Abstract:
An air inlet assembly comprising an entrance region configured to receive an airflow, an exit region configured to output the airflow, and an attenuation region disposed between the entrance region and the exit region. The attenuation region has a cross section aspect ratio of at least about 5-to-1, and comprises acoustic absorbing material.

Description:
BACKGROUND 
     The present invention relates to noise attenuation systems. In particular, the present invention relates to noise attenuation systems for use with gas turbine engines such as aircraft auxiliary power unit (APU) turbine engines. 
     Large commercial aircraft typically include on-board APU turbine engines, located in the tail sections of the aircraft, to provide electrical power and/or 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 turbine engine provides the main source of power for a variety of systems, such as the environmental control systems, hydraulic pumps, electrical systems, and main engine starters. The APU turbine engine may also provide power during in-flight operations, such as for electrical and pneumatic systems. 
     In many gas turbine engine applications, particularly those in which the engine is used in conjunction with a commercial passenger aircraft, there is a widespread demand by the airline industry to maintain noise levels below defined limits. This is particularly important at ground service stations for the aircraft, where ground crew load and unload luggage, fuel and provision the aircraft, and remove waste materials from the aircraft. Under these conditions, the aircraft APU is the turbine engine of interest. 
     Noise generated during the operation of an APU turbine engine typically includes low frequency noise generated during the combustion process within the turbine engine, and high frequency noise generated by the interaction with inlet air at the compressor portion of the turbine engine. The low frequency noise is typically attenuated with an exhaust silencer placed 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 high frequency noise, however, is typically attenuated with the use of an inlet noise silencer disposed in an air inlet duct, where the air inlet duct is located upstream relative to the APU turbine engine. To provide effective attenuation of high frequency noise, the inlet noise silencer desirably has a narrow passage width that is comparable in size with the wavelengths of the high frequency noise. If the passageway is too large (e.g., greater than about twice the average noise wavelength), the level of noise attenuation decreases. 
     One common technique to provide small passageways in an air inlet duct involves segregating the duct into separate parallel passages with the use of acoustically lined splitters. Unfortunately, the splitters can collect ice on their leading edges, which may clog the air inlet duct, thereby hindering certification of the APU turbine engine for operation in icing conditions. Furthermore, the use of splitters increases the cost of the inlet noise silencer. As such, there is a need for an inlet noise attenuation system that is effective for attenuating high frequency noise without the use of splitters or other components that may otherwise hinder the use of the APU turbine engine. 
     SUMMARY 
     The present invention relates to an air inlet assembly that includes an entrance region configured to receive an airflow from an intake opening of an aircraft, an exit region configured to output the airflow to a turbine engine, and an attenuation region disposed between the entrance region and the exit region. The attenuation region has a cross section aspect ratio of at least about 5-to-1, and includes acoustic absorbing material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top schematic view of an aircraft tail section, which includes an air inlet assembly in use with an on-board APU turbine engine. 
         FIG. 2  is an expanded top schematic view of the air inlet assembly in the aircraft tail section, illustrating the interior of the air inlet assembly. 
         FIG. 3  is an expanded front schematic view of the air inlet assembly the aircraft tail section, further illustrating the interior of the air inlet assembly. 
         FIG. 4  is a sectional view of section  4 - 4  taken in  FIG. 3 , illustrating an attenuation region of the air inlet assembly. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a top schematic view of aircraft tail section  10 , which includes exterior structure  12  and APU nacelle  14 . Exterior structure  12  is an exterior skin of the aircraft, which includes intake opening  16  and door  18 . Intake opening  16  extends through exterior structure  12  along the side of aircraft tail section  10  for receiving air. Door  18  is a standard compartment door pivotally connected to exterior structure  12  at intake opening  16 , and desirably remains open during the course of operation. APU nacelle  14  is a compartment within exterior structure  12  that contains air inlet assembly  20 , APU  22 , eductor  24 , and exhaust pipe  26 . 
     Air inlet assembly  20  functions as an air inlet duct that extends between intake opening  16  and APU  22  for directing airflow into APU  22 . As shown, air inlet assembly  20  includes top wall  28 , front wall  30 , and rear wall  32 , which are exterior walls that divide air inlet assembly  20  into entrance region  34 , attenuation region  36 , and exit region  38 . Entrance region  34  is the upstream portion of air inlet assembly  20  and is disposed adjacent intake opening  16 . Attenuation region  36  is the middle portion of air inlet assembly  20  and is disposed downstream of entrance region  34 . Exit region  38  is accordingly disposed downstream of attenuation region  36 , adjacent APU  22 . The terms “upstream” and “downstream” are used herein with reference to the direction of airflow through air inlet assembly  20  from intake opening  16  to APU  22 . Air inlet assembly  20  also includes a bottom exterior wall (not shown in  FIG. 1 ) that is an opposing exterior wall to top wall  28 . 
     APU  22  is an on-board gas turbine engine that includes a core engine portion  40  (e.g. including a compressor, combustor and turbine), intake plenum  42 , and exhaust diffuser  44 , and which provides electrical and/or pneumatic power to the aircraft. Intake plenum  42  connects exit region  38  of air inlet assembly  20  to turbine portion  40 , thereby allowing a flow of air to reach core engine portion  40 . APU  22  can also include additional components (not shown) that facilitate the operation of APU  22  and the transfer of electrical and/or pneumatic power (e.g., inlet air ducts, gearboxes, generators, and bleed air ducts). While shown in aircraft tail section  10 , air inlet assembly  20  and APU  22  may alternatively be located in any suitable location on an aircraft. 
     Eductor  24  is an airflow system that extends annularly around at least a portion of exhaust diffuser  44  and draws cooling air through APU nacelle  14  and/or an oil cooler (not shown). In alternative embodiments, eductor  24  may be omitted and/or replaced with other cooling airflow systems. Exhaust pipe  26  extends from eductor  24 , and provides a channel for expelling gases from aircraft tail section  10 . During the course of operation, air flows through air inlet assembly  20  to intake plenum  42  and core engine portion  40  of APU  22 . Core engine portion  40  compresses the received air, adds fuel, and combusts the resulting fuel/air mixture. The resulting hot, high-pressure combustion gas then expands through a turbine stage (not shown) within turbine portion  40 . The resulting rotation of the turbines is used to generate electrical power for associated devices of the aircraft (not shown). The spent combustion gases exit through exhaust diffuser  44 , and are expelled from aircraft tail section  10  through exhaust pipe  26 . APU nacelle  14  may also include an exhaust silencer (not shown) located downstream from eductor  24  for attenuating low frequency noise. 
     While operating, core engine portion  40  generates high frequency noise (e.g., greater than about 4,000 Hertz, with a peak frequency of about 11,000 Hertz) due to the inducer blade passing frequency of the compressor. However, as discussed below, top wall  34 , front wall  36 , rear wall  38 , and the bottom exterior wall of air inlet assembly  20  provide a narrow passage width at attenuation region  36  that is comparable in size with the wavelengths of the high frequency noise. This allows attenuation region  36  to reduce the high frequency noise passing upstream from APU  22 . Additionally, attenuation region  36  has a cross section with a high aspect ratio, thereby substantially preserving the volumetric flow rate of air through air inlet assembly  20  without the use of segregating splitters that are typical with conventional inlet noise silencers. Accordingly, air inlet assembly  20  has a reduced risk of ice build up, thereby allowing air inlet assembly  20  to be used in icing conditions while providing compliance with aviation noise standards. 
       FIG. 2  is an expanded top schematic view of air inlet assembly  20  in aircraft tail section  10 , illustrating the interior of air inlet assembly  20 . As shown, air inlet assembly  20  further includes bottom wall  46 , liners  48 ,  50 , and  52 , and supports  54 , where bottom wall  46  is the bottom exterior wall that opposes top wall  28  (shown in  FIG. 1 ). Liners  48 ,  50 , and  52  are acoustic liners respectively secured to, or integrated with, front wall  30 , rear wall  32 , and bottom wall  46  at attenuation region  36 , and are configured to attenuate noise that travels upstream through attenuation region  36  from APU  22 . An additional acoustic liner (not shown in  FIG. 2 ) is also desirably secured to the interior surface of top wall  28  (shown in  FIG. 1 ) at attenuation region  36 . Examples of suitable liners for liners  48 ,  50 ,  52 , and the additional liner secured to top wall  28  include layers capable of dissipating acoustic energy, such as porous sheets, honeycomb matrices, acoustic-absorbent layers, and combinations thereof. In one embodiment, liners  48 ,  50 ,  52 , and the additional liner each include a multi-layer structure containing a honeycomb core disposed between a backing layer (e.g., an aluminum backing sheet) and an outer plate formed from a porous acoustic media. 
