Patent Description:
Acoustic liners and their use in gas turbine engines for acoustic treatment are well known. Acoustic liners typically consist of a perforated facing sheet, a backing sheet and a plurality of cells arranged between the facing sheet and the backing sheet. The cells have a cell depth and each define a cavity.

Generally, two damping mechanisms are effective in an acoustic liner, one being resistive damping caused by the conversion of acoustic energy of the incident sound waves into heat by causing particle oscillations in the pores of the absorber material, and the other being reactive cancellation in which incident sound waves are extinguished by waves reflected from the backing sheet. For reactive cancellation, only wavelengths of a certain range are cancelled depending on the depth of the individual cells of the acoustic liner. More particularly, incoming waves are absorbed by interference if the cell depth is equal to λ/<NUM> or an odd-numbered multiple of λ/<NUM>, with λ being the wavelength of the sound waves. Acoustic cells that absorb sound waves with a wavelength of λ are also called λ/<NUM> resonators or λ/<NUM> absorbers. They absorb sound waves in a relatively narrow frequency range which lies around the resonant frequency of the cells. Accordingly, the thickness of an acoustic liner depends on the dominant frequency range of the noise that shall be attenuated. Both resistive damping and reactive cancellation are present in a damping process.

There is an ongoing need to improve the reduction of noise of a gas turbine engine and, accordingly, an ongoing need to improve the quality of acoustic liners. For example, noise reduction is a particular challenge in a gas turbine engine in which the speed of the turbines is increased, such as in a geared gas turbine engine, and/or in a gas turbine engine in which the length of the nacelle is a decreased. There is a particular challenge if the noise that has to be reduced has relatively low frequencies as relatively low frequencies result in a required larger cell depth for λ/<NUM> absorption and thus thicker acoustic liners. However, available space for attachment of acoustic liners is limited in a gas turbine engine, in particular in the turbine and nozzle sections of a gas turbine engine.

Document <CIT> discloses a geared gas turbine engine in which a plurality of discrete acoustic liner segments with varied geometric properties are disposed along the bypass duct of the engine. Document <CIT> discusses the use of 3D-printing when manufacturing acoustic liners.

Document <CIT> discloses an acoustic liner with the features of the preamble of claim <NUM>.

The problem underlying the present invention is to provide for a low profile acoustic liner which is suitable to mitigate turbine tones generated in the turbine section of a geared gas turbine engine. Further, a gas turbine engine shall be provided with reduced noise level from the turbine section.

This problem is solved by an acoustic liner with the features of claim <NUM>, a gas turbine engine with the features of claim <NUM> and a gas turbine engine with the features of claim <NUM>. Embodiments of the invention are identified in the dependent claims.

According to first aspect of the invention, an acoustic liner is provided which comprises a facing sheet, a backing sheet and a plurality of cells arranged between the facing sheet and the backing sheet. The cells have a cell depth and each define a cavity. The facing sheet comprises a plurality of holes which define the porosity of the facing sheet, the porosity being defined by the ratio of the open area of the facing sheet (defined by the holes) to the overall area of the facing sheet. The facing sheet further has a facing sheet thickness.

A plurality of internal necks extending from the inner side of the facing sheet towards the backing sheet are provided, each internal neck being located around a hole of the facing sheet, thereby extending the longitudinal length of the hole. Accordingly, the longitudinal length of the hole is partly defined by the facing sheet thickness and partly defined by the length of the internal neck. The acoustic liner is adapted to have a peak attenuation at a frequency in the range between <NUM> and <NUM>.

The first aspect of the invention is thus based on the idea to provide for an extension of the longitudinal length of the holes provided in the facing sheet without increasing the overall thickness of the facing sheet. Instead, necks, which may also be referred to as projecting rims, are located around the holes and extend from the inner side of the facing sheet towards the backing sheet. By increasing the longitudinal length of the holes, the vibrating column of air that is located in the holes is extended in length. Accordingly, the air mass that is oscillating is increased. This again decreases the resonance frequency at which absorption of sound waves is at a maximum.

In other words, the inertance of the air mass in the facing sheet is increased, thereby reducing the resonance frequency. In particular, according to a simplified physical model, the resonance frequency fres is proportional to the root from <NUM>/L<NUM>, L<NUM> being the length of the hole and, thus, the length of the air column in the hole. Accordingly, the resonance frequency decreases if the length of hole increases.

