Patent Application
20100193605
The present invention relates generally to aero structures, and, more specifically, to aero structures such as exhaust nozzles and chevrons for gas turbine engines, and heat shields.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the gases in a high pressure turbine (HPT) which powers the compressor, and, additional energy is extracted from the gases in a low pressure turbine (LPT) which powers an upstream fan in a turbofan aircraft engine application.
In the turbofan engine, a bypass duct surrounds the core engine and bypasses pressurized fan air through a fan nozzle for providing a large portion of propulsion thrust. Some of the fan air enters the core engine wherein it is further pressurized to generate the hot combustion gases which are discharged through the primary or core exhaust nozzle to provide additional propulsion thrust concentrically inside the surrounding fan air stream.
During takeoff operation of the engine in an aircraft, the high velocity core exhaust and fan exhaust generate significant noise as the exhaust flows mix with the ambient airflow. Noise attenuation in commercial aircraft engines is a significant design objective that may adversely impact engine efficiency, which is the paramount design objective in commercial aircraft.
The typical core and fan exhaust nozzles are conical and taper in diameter aft to thin, annular trailing edges. The nozzles may be single-ply sheet metal, or may be two-ply sheet metal with a honeycomb strengthening core laminated therebetween.
The nozzles are also typically formed as full, or substantially complete, annular rings which enhances their structural rigidity and strength for accommodating the large pressure loads developed during operation as the core and fan exhaust streams are discharged from the engine at high velocity.
A significant advancement in noise attenuation while maintaining aerodynamic efficiency is found in the chevron exhaust nozzle disclosed in U.S. Pat. No. 6,360,528, assigned to the present assignee. In this Patent, a row of triangular chevrons form the exhaust nozzle for enhancing mixing between the high velocity exhaust flow and the lower velocity surrounding stream. The individual chevrons are integrally formed at the aft end of a supporting annular exhaust duct and enjoy the combined structural rigidity and strength therewith.
However, since each chevron in the primary core nozzle is cantilevered over the hot exhaust flow, it is subject to large differential temperature over its radially opposite surfaces, especially during transient takeoff operation of the aircraft.
These differential temperatures can then effect temperature gradients radially through the chevron, with corresponding thermal distortion and stress depending on the particular chevron construction. And, the thermal distortion can significantly change the geometry of the nozzle and therefore affect both its aerodynamic performance and noise attenuation effectiveness.
For example, the cantilevered chevron is subject to undesirable tip curling of its aft apex end due to temperature gradients, and that curling changes the chevron geometry, including the effective flow area of the chevron nozzle.
By forming the chevrons in single-ply metal as found in the above-identified patent, the temperature gradients therein can be minimized, which in turn will minimize undesirable changes in nozzle geometry.
Single-ply construction for the primary exhaust nozzle requires a strong material having high strength at the high temperatures experienced during operation, and Titanium may therefore be used for that application.
However, Titanium metal is quite expensive and difficult to fabricate, and increases the cost of manufacture, although it also enjoys the benefit of low weight, which is especially important for aircraft engines.
Accordingly, it is desired to provide an improved aero structure such as a chevron exhaust nozzle for addressing these cost and operational problems.
An aero structure includes inner and outer skins joined together by a core. The core has a different thermal conductivity than the inner skin to balance heat conduction therethrough.
The invention, in accordance with preferred and exemplary embodiments, together with further objects and advantages thereof, is more particularly described in the following detailed description taken in conjunction with the accompanying drawings in which:
The engine also includes a core nacelle or cowl 26 surrounding the core engine and LPT, and a fan nacelle or cowl 28 surrounding the fan and the forward part of the core cowl and spaced radially outwardly therefrom to define a fan bypass duct 30. A conventional centerbody or plug 32 extends aft from the LPT and is spaced radially inwardly from the aft end of the core cowl.
During operation, ambient air 34 flows into the fan 14 as well as around the fan nacelle. The air is pressurized by the fan and discharged through the fan duct as fan exhaust for producing thrust. A portion of the air channeled past the fan is compressed in the core engine and suitably mixed with fuel and ignited for generating hot combustion gases 36 which are discharged from the core engine as core exhaust.
