Abstract:
An exhaust silencer assembly includes an exhaust duct and an exhaust silencer. The exhaust silencer is disposed about the exhaust duct and has a plurality of solid baffles and at least one perforated baffle The exhaust silencer assembly is disposed downstream of an auxiliary power unit and at least partially attenuates the downstream noise of the combustion gases that result from operation of the auxiliary power unit. In one embodiment, at least one of the plurality of solid baffles is disposed to extend axially between generally radially extending solid baffles, perforated baffle(s), or partially perforated baffles. The axially extending solid baffle allows for cavity depth variation in the silencer to optimize tuning of particular frequencies from the auxiliary power unit.

Description:
BACKGROUND 
       [0001]    The present invention relates to noise suppression systems. In particular, the present invention relates to noise suppression systems for use with gas turbines such as aircraft auxiliary power units (APUs). 
         [0002]    Large commercial aircraft typically include on-board APUs, located in the tail sections of the aircraft, to provide electrical power and compressed air for systems throughout the aircraft. When an aircraft is on the ground, the primary propulsion engines of the aircraft are shut down, and the APU provides the main power source for a variety of systems and serves as a main engine starter. The APU may also provide power during in-flight operations for various systems. 
         [0003]    For commercial passenger aircraft in particular, there is widespread demand by the airline industry to maintain noise levels generated by turbines below defined limits. This is particularly important at ground service stations for the aircraft, where ground crews load and unload luggage, fuel and provision the aircraft. When the aircraft is on the ground, minimizing APU noise is therefore desirable. 
         [0004]    One technique for attenuating the exhaust noise of an APU involves placing an exhaust silencer directly downstream from the APU exhaust diffuser. This allows the exhaust silencer to attenuate the noise of the combustion gases as the gases exit the exhaust diffusers. The exhaust silencer is typically placed directly downstream from the APU exhaust diffuser to minimize the overall axial length of the APU. However, the combustion gases exit the APU at high velocities, and create a turbulent mixing downstream from the exhaust diffuser. The turbulent mixing generates a substantial amount of additional noise downstream from the exhaust diffuser. 
         [0005]    To address this issue, it is common to increase the size of the exhaust silencer, and thereby increase the size of the APU housing structure so as to suppress noise generated downstream of the exhaust diffuser. However, such a solution undesirably increases the overall size and weight of the aircraft tail section. 
       SUMMARY 
       [0006]    An exhaust silencer assembly includes an exhaust duct and an exhaust silencer. The exhaust silencer is disposed about the exhaust duct and has a plurality of solid baffles and at least one perforated baffle. The exhaust silencer assembly is disposed downstream of an auxiliary power unit and at least partially attenuates the downstream noise of the combustion gases that result from operation of the auxiliary power unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic view of an aircraft tail section, which includes an exhaust silencer assembly in use with an on-board APU. 
           [0008]      FIG. 1A  is a schematic view of a perforated baffle of the exhaust silencer assembly of  FIG. 1 . 
           [0009]      FIG. 2  is a schematic view of an aircraft tail section, which includes an alternative exhaust silencer assembly in use with an on-board APU. 
           [0010]      FIG. 2A  is a schematic view of a partially perforated baffle and an axial baffle of the exhaust silencer assembly of  FIG. 2 . 
           [0011]      FIG. 2B  is a schematic view of another embodiment of a partially perforated baffle and an axial baffle. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIG. 1  is a schematic view of aircraft tail section  10 A, which includes an exterior structure  12 A and an APU nacelle  14 A. APU nacelle  14 A is a compartment within exterior structure  12 A that contains an APU  16 A, an eductor  18 A, an exhaust silencer  20 A, and exhaust duct  22 A. Exhaust silencer  20 A includes solid baffles  24   a ,  24   b ,  24   c ,  24   d ,  24   e , and  24   f  and at least one perforated baffle  26 . Together solid baffles  24   a ,  24   b ,  24   c ,  24   d ,  24   e , and  24   f  form a plurality of tuned chambers  28   a ,  28   b ,  28   c ,  28   d , and  28   e.    
