Abstract:
A compressor diffuser for a gas turbine engine includes a plurality of diffuser pipes each having a diverging tubular body defining a flow passage extending fully therethrough. The tubular body includes a first portion extending in a first direction, a second portion extending in a second direction different from the first direction, and a curved portion interconnecting the first portion and the second portion. At least one splitter vane extends into the flow passage and disposed at least partially within the curved portion of the tubular body.

Description:
TECHNICAL FIELD 
       [0001]    The application relates generally to gas turbine engines and, more particularly, to compressor diffusers therefor. 
       BACKGROUND 
       [0002]    Diffuser pipes are provided in gas turbine engines for directing flow of compressed air from a centrifugal compressor to an annular chamber containing the combustor, while diffusing the high speed air. The diffuser pipes are typically circumferentially arranged at a periphery of an impeller, and are designed to transform kinetic energy of the flow into pressure energy. Diffuser pipes may provide a uniform exit flow with minimal distortion, as it is preferable for flame stability, low combustor loss, reduced hot spots etc. While longer diffuser pipes may accomplish better diffusion, spatial constraints in the gas turbine engine may restrict their length. Large flow diffusion in diffuser pipes over insufficient pipe length may result in thick and weak boundary layers built up on the pipe wall. To compensate for a shorter length, many diffuser pipes have a tight bend. Turbulence and other non-streamline behavior of the flow at the bend may lead to pressure losses and decrease efficiency of the diffuser pipe. 
       SUMMARY 
       [0003]    In one aspect, there is provided a compressor diffuser for a gas turbine engine, the diffuser having a plurality of diffuser pipes each comprising: a diverging tubular body defining a flow passage extending fully therethrough, the tubular body including a first portion extending in a first direction, a second portion extending in a second direction different from the first direction, and a curved portion interconnecting the first portion and the second portion; and at least one splitter vane extending into the flow passage and disposed at least partially within the curved portion of the tubular body. 
         [0004]    In another aspect, there is provided a gas turbine engine comprising a centrifugal compressor including an impeller case and a plurality of diffuser pipes downstream of the impeller and receiving compressed air therefrom, each of the diffuser pipes having a diverging tubular body defining a flow passage extending therethrough, the tubular body of the diffuser pipes extending from the periphery of the impeller case and including a radial portion and an axial portion connected by a curved portion, the curved portion having at least one splitter vane disposed at least partially within the flow passage. 
         [0005]    In a further aspect, there is provided a method of manufacturing a diffuser pipe for a centrifugal compressor of a gas turbine engine, the method comprising: forming a tubular body out of a sheet metal, the tubular body having a first portion extending in a first direction, a second portion extending in a second direction different from the first direction, and a curved portion between the first portion and the second portion; inserting a splitter vane at least partially into the curved portion of the tubular body and aligning sides of the splitter vane in a desired position between opposed walls of the curved portion; and fixing the sides of the splitter vane to the opposed walls within the curved portion. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]    Reference is now made to the accompanying figures in which: 
           [0007]      FIG. 1  is a schematic cross-sectional view of a gas turbine engine; 
           [0008]      FIG. 2  is a schematic perspective view of an impeller and corresponding plurality of radially disposed diffuser pipes; 
           [0009]      FIG. 3  is a schematic perspective view of one of the diffuser pipes having a splitter vane; 
           [0010]      FIG. 4  is a schematic cross-sectional view of the diffuser pipe of  FIG. 3 ; 
           [0011]      FIG. 5  is another schematic cross-sectional view (partial) of the diffuser pipe of  FIG. 3 ; 
           [0012]      FIG. 6  is a schematic side elevation view another diffuser pipe having two splitter vanes, and shown with shading to illustrate streamline of the flow having various velocities; and 
           [0013]      FIG. 7  is a schematic top view of the diffuser pipe of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  illustrates a gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication along an engine axis  11 : a fan  12  through which ambient air is propelled, a compressor section  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 section  18  for extracting energy from the combustion gases. The compressor section  14  includes a plurality of stators  13  and rotors  15  (only one stator  13  and rotor  15  being shown in  FIG. 1 ), and an impeller  17 . A plurality of diffuser pipes  20  are circumferentially disposed at a periphery of the impeller  17  and redirect the exhaust gases from a radial orientation to an axial orientation (i.e. aligned with the engine axis  11 ). Diffusers, such as the diffuser pipes  20 , convert high kinetic energy at impeller  17  exit to static pressure by slowing down fluid flow. In most cases, a Mach number of the flow entering the diffuser pipe  20  may be at or near sonic, while a Mach number exiting the diffuser pipe  20  may be in the range of 0.2-0.25 to enable stable air/fuel mixing, light/re-light in the combustor  16 . 
         [0015]    Turning now to  FIG. 2 , a front perspective view of the impeller  17  shows the plurality of diffuser pipes  20 , commonly known as “fishtail diffuser pipes”. Each of the diffuser pipes  20  includes a tubular body  22 , formed, in one embodiment, of sheet metal. The body  22  includes a first portion  24  extending generally tangentially from the periphery of the impeller  17 . The first portion  24  has an open end forming an inlet I (shown in  FIG. 4 ) of the diffuser pipe  20 . The first portion  24  is inclined at an angle θ 1  relative to a radial axis R. The angle θ 1  may be at least partially tangential, or even substantially tangentially, and may further correspond to a direction of airflow at the exit of the blades of the impeller  17 , to facilitate transition of the flow F (shown in  FIG. 3 ) from the impeller  17  to the diffuser pipes  20 . The first portion  24  could alternatively extend more substantially along the radial axis R. 
         [0016]    A second portion  26  is disposed generally axially and is connected to the first portion  24  by an out-of-plane bend or curved portion  28 . The second portion  26  includes an open end forming an outlet O (shown in  FIG. 4 ) of the diffuser pipe  20 . 
         [0017]    High swirl of the flow F exiting the impeller  17 , and therefore entering the first portion  24  of each of the diffuser pipes  20 , may be removed by shaping the diffuser pipe  20  with the curved portion  28 , such that the flow F is redirected axially before existing to the combustor  16 . For a given impeller exit Mach number and swirl of the flow F, the effectiveness of a diffuser pipe may be dependent upon its length. For a fishtail pipe type diffuser, such as the one described herein, the greater the length the easier it is for the pipe to diffuse flow efficiently without, or with minimal, flow separation at the curved portion  28 . Length can be obtained by growing pipe radially or axially or both. Longer diffuser pipes are however disadvantaged in that they can potentially increase both weight and size of the engine. In addition, a required gap between the outlet and fuel nozzle locations is another constraint that put a physical limit on radial/axial extension of the diffuser pipes  20 . As a result, the diffuser pipe  20  may be designed to have a tight  90  degrees bend to compensate for a reduced length. 
         [0018]    In the depicted embodiment, the cross-sectional area of the diffuser pipe  20  increases gradually and continuously along its length, from the inlet I to the outlet O. The first portion  24  has a generally circular cross-section C 1  (shown in  FIG. 4 ), while the second portion  26  has generally a flattened oval (or oblong) cross-section C 2  (shown in  FIG. 4 ). Other types of cross-sections for the first portion  24  and the second portion  26  are contemplated. 
         [0019]    Referring now to  FIGS. 3 to 5 , each of the diffuser pipes  20  includes within its interior passage a guide or splitter vane  30 , disposed between inner wall  28   a  and outer wall  28   b  of the diffuser pipe  20 . In the present embodiment, the splitter vane  30  is disposed within the interior passage at the curved or bent portion  28  of the pipe. The curved portion  28  may be defined by a zone of redirection between the first portion  24  and the second portion  26 , as illustrated by the two dotted lines joined by the bracket  28  in  FIGS. 3 and 4 . It is contemplated that the splitter vane  30  could be only partially disposed in the curved portion  28 , and therefore extend at least partially into the first or the second portion  24 ,  26 . However, in one particular contemplated embodiment, a majority of the total length of the splitter vane  30  is disposed within the redirection zone defined at the curved portion  28 . The presence of the splitter vane  30  may at least reduce some of the drawbacks associated with the tight bend of the curved portion  28 , as noted below. 
         [0020]    The curvature of the curved portion  28  may tend to detach the flow F from the walls  28   a ,  28   b , which can result in pressure losses and non-uniform flow at the outlet O. Mixing loss may contribute to overall diffuser performance. Flow separation in the diffuser pipe  20  starting at the curved portion  28  may not only be potentially detrimental to the compressor section  17  performance and operability, but also to its structural integrity as flow separation can be destructive in nature and can lead to premature pipe breakage, fatigue, cracking, noise, flame instability etc. 
         [0021]    The diffuser pipe  20  of the present disclosure may relieve the pressure gradient at the curved portion  28  by the presence of the splitter vane  30 . While the splitter vane  30  may provide additional aerodynamic friction loss, the reduction in overall mixing loss may more than offset this increase. 