This aspect of the invention allows to substantially reduce the resonance frequency at which the absorption of the acoustic liner is at a maximum compared to the λ/<NUM> resonance frequency solely defined by the cell depth. In embodiments, peak attenuation is less than <NUM>/<NUM> of the λ/<NUM> resonance frequency of the liner cell depth (at a temperature the liner was designed for, such as the temperature present at a location of the turbine section of a jet engine). Such reduction of the resonance frequency is achieved without the need to provide for a thick facing sheet which would negatively add to weight, specific fuel consumption and occupation of space. Instead, by providing local necks around the holes in the facing sheet, the facing sheet can be kept relatively thin while at the same time providing for an extended longitudinal length of the holes.

Aspects of the invention thus provide for a low profile acoustic liner which is substantially thinner than prior art acoustic liners. Such liner can be implemented particularly in a geared gas turbine engine, in particular the turbine section of a geared gas turbine engine. However, in principle, such liner can be implemented in any part of a gas turbine engine in which a low profile acoustic liner panel is of advantage, in particular where conventional panels require a much larger thickness at high temperatures (the conventional λ/<NUM> cell depth increases with temperature as the sound velocity increases with temperature). According to an embodiment, the facing sheet has a porosity in the range between <NUM> % and <NUM> %. In particular, the porosity of the facing sheet may be equal to or smaller than <NUM> %. According to this aspect, a very low porosity of the facing sheet is implemented. By reducing the porosity, i.e., the open area of the facing sheet, a high facing sheet air mass inertance is provided for and, accordingly, the resonance frequency is further decreased. According to a further embodiment, the facing sheet thickness is in the range between <NUM> and <NUM>. For example, the facing sheet thickness is in the range between <NUM> and <NUM>. As discussed, by providing internal necks around the holes in the facing sheet, the facing sheet thickness can be reduced to small values of about <NUM>. However, to further increase the length of the air column in the holes, in addition, the sheet thickness may be increased to up to <NUM>.

According to the invention, the cell depth of the cells is in the range between <NUM> and <NUM>. For example, the cell depth of the cells is equal to or smaller than <NUM>. Accordingly, a very low cell depth is provided for, considering that, according to the invention, the acoustic liner is adapted to have its peak attenuation at a frequency in the range between <NUM> and <NUM>. In particular, the frequency range in which the liner is adapted to have its peak attenuation may be between <NUM> and <NUM>, in particular between <NUM> and <NUM>. The very low cell depth can be achieved by implementing the necks that increase the longitudinal length of the cells without increasing the facing sheet thickness. According to the invention, the neck has a length up to <NUM>. The neck may be formed hollow cylindrical. This is associated with the advantage that the neck is a straight extension of the respective hole in the facing sheet. However, in principle, the neck may have other forms such as the form of a torus.

According to an embodiment, the holes in the facing sheet have a diameter in the range between <NUM> and <NUM>. In particular, the holes may have a diameter equal to or smaller than <NUM>. If the holes are not circular, which may be the case in embodiments, the diameter is the largest diameter of the hole. By providing for a small hole diameter, the facing sheet air mass inertance is further increased and the resonance frequency is further decreased.

In a further embodiment, the diameter of the cells is in the range between <NUM> and <NUM>. in particular, the diameter of the cells may be in the range between <NUM> and <NUM>. If the cells are not circular, which typically is the case, the diameter is the largest diameter of the cell.

Generally, the acoustic liner is designed in sandwich construction, with the facing sheet on the outside of a cell layer and the backing sheet on the inside. The backing sheet is of rigid nature in the sense that it reflects the incoming sound waves. It may also take on structural tasks. The cells may be prismatic cells and formed as a honeycomb structure. Instead of a hexagonal honeycomb structure, square, rectangular, rhombic or complex cell structures are also possible according to the present invention. Also, cells with a circular cross-section may be implemented.

If the cells of the acoustic liner are arranged in a honeycomb structure, the walls of the honeycomb structure may have a wall thickness in the range between <NUM> and <NUM>.

Generally, when referring to a range or interval, the boundary values of the range/interval are also included in the range/interval.

According to the invention, each cell of the acoustic liner is associated with a plurality of holes in the facing sheet. For example, the holes in the facing sheet are arranged such that between <NUM> and <NUM> holes are associated which each cell. The holes may be arranged in a certain geometric configuration such as, in the case of <NUM> holes, in a hexagonal form, or, in the case of the <NUM> holes, in a cross-like form.

The overall thickness of the liner including the thickness of the facing sheet and the thickness of the backing sheet may be in the range between <NUM> and <NUM>, in particular in the range between <NUM> and <NUM>, according to aspects of the present invention.