     Supports  54  could be rigid pins secured between front wall  30  and rear wall  32  that provide reinforcing support for attenuation region  36 . Supports  54  are also desirably layered with acoustic liners to further dissipate acoustic energy traveling upstream through attenuation region  36 . Examples of suitable liners for supports  54  include those discussed above for liners  48 ,  50 , and  52 . Each support  54  desirably has a small surface area, thereby reducing the risk of ice build up within attenuation region  36 . 
     As shown at entrance region  34 , front wall  30  and rear wall  32  are secured to exterior structure  12  at intake opening  16 , and converge along entrance region  34  toward attenuation region  36 . Similarly, at exit region  38 , front wall  30  and rear wall  32  are secured to intake plenum  42  of APU  18 , and converge along exit region  38  toward attenuation region  36 . At attenuation region  36 , front wall  30  and rear wall  32  have interior surfaces offset along lateral axis  56 , where lateral axis  56  is perpendicular to the airflow direction through attenuation region  36 . The interior surfaces of front wall  30  and rear wall  32  are offset by an average width along lateral axis  56  (referred to as offset width  58 ). Offset width  58  provides a narrow gap between front wall  30  and rear wall  32 , thereby positioning liners  48  and  50  at positions to attenuate the high frequency noise traveling upstream through attenuation region  36 . 
     As discussed above, high frequency noise waves generated from APU  18  have small wavelengths (e.g., from about 4,500 Hertz to about 20,000 Hertz). These small wavelengths may be attenuated with the use of acoustically-lined passageways having small passage widths that are comparable in size to the wavelengths. Accordingly, examples of suitable average distances for offset width  58  range from about 15 millimeters to about 150 millimeters, with particularly suitable average distances for offset width  58  ranging from about 20 millimeters to about 100 millimeters, and with particularly suitable average distances for offset width  58  ranging from about 25 millimeters to about 50 millimeters. In one embodiment, front wall  30  and rear wall  32  are substantially parallel at attenuation region  36 . In alternative embodiments, one or more of top wall  28 , front wall  30 , rear wall  32 , and bottom wall  46  have non-planar portions, and may intersect each other with rounded corners. In these embodiments, front wall  30  and rear wall  32  each desirably have planar portions at attenuation region  36  to which liners  48  and  50  are secured for attenuating noise. Accordingly, the average distance of offset height  58  is measured from these planar portions of front wall  30  and rear wall  32 . 
     Because of the narrow gap between front wall  30  and rear wall  32  (i.e., offset width  58 ), attenuation region  36  is free of segregating splitters that are typical with conventional inlet noise silencers. As discussed above, this reduces the risk of ice build up within attenuation region  36 . As such, air inlet assembly  20  is suitable for use in icing conditions while providing compliance with aviation noise standards. 
       FIG. 3  is an expanded front schematic view of air inlet assembly  20  in aircraft tail section  10 , further illustrating the interior of air inlet assembly  20 . As shown, air inlet assembly  20  further includes liner  60 , which is the additional acoustic liner secured to top wall  28  at attenuation region  36 . As discussed above, the narrow gap between front wall  30  (shown in  FIG. 2 ) and rear wall  32  positions liner  48  (shown in  FIG. 2 ) and liner  50  to attenuate high frequency noise traveling upstream through attenuation region  36 . Liners  52  and  60  increase the acoustically-lined surface area of attenuation region  36 , thereby further dissipating acoustic energy traveling upstream through attenuation region  36 . 
     Attenuation region  36  is also shown in use with six support pins  54 , which is a suitable number of support pins  54  for reinforcing attenuation region  36  while also allowing support pins  54  to have small surface areas. In alternative embodiments, fewer or greater numbers of support pins  54  may be used. Examples of suitable numbers of support pins  54  for reinforcing attenuation region  36  range from one to about ten pins, with particularly suitable numbers of support pins  54  ranging from about four to about eight pins. In additional embodiments, attenuation region  36  may be reinforced with alternative structural supports, such as strengthened walls (e.g., top wall  28  and bottom wall  46 ). In these additional embodiments, support pins  54  may be omitted. 
     As shown at entrance region  34 , top wall  28  and bottom wall  46  are secured to exterior structure  12  at intake opening  16 , and diverge along entrance region  34  toward attenuation region  36 . Similarly, at exit region  38 , top wall  28  and bottom wall  46  are secured to intake plenum  42  of APU  18 , and diverge along exit region  38  toward attenuation region  36 . In contrast to the narrow gap between front wall  30  (shown in  FIG. 2 ) and rear wall  32 , at attenuation region  36 , top wall  28  and bottom wall  46  have interior surfaces that are offset along vertical axis  62  by an average height (referred to as offset height  64 ), where vertical axis  62  is perpendicular to the direction of air flow through attenuation region  36  and to lateral axis  56  (shown in  FIG. 2 ). Offset height  64  desirably provides a wide gap between top wall  28  and bottom wall  46 , which preserves the volumetric flow rate of air through air inlet assembly  20  despite the narrow gap between front wall  30  and rear wall  32 . 
       FIG. 4  is a sectional view of section  4 - 4  taken in  FIG. 3 , which illustrates a cross section of attenuation region  36  substantially perpendicular to the direction of airflow. As shown, the narrow gap between front wall  30  and rear wall  32  along lateral axis  56  (i.e., offset width  58 ) and the wide gap between top wall  28  and bottom wall  46  along vertical axis  62  (i.e., offset height  64 ) provide attenuation region  36  with a high aspect ratio cross section. As discussed above, the high aspect ratio cross section is obtained by the convergence of front wall  30  and rear wall  32  as they progress toward attenuation region  36  combined with the divergence of top wall  28  and bottom wall  46  as they progress toward attenuation region  36 . Accordingly, the high aspect ratio cross section allows attenuation region  36  to have a narrow passage width for attenuating noise traveling upstream from APU  18  (shown in  FIG. 1 ) without the use of segregating splitters, while also preserving the volumetric flow rate of air through air inlet assembly  20 . 
     In the embodiment shown in  FIG. 4 , the aspect ratio of the cross section of attenuation region  36  is measured as a ratio of offset height  64  relative to offset width  58 . Examples of suitable aspect ratios for the cross section of attenuation region  36  include ratios of at least about 5-to-1, with particularly suitable aspect ratios including ratios of at least about 7-to-1, and with even more particularly suitable aspect ratios including ratios of at least about 10-to-1. The aspect ratio of the cross section of attenuation region  36  is also desirably low enough such that offset height  64  (i.e., the distance between top wall  28  and bottom wall  46 ) fits within APU nacelle  14  (shown in  FIG. 1 ). Accordingly, additional examples of suitable maximum aspect ratios for the cross section of attenuation region  36  include ratios of about 20-to-1, with particularly suitable maximum aspect ratios including ratios of about 17-to-1, and with even more particularly suitable maximum aspect ratios including ratios of about 15-to-1. 
     In one embodiment, the convergence and divergence of the walls of air inlet assembly  20  allow the varying cross sections along air inlet assembly  20  to have substantially the same cross sectional areas. In this embodiment, while the aspect ratios of the cross sections (in planes perpendicular to the airflow) vary over the length of air inlet assembly  20 , the areas of the cross sections remain substantially the same. For example, the cross sections of entrance region  34  at intake opening  16  (shown in  FIG. 2 ) and of exit region  38  at intake plenum  42  (shown in  FIG. 2 ) each have low aspect ratios relative to the cross section of attenuation region  36 . However, in this embodiment, these cross sections have substantially the same areas. This preserves the volumetric flow rate of air through air inlet assembly  20 , thereby preventing a bottleneck effect within air inlet assembly  20 . Examples of suitable variations in cross sectional areas for attenuation region  36  relative to entrance region  34  and exit region  38  include variations of about 10% or less (i.e., the cross sectional area of attenuation region  36  is at least about 90% of the cross sectional areas of entrance region  34  and exit region  38 , with particularly suitable area variations including variations of about 5% or less, and with even more particularly suitable area variations including variations of about 1% or less. 
     The convergence and divergence of the walls of air inlet assembly  20  also desirably allows air inlet assembly  20  to be installed within aircraft without substantial modifications to the dimensions of the intake opening (e.g., intake opening  16 ) and the intake plenum (e.g., intake plenum  42 ) of the aircraft. As such, air inlet assembly  20  may be designed around standard APU nacelle dimensions. For example, offset width  58  of attenuation region  36  may be set to attenuate a desired noise frequency from an APU turbine engine (e.g., APU  18 ). The aspect ratio of the cross section of attenuation region  36  may then be set to match a desired volumetric flow rate through attenuation region  36 . The convergence and divergence of the walls of entrance region  34  and exit region  38  may then be set for installation with the dimensions of the aircraft intake opening and intake plenum. This further increases the versatility of air inlet assembly  20  for use with a variety of different turbine engine designs. 
     While the components of air inlet assembly  20  are discussed herein as having directional orientations (e.g., front, rear, top, bottom, height, width, lateral, and vertical), such orientations are used for ease of discussion, and are not intended to be limiting to any particular spatial orientation within an aircraft. Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.