Patent Description:
Gas turbine engines generate audible frequencies during operation, commonly considered to be "noise. " Acoustic liners are used to attenuate such noise. A typical acoustic liner includes a cellular structure sandwiched between a back sheet and a perforated face sheet. The liner may be used in the fan case or nacelle of the engine.

<CIT> discloses a noise attenuation panel used to attenuate noise in aircraft engines includes a cellular core and a facing sheet formed with an array of holes.

From a first aspect, the invention provides an acoustic liner for a gas turbine engine as claimed in claim <NUM>.

In a further embodiment of any of the foregoing embodiments, each elongated slot defines a slot length (L) and a slot width (W), and a ratio of L to W is from <NUM> to <NUM>.

In a further embodiment of any of the foregoing embodiments, the elongated slots are arranged in multiple circumferential rows.

In a further embodiment of any of the foregoing embodiments, each elongated slot defines a slot width (W), the elongated slots define a slot-to-slot spacing (S), and a ratio of S to W is from <NUM> to <NUM>.

In a further embodiment of any of the foregoing embodiments, the elongated slots are linear.

In a further embodiment of any of the foregoing embodiments, the elongated slots are parallel to each other.

In a further embodiment of any of the foregoing embodiments, the oblique angles are from <NUM>° to <NUM>°.

In a further embodiment of any of the foregoing embodiments, each elongated slot defines a slot width (W), face sheet defines a thickness (T), width (W) is from <NUM> to <NUM> inches (about <NUM> to <NUM>), and thickness (T) is from <NUM> to <NUM> inches (about <NUM> to <NUM>).

In a further embodiment of any of the foregoing embodiments, elongated slots are arranged in adjacent slot patterns, each elongated slot defines a slot width (W), each one of the adjacent slot patterns defines a minimum pattern-to-pattern gap (G), and a ratio of the pattern-to-pattern gap (G) to the slot width (W) is <NUM> or greater.

In a further embodiment of any of the foregoing embodiments, the elongated slots are aligned with cells of the cellular structure.

From a further aspect, the invention provides a gas turbine engine as claimed claim in <NUM>.

From a still further aspect, the present invention provides a method of fabricating a face sheet for an acoustic liner as claimed in claim <NUM>. In a further embodiment of any of the foregoing embodiments, the face sheet is formed on a cellular structure and the slot centerlines are sloped at oblique angles to the central axis.

In a further embodiment of any of the foregoing embodiments, the elongated slots are formed in alignment with cells of the cellular structure.

In a further embodiment of any of the foregoing embodiments, the face sheet is fused directly to the cellular structure without an adhesive.

The engine <NUM> generally includes a fan <NUM>, a compressor section <NUM>, a combustor <NUM>, and a turbine section <NUM>. The turbine section <NUM> is coupled by a shaft <NUM> to the compressor section <NUM> and the fan <NUM>, which are all rotatable about a central engine axis A. The shaft <NUM> may be coupled through a speed reduction gear <NUM> to the fan <NUM>. A housing <NUM>, such as a casing and/or nacelle, surrounds the fan <NUM>. A core case <NUM> surrounds the core engine components, such as the compressor section <NUM> and the turbine section <NUM>. The portion of the housing <NUM> aft of the fan <NUM> and the core case <NUM> define a bypass passage B there between. The portion of the housing <NUM> forward of the fan <NUM> defines an inlet region <NUM>.

A portion of incoming air from the inlet region <NUM> enters the core engine and is pressurized in the compressor section <NUM>. The pressurized air is provided to the combustor <NUM>, where the air is mixed with fuel and ignited to produce a high velocity gas flow that expands through the turbine section <NUM>. The turbine section <NUM> rotationally drives the compressor section <NUM> and the fan <NUM> via the shaft <NUM>. The rotation of the fan <NUM> moves air from the inlet region <NUM> through the bypass passage B to provide thrust. Although shown schematically, this disclosure is not limited to the depicted engine architecture.

