Abstract:
An acoustic treatment for the air ducts of a gas turbine engine. The acoustic treatment generally includes a facesheet having a plurality of holes therein, a backplate spaced apart from the facesheet, and a plurality of interconnected cells between the facesheet and backplate. Each of the cells is defined by walls attached to the facesheet and the backplate, and at least some of the walls are formed of a porous material so that air is able to flow through the cells in a direction parallel to the facesheet and backplate.

Description:
FIELD OF THE INVENTION 
     The present invention relates to materials and structures for noise suppression. More particularly, this invention relates to an acoustic treatment panel for suppressing radiated noise in an aircraft engine nacelle, with the panel being configured as a sandwich panel that exhibits acoustic properties similar to that of bulk absorber material. 
     BACKGROUND OF THE INVENTION 
     Gas turbine engines operate over a broad range of speeds and thrusts, and as a result generate a broad range of noise frequencies. Acoustic treatments in the form of acoustic liners that line the fan and exhaust ducts of gas turbine engines are widely used to suppress aircraft engine noise beyond those levels that can be achieved by the particular design of the turbo machinery. In view of stringent noise abatement requirements around the world, considerable effort has been directed to designing acoustic liners that are capable of absorbing noise over a broad range of frequencies, while also being durable, relatively low-weight, readily fabricated, and having minimal impact on engine performance. 
     There are two primary sources of aircraft-generated noise. One source is the viscous shearing that takes place between the rapid exhaust gases and the relatively quiescent surrounding air, while the second source is the rotating blades of the fan, compressor and turbines, and the resulting air flow past the vanes and other stationary objects within the engine air flow path. Acoustic treatments for suppressing noise produced by the latter source can generally be categorized as bulk absorbers or resonator-type absorbers. 
     A bulk absorber  10  is represented in FIG.  1 . With this type of treatment, a porous material  12 , such as a fibrous or rigid foam material, fills a cavity between two sheets  14  and  16 . The sheet  14  is formed of an air-permeable material that forms the walls of a nacelle flow duct of a gas turbine engine, e.g., the fan inlet and fan exhaust ducts and the turbine exhaust duct. The sheet  14  and the bulk absorber  10  absorb sound waves that impact these walls as the waves propagate through the duct. Examples of suitable materials for the sheet  14  include sheet fabricated from sintered or felted metal, or other porous materials having suitable flow resistances. The back sheet  16  is typically rigid and air-impermeable. 
     Acoustic treatments referred to as resonator-type absorbers include Helmholtz resonator chambers or compartments. A double-layer resonator absorber  20  of this type is represented in FIG. 2 as having a compartmented airspace core with an air-permeable facesheet  22  and an air-impermeable back sheet  24 , between which there are a number of compartments or cells  26 . The facesheet  22  typically has perforations  30  within which sound absorption occurs. In the double layer resonator  20  shown in FIG. 2, a porous septum  32  is present between and parallel to the facesheet  22  and back sheet  24 . Conventional methods by which the resonator  20  is manufactured typically entail individually forming the resonator layers separated by the septum  32 , and then bonding the layers and the septum  32  together. As a result, misalignment often occurs between the cells  28  of these layers. In a single-layer resonator (not shown), the porous septum  32  is omitted. 
     As a rule, the cells  26  of resonator-type absorbers have been defined by hard, air-impermeable walls  28 , which are often configured so that the cells  26  have a hexagonal-shaped cross-section that yields a honeycomb cell pattern. Passages between resonator cells  26  have been proposed, as shown in U.S. Pat. Nos. 3,972,383 and 4,189,027. However, the former resonator absorber relies on air being forced through the cells  26  from an exterior source in order to tune the facesheet  22 , while the latter absorber requires adjacent cells  26  to be asymmetric, which causes air pumping between cells  26  when air flows over the perforations  30  in the facesheet  22 . 
     There are known advantages and shortcomings with each of the acoustic treatments described above. The double-layer resonator-type absorber  20  represented in FIG. 2 provides good noise attenuation over a relatively wide band of frequencies centered about a particular frequency to which the cells  26  are tuned, based in part on their depth. To achieve a broadband capability, a resonator-type absorber must have a variety of cavity sizes to cover the frequency range of concern, or must be capable of mechanically changing the sizes of the cells. Both of the approaches are mechanically complex and contribute undesirable weight to the engine. 
