Abstract:
A structure and a method to form the structure are provided for an impeller bleed passage of a compressor for a gas turbine engine. The compressor has an impeller assembly which includes an impeller rotor rotatably supported within an annular shroud having an inlet and an outlet. The shroud is made of two separate annular segments which are axially spaced apart. Each of the segments is supported separately and independently in a cantilevered manner. Such that a circumferentially continuous, uninterrupted annular slot is formed between the two segments and air passes through the slot without causing a dynamic component to affect the impeller rotor. The width of the slot is adjustable for different engines depending on the requirements of use of a particular engine. The width of the slot is also self-regulating in response to changes in the air pressure within the shroud because of the deformation of the segments. The structure is relatively simple and inexpensive to manufacture.

Description:
TECHNICAL FIELD 
     This invention relates to compressors for use in gas turbine engines and, more particularly, to centrifugal compressors including air bleed in association therewith for regulating the operating characteristics of the compressor. 
     BACKGROUND OF THE INVENTION 
     In gas turbine engines for use in powering aircraft, air is directed through multiple stage compressors as it flows axially or axially and radially through the engine to a burner. As the air passes through each successive compressor stage, the pressure of the air is increased. Under certain conditions, such as when the engine is throttled back or during start-up, the compressor pumping capacity is significantly reduced. In this condition, an engine surge or blow-out may occur, endangering the operation of the engine and the associated aircraft. In the past, it has been recognized that inadequate surge margin in such compressors could be eliminated by bleeding a substantial percentage of the compressor air flow at strategic locations along the gas path. 
     It has been proposed in U.S. Pat. No. 4,248,566 which is entitled DUAL FUNCTION COMPRESSOR BLEED and issued to Chapman et al. on Feb. 3, 1981, to form an annular control slot in the stationary shroud so as to allow the inflow of air from outside the shroud to the rotor chamber under high r.p.m. conditions of the compressor operations and to allow air flow to bleed from the rotor chamber to the exterior of the shroud when the rotor is operating at a low r.p.m. whereby to stabilise the flow of the rotor at low r.p.m. operation. Nevertheless, the annular slot disclosed in this patent is not circumferentially continuous and the radial air flow is affected by reinforcing bridges on the shroud. The reinforcing bridges connect the two parts of the shroud separated by the slot and serve to carry structural roads. 
     It is also suggested that separate holes in a circumferential row could replace the annular slot as long as the desired bleed flow area is maintained. The outer tip of the impeller bleed will be effected by the local pressure variation when the outer tip of each blade sweeps from an area having open bleed passages to an area without bleed passages or blocked by the bridges, which is an undesirable dynamic component to the compressor operation. 
     To increase the engine r.p.m. over which compressors can operate in a stable manner, U.S. Pat. No. 4,743,161 entitled COMPRESSORS which issued to Fisher et al. on May 10, 1998, discloses a compressor having an air bleed passage in communication with the normal intake so that the air is thus not bled to the exterior of the impeller housing, and thus atmosphere, nor drawn in from the exterior atmosphere separately from the normal gas intake to the compressor, as in U.S. Pat. No. 4,248,566, but is bled back to the normal intake or is drawn from the normal intake. In one embodiment illustrated in FIG. 5 of U.S. Pat. No. 4,743,161, a circumferentially continuous annular slot is provided for communication with the chamber in which the impeller wheel rotates and an annular chamber. The annular chamber also communicates with the intake through a series of holes. However, the gas pressure is released in the intake rather than the annular chamber. The gas bleed passage includes not only the annular slot but also the annular chamber and the series of holes. The bleed gas flow is not circumferentially even because of the holes and the circumferential pressure variation causes the dynamic component and affects the outer tips of the impeller, particularly, in the case where the holes are close to the outer tip of the blade, which is illustrated in the Figure. 
