Abstract:
A device for mounting a dividing wall for separating the main air stream and the bypass air in a bypass turbojet engine afterburner is disclosed. The device includes an attachment unit which attaches the upstream end of the wall to a guide vane casing and a supporting unit which supports the downstream end of the wall provided on flame holder arms of the afterburner radially on the inside of the dividing wall.

Description:
BACKGROUND OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART 
     The present invention relates to a device for mounting a dividing wall for separating the main air stream and the bypass air in a bypass turbojet engine afterburner. 
     This wall is arranged inside a cylindrical guide vane casing of the turbojet engine and therewith delineates an annular passage in which the bypass air can flow, which bypass air is intended to be mixed partially with the main air stream flowing inside the wall, downstream of the afterburner. 
     The dividing wall is fixed at its upstream end to the guide vane casing by means of cylindrical studs which extend radially into the bypass flow and the external end of which is fixed to the guide vane casing and the internal end of which is mounted, with sealing, in a corresponding bore in the wall. 
     At its downstream end the wall has notches oriented downstream and penetrated by flame holder arms which extend radially from the guide vane casing into the main air stream. Each arm extends some distance from the edge of the corresponding notch in the wall and with this edge defines a cross section through which air from the bypass stream can escape into the main air stream. 
     The downstream part of this wall is thus mounted with an overhang and can expand freely under the effect of the rise in temperature when the turbojet engine is running. 
     However, the pressure of the bypass stream is higher than that of the main air stream from the turbojet engine turbine, and this gives rise to significant loadings on the downstream part of the dividing wall and results in local deformations of this downstream part toward the axis of the turbojet engine and in an increase in the aforementioned leakage cross section, thus reducing engine performance. 
     One known solution to this problem is to stiffen the dividing wall by increasing its thickness and forming stiffeners in its downstream end part. However, this solution is not satisfactory because it is complicated and expensive to achieve and leads to an increase in the mass of the dividing wall—something which is disadvantageous in the aeronautical industry. 
     SUMMARY OF THE INVENTION 
     It is a particular object of the invention to provide a simpler and more economical alternative solution to this problem. 
     To this end, the invention proposes an afterburner for a bypass turbojet engine, comprising a substantially cylindrical dividing wall separating the main air stream from the bypass air, means of attaching the upstream end of this wall to an external casing, and means for supporting the downstream end of this wall, these support means being provided on flame holder arms extending radially with respect to the axis of the afterburner, wherein the dividing wall comprises orifices or notches through which the flame holder arms pass, each comprising a substantially radial flange extending inside the dividing wall and forming a surface for supporting the edge of a corresponding orifice or notch in the dividing wall, and a cylindrical rim substantially coaxial with the arm is formed on the flange and extends radially outward in the orifice or the notch in the dividing wall and along the edge of the orifice or of the notch. 
     According to the invention, the downstream part of the dividing wall is positively supported by structural elements of the afterburner allowing the pressure loadings applied to this downstream part to be transmitted to the structural elements and preventing it from deforming inward during operation while at the same time affording the wall the freedom of thermal expansion with respect to the structural elements. 
     It is thus possible to produce afterburner systems with high extraction ratios capable of withstanding high pressure loadings across the dividing wall. 
     The support means are formed on flame holder arms which extend radially with respect to the axis of the chamber through the orifices or notches in the downstream part of the wall. Each flame holder arm comprises a substantially radial flange which extends inside the dividing wall and forms a support surface supporting the edge of a corresponding orifice or notch in the dividing wall. This flange supports the pressure loadings applied to the wall and, in operation, extends a short distance from the dividing wall so as to reduce the cross section for leakage between the flame holder arm and the wall. 
     The flange may be formed as a single piece with the flame holder arm or be attached to the flame holder arm. 
     Advantageously, a cylindrical rim substantially coaxial with the arm is formed on the flange and extends radially outward in the orifice or the notch in the dividing wall and along the edge of the orifice or of the notch. 
     This cylindrical rim is preferably tall enough to prevent hot gases from the main air stream and passing between the flange and the wall from entering means of ventilation of the flame holder arm. 