Operation of the engine <NUM> may produce noise. In this regard, the housing <NUM> includes one or more acoustic liners <NUM> for reducing noise. In the example shown, the housing <NUM> includes acoustic liners <NUM> forward of the fan <NUM> in the inlet region <NUM> and forward fan case, another acoustic liner <NUM> at an inter-stage location between the fan <NUM> and exit guide vanes <NUM>, a trailing acoustic liner <NUM> aft of the exit guide vanes <NUM>, and an acoustic liner <NUM> in the tailpipe. The liners <NUM> in the housing <NUM> are located on the outer boundary of the inlet region <NUM> and bypass passage B. Additionally, the core case <NUM> can include an acoustic liner <NUM> on the inner boundary of the bypass passage B and/or in the tailpipe. As will be appreciated, such locations are for example only, and acoustic liners <NUM> may be excluded from one or more of the locations shown and/or additionally used elsewhere in the engine <NUM>.

The acoustic liner <NUM> is generally provided as an annular structure. In this regard, the acoustic liner <NUM> can be constructed of one or more acoustic panels <NUM>, a representative one of which is shown in cross-section in <FIG>. The one or more panels <NUM> are curved about a central axis which is coincident with the central engine axis A. The panel <NUM> may be attached to a structural portion of the housing <NUM> or core case <NUM> in a known manner, such as with fasteners. For instance, panels <NUM> may be provided as arc segments that are attached to provide the annular structure.

Referring also to <FIG> that illustrates a cutaway view of a portion of the panel <NUM> and <FIG> that illustrates a plan view of a portion of the panel <NUM>, the panel <NUM> includes a support backing <NUM> (e.g., a back sheet), a face sheet <NUM>, and a cellular structure <NUM> (e.g., a honeycomb structure) disposed between the support backing <NUM> and the face sheet <NUM>. The face sheet <NUM> faces toward, and is exposed to, the acoustic environment, which in the illustrated engine <NUM> is the bypass passage B or inlet region <NUM>.

The support backing <NUM>, the face sheet <NUM>, and the cellular structure <NUM> can be composed of metal alloys, polymers, or composites, and may be attached together, such as with an adhesive, soldering, or brazing, as applicable. As shown, the architecture of the panel <NUM>, having a single layer of the cellular structure <NUM>, is what is known as a "single degree of freedom" construction. As will be appreciated, this disclosure is not limited to such architectures and the examples herein can also be applied to "double degree of freedom" constructions, "three degree of freedom" constructions, as well as other architectures.

The face sheet <NUM> defines elongated slots <NUM> that extend along respective slot centerlines C in the plane of the face sheet <NUM>. The slots <NUM> of the face sheet <NUM> combined with the cellular structure <NUM> produces a resonant acoustic liner which dissipates acoustic energy as air alternately pumps into and out of the face sheet <NUM> due to acoustic excitation of the resonant liner. Each slot <NUM> defines a first, or axially aft, slot end 50a, a second, or axially forward, slot end 50b, and sides 50c/50d that join the ends 50a/50b. In the illustrated example, the slots <NUM> are generally rectangular in that the sides 50c/50d are parallel along the length of the slot <NUM> and the ends 50a/50b are squared. However, the slots <NUM> are not limited to such a shape. For instance, the ends 50a/50b may be rounded to eliminate definitive corners. Most typically, however, at least the sides 50c/50d will be parallel to facilitate manufacturing and slot spacing. The slots <NUM> may be formed in the face sheet <NUM> by machining, laser cutting, stamping, additive manufacturing or other similar technique.