     Bleed valves are also used for gas turbine engines to provide adjustable bleed passages. U.S. Pat. No. 5,380,151 which issued to Kostka et al. on Jan. 10, 1995 and entitled AXIALLY OPENING CYLINDRICAL BLEED VALVE, is an example. In this patent, Kostka discloses a bleed valve for a gas turbine engine having a housing made of two segments and which forms a gas flow path through the compressor. A first segment is moveable from the second segment thereby creating an opening therebetween. The moveable segment has one or more arms with rollers attached thereto where the stationary segment defines recessed paths in which the rollers travel. The moveable segment is caused to move away from the stationary segment thereby opening the valve. Because the arms extend across the annular opening between the two segments to moveably connect the two segments, the bleed passage provided by the valve is faced with the same problem as discussed in the above prior art, that is, a dynamic component is created to affect the blades when the air passes through the bleed passage. Further, the arms, rollers and the travel path fixed to the bleed valve segments add weight and machining operations to the construction of the valve which translates into additional manufacturing costs. 
     Therefore, there exists a need for a structure for an impeller bleed passage of a compressor for a gas turbine engine which eliminates the dynamic component that affects the blades of the impeller when air passes through the bleed passage. It is also desirable to provide a structure for an adjustable bleed passage that is relatively simple and inexpensive to manufacture. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a structure for an impeller bleed passage of a compressor for a gas turbine engine, to minimise dynamic components which affect the impeller blades when air passes through the bleed passage. 
     Another object of the invention is to provide a structure for an impeller bleed passage of a compressor for a gas turbine engine, having a minimum width of the bleed passage to decrease operational inefficiency of the compressor caused by the air bleed. 
     Another object of the invention is to provide a structure for an impeller bleed passage of a compressor for a gas turbine engine, having a width of the bleed passage that is adjustable for different engines depending on the requirements of use of a particular engine. 
     Yet another object of the invention is to provide a structure for an impeller bleed passage of a compressor for a gas turbine engine, having a width of the bleed passage that is self-regulating in response to changes in the air pressure within the impeller chamber. 
     A further object of the invention is to provide a structure for impeller bleed passage of a compressor for a gas turbine engine that is relatively simple and inexpensive to manufacture. 
     In accordance with one aspect of the invention a compressor for a gas turbine engine is provided, which includes an annular shroud having an inlet end, an outlet end and an inner surface; a compressor rotor located within the shroud including a plurality of blades directed radially and outwardly from the rotor. The annular shroud comprising: 
     an upstream annular segment and a downstream annular segment independently supported and axially spaced apart to form a circumferentially continuous uninterrupted annular slot therebetween, such that the annular slot extends through the shroud. 
     Preferably, at least one of the segments being elastically deformable so that a width of the slot changes in response to changes in air pressure within the shroud during operation of the compressor. Preferably, the downstream annular segment is elastically deformable. 
     The slot width is preferably adjustable for different engines depending on requirements of use of a particular engine. 
     In accordance with another aspect of the invention, a compressor for a gas turbine engine is provided, which includes a stationary annular shroud having an inlet end and an outlet end and an inner surface; a rotor located within the shroud including a plurality of blades directed radially and outwardly from the rotor, each blade having an outer tip that is of similar contour to and located in a close spaced relationship to the inner surface of the shroud; the annular shroud comprising: 
     an upstream annular segment and a downstream annular segment axially spaced apart to form a circumferentially continuous uninterrupted annular slot therebetween, the annular slot extending through the shroud, the upstream annular segment being supported by a first structure and the downstream annular segment being supported by a second structure, each of the upstream and downstream annular segments being independent and self-supporting at a peripheral edge adjacent the slot so that when the compressor is in operation, air passes through the continuous annular slot without causing a dynamic component which affects the blades. 
     The first structure is preferably an inducer which includes an annular passage in communication with the shroud at the inlet end for introducing air flow through the shroud. The second structure is preferably a casing by which the rotor is rotatably supported. 
     In accordance with a further aspect of the invention there is provided a method for providing an air bleed passage in association with a compressor for use in gas turbine engines, the compressor having an impeller assembly which including an impeller rotor rotatably supported within an annular shroud having an inlet and an outlet, comprising: 
     producing the impeller shroud in two separate annular segments having an upstream annular segment and downstream annular segment; 
     supporting the upstream and downstream annular segments separately and independently in an axially spaced apart relationship to form a circumferentially continuous, uninterrupted annular slot therebetween, such that the annular slot extends through the shroud. 