According to the invention, the complete acoustic liner is manufactured by 3D printing, including the backing sheet, the cells and the facing sheet. By implementing 3D printing, the neck can be provided for in an exact and efficient manner around the holes in the facing sheet. The acoustic liner may be printed from an organic material or a plastic material in embodiments. In other embodiments, the acoustic liner is printed from a nickel-base alloy.

However, the acoustic liner of the present invention is not limited to any particular method of manufacture. For example, in alternative embodiments, the acoustic liner may be a sintered material. More particularly, the acoustic liner may alternatively be formed by metal injection molding and sintering.

When manufacturing the acoustic liner by means of 3D printing, in an embodiment, the backing sheet comprises holes which are sized such that powder from the cells produced during 3D printing can be removed through such holes. The holes are sized and provided in an amount such that the rigid nature of the backing sheet required for reflecting the acoustic waves is not negatively affected by the holes.

According to a second aspect of the invention, a gas turbine engine is provided for. The gas turbine engine comprises an engine core with a compressor, a combustion equipment and a turbine. An acoustic liner in accordance with the present invention is provided for and attached to at least one surface of the turbine.

According to this aspect of the invention, the acoustic liner of the present invention is located in the turbine section of a gas turbine engine. As the acoustic liner has a low profile, it can be implemented in an efficient manner in such turbine section. The acoustic liner may be implemented in the high-pressure turbine and/or the low-pressure turbine.

In an example embodiment, the acoustic liner is attached to an annular structure surrounding the main gas path through the turbine. For example, it is attached to an outer annular structure and/or an inner annular structure surrounding the main gas path through the turbine. In an embodiment, it may be attached to an annulus structure of the low-pressure turbine of the turbine section. For example, it may be attached to an annulus structure of the outlet guide vane (OGV) of the most downstream turbine (the low-pressure turbine).

In another embodiment, the acoustic liner is attached to struts or guide vanes of the turbine. For example, it may be attached to the outlet guide vanes of the low-pressure turbine. The acoustic liner may be attached to the pressure side and/or the suction side of the guide vanes.

The acoustic liner may be attached to struts or guide vanes by welding, bolting or gluing according to different embodiments.

The gas turbine engine may be a geared gas turbine engine. Accordingly, the gas turbine engine further comprises a core shaft connecting the turbine to the compressor, a fan located upstream of the engine core, and a gearbox that receives an input from the core shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft. In a geared gas turbine engine, due to the higher-speed and smaller diameter of the turbine, a low noise cut off design of the turbine is challenging to achieve. It is, therefore, of advantage to implement the acoustic panel in the turbine section of a geared gas turbine engine.

In a third aspect of the invention, a gas turbine engine for an aircraft is provided which comprises guide vanes in a compressor section and in a turbine section of the gas turbine engine. It is provided that an acoustical liner in accordance with claim <NUM> is attached to struts and/or to guide vanes of at least one guide wheel in at least one of the compressor section and the turbine section of the gas turbine engine.

This aspect of the present invention is thus based on the idea to place an acoustic liner directly on a guide vane or a strut. Thereby, the production of noise is reduced at an early stage before the propagating sound waves are reflected at the boundaries of the gas path. This aspect of the invention, therefore, provides for effective noise reduction. The acoustic liner may be any acoustic liner.

It should be noted that the present invention is described in terms of a cylindrical coordinate system having the coordinates x, r and ϕ. Here x indicates the axial direction, r the radial direction and ϕ the angle in the circumferential direction. The axial direction is defined by the machine axis of the gas turbine engine in which the acoustic liner of the present invention is implemented, with the axial direction pointing from the engine inlet to the engine outlet. The axial direction of the planetary gearbox is identical to the axial direction of the gas turbine engine. Starting from the x-axis, the radial direction points radially outwards.

Terms such as "in front of" and "behind" refer to the axial direction or flow direction in the engine. Terms such as "outer" or "inner" refer to the radial direction.

Each fan blade may be defined as having a radial span extending from a root (or hub) at a radially inner gas-washed location, or <NUM>% span position, to a tip at a <NUM>% span position. The ratio of the radius of the fan blade at the hub to the radius of the fan blade at the tip may be less than (or on the order of) any of: <NUM>, <NUM>, <NUM><NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>. The ratio of the radius of the fan blade at the hub to the radius of the fan blade at the tip may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds). These ratios may commonly be referred to as the hub-to-tip ratio. The radius at the hub and the radius at the tip may both be measured at the leading edge (or axially forwardmost) part of the blade. The hub-to-tip ratio refers, of course, to the gas-washed portion of the fan blade, i.e. the portion radially outside any platform.