More specifically, the core engine includes an aero structure in the form of a primary or core exhaust nozzle 38 at the aft end thereof which surrounds the center plug 32 for discharging the core exhaust gases. The core nozzle 38 is generally axisymmetric about the axial centerline axis of the engine in the exemplary embodiment illustrated in
If desired, another form of aero structure such as the chevron exhaust nozzle may be used for the fan nozzle 40 at the aft end of the fan nacelle 28 for discharging the pressurized fan air around the core cowl 26 where it also meets and mixes with the ambient airflow as the aircraft is propelled during flight.
The primary exhaust nozzle 38 is illustrated in isolation in
More specifically, the nozzle 38 includes an annular exhaust duct 44 having an annular mounting flange 46 integrally formed at the forward end thereof as illustrated in
The exhaust duct 44 extends axially aft and terminates in a converging cone portion for discharging the core exhaust 36 around the center plug 32 as shown in
An annular fairing 50 surrounds the duct 44 and is spaced radially outwardly therefrom, and terminates therewith at the common support flange 48. The fairing 50 increases in outer diameter in the upstream direction from the aft support flange 48 and suitably blends flush with the aft end of the core cowl 26 to provide an aerodynamically smooth surface over which the fan air 34 is discharged.
The aft ends of the exhaust duct 44 and the fairing 50 where they join the common annular support flange 48 is best illustrated in
The collective assembly of these three elements provides a full annular ring of considerable rigidity and strength, all of these components being suspended in turn from the common mounting flange 46 attached to the turbine rear frame.
The common annular support flange 48 initially illustrated in part in
More specifically, each chevron 52 is a dual skin fabrication including a radially inner skin 54 and a radially outer skin 56 having similar triangular configurations. The two skins are laminated together by a hollow structural core 58 extending radially therebetween.
For the primary nozzle configuration, the two skins may be formed of conventional, thin sheet metal for providing smooth aerodynamic surfaces. And, the core itself may also be formed of thin sheet metal for reducing weight while maintaining strength.
The skins and core may be made of metal alloys suitable for withstanding the high temperature of the core gases 36, and may be conventionally brazed together in an integrally joined unitary assembly for enhanced rigidity and strength. The so bonded assembly of metal components ensures a direct thermal path from the inner skin and through the core to the outer skin for thermally conducting heat therethrough.
The chevrons 52 share common configurations in different sizes as desired, including a circumferentially or laterally wide base end 60 and decrease laterally in width W to a preferably arcuate apex 62 at the opposite aft end thereof to define the triangular profile thereof as illustrated in
The two skins are fixedly joined together on opposite sides of an arcuate base flange 64 shown in
Each chevron 52 illustrated in
Correspondingly, as the individual chevrons converge in width in the downstream direction, diverging slots 68 are defined between adjacent chevrons and increase in lateral width in the downstream direction along the opposite portions of opposing trailing edges of the chevrons.
As shown in
Each chevron is therefore an aero structure which is a modular or unitary assembly of individual subcomponents which may be conveniently manufactured independently of the entire primary nozzle. The individual chevrons share the common modular features of dual skins, core, support flange, and perimeter rim, yet may conveniently vary in size for maximizing aerodynamic performance of the entire complement of chevrons in the nozzle.
Since each chevron 52 illustrated in
The lateral or circumferential taper is best illustrated in
Each skin is preferably thin sheet metal having a nominal thickness of about 12 mils (0.30 mm) which is substantially thinner than the thickness of the exhaust duct 44 and fairing 50 which integrally support the support flange 48.
And, the thickness T of the chevron has a maximum value T1 as illustrated in
The aerodynamic moment loads are in turn carried from the base flange 64 into the annular support flange 48, and in turn carried upstream along the exhaust duct 44 to the turbine rear frame.