     The upstream and downstream annular segments are preferably mounted respectively to a first and a second structures in a cantilevered manner, each of the upstream and downstream annular segments independent and self-supporting at a peripheral edge adjacent the slot so that when the compressor is in operation, air passes through the continuous, uninterrupted annular slot without causing a dynamic component which affects the impeller rotor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood from the following description of a preferred embodiment, as an example only, in conjunction with reference to the accompanying drawings, in which: 
     FIG. 1 is a fragmentary, longitudinal section of a compressor including the preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawing, a compressor  10  is shown in FIG.  1 . It includes an upstream support assembly  12  and a downstream support assembly  14  for physically locating a compressor impeller assembly  16  of the compressor  10 , in a manner to be discussed. More particularly, the upstream support assembly  12  is made up of an annular inducer  18  for introduction of air flow to the compressor impeller assembly  16 . The inducer  18  has a plurality of circumferentially spaced radial stator vanes  20  located in a generally axial direction across an annular, radial passage  22  for directing air to the compressor impeller assembly  16  which is interposed between the upstream support assembly  12  and the downstream support assembly  14 . 
     The annular radial passage  22  includes a outer annular shroud  24  having a stepped shoulder  26  on the downstream end thereof for accommodating an inlet end  28  of a impeller shroud  30 . The outer annular shroud  24  has a contour that defines a smooth path surface  32  of the inducer fluid path  34  that extends smoothly from a radial direction to an axial direction to prevent abrupt flow changes upstream of a contoured inner surface  36  of the impeller shroud  30 . Likewise, the annular radial passage  22  includes an inner annular shroud  38  that extends smoothly from a radial direction to an axial direction and defines a smooth surface  40  of the flow path  34  to avoid abrupt flow changes through the flow path  34  to the contoured hub surface  42  on an impeller hub  44  of the compressor impeller assembly  16 . 
     An abradable annular seal assembly  46  is provided between the inner annular shroud  38  and the impeller hub  44  and includes a contoured surface  48  that defines a smooth transition between the surface  40  of the inner annular shroud  38  and the hub surface  42 . The abradable seal assembly  46  is attached to the inner annular shroud  38  at the downstream end thereof and held in position by a spring ring  50 . The abradable seal assembly  46  includes a labyrinth seal member  52  on the impeller hub  44  to seal the internal air flow path through the compressor assembly  10  from low pressure cavities within the compressor. 
     The air flow path through the compressor impeller assembly  16  is arranged to produce as uniform a flow as possible from the inducer  18  to an annular impeller chamber  54  defined by the impeller hub  44  and the impeller shroud. 
     More particularly, the impeller chamber  54  is formed between the inner surface  36  of the impeller shroud  30  and the hub surface  42  of the impeller hub  44 . A plurality of impeller blades  56  extend radially and axially from the impeller hub  44 . Each of the blades  56  includes a leading edge  58 , a trailing edge  60  and an outer tip  62 . The leading edge  58  of the impeller blade  56  is located at the inlet end  28  of the impeller shroud  30  and the trailing edge  60  is located at an outlet end  64  of the impeller shroud  30 . The outer tip  62  of the impeller blade  56  extends, starting from the leading edge  58  and ending to the trailing edge  60 , smoothly from an axial direction to an outwardly radial direction and follows the contour of the inner surface  36 . 
