Gas turbine engine fuel manifold

A fuel manifold is disclosed for providing fuel to a combustor disposed radially inside an annular casing. The manifold is disposed inside the casing and includes an arcuate manifold tube, a thermal insulation layer surrounding the tube, and a cover layer surrounding the insulation layer which is substantially rigid for protecting the insulation layer from physical damage.

TECHNICAL FIELD 
The present invention relates generally to gas turbine engines, and, more 
specifically, to fuel manifolds for providing fuel to a combustor thereof. 
BACKGROUND ART 
Conventional gas turbine engines include an annular combustor having a 
plurality of circumferentially spaced carburetors disposed in an annular 
dome at an upstream end thereof. The combustor is disposed radially inside 
an annular casing and is provided with hot compressor discharge air which, 
depending on particular engine designs, may be from about 1,000.degree. F. 
(538.degree. C.) to about 1,400.degree. F. (760.degree. C.). To channel 
fuel to the carburetors within the annular casing, conventional fuel stems 
extend to the carburetors from an annular manifold surrounding the casing. 
The fuel stems are exposed to the hot compressor discharge air and must be 
suitably constructed for channeling the fuel without adverse effect. 
For example, conventional fuel manifolds and the fuel stems are made from 
stainless steel or Hastolly X to provide adequate strength for 
withstanding the fuel pressure therein. These metals are also preferred 
since they do not react chemically with the fuel at the relatively high 
temperatures in the environment of the combustor. Although these metals 
have these advantages, they also have the disadvantage of having 
relatively high thermal conductivity which allows heat to be transferred 
to the fuel flowing in the manifold and fuel stems, which will raise the 
temperature of the fuel unless suitable means are provided for protecting 
the fuel. 
The fuel stems exposed to the compressor discharge air are typically 
designed to be cooled, for example by the fuel itself or by insulating air 
gaps, to ensure that the temperature of the fuel being channeled 
therethrough does not reach unacceptably high temperatures which would 
cause undesirable coking of the fuel inside the fuel channels. It is 
desirable to maintain the internal wetted wall temperature of the fuel 
tubes or conduits less than about 350.degree. F. (177.degree. C.) for 
preventing coking of the fuel therein. 
In order to meet this requirement, conventional fuel manifolds are mounted 
outside the casing surrounding the combustor wherein the environment is 
substantially cooler than that inside the casing wherein the compressor 
discharge air flows. Since conventional fuel manifolds typically extend 
about 360.degree. around the engine centerline and the casing surrounding 
the combustor, they are relatively large diameter structures and therefore 
increase the weight and envelope of the engine. Although the fuel stems 
connecting the manifold to the fuel injectors in the carburetors extend 
through the casing to the combustor in the environment of the hot 
compressor discharge air, they typically have relatively small diameters 
and are relatively short, and therefore may be more effectively cooled by 
the fuel channeled therein, or by insulating air gaps therein. 
OBJECTS OF THE INVENTION 
Accordingly, one object of the present invention is to provide a new and 
improved fuel manifold for a gas turbine engine. 
Another object of the present invention is to provide a fuel manifold which 
may be mounted internal to the casing surrounding the combustor for 
reducing the size of the manifold and, therefore, engine weight and engine 
envelope. 
Another object of the present invention is to provide a fuel manifold 
effective for withstanding compressor discharge air temperatures without 
coking of the fuel flowing therein. 
DISCLOSURE OF THE INVENTION 
A fuel manifold is disclosed for providing fuel to a combustor disposed 
radially inside an annular casing in which flows hot compressor discharge 
air. The manifold is disposed inside the casing and includes an arcuate 
manifold tube, a thermal insulation layer surrounding the tube, and a 
cover layer surrounding the insulation layer which is substantially rigid 
for protecting the insulation layer from physical damage.

MODE(S) FOR CARRYING OUT THE INVENTION 
Illustrated in FIG. 1 is a schematic representation of an exemplary high 
bypass turbofan engine 10. The engine 10 includes a longitudinal 
centerline axis 12 around which are coaxially disposed in serial flow 
communication a conventional fan 14, a conventional compressor 16, a 
conventional combustor 18, a conventional high pressure turbine nozzle 20, 
a conventional high pressure turbine 22 suitably joined by a shaft to the 
compressor 16, and a conventional low pressure turbine 24 which 
conventionally drives the fan 14 through a shaft. 