         [0022]    As seen in  FIG. 4 , the splitter vane  30  is, in this embodiment, airfoil shaped and includes a leading edge  32  and a trailing edge  34 . The airfoil of the splitter vane  30  therefore defines a pressure side  36  and a suction side  38 , as conventionally known for airfoils. The splitter vane  30  is oriented in the diffuser pipe  20  so that the leading edge  32  receives the incoming flow F, and a curvature of the airfoil shaped splitter vane  30  is in a same direction as the curved portion  28  of the diffuser pipe  20 . In other words, the pressure side  36  of the airfoil  30  faces the inner wall  28   a . The splitter vane  30  is generally disposed to conform to the flow F (i.e. streamlined) so that there is minimal separation when the flow F encounters the splitter vane  30 . Structurally the splitter vane  30  may also act as stiffener and help to strengthen diffuser pipe  20 . Splitter vane (s) can thus be used to replace traditional stiffening ribs that are normally stamped on pipe wall. 
         [0023]    The splitter vane  30  extends across the diffuser pipe  20 , wall-to-wall. In the example shown in  FIGS. 3 to 5 , the splitter vane  30  is disposed at a lateral midpoint between opposed walls  28   a  and  28   b , i.e. half way across the bend of the diffuser pipe  20 . It is however contemplated that the splitter vane  30  could be disposed more toward the inner wall  28   a  of the curved portion  28 , or more toward the outer wall  28   b  of the curved portion  28  (i.e. not centrally disposed). 
         [0024]    Referring now to  FIGS. 6 and 7 , a diffuser pipe  120  of an alternate embodiment includes within its interior flow passage two splitter vanes  130  and  130 ′. The diffuser pipe  120  is similar to the diffuser pipe  20 , and the splitter vanes  130  and  130 ′ are similar to the splitter vane  30 . Details of the diffuser pipe  120  and the splitter vane  130 ,  130 ′ will thus not be described in great detail herein again. 
         [0025]    The splitter vanes  130 ,  130 ′ are disposed in a curved portion  128  of the diffuser pipe  120 , with the splitter vane  130  being upstream relative to the splitter vane  130 ′. The curved portion  128  of the diffuser pipe  120  may be longer than the curved portion  28  of the diffuser pipe  20 , in order to accommodate the multitude of splitter vanes  130 ,  130 ′. The splitter vanes  130 ,  130 ′ have a same orientation and disposition as the splitter vane  30 . As best seen in  FIG. 6 , in this embodiment, the splitter vane  130  overlaps with a portion of the splitter vane  130 ′, i.e. a trailing edge  134  of the upstream splitter vane  130  is located downstream relative to a leading edge  132 ′ of the downstream splitter vane  130 ′. It is contemplated that the splitter vanes  130 ,  130 ′ could alternatively not overlap. It is also contemplated that more than two splitter vanes could be disposed in the curved portion  128 . It is also contemplated that the splitter vanes  130 ,  130 ′ could have various dispositions relative to each other. For example, the splitter vanes  130 ,  130 ′ could totally overlap. 
         [0026]    Because of the diffusion process, the diffuser pipes  20 ,  120  experience adverse pressure gradients in the direction of flow F, with endwall boundary layer being built up as the result. The buildup may lead to increased blockage, diminished pressure recovery and eventually lead to flow separation. The flow separation usually starts at the diffuser bend  28 ,  128  where the curvature is at its maximum. The splitter vane(s)  30 ,  130 ,  130 ′ may reduce pressure gradient across the curved portion  28 ,  128  and help the flow F to negotiate the tight turn more efficiently. The airfoil splitter vanes  30 ,  130 ,  130 ′ described herein may also facilitate swirl removal. Computational fluid models can be used to optimize the splitter vane  30 ,  130 ,  130 ′ length and/or location, while the inner and outer walls  28   a ,  28   b ,  128   a ,  128   b  can be shaped in accordance with the splitter vane  30 ,  130 ,  130 ′ to best conform to a stator pitch. 
         [0027]    The diffuser pipes  20 ,  120  with splitter vane(s)  30 ,  130 ,  130 ′ at the curved portions  28 ,  128  thereof may at least reduce flow separation from initiating. Since mixing losses may be a prominent contributor to diffuser pipe loss and is initiated mostly at the curved portion  28 ,  128 , employing splitter vane(s)  30 ,  130 ,  130 ′ at that location may be more effective than anywhere else in the diffuser pipe  20 ,  120 . 
         [0028]    One way to manufacture any of the above sheet metal diffuser pipes with internal vanes is to laser drill slots on the sheet metal forming the diffuser pipes, at a location where the splitter vane is to be disposed in the curved portion. The splitter vane(s) may then be inserted inside the diffuser pipe, for example from the outlet end O thereof, and brazed at both ends onto the inner wall(s) of the diffuser pipe where the slots are formed. Alternatively, no slots may be need to be formed and the splitter vanes may be simply brazed in place within the portion of each diffuser pipe. 
         [0029]    The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. Other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.