     The compressor impeller assembly  16  is supported for rotation with respect to the contoured inner surface  36  of the impeller shroud  30  by a rear bearing assembly  66  and a front bearing assembly  68 . The rear bearing assembly  66  supports a rear hub extension  70 . The impeller hub  44  is mounted on a compressor drive shaft, not shown, and is driven by the drive shaft during compressor operation. The downstream support assembly  14  includes a casing  72 , a bearing support  74  and an abradable seal land member  76 . Both the bearing support  74  and the abradable seal land member  76  are formed integrally with the casing  72 . The bearing support  74  receives and supports the bearing assembly  66 . The abradable seal land member  76  cooperates with a labyrinth seal  78  on the impeller hub  44  to seal the internal air flow path through the compressor assembly  10  from low pressure cavities within the compressor. The casing  72  includes a front flange  80  that is connected with a rear flange  82  of the inducer  18  for supporting the inducer  18 . An annular diffuser groove  84  is formed in the casing  72  and in the same radial plane as the outlet end  64 . The air flow passes through a pipe diffuser  86 , to eventually communicate with the combustion chamber of the engine, as well as provide cooling for the compressor assembly, not shown. 
     The front bearing assembly  68  supports a front hub extension  88  to permit the rotation of the compressor impeller assembly  16 . The front bearing assembly  68  in turn is received and supported by a front bearing support  90  that is supported with respect to a stationary structure of the compressor, not shown. 
     The impeller shroud  30  includes an upstream annular segment  92  and a downstream annular segment  94  which are axially spaced apart, forming a circumferentially continuous uninterrupted annular slot  96  between the two segments  92 ,  94 . The upstream annular segment  92  has an cylindrical portion  98  and a radial flange  100  extending outwardly from the cylindrical portion  98 . The upstream end of the cylindrical portion  98  is snugly fit in the stepped shoulder  26  of the outer annular shroud  24  of the inducer  18 , forming the inlet end  28  of the impeller shroud  30 . 
     The downstream end of the cylindrical portion  98  has frusto-conical surface  102  extending outwardly and rearwardly. A plurality of holes, not shown, extend through the radial flange  100 , circumferentially and equally spaced apart for receiving studs and nuts  104 . 
     The studs are respectively secured in screw holes, not shown, in a plurality of bosses  106  that are circumferentially formed on the outer annular shroud  24  at the downstream end thereof. The cylindrical portion  98  of the upstream annular segment  92  is short in axial length relative to the full length of the outer tip  62  of the impeller blade  56  and the annular slot  96  is therefore located in a position so as to allow an inflow of air from outside of the impeller shroud  30  to the impeller chamber  54  under high r.p.m. conditions of compressor operations and to allow air flow to bleed from the impeller chamber  54  to the exterior of the impeller shroud  30  when the compressor is operating at a lower r.p.m. to stabilise the flow to the impeller rotor at part r.p.m. operation, which is disclosed in U.S. Pat. No. 4,248,566. The downstream annular segment  94  includes a contoured section  108  which is a major section of the inner surface  36  of the impeller shroud  30 . The inner surface  36  is contoured to the outer tip  62  of the impeller blade  56 . The downstream annular segment  94  further includes a cylindrical portion  110  and a flange  112  on the downstream end thereof to be supported by the casing  72  in a cantilevered manner. A plurality of holes, not shown, are circumferentially and equally spaced apart and extend through the flange  112  for receiving connection bolts  114 . A plurality of corresponding holes, not shown, are provided respectively in a plurality of scallops  116  which are circumferentially and equally spaced apart, formed integrally with the casing  72  and connected to the flange  112 . The connection bolts  114  co-operate with nuts  118  to fasten the flange  112  and the scallops  116  together. Edge  120  formed at the juncture of the contoured section  108  and the cylindrical portion  110  defines the outlet end  64  of the impeller shroud  30 . 
     The downstream annular segment  94  defines a rein on the upstream end with a ramp (frusto-conical) surface  122  thereon. The ramp surface  122  is parallel to the frusto-conical surface  102  of the upstream annular segment  92  and is spaced apart therefrom to form the annular slot  96 . 
     Because the upstream annular segment  92  is fixed to the inducer  18  and the downstream annular segment  94  is mounted to the casing  72 , there is no connecting member to directly bridge the two segments, each segment being independent and self-supporting at a peripheral edge adjacent the slot. Thus when the compressor is in operation, air passes through the continuous annular slot  96  without causing a dynamic component to affect the blades as discussed. 
