Abstract:
The present invention is concerned with an annular combustion chamber of a gas turbine engine with an external annular wall and an internal annular wall, including an upstream fairing for separating the gas stream at the inlet of the chamber into a combustion stream and a bypass stream which bypasses the inlet of the chamber, the fairing including an annular wall forming a cap, which includes a downstream portion for fastening to a wall of the chamber and an upstream portion forming an edge of the flow cross section for the combustion stream, wherein the upstream portion is continued into at least one additional downstream portion for fastening to the wall of the chamber.

Description:
BACKGROUND OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART 
   The invention relates to a combustion chamber of a gas turbine engine with an upstream fairing for separating the gas stream, to an annular wall forming a cap of the upstream fairing of the chamber, and to a gas turbine engine with the chamber. 
   A turbojet comprises, from upstream to downstream in the direction of gas flow, a fan, one or more compressor stages, a combustion chamber, one or more turbine stages and a gas exhaust nozzle. The terms “external” and “internal” are intended to mean radially external and internal with respect to the axis of the turbojet. The terms “outer” and “inner” are intended to mean the outer side and the inner side of the combustion chamber. 
   With reference to  FIG. 1 , which represents a combustion chamber  1  of the prior art, the combustion chamber  1  is generally annular around the axis of the turbojet. It comprises, in its upstream portion, a chamber end section  2  with injection systems supplied with fuel by injectors  3  connected to a supply line  4 . The injection systems are distributed along the chamber end section  2 . The gas of the primary stream emerges upstream of the chamber  1  via a diffuser  5 , from which the gas stream is separated into a stream  6  passing into the combustion chamber  1  to allow combustion of the fuel injected by the injector  3 , referred to as combustion stream  6 , into an external bypass stream  7  which externally bypasses the inlet of the chamber  1 , and into an internal bypass stream  8  which internally bypasses the inlet of the chamber  1 . The streams  7 ,  8  which bypass the inlet of the chamber are used for cooling the chamber  1 , in particular. 
   The primary gas stream is separated at a fairing  9 . This fairing  9  comprises two parts, called an external cap  10  and an internal cap  11 . The external cap  10  takes the form of an annular metal sheet domed toward the upstream side, fastened to the combustion chamber  1  at an outer downstream surface portion  15 , and the inner upstream edge  12  of which forms a fold in the downstream direction, thus forming an aerodynamic surface for separation into an external bypass stream  7  and a combustion stream  6 . Likewise, the internal cap  11  takes the form of an annular metal sheet domed toward the upstream side, fastened to the combustion chamber  1  at an outer downstream surface portion  16 , and the inner upstream edge  13  of which forms a fold in the downstream direction, forming an aerodynamic surface for separating the internal bypass stream  8  and the combustion stream  6 . 
   The external  10  and internal  11  caps are fastened on the outer side of the external  31  and internal  32  wall, respectively, of the combustion chamber  1 , at their outer downstream surface portion  15 ,  16 , respectively, by bolts  14 . The external  10  and internal  11  caps are therefore mounted in cantilever fashion on the combustion chamber  1 . 
   The combustion chamber is subjected to vibrational stresses, particularly as a result of the combustion and the engine speed. The caps  10 ,  11  are therefore subjected to these vibrations, in particular the external cap  10 . The caps  10 ,  11  are also subjected to other dynamic excitation frequencies, in particular certain harmonic frequencies of the rotational speed of the rotating elements of the turbojet. The caps  10 ,  11 , mounted in cantilever fashion, may have resonance modes close to the aforementioned frequencies and are therefore subjected to high mechanical dynamic stresses. The caps  10 ,  11  are consequently exposed to the risks of breaking or cracking. 
   Various solutions to this problem have been proposed. 
   A first solution involves providing an annular damping ring, housed in the fold  12 ,  13  of the caps  10 ,  11  (or only in the fold of the external cap which is most subjected to the vibrational stresses); the fold  12 ,  13  is to this end wrapped around the ring so as to hold it in place. The friction caused by the presence of the ring provides an effect of damping and therefore of shifting the frequencies of the resonance modes of the caps  10 ,  11 , which enables them to be distanced from the vibrational frequencies to which the caps  10 ,  11  are subjected. However, such a device has the disadvantage of low mechanical strength. There is a risk of the ring loosening, or even breaking (on account of the vibrations to which it is subjected), which diminishes or cancels out its effectiveness. 
   A second solution involves providing an integrated fairing  9 , which will thus be termed a covering. The external  10  and internal  11  caps are then formed in a single piece, with connection tabs between them at their inner upstream edges  12 ,  13 . Such a device has two disadvantages. First, given the connecting tabs between the caps  10 ,  11 , the flow cross section for the combustion stream  6  is reduced; now it is an established fact that this cross section must be as large as possible so as to promote the flow of this stream in order to achieve better combustion efficiency. Second, it is appropriate for the cutouts between the tabs to be formed by laser cutting, these cutouts having to have the equivalent of the folds  12 ,  13  around their contour. Producing such an integrated covering is very difficult and therefore expensive. 
   SUMMARY OF THE INVENTION 
   The invention aims to overcome these disadvantages and to provide a fairing sufficiently withstanding the vibrational stresses, complying with the aerodynamic criteria of stream separation and at the same time having a maximum cross section for the flow of the combustion stream  6 , and being able to be produced simply and at low cost. 
   To this end, the invention relates to an annular combustion chamber of a gas turbine engine with an external annular wall and an internal annular wall, comprising an upstream fairing for separating the gas stream at the inlet of the chamber into a combustion stream and a bypass stream which bypasses the inlet of the chamber, the fairing comprising an annular wall forming a cap, which comprises a downstream portion for fastening to a wall of the chamber and an upstream portion forming an edge of the flow cross section for the combustion stream, wherein the upstream portion is continued into at least one additional downstream portion for fastening to the wall of the chamber. 
   By virtue of the invention, the cap-forming annular wall, which is fastened not only at its downstream fastening portion but also at the additional downstream fastening portion integral with its upstream portion, is stiffened, which increases the frequency of its resonance modes, which do not intersect with the vibration frequencies to which the flange is subjected. The cantilever effect is attenuated. Moreover, such a cap is mechanically solid, which avoids the disadvantages associated with the presence of a ring, while it allows the fairing for separating the air stream upstream of the chamber to be formed as an external cap and an internal cap, thereby providing an optimum flow cross section for the combustion stream. 
   Preferably, the downstream fastening portion of the cap is fastened on the outer side of the wall of the chamber and the additional downstream fastening portion of the cap is fastened on the inner side of the wall of the chamber. 
   Advantageously in this case, with the combustion chamber comprising a chamber end section, the additional downstream fastening portion is fastened to a flange of the chamber end section, which flange is fastened to the wall of the chamber on its inner side. 
   Preferably again, the downstream fastening portion of the cap, the combustion chamber and the additional downstream fastening portion of the cap are fastened by fastening bolts. 
   Advantageously in this case, the flange of the chamber end section is also fastened by the fastening bolts. 
   According to a first embodiment, the additional downstream fastening portion comprises a downstream wall with a downstream portion for fastening to the wall of the chamber. 
   In a particular embodiment, this downstream wall has cutouts. 
   According to a second embodiment, the additional downstream fastening portion comprises fastening tabs, extending from the upstream edge, comprising a downstream portion for fastening to the wall of the chamber. 
   According to a third embodiment, the additional downstream fastening portion comprises reinforcing tabs connected by a downstream rim which supports tabs for fastening to the wall of the chamber. 
   The invention also relates to an annular wall forming a cap of the upstream fairing of the combustion chamber presented above. 
   The invention further relates to a gas turbine engine comprising the combustion chamber presented above. 
   Preferably, the annular wall forming the external cap of the upstream fairing of the combustion chamber is the one according to the invention, because it is this wall which is most subjected to the vibrational stresses. The invention also applies to the internal cap. 