         [0013]    APU nacelle  14 A houses APU  16 A therein. Eductor  18 A is arranged downstream of and is in fluid communication with APU  16 A. Eductor  18 A is an airflow system that extends annularly around at least a portion of an exhaust diffuser portion of the APU  16 A. Eductor  18 A extends axially to connect to and communicate with exhaust duct  22 A. Exhaust silencer  20 A is disposed annularly around exhaust duct  22 A and in some instances eductor  18 A, and is configured to attenuate the noise of APU  16 A. More particularly, exhaust silencer  20 A has an array of solid baffles  24   a ,  24   b ,  24   c ,  24   d , and  24   e , and  24   f  spaced apart axially from one another and extending annularly around educator  18 A and exhaust duct  22 A to form tuned chambers  28   a ,  28   b ,  28   c ,  28   d , and  28   e  to attenuate noise that results from the combustion gases of APU  16 A. As shown in  FIG. 1 , solid baffles  24   a ,  24   b ,  24   c ,  24   d , and  24   e , and  24   f  and first perforated baffle  26  extend generally radially around exhaust duct  22 A to exterior structure  12 A. 
         [0014]    In the embodiment shown in  FIG. 1 , perforated baffle  26  connects to and extends annularly about exhaust duct  22 A downstream of first solid baffle  24   a  and is disposed upstream of second solid baffle  24   b . Perforated baffle  26  has a plurality of holes extending therethrough to allow sound of a certain frequency to pass therethrough. Together first solid baffle  24   a  and second solid baffle  24   b  connect to exhaust duct  22 A and form first tuned chamber  28   a  with first perforated baffle  26  disposed therein. Similarly, second solid baffle  24   b  and third solid baffle  24   c  form second tuned chamber  28   b  axially downstream of first tuned chamber  28   a . Third solid baffle  24   c  and fourth solid baffle  24   d  form third tuned chamber  28   c  downstream of second tuned chamber  28   b . Fourth tuned chamber  28   d , and fifth tuned chamber  28   e  are formed in a similar manner by solid baffles  24   d ,  24   e , and  24   f , respectively. 
         [0015]    In the embodiment shown in  FIG. 1 , first solid baffle  24   a  is disposed substantially flush with leading edge of eductor  18 A. In other embodiments, first solid baffle  24   a  can be disposed downstream (as defined by direction of combustion air flow from APU  16 A) of eductor  18 A along exhaust duct  22 A. In the embodiment shown in  FIG. 1 , perforated baffle  26  is disposed downstream of the exhaust portion of APU  16 A at a distance that is between about 100% of the inner diameter of the exhaust duct  22 A to about 150% of the inner diameter of the exhaust duct  22 A. In other embodiments, the perforated baffle  26  can be disposed in a different location further upstream or downstream relative the APU  16 A, and perforated baffle  26  can comprise a plurality of baffles. Additionally, the number of solid baffles and chambers can be varied in order to achieve desired noise attenuation in particular frequency ranges so as to meet aviation noise standards. 
         [0016]    APU  16 A is an on-board APU turbine engine that includes a turbine portion and exhaust diffuser (not shown) and provides electrical power to the aircraft. APU  16 A also includes additional components (not shown) that facilitate transfer of electrical power such as generators and gearboxes during the operation of APU  16 A. Eductor  18 A draws cooling air from APU nacelle  14 A and mixes the cooling air with combustion gases from APU  16 A. Exhaust duct  22 A communicates with eductor  18 A and provides a channel for expelling the combustion and cooling gases from aircraft tail section  10 . Exhaust duct  22 A is an annular metal tube connected to eductor  18 A. In the embodiment shown in  FIG. 1 , exhaust duct  22 A is perforated in a manner similar to perforated baffle  26  downstream of perforated baffle  26  and does not have perforations (i.e. is a solid sheet metal tube) upstream thereof. 
         [0017]    The turbulent mixing of the combustion gases and the cooling gases generates a substantial amount of downstream noise. Exhaust duct  22 A provides a suitable residence time for the combustion gases to generate the downstream noise before reaching desired portions of exhaust silencer  20 A. As is know in the art and disclosed by Sheoran et al., U.S. Patent Publication No. 2002/0139120 and Napier et al., U.S. Patent Publication No. 2008/0236939, both of which are incorporated herein by reference, chambers  28   a ,  28   b ,  28   c ,  28   d , and  28   e  of exhaust silencer  20 A are tuned to resonances of the combustion and cooling gases to attenuate noise and meet aviation noise standards. Disposing first solid baffle  24   a  at or adjacent eductor  18 A and utilizing perforated baffle  26  disposed in first chamber  28   a  upstream of perforated exhaust duct  22 A allows exhaust silencer  20 A additional space in exterior structure  12 A so as to more effectively attenuate noise and thereby meet aviation noise standards. 
