Abstract:
A multi-lobe exhaust mixer has an annular body composed of a plurality of circumferentially adjacent lobe segments. The lobe segments may be made of a ceramic matrix composite material to reduce the weight of the mixer and ensure proper behavior when exposed to high thermal gradients. Each lobe segment may have partial lobes at circumferentially opposed ends thereof and at least one complete lobe therebetween. The partial lobes of the circumferentially adjacent lobe segments combining to conjointly form complete lobes at the junction between the circumferentially adjacent lobe segments. The partial lobes may be nested into each other to dampen vibrations.

Description:
TECHNICAL FIELD 
       [0001]    The application relates generally to aircraft gas turbine engines and, more particularly, to a multi-lobe exhaust mixer. 
       BACKGROUND OF THE ART 
       [0002]    In turbofan engines, high velocity air from the turbofan core is mixed with low velocity air from the bypass duct, and this mixed air is then exhausted from the engine. Turbofan engines generally use exhaust mixers in order to increase the mixing of the high and low velocity fluid flows. 
         [0003]    For manufacturability reasons, exhaust mixers are typically made out of metal, such as nickel alloy. However, such metal exhaust mixers add non-negligible weight to the engines. Also exhaust mixers are exposed to important thermal gradients and, thus, the ability of being able to use thermo structural composite materials, such as a ceramic matrix composite material (CMC), would be beneficial. There is, thus, a need for a new multi-lobe exhaust mixer construction allowing for the use of different thermally stable materials in the fabrication of an exhaust mixer. 
       SUMMARY 
       [0004]    In one aspect, there is provided a multi-lobe exhaust mixer for a gas turbine engine, the multi-lobe exhaust mixer comprising: an annular body having an array of circumferentially distributed alternating inner and outer lobes, the inner lobes including troughs forming an inner radial portion thereof and the outer lobes including crests forming an outer radial portion thereof, the annular body being segmented into a plurality of individual lobe segments, and wherein the individual lobe segments overlap at the crests or the troughs. 
         [0005]    In another aspect, there is provided a multi-lobe exhaust mixer for a gas turbine engine, the multi-lobe exhaust mixer comprising: an annular body composed of a plurality of circumferentially adjacent lobe segments, each lobe segment having partial lobes at circumferentially opposed ends thereof and at least one complete lobe therebetween, the partial lobes of the circumferentially adjacent lobe segments combining to conjointly form complete lobes at the junction between the circumferentially adjacent lobe segments. 
         [0006]    In a further aspect, there is provided a multi-lobe exhaust mixer for a gas turbine engine of the type having an annular core flow passage for channelling a high temperature 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 multi-lobe exhaust mixer comprising: an annular body having an array of circumferentially distributed alternating inner and outer lobes, the outer lobes protruding radially outwardly into the bypass passage and the inner lobes protruding radially inwardly into the core flow passage, the annular body being composed of a plurality of individual lobe segments which alternately radially outwardly and radially inwardly overlap each other around the annular body. 
         [0007]    In a still further aspect, there is provided a lobe segment adapted to be assembled to similar circumferentially adjacent lobe segments to form a multi-lobe exhaust mixer of a gas turbine engine, the lobe segment having partial lobes at circumferentially opposed ends thereof and at least one complete lobe therebetween, the partial lobes of the lobe segment being nestable into corresponding partial lobes of circumferentially adjacent lobe segments. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0008]    Reference is now made to the accompanying figures in which: 
           [0009]      FIG. 1  is a schematic cross-sectional view of a turbofan gas turbine engine having a segmented multi-lobe exhaust mixer; 
           [0010]      FIG. 2  is a front isometric view of the segmented multi-lobe exhaust mixer; 
           [0011]      FIG. 3  is an enlarged view of a portion of the exhaust mixer illustrating the nesting assembly of individual loge segments; 
           [0012]      FIG. 4 a    is a front isometric view of an example of an individual lobe segment comprising a pair of partial outer lobes at circumferentially opposed ends thereof and a complete inner lobe therebetween; 
           [0013]      FIG. 4 b    is a rear isometric view of the individual lobe segment shown in  FIG. 4   a;    
           [0014]      FIG. 5  is an enlarged rear isometric view of an individual lobe segment mounted at an upstream end thereof to a support ring or flange adapted to be bolted to the turbine exhaust casing; and 
           [0015]      FIG. 6  is an enlarged side cross-section view illustrating one possible mounting arrangement of the upstream end of the lobe segments. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      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. 
         [0017]    The gas turbine engine  10  includes a first casing  20  which encloses the turbo machinery of the engine  10 , 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. 
         [0018]    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 an inner wall of the core flow path  26  so that the combustion gases flow therearound. A multi-lobe 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. 
         [0019]    Referring to  FIG. 2 , the mixer  32  has a generally annular body  34  extending from an upstream end  36  along which the flows from the core flow path  26  and from the bypass passage  24  are received to a downstream end  38  where the two flows meet and are mixed together. The annular body  34  defines a plurality of circumferentially distributed lobes. The lobes include alternating inner and outer lobes  42 ,  44 , with the outer lobes  44  extending radially outwardly into the bypass passage  24  and the inner lobes  42  extending radially inwardly into the core flow path  26 . Each inner lobe  42  has a trough  42   a  forming an inner radial portion thereof. Likewise, each outer lobe  44  has a crest  44   a  forming an outer radial portion thereof. Adjacent inner and outer lobes  42 ,  44  have radially extending sidewalls  46  interconnecting adjacent troughs  42   a  and crests  44   a,  one sidewall being positioned between each trough  42   a  and each crest  44   a,  a complete lobe being formed by a pair of sidewalls and a trough or a crest. 
         [0020]    Referring now concurrently to  FIGS. 3, 4   a  and  4   b , it can be appreciated that the annular body  34  is segmented into a plurality of individual lobe segments  50   a,    50   b,    50   c,    50   d  . . . etc. According to the illustrated embodiment, each lobe segment has a pair of partial outer lobes  44 ′ ( FIGS. 4 a  and 4 b   ) at circumferentially opposed ends and a complete inner lobe  42  therebetween. However, it is understood each lobe segment  50  could comprise one or more complete outer lobe bordered by two partial inner lobes. It is also understood that each lobe segment  50   a,    50   b,    50   c,    50   d  . . . could include more than one complete lobe. For instance, one segment could comprise a series of three lobes bordered at opposed circumferential ends by a pair of partial lobes. In the illustrated embodiment, the partial outer lobes  44 ′ include one sidewall  46  and a crest  44   a.    
         [0021]    As shown in  FIG. 3 , the lobe segments  50  are connected together at the crests  44   a  of the partial outer lobes  44 ′. The crests  44   a  of the partial outer lobes  44 ′ of adjacent lobe segments  50   a,    50   b,    50   c,    50   d  . . . overlap each other. According to the illustrated embodiment, the partial outer lobes  44 ′ are nested into each other. The partial outer lobes  44 ′ of adjacent lobe segments combine to form a complete outer lobe  44  with the sidewalls of the complete lobe forming part of different lobe segments  50   a,    50   a,    50   b,    50   c,    50   d.  This nesting arrangement provides damping benefits when the exhaust mixer  32  is subjected to vibrations. As illustrated in the enlarged views of  FIG. 3 , the partial lobe  144 ′ at a first circumferential end of a given lobe segment  50   b  may extend radially outwardly over an adjoining partial lobe  244 ′ of a first adjacent lobe segment  50   a  while the partial lobe  344 ′ at the opposed circumferential end of the lobe segment  50   b  extends radially inwardly underneath the adjoining partial lobe  444 ′ of a second adjacent lobe segment  50   c.  The radially inwardly and radially outwardly overlapping arrangement alternate all around the segmented multi-lobe exhaust mixer  32   
         [0022]    It is understood that if the lobe segments are configured to comprise at least one complete outer lobe bordered by two partial inner lobes then the above described overlapping and nesting relationships of the individual lobe segments would take place at the troughs  42   a  instead of at the crests  44   a.    
         [0023]    The nested crest or trough arrangement provides for a stable while flexible lobe assembly which is well suited for accommodating thermal stresses and vibrations. Also, the modularity of the multi-lobe exhaust mixer  32  simplify the fabrication of the lobe structure, thereby providing more flexibility in the choice of material that can be used for manufacturing the mixer  32 . According to one embodiment, each individual lobe segment  50   a,    50   b,    50   c,    50   d  . . . can be made of a thermo-structural composite material known to have good mechanical properties at high temperature. For instance, the individual lobe segments could be made of a ceramic matrix composite (CMC) material, i.e. a material made of refractory reinforcing fibers (e.g. carbon or ceramic fibers) densified by a matrix constituted at least in part by ceramic. The use of a CMC material allows to reduce the weight of the exhaust mixer  32  compared with conventional metal exhaust mixers. 
         [0024]    The modularity also allows to individually replace the lobes, thereby reducing replacement and maintenance costs. The use of individual lobe segments also provides for variable pitch and lobe shapes around the multi-lobe exhaust mixer  32 . For instance, the lobes could have various curvatures to perform a tailored de-swirling function. 
         [0025]    As shown in  FIGS. 5 and 6 , the individual lobe segments  50   a,    50   b,    50   c,    50   d  . . . are mounted at their upstream end  36  to a support ring  60  adapted to be bolted or otherwise secured to the turbine exhaust case (not shown) of the engine  10 . The support ring  60  may have a sheet metal spring loaded flange  62  upon which the individual lobe segments  50   a,    50   b,    50   c,    50   d  . . . rest, thereby providing a vibration damping function. A separate flange  64  may be bolted or otherwise detachably mounted to the support ring  60  to axially retain and clamp the individual lobe segments  50   a,    50   b,    50   c,    50   d  . . . on the support ring  60 . A damping material, such as a resilient and high temperature resistant tape  66  or rope seal, may also be wrapped around the upstream end  36  of the individual lobe segments  50   a,    50   b,    50   c,    50   d  . . . on the support ring  60  to increase radial damping. The resilient and high temperature resistant tape  66  may consist of a ceramic fiber tape, such as the one commercialized under the trade mark CerMax™. 
         [0026]    Other lobe supporting structures (not shown) could be provided as well. For instance, a second support or stiffener ring (not shown) could be provided at the downstream end  38  of the multi-lobe exhaust mixer  32 . 
         [0027]    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.