The radius of the fan may be measured between the engine centreline and the tip of a fan blade at its leading edge. The fan diameter (which may simply be twice the radius of the fan) may be greater than (or on the order of) any of: <NUM> (around <NUM> inches), <NUM>, <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM>, <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM>, <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM> (around <NUM> inches) cm or <NUM> (around <NUM> inches). The fan diameter may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds).

The rotational speed of the fan may vary in use. Generally, the rotational speed is lower for fans with a higher diameter. Purely by way of non-limitative example, the rotational speed of the fan at cruise conditions may be less than <NUM> rpm, for example less than <NUM> rpm. Purely by way of further non-limitative example, the rotational speed of the fan at cruise conditions for an engine having a fan diameter in the range of from <NUM> to <NUM> (for example <NUM> to <NUM>) may be in the range of from <NUM> rpm to <NUM> rpm, for example in the range of from <NUM> rpm to <NUM> rpm, for example in the range of from <NUM> rpm to <NUM> rpm. Purely by way of further non-limitative example, the rotational speed of the fan at cruise conditions for an engine having a fan diameter in the range of from <NUM> to <NUM> may be in the range of from <NUM> rpm to <NUM> rpm, for example in the range of from <NUM> rpm to <NUM> rpm, for example in the range of from <NUM> rpm to <NUM> rpm.

In use of the gas turbine engine, the fan (with associated fan blades) rotates about a rotational axis. This rotation results in the tip of the fan blade moving with a velocity Utip. The work done by the fan blades <NUM> on the flow results in an enthalpy rise dH of the flow. A fan tip loading may be defined as dH/Utip2, where dH is the enthalpy rise (for example the <NUM>-D average enthalpy rise) across the fan and Utip is the (translational) velocity of the fan tip, for example at the leading edge of the tip (which may be defined as fan tip radius at leading edge multiplied by angular speed). The fan tip loading at cruise conditions may be greater than (or on the order of) any of: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM> (all units in this paragraph being Jkg-<NUM>-<NUM>/(ms-<NUM>)<NUM>). The fan tip loading may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds).

Gas turbine engines in accordance with the present disclosure may have any desired bypass ratio, where the bypass ratio is defined as the ratio of the mass flow rate of the flow through the bypass duct to the mass flow rate of the flow through the core at cruise conditions. In some arrangements the bypass ratio may be greater than (or on the order of) any of the following: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>. The bypass ratio may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds). The bypass duct may be substantially annular. The bypass duct may be radially outside the core engine. The radially outer surface of the bypass duct may be defined by a nacelle and/or a fan case.

The overall pressure ratio of a gas turbine engine as described and/or claimed herein may be defined as the ratio of the stagnation pressure upstream of the fan to the stagnation pressure at the exit of the highest pressure compressor (before entry into the combustor). By way of non-limitative example, the overall pressure ratio of a gas turbine engine as described and/or claimed herein at cruise may be greater than (or on the order of) any of the following: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. The overall pressure ratio may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds).

Specific thrust of an engine may be defined as the net thrust of the engine divided by the total mass flow through the engine. At cruise conditions, the specific thrust of an engine described and/or claimed herein may be less than (or on the order of) any of the following: <NUM> Nkg-<NUM>, <NUM> Nkg-<NUM>, <NUM> Nkg-<NUM>, <NUM> Nkg-<NUM>, <NUM> Nkg-<NUM>, <NUM> Nkg-<NUM> or <NUM> Nkg-<NUM>. The specific thrust may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds). Such engines may be particularly efficient in comparison with conventional gas turbine engines.

A gas turbine engine as described and/or claimed herein may have any desired maximum thrust. Purely by way of non-limitative example, a gas turbine as described and/or claimed herein may be capable of producing a maximum thrust of at least (or on the order of) any of the following: 160kN, 170kN, 180kN, 190kN, 200kN, 250kN, 300kN, 350kN, 400kN, 450kN, 500kN, or 550kN. The maximum thrust may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds). The thrust referred to above may be the maximum net thrust at standard atmospheric conditions at sea level plus <NUM> deg C (ambient pressure <NUM>. 3kPa, temperature <NUM> deg C), with the engine static.

In use, the temperature of the flow at the entry to the high pressure turbine may be particularly high. This temperature, which may be referred to as TET, may be measured at the exit to the combustor, for example immediately upstream of the first turbine vane, which itself may be referred to as a nozzle guide vane. At cruise, the TET may be at least (or on the order of) any of the following: <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>. The TET at cruise may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds). The maximum TET in use of the engine may be, for example, at least (or on the order of) any of the following: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>. The maximum TET may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds). The maximum TET may occur, for example, at a high thrust condition, for example at a maximum take-off (MTO) condition.