With reference to <FIG>, the slots <NUM> are provided in a pattern as represented at <NUM> that is defined by the number of slot rows <NUM>, the slot width (W), the slot angle (OA) relative to the engine axis A, the ratio of the slot length (L) to slot width (W), the ratio of the slot spacing (S) to slot width (W), and the ratio of slot offset (Q) to slot length (L). The slot length (L) is the end-to-end length between the ends 50a/50b taken along the slot centerlines C, while the slot width (W) is the distance between the sides 50c/50d taken perpendicular to the slot centerline C. The slot-to-slot spacing (S) is the distance between the side 50c of one slot <NUM> and the side 50d of the next adjacent slot <NUM> taken perpendicular to the slot centerlines C. The slot offset (Q) is the circumferential spacing between ends 50a of adjacent slots <NUM>, measured in the direction of fan rotation, which is denoted by R in <FIG>. <FIG>, <FIG>, and <FIG> show, respectively, three exemplary slot patterns. In <FIG> the slot pattern has a positive offset, in <FIG> the slot pattern has a zero offset, and in <FIG> the slot pattern has a negative offset. The positive or negative offsets are taken with regard to the direction of fan rotation R, in which offset in the direction of fan rotation R is positive and offset in the direction opposite the fan direction is negative.

In the illustrated example, the parameters that define the slot pattern are the same for each slot. However, the slot pattern is not limited to such a configuration and may contain slots of different width, angle, length, spacing and offset in order to reduce aerodynamic losses, increase acoustic attenuation and improve impact resistance of the face sheet. For example, <FIG> shows a slot pattern with slot length (L) and slot angle (OA) varying between the rows <NUM>. Such a configuration may be desirable to strengthen the face sheet near the fan trailing edge where impact resistance is most important, and to better align the slots as the airflow angle evolves downstream from the fan.

Also with reference to <FIG>, the face sheet <NUM> may have one or more slot patterns, for example <NUM>, <NUM>, <NUM> and <NUM>, which are circumferentially spaced by the pitch (P). The pitch (P) establishes the smallest or minimum pattern-to-pattern gap (G) between the two closest points, 50e and 50f, on adjacent patterns, for example slot patterns <NUM> and <NUM>. In the illustrated example, the slot patterns <NUM>, <NUM>, <NUM> and <NUM> are identical to each other and uniformly spaced. However, the face sheet is not limited to such a configuration and may contain different slot patterns and pitches.

With reference to <FIG>, the slotted face sheet <NUM> has thickness (T), which is the distance between surface 46a and surface 46b taken perpendicular to surface 46a. To improve impact resistance, the thickness (T) may be variable in a direction D1, which may be parallel to the engine axis A. For example, the slotted face sheet <NUM> may have its thickest value near the fan <NUM> trailing edge TE to improve its capability to withstand impact from ice that may be shed from the fan <NUM>.

As shown in the figures, the slots <NUM>, or at least groups of the slots <NUM>, are parallel to each other. Additionally, the slots <NUM>, or at least groups of the slots <NUM>, are all of the same length and width. As also shown in <FIG>, the slots <NUM>, or groups of slots <NUM>, can be arranged in a circumferential row <NUM> (the central engine axis A is shown for reference). In the illustrated example, the slots <NUM> are arranged in six circumferential rows, but additional or fewer circumferential rows may be used.

Perforations in acoustic liners can create drag that debits aerodynamic performance of an engine. In this regard, the slots <NUM> of the panel <NUM> are sloped with regard the angular orientations of the slots <NUM> to the central engine axis A to reduce drag. For example, each slot <NUM> is sloped at an oblique angle (OA) to the central axis about which the panel <NUM> is curved (i.e., the central engine axis A). For instance, the angles are defined by the slot centerlines C and the central engine axis A. As an example, the angles (OA) are from <NUM>° to <NUM>°. In this regard, because of the slope of the slots <NUM>, the axially aft slot end 50a of each slot <NUM> is circumferentially offset from the axially forward slot end 50b of the slot <NUM>.

In embodiments, the slots <NUM>, or at least groups of the slots <NUM>, are all of the same angle (OA). In further examples, a plurality of circumferentially consecutive slots <NUM> in each circumferential row <NUM>, such as five slots, ten slots, twenty-five slots, or all of the slots <NUM> in the circumferential row <NUM>, have the same angle (OA). In yet a further example, all or at least a group of slots <NUM> in a circumferential row <NUM> have the same angle (OA) as all or at least a group of slots <NUM> in another circumferential row <NUM>.