As initially shown in
Each chevron 52 illustrated in
Furthermore, each chevron may additionally be arcuate in the axial or longitudinal direction for providing the compound arcuate or bowl configuration of the original single-ply chevrons. In particular, the chevron inner skin 54 has a radius of curvature R in the axial plane or section illustrated in
Correspondingly, the outer skin 56 is similarly axially convex outwardly in addition to being circumferentially convex outwardly.
The compound curvature of the inner and outer skins 54,56 may be used to advantage for maximizing aerodynamic performance, with the additional design variable of varying the radial thickness T of the chevron between its base or root end where it is mounted on the common support flange 48 to its aft or distal end at the corresponding apex 62.
In the preferred embodiment illustrated in the several Figures, the thickness T of the chevron remains constant in the circumferential direction while varying or tapering thinner in the axial direction between the base and apex ends thereof.
To further enhance the strength of the individual chevrons 52, the hollow core 58 is in the preferred form of a metal honeycomb laminated, by brazing for example, between the dual skins 54,56 as shown in
The honeycomb core 58 preferably extends over substantially the entire surface area of the laminated skins illustrated in
A preferred embodiment of the chevron trailing edge rim 66 is illustrated in
The honeycomb core and sheet metal rim may be brazed to the inner and outer skins to form a unitary and modular chevron with enhanced rigidity and strength, while still being exceptionally lightweight.
Each chevron 52 includes a row of apertures extending transversely or radially through the skins and base flange 64, and aligned with corresponding apertures through the support flange 48. Individual fasteners, such as conventional rivets, may be used in each aperture to fixedly and independently mount each of the chevrons on the support flange 48 with the tongue-and-groove joints therewith.
Accordingly, each chevron 52 is securely mounted to the annular supporting flange 48 at the aft end of the exhaust duct 44 and suitably mixes the hot core exhaust 36 with the cooler fan exhaust 34 for attenuating noise during operation.
Since each chevron is cantilevered from the common supporting flange 48, it independently withstands the substantial pressure loads exerted radially thereacross.
However, the large radial temperature difference between the hot core exhaust 36 and cool fan exhaust 34 subjects the cantilevered chevrons to the undesirable tip curling problem disclosed in the Background section.
In particular, the hot inner skin 54 tends to thermally expand greater than the thermal expansion of the cooler outer skin 56, which differential expansion can result in substantial tip curling in the laminated chevron configuration disclosed above when the components thereof are made from a single metal alloy.
Development testing has shown that tip curling of the chevron can significantly alter nozzle geometry, and therefore reduce nozzle aerodynamic performance and efficiency, and, significantly reduce noise attenuation of the chevron nozzle itself Tip curling will be most pronounced under transient operation of the engine where exhaust temperature changes are greatest, but can also occur during steady state operation, such as cruise, whenever temperature gradients are effected across the chevrons.
To minimize and correspondingly control the differential thermal expansion of the chevron skins, those skins, and the honeycomb core, are preferably made from selectively different materials illustrated schematically in
More specifically, since the core 58 itself is hollow for reducing chevron weight, while nevertheless maintaining strength and rigidity thereof, it necessarily separates radially the two skins over the requisite radial thickness T of the chevron and therefore effects a substantial radial temperature gradient through the chevron, especially in transient operation corresponding with aircraft takeoff where most noise attenuation is desired.
That temperature gradient in turn creates corresponding thermal strain and stress, and subjects the two skins to correspondingly different thermal expansion during operation which can lead to the undesirable tip curling problem and change of nozzle flow area geometry.
However, by preferentially selecting the core material, C for example, to be different than the material A of the inner skin 54, thermal conduction through the core 58 may be preferentially controlled.
For the core nozzle 38, it is desirable to incorporate a core material C having a thermal conductivity greater than that of the inner skin 54 for significantly increasing the thermal conduction from the inner skin 54 and through the core 58 to the outer skin 56, which in turn better balances heat distribution throughout the chevron. The core may comprise a structural portion and a conductive portion, with the conductive portion having a greater thermal conductivity than both inner and outer skins.
