Abstract:
An integrated mixer/TEC having structural features that may be individually used or combined to achieve a robust and durable design.

Description:
RELATED APPLICATION 
       [0001]    The present application claims priority on U.S. Provisional Application No. 62/238,898 filed on Oct. 8, 2015, the entire content of which is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The application relates generally to gas turbine engines and, more particularly, to an integrated gas turbine exhaust case (TEC) mixer. 
       BACKGROUND 
       [0003]    Integrated turbine exhaust case mixers typically comprise struts extending between the mixer lobe valleys and an inner shroud. Nominally, such a design is not considered structurally robust since radial operational loading are directed through the struts to non-structural and flexible structures of both the inner shroud and the mixer valleys. Heretofore, high resulting tensile stresses under operational loading have, thus, limited the durability of conventional exhaust mixers. 
       SUMMARY 
       [0004]    In accordance with a general aspect, there is provided a turbine exhaust case (TEC) mixer comprising: an inner shroud, an outer shroud including a multi-lobe mixer, the multi-lobe mixer having circumferentially alternating inner and outer lobes, each of the inner lobes having a valley and first and second sidewalls extending radially outwardly from the valley, circumferentially spaced-apart struts depending radially inwardly from the valley of associated ones of the inner lobes, each strut having a leading edge and a trailing edge, wherein for at least one of the inner lobes, the leading edge of an associated one of the struts is off-centered between the sidewalls of the at least one inner lobe as viewed in plane containing the leading edge. 
         [0005]    In accordance with another general aspect, there is provided a gas turbine engine comprising a compressor for pressurizing incoming air, a combustor in which air compressed by the compressor is mixed with fuel and ignited for generating a stream of combustion gases, a turbine for extracting energy from the combustion gases, and a turbine exhaust case (TEC) mixer disposed downstream of the turbine, the TEC mixer comprising: an inner shroud, an outer shroud including a multi-lobe mixer, the multi-lobe mixer having circumferentially alternating inner and outer lobes, each of the inner lobes having a valley bordered by first and second sidewalls extending radially outwardly from the valley, circumferentially spaced-apart struts extending radially inwardly from the valley of associated ones of the inner lobes to the inner shroud, at least some of the struts being respectively spaced from the first and second sidewalls of the associated inner lobes by a distance T 1  and a distance T 2 , and wherein T 1 &lt;T 2 . 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]    Reference is now made to the accompanying figures, in which: 
           [0007]      FIG. 1  is a schematic cross-section view of a turbofan gas turbine engine; 
           [0008]      FIG. 2  is a front isometric view of an integrated TEC-Mixer (ITM) forming part of the engine shown in  FIG. 1 ; 
           [0009]      FIG. 3  is a front isometric enlarged view of a strut depending radially inwardly from a valley of an inner lobe of the mixer of the ITM shown in  FIG. 2 ; 
           [0010]      FIG. 4  is a front isometric cross-section view illustrating the transition between one inner lobe and an associated strut; 
           [0011]      FIG. 5  is a front isometric cross-section view similar to  FIG. 4  but illustrating the wall thickness transition between the strut walls, the valley and the radial wall of the lobe; 
           [0012]      FIG. 6  is a front isometric cross-section view similar to  FIGS. 4 and 5  but illustrating the off-centered positioning of the strut relative to the valley of the inner lobe; 
           [0013]      FIG. 7  is a front isometric cross-section view similar to  FIGS. 4-6  but illustrating the positioning of the center of the fillet radii in relation to the distance between the walls of the struts and the corresponding walls of the lobe; 
           [0014]      FIG. 8  is an enlarged front isometric view illustrating the fillet radii between the strut and the lobe along the axial and tangential directions; 
           [0015]      FIG. 9  is a front isometric cross-section view of a hollow TEC strut; 
           [0016]      FIG. 10  is a bottom isometric view illustrating a cooling opening at the base of a hollow TEC strut; 
           [0017]      FIG. 11  is an isometric view illustrating internal fillet radii at the radially outer and radially inner end of the strut; 
           [0018]      FIG. 12  is an isometric view illustrating a method of joining a strut to the rest of the ITM structure; and 
           [0019]      FIG. 13  is a front isometric view illustrating a weld assembly between adjacent ITM sectors. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  illustrates an example of a turbofan gas turbine engine generally comprising a housing or nacelle  10 ; a low pressure spool assembly  12  including a fan  11 , a low pressure compressor  13  and a low pressure turbine  15 ; a high pressure spool assembly  14  including a high pressure compressor  17 , and a high pressure turbine  19 ; and a combustor  23  including fuel injecting means  21 . 