A fan blade and/or aerofoil portion of a fan blade described and/or claimed herein may be manufactured from any suitable material or combination of materials. For example, at least a part of the fan blade and/or aerofoil may be manufactured at least in part from a composite, for example a metal matrix composite and/or an organic matrix composite, such as carbon fibre. By way of further example at least a part of the fan blade and/or aerofoil may be manufactured at least in part from a metal, such as a titanium based metal or an aluminium based material (such as an aluminium-lithium alloy) or a steel based material. The fan blade may comprise at least two regions manufactured using different materials. For example, the fan blade may have a protective leading edge, which may be manufactured using a material that is better able to resist impact (for example from birds, ice or other material) than the rest of the blade. Such a leading edge may, for example, be manufactured using titanium or a titanium-based alloy. Thus, purely by way of example, the fan blade may have a carbon-fibre or aluminium based body (such as an aluminium lithium alloy) with a titanium leading edge.

A fan as described and/or claimed herein may comprise a central portion, from which the fan blades may extend, for example in a radial direction. The fan blades may be attached to the central portion in any desired manner. For example, each fan blade may comprise a fixture which may engage a corresponding slot in the hub (or disc). Purely by way of example, such a fixture may be in the form of a dovetail that may slot into and/or engage a corresponding slot in the hub/disc in order to fix the fan blade to the hub/disc. By way of further example, the fan blades maybe formed integrally with a central portion. Such an arrangement may be referred to as a blisk or a bling. Any suitable method may be used to manufacture such a blisk or bling. For example, at least a part of the fan blades may be machined from a block and/or at least part of the fan blades may be attached to the hub/disc by welding, such as linear friction welding.

The gas turbine engines described and/or claimed herein may or may not be provided with a variable area nozzle (VAN). Such a variable area nozzle may allow the exit area of the bypass duct to be varied in use. The general principles of the present disclosure may apply to engines with or without a VAN.

The fan of a gas turbine as described and/or claimed herein may have any desired number of fan blades, for example <NUM>, <NUM>, <NUM>, or <NUM> fan blades.

As used herein, cruise conditions may mean cruise conditions of an aircraft to which the gas turbine engine is attached. Such cruise conditions may be conventionally defined as the conditions at mid-cruise, for example the conditions experienced by the aircraft and/or engine at the midpoint (in terms of time and/or distance) between top of climb and start of decent.

Purely by way of example, the forward speed at the cruise condition may be any point in the range of from Mach <NUM> to <NUM>, for example <NUM> to <NUM>, for example <NUM> to <NUM>, for example <NUM> to <NUM>, for example <NUM> to <NUM>, for example <NUM> to <NUM>, for example on the order of Mach <NUM>, on the order of Mach <NUM> or in the range of from <NUM> to <NUM>. Any single speed within these ranges may be the cruise condition. For some aircraft, the cruise conditions may be outside these ranges, for example below Mach <NUM> or above Mach <NUM>.

Purely by way of example, the cruise conditions may correspond to standard atmospheric conditions at an altitude that is in the range of from <NUM> to <NUM>, for example in the range of from <NUM> to <NUM>, for example in the range of from <NUM> to <NUM> (around <NUM> ft), for example in the range of from <NUM> to <NUM>, for example in the range of from <NUM> to <NUM>, for example in the range of from <NUM> (around <NUM> ft) to <NUM>, for example in the range of from <NUM> to <NUM>, for example in the range of from <NUM> to <NUM>, for example on the order of <NUM>. The cruise conditions may correspond to standard atmospheric conditions at any given altitude in these ranges.

Purely by way of example, the cruise conditions may correspond to: a forward Mach number of <NUM>; a pressure of <NUM> Pa; and a temperature of -<NUM> deg C.

As used anywhere herein, "cruise" or "cruise conditions" may mean the aerodynamic design point. Such an aerodynamic design point (or ADP) may correspond to the conditions (comprising, for example, one or more of the Mach Number, environmental conditions and thrust requirement) for which the fan is designed to operate. This may mean, for example, the conditions at which the fan (or gas turbine engine) is designed to have optimum efficiency.

In use, a gas turbine engine described and/or claimed herein may operate at the cruise conditions defined elsewhere herein. Such cruise conditions may be determined by the cruise conditions (for example the mid-cruise conditions) of an aircraft to which at least one (for example <NUM> or <NUM>) gas turbine engine may be mounted in order to provide propulsive thrust.