In this way, the temperature gradient between the two skins, and core, can be significantly reduced, and the thermal expansion of the outer skin 56 may be increased to better match the thermal expansion of the inner skin 54, and thereby reduce the differential expansion therebetween, and thusly minimize the undesirable tip curling. Accordingly, aero structures such as nozzles, chevrons, heat shields, or the like may be fabricated from a thermally balanced material having inner and outer skins and a core.
Thermal conductivity is one common material property of a metal, and is expressed in Watts per meter-degree(K), at room temperature for example; and the Coefficient of Thermal Expansion (CTE), expressed in mm per mm-degree(F.), is another common material property that is indicative of increasing length or expansion as temperature rises.
For a common material and common temperature, the resulting thermal expansion will be the same. However, for the common material and different temperatures, the resulting thermal expansion will be different.
Accordingly, independently of the particular material compositions of the two skins, be they the same or different, if the difference in operating temperatures thereof is reduced, then the difference in thermal expansion thereof will correspondingly be reduced, and this can be used to effectively reduce the undesirable tip curling of the chevron.
The higher thermal conductivity core 58 in conjunction with the lower thermal conductivity inner skin 54 in particular, as well as the lower thermal conductivity outer skin 56, may be used to particular advantage in reducing the undesirable tip curling of the modular chevron during transient operation, as well as during steady state operation. Thus, thermally balanced materials such as those described herein may be utilized to fabricate thermally balanced aero structures, such as exhaust nozzles, chevrons, heat shields, etc., with desirable thermal geometric properties.
Since the core 58 is integral to the collective strength of the modular chevron 52, that core 58 must have sufficient strength, notwithstanding the desire to increase its thermal conductivity. In other words, increased thermal conductivity must not be effected with any undesirable decrease in core strength.
Accordingly, one configuration for selectively increasing thermal conductivity of the core 58 is to form the honeycomb thereof in two, or more, plies.
Each of the two plies 72,74 is preferably thin sheet metal with different material compositions, with the first ply 72 being made from material C and the second ply being made from a different material D.
In particular, the first ply 72 has a thermal conductivity substantially greater than the thermal conductivity of the inner skin 54, as well as that of the outer skin 56 and the second ply 74.
The different metal components of the chevron 52 may therefore be formed of different materials having different material compositions and different material properties individually selected for enhancing strength of the modular chevron while minimizing undesirable changes in geometry thereof due to temperature gradients therein.
At least one of the honeycomb plies 72,74 preferably has the higher thermal conductivity than the inner skin 54, although higher thermal conductivity of the core 58 may otherwise be introduced therein. The advantage of the higher thermal conductivity first ply 72 is the simplicity of maintaining the honeycomb configuration for low-weight strength thereof, with the first ply 72 providing primarily the increased thermal conduction and the second ply 74 providing the requisite strength.
The two-ply honeycomb core 58 may be readily fabricated in sheet metal like the sheet metal skins 54,56. The two plies 72,74 may be laminated into half-cell strips, and the half-cell strips may abut each other, at four plies, to form the hexagonal cells.
The honeycomb strips are sandwiched between the two skins and bonded together by conventional brazing into an integrated and unitary module. Full surface braze joints are formed laterally between the abutting core plies 72,74 themselves, with corresponding braze joints between the edges of the plies and the bounding skins 54,56.
The use of selectively different materials for aero structures such as the chevron components may be used for additional advantage to further improve thermal response, and further decrease undesirable tip curling if desired.
For example, the two skins 54,56 may selectively have different coefficients of thermal expansion, with the outer skin 56 have a greater CTE than the inner skin 54.
For the core nozzle 38 configuration illustrated in
Accordingly, by using a higher or greater coefficient of thermal expansion for the cooler outer skin 56, that outer skin 56 will thermally expand more than it otherwise would, and thereby reduce the differential expansion with the hotter inner skin 54.