         [0021]    Referring to  FIGS. 1 and 2 , the gas turbine engine  10  further comprises an integrated turbine exhaust case (TEC)-Mixer (ITM)  25  disposed immediately downstream of the last stage of low pressure turbine blades for receiving hot gases from the low pressure turbine  15  and exhausting the hot gases to the atmosphere. The ITM  25  comprises an annular inner shroud  27  (an inner cone in the illustrated embodiment) concentrically mounted about the central axis A ( FIG. 1 ) of the engine, an annular outer shroud  29  concentrically mounted about the central axis A of the engine and the inner shroud  27 . The inner and outer shrouds  27 ,  29  define therebetween an annular gaspath  33  for channelling the engine core flow. The inner and outer shrouds  27 ,  29  may be made out of sheet metal. A plurality of circumferentially spaced-apart struts  31  extends radially between the inner and outer shrouds  27 ,  29  across the gaspath  33 . The struts  31  may not only serve as structural components, they may have an airfoil profile to serve as vanes for directing/straightening the incoming flow of hot gases. The struts  31  may also have a hollow body. The ITM  25  further comprises a multi-lobed mixer  37  integrated to the outer shroud  29  and extending axially rearwardly therefrom. The mixer  37  defines a plurality of circumferentially distributed lobes. The lobes include alternating outer and inner lobes. The outer lobes extend radially outwardly into the bypass passage of the engine while the inner lobes extend radially inwardly into the core flow path. 
         [0022]    A mounting flange  39  may be provided at the front end of the outer shroud  29  for securing the ITM  25  to the engine case  41  ( FIG. 1 ) which, in turn, may be structurally connected to the nacelle  10  through a plurality struts  43  ( FIG. 1 ) extending radially through the bypass passage of the engine. Referring more specifically to  FIG. 1 , it may also be appreciated that a tail cone  35  may be mounted to the aft end of the inner shroud  27  of the turbine exhaust case  25 . The tail cone  35  may be bolted or other suitably removably connected to the inner shroud  27 . 
         [0023]    In operation, combustion gases discharged from the combustor  23  power the high and low pressure turbines  19  and  15 , and are then exhausted into the annular hot gaspath  33  defined between the inner and outer shrouds  27 ,  29  of the ITM  25 . The tangential components included in the exhaust gases may be de-swirled by the struts  31  or similar de-swirling airfoil structures which may be integrated in the ITM  25 , and then the exhaust gases are discharged into the atmosphere through the mixer  37  which facilitates the mixing of the exhaust gases with the outer air flow from the bypass passage. 
         [0024]    In an integrated mixer/TEC, the struts  31  extend between the inner lobes (outer radius) and the TEC shroud  27  (inner radius). Typically, the number of struts is less than the number of inner lobes (i.e. there is not a strut at each inner lobes). Nominally, this design is not robust structurally since radial operational loading will be directed through the TEC struts  31  to the weak and flexible structures of both the inner shroud  27  and the mixer inner lobes (forming part of the outer shroud  29 ). High resulting tensile stresses under operational loading will limit the durability of the nominal design. 
         [0025]    The features described hereinbelow are designed to direct radial loads in the TEC struts to (1) the radial mixer lobe walls between alternating valleys and crests and (2) a reinforced inner shroud  27 . The resulting load path directs radial loads into regions low concentrations compressive stress, thus, achieving durability under operational loads. 
         [0026]    It is understood that various combinations and sub-combinations of the features described hereinbelow with reference to  FIGS. 3 to 13  are possible to provide added durability. 
         [0027]    As shown in  FIG. 3 , the inner shroud  27  can be reinforced with stiffening bands  60  provided below the leading edge  61  and trailing edge  63  of the TEC struts  31 . The stiffening bands  60  may be suitably joined to the radially inwardly facing surface of the inner shroud  27  at axial locations generally aligned with the leading and trailing edges of the struts  31 . According to one embodiment, the bands  60  are provided by additive manufacturing. The bands  60  could also be brazed to the inner shroud  27 . The bands  60  can be circumferentially continuous or segmented. According to one embodiment, the bands  60  are only provided underneath the struts (i.e. no band is provided on the inner shroud circumferentially between the struts). 
         [0028]    As shown in  FIG. 4 , each inner lobe of the mixer comprises a valley  64  and a pair of sidewalls  66  extending radially outwardly from the valley  64 . The sidewalls  66  interconnect adjacent inner and outer lobes. As can be appreciated from  FIG. 4 , it is contemplated to provide additional thickness in the material forming the base of each of the valley  64  of the mixer inner lobe. Indeed, as shown in  FIG. 4 , the thickness t 1  of the sidewalls  66  is less than the thickness t 2  of the valley  64 . That is the base of the valleys  64  where the struts  31  extend from could be made thicker than the radial mixer lobe walls  66 . According to one possible embodiment, the ratio t 1 /t 2  ranges from about 0.2 to about 1. In one particular application, the ratio t 1 /t 2  is about 0.6. 