     The air surrounding the exterior of the impeller shroud  30  is in communication with the ambient air through a plurality of openings  124  in an annular frame  126  that extends downstream from the outer annular shroud  24  of the inducer  18  to mount the rear flange  82 . The annular frame  126  is located relatively remote from the annular slot  96  and there is plenty of air volume between the annular frame  126  and the exterior of the impeller shroud  30  to eliminate any dynamic component caused by the annular frame  126 , if any, which can affect the impeller blade  56  when the air passes through the annular slot  96  and the openings  124 . 
     A rear spacer  128  with a predetermined thickness is provided between the flange  112  of the downstream annular segment  94  and the scallops  116  of the casing  72  at each bolt connection to set an axial location of the downstream annular segment  94 . The inner surface  36  of the impeller shroud  30  is set in closely spaced relationship with the outer tips  62  of the impeller blades  56 . A spacer  130  of predetermined thickness is provided between each boss  106  and the radial flange  100  of the upstream annular segment  92 . The axial position of the upstream annular segment  92  is set by the selection of the thickness of the spacer  130  so that the width of the annular slot  96  is adjusted depending on the engine specification determined by the use of a particular engine when the position of the downstream annular segment  94  is fixed. 
     The downstream annular segment  94  has a crateriform shape and is cantilevered (supported only by flange  112 ), and has an appropriate thickness so that the downstream annular segment  94  is elastically deformable when the air pressure within the impeller chamber  54  changes and, as a result, the width of the annular slot  96  changes in response to the changes in air pressure within the impeller chamber  54  during the operation of the compressor. The rein of the downstream annular segment  94  may be displaced axially. The annular slot  96  is defined between the end surface  102  and ramp surface  122  on the rein so that the displacement of the rein in the axial direction causes the change of the width of the slot  96 . 
     The advantages of the single, annular, uninterrupted slot of the impeller bleed passage will now be described. The continuous annular single slot compares favourably to a series of bleed holes, in the prior art, because a series of holes with the same effective area would need to be larger in diameter than the width of a single slot. The length of the outer tip of the blade corresponding to the width of the blade passage is affected from the perspective of performance efficiency. The provision of a minimum possible width of this bleed passage, therefore, also provides the minimum possible length of the outer tip of the blade to be affected and, as a result, the impeller performance is improved. 
     The use of selective spacers to adjust the width of the annular slot during the assembly of the compressor advantageously extends this invention to broader applications and enable it to meet different engine requirements. For example, if the engine is being used on an aircraft to power the aircraft by means of a propeller, then the surges and pressure changes in the impeller during idle or cruising speeds may vary considerably. On the other hand, if the engine is being used as an auxiliary engine, for instance, in a Boeing  747  to power the hydraulics and electricals, then the requirements are quite different and the slot may be adjusted differently. 
     Furthermore, the elastically deformable downstream annular segment provides a self-regulating feature to the impeller bleed passage, that is, as pressure increases within the impeller chamber of the compressor, the slot width is reduced. 
     Another advantage of the invention is that the dynamic component caused by the pressure differential circle is eliminated because each of the upstream and downstream annular segments is independent and self-supporting at a peripheral edge adjacent the slot, without any bridge members crossing the slot which usually causes the pressure differential circle, as discussed previously. 
     The structure for the annular slot bleed passage is relatively simple, in contrast to the prior art, and less components and parts need to be used. For example, an O-ring seal is omitted in the present invention. The O-ring seal is used in the prior art to seal a socket connection between the inducer and the shroud. The O-ring seal prevents the pressurized air bled from the bleed holes from entering the inlet end of the shroud to cause a re-ingestion. This re-ingestion causes an impeller performance loss. However, since the upstream annular segment of the shroud, in this invention, is securely connected to the inducer using screw fasteners so that the possible clearance between the inlet end of the shroud and the inducer is eliminated. The simple structure provides a possibility to reduce the manufacturing costs. 
     Although a preferred embodiment of the invention has been disclosed, it should be apparent to those skilled in the art that the invention may be practical in other forms without departing from its spirit and scope which are only defined by the appended claims.