The compressor 16 and the combustor 18 are disposed radially inside an 
annular casing 26, and in this exemplary embodiment of the engine 10, an 
annular outer casing 28 surrounds the casing 26 to define an annular 
bypass duct 30. In conventional operation, ambient air 32 is channeled 
through the fan 14, a first portion 32a of which is channeled into the 
compressor 16, a second portion 32b is channeled through the bypass duct 
30, and a third portion 32c is discharged from the fan 14 over the outer 
casing 28 for providing thrust for powering an aircraft (not shown) in 
flight. The air first portion 32a is compressed in the compressor 16 and 
forms relatively hot compressor discharge air 34 which is channeled to the 
combustor 18. A fuel manifold 36 in accordance with one embodiment of the 
present invention provides fuel to the combustor 18 which is mixed with a 
portion of the compressor discharge air 34 and conventionally ignited for 
generating combustion discharge gases 38 which flow through the nozzle 20, 
and turbines 22 and 24 for powering the fan 14 and the compressor 16. 
Illustrated in more particularity in FIG. 2 is an enlarged view of the 
region of the combustor 18 including the fuel manifold 36 of the present 
invention. The combustor 18 conventionally includes an annular, radially 
outer liner 40 and an annular, radially inner liner 42 spaced therefrom 
which are conventionally joined at upstream ends thereof to a conventional 
annular dome 44. Conventional outer and inner cowls 46 and 48 extend 
upstream from the dome 44 for conventionally channeling a portion of the 
compressor discharge air 34 to the dome 44 and portions over the outer 
surfaces of the liners 40 and 42. 
In this exemplary embodiment of the combustor 18, the dome 44 is a double 
dome having a plurality of circumferentially spaced outer carburetors 50 
and a plurality of circumferentially spaced inner carburetors 52. Each of 
the carburetors 50 and 52 includes a conventional air swirler 54 and a 
conventional fuel injector 56. Fuel is conventionally discharged from the 
fuel injector 56 and mixed with a portion of the compressor discharge air 
34 in the swirler 54 for creating fuel/air mixtures which are 
conventionally ignited for forming the combustion gases 38. 
In this exemplary embodiment of the present invention, the fuel manifold 36 
includes a radially inner manifold 36a providing fuel to the inner 
carburetors 52, and a radially outer manifold 36b providing fuel to the 
outer carburetors 50. The inner and outer manifolds 36a and 36b are 
substantially identical to each other except in size and orientation and 
the following detailed description of the inner manifold 36a applies 
equally as well to the outer manifold 36b wherein certain like elements 
are identified by the suffix "b" added to the corresponding element 
numeral designation. Of course, the invention may also be practiced for a 
conventional single dome combustor having solely one row of 
circumferentially spaced carburetors. 
FIG. 3 illustrates an axial, aft facing view of the fuel manifolds 36a, 36b 
and the combustor 18. Only four outer and inner carburetors 50 and 52 are 
shown for clarity, it being understood that the number of 
circumferentially spaced carburetors is conventionally determined for each 
particular engine application. The fuel manifold 36a includes at least 
one, and in this exemplary embodiment two, arcuate inner manifold tubes 
58a. The tubes 58a in the preferred embodiment are formed of conventional 
stainless steel such as 321 stainless steel having the designation AMS 
5557, for enjoying the conventional advantages thereof. 
In accordance with one feature of the present invention, the manifold tubes 
58a are preferably disposed radially inside the casing 26 surrounding the 
combustor 18 for reducing their size, and therefore engine weight and 
engine envelope. For example, the two manifold tubes 58a are preferably 
disposed coaxially with each other at a common radius around the engine 
centerline axis 12 and together extend circumferentially about 360.degree. 
for forming a ring. Also in the preferred embodiment, each of the manifold 
tubes 58a extends circumferentially about 180.degree.. The manifold tubes 
58a, as more particularly illustrated in FIG. 2, are also preferably 
disposed upstream of the dome 44 and radially between the outer and inner 
liners 40 and 42, and radially between the outer and inner cowls 46 and 
48. In this way, the manifold tubes 58a are located relatively close to 
the carburetors 52, and thusly reduce the size of the fuel manifold 36a 
for reducing engine weight and envelope. 
However, since the manifold 36a is disposed inside the casing 26 and 
upstream of the dome 44 it is subject to the high temperature compressor 
discharge air 34 which would lead to unacceptable coking of the fuel 
therein but for the provisions of the present invention. 
