Abstract:
An ejector/mixer for a gas turbine engine includes an annular wall having upstream end adapted to be fastened to an engine case and a downstream end forming a plurality of lobes. A support member interconnects the lobes, and includes an annular blade located radially inwardly of the bight of the lobes. The lobes extend radially inwardly downstream relative to the annular wall and the support member includes an annular blade and has spaced apart joint surfaces spaced apart to coincide with the joint surfaces of a respective lobes. The spaced-apart joint surfaces of the support member being profiled to mate with the corresponding joint surface of the lobes.

Description:
TECHNICAL FIELD 
     The application relates generally to aircraft gas turbine engines and, more particularly, to aft section of the engine including an ejector mixer. 
     BACKGROUND OF THE ART 
     In gas turbine engines, hot high velocity air exits from the turbine through the core gas path. The exhaust gases may be constrained by an exhaust case section in the form of a corrugated annular case extension having ejector/mixer lobes. Turbofan engines generally use exhaust mixers in order to increase the mixing of the high and low velocity exhaust gas flows. Turbo-shaft engines may be provided with similar devices sometimes referred to as ejectors. Exhaust mixers/ejectors may experience thermal variation and/or radial deflection due to exposure to the high and low velocity flows. In addition, exhaust ejector/mixers may be prone to vibrations, which has negative consequences for the surrounding hardware. As such, it is generally desirable to increase the stiffness or rigidity of the exhaust case. Various configurations of exhaust ejector/mixers have been proposed to date in order to try to increase the stiffness or reduce deflection thereof. 
     However, there remains a need for an improved exhaust ejector/mixer for a gas turbine engine. 
     SUMMARY 
     In one aspect, there is provided a gas turbine engine having an engine casing enclosing a compressor section, a combustor and a turbine section defining a main gas path serially extending therethrough, and comprising: an exhaust cone disposed downstream of the turbine section; an ejector/mixer cantilevered from an aft end of the engine casing, the ejector/mixer at least partially surrounding the exhaust cone such as to define a portion of the main gas path between an outer surface of the exhaust cone and the ejector/mixer; the ejector/mixer having a plurality of circumferentially distributed lobes; and a support member connected to at least a number of the lobes; each of the at least number of lobes formed with a trough presenting a joint surface; the support member having corresponding concave joint surfaces profiled for matingly engaging the corresponding joint surfaces of the lobes. 
     In another aspect there is an exhaust ejector/mixer for a gas turbine engine adapted to be mounted to a casing at an exhaust end of the gas turbine engine such as to at least partially surround an exhaust cone, the exhaust ejector/mixer comprising: an annular wall having an upstream end adapted to be fastened to an engine case and a downstream end forming a plurality of circumferentially distributed lobes; and a support member disposed towards the downstream end of the annular wall and interconnecting at least a number of the lobes, each of the at least number of lobes formed with a trough with an convex bight radially inward thereof presenting a joint surface; the support member having corresponding concave joint surfaces adapted to be joined to the mating convex joint surfaces of the lobes. 
     The exhaust ejector/mixer may be provided for a turbofan engine where alternating lobes extend alternatively radially outwardly and radially inwardly. In the this case the support member is joined to the inwardly extending members only. For a turbo-shaft engine, the lobes might extend inwardly only, in which case the support member is joined to every lobe. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures in which: 
         FIG. 1  is a schematic cross-sectional view of a turbo-shaft gas turbine engine; 
         FIG. 2  is a rear isometric view of an exhaust ejector/mixer, having a support member connected to the ejector/mixer lobes thereof, in accordance with one embodiment of the present disclosure; 
         FIG. 3  is an enlarged fragmentary, isometric view of a lobe and support member according to  FIG. 2 ; 
         FIG. 4  is a fragmentary rear isometric view an ejector/mixer, having a support member connected to the lobes thereof, in accordance with another embodiment; 
         FIG. 5  is an enlarged fragmentary, isometric view of a lobe and support member according to  FIG. 4 ; 
         FIG. 6  is a schematic, axial cross section of a portion of the ejector/mixer showing the main gas path, and the support member located and oriented in the gas path; and 
         FIG. 7  is a schematic, radial cross section of a portion of the ejector/mixer showing the hot main gas path and the induced cool air in the lobes; and illustrating the relative location of the support member. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a turbo-shaft gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication 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 gas turbine engine  10  includes a core engine casing  20  which encloses the turbo machinery of the engine. The main air flow passes through the core of the engine via a main gas path  26 , which is circumscribed by the core engine casing  20  and allows the flow to circulate through the multistage compressor  14 , combustor  16  and turbine section  18  as described above. 
     At the aft end of the engine  10 , an exhaust cone  22  is centered about a longitudinal axis X of the engine  10 , the exhaust cone  22  being connected to an aft end of the turbine section  18 . The exhaust cone  22  has an outer surface, which defines an inner wall of the main gas path  26  so that the combustion gases flow therearound. An ejector/mixer  30  forms the outer wall of the aft end of the main gas path  26 . As best seen in  FIG. 2 , the ejector/mixer  30  includes an annular wall  34  with a radial fastening ring or flange  32  upstream thereof. The fastening ring  32  is adapted to be mechanically fastened to the aft portion  20   a  ( FIG. 1 ) of the casing  20 . 