The invention will be explained in more detail on the basis of exemplary embodiments with reference to the accompanying drawings in which:.

The planet carrier <NUM> constrains the planet gears <NUM> to process around the sun gear <NUM> in synchronicity whilst enabling each planet gear <NUM> to rotate about its own axis.

Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in <FIG> has a split flow nozzle <NUM>, <NUM> meaning that the flow through the bypass duct <NUM> has its own nozzle that is separate to and radially outside the core engine nozzle <NUM>. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct <NUM> and the flow through the core <NUM> are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine <NUM> may not comprise a gearbox <NUM>.

In the context of the present invention the provision of an acoustic liner is of relevance, in particular the provision of an acoustic liner arranged in the high-pressure turbine <NUM> and/or the low pressure turbine <NUM> of the gas turbine engine.

<FIG> shows an embodiment of an acoustic liner <NUM>. The acoustic liner <NUM> has a sandwich construction with a top facing sheet <NUM>, a bottom backing sheet <NUM> and a plurality of cells <NUM> arranged between the facing sheet <NUM> and the backing sheet <NUM>. The backing sheet <NUM> may be attached to a further structure <NUM>.

The facing sheet <NUM> has an upper side <NUM> and an inner side <NUM> that faces towards the backing sheet <NUM>. A thickness d of the facing sheet <NUM> is defined by the distance between the upper side <NUM> and the inner side <NUM> of the facing sheet <NUM>. The facing sheet <NUM> comprises a plurality of holes <NUM>. The holes <NUM> define a porosity of the facing sheet <NUM>, the porosity being defined by the ratio of the open area of the facing sheet (given by the holes <NUM>) to the complete area of the facing sheet. As will be discussed with respect to <FIG>, a plurality of holes <NUM> are associated with each cell, wherein typically between three and six holes <NUM> are associated with each cell <NUM>.

The individual cells <NUM> are prismatic cells. They may have a hexagonal, square, rectangular, rhombic or other cell structure. The cells <NUM> form a honeycomb structure. The individual cells <NUM> are separated by walls <NUM>. Each cell <NUM> has a cell depth L<NUM> which is defined by the distance between the inner side <NUM> of the facing sheet <NUM> and the upper side of the backing sheet <NUM>.

The backing sheet <NUM> is of rigid nature such that sound waves entering the cells <NUM> through the holes <NUM> are reflected by the backing sheet <NUM>.

The acoustic liner <NUM> further comprises a plurality of internal necks <NUM>. Each neck <NUM> is located around one of the holes <NUM> and extends from the inner side <NUM> of the facing sheet <NUM> towards the backing sheet <NUM>. The neck <NUM> has a length which is less than the cell depth L<NUM>. The neck <NUM> increases the longitudinal length L<NUM> of the respective hole <NUM>. The longitudinal length L<NUM> of the respective hole <NUM> is the sum of the facing sheet thickness d and the length of the neck <NUM>, which is L<NUM> minus d.

As shown in <FIG>, the neck <NUM> has a hollow cylindrical wall <NUM>, the cross-section of which is identical to the cross section of the section of the hole <NUM> that is formed in the facing sheet <NUM>. Accordingly, there is a smooth transition from the section of the hole <NUM> that is formed in the facing sheet <NUM> to the section of the hole <NUM> that is defined by the neck <NUM>.

However, in other embodiments, the inner diameter and/or cross-section of the hole <NUM> may differ in the section of the hole <NUM> surrounded by the neck <NUM> and the section of the hole <NUM> surrounded by the facing sheet <NUM>. Also, the form of the neck <NUM> may be different from a hollow cylindrical form. The lower edge of the neck <NUM> may be rounded.

In an embodiment, the acoustic liner <NUM> has been manufactured by 3D printing. In particular, the neck <NUM> has been 3D printed together with the facing sheet <NUM>, the walls <NUM> of the cells <NUM> and the backing sheet <NUM>. This allows for an exact placement of the neck <NUM> at the inner side <NUM> of the facing sheet <NUM>. In alternative embodiments, the acoustic liner <NUM> may be manufactured by metal injection molding and sintering.

By providing an extended length L<NUM> of the hole <NUM>, the column of air in the hole <NUM> is also increased, thereby reducing the resonance frequency of the cell <NUM>. This is achieved without the necessity to increase the depth L<NUM> of the cells <NUM> and without increasing the facing sheet thickness d.

<FIG> show two alternative examples which implement a different structure to achieve a resonance frequency comparable to that of the embodiment of <FIG>. In <FIG>, the cell depth L<NUM>' has been increased to be longer than the cell length L<NUM> of <FIG>. This is associated with the disadvantage of a larger overall thickness of the liner. In <FIG>, the facing sheet thickness d' has been increased to provide for a comparable longitudinal length of the holes <NUM>. This is associated with the disadvantage of requiring both a thicker facing sheet <NUM> and a larger overall thickness of the liner.

It is pointed out that <FIG> is a schematic cross-sectional view of an acoustic liner <NUM> and that the dimensions of a manufactured acoustic liner and its components may be different.

<FIG> show different embodiments of an acoustic liner which differ in the form of the acoustic cells <NUM> and the number and arrangement of the holes <NUM>.

In the embodiment of <FIG>, the cells <NUM> are rhombic cells and each cell is associated with six holes <NUM> in the facing sheet <NUM>, the six holes <NUM> being arranged in a hexagonal manner. The holes <NUM> are each provided with an internal neck <NUM>. <FIG> also shows a cross-sectional view of one of the cells <NUM> along the line indicated in <FIG>. The cross-sectional structure is identical to that discussed with respect to <FIG>.

In the embodiment of <FIG>, the cells <NUM> are hexagonal cells and each cell is associated with six holes <NUM> in the facing sheet <NUM>, the six holes <NUM> also being arranged in a hexagonal manner.

In the embodiment of <FIG>, the cells are rhombic cells as in <FIG>, wherein each cell is associated with five holes <NUM> in the facing sheet <NUM>, the five holes <NUM> being arranged in a cross-like manner.

In the embodiment of <FIG>, the cells are hexagonal cells and each cell is associated with five holes <NUM> in the facing sheet <NUM>, the five holes being arranged in a cross-like manner.

In each of these embodiments, the holes <NUM> are provided with internal necks <NUM> as discussed with respect to <FIG>.

In the following, several examples are given for combinations of parameters that an acoustic liner <NUM> built in accordance with <FIG> may implement.

According to a first embodiment, the porosity of the facing sheet <NUM> is <NUM> percent. The facing sheet thickness d is <NUM> millimeter. The diameter of the holes <NUM> is <NUM> millimeter. The length of the neck <NUM> is <NUM> millimeter. The cell depth L<NUM> is <NUM> millimeter. The cell diameter is <NUM> millimeter. The wall thickness of the honeycomb walls <NUM> is <NUM> millimeter. This leads to an overall thickness of the liner of about <NUM> millimeter. The number of holes per cell may is <NUM> or <NUM>. Such acoustic liner has a resonance frequency of about <NUM>.

In a second embodiment, the length of the neck <NUM> is increased to <NUM> millimeter. This results in a decreased resonance frequency of about <NUM>.

In a third embodiment, the length of the neck <NUM> is increased to <NUM> millimeter. This results in a further decreased resonance frequency of about <NUM>.

In a fourth embodiment, the length of the neck <NUM> is increased to <NUM> millimeter. This results in a further decreased resonance frequency of about <NUM>.

It is pointed out that the measurements of the resonance frequency in the first to fourth embodiments discussed above were carried out in a cold condition. In the hot gas of a low-pressure turbine the sound velocity is about <NUM>/s, compared to about <NUM>/s at <NUM>. Accordingly, it can be assumed that in the hot temperature condition the resonance frequencies are about <NUM> times larger, thus ranging from about <NUM> in the first embodiment to <NUM> in the fourth embodiment, this showing the effect of an increased neck length to reduce the resonance frequency.

According to a fifth embodiment, the porosity of the facing sheet <NUM> is <NUM> percent. The facing sheet thickness d is <NUM> millimeter. The diameter of the holes <NUM> is <NUM> millimeter. The length of the neck <NUM> is <NUM> millimeter. The cell depth L<NUM> is <NUM> millimeter. The cell diameter is <NUM> millimeter. The wall thickness of the honeycomb walls <NUM> is <NUM> millimeter. This leads to an overall thickness of the liner of about <NUM> millimeter. The number of holes per cell may is <NUM> or <NUM>. Such acoustic liner has a resonance frequency of about <NUM>,<NUM> in a cold condition.

