Patent Publication Number: US-2015075169-A1

Title: Integrated turbine exhaust struts and mixer of turbofan engine

Description:
TECHNICAL FIELD 
     The application relates generally to turbofan aero-engines and, more particularly to an improved turbine exhaust case including a mixer for such engines. 
     BACKGROUND OF THE ART 
     In order to increase the effective thrust of turbojet engines, bladed fans have been added to a turbine driven shaft thereof to affect the flow of a quantity of atmospheric air through an annular bypass duct surrounding the turbojet. Hot gases exhausted from the engine core and the bypass airstream are mixed together before expulsion through a single nozzle. In order to perform the mixing function, mixers have been attached to the downstream end of a shroud of the turbine exhaust case (TEC). A swirling flow of exhaust gases from the turbine exit is conventionally deswirled by a plurality of deswirling struts located within the TEC, upstream of the mixer as shown in  FIG. 6 , such that the exhausted gases are substantially deswirled prior to entering the mixer in order to maximize the performance of the struts and mixer individually and to promote efficient mixing with minimum pressure losses. Nevertheless, there is room for improvement of such a conventional configuration of deswirling struts and mixer. 
     Accordingly there is a need to provide an improved mixer. 
     SUMMARY 
     In one aspect, there is provided a turbine exhaust case (TEC) of a turbofan aeroengine including a mixer for mixing exhaust gases with a bypass air stream, the TEC comprising an annular hub and an annular shroud with said mixer located at a downstream end of the shroud, the mixer surrounding the hub to form an annular exhaust gas duct positioned radially therebetween, a plurality of deswirling struts circumferentially spaced apart with respect to a central axis of the TEC and located within an axial length of the mixer between an upstream end where exhaust gases enter the mixer and a downstream end of the mixer where exhaust gases are discharged from the mixer and mixes with the bypass air stream, the deswirling struts each having a cambered profile and extending radially across the annular exhaust gas duct and interconnecting the mixer and the hub. 
     In another aspect, there is provided a turbofan aeroengine comprising a turbine exhaust case (TEC) positioned downstream of a turbine section for directing a flow of gases exhausted from the turbine section, the TEC including an inner annular hub surrounded by an annular outer wall, a downstream end section of the outer wall being in a circumferential wavy configuration to form a plurality of axially extending lobes defining alternative crests and valleys extending divergently to a downstream end of the TEC, the crests defining internal axial and radially-outward passages for directing gases exiting from the turbine section to pass through the TEC, and the valleys defining external axial and radially-inward passages for directing a bypass air stream to pass along an external surface of the TEC, resulting in mixing of the gases with the bypass air stream, a plurality of circumferentially spaced deswirling struts each having a cambered profile and located within an axial length of the wavy configuration for deswirling a rotational component of the gases passing through the TEC. 
     Further details of these and other aspects of the described subject matter will be apparent from the detailed description and drawings included below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures in which: 
         FIG. 1  is a schematic side cross-sectional view of an examplary turbofan aeroengine showing an application of the described subject matter according to one embodiment; 
         FIG. 2  is a perspective view of a turbine exhaust case mixer according to one embodiment which may be used in the engine of  FIG. 1 ; 
         FIG. 3  is a partial cross-sectional view of the engine of  FIG. 1 , showing another embodiment of the mixer integrated with deswirling struts in an enlarged scale; 
         FIG. 4  is a perspective view of the mixer incorporated with the deswirling strut of  FIG. 3 , with the deswirling strut partially cut away to show a cross-section thereof; 
         FIG. 5  is a cross-sectional view of the deswirling strut to show the cross-section of the deswirling strut in  FIG. 4  having an aerofoil profile; and 
         FIG. 6  is a partial cross-sectional view of a turbine exhaust case mixer conventionally attached to the turbine exhaust case downstream of deswirling struts installed within the turbine exhaust case. 
     
    
    