     In contrast, bulk absorbers of the type shown in FIG. 1 offer higher suppression performance than either single-layer or double-layer resonator-type treatments by their ability to absorb noise over a wider frequency range. In spite of this performance advantage, bulk absorbers are not widely used in aircraft engines due to disadvantages inherent in she material properties. Specifically, the conventional concern is that fibrous materials will disintegrate with aging and the high dynamic vibration levels within gas turbine engines, and may wick liquids that could create a fire hazard. Another drawback of bulk absorbers is their poor serviceability. 
     In view of the above, it can be seen that it would be desirable if an acoustic treatment were available for gas turbine engines, by which a broad band of noise suppression was possible along with structural integrity compatible with air flow conditions of the gas turbine engine environment. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the present invention, there is provided an acoustic treatment for the air ducts of a gas turbine engine. The acoustic treatment generally includes a facesheet having a plurality of holes therein, a backplate spaced apart from the facesheet, and a plurality of interconnected cells between the facesheet and backplate. Each of the cells is defined by walls attached to the facesheet and the backplate, and at least some of the walls are formed of a porous material that provides flow resistance therethrough and allows acoustic propagation in a direction parallel to the facesheet and backplate. 
     A significant advantage of the above construction is that the acoustic treatment of this invention is able to exhibit the suppression performance advantages of bulk absorbers, yet has the structural advantages of a resonator-type absorber. Specifically, the porous walls of the cells allow acoustic waves to travel in a direction parallel to the facesheet, which provides the acoustic treatment with the noise suppression properties of a bulk absorber. On the other hand, the rigid facesheet and backplate provide a sandwich structure that is resistant to the hostile thermal, chemical and mechanical environment of a gas turbine engine. The porous material of the cell walls is also able to contribute to the structural integrity of the treatment without unduly restricting airflow between adjacent cells. With this construction, cell size and cell wall porosity can both be controlled in order to achieve the desired acoustic and structural properties for a particular acoustical environment. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 represent cross-sections through prior art bulk and resonator-type absorbers, respectively; 
     FIG. 3 represents a cross-section through an acoustic treatment in accordance with the present invention; 
     FIG. 4 is a perspective view of a section of an acoustic treatment in accordance with this invention; 
     FIG. 5 is a perspective view of a small section of the acoustic treatment of FIG. 4; and 
     FIG. 6 depicts the acoustic treatment of the present invention installed in a turbofan gas turbine engine. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 6 depicts a turbofan gas turbine engine  50  of the type used to power an aircraft (not shown). The engine  50  includes a nacelle  52  surrounding a fan  54  that is powered by a turbine (not shown) downstream in the engine  50 . The nacelle  52  includes an inlet duct  56  that receives ambient air  58 , which then flows downstream through the fan  54 . During engine operation, and particularly during takeoff of the aircraft when the fan blades reach transonic and supersonic velocities, noise is generated that propagates upstream and out through the inlet duct  56 . In order to attenuate the noise radiated within the nacelle  52 , an acoustic treatment panel  100  in accordance with this invention is positioned upstream of the fan  54 , as shown in FIG.  6 . The acoustic treatment panel  100  is preferably configured for attenuating noise over a relatively wide frequency range, preferable from about 800 to about 8000 Hertz, though it is foreseeable that the panel  100  could be adapted to attenuate a broader or narrower range of frequencies. 
     FIG. 3 schematically illustrates a cross-section of the panel  100 , while FIGS. 4 and 5 are perspective views of portions of the panel  100 . As shown in FIGS. 3 through 5, the acoustic treatment panel  100  of this invention includes a perforated facesheet  102 , a rigid backplate  104  generally parallel to and spaced apart from the facesheet  102 , and an acoustic filler  105  therebetween formed by a number of compartments or cells  106 . Each cell  106  is defined by walls  108  that, in accordance with this invention, are porous, and more preferably are formed entirely of a porous material. While the cells  106  are each depicted as being formed by six walls  108  so as to have a hexagonal cross-sectional shape that yields a honeycomb-like cell pattern, the cells  106  could be formed by any number of walls  108  to have any desired shape. The cells  106  are preferably identical in shape and size, so that the permeability of the walls  108  enables uniform coupling to become established through the cells  106  and parallel to the facesheet  102  and backplate  104 . Alternatively, only selected walls  108  of each cell  106  could be formed of porous material, so that their permeability establishes directional coupling through the cells  106  parallel to the facesheet  102  and backplate  104 . 
     Shown in FIG. 4 is an optional grid of partitions  114  that can be used to separate the panel  100  into larger cells, each containing a number of the hexagonal cells  106 . The partitions  114  are preferably air-impermeable and attached to the facesheet  102  and backplate  104 . As shown, the partitions  114  define a grid of rectangular partitioned regions, each of which surrounds a number of the cells  106 . The partitions  114  contain the acoustic field that propagates parallel to the facesheet  102  and back plate  104  to a limited region of the panel  100 , with the effect that a suppression advantage is achieved at certain frequencies of operation. 
     The facesheet  102  is formed to have a number of orifices  110  that fluidically communicate with each of the cells  106 , though it is foreseeable that only some of the cells  106  could be paired with an orifice  110 . The facesheet  102  can be formed of any suitable material, including metals and composite materials, chosen on the basis of weight and structural considerations. The facesheet  102  is preferably bonded directly to the cells  106  by such methods as reticulated adhesion bonding of a type known in the art. A wire mesh  112  (FIG. 3) may be bonded to the facesheet  102  to achieve added acoustic resistance. 
     The backplate  134  is preferably formed of a suitable metal or composite material that renders the backplate  104  acoustically rigid. A preferred material for this purpose is aluminum and its alloys. Similar to the facesheet  102 , the backplate  104  is preferably bonded directly to the ends of the cells  106  opposite the facesheet  102  with an adhesive. The backplate  104  is assembled with the facesheet  102  and the porous-walled honeycomb acoustic filler  105  formed by the cells  106  to form a rigid sandwich panel acoustic treatment, which is then mounted within the inlet duct  56  as depicted in FIG.  6 . 
     According to this invention, the porosity of the cell walls  108  enables the acoustic treatment panel  100  to exhibit acoustical properties very near that of the bulk absorber  10  of FIG.  1 . To achieve this capability, each of the walls  108  of the cells  106  is preferably formed of a porous material that provides a desired level of resistance to air flow, and is sufficiently rigid to promote the structural rigidity and integrity of the panel  100 . Suitable materials for this purpose include metallic and composite materials, with preferred materials being those that can easily be made permeable with the required resistance to air flow. The porous honeycomb acoustic filler  105  of this invention is preferably comparable in weight and strength to those air-impermeable honeycomb structures of the prior art. While a variety of materials can be processed to have the desired mechanical and physical properties described above, it is believed that aluminum-based and fiberglass based materials are particularly suitable. 
     Those skilled in the art will appreciate that the dimensional characteristics of the facesheet  102 , backplate  104  and cells  106  will determine the acoustical properties of the panel  100 . In particular, the thickness of the facesheet  102 , the diameters of the orifices  110 , and the open area ratio of the facesheet  102  resulting from the orifices  110  are specified according to known acoustic design methods. The size and shape of the impermeable partitions  114  are also acoustical design parameters. Furthermore, the depth and cross-sectional area of each cell  106  and the thickness of the cell walls  108  are to be specified according to acoustic design principles. However, the porosity of the cell walls  108  is an additional design parameter of this invention, and must be tailored to achieve a desired level of air flow resistance through the cells  106  in a direction parallel to the facesheet  102  and backplate  104 . Generally, air flow resistance is specified as the steady (DC) flow resistance of the material, corresponding to a specified air flow rate through the material. In a preferred embodiment, this steady flow resistance is about 20 to about 120 CGS Rayl as determined by standard test methods. 
     While the above dimensions will typically be determined for a particular application, suitable dimensions for the panel  100  depicted in FIG. 5 are believed to include a facesheet thickness of about 0.40 to about 3.00 mm, orifice diameters of about 0.5 to about 2.0 mm, a facesheet open area ratio of about 5% to about 20%, a cell depth of about 12 to about 50 mm, a. cell cross-sectional area of about 30 to about 130 mm 2 , and a cell wall thickness of about 0.075 to about 0.150 mm. 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.