<FIG> discuss locations in a gas turbine engine in which the acoustic liner of <FIG> may be arranged. <FIG> shows a section of a low-pressure turbine such as of the low-pressure turbine <NUM> of <FIG>. The section that is shown is a static (non-rotating) section <NUM> which comprises a static outer casing <NUM> and a static inner casing <NUM>. Between the inner casing <NUM> and the outer casing <NUM> runs the main flow path <NUM> of the core engine. A plurality of airfoils <NUM> extend in the radial direction through the main flow path <NUM> between the inner casing <NUM> and the outer casing <NUM>. For example, the airfoils <NUM> are guide vanes, in particular outlet guide vanes of the low-pressure turbine. In other embodiments, the airfoils may be struts that carry loads between the inner casing <NUM> and the outer casing <NUM>. Also, it may be provided that guide vanes at least partly also fulfill the function of struts.

As schematically depicted, acoustic liners <NUM> having a structure as discussed with respect to <FIG> may be arranged on the inner casing <NUM> and/or the outer casing <NUM> facing the main flow path <NUM>.

<FIG> shows a detail of the low-pressure turbine section of <FIG> showing the inner casing <NUM>, the outer casing <NUM>, the main flow path <NUM> and several airfoils <NUM>. The airfoils <NUM> each have a pressure side <NUM> and a section side <NUM>. In the embodiment of <FIG>, an acoustic liner <NUM> with a structure as discussed with respect to <FIG> is attached to the pressure side <NUM> of at least one airfoil <NUM>. In addition, or alternatively, the acoustic liner <NUM> may be attached to the suction side <NUM> of the airfoil <NUM>. The acoustic liner <NUM> may, e.g., be welded, bolted or glued to the airfoil <NUM>.

<FIG> is a perspective enlarged view of a section of the acoustic liner <NUM> of <FIG>, in accordance with reference numeral X1 of <FIG>, wherein the view is onto the facing sheet <NUM>. Between the facing sheet <NUM> and the backing sheet <NUM> a plurality of hexagonal cells <NUM> with walls <NUM> are implemented. Holes <NUM> in the facing sheet are extended by means of internal necks <NUM> as discussed with respect to <FIG>.

<FIG> is a perspective enlarged view of a section of the acoustic liner <NUM> of <FIG>, in accordance with reference numeral X2 of <FIG>, wherein the view is onto the backing sheet <NUM>. In <FIG>, the necks <NUM> formed as hollow cylindrical structures that extend from the underside of the facing sheet <NUM> are well shown.

In the embodiment of <FIG>, the facing sheet <NUM> with holes <NUM> is facing towards the main gas path <NUM>.

In an alternative embodiment, the acoustic liner <NUM> that is attached to at least one airfoil <NUM> may be any liner and is not necessarily a liner as shown in <FIG>.

<FIG> shows a further embodiment of an acoustic liner <NUM> that includes necks <NUM> which are located around holes <NUM> in the facing sheet <NUM>. The difference with respect to the embodiment of <FIG> lies in that the backing sheet <NUM> also comprises holes <NUM>. These holes <NUM> are provided and configured to be able to remove powder from the cells <NUM> left during manufacture of the acoustic liner <NUM> by 3D printing. In an embodiment, all elements of the acoustic liner <NUM> including the facing sheet <NUM>, the necks <NUM>, the cell walls <NUM> and the backing sheet <NUM> are produced in one manufacturing process by 3D printing.

Claim 1:
Acoustic liner (<NUM>) which comprises:
- a facing sheet (<NUM>) that comprises holes (<NUM>), the facing sheet (<NUM>) having a porosity and a facing sheet thickness (d),
- a backing sheet (<NUM>),
- a plurality of cells (<NUM>) arranged between the facing sheet (<NUM>) and the backing sheet (<NUM>), the cells (<NUM>) having a cell depth (L<NUM>) and each cell defining a cavity, and
- a plurality of internal necks (<NUM>) extending from the inner side (<NUM>) of the facing sheet (<NUM>) towards the backing sheet (<NUM>), each internal neck (<NUM>) being located around a hole (<NUM>) of the facing sheet (<NUM>), thereby extending the longitudinal length (L<NUM>) of the hole (<NUM>),
characterized in that
- the acoustic liner (<NUM>) is adapted to have a peak attenuation at a frequency in the range between <NUM> and <NUM>,
- the cell depth (L<NUM>) of the cells (<NUM>) is in the range between <NUM> and <NUM>,
- the neck (<NUM>) has a length up to <NUM> millimeter,
- the facing sheet (<NUM>) including the plurality of internal necks (<NUM>), the backing sheet (<NUM>) and the cells (<NUM>) have been manufactured by 3D printing, and
- each cell (<NUM>) of the acoustic liner is associated with a plurality of holes (<NUM>) in the facing sheet (<NUM>), each of the holes (<NUM>) provided with an internal neck (<NUM>).