
Figure 9;
- Figures 13 and 14 are sectional views taken
respectively along lines H-H and J-J of Figure 12;
Figure 15 is a sectional view taken along line K-K of
Figure 9;
Figure 16 is a sectional view taken along line L-L of
Figure 9; and
Figures 17 through 22 are sectional views respectively
taken serially through lines M-M through R-R of Figure 11.
In the various figures of the drawing, like reference
characters designate like parts.
DEl`AILED DESORIPTION OF TH13 PREFERRED EMBODIMENT~:
The present invention will now be described in
conjunction with the drawings, beginning with Figures 9 and
10 which show a metal platform 66 connected to a rotor disk
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20S1~83
38 by a multiple tang dovetail 68. Dovetail 68 extends
axially, without curvature, on the platform 66 and is
dimensioned for secure insertion into a matching straight
dovetail slot 70 in the rotor disk 38.
The dovetail 68 and dovetail ~lot 70 preferably run the
full length of the rotor disX rim 72. The centerline 73 of
dovetail 68 is shown in Figures ll and 16 to form an angle
75 of about 20 degrees with respect to the centerline 77 of
rotor disk 38.
Cooling air 39, such as compressor discharge pressure
air, is used to cool the platform 66 and rotor disk rim 72.
The cooling air 39 enters the plenum 74 formed by the
forward blade seal and retainer 34 and rotor disk 38 and
passes into a cavity 76 formed between the bottom of the
disk dovetail slot 70 and the base of the platform dovetail
68. From cavity 76, the cooling air 39 flows up through
bore holes or channels 78 formed in the platform dovetail 68
and then into a platform chamber 80.
The cooling air 39 is used to convection cool the
platform 66 before it passes out through film cooling holes
82 formed in the top wall or roof 81 of platform 66 which
defines the inner surface of the gas stream flowpath. Film
cooling holes 82 may be placed anywhere it is deemed
necassary to help cool the platform 66, rotor blade 84, or
disk rim 72.
The disk rim 72 will run cooler than prior designs
because the rotor blades 84 are separated from the disk rim
72 and will not conduct heat from the hot gas stream via
blade airfoils or blade dovetails.
The rotor blade 84 does not have a conventional shank
portion where conventional airfoils transition to a dovetail
neck. Instead, the airfoil 86 leads smoothly and directly
into a dovetail 88. This is best seen in Figures 12, 13 and
14, and 17 through 22. It should be noted that there are no
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20619~3
kinks, doglegs, or offset angles in the continuous, smooth,
even contour of airfoil 86 as it joins the dovetail 88.
As further seen in Figures 17 through 22, the platforms
66 are provided with angled arcuate or airfoil shaped
axially extendinq support surfaces 90 and 92 that compliment
and mate with the curved or airfoil shaped blade dovetail
88. These support surfaces retain the rotor blade 84 as
described earlier. The platforms 66 are also provided with
optional transverse support columns 94 as seen in Figures 9,
15, 18 and 19 that may be required to help support the
angled surfaces 9o and 92.
The upright concave side wall 96 and convex side wall
98 seen in Figures 16, 17 and 18 along with the flat or
planar forward wall 100 and flat or planar aft wall 102
provide all around support for the slightly arched platform
roof 81 and help form the pressure chamber needed to contain
the cooling air 39.
Because the blade is supported and located by the
angled surfaces 90 and 92 the concave edge 104 and convex
edge 106 (Figure 16~ on the platform 66 can be easily sized
to come close to but not touch the more delicate nonmetallic
blade airfoil 86. This will prevent fretting of the blade
due to friction.
There has been disclosed a heretofore the best
embodiment of the invention presently contemplated.
However, it is to be understood that various changes and
modifications may be made thereto without departing from the
spirit of the invention. For example, platforms 66 could
include serpentine cooling passages. Moreover, platforms 66
need not necessarily be formed exclusively of metal in which
case air cooling could be optional.
