Abstract:
An exhaust mixer for a gas turbine engine has a lobe cross-over offset. The proposed geometric feature leads to improved exhaust performance and potential weight reduction.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority on U.S. provisional patent application No. 62/234,192 filed on Sep. 29, 2015, the content of which is herein incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The application relates generally to aircraft gas turbine engines and, more particularly, to an exhaust mixer suitable for bypass gas turbine engines. 
       BACKGROUND OF THE ART 
       [0003]    A typical turbofan forced mixer consists of a number of mixer lobes that alternately extend radially outwards (crests) and inwards (valleys) to create the geometry necessary for forced mixing. In the current designs of turbofan mixers, the transition from annular gaspath to the forced mixer is designed such that the curve that forms the crest line and the curve that forms the valley line depart from the annular gas path at the same axial location, defined as the cross-over point. Applicant has found that this may lead to diffusion problems. 
       SUMMARY 
       [0004]    In one general aspect, there is provided an exhaust mixer for a gas turbine engine, the mixer having a lobe cross-over offset. That is instead of the curves that define the crests and valleys originating from the same axial location, the respective points of origin of the valleys and crests are offset axially relative to each other. 
         [0005]    In accordance with another general aspect, there is provided a turbofan forced mixer comprising a circumferential array of alternating crests and valleys, the respective points of origin of the valleys and crests being axially offset relative to each other. 
         [0006]    In accordance with another general aspect, there is provided a turbofan exhaust mixer comprising an annular wall extending around a central axis, the annular wall extending axially from an upstream end to a downstream end and defining a plurality of circumferentially distributed lobes, the lobes including alternating valleys and crests, the valleys and crests having respective upstream ends, the upstream ends of at least some of the valleys being axially spaced-apart from the upstream ends of the crests by a lobe cross-over offset distance (O). 
         [0007]    In accordance with a further general aspect, there is provided a gas turbine engine comprising: an annular core flow passage for channelling a core flow along an axis of the engine, a bypass passage extending concentrically about the core flow passage for axially channelling bypass air; and an exhaust mixer, the exhaust mixer having an annular wall concentrically disposed relative to the axis of the engine and extending axially between an upstream end and a downstream end, the annular wall defining a plurality of circumferentially distributed lobes forming alternating crests and valleys, the crests protruding radially outwardly into the bypass passage and the valleys protruding radially inwardly into the core flow passage, wherein an origin of the crests at the upstream end of the annular wall of the exhaust mixer is axially offset from an origin of the valleys by a lobe cross-over offset distance (O). 
         [0008]    In accordance with a still further general aspect, there is provided an exhaust mixer for a gas turbine engine of the type having an annular core flow passage for channelling a core flow along an axis of the engine, and a bypass passage extending concentrically about the core flow passage for axially channelling bypass air; the exhaust mixer comprising: an annular wall adapted to be concentrically disposed relative to the axis of the engine and extending axially between an upstream end and a downstream end, the annular wall defining a plurality of circumferentially distributed alternating crests and valleys, the crests being configured to protrude radially outwardly into the bypass passage while the valleys are configured to protrude radially inwardly into the core flow passage, wherein an upstream end of at least some of the crests is axially offset from an upstream end of the valleys. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0009]    Reference is now made to the accompanying figures, in which: 
           [0010]      FIG. 1  is a schematic cross-section view of a turbofan gas turbine engine having an exhaust mixer; 
           [0011]      FIG. 2  is a rear isometric view of a forced exhaust mixer; 
           [0012]      FIGS. 3 a  and 3 b    are respectively isometric and side views of a sector of a conventional mixer having a single-point cross-over; 
           [0013]      FIGS. 4 a  and 4 b    are respectively isometric and side views of a sector of a mixer having a lobe cross-over offset in accordance with an embodiment of the present disclosure; and 
           [0014]      FIG. 5  is a graph representation of a sample area distribution along the mixer for a core stream. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  illustrates a turbofan gas turbine engine  10  of a type preferably provided for use in subsonic flight, 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 section  18  for extracting energy from the combustion gases. 
         [0016]    The gas turbine engine  10  includes a first casing  20  which encloses the turbo machinery of the engine, and a second, outer casing  22  extending outwardly of the first casing  20  such as to define an annular bypass passage  24  therebetween. The air propelled by the fan  12  is split into a first portion which flows around the first casing  20  within the bypass passage  24 , and a second portion which flows through a core flow path  26  which is defined within the first casing  20  and allows the flow to circulate through the multistage compressor  14 , combustor  16  and turbine section  18  as described above. 
         [0017]    At the aft end of the engine  10 , an axisymmetrical bullet  28  is centered on a longitudinal axis  30  of the engine  10  and defines a downstream portion of an inner wall of the core flow path  26  so that the combustion gases flow therearound. An annular exhaust mixer  32  surrounds at least a portion of the bullet  28 , the mixer  32  acting as a rearmost portion of the outer wall defining the core flow path  26  and a rearmost portion of the inner wall defining the bypass passage  24 . The hot combustion gases from the core flow path  26  and the cooler air from the bypass passage  24  are, thus, mixed together by the mixer  32  at the exit thereof such as to produce an exhaust with a reduced temperature. 
         [0018]    As shown in  FIG. 2 , the mixer  32  may include an annular sheet metal wall  34  defining an upstream end  36  of the mixer  32  along which the flows from the core flow path  26  and from the bypass passage  24  are received, and a downstream end  38  where the two flows meet and are mixed together. The annular wall  34  defines a plurality of circumferentially distributed lobes. The lobes include alternating inner and outer lobes or valleys and crests  42 ,  44 , with the crests  44  extending radially outwardly into the bypass passage  24  and the valleys  42  extending radially inwardly into the core flow path  26 . As such, the valleys  42  define troughs in the bypass passage  24  in between adjacent ones of the crests  44 , while the crests  44  define troughs in the core flow path  26  in between adjacent ones of the valleys  42 . 
         [0019]    As can be appreciated from  FIGS. 3 a  and 3 b   , conventional turbofan forced mixers have a single-point cross-over P. That is the curve C 1  that forms the crest line and the curve C 2  that forms the valley line depart from the annular gaspath at a same axial location. Due to the 3-D geometrical shape that results from this construction, there is an inherent axial variation in cross-sectional area and Mach number on the core and bypass streams that is detrimental to the aerodynamic performance of the mixer. Specifically, the area of the core is reduced which must then be recovered through additional diffusion in the mixer. This requirement of extra diffusion limits the geometrical design of the mixer, often requiring additional length. 
         [0020]    In contrast, the embodiment shown in  FIGS. 4 a  and 4 b    provides for a lobe cross-over offset O. Indeed, instead of the curves that define the crest and valley originating from the same axial location, the start of at least some of the valleys and crests may be offset axially relative to each other. In the illustrated embodiment, the curve C 3  of the crests starts axially upstream of the curve C 4  of the valleys. However, it is understood that the lobe-cross-over offset may be in the opposite direction (i.e. the valleys could start upstream of the crests). Also, a mixer could have more than one lobe cross-over offset O. That is the length of the lobe cross-over offsets could differ between lobes of a same mixer. For example, if de-swirling struts counts (either in conventional or integrated configuration) would not match the number of mixer lobes, one may apply a different lobe cross-over offset for the set of the valley lobes aligning with de-swirling strut than for the remaining valley lobes. 
         [0021]    The 3-D shape that results from the lobe distribution shown in  FIGS. 4 a  and 4 b    allows for a uniform axial distribution of area at the start (i.e. upstream end) of the mixer. It may lead to specific fuel consumption (SFC) improvements. With the elimination of the area variation of the core and bypass, an efficient control of area distribution through the entire mixer can be achieved for improved aerodynamic performance of the exhaust by: 
         [0022]    1. Reduced losses in the core stream 
         [0023]    2. More efficient control of mass flow distribution of the hot stream 
         [0024]    3. Allowing for shorter mixer, thus, longer mixing length for fixed exhaust length increasing the mixing efficiency of the exhaust system. 
         [0000]      FIG. 5  graphically illustrates sample area distributions along the mixer for the core stream; both, distributions from standard single point cross-over design ( FIGS. 3 a -3 b   ) and design based on the cross-over offset ( FIGS. 4 a -4 b   ) being depicted. Sample reduction in local diffusion in the core stream and opportunity for reduced mixer length allowed by proposed features are highlighted; these can yield significant improvement in engine performance. Furthermore, the mixer length reduction allow for weight reduction. 
         [0025]    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. 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.