     It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary turbofan aeroengine which includes a nacelle configuration  10 , a core casing  13 , a low pressure spool assembly seen generally at  12  which includes a fan assembly  14 , a low pressure compressor assembly  16  and a low pressure turbine assembly  18 , and a high pressure spool assembly seen generally at  20  which includes a high pressure compressor assembly  22  and a high pressure turbine assembly  24 . The core casing  13  surrounds the low and high pressure spool assemblies  12  and  20  in order to define a main fluid path (not numbered) therethrough. In the main fluid path there is provided a combustion chamber  26  in which a combustion process produces combustion gases to power the high and low turbine pressure assemblies  24  and  18 . A turbine exhaust case (TEC)  28  is provided to form a downstream end of the core casing  13  and a mixer  30  is attached to the downstream end of the TEC  28  for mixing hot exhaust gases discharged from the high and low pressure turbine assemblies  24 ,  18  through the main fluid path, with a bypass airstream driven by the fan assembly  14  through an annular bypass duct  32  which is defined radially between the nacelle configuration  10  and the core casing  13 . 
     Referring to  FIGS. 1-3 , the TEC  28  and the mixer  30  define a common central axis  34  which substantially superposes a central rotation axis of the aeroengine. The TEC  28  includes an annular hub  36  and an annular shroud  38  with the annular mixer  30  attached to a downstream end of the shroud  38 . The shroud  38  and the mixer  30  surround the hub  36  to form an annular exhaust gas duct  40  disposed radially therebetween. 
     It should be noted that the terms “upstream” and “downstream” used herein and hereinafter refer to the direction of a fluid flow passing through the main fluid path of the engine. It should also be noted that the terms “axial”, “radial” and “circumferential” are used with respect to the central axis  34 . 
     The mixer  30  according to one embodiment such as shown in  FIG. 2 , may define an annular wavy configuration around the central axis  34  and may axially extend between an upstream end  42  and a downstream end  44  thereof. The mixer  30  may include inner and outer circumferential flow surfaces  46 ,  48  extending between the upstream and downstream ends  42 ,  44  of the mixer  30 . The inner and outer flow surfaces  46 ,  48  may be in a circumferentially wavy or twisted annular configuration to thereby form a plurality of lobes  50  of the mixer  30 . The lobes  50  may be axially extending or axially straight and may define a plurality of alternating crests  52  and valleys  54 . In a cross-sectional view as shown in  FIG. 3 , adjacent crest  52  and valley  54  extend from an axial start point  56  which is close to the upstream end  42  (more clearly shown in  FIG. 3 ) and diverging to the downstream end  44  of the mixer  30 . 
     The inner surface  46  may define inner passageways (not numbered) axially and radially-outwardly along the respective crests  52  for directing the exhaust gases flowing through the annular exhaust gas duct  40 . The outer flow surface  48  may define external passageways (not numbered) axially and radially-inwardly along the respective valleys  54  for directing the bypass airstream coming through the annular bypass air duct  32  to flow through the mixer  30 . Therefore, the internal and external passageways of the mixer  30  may in combination establish a vortex system downstream of the mixer  30  to encourage mixing between the bypass airstream and the turbine exhaust gases during operation of the aeroengine. 
     Referring to FIGS.  1  and  3 - 5 , the mixer  30  according to one embodiment may include a plurality of deswirling struts  58  circumferentially spaced apart with respect to the central axis  34 . The deswirling struts  58  may be disposed within an axial length of the mixer  30 , between the upstream end  42  and the downstream end  44  of the mixer  30 . The deswirling struts  58  may extend radially across the annular exhaust gas duct  40  and may interconnect the mixer  30  and the hub  36  of the TEC  28 . 
     The deswirling struts  58  each include a leading edge  60  and a trailing edge  62 . The trailing edge  62  of each deswirling strut  58  according to one embodiment may circumferentially align with a bottom of the valley  54  such as a bottom line  64  (see  FIG. 4 ) which is a center line of the valley  54 . The deswirling struts  58  according to one embodiment may be axially located in a middle area of the mixer  30  such that the leading edges  60  of the respective deswirling struts  58  are axially spaced away from the starting point  56  of the divergently extending crests  52  and valleys  54  and such that the trailing edges  62  of the respective deswirling struts  58  are axially spaced away from a downstream end of the respective valleys  54  of the mixer  30 . The downstream end of the respective valleys  54  according to this embodiment are the downstream end  44  of the mixer  30  because the crests  52  and valleys  54  have a substantially equal axial length as shown in  FIG. 3 . However, if the axial length of the valleys  54  is less than the axial length of the crests  52 , such as illustrated in the embodiment shown in  FIG. 2 , the downstream end of the valleys  54  will not be the downstream end of the mixer  30 . 
     Optionally, the deswirling struts  58  may each have a cambered profile, for example including a convex side  66  and a concave side  68  extending between the leading and trailing edges  60  and  62  as shown in the cross-sectional view of the deswirling strut  58  in  FIG. 5 . The struts  58  are cambered in the direction of an incoming swirling flow of the exhaust gases, as indicated by arrow  70  in  FIG. 5 . 
     According to one embodiment the deswirling of the swirling flow  70  of the exhaust gases discharged from the low pressure turbine assembly  38  and passing through the annular exhaust gas duct  40 , may be accomplished within the mixer  30  by both the deswirling struts  58  and the mixer lobes  50 . The swirling flow  70  of exhaust gases passing through the annular exhaust gas duct  40  near the hub  36  may be deswirled by the deswirling struts  58 . The swirling flow  70  of the exhaust gases passing through the annular exhaust gas duct  40  near the shroud  38  may be deswirled by the lobes  50  of the mixer  30 . With the configuration as described in the above embodiments, the deswirling and mixing functions may be accomplished within a much shorter axial length of the TEC and mixer in contrast to conventional TEC and mixer configurations, thereby advantageously saving engine and nacelle weight. The configuration of the above-described embodiments, can deswirl the swirling flow of exhaust gases and mix the exhaust gases with the bypass air stream with a performance equivalent to or better than that of conventional separate mixer and TEC struts. 
     The size, shape and position of the deswirling struts may be optimized based on the application and are dependent on the flow conditions including the residual swirl condition from the low pressure turbine assembly  18 . The deswirling struts according to the described embodiments may be incorporated into any conventional TEC mixer when the swirl in the exhausted gases is required to be removed. For example, some of the described embodiments may be applicable to TEC mixers in which the axial length of the valleys of the mixers are longer than the axial length of the crests of the mixers. 
     Alternatively, the deswirling struts  58  may be axially located within the mixer  30  such that the leading edge  60  of each of the deswirling struts  58  axially aligns with the start point  56  of the divergently extending crests  52  and valleys  54 , as shown by broken line  60   a  in  FIG. 3 . Also alternatively, the deswirling struts  58  may be axially located within the mixer  30  such that the trailing edge  62  of each of the deswirling struts  58  axially aligns with the downstream end of the respective valleys  54 , as indicated by broken line  62   b  in  FIG. 3 . 
     Optionally, each of the valleys  54  of the mixer  30  may be connected with one of the deswirling struts. Also optionally, every second one of the valleys  54  of the mixer  30  may be connected with one of the deswirling struts. Furthermore, the deswirling struts may be circumferentially located at other intervals of the valleys  54  of the mixer  30 . 
     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 described subject matter. Modifications which fall within the scope of the described subject matter 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.