To reduce drag, the slots <NUM> are oriented at the angles (OA) such that the slots <NUM> are approximately perpendicular to an expected airflow direction, represented at AD in <FIG>. In this regard, the panel <NUM> may especially be employed in locations aft of the fan <NUM> where there is directional oblique airflow from the fan and, in particular, at an inter-stage location between the fan <NUM> and the exit guide vanes <NUM>. The airflow in this inter-stage location, prior to being straightened by the exit guide vanes, is non-axial. The slots <NUM> are angled such that the non-axial airflow moves across the slots <NUM> rather than travelling substantially along the length of the slots <NUM>.

For instance, the airflow direction may be the airflow from the fan <NUM> at an acoustic certification condition, such as the approach, lateral or flyover condition, and may be determined from computer simulation and/or engine testing. As an example, the airflow direction is from about <NUM>° to <NUM>° with respect to the angle formed with the engine central axis A. The orientation of the slots <NUM> to reduce drag may also be represented with regard to a slot texture <NUM> (e.g., see <FIG>). The slots <NUM> collectively define the slot texture <NUM>, schematically shown with single-headed arrows aligned with the slot centerlines C. The slot texture <NUM> is determined by the angles (OA) of the slots <NUM> and the relative axial and circumferential positions of the ends 50a/50b. For instance, in <FIG>, the axially forward slot end 50b is situated axially upstream, or forward, of the axially aft slot end 50a with respect to the airflow through the engine <NUM> from the front of the engine (at the inlet region <NUM>) to the back of the engine <NUM>. The ends 50a/50b are also circumferentially offset. Such a configuration establishes a sloped directionality along the slot centerlines C from the axially forward slot end 50b to the axially aft slot end 50a. The single-headed arrows represent this directionality and thus the slot texture <NUM>. As will be appreciated, the slot texture <NUM> is not limited to texture of rectangular elongated slots <NUM>, and slots having other geometries with upstream and downstream ends may likewise define a slot texture.

The fan <NUM> rotates in a rotational direction, which is represented at R in <FIG>. The rotational direction R establishes a circumferentially forward direction in the direction of the rotation and a circumferentially aft direction opposite to the direction of the rotation. The rotational direction R of the fan <NUM> is against the slot texture <NUM>. That is, the axially forward slot ends 50b are circumferentially forward of the axially aft slot ends relative to the rotational direction R of the fan <NUM>. Thus, for a given rotating blade on the fan <NUM> and each given slot <NUM>, the blade passes by the circumferential position of the axial aft slot end 50a of the slot <NUM> before then passing by the circumferential position of the axially forward slot end 50b of that slot <NUM>. Airflow coming off the fan <NUM> at an oblique angle that lies between the rotation direction R and the axis A must therefore pass across the slots <NUM> rather than substantially down the lengths of the slots <NUM>, which facilitates a reduction in drag. In this regard, even if the airflow is not substantially perpendicular to the slots <NUM>, having rotational direction R of the fan <NUM> be against the slot texture <NUM> ensures that much of the airflow will have to pass across the slots <NUM> as desired for reduced drag.

The face sheet <NUM> and slots <NUM> may be designed to enhance acoustic attenuation and drag reduction, and must be able to withstand impact from ice that may shed from the fan. To achieve those objectives, for example, the face sheet thickness (T) is from <NUM> to <NUM> inches (about <NUM> to <NUM>), the slot width (W) is from <NUM> to <NUM> inches (about <NUM> to <NUM>), the ratio of the slot length (L) to the slot width (W) is from <NUM> to <NUM>, the ratio of the slot-to-slot spacing (S) to the slot width (W) is from <NUM> to <NUM>, and the ratio of pattern-to-pattern gap (G) to slot width (W) is <NUM> or greater (where each parameter is measured in inches or equivalent units and each of the ratios specified is non-dimensional).