         [0029]      FIG. 5  shows the transition between a strut  31  and an inner lobe of the mixer  37 . In the illustrated example, the strut  31  is hollow and has a pair of radially extending walls  68  merging at a radially outer end thereof with the valley  64  of the associated inner lobe of the mixer  37 . As can be appreciated from  FIG. 5 , a smooth material thickness transition in the material between the radially extending walls  68  of the TEC strut  31  and the radial mixer lobe walls  66  may be provided. The material is thicker at the base of the valley  64  and gradually becomes thinner has it merges with the walls  68  of the strut  31 . For instance, a variation of thickness of less than about 10% per unit length can be provided. According to one possible application, a variation of thickness of about 6% per unit length may be implemented. 
         [0030]    According to the embodiment shown in  FIG. 6 , the leading edge (LE) of TEC struts  31  may be positioned to minimize the offset between the strut  31  and the closest radial mixer wall  66 . That is in the embodiment shown in  FIG. 6 ; the strut  31  is non-centered relative to the valley of the inner lobe (the leading edge is laterally offset relative to a radial median between the sidewalls  66 ). The strut  31  is rather positioned generally closer to one of the inner lobe sidewalls  66  to provide for a more direct load path between the lobe sidewall and the strut. 
         [0031]    Furthermore, as shown in  FIG. 7 , at the radially outer end of the TEC struts  31 , in the tangential direction (i.e. direction perpendicular to the axial and radial directions of the engine), the center of the fillet radii R 1  and R 2  may vary in relation to the distance T 1  and T 2  between the TEC strut position in the mixer valley and the corresponding radial mixer lobe wall  66   a,    66   b.  For instance, according to the illustrated embodiment, the distance T 2  is greater than distance T 1  and the center of fillet radius R 2  is spaced radially inwardly from the center of the fillet radius R 1  and spaced further way from the associated side of the strut along the illustrated tangential axis. This provides for additional material on the side of the strut  31  that is disposed farther from the corresponding lobe wall  66  (at the transition between walls  66   b  and  68   b  in the illustrated embodiment). From,  FIG. 7 , it can be appreciated that the wall profile thickness of the valley  64  is non-symmetric relative to a central radial axis of the lobe. As mentioned hereinabove, more material is provided on the side of the strut  31  which is farther from the associated lobe wall (there is more material on the left hand side haft of the valley in the embodiment depicted on  FIG. 7 ). It is understood that the wall thickness profile of the valley is function of the positioning of the strut  31  relative to the lobe walls  66   a,    66   b.    
         [0032]    Referring now to  FIG. 8 , it can be appreciated that in the axial direction, the fillet radii of magnitude can be in general unequal to the radii in the tangential direction, with a smooth transition between the radial and axial fillet radii. 
         [0033]    Also as shown in  FIG. 9 , each strut  31  may be provided in the form of a hollow TEC strut with thin walls, except in the area of the fillet radii between the TEC cone  27  and mixer lobe. 
         [0034]    It is also contemplated to provide a platform  70  on the base of a hollow TEC strut  31 , the platform  70  defining a small opening  72  for air/heat exchange in the TEC strut cavity (see  FIG. 10 ). The opening has a surface area which is significantly smaller than a cross-section area of the internal volume of the strut. This provide for a more structural robust designed compared to an open end strut. Cooling air can flow through the opening  72  to internally cool the hollow strut  31 . The opening  72  can be minimized to maximize strength. 
         [0035]    Also, as shown in the embodiment of  FIG. 11 , fillet radii r 3 , r 4 , r 5 , r 6  may be defined on the inner side of the TEC strut walls  68  between the mixer lobe valley  64  and the platform  70  at the base (radially inner end) of each TEC strut. These internal fillets contribute to further strengthen the struts. 
         [0036]    The features described herein above with respect to  FIGS. 3 to 11  can be created on the strut  31  and then assembled into a circumferential section of mixer  29  and TEC cone  27  via brazing, as for instance shown in  FIG. 12 . Alternately, the full strut, mixer &amp; TEC cone can be manufactured into one circumferential section ( FIG. 13 ). In both cases, the circumferential sections are welded together along weld lines W to form the integrated mixer/TEC. 
         [0037]    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. While all the inner lobes and associated struts could include all or some of the features disclosed in  FIGS. 3 to 12 , it is understood that not necessary all the inner lobes and struts need to be strengthened. For instance, the off-centre positioning of the leading edge of the struts could only be applied to some of the lobes of a given TEC mixer. Not all the lobes and struts needs to have the same features. Various combinations are contemplated. 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 scope and spirit of the present invention.