More specifically, and in accordance with the present invention, a thermal 
insulation layer 60 as shown in more particularity in FIGS. 4 and 5 
preferably completely surrounds the manifold tube 58a for providing 
thermal protection thereof. The thermal insulation layer 60 preferably has 
a thermal conductivity which is as low as possible. In one embodiment of 
the invention, commercially available "MIN-K" (trademark) brand insulation 
material manufactured by Johns Manville may be used and has a thermal 
conductivity of about 0.30 BTU-In/Sq. Ft.-Hr-.degree.F. (0.04327 
W/M-.degree.C.). Since effective thermal insulation layers, such as layer 
60, are typically non-structural layers, the fuel manifold 36a preferably 
also includes a cover layer 62 completely surrounding the insulation layer 
60 which is substantially rigid, for example, for protecting the 
insulation layer 60 from physical damage. In the preferred embodiment of 
the present invention, the cover layer 62 is also preferably impervious to 
the fuel so that any leaks of fuel from the manifold tube 58a are 
contained by the cover layer 62. The cover layer 62 in accordance with one 
embodiment of the present invention is preferably a composite prepreg such 
as a ceramic matrix composite in the form of tape or cloth which provides 
additional thermal insulation of the tube 58a and the fuel flowable 
therein. 
As illustrated in FIG. 6, both the insulation layer 60 and the cover layer 
62 are preferably in the form of tapes which are conventionally wound 
around the manifold tube 58a. The cover layer 62 is conventionally 
impregnated with a conventional hardening agent and baked at a suitably 
high temperature in a conventional manner for curing the cover layer 62 
for forming a relatively hard cover layer 62 for protecting the insulation 
layer 60. 
Referring again to FIGS. 2 and 3, since the manifold tubes 58a and 58b are 
preferably mounted inside the casing 26, respective fuel inlet conduits 
64a, 64b extend from outside the casing 26, through the casing 26, and are 
conventionally joined in flow communication with the manifold tubes 58a, 
58b, respectively, for providing fuel thereto. The inlet conduits 64a are 
conventionally joined to a conventional first fuel supply 66a 
conventionally mounted outside the casing 26, and the fuel inlet conduits 
64b are similarly conventionally joined to conventional second fuel 
supplies 66b also conventionally disposed outside of the casing 26. 
As illustrated in FIGS. 3 and 5, each of the fuel manifolds 36a, 36b has 
its own respective dimensions. For example, the radially inner manifold 
tube 58a has a nominal radius R measured from the engine centerline 12, a 
circumferential length C, and an outside diameter D.sub.t. Of course, the 
cover layer 62 has similar dimensions including, for example, an outer 
diameter D.sub.c, which is greater than D.sub.t. 
It is conventionally known that thermal expansion and contraction of a 
material is equal to the product of a linear dimension of that material, 
such as R, C, and D.sub.t ; the conventional thermal coefficient of 
expansion; and the differential temperature experienced by the material. 
Since the cover layer 62 is preferably substantially rigid in the 
preferred embodiment of the present invention, and is formed from a 
different material than that of the manifold tube 58a, thermally induced 
differential movement of these two members during operation of the 
combustor 18 may damage the cover layer 62, which in the preferred 
embodiment of the present invention is ceramic. 
Accordingly, in order to reduce or eliminate the differential thermal 
movement between the manifold tube 58a and the rigid cover layer 62, the 
cover layer 62 preferably has a thermal coefficient of expansion which is 
preselected so that the product of a linear dimension of the cover layer 
62, its thermal coefficient of expansion, and the expected differential 
temperature during operation of the combustor 18 matches or substantially 
matches the product of a respective linear dimension of the manifold tube 
58a, its thermal coefficient of expansion, and its expected differential 
temperature during operation of the combustor 18. Since the fuel channeled 
through the manifold tube 58a is relatively cool, and since the compressor 
discharge air 34 channeled over the cover layer 62 is relatively hot, the 
differential temperatures experienced by the manifold tube 58a will be 
less than the differential temperature experienced by the cover layer 62 
during operation. Accordingly, the cover layer 62 preferably has a thermal 
coefficient of expansion less than the thermal coefficient of expansion of 
the manifold tube 58a for reducing the thermal differential movement 
between the manifold tube 58a and the cover layer 62. 
For example, since the radius R of the manifold tube 58a is equal to the 
corresponding radius of the cover layer 62, then the two operative 
parameters for obtaining differential thermal movement therebetween are 
the thermal coefficients of expansion and the differential temperatures. 