         [0018]      FIG. 1A  shows perforated baffle  26  from exhaust silencer  20 A of  FIG. 1 . Perforated baffle  26  includes an exhaust duct hole  30  and a plurality of noise attenuating holes  32 . 
         [0019]    Like solid baffles  24   a ,  24   b ,  24   c ,  24   d ,  24   e , and  24   f  ( FIG. 1 ), perforated baffle  26  comprises a sheet metal most commonly constructed of stainless steel, titanium, or nickel based alloy. Exhaust duct hole  30  is sized to receive exhaust duct  22 A ( FIG. 1 ) when installed. In the embodiment shown, noise attenuating holes  32  extend through perforated baffle  26  and are disposed across substantially the entire surface thereof. As illustrated in  FIG. 1A , noise attenuating holes  32  are about 0.125 inches (0.32 cm) in diameter and are patterned so that the noise attenuating holes  32  take up between 20 and 30 percent of the surface area of perforated baffle  26 . However, the size, shape, and pattern of noise attenuating holes  30  can vary from embodiment to embodiment depending on how exhaust silencer  20 A ( FIG. 1 ) is tuned to achieve desired noise attenuation. 
         [0020]      FIG. 2  a schematic view of an alternative exhaust silencer  20 B disposed in aircraft tail section  10 B. Aircraft tail section  10 B includes an exterior structure  12 B and an APU nacelle  14 B. APU nacelle  14 B contains an APU  16 B, an eductor  18 B, an exhaust silencer  20 B, and exhaust duct  22 B. Exhaust silencer  20 B includes solid baffles  34   a ,  34   b ,  34   c ,  34   d , and  34   e , partially perforated baffles  36   a  and  36   b , and a perforated baffle  38 . Together solid baffles  34   a ,  34   b ,  34   c ,  34   d , and  34   e  and solid portions  36   s  and  36   ss  of partially perforated baffles  36   a  and  36   b  form a plurality of tuned chambers  40   a ,  40   b ,  40   c ,  40   d , and  40   e . As shown in  FIG. 2 , solid baffles  34   a ,  34   b ,  34   c , and  34   d , partially perforated baffles  36   a  and  36   b , and a perforated baffle  38  extend generally radially around exhaust duct  22 B to exterior structure  12 B. 
         [0021]    Exhaust silencer  20 B is constructed in a manner similar to exhaust silencer  20 A and can be used as an alternative or in addition to exhaust silencer  20 A in order to achieve noise attenuation in particular frequency ranges so as to meet aviation noise standards. 
         [0022]    As shown in  FIG. 2 , APU nacelle  14 B houses APU  16 B therein and exterior structure  12 B houses all components including exhaust silencer  20 B. Eductor  18 B is arranged downstream of and is in fluid communication with APU  16 B. Eductor  18 B extends axially to connect to and communicate with exhaust duct  22 B. Exhaust silencer  20 B is disposed annularly around exhaust duct  22 B adjacent eductor  18 B, and is configured to attenuate the noise of combustion gases generated by APU  16 B. 
         [0023]    More particularly, exhaust silencer  20 B has an array of solid baffles  34   a ,  34   b ,  34   c , and  34   d  spaced apart axially from one another and extending annularly around exhaust duct  22 B. Additionally, solid baffle  34   e  extends generally axially between partially perforated baffles  36   a  and  36   b  and between partially perforated baffle  36   b  and fourth solid baffle  34   d . In the embodiment shown in  FIG. 2 , partially perforated baffles  36   a  and  36   b  have portions of solid construction  36   s  and  36   ss  radially above (as defined by the centerline of exhaust duct  22 B) the connection with solid baffle  34   e . Together solid baffles  34   a ,  34   b ,  34   c ,  34   d , and  34   e  and the solid portions  36   s  and  36   ss  of partially perforated baffles  36   a  and  36   b  form tuned chambers  40   a ,  40   b ,  40   c ,  40   d , and  40   e.    
         [0024]    In the embodiment shown in  FIG. 2 , perforated baffle  38  extends annularly about exhaust duct  22 B downstream of first solid baffle  34   a  and upstream of second solid baffle  34   b . Perforated baffle  38  has a plurality of holes extending therethrough to allow noise of a certain frequency to pass therethrough. Together first solid baffle  34   a  and second solid baffle  34   b  form first tuned chamber  40   a  with first perforated baffle  38  disposed therein. Similarly, second solid baffle  34   b  and third solid baffle  34   c  form second tuned chamber  40   b  axially downstream of first tuned chamber  40   a . Third solid baffle  34   c , solid portion  36   s  of perforated baffle  36   a , fourth solid baffle  34   d , and fifth solid baffle  34   e  form L shaped third tuned chamber  40   c  downstream of second tuned chamber  40   b . Perforated portions  36   p  and  36   pp  of partially perforated baffles  36   a  and  36   b  are disposed in third tuned chamber  40   c . Fourth tuned chamber  40   d  is disposed radially above third tuned chamber  40   c , extends about exhaust duct  22 B, and is formed by solid portions  36   s  and  36   ss  of partially perforated baffles  36   a  and  36   b  and fifth solid baffle  34   e . Similarly, fifth tuned chamber  40   e  is formed by solid portion  36   ss  of partially perforated baffle  36   b , fourth solid baffle  34   d , and fifth solid baffle  34   e , and is disposed downstream of fourth tuned chamber  40   d . The number of baffles and chambers, and the location of the chambers relative to exhaust duct  22 B can be varied in order to achieve noise attenuation in particular frequency ranges so as to meet aviation noise standards. The axial distance of the solid baffle  34   e  can also be varied from the outer diameter of the exhaust duct  22 B, this allows exhaust silencer  20 B to be tuned to specific frequencies to reduce noise from APU  16 B. 
         [0025]    First solid baffle  34   a  is disposed substantially adjacent a trailing edge of eductor  18 B around exhaust duct  22 B. In other embodiments, first solid baffle  34   a  can be disposed flush with leading edge of eductor  18 B or along exhaust duct  22 B at various desired lengths from APU  16 B. In the embodiment shown in  FIG. 2 , perforated baffle  38  is disposed downstream of the exhaust portion of APU  16 B at a distance that is between about 100% of the inner diameter of the exhaust duct  22 B to about 150% of the inner diameter of the exhaust duct  22 B. Exhaust duct  22 B is perforated in a manner similar to perforated baffle  38  downstream of perforated baffle  38  and does not have perforations (i.e. is solid) upstream thereof. In other embodiments, the perforated baffle  38  and partially perforated baffles  36   a  and  36   b  can be disposed in a different location further upstream or downstream relative the APU  16 B and perforated baffle  38  can comprise a plurality of baffles in variously sized chambers in order to achieve noise attenuation in particular frequency ranges so as to meet aviation noise standards. 
         [0026]    Chambers  40   a ,  40   b ,  40   c ,  40   d , and  40   e  of exhaust silencer  20 B are tuned to attenuate the noise frequency spectrum of the APU  16 B to meet aviation noise standards. Disposing first solid baffle  34   a  adjacent eductor  18 B and utilizing perforated baffle  38  disposed in first chamber  40   a  upstream of perforated exhaust duct  22 B allows exhaust silencer  20 B additional space in exterior structure  12 B so as to more effectively attenuate noise and thereby meet aviation noise standards. Additionally, utilizing partially perforated baffles  36   a  and  36   b  allows exhaust silencer  20 B to achieve noise attenuation in particular frequency ranges so as to meet aviation noise standards. 
         [0027]      FIG. 2A  shows partially perforated baffle  36   b  from exhaust silencer  20 B of  FIG. 2 . Partially perforated baffle  36   b  includes exhaust duct hole  30  and noise attenuating holes  32 . 
         [0028]    Noise attenuating holes  32  extend through perforated portion  36   pp  of partially perforated baffle  36   b . Axially extending solid baffle  34   e  connects to partially perforated baffle  36   b  and divides perforated portion  36   pp  from solid portion  36   ss . As shown in  FIG. 2A , solid baffle  34   e  extends in an arc at a distance from exhaust duct hole  30  and divides chamber  40   e  from  40   c  ( FIG. 2 ). 
         [0029]      FIG. 2B  shows another embodiment of partially perforated baffle  42 . Partially perforated baffle  42  can be substituted for partially perforated baffle  36   b  and  36   a  of  FIGS. 2 and 2A  and includes exhaust duct hole  30  and noise attenuating holes  32 . 
         [0030]    Noise attenuating holes  32  extend through perforated portion  44  of partially perforated baffle  42 . Axially extending solid baffle  46  connects to partially perforated baffle  42  and divides perforated portion  44  from solid portion  48 . As shown in  FIG. 2B , solid baffle  46  is disposed entirely around exhaust duct hole  30 , and therefore, forms an attenuating chamber  50 . Solid baffle  46  extends from partially perforated baffle  42  to adjacent solid baffles (not shown). 
         [0031]    While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.