Abstract:
A gas turbine engine diffuser comprises a bowl-shaped diffuser casing and a cover nested into the bowl-shaped diffuser casing and cooperating therewith in defining a diffuser passage having a channeled entry portion in fluid flow communication with a vaned exit portion via a vaneless intermediate portion. The channeled entry portion is divided into an array of inlet flowpaths by a first set of vanes. Likewise, the vaned exit portion is divided into an array of outlet flowpaths by a second set of vanes.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to gas turbine engines and, more particularly, to a diffuser for directing a flow of compressed air with a radial component to a diffused annular flow having an axial component. 
   2. Description of the Prior Art 
   Conventional gas turbine engine diffusers comprise a machined ring which surrounds the periphery of a compressor impeller for capturing a radial flow of compressed air and redirecting it through generally tangential orifices into an array of diffuser pipes. Fabrication of the diffuser pipes is extremely complex since they have a flared internal pathway that curves from a generally radial tangential direction to an axial rearward direction. Each pipe must be manufactured to close tolerances individually and then assembled to the machined diffuser ring. Complex tooling and labor intensive manufacturing procedures result in a relatively high cost for the preparation of the diffusers. 
   In an attempt to reduce the tooling and the manufacturing costs, it has been proposed to manufacture a diffuser from two concentric nested shells see U.S. Pat. No. 6,471,475, secured together by brazing, one of the shells being provided with grooves separated by seam edges while the other shell is provided with a smooth surface of revolution. The groove on the one shell are closed by the other shell when the shells are nested together and the seam edges are secured to the smooth surface thus defining individual ducts extending continuously from the compressor impeller to the outer shell edges. 
   Although the above-described diffuser design greatly reduces the tooling and manufacturing costs associated with prior art diffuser assemblies, the pursuit of increased efficiency at decreased cost makes improvement ever-desirable. 
   SUMMARY OF THE INVENTION 
   It is therefore an aim of the present invention to simplify the fabrication of a gas turbine engine diffuser. 
   Therefore, in accordance with the present invention, there is provided a gas turbine engine diffuser comprising a bowl-shaped diffuser casing and a cover nested into the bowl-shaped diffuser casing and cooperating therewith in defining a diffuser passage having a channeled entry portion in fluid flow communication with a vaned exit portion via a vaneless intermediate portion, said channeled entry portion being divided into an array of inlet flowpaths by a first set of vanes, and wherein said vaned exit portion is divided into an array of outlet flowpaths by a second set of vanes. 
   In accordance with a further general aspect of the present invention, there is provided a diffuser for directing a flow of compressed air with a radial component to a diffused annular flow having an axial component, the diffuser comprising a diffuser casing including: a generally radially extending surface having a first array of vanes integrally formed on a rearwardly facing side thereof, and a generally axially extending annulus projecting rearwardly from a periphery of said radially extending surface, said annulus being provided with a second array of vanes defining a plurality of exit air passages through said annulus; and a cover adapted to cooperate with said first array of vanes when secured to said diffuser casing in order to define therewith a plurality of entry air passages in communication with said exit air passages. 
   In accordance with a further general aspect of the present invention, there is provided a diffuser comprising an integrated opened island diffuser casing having a plurality of island vanes, the opened island diffuser casing being closed by a cover, the island vanes and the cover cooperating to define a plurality of D-shaped entry passages leading to a vaneless annular bend, the vaneless annular bend opening to an annular array of exit passages defined by a set of deswirl vanes. 
   In accordance with a further general aspect of the present invention, there is provided a method of making a diffuser for directing a flow of compressed air with a radial component to a diffused annular flow having an axial component, the method comprising the steps of: providing a bowl-shaped casing having an annular disc surface provided with a circumferential array of island vanes, and an annulus projecting axially from a periphery of the disc surface, said annulus defining a circumferential array of axially extending exit passages, and securely nesting a cover in said bowl-shaped casing to cooperate with said island vanes to form a circumferential array of generally radially oriented inlet passages in fluid flow communication with said axially extending exit passages. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof, and in which: 
       FIG. 1  is a side view, partly broken away, of a gas turbine engine to which an embodiment of the present invention is applied; 
       FIG. 2  is a perspective view, partly broken away, of a portion of a diffuser assembly according to a preferred embodiment of the present invention; 
       FIG. 3   a  is a partial radial sectional view of the diffuser assembly shown in  FIG. 2 ; 
       FIG. 3   b  is a partial radial sectional view of another embodiment of the diffuser assembly; and 
       FIG. 4  is a schematic view of a cross-section of one air entry passage of the diffuser assembly. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a subsonic gas turbine engine  10  generally comprising in serial flow communication a fan  12  through which ambient air is propelled, a multistage compressor  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine  18  for extracting energy from the combustion gases. 
   The last stage of the illustrated compressor  14  comprises a high-pressure centrifugal impeller  19 . The centrifugal impeller  19  directs the compressed air radially outwardly into a diffuser  20 . The diffuser  20  redirects the compressed air from a radial direction to a diffused annular axial rearward flow into the combustor  16 . 
   As shown in  FIG. 2 , the diffuser  20  according to the present invention is preferably of a two-piece construction and generally comprises an integrated opened island diffuser casing  22  and a separate annular cover  24 . The casing  22  is bowl-shaped and the cover  24  is concentrically nested in the bowl-shaped diffuser casing  22  and secured thereto. 
   As will be seen hereinafter, the diffuser casing  22  can be provided in the form of a one-piece casting ( FIG. 3   a ) or, alternatively, designed as an assembly of machined pieces and sheet metal pieces ( FIG. 3   b ). 