Accordingly, the coefficients of expansion of the tube 58a and the cover 
layer 62 are preselected so that the product of the tube thermal 
coefficient of expansion and the tube differential temperature is 
substantially equal to the product of the thermal coefficient of expansion 
of the cover layer 62 and the cover layer differential temperature. In 
this way, differential radial movement between the tube 58a and the cover 
layer 62 is reduced or eliminated for thusly protecting the cover layer 
62. Such differential radial movement is more significant in an embodiment 
of the invention having a completely annular, single manifold tube 58a. 
However, by using discrete and unconnected manifold tube segments such as 
the two segments 58a, differential radial movement is substantially 
reduced. 
Similarly, differential thermal movement between the tube 58a and cover 
layer 62 also occurs along the circumferential length C as illustrated in 
FIG. 3 and, with the linear dimension C being substituted for the linear 
dimension R, the preferred coefficients of expansion as above described 
also reduce or eliminate differential thermal movement along the 
circumferential direction. And, thermal differential movement is also 
reduced or eliminated in the radial direction through a cross section of 
the tube 58a and the cover layer 62 for the linear dimensions D.sub.t and 
D.sub.c as illustrated in FIG. 5. 
As illustrated in FIGS. 4 and 5, the manifold tube 58a includes a plurality 
of relatively short manifold stems 68 to which the fuel injector 56 is 
conventionally joined. For example, the fuel injector 56 includes a 
relatively short injector stem 70 suitably joined to the manifold stem 68, 
by brazing for example, and an injector tip 72 suitably joined to the stem 
70, also by brazing for example. Referring also to FIG. 3, the manifold 
stems 68 of the inner manifold tubes 58a extend radially inwardly, and the 
manifold stems 68 of the outer manifold tubes 58b extend radially 
outwardly. In the preferred embodiment of the present invention, the 
thermal insulation layer 60 and the cover layer 62 are disposed also on 
the manifold and fuel injector stems 68 and 70, as well as on the fuel 
inlet conduits 64a and 64b. In the preferred embodiment of the present 
invention, the thermal insulation layer 60 and the ceramic cover layer 62 
are preselected for having suitably low thermal conductivity and suitable 
number of layers for providing thermal insulation of the manifold tube 58a 
for maintaining an internal wetted wall temperature of the tube below 
about 350.degree. F. (177.degree. C.) to prevent coking of the fuel 
therein, even though the manifold 36a is subject to the hot compressor 
discharge air 34 which may be at least 1000.degree. F. (538.degree. C.). 
As illustrated in FIGS. 3 and 4, each of the manifold tubes 58a, 58b also 
includes an end cap 74 at each circumferential end thereof, around which 
both the thermal insulation layer 60 and the cover layer 62 are also 
provided. Adjacent end caps 74 of adjacent manifold tubes 58a, and 58b, 
respectively, form gaps G therebetween. Although a fully annular, 
360.degree. manifold tube 58a or 58b could be used in other embodiments of 
the present invention, it is preferred that the manifold tubes are 
provided in arcuate, segmented portions which, for example, reduces the 
differential thermal movement between the manifold tubes and the cover 
layer 62 in the radial direction relative to the engine centerline axis 12 
as described above. 
Also as illustrated in FIG. 3, wherein the double annular dome 44 is used, 
the two radially inner manifold tubes 58a and respective gaps G are 
disposed or oriented, relative to the centerline axis 12, about 90.degree. 
relative to the orientation or position of the two radially outer manifold 
tubes 58b and respective gaps G so that the fuel inlet conduits 64a of the 
inner manifold 36a pass between the gaps G formed between adjacent ones of 
the outer manifold tubes 58b and extend to the casing 26. This provides 
for a more compact arrangement of the inner and outer manifolds 36a and 
36b which allows the fuel inlet conduits 64a of the inner manifold tubes 
58a to pass between the outer manifold tubes 58b to reach the first fuel 
supply 66 outside of the casing 26. 
Furthermore, as illustrated additionally in FIG. 2, the two manifolds 36a 
and 36b are preferably positioned near the center of the dome 44 radially 
between the outer and inner carburetors 50 and 52 for further increasing 
the compactness thereof, and therefore reducing the fuel flow distance 
between the manifolds 36a, 36b and the corresponding fuel injectors 56. 
While there has been described herein what is considered to be a preferred 
embodiment 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. 
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: