Abstract:
A swirl generator for creating bulk swirl in a compressor plenum in the opposite direction to the direction of rotation of the compressor rotor. The generator includes a continuous flow passageway formed by an inlet duct and a compressor forming an angle in the range of about 0°-180° to one another. A non-uniform velocity gradient is created by forming an area of non-uniform cross-sectional area within the flow passageway. The non-uniform area may comprise a ramp with a tapered end line, a rounded line of intersection, a fillet or by lengthening one side wall of the inlet duct relative to the opposite side wall. Preferably, the non-uniform cross sectional area uniformly tapers from ones sidewall of the flow passageway to the opposite sidewall.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to an Auxiliary Power Unit (APU) system of the type utilized for starting a jet engine and providing environmental control for commercial aircraft. More particularly, the present invention is directed to an inlet duct and interfacing plenum capable of significantly reducing Exhaust Gas Temperature (EGT) which directly translates to reducing Turbine Inlet Temperature (TIT) and, consequently, increasing APU turbine life. 
     It has been observed in prior systems that plenums that receive uniform airflow tend to produce a pair of vortices that cause a 2 per rev mechanical and aerodynamic distortion at the compressor eye  10  as shown in FIG.  1 . For each revolution of the compressor  12 , a blade leading edge  14  experiences a reversal of flow incidence two separate times. This distortion causes a decrement in compressor performance that leads to increased TIT and ultimately reduced turbine life as well as reduced operating efficiency of the APU. 
     Compressors prefer a reasonable amount of counter-rotating bulk swirl. Any non-uniform flow entering a plenum generates bulk swirl. Air having a controlled velocity profile entering the plenum can be used to create such a favorable counter-rotating bulk swirl. It has been further observed that a simple straight-walled inlet duct that turns substantially 90° into a plenum tends to produce a uniform plenum flow resulting in the reduced life of the turbine blades and ultimately the APU system. 
     There is a need for a bulk swirl generator having an inlet into a plenum that generates air flow having a predictable and prescribed velocity gradient entering the plenum and that assures the desirable bulk swirl is maintained within an APU plenum to reduce EGT and ultimately TIT in order to maximize life of the turbine blades. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a bulk swirl generator comprises an airflow passageway leading to an APU plenum, which feeds to a compressor. The passageway creates a non-uniform air flow profile or gradient entering the compressor. In particular, a tapered ramp extends across the width of an inlet duct at the interface of the duct and the APU plenum. The ramp has a maximum turn radius located at one side of the inlet duct, steadily decreasing in radius or taper until a sharp corner is formed at the opposite side of the inlet duct. The turn angle of the radius is preferably between about 45°-90°. The tapered ramp creates the non-uniform velocity gradient across the mouth of the compressor plenum with the greater velocity existing at the side of the plenum inlet duct having the greatest turn radius. The intersecting surfaces of the inlet duct and the plenum oppositely disposed from the ramp form an angle of substantially 90°. The velocity of the stream of air entering the plenum steadily decreases as the radius of the ramp decreases, creating a non-uniform velocity gradient entering the plenum. This, in turn, creates the desired bulk swirl in the opposite direction to the rotation of the compressor rotor. 
     In another aspect of the invention, an air flow passageway comprises a compressor plenum and attached inlet duct wherein a fillet is created at the interface between the inlet duct and the mouth of the plenum. The fillet tapers from a maximum radius at one side of the inlet duct to a sharp corner at the other side of the inlet duct. The opposite interface between the inlet duct and plenum forms an angle of approximately 90°. The tapering fillet creates a non-uniform velocity gradient entering the plenum that, in turn, creates the desirable bulk swirl flow in the direction opposite to the rotation of the compressor rotor. 
     In still another aspect of the invention, a flow passageway generator includes a rounded interface between the inlet duct and the compressor plenum. The rounded interface uniformly tapers from having a maximum radius at one side of the inlet duct to having a sharp corner at the other side of the inlet duct. The inlet duct and plenum interface oppositely disposed from the rounded interface form an angle of substantially 90°. This configuration creates a flow passageway having a non-uniform gradient wherein the maximum airflow velocity is created adjacent the portion of the rounded interface having the maximum radius; with the air flow velocity decreasing toward the opposite side with the sharp radius or corner. 
     In a further aspect of the present invention, a generator comprises an air flow passageway wherein a ramp is disposed in the wall of a compressor plenum, with the ramp extending upstream towards the interface with an inlet duct attached to the plenum. The leading edge of the ramp extends across the mouth of the plenum and is formed with a maximum turn radius located at one side of the plenum. The turn radius steadily decreases in radius or taper until reaching a sharp corner at the opposite side of the plenum. The interface inlet duct and plenum interface opposite the ramp forms an angle of substantially 90°. The radius of the ramp turn angle is preferably between about 45°-90°. During operation, air flows through the inlet duct and across the plenum ramp toward the compressor rotor. The tapered ramp creates a non-uniform velocity gradient that, in turn, creates the desirable bulk swirl in the opposite direction to the rotation of the compressor rotor. The bulk swirl reduces the EGT and ultimately the TIT, significantly increasing the life of the APU turbine blades as well as improving the operating efficiency of the APU by decreasing the amount of fuel needed to run the system. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a uniform velocity flow gradient at the entry of a compressor plenum in a prior art design; 
     FIG. 2 is a perspective view of a non-uniform velocity flow gradient at the entry of a compressor plenum in accordance with the present invention; 
     FIG. 3 is a perspective view of an inlet duct and plenum formed in accordance with the present invention; 
     FIG. 4 is a cross-sectional view of inlet duct and plenum formed in accordance with the embodiment of FIG. 3; 
     FIG. 5 is a perspective view of a further inlet duct formed in accordance with the present invention; 
     FIG. 6 is a cross-sectional view of the inlet duct of FIG. 5 formed in accordance with the present invention; 
     FIG. 7 is a cross-sectional view of another inlet formed in accordance with the present invention; 
     FIG. 8 is a cross-sectional view of yet another inlet formed in accordance with the present invention; and 
     FIG. 9 is a perspective view of yet another inlet formed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out the present invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Referring again to FIG. 1 that depicts a prior art design, the uniform velocity gradient is identified by arrows  20  of equal length across the mouth of plenum  22 . Because the flow into plenum  22  is substantially uniform, the flow velocity from a first side  24  to a second side  26  is substantially constant as identified at  28 . In particular, the rectangular shape of the pressure contour indicative of constant velocity extends across the entire width of plenum  22 . In contrast, a non-uniform flow into plenum in FIG. 2 in accordance with the present invention is represented by arrows  30  of decreasing lengths across the mouth of plenum  22 . This creates a velocity contour of generally elliptical shape  31 , having the bulk of contour located where the arrows  30  are of minimum length, indicating reduced velocity. The primary difference between the flow through plenum  22  in FIGS. 1 and 2 is the creation of secondary flows that will directly affect a compressor  12 . 
     As discussed above, the uniform velocity gradient  20  in FIG. 1 creates a pair of counter-rotating vortices  32  and  34  that create a 2 per rev mechanical and aerodynamic distortion to a compressor rotator  12  regardless of its direction of rotation. In comparison, if the non-uniform velocity gradient  30  decreases from first side  24  to second side  26  of plenum  22  and compressor rotator  12  rotates in the counter-clockwise direction shown in FIG. 2, a counter swirl  36  is created in the direction counter to the direction of rotation of compressor rotator  12 . This counter swirl  36  has shown to significantly improve compressor performance thereby reducing EGT substantially (i.e., about 10-14° F.). 
     Turning now to FIGS. 3 and 4, an inlet duct  40  extends at substantially about 90° to an intersection with a mouth of a compressor plenum  22 . However, the scope of the present invention is considered to include an inlet duct  40  and attached plenum  22  that form any angle up to about 180°. In order to achieve the desired non-uniform velocity gradient  30 , a ramp  42  extends within inlet duct  40  in the direction of the mouth of plenum  22  as formed by plenum sidewalls  24  and  26 , bottom wall  23 , and top wall  25 , respectively. Ramp  42  extends from a first sidewall  44  of inlet duct  40  to a second, opposite sidewall  46 . It is noted that velocity gradient  30  decreases in value from side wall  44  to side wall  46  in FIG. 3, thus creating the elliptical area  31  of low velocity toward the side  26  of plenum  22 . 
     Ramp  42  is preferably formed with an angle φ of substantially about 8 to 10° rise compared with wall  43  of inlet duct  40 . The maximum radius of curvature  48  of ramp  42  as measured from bottom wall  43  is located adjacent to side wall  44  and is preferably about 10% to 25% of the height of side wall  44 , while a minimum radius is located adjacent an opposite wall  46  and is preferably a close to 0 as possible to create a sharp corner between inlet duct  40  and plenum  22 . The radius of curvature  48  steadily decreases in value from its maximum value until approaching side wall  46  where it makes a sharp, substantially 90° corner with plenum  22 , as denoted at  47 . 
     It is to be understood that if compressor rotator  12  were to rotate in the opposite, or clockwise direction, the velocity gradient  30  should also run in the opposite direction wherein the largest value is adjacent side wall  46  and the smallest value adjacent side wall  44 . This would mean that ramp  42  would have the greatest radius of curvature  48  adjacent to side wall  46  and a sharp corner adjacent side wall  44 . 
     Regardless of the direction of the taper of ramp  42 , some of the air will closely follow the curved ramp  42  while other air will separate at the sharp corner between inlet duct  40  and plenum  22 . By tapering the ramp  42  as discussed, inlet duct  40  creates the non-uniform velocity and flow gradient  30  in the same direction as the taper, as well as the direction of rotation of compressor rotator  12 . While the ramp  42  is shown extending outwardly from a bottom side  43  of inlet duct  40  (FIG.  4 ), it is well within the scope of the present invention to position ramp  42  on the opposite or topside  45  of inlet duct  40 . present invention to position ramp  42  on the opposite or topside  45  of inlet duct  40 . 
     In another aspect of the invention, the favorable bulk swirl in the plenum  22  may be created by filleting the intersection between plenum side  23  and bottom side  43  of inlet duct  40 . As shown in FIGS. 5 and 6, a fillet  50  extends between inlet bottom side  43  and plenum side  23 . The fillet  50  has its maximum radius at the intersection of inlet sidewall  44  and inlet bottom wall  43  as best shown at  50   a  in FIG.  5 . The radius of curvature of fillet  50  steadily decreases or tapers from a value at  50   a  to lesser values at the intersection of side wall  46  and bottom wall  43 , identified at  50   b  and in the direction of the top wall  45  of inlet  40  identified as  50   c.  The taper should smoothly blend in with the adjacent walls  43  and  23  of duct  40  and plenum  22 , respectively. The maximum radius of curvature of the fillet  50  located at  50   a  would be about 10%-25% of the height of adjacent side wall  44 , while the minimum radius of curvature at  50   b  would be as close to 0 as practical to create a sharp corner of substantially 90° between inlet duct bottom wall  43  and plenum wall  23 . 
     In a yet further aspect of the present invention shown in FIG. 7, the non-uniform flow gradient across the mouth of plenum  22  is formed by rounding the intersecting walls  43  of inlet duct  40  and  23  of plenum  22 . The rounded line of intersection  60  would taper between a fully rounded configuration adjacent inlet duct sidewall  44  to a sharp corner adjacent sidewall  46 . The maximum radius of curvature of the rounded line of intersection  60  would be about 10%-25% of the height of adjacent side wall  44 , while the minimum radius of curvature adjacent side wall  46  would be as close to 0 as practical to create a sharp corner of substantially 90° between inlet duct bottom wall  43  and plenum wall  23 . 
     Attention is respectfully directed to FIG. 8, wherein a further aspect of the invention is shown. A ramp  72 , somewhat similar to ramp  42  in FIG. 3, is positioned on the side  23  of plenum  22  facing toward the line of intersection with inlet duct  40 . Ramp  72  relies on the Coanda effect which states that flow will naturally follow a curved surface. Ramp  72  would preferably taper from having a maximum radius adjacent to the sidewall  24  of plenum to a sharp corner adjacent to the side  26  of plenum  22 . Ramp  72  preferably forms an angle φ of substantially about 8 to 10° rise compared with wall  23  of plenum  22 . The maximum radius of curvature of ramp  72  is preferably about 10% to 25% of the height of sidewall  24  of plenum  22 , while the minimum radius of curvature is as close to 0 as practical at a location adjacent to sidewall  26  of plenum  22 . 
     Attention is directed to FIG. 9, wherein a further aspect of the present invention is shown. The inlet duct  90  forms a substantially right angle of 90° with plenum  92 . At the entryway to duct  90 , the sidewalls  94  and  96  are similar in shape and size. This creates a rectangular cross-sectional area  91  of uniform shape. However, proceeding through duct  90 , sidewall  96  gradually narrows relative to sidewall  94 . The cross-sectional shape of duct  90  varies from that of a uniform rectangle  91  at the entry to a non-uniform shape  97  at the intersection with plenum  92 . In particular, the cross-sectional area  97  of inlet duct  90  tapers from side  94  to side  96 . When air flows through duct  90  into plenum  92 , a non-uniform velocity profile gradient  98  is created which generates the desired bulk swirl within plenum  92 . 
     Each of the aspects of the present invention alters the airflow gradient by varying the shape of a line of intersection between the duct and plenum. At one location along the interface, the duct and plenum have intersecting walls that meet along a radius of curvature. At another location, the walls meet in a sharp corner. The line of intersection tapers in a uniform manner from the rounded intersection to the square edge. This taper changes the air flow passageway which causes a corresponding change in the flow velocity of the air flowing from the inlet duct into plenum before encountering compressor rotator. The non-uniform gradient created in the plenum causes desirable bulk swirl flow, reducing exhaust gas temperature and ultimately turbine inlet temperature, thus increasing turbine blade life and overall performance of the APU. 
     It should be understood, of course, that the foregoing relates to preferred embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.