Abstract:
The invention relates to a flame holder ( 10 ) for an afterburner duct of a jet engine, comprising an arm ( 100 ) in the form of a gutter forming a cavity, a shield ( 110 ) for protecting the cavity of the arm ( 100 ) from heat, and an air supply baffle ( 120 ) housed in the cavity. The arm ( 100 ), the protective shield ( 110 ) and the air supply baffle ( 120 ) are held together by a one-piece spacer shoe ( 150 ).

Description:
BACKGROUND OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART 
       [0001]    The present invention relates to the field of jet engines comprising an afterburner duct for reheating the gases emanating from the gas generator. 
         [0002]    Jet engines with afterburners comprise, from upstream to downstream, a gas generator, consisting of a gas turbine engine, producing gases heated by combustion, an afterburner duct, and an exhaust nozzle for exhausting the gases into the atmosphere. The engine is usually of the double-flow type, with a central primary flow and a peripheral secondary flow. 
         [0003]    The afterburner duct is provided with a liner made of a material which is resistant to the gas combustion temperature, said liner being suitably cooled. At the inlet of the duct, fuel injection means are arranged in the gas flow path, combined with means forming flame holders. 
         [0004]    With reference to  FIG. 1 , the flame holder means  10  are in the form of radial arms which are arranged in a star pattern with respect to the engine axis and which pass through the two flow paths for the primary and secondary flows, said arms being connected to one another by elements in the form of annular sectors  9 . With reference to  FIG. 2 , the integrally cast arms  100  in the form of gutters have a U- or V-shaped cross section which is open in the downstream direction so as to create a negative pressure region capable of stabilizing the combustion therein. In at least part of the flame holders  10 , fuel injectors  130  are placed inside within the cavity formed between the walls, upstream and in the vicinity of the apex, together with air ventilation baffles  120 . Air is bled from the secondary flow and distributed by the baffles  120  toward the injectors  130 . In order to protect these elements, a protective heat shield  110  is placed as a covering over this part of the arm  100  containing the fuel injectors  130  and the ventilation baffle  120 . 
         [0005]    Traditionally, as represented in  FIG. 2 , the air ventilation baffle  120  is centered at its upper part and at its lower part in the cavity of the arm  100 . It is held in a radial position via a tenon  5  on the base of the tube which passes through the protective heat shield  110  and thereby rotationally immobilizes the baffle  120  in the arm  100 . 
         [0006]    However, it is not desirable to weaken the heat shield  110  by piercing it in order to retain the air baffle  120 . Specifically, the protective heat shield  110 , which is generally made of CMC (ceramic matrix composite), is damaged by peening and delamination, something which is particularly detrimental during vibratory operation. 
         [0007]    Similarly, piercing and machining operations performed on protective metal shields lead to a concentration of stresses, thereby reducing the efficiency and useful life of said shields. 
         [0008]    It is also known practice to pierce the arm  100  at its lower end so as to introduce there the air ventilation baffle  120 , which is fastened via a washer  16  welded to the lower end of the air ventilation baffle  120 , outside the arm  100 , as represented in  FIG. 3 . 
         [0009]    This alternative is not satisfactory since it requires piercing the arm  100  and therefore entails all of the disadvantages mentioned above. 
       SUMMARY OF THE INVENTION 
       [0010]    In order to rectify at least some of these disadvantages, the Applicant proposes an afterburner flame holder which does not require performing machining operations on the body of the arm  100  and/or on the heat shield  120  that mechanically and/or thermally weaken the flame holder. Such a flame holder is simple in design and easy to assemble, thereby reducing its production cost. Moreover, it offers increased thermal resistance and mechanical strength owing to the absence of weak spots. 
         [0011]    The invention relates to a flame holder for an afterburner duct of a jet engine, comprising an arm in the form of a gutter forming a cavity, a shield for protecting the cavity of the arm from heat, and an air supply baffle housed in the cavity, wherein the arm, the protective shield and the air supply baffle are held together by a one-piece spacer shoe comprising at least one spacer lug designed to form a gap (e) between the protective heat shield and at least one wall of the arm. 
         [0012]    Such a shoe makes it possible to maintain separations between the various elements, thus preventing wear during a vibratory operation. 
         [0013]    The gap-producing lug advantageously makes it possible to form a channel between the protective shield and the arm so as to allow carbureted air to pass through. 
         [0014]    Preferably still, the shoe comprises a means for retaining the baffle. 
         [0015]    The shoe advantageously makes it possible to retain the baffle without weakening the arm or its protective heat shield. 
         [0016]    According to one embodiment of the invention, a fuel injector is placed inside said cavity of the arm. 
         [0017]    Preferably, the shoe comprises a means for centering the injector. 
         [0018]    The shoe advantageously makes it possible to center the injector without weakening the arm or its protective heat shield. That makes it possible, advantageously still, to maintain a constant distance between the injector and the air baffle, thereby facilitating the distribution of air from the air baffle toward the injectors. 
         [0019]    Preferably, the shoe is welded to the air supply baffle, thus making it possible to avoid any translational movement of the baffle in the arm. 
         [0020]    Preferably still, the shoe takes the form of a Y defining a central branch and two lateral branches, the central branch comprising a through orifice for retaining the baffle. 
         [0021]    The shape of the shoe advantageously makes it possible to overcome differential expansions between the arm and the shoe. 
         [0022]    The through orifice for retaining the baffle advantageously makes it possible to guide and to lock the air baffle. 
         [0023]    Preferably still, the central branch comprises a through orifice for centering the injector. 
         [0024]    The invention relates to a jet engine afterburner duct comprising at least one such flame holder. 
         [0025]    The invention also relates to a jet engine comprising such an afterburner duct. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The invention will be better understood with reference to the appended drawings, in which: 
           [0027]      FIG. 1  represents a perspective view of an afterburner case with flame holders; 
           [0028]      FIG. 2  represents a sectional view of part of a turbofan engine with a flame holder according to a first prior art; 
           [0029]      FIG. 3  represents a sectional view of the lower part of a flame holder according to a second prior art; 
           [0030]      FIG. 4  represents a sectional view of a flame holder according to the invention; 
           [0031]      FIG. 5  represents a sectional view of the flame holder in  FIG. 4  in the direction II-II; 
           [0032]      FIG. 6  represents a perspective view of a spacer shoe according to the invention; 
           [0033]      FIG. 7  represents a perspective view of the shoe in  FIG. 6 , retaining an air supply baffle; 
           [0034]      FIG. 8A  represents a partial perspective view of the shoe and the baffle in  FIG. 7 , which are mounted in a flame holder; and 
           [0035]      FIG. 8B  is a close-up view of  FIG. 8A  representing the connection, produced by the shoe, between a flame holder arm and its protective heat shield. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0036]      FIG. 2  represents part of a turbofan engine. All that can be seen of this engine is the ogive shape of the exhaust case  3  at the rear of the gas turbine engine, inside the outer cylindrical casing  4 . The engine supplies a hot primary gas flow, represented by the arrow P, at the outlet of the turbine of the gas turbine engine. Some of the air sucked in by the engine is simply compressed and bypasses the combustion chamber of the engine. This air constitutes the secondary flow, represented by the arrow S. The two flows here mix downstream of a confluence section formed by an annular metal sheet. 
         [0037]    This engine section is extended, particularly in military-type aircraft, by a cylindrical afterburner duct  1  for reheating the gases that is situated ahead of the exhaust nozzle. The upstream part of the afterburner duct  1  is shown, but not the nozzle. In certain flight phases, it is necessary to supply additional energy to the gases producing the thrust. This is provided by reheating, or afterburning, the gases in the afterburner duct. Fuel is injected into the gases by injectors like those represented at  7 , radially passing through the two, primary and secondary, flow paths. Downstream of these injectors, means forming flame holders  10  are configured so as to allow the gases to be retained during their combustion. These means comprise partly rectilinear flame holders  10  arranged radially in a star pattern in a plane substantially perpendicular to the engine axis, in this case immediately downstream of the confluence of the two primary and secondary flows. They are connected by arms  9  in the form of a ring sector which, in this type of afterburner device, are on the secondary flow path side. 
         [0038]    With reference to  FIG. 4 , the radial flame holders  10  are formed by radial arms  100  in the form of gutters, with a V or U-shaped cross section, whose apex faces in the upstream direction with respect to the gas flow direction. The structure of the flame holder  10  is visible from the sectional view in  FIG. 5  in the direction II-II shown in  FIG. 4 . The arm  100  defines a cavity, between the upstream apex and the free downstream edges of the two walls  101   a  and  101   b , which is occupied by a baffle  120  which supplies air bled from the flow path for the secondary flow, a tubular fuel injector  130 , and a protective heat shield  110 . The shield  110  is in the form of a curved metal sheet whose concave side faces in the downstream direction. 
         [0039]    The air supply baffle  120 , the tubular fuel injector  130  and the protective heat shield  110  are held in the arm  100 , at the top, in the afterburner case and, at the bottom, by a spacer shoe  150  whose function is to prevent them from coming into contact, in particular during vibratory operation. The shoe  150  makes it possible to fasten the air supply baffle  120  and the injector  130  in the arm  100  without the need for piercings in thermally stressed regions of both the shield  110  and the arm  100 , thus making it possible to limit wear. 
         [0040]    In this example, the arm  100  and the tubular fuel injector  130  are directly fastened in the afterburner case. 
         [0041]    With reference to  FIG. 6 , the spacer shoe  150  substantially takes the form of a Y comprising a central branch  150   c , extending lengthwise in a direction X, from which there extend two substantially parallel lateral branches  150   a ,  150   b . Each lateral branch  150   a ,  150   b , extending in the direction X, is terminated at its end by a circular spacer lug  151   a ,  151   b  each comprising a fastening orifice  152   a ,  152   b  formed in the lateral branch  150   a ,  150   b  in a direction Y orthogonal to the direction X. 
         [0042]    The lateral branches  150   a ,  150   b  have a small thickness so as to make it possible to overcome differential expansions between the arm  100  made of CMC and the shoe  150  made of metal. This equally applies to an arm made of a metal material, with differential expansions occurring due to temperature differences between the metal components. 
         [0043]    The central branch  150   c  of the spacer shoe  150 , which is wider than the lateral branches  150   a ,  150   b , is pierced at two locations within its thickness in a direction Z. A first retaining through orifice  154  is formed at the base of the central branch  150   c  and is intended to retain an axial portion of the air supply baffle  120 . This orifice will be designated hereinafter as baffle retention orifice  154 . A second retaining through orifice  156  is formed in the central branch  150   c , between the baffle retention orifice  154  and the lateral branches  150   a ,  150   b . This orifice  156  is intended to center the tubular fuel injector  130  in the arm  100 . This orifice will be designated hereinafter as injector centering orifice  156 . 
         [0044]    Still with reference to  FIG. 6 , the spacer shoe  150  also comprises locking through orifices  155  formed within the width of the central branch  150   c  in the direction Y, orthogonally to the baffle retention orifice  154 . Thus, with reference now to  FIG. 7 , when the air supply baffle  120  is introduced into the spacer shoe  150  via its baffle retention orifice  154 , the outer surface of the baffle  120  is visible via the locking orifices  155 , the function of which will be described in detail hereinafter. 
         [0045]    The invention will be understood better still from the description of the mounting of the spacer shoe  150  in the arm  100 , as represented in  FIGS. 4 ,  5 ,  7 ,  8   a  and  8   b.    
         [0046]    With reference to  FIG. 4 , the air supply baffle  120  and the tubular fuel injector  130  are centered, at the top, in the flame holder  10  and held, at the bottom, by the spacer shoe  150 . In this example, the air baffle  120  comprises, at the top, a swiveling head enabling it to be centered in the arm. The air supply baffle  120  is introduced axially, in the direction Z, into the spacer shoe  150  via its baffle retention orifice  154  and is locked axially with the shoe  150  by welding. During the welding step, material is applied to the outer surface of the baffle  120 , which is visible via the locking orifices  155 . The air baffle  120  is then fixedly retained by the shoe  150 , as represented in  FIG. 7 . 
         [0047]    The tubular fuel injector  130  is, for its part, introduced axially in the direction Z into the injector centering orifice  156  in order to maintain it at a distance from the air supply baffle  120 . The distance between the baffle  120  and the injector can be set by adapting the spacing between the injector centering orifice  156  and the baffle retention orifice  154  of the shoe  150 . 
         [0048]    During the mounting operation, the circular gap-producing lugs  151   a ,  151   b  of the spacer shoe  150  are arranged between the walls of the protective shield  110  and the walls of the arm  100 . With reference to  FIG. 5 , the lug  151   a  is interposed between the wall  111   a  of the protective shield  110  and the wall  101   a  of the arm  100 , the thickness of the lug  151   a  defining a gap thickness (e) between said walls  101   a ,  111   a . The gap (e) forms a channel allowing carbureted air to pass through. 
         [0049]    For each spacer lug of the shoe  150 , a cylindrical stud  160  successively passes through the wall  111   a  of the shield  110 , the fastening orifice  152   a  of the lug, and the wall  101   a  of the arm  100 . The stud  160  is retained by a washer  161  welded to the outside of the arm  100 . The assembly formed by the wall  101   a  of the arm  100 , the wall  111   a  of the shield  110  and the lug  151   a  is held clamped between the welded washer  161  and the head  162  of the cylindrical stud. 
         [0050]    It goes without saying that other fastening means could also be suitable, such as those described in application FR0655241. 
         [0051]    What has been described is the mounting of a protective heat shield on a flame holder of rectilinear shape. The invention is not limited to this application. This type of mounting is also valid for mounting a protective heat shield in a flame holder in the form of a ring sector such as those connecting the radial arms.