   Indeed, the diffuser casing  22  can be provided in the form of a one-piece casting comprising an open-vaned disc  26  having an inner rim  28  circumscribing a central impeller opening. A circumferential array of island vanes  29  (i.e. wide vanes) is integrally formed on an inner surface of the disc  26 . The island vanes  29  extend between the inner rim  28  and the periphery of the disc  26  to define a series of radial diffuser grooves  31  having cross-sectional areas of increasing magnitude in a direction away from the inner rim  28 . The outer periphery of the open vaned disc  26  merges into an arcuate vaneless annular wall portion  30  defining a 90 degrees bend from radial to axial (see  FIG. 3 ). The annular arcuate wall portion  30  merges into an axially extending annular outer wall portion  32  which cooperates with a concentric axially extending annular inner wall portion  34  to bound a series of deswirl vanes  36  integrally cast therebetween. 
   To ensure accurate throat and a consistent leading edge shape, the island vanes  29  or at least the entrance surfaces thereof are preferably machined to appropriate finished surface tolerances before the cover  24  be attached thereto. The other cast part of the diffuser casing  22  do not generally requires machining as they have a less critical impact on the flow of compressed air. 
   Instead of being made from a one-piece casting, the diffuser casing  22  could be designed as an assembly in which the vaned disc  26 , the situate wall  30 , the straight outer annular wall portion  32 , the inner annular wall portion  34  and the deswirl vanes  36  are separate pieces adapted to be assembled together in a diffuser casing configuration (see  FIG. 3   b ). The vaned disc  26  could be machined in a solid block of material, whereas the arcuate annular wall  30 , the annular outer wall portion  32 , the annular wall portion  34  and the deswirl vanes  36  could be made from sheet metal. These various pieces could be assembled together as by welding. Such an assembly is advantageous in that it allows reducing the weight of the diffuser by using sheet metal against casting. 
   The cover  24  has a smooth surface of revolution  38  adapted to be uniformly seated against the free distal end surface  40  of the island vanes  29  in order to close the entry grooves  31  and, thus, form a circumferential array of radial entry passages. For simplicity, the cover  24  may be a sheet metal part, joined mechanically on the diffuser casing  22  e.g. brazed, welded, bolted, etc. For the brazed version, to ensure a good contact during brazing, the cover  24  may be press-fit mounted. Equidistant slots  42  may be cut in the opposite region of each island vane  29 , which may be filled with the brazing paste during the brazing process. In the illustrated embodiment, the cover  24  is a simple body of revolution, which is advantageously easy to make. The cover  24  is provided with a peripheral annular ridge  44  for sealing engagement with the inner annular wall portion  34  of the diffuser casing  22 . 
   As shown in  FIG. 3 , the cover  24  cooperates with the arcuate annular wall portion  30  to define a vaneless intermediate annular passage for receiving the compressed air from the radial air passages defined by the island vanes  29  and the cover  24 . The compressed air then passes through a circumferential array of axially extending exit air passages defined by the deswirl vanes  36  and the concentric inner and outer annular wall portions  32  and  34 . 
   As shown in  FIG. 2 , the cross-section of the grooves  31  on the disc portion has a “D” shape to facilitate casting or machining. Each groove  31  has a cross-section of variable area by variation of the width and of the height as well. As shown in  FIG. 4 , the sidewalls  33  of each groove  31  are inclined outwardly at an angle α from the vertical. The angle α is preferably about 10 degrees. The sidewalls  33  should not be perpendicular to the bottom  35  of the groove  31  and the cover  24  (i.e. α&gt;0°). Furthermore, sharp corners at the junction of the sidewalls  33  and the bottom  35  of each groove  31  should also be avoided. A curvature of radius R is preferably provided at the junction of the sidewalls  33  and the bottom wall  35  of each groove  31  in order to provide for a smooth transition between the sidewalls  33  and the bottom wall  35  of the groove  31 . 
   The internal vaneless space geometry, which is formed by the repeated intersection of the “D” shaped passages, is chosen because of the advantageous match between the air angle distribution exiting the impeller  19  and the metal angles formed by the leading edges. The D-shaped cross-section has been found to provide unique aerodynamic benefits. 
   In order to further increase the aerodynamic efficiency, the axial deswirl vanes  36  may be bowed and provided with different leading edge profile such as, straight having an angle different of 90 degrees with the airflow direction, convex, concave, “S”, reverse “S”, etc. 
   The present design advantageously provides for easy fabrication as well as a simpler manner of obtaining a diffuser having a hybrid combination of inlet island vanes and outlet deswirl vanes. In contrast to known island vane diffusers in which the vanes have to be sealed to a surrounding turbine structure, the island vanes  29  and the deswirl vanes  36  are integrated to a bowl-shaped diffuser casing and the grooves between the island vanes  29  are closed by nesting a simple dedicated cover in the bowl-shaped diffuser casing. Also the axially extending exit passages formed between the deswirl vanes  36  are radially bounded on opposed sides thereof by a pair of concentric inner and outer annular walls which forms part of the diffuser casing, thereby obviating the need for sealing the deswirl vane to a surrounding turbine casing structure of the gas turbine engine. 
   It is noted that the integrated opened island diffuser casing  22  may be a module of the gas generator case, integrated into it. 
   The present diffuser, therefore, is just two parts which are easy to make, especially since no manual work for the casting version (i.e. no welding and no adjustment). The invention also eliminates expensive tooling and leads to good process control, little manual work and a compact structure which reduces vibrations and improves life. 
   Modifications and improvements to the above-described embodiment of the present invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.