Abstract:
An inertial particle separator (IPS) for an inlet bell-mouth that couples an inlet air plenum to a compressor in a gas turbine engine, wherein the IPS removes air particles within reverse air flow passing through at least one bell-mouth aperture in the inlet bell-mouth into shroud bleed apertures in a shroud for the compressor, comprising: at least one baffle that protrudes from each bell-mouth aperture positioned to bend a reverse air flow stream through the bell-mouth aperture to a degree that forces particles out of the reverse air flow stream and into the inlet air plenum.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to gas turbine engines, and more particularly to a gas turbine engine that has bleed slots arranged around its compressor shroud. 
       BACKGROUND OF THE INVENTION 
       [0002]    Gas turbine engines for vehicles, such as helicopters and tanks, that operate in environments with significant particle loading due to atmospheric particulates, such as dust and sand, generally have an inlet design that employs an Inertial Particle Separator (IPS). Commercial aircraft do not generally operate in atmospheric conditions with high particle loading or concentration. Therefore, gas turbine engines for commercial aircraft, such as those employed as an auxiliary power unit (APU), generally do not include an IPS system since damage to the leading edges of their compressor blades due to ingestion of particles such as sand and dust is low. 
         [0003]    This is true as long as the compressor ingests airflow through its impeller/inducer inlet plane. In this case, the compressor continuously accelerates the airflow towards the impeller and then directs it through the flow passages in between the rotating blades. Consequently, if particle impact with the blades takes place in this context, impact angles are shallow and/or relative impact velocity is low. 
         [0004]    In order to increase operating range, many state-of-the-art gas turbine engines employed as an APU include an aerodynamic control feature referred to as “shroud bleed”. A plurality of shroud-bleed apertures that each penetrate through the outer shroud for the compressor impeller somewhat downstream of the impeller throat allow a certain proportion of compressed air to escape from the compressor shroud and recirculate back through the engine inlet to improve the surge resistance of the compressor under heavy shaft loading conditions. This recirculated air passes through a plurality of bell-mouth apertures that penetrates a bell-mouth that couples the inlet plenum to the compressor. Under certain operating conditions, air flow may enter the compressor stage not only through the compressor inlet plane but also through the shroud-bleed apertures. Such air flow entering through the shroud-bleed apertures into the compressor passages between the impeller blades accelerates from virtually zero velocity to blade-tip velocity. Consequently, the bulk of any particles present within this reverse shroud-bleed air flow shall collide with the rotating impeller blade tips due to their inertia, thereby giving rise to blade erosion or damage. 
       SUMMARY OF THE INVENTION 
       [0005]    Generally, the invention comprises an IPS for an inlet bell-mouth that couples an inlet air plenum to a compressor in a gas turbine engine, wherein the IPS removes air particles within reverse air flow passing through at least one bell-mouth aperture in the inlet bell-mouth into shroud bleed apertures in a shroud for the compressor, comprising: at least one baffle that protrudes from each bell-mouth aperture positioned to bend a reverse air flow stream through the bell-mouth aperture to a degree that forces particles out of the reverse air flow stream and into the inlet air plenum. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a partial cut-away side view of a typical gas turbine engine that has an inlet plenum coupled to a compressor by way of an inlet bell-mouth according to the prior art. 
           [0007]      FIG. 2  is an end view of the inlet bell-mouth that couples the inlet plenum to the compressor for the gas turbine engine shown in  FIG. 1  according to the prior art. 
           [0008]      FIG. 3  is a partial cut-away side view of a gas turbine engine that has an inlet plenum coupled to a compressor by way of an inlet bell-mouth according to a first possible embodiment of the invention. 
           [0009]      FIG. 4  is an end view of the inlet bell-mouth that couples the inlet plenum to the compressor for the gas turbine engine shown in  FIG. 3  according to a first possible embodiment of the invention. 
           [0010]      FIG. 5  is a partial cut-away side view of the inlet bell-mouth that couples the inlet plenum to the compressor for the gas turbine engine shown in  FIG. 3  that shows a bell-mouth aperture for the inlet bell-mouth according to a first possible embodiment of the invention. 
           [0011]      FIG. 6  is a partial cut-away side view of a gas turbine engine that has an inlet plenum coupled to a compressor by way of an inlet bell-mouth according to a second possible embodiment of the invention. 
           [0012]      FIG. 7  is a partial cut-away side view of the inlet bell-mouth that couples the inlet plenum to the compressor for the gas turbine engine shown in  FIG. 6  that shows a bell-mouth aperture for the inlet bell-mouth according to a second possible embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]      FIG. 1  is a partial cut-away side view of a typical gas turbine engine  2  that has an inlet plenum  4  coupled to a compressor  6  by way of an inlet bell-mouth  8  according to the prior art.  FIG. 2  is an end view of the inlet bell-mouth  8  that couples the inlet plenum  4  to the compressor  6  for the gas turbine engine  2  shown in  FIG. 1 . The inlet plenum  4  allows ambient air to pass through a plurality of inlet air apertures  10 . A compressor shaft  12  rotates a compressor impeller  14  within a compressor shroud  16  to suck air within the inlet plenum  4  into a generally axial impeller inlet  18 , compress it and discharge compressed air from a generally radial compressor outlet  20 . 
         [0014]    A plurality of shroud-bleed apertures  22  penetrates through the compressor shroud  16  somewhat downstream of the impeller inlet  18  to allow a certain proportion of compressed air to escape from the compressor shroud  16 . This compressed air normally recirculates back through the inlet plenum  4  by way of a plurality of bell-mouth apertures  24  that penetrates the bell-mouth  8 . Under certain operating conditions, reverse air flow may flow from the inlet plenum  4  through the bell-mouth apertures  24  and into the shroud-bleed apertures  22 . Such air flow entering through the shroud-bleed apertures  22  into compressor passages between blades of the compressor impeller  14  accelerates from virtually zero velocity to blade-tip velocity. Consequently, the bulk of any particles present within this reverse shroud-bleed air flow will collide with rotating impeller blade tips of the compressor impeller  14  due to particle inertia, thereby giving rise to blade erosion or damage of the compressor impeller  14 . Line  26  represents a possible path of typical particles that may find their way from the inlet plenum  2  into the shroud bleed apertures  22  in this manner. 
         [0015]      FIG. 3  is a partial cut-away side view of a gas turbine engine  2  that has an inlet plenum  4  coupled to a compressor  6  by way of an inlet bell-mouth  28  according to a first possible embodiment of the invention.  FIG. 4  is an end view of the inlet bell-mouth  28  that couples the inlet plenum  4  to the compressor  6  for the gas turbine engine  2  shown in  FIG. 3  according to a possible embodiment of the invention. According to this embodiment, the inlet bell-mouth  28  has a plurality of bell-mouth apertures  30 . Each bell-mouth aperture  30  has an associated particle-deflecting baffle or louvre  32  along a compressor side  34  of the inlet bell-mouth  28 . In one possible embodiment the apertures  30  have a generally rectangular shape. Each baffle  32  extends from the compressor side  34  of the inlet bell-mouth  28  to an outlet end  36 . 
         [0016]      FIG. 5  is a partial cut-away side view of the inlet bell-mouth  28  that couples the inlet plenum  4  to the compressor  6  for the gas turbine engine  2  shown in  FIG. 3  that shows one of the bell-mouth apertures  30  with its associated baffle  32  in detail. Lines  38  represent streamlines of reverse air flow through the bell-mouth aperture  30  and associated baffle  32 . Curvature of the streamlines  38  increases significantly as the acceleration of the reverse air flow increases from inside the inlet plenum  4  toward the bell-mouth aperture  30 . The reverse air flow streamlines  38  penetrate the bell-mouth aperture  30  and bend around an inner surface  40  of the baffle  32  to a degree that any particle with a path that initially follows the reverse air flow, as represented by line  42 , can no longer do so due to its inertia. The baffle  32  thereby forces the particle out of the reverse air flow and it deflects off of the baffle  32  back into the inlet plenum  4  downstream of the inlet air apertures  10 . 
         [0017]    Thus, the baffle  32  bends the reverse air flow to an extent that particles within the reverse airflow remain within the inlet plenum  4 . It is possible to optimise the height H of the bell-mouth aperture  30 , as represented by line  44 , and the length L between the inlet side  34  of the inlet bell-mouth  28  and the outlet end  36  of the baffle  32 , as represented by a line  46 , to effectively eliminate ingestion of particles in this manner that are larger than a given size. 
         [0018]    Although each baffle  32  may have a generally rectangular or wedge-like shape that extends from the compressor side  34  of the inlet bell-mouth  28  to the outlet end  36  of the baffle  32 , alternatively each baffle  32  may have different or more complex shapes that perform the same function. For instance, the inner surface  40  of each baffle  32  may be generally curvilinear rather than generally flat as shown in  FIG. 5 . Each bell-mouth aperture  30  may also have a variety of shapes, such as generally triangular or semicircular, in which case each associated baffle  32  may have a corresponding shape, such as a generally truncated cone or cup-like shape that extends from the compressor side  34  of the inlet bell-mouth  28 . Finally, each baffle  32  may comprise a plurality of inner surfaces  40  that deflects particles in the reverse air flow stream back into the inlet plenum  4 . 
         [0019]      FIG. 6  is a partial cut-away side view of a gas turbine engine  2  that has an inlet plenum  4  coupled to a compressor  6  by way of an inlet bell-mouth  48  according to a second possible embodiment of the invention. It is similar in appearance to the inlet bell-mouth  28  shown in  FIG. 3 , but it has a plurality of bell-mouth apertures  50 . Each bell-mouth aperture  50  has an associated particle-deflecting baffle or louvre  52  along an inlet side  54  of the inlet bell-mouth  48 . In one possible embodiment the apertures  50  have a generally rectangular shape. Each baffle  52  extends from the inlet side  54  of the inlet bell-mouth  48  to an inlet end  56 . 
         [0020]      FIG. 7  is a partial cut-away side view of the inlet bell-mouth  48  that couples the inlet plenum to the compressor for the gas turbine engine shown in  FIG. 6  that shows one of the bell-mouth apertures  50  with its associated baffle  52  in detail. Lines  58  represent streamlines of reverse air flow through the bell-mouth aperture  50  and associated baffle  52 . Curvature of the streamlines  58  increases significantly as the acceleration of the reverse air flow increases from inside the inlet plenum  4  toward the bell-mouth aperture  50 . The reverse air flow streamlines  58  penetrate the bell-mouth aperture  50  and bend around an inner surface  60  of the baffle  52  to a degree that any particle with a path that initially follows the reverse air flow, as represented by line  62 , can no longer do so due to its inertia. The baffle  52  thereby forces the particle out of the reverse air flow and it then continues its path within the inlet plenum  4 . 
         [0021]    Thus, the baffle  52  bends the reverse air flow to an extent that particles within the reverse airflow remain within the inlet plenum  4 . Again, it is possible to optimise the height H of the bell-mouth aperture  50 , as represented by line  64 , and the length L between the inlet side  54  of the inlet bell-mouth  48  and the inlet end  56  of the baffle  52 , as represented by line  66 , to effectively eliminate ingestion of particles in this manner that are larger than a given size. 
         [0022]    Once again, although each baffle  52  may have a generally rectangular or wedge-like shape that extends from the inlet side  54  of the inlet bell-mouth  48  to the inlet end  56  of the baffle  52 , alternatively each baffle  52  may have different or more complex shapes that perform the same function. For instance, the inner surface  62  of each baffle  52  may be generally curvilinear rather than generally flat as shown in  FIG. 7 . Each bell-mouth aperture  50  may also have a variety of shapes, such as generally triangular or semicircular, in which case each associated baffle  52  may have a corresponding shape, such as a generally truncated cone or cup-like shape that extends from the inlet side  54  of the inlet bell-mouth  48 . Finally, each baffle  52  may comprise a plurality of inner surfaces  62  that separate particles out of the reverse air flow stream so that they remain within the inlet plenum  4 . 
         [0023]    Any embodiment of the invention, such as the inlet bell-mouth  28  or the inlet bell-mouth  48  hereinbefore described, may comprise a stamping or weldment, such as of sheet metal, or a moulding, such as of plastic or a composite material. The described embodiments of the invention are only illustrative implementations of the invention wherein changes and substitutions of the various parts and arrangement thereof are within the scope of the invention as set forth in the attached claims.