BACKGROUND OF THE INVENTION
Field of the Invention
[0001] 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
[0002] 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.
[0003] 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.
[0004] Metal 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 reliably sustaining such loads. This has 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.
[0005] As seen in Figures 3 through 6, a separate metal 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 equipped
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.
[0006] 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.
[0007] 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.
[0008] 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 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.
[0009] 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.
[0010] 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.
[0011] In order to improve the shank bending problem and loading problem 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 the offset angle 54 in the shank area 46 adjacent the straight dovetail
58 of Figure 1.
[0012] 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 profile to a low camber
profile, a significant loss in performance results.
[0013] 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.
[0014] 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.
[0015] 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.
SUMMARY OF THE INVENTION
[0016] 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.
[0017] 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
and directly into a blade dovetail without kinks, doglegs or offsets in the blade
root and dovetail areas.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] Briefly, 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.
[0023] 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 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.
[0024] 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.
[0025] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] 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 4 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;
Figure 6 is a fragmental view of the trailing edge of the rotor disk rim and the metal
platform dovetails of Figure 5 with the airfoils 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.
[0027] In the various figures of the drawing, like reference characters designate like parts.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] 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 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.
[0029] The dovetail 68 and dovetail slot 70 preferably run the full length of the rotor
disk rim 72. The centerline 73 of dovetail 68 is shown in Figures 11 and 16 to form
an angle 75 of about 20 degrees with respect to the centerline 77 of rotor disk 38.
[0030] 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.
[0031] 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 necessary to help cool the platform 66, rotor blade 84,
or disk rim 72.
[0032] 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.
[0033] 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 kinks, doglegs, or offset angles in the continuous,
smooth, even contour of airfoil 86 as it joins the dovetail 88.
[0034] As further seen in Figures 17 through 22, the platforms 66 are provided with angled
arcuate or airfoil shaped axially extending 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 90 and 92.
[0035] 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.
[0036] 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.
[0037] 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.
1. A platform member for attaching airfoil blades to a rotor disk, said platform member
comprising a tail portion for engaging said disk, a first arcuate blade support surface
connected to said tail portion for supporting one airfoil blade and a second arcuate
blade support surface connected to said tail portion for supporting another airfoil
blade.
2. The platform of claim 1 wherein said first arcuate blade support surface comprises
a concave surface and wherein said second arcuate blade support surface comprises
a convex surface.
3. The platform of claim 1, wherein said dovetail portion is formed with internal channels
for conducting cooling air to said blades.
4. The platform of claim 2, further comprising support means extending between said first
and second support surfaces.
5. The platform of claim 4, wherein said support means comprises a plurality of columns.
6. The platform of claim 1, wherein said first and second blade support surfaces diverge
from said tail portion toward said airfoil blades.
7. The platform of claim 1, further comprising a first arcuate side wall connected to
said first blade support surface, a second arcuate side wall connected to said second
blade support surface and a top wall extending between said first and second side
walls.
8. The platform of claim 7, further comprising a forward wall and an aft wall each connected
to said first and second side walls and to said top wall so as to form a chamber within
said platform.
9. The platform of claim 8, wherein said forward wall and said aft wall each comprises
planar wall portions.
10. The platform of claim 8, wherein said top wall includes a plurality of cooling air
holes formed therein.
11. A platform assembly for mounting a plurality of nonmetallic or ceramic airfoil blades
to a rotor disk, said assembly comprising a plurality of platform members secured
to said disk, said platform members having arcuate support surfaces engaging said
airfoil blades and securing said blades to said disk.
12. The platform assembly of claim 11, wherein said airfoil blades comprise arcuate dovetails
and wherein said arcuate support surfaces of said platform members compliment and
engage said arcuate dovetails.
13. The platform assembly of claim 12, wherein said airfoil blades comprise airfoil portions
which transition directly and smoothly into said arcuate dovetails.
14. The platform assembly of claim 13, wherein said airfoil blades are formed without
platforms.
15. The platform assembly of claim 13, wherein said airfoil blades are formed with continuous
smooth profiles throughout their entire length up to said arcuate dovetails.
16. The platform assembly of claim 11, wherein said platform members comprise straight
axially-extending dovetails.
17. A platformless, nonmetallic rotor blade comprising:
an airfoil portion having a tip portion and a root portion; and
a dovetail portion connected directly to said airfoil portion at said root portion,
said airfoil portion comprising a smooth continuous contour from said tip portion
to said dovetail portion such that said rotor blade substantially avoids the formation
of offset angles between said airfoil portion and said dovetail portion.
18. The rotor blade of claim 17, wherein said dovetail portion comprises an axially-extending
arcuate surface portion.
19. The rotor blade of claim 18, wherein said dovetail portion consists of a single dovetail.