     In a preferred embodiment of the invention, the cylindrical rim extends along the upstream edge of the flange and along at least part of the downstream edge of the flange and is attached to lateral lugs used for fixing the flame holder arm to the casing. 
     The part of the arm that extends radially inside the flange advantageously has an aerodynamic profile to guarantee that the flow of hot gases around the arm is clean and stable, that is to say with no separation or recirculation. 
     When the dividing wall extends at least partly inside a cylindrical sleeve that provides thermal protection to an afterburner jetpipe, the downstream end of the dividing wall comprises means for bearing radially against this sleeve in order to limit the outward deformations of the downstream end of the wall. 
     These radial-bearing means are, for example, formed of small bridges attached to the downstream end of the dividing wall. 
     This appreciably improves the dynamic behavior of the dividing wall, of which the movements at its downstream end are thus radially limited inward and outward. 
     In another embodiment of the invention, the afterburner involves annular afterburning and the dividing wall is supported by small bridges attached to the trailing edges of the flame holder rings. 
     The invention also relates to a bypass turbojet engine which comprises an afterburner as described hereinabove. 
     The invention also relates to a flame holder arm for an afterburner of the aforementioned type comprising, at one end, fixing lugs and which comprises, at the base of these lugs, a substantially radial external flange formed with a cylindrical rim attached to the fixing lugs that secure the arm. This flange may be formed as one piece with the arm or be attached thereto. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and other details, characteristics and advantages thereof will become apparent from reading the following description given by way of nonlimiting example and with reference to the attached drawings in which: 
         FIG. 1  is a schematic half view in axial section of a bypass turbojet engine afterburner; 
         FIG. 2  is a schematic perspective view of a dividing wall according to the prior art; 
         FIG. 3  is a schematic perspective view of a dividing wall according to the invention; 
         FIG. 4  is a schematic front view of a flame holder arm according to the invention, viewed from the downstream end; 
         FIGS. 5 to 7  are partial schematic perspective views of the device according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference is made first of all to  FIG. 1  which depicts a bypass turbojet engine afterburner  10  situated downstream of the turbine and upstream of the turbojet engine jetpipe. 
     The afterburner  10  comprises a substantially cylindrical wall  12  (also known as a “confluence”) that separates the main air stream from the bypass air and is mounted inside an external cylindrical exhaust casing  14  and around an exhaust cone  18  of the turbojet engine. The wall  12  and the casing  14  between them delimit an external annular passage through which the cold stream or bypass air stream  16  of the turbojet engine, generated by a fan upstream of the turbojet engine and used to increase the thrust and to ventilate parts of the turbojet engine, flows. The wall  12  delimits with the exhaust cone  18  an internal annular passage through which the hot stream or main air stream  20  of the turbojet engine, which consists of the exhaust gases from the combustion chamber of the turbojet engine, flows. The main air stream  20  and the bypass air  16  are partially mixed downstream of the wall  12  in order to increase the thrust of the turbojet engine. 
     The wall  12  is engaged axially at its upstream end on a shroud  22  of a part of the turbojet engine situated upstream of the afterburner  10  and is fixed to the guide vane casing  14  by three cylindrical pegs  24  extending radially into the bypass air stream  16  between the wall  12  and the casing  14  and uniformly distributed about the axis  25  of the turbojet engine. 
     Each peg  24  has a radially external end fixed by bolts to the casing  14  and a radially internal end mounted, with sealing, in an external radial bore  26  formed on an upstream end part of the wall  12  as is visible in  FIGS. 1 and 2 . The internal ends of the pegs  24  are slightly flared so as to allow the ends of the pegs to slide and swivel slightly in the bores  26  in the wall when there is differential thermal expansion of the wall and of the casing. 
     The wall  12  also comprises, at its downstream end, U-shaped notches  28 , the openings of which are directed downstream and through which pass flame holder arms  30  which extend radially with respect to the axis  25  of the turbojet engine and obliquely with respect thereto, their radially external end being fixed to the guide vane casing  14  and their radially internal end being offset in the downstream direction and located downstream of the exhaust cone  18 . The flame holder arms  30  pass with clearance through the notches  28  to allow freedom of thermal expansion of the wall  12  with respect to the arms  30  and together with the edges of the notches define a cross section for the leakage of air from the bypass stream toward the main air stream. 
     The radially internal part of each arm  30  which extends into the main air stream  20  is in the form of a hollow dihedron, the vertex of which is directed upstream, and inside which there extends a fuel injection harness (not depicted), the radially external end of which is fixed to the guide vane casing  14  and connected to fuel supply means. The arm is fixed to the guide vane casing  14  by means of lateral lugs  32  which extend between the wall  12  and the casing  14  and between which air from the bypass air stream  16  circulates, it being possible for some of this air to enter the arm and be diffused over the fuel injection harness by way of ventilation means (not depicted). 
     The fixing lugs  32  comprise, downstream, a housing for securing a C-section burner ring  34 , the opening of which is directed downstream and which contains a fuel injection harness  36  which is coupled to the aforesaid fuel supply means by elbowed pipes  38  passing axially between the fixing lugs  32  for fixing the arms. 
     A cylindrical sleeve  39  is fixed for example by rivets to a cylindrical afterburner jetpipe  15  fixed to the downstream end of the guide vane casing  14  to afford this pipe  15  thermal protection against the increase in temperature caused by the burning of the mixture of gas and fuel injected into the chamber  10 . 
     During operation, the pressure of the bypass air stream  16  is higher than that of the main air stream  20  and this gives rise to significant loadings on the downstream end part of the wall  12  and results in local deformations of this part toward the axis  25  of the turbojet engine and in an increase in the aforesaid leakage cross section, thus reducing engine performance. 
     In the known art, attempts are made at limiting these deformations using stiffeners and by thickening the wall. In the example depicted in  FIG. 2 , the wall  12  is thick, its downstream part comprises axial stiffening ribs  40  uniformly distributed about the axis, and the notches  28  have edges projecting toward the inside of the wall. 
     However, this solution is not entirely satisfactory; it is complicated and expensive to achieve and leads to an increase in the mass of the wall  12 . 
     The invention allows the aforesaid problem to be solved using means for supporting the downstream end of the dividing wall which are provided on structural elements of the afterburner  10  and situated radially on the inside of the wall. 
     In the embodiment of the invention depicted in  FIGS. 3 to 7  where the elements from  FIGS. 1 and 2  are denoted by the same reference numerals increased by one hundred, the flame holder arms  130  of the afterburner comprise external flanges  150  forming means of supporting the downstream end of the dividing wall  112 . 
     This wall  112  has a substantially biconical shape, its ends being flared outward ( FIG. 3 ). As in the prior art, the upstream end part of the wall  112  comprises radial bores  126  for housing cylindrical pegs  24  for attaching the wall to the guide vane casing  14  and orifices  127  for the passage of fuel injectors. 
     The downstream end part of the wall  112  comprises orifices  128  through which the flame holder arms  130  pass and the edges of which bear on the external flanges  150  of these arms. 
     The external flange  150  of each arm is formed at the base of the fixing lugs  132  and extends over 360° about the axis of the arm inside the wall  112  to form an annular surface for supporting the wall  112 . 
     In the example depicted, the flange  150  is formed as a single piece with the arm  130  and is connected to the dihedron of the arm by a fillet  158  on the inside ( FIG. 6 ). The flange has a substantially polygonal contour and its dimensions exceed those of the corresponding orifice  128  in the wall  112  so that the edge of this orifice is fully supported by the flange. The thickness of the flange  150  is determined in such a way as to prevent it from deforming itself when the wall transmits to it the pressure loadings to which it is subjected during operation and, for example, its thickness exceeds that of the wall and is substantially identical to that of the walls of the dihedron. 
     The flange is shaped to run parallel to the wall  112  and a short distance therefrom ( FIG. 7 ), in order to limit the leakage cross section  160  for the leakage from the bypass air stream to the main air stream and the leakage of hot gases from the bypass air stream to the main air stream, this leakage cross section being appreciably smaller than the leakage cross section in the prior art which was defined by the edge of the notch in the wall and the arm, as schematically depicted by the arrow  162  in  FIG. 7 . 
     An external cylindrical rim  164  is formed on the flange  150  on the opposite side to the fillet  158  and extends substantially coaxially with respect to the arm and inside the corresponding orifice  128  in the wall  112 . The radial distance with respect to the axis of the arm between the cylindrical rim  164  and the edge of the orifice  128  is determined in such a way as to allow free thermal expansion of the wall with respect to the arm. 
     In the example depicted, the cylindrical rim  164  extends along the upstream edge and along the downstream edge of the flange  150  and is connected to the fixing lugs  132  used to fix the arm to the guide vane casing  14 . 
     This rim  164  has a height or an axial dimension with respect to the axis of the arm which is determined such that the hot gases entering the bypass stream by passing through the aforesaid leakage cross section between the upstream edge of the flange  150  and the wall  112  are deflected by the rim  164  and flow around the arm as depicted by the arrows  166  in  FIG. 6 , thus preventing these hot gases from entering the aforesaid ventilation means as this would be to the detriment of the cooling of the arms and of their fuel injection harness. 
     The flame holder arm  130  comprises, downstream of the dihedron and near the flange, a housing  168  for attaching a burner ring  34  similar to that of  FIG. 1 . The part  170  of the dihedron which extends radially between the housing  168  and the flange  150  has an aerodynamic profile so as not to impede the flow of the main air stream  20  between the burner ring and the flange and so as not to cause any separation or recirculation of the flow. 
     In the example depicted, the wall  112  has an axial dimension greater than that of the wall  12  of  FIG. 2  and its upstream end extends inside the cylindrical sleeve  39  affording thermal protection to the afterburner jetpipe  15  and comprises small bridges  152  fixed to its external periphery and uniformly distributed about the axis of the wall, these small bridges  152  being intended to bear radially against the sleeve  39  to limit the outward deformations of the wall while at the same time allowing the bypass air to pass between the wall  112  and the sleeve  39 . 
     The small bridges  152  are in the shape of an Ω of an inverted U and are fixed to the wall at their ends  156  by welding or brazing. The small bridges are, for example, 27 in number. The bypass air can flow inside the small bridges or between the small bridges. 
     The thickness of the wall  112  is less than that of the wall of  FIG. 2  and for example is between about 1 and 2 mm. 
     In operation, the wall  112  expands radially outward and is no longer supported or is supported locally by the flanges of the flame holder arms. The pressure difference between the main air stream and the bypass air stream is applied to the downstream end of the wall which deforms slightly inward and comes to bear radially on the flanges of the arms  130  to limit this deformation. The downstream end of the wall can also come to bear radially against the heat-protection sleeve  39  in order also to limit the outward deformation of the wall. The means for supporting the downstream end of the wall and for it to bear against thus allow the dynamic behavior of this end of the wall  112  to be improved. 
     Of course, the invention is not restricted to the embodiment described in the foregoing and depicted in the attached drawings. For example, the wall  112  may comprise notches through which the flame holder arms pass so that the flanges of the arms form means of supporting the edges of the notches. 
     It is also possible for the flange to be attached and fixed to the arm by any appropriate technique. The flange is, for example, made of a ceramic matrix composite (CMC) material and fixed by rivets or screws to a flame holder arm also made of CMC. 
     The flange may equally extend along just part of the edge of the orifice or of the notch in the wall. 
     It is also possible for the housing for securing the ring sector of the flame holder arm  130  to be provided on the arm fixing lugs, as is the case in the prior art depicted in  FIG. 1 . 
     In another embodiment, not depicted, the afterburner involves annular afterburning formed, for example, using coaxial flame holder rings, and the means of supporting the downstream end of the wall are formed by small bridges fixed to the trailing edges of one of the flame holder rings. These small bridges may be of the same type as the small bridges  152  fixed to the downstream end of the wall  112 .