     Referring to  FIGS. 2 and 3 , in further detail, the annular wall  34  of the ejector/mixer  30 , includes and defines a plurality of circumferentially distributed radially inner lobes  36  extending axially rearwardly from a front frusto-conical portion of the annular wall  34  to a downstream edge  37 , i.e. a trailing edge thereof. The lobes  36  include side, radially-extending, walls  38  with a bight forming an arcuate trough  40 . The trough  40  presents a convex surface  41  on the radially inner or central side of the annular wall  34 . 
     An annular support member includes a blade  42  extending concentrically about the longitudinal axis X of the engine  10 . In the embodiment shown, the blade  42  comprises an annular longitudinal, flat bar. The blade  42  is interrupted only at form-fitting joint areas  44 . The joint areas  44  are located on the blade  42  to correspond with the convex surfaces  41  of the lobes  36 . The joint areas  44  are curved so that it complements the convex surface  41 , as shown in  FIG. 3 . The curved joint area  44  is concave relative to the convex surface  41  of the lobe  36 . The blade  42  is spaced radially outwardly and independent from the exhaust cone  22  and floats with respect thereto. The blade  42  in one embodiment is a thin sheet metal strip capable of being welded to the sheet metal forming the annular wall  34 . In the embodiment shown in  FIGS. 2 and 3 , the thin sheet metal strip is formed into a continuous ring. 
     As mentioned, the ejector/mixer  30  is solely connected to the engine  10  at the aft end  20   a  of the core engine casing  20 , and so, the ejector/mixer  30  is effectively cantilevered from the core engine casing  20 . This cantilevered configuration allows the lobes  36  of the exhaust ejector/mixer  30  to vibrate at one or more modes in the engine operating frequency range, while remaining relatively stiff. In addition, the thermal variations in the exhaust mixer  32  due to the high and low velocity flows through the main gas path  26  may cause axial and radial displacements in the ejector/mixer  30 , which can accordingly be absorbed by the exhaust ejector/mixer  30 . Moreover, the downstream end  37  of the ejector/mixer  30 , which would otherwise be prone to deflection, is reinforced by the blade  42  which serves to increase the rigidity of the exhaust ejector/mixer  30  and thus inhibit movement at the downstream end  37  thereof. By joining all the lobes  36  together with the blade  42 , any movement of the ejector/mixer  30  is reduced, as are the vibrations thereof. In addition, by providing a blade  42  which is independent of the exhaust cone  28 , i.e. it is free to move relative thereto such as to absorb any vibrations or thermal growth mismatches therebetween. The blade  42  is able to accommodate any axial or radial displacements due to such thermal variations. As such, the ejector/mixer  30  provides enhanced rigidity and may accommodate thermal variations, vibrations and other displacements, as required. 
     Another embodiment is shown in  FIGS. 4 and 5 . In this case, the blade is made up of blade segments  142   a ,  142   b  . . .  142   n . Each segment has a length corresponding to the distance between the center lines of two adjacent lobes  36 . Each end of the segment terminates in a partially formed concave curve to complement part of the convex surface  41  of the lobe  36 . For instance, as shown in  FIG. 5 , corresponding ends of segments  142   a  and  142   b  make-up the form fitting joint area  144 . 
     The blade  42 ,  142  may be located at different axial positions along the convex surfaces  41  of the lobe  36 .  FIG. 3  illustrates a blade  42  being spaced upstream from the trailing edge  37 , of the lobe  36 . As shown in  FIG. 5 , the blade  142  is located at or slightly downstream from the trailing edge  37 , of the lobe  36 . The blade  42 ,  142  may be fixed to the convex surfaces  41  of the lobes  36  at joint areas  44 ,  144  using a combination of resistance, fusion or ball tack welding with a brazing application, or other types of suitable bonding that are well known in the art. 
     The injector/mixer  30 , in the present embodiment, acts to induce cool air, exterior of the engine casing  20 , to be drawn radially inwardly through the lobes  36  to cool the mechanical parts of the injector/mixer  30 . As previously mentioned, the support member is often, according to the prior art, subject to thermal stresses caused by the entrained cool air and of the hot air exiting the turbine  18 .  FIGS. 6 and 7  show the gases flow in the ejector/mixer  30 . The blade  42 ,  142  is disposed directly in the main gas path  26  and is shaped to be laminar with the flow of the hot gases, as can be seen in both  FIGS. 6 and 7 . The blade  42  is essentially exposed only to the hot gases flowing in the main gas path  26 . This reduces the thermal gradient in the blade  42 ,  142 . 
     The embodiments described show a turbo-shaft engine. However, in the case of a turbofan engine, cool air from the fan is directed to the ejector/mixer  30  which in such a case would have inner and outer alternating lobes to best mix the hot gases with the cool air. U.S. Pat. No. 5,265,807 Steckbeck et al 1993; U.S. Pat. No. 7,677,026 Conete et al 2010; and U.S. Pat. No. 8,739,513 Lefebvre et al 2014 describe exhaust mixers which are herewith incorporated by reference. 
     The above described embodiments provides an improved exhaust ejector/mixer for a gas turbine engine where the thermal stresses on the support member are reduced for improved longevity. 
     It is noted that the ejector/mixer and the support member could be made by additive manufacturing processes, such as direct metal laser sintering. Therefore, the ejector/mixer and the support member could be made monolithically. 
     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. For example, the invention may be used with various types of gas turbine engines where cool and hot gases may simultaneously be in contact with the machinery involved. Still 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.