The effect of different CTE for the two skins complements the higher thermal conductivity of the core, and collectively these two effects may be used to tailor the resulting tip curl of the chevron. Significant reduction in the curl, which would otherwise be effected for identical material throughout the chevron, may be obtained by selecting different materials as described above, with tip curl reduction being reduced to about zero if desired, or even having tip curl reversing direction from radially out to radially in, if so desired.
In one embodiment analyzed, total tip curl, measured by radial displacement at the tip or apex 62 of the chevron, could be as large as about 5 percent of the chevron length for a single-material chevron. But, for the multiple-material chevrons disclosed above, that tip curl could be reduced to a few mils, or zero, in the radially outwardly direction, and even reversed to the radially inward direction in a magnitude approaching −1 percent.
Accordingly, the thermal effects of material selection for the modular chevron are pronounced and allow further variation in chevron design at desired design points like takeoff or cruise for example.
Since the core nozzle 38 is subject to the high temperatures of the core exhaust 34, the multiple materials of the modular chevron 52 may be used to advantage to balance thermal performance thereof, and preferentially reduce the undesirable tip curl.
Inconel (or Inco) is a nickel-based metal alloy commonly used in the production of modern gas turbine engines, especially for components thereof exposed to the hot combustion gases. It is less expensive than Titanium, but does not enjoy the strength-to-weight advantage of Titanium.
The chevrons may nevertheless be manufactured from Inconel in multi-ply sheet metal modular form for replacing the more expensive single-ply Titanium chevrons disclosed above.
For example, the inner and outer skins 54,56 may be formed of Inco 625 or AMS 5599 which has a thermal conductivity of 9.8, and a CTE of 7.1×10−6, which material is less expensive that Titanium.
For further reducing cost, the outer skin 56 may also be formed of a suitable stainless steel, like AISI 347, which has a thermal conductivity of 16; and a CTE of 9.6×10−6, which is still suitably larger than the CTE of the inner skin.
The inner skin 54 may also be formed of other materials, like Inco 909, having a thermal conductivity of 14.8.
The honeycomb core 58 may be formed of a suitably different material, like copper for the first ply 72 for its large thermal conductivity of 385, while the second ply 74 being Inco 625 with its smaller thermal conductivity of 9.8. However, the combined thermal conductivity of the two different core plies 72,74 is still quite large at about 197, and is effectively larger than that of the inner skin 54.
In one combination of materials having enhanced performance for the core nozzle 38, material A for the inner skin is Inco 625, material B for the outer skin 56 is AISI 347, material C for the first core ply 72 is two mil (0.05 mm) thick copper, and the material D for the second core ply 74 is two mil (0.05 mm) thick Inco 625.
This combination of materials results in a modular chevron 52 of the core nozzle 38 having negligible tip curl during the transient takeoff operating condition.
And, different material combinations may be used for different operating conditions and operating environments as desired.
Since the chevron fan nozzle 40 illustrated in
Nevertheless, the modular chevrons for the fan nozzle 40 may also be formed with suitably different materials, additionally including composite materials, for reducing changes in geometry thereof during operation.
The modular configuration of the individual chevrons 52 disclosed above provides strong, lightweight chevron modules which may be conveniently and economically premanufactured individually for later assembly. The common support flange 48 provides a fully annular supporting structure having enhanced rigidity and strength to which the individual modular chevrons may be attached or removed as desired.
The modular configuration of the chevrons also permits the use of different materials in the fabrication of the different components thereof, from the preferred multiple metal configurations disclosed above to advanced composite materials if desired. Such multiple materials may therefore be used to thermally balance operating temperatures and reduce thermal stress, distortion, and undesirable tip curl.
While much of the foregoing discussion has focused on exhaust nozzles and chevrons for gas turbine engines, it should be understood that the multilayer materials described herein may be employed in the fabrication of a wide variety of other structures, including but not limited to aero structures such as the exhaust nozzles and chevrons described herein but also to heat shields and other structures where the thermal balance and stability provided by such materials may be employed to advantage.
While there have been described herein what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein, and it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention.
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 in which we claim:
