Abstract:
An inner bypass duct of a gas turbine engine includes a plurality of non-structural front panels arranged circumferentially. Each front panel has an outer surface and an inner surface. The inner surface is at least partially covered by a composite structure. The composite structure includes a fireproof layer and an acoustic layer disposed between the fireproof layer and the inner surface. A non-structural panel for an inner bypass duct and a method of forming such panel are also presented.

Description:
TECHNICAL FIELD 
       [0001]    The application relates generally to gas turbine engines and, more particularly, to inner bypass ducts. 
       BACKGROUND OF THE ART 
       [0002]    Turbofan engines generally comprise an annular bypass air passage defined between radially outer and radially inner bypass ducts. The annular bypass air passage directs a bypass air flow drawn by the fan. The inner bypass duct surrounds an engine core including a compressor section, a combustor and a turbine section. 
       SUMMARY 
       [0003]    In one aspect, there is provided an inner bypass duct of a gas turbine engine, the inner bypass duct comprising: a plurality of non-structural front panels arranged circumferentially, each front panel having an outer surface and an inner surface, the inner surface being at least partially covered by a composite structure, the composite structure including a fireproof layer and an acoustic layer disposed between the fireproof layer and the inner surface. 
         [0004]    In another aspect, there is provided a non-structural panel for an inner bypass duct of a gas turbine engine, the panel comprising: an inner surface at least partially covered by a composite structure, the composite structure including a fireproof layer and an acoustic layer disposed between the fireproof layer and the inner surface. 
         [0005]    In a further aspect, there is provided a method of forming a non-structural panel for an inner bypass duct for a gas turbine engine, the method including: disposing an acoustic layer onto an inner surface of the panel; and bonding a fireproof layer on top of the acoustic layer. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]    Reference is now made to the accompanying figures in which: 
           [0007]      FIG. 1  is a partial schematic cross-sectional view of a gas turbine engine; 
           [0008]      FIG. 2  is a schematic partial perspective view of an outside of an inner bypass duct for the gas turbine engine of  FIG. 1 ; 
           [0009]      FIG. 3  is a schematic partial perspective view of an inside of the inner bypass duct showing a composite structure on front panels; 
           [0010]      FIG. 4  is a schematic cross-sectional view of one of the front panels with the composite structure according to one embodiment; and 
           [0011]      FIG. 5  is a schematic cross-sectional view of one of the front panels with the composite structure according to another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIG. 1  illustrates a gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication along a centerline  11 : a fan  12  through which ambient air is propelled, a compressor section  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. The compressor section  14  and the turbine section  18  form part of an engine core  20 . The engine core  20  defines a main fluid path  22  in which the combustor  16  is provided. The engine core  20  is coaxially positioned within an annular bypass duct  24  including an annular radially outer bypass duct  26  and an annular radially inner bypass duct  28 . The radially outer and inner bypass duct walls  26  and  28  define therebetween an annular bypass air passage  30  for directing a bypass air flow  32  drawn by the fan  12 . In contrast to the outer bypass duct  26  which perform a structural function in supporting and centering the engine core  20 , the inner bypass duct  28  is typically not a load transmitting component. The inner bypass duct  28  includes an inner surface  28   a  and an outer surface  28   b  facing the outer bypass duct  26 . In one embodiment, the inner bypass duct  28  is made of aluminum. 
         [0013]    Turning now to  FIGS. 2 and 3 , the inner bypass duct  28  may comprise a circumferential array of front panels  34  and a circumferential array of rear panels  36 . By way of example, the circumferential array of front panels  34  may comprise four side panels  34   a,  a top panel  34   b,  and a bottom panel  34   c  assembled together to form a continuous cylindrical wall structure. The front panels  34  may be riveted, bolted or otherwise suitably attached to one another along adjoining circumferential edges. In the embodiment shown in the Figures, the circumferential array of rear panels  36  comprises three arcuate panels. However, it is understood that a different number of rear panels  36  could be provided. The rear panels  36  may be provided in the form of sheet metal. Like the front panels  34 , the rear panels  36  may be riveted, bolted or otherwise suitably joined along circumferentially adjacent edges. Openings or passages may be defined in the front and rear panels  34  and  36  for allowing mounting of equipment to the engine core or to provide access thereto (see for instance the elongated openings defined in the top front panels  34   b ). Openings may also be defined in the panels  34  and  36  for allowing the passage of structural elements. 
         [0014]    Turning now to  FIG. 4 , an embodiment of the front panels  34  including a composite structure  40  will be described. 
         [0015]    The composite structure  40  is formed on the inner surface  28   a  of the inner bypass duct  28  at the front panels  34 . While only the front panels  34  are shown herein having the composite structure  40 , it is contemplated that some or all of the rear panels  36  could also have a composite structure, which may or may not be similar to the composite structure  40 . The side panels  34   a,  top panels  34   b,  and bottom panels  34   c  of the front panels  34 , may have the same composite structure  40 . It is however contemplated that the composite structure may differ between the front panels  34 , according to, for example, given acoustic and/or fireproof requirements. The composite structure  40  is provided on the inner surface  28   a  to protect the front panels  34  from a potential fire hazard originating from the engine core  20 . The composite structure  40  may be bonded to the inner surface  28   a  by an adhesive or otherwise suitably attached. The composite structure  40  may cover a majority of a surface area of the inner surface  28   a  at the front panels  34 . In one embodiment, the composite structure  40  covers almost up to a width W (shown in  FIG. 3 ) and a length L (shown in  FIG. 3 ) of each of the front panels  34 . 
         [0016]    The composite structure  40  includes an acoustic layer  42  and a fireproof layer  44  covering the acoustic layer  42 . The acoustic layer  42  may have a thickness T that may be at least twice the thickness t of the front panels  34 . The acoustic layer  42  may be made of different materials having different acoustic properties depending on given requirements. In one embodiment, the acoustic layer  42  is made of a honeycomb structure. In one embodiment, the acoustic layer  42  includes an Acousti-Cap®. In one embodiment, the honeycomb is made of aluminum. The acoustic layer  42  may be made using combinations of honeycomb thickness, septum, face sheet hole size and hole pattern etc. to attenuate a particular set of frequencies. In yet another embodiment, shown in  FIG. 5 , an acoustic layer  42 ′ may be made to include a double degree of freedom Acousti-Cap®. A double degree of freedom acoustic treatment may include a first acoustic layer  43   a ′, and a second acoustic layer  43   b ′ separated by a septum  43   c ′. The septum  43   c ′ may be disposed parallel to the face sheet  28   a / 28   b.  The first acoustic layer  43   a ′ and second acoustic layer  43   b ′ have different thickness to attenuate different frequencies. In one embodiment, the septum  43   c ′ is made of aluminum. A thickness T 1 ′ of the first acoustic layer  43   a ′ may be bigger than a thickness T 2 ′ of the second acoustic layer  43   b ′. It is contemplated that the first acoustic layer  43   a ′ could have a thickness thinner or thicker than the second acoustic layer  43   b′.    
         [0017]    Referring back to  FIG. 4 , the fireproof layer  44  forms a protective capsule to the acoustic layer  42 . In one embodiment, the fireproof layer  44  is bonded to the inner surface  28   a  and to the acoustic layer  42  by adhesives. A foam adhesive  46  may be disposed axially laterally between along the edges of the acoustic layer  42  and between the fireproof layer  44  to close out the cells along the edges. 
         [0018]    The fireproof layer  44  includes, in one embodiment, a first layer  48  of fiberglass, a layer  50  of fireproof fabric and a second layer  52  of fiberglass. Other layering and types of fireproof layer  44  are contemplated. For example, there could be more than one layer of fireproof fabric  50  separated or not by yet another layer of fiberglass. In another example, the fireproof fabric  50  is a ceramic fiber material. The first layer  48  of fiberglass may completely cover the acoustic layer  42 , while the second layer  52  of fiberglass may have one or more openings exposing a portion of the fireproof fabric  50 , such as opening  54  to reduce a total weight of the composite structure  40 . The second layer  52  could have one big opening or a plurality of smaller openings throughout. It is also contemplated that only a portion of the acoustic layer  42  could be covered by the fiberglass layer  44 . It is also contemplated that the second layer  52  of fiberglass may completely cover the layer  50  of fireproof fabric. The first layer  48  and second layer  52  of fiberglass sandwich the layer of fireproof fabric  50  and are bonded to it by an adhesive or otherwise suitably attached. The first layer  48  and second layer  52  may be rigid while the fireproof fabric  50  may be flexible. It is contemplated that one or both of the first layer  48  and second layer  52  of fiberglass could be omitted. 
         [0019]    The use of fiberglass and fireproof fabric may allow for a light yet resistant construction of the composite structure. In addition, the use of adhesives may also reduce the weight of the composite structure. The composite structure may be easily incorporated into the panels without burdensome costs. 
         [0020]    The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. Other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.