The face sheet <NUM> and slots <NUM> can be fabricated by subtractive machining, such as by using shaped tools and secondary processes to cut the slots <NUM> and smooth the edges of the slots <NUM>. Alternatively, additive manufacturing can be used to fabricate the face sheet <NUM> and slots <NUM>. For instance, additive manufacturing may enable smoother surfaces and edges that are free or substantially free of burrs to further reduce drag, as well as more complex geometries. Additionally, additive manufacturing may enable the pattern of the slots <NUM> to be produced in alignment with the cells of the cellular structure <NUM> and avoid or reduce configurations in which the slots <NUM> overlap the walls of the cellular structure <NUM> so that the walls do not block the slots <NUM>. Additive manufacturing also enables a wider variety of possible materials for the face sheet <NUM>, as additive manufacturing can be conducted for metals or polymers. In this regard, the face sheet <NUM> can be composed of a relatively low hardness or durometer polymer, which can serve to further absorb sound and/or enhance foreign object impact resistance. Additionally, in bonded covers, the adhesive may fill the cell cavity or block the face sheet opening. Additive manufacturing eliminates the need for adhesive, thereby avoiding reduction in cavity volume and blockage of the slots <NUM>.

In general terms, additive manufacturing techniques allow for the creation of a component, such as the face sheet <NUM>, by building the component with successively added layers; e.g., layers of powdered material. In the additive manufacturing process, one or more materials are deposited on a surface in a layer. In some instances, the layers are subsequently compacted. The powder material(s) of the layer may be subsequently fused using any one of a number of known processes (e.g., laser, electron beam, etc.). Typically, the deposition of the material (i.e. the geometry of the deposition later for each of the materials) is computer controlled using a three-dimensional computer aided design (CAD) model. The three-dimensional (3D) model is converted into a plurality of slices, with each slice defining a cross section of the component for a predetermined height (i.e. layer) of the 3D model. The additively manufactured component is then "built" layer by layer; e.g., a layer of powdered material(s) is deposited and then fused, and then the process is repeated for the next layer.

Examples of additive manufacturing processes that can be used with the present disclosure include, but are not limited to, Stereolithography (SLS), Direct Selective Laser Sintering (DSLS), Electron Beam Sintering (EBS), Electron Beam Melting (EBM), Laser Engineered Net Shaping (LENS), Laser Net Shape Manufacturing (LNSM), Direct Metal Deposition (DMD), Direct Metal Laser Sintering (DMLS), "material extrusion" or fused filament fabrication (FFF), "vat polymerization" such as stereolithography (SLA) and digital light projection (DLP), polyjet, and selective laser sintering (SLS), and others. The present disclosure is not limited to using any particular type of additive manufacturing process.

As an example, <FIG> shows a sectioned view of a further example of the panel <NUM> in which the face sheet <NUM> has been fabricated using an additive manufacturing process. For example, the face sheet <NUM> is formed on the cellular structure <NUM> such that the face sheet <NUM> is fused directly to the walls of the cellular structure such that no adhesive is required. The slots <NUM> of the face sheet <NUM> are aligned with the cells of the cellular structure <NUM> and do not overlap the walls of the cells. The face sheet <NUM> is also made of a polymer that varies in hardness. For example, the region at 64a has a first hardness and the region at 64b has a second, different hardness. The variation in hardness can be tailored to provide toughening to endure harsh environments (i.e., ice impact). This may further enable a greater area of a liner to be acoustically treatable to absorb sound while also being durable against ice impact.

Claim 1:
An acoustic liner (<NUM>) for a gas turbine engine (<NUM>), comprising:
an acoustic panel (<NUM>) that is curved about a central axis, the acoustic panel (<NUM>) including,
a support backing (<NUM>),
a face sheet (<NUM>), and
a cellular structure (<NUM>) disposed between the support backing (<NUM>) and the face sheet (<NUM>),
the face sheet (<NUM>) having elongated slots (<NUM>) extending along respective slot centerlines (C) in the plane of the face sheet (<NUM>), the slot centerlines (C) being sloped at oblique angles to the central axis,
characterized in that the face sheet (<NUM>) varies in hardness.