   The invention is here described in relation to a turbojet, but it goes without saying that it applies to any gas turbine engine comprising a combustion chamber. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood with the aid of the description which follows of the preferred embodiment of the fairing of the invention, with reference to the appended plates, in which: 
       FIG. 1  represents a schematic view in section of a combustion chamber of the prior art; 
       FIG. 2  represents a view in perspective and in partial schematic section of a first embodiment of the fairing of the invention, seen from the downstream direction; 
       FIG. 3  represents a view in perspective and in partial schematic section of the fairing of  FIG. 2 , fastened to a combustion chamber, seen from the downstream direction; 
       FIG. 4  represents a view in perspective and in partial schematic section of a second embodiment of the fairing of the invention, seen from the downstream direction; 
       FIG. 5  represents a view in perspective and in partial schematic section of the fairing of  FIG. 4 , fastened to a combustion chamber, seen from the downstream direction; 
       FIG. 6  represents a view in perspective and in partial schematic section of a third embodiment of the fairing of the invention, seen from the downstream direction, and 
       FIG. 7  represents a view in perspective and in partial schematic section of the fairing of  FIG. 6 , fastened to a combustion chamber and seen from the upstream direction. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the three embodiments of the fairing  9  described hereinafter, only the annular wall forming the external cap of the fairing  9  of the combustion chamber is in accordance with the invention, the annular wall forming the internal cap being in accordance with the caps of the prior art, since it is the external cap which is most subjected to the vibrational stresses. It goes without saying that provision can also be made for the internal cap to be in accordance with the invention by simply transposing the characteristics of the external cap to the internal cap. 
   In the description which follows, the elements of the turbojet which are similar will be denoted by the same references as in  FIG. 1 . In particular, for the sake of simplification, the upstream fairing  9  of the combustion chamber  1 , comprising an external cap and an internal cap, is always denoted by the reference “ 9 ”. The same applies to the internal cap  11 , which is similar to the internal cap  11  of  FIG. 1 , and to elements which are common to the various embodiments of the external caps. 
   With reference to  FIGS. 2 to 7 , and according to the three embodiments described here, the external cap  20 ,  20 ′,  20 ″ takes the form of a shaped metal sheet of constant thickness (it is thus less expensive and simpler to manufacture). This sheet may be made of any suitable material, for example the same material as the walls of the combustion chamber, in this instance a nickel- or cobalt-based alloy. 
   The external cap  20 ,  20 ′,  20 ″ comprises an upstream annular wall  21  for separating the primary gas stream into a combustion steam  6  and a bypass stream  7 , an external bypass stream here, which bypasses the inlet of the combustion chamber  1 . This upstream wall  21  has a surface similar to that of the caps of the prior art, shaped to allow good separation of the primary gas stream. 
   On its downstream and outer side, the upstream wall  21  comprises a downstream portion  22  for fastening to the external wall  31  of the combustion chamber  1 , on its outer side here. This downstream portion  22  is in this case planar and obtained by folding the sheet metal on the downstream side, in the same way as in the prior art. It comprises holes  23  for the insertion of a bolt (not shown) so that it can be fastened to the external wall  31  of the combustion chamber  1 , which comprises corresponding holes  33  for the insertion of the fastening bolts. 
   On its upstream and inner side, the upstream wall  21  of the external cap  20 ,  20 ′,  20 ″ comprises an upstream portion  24  folded in the downstream direction, termed upstream edge  24 , forming an edge of the flow cross section for the combustion stream  6 , in this instance the external edge of this cross section. This downstream portion  24  is continued, on the downstream side, into an additional portion  25  for fastening to a wall of the combustion chamber  1 , in this instance the external wall  31 , which will be referred to as the additional downstream fastening portion  25 . 
   In the first embodiment of  FIGS. 2 and 3 , the additional downstream fastening portion  25  comprises a downstream annular wall  26  which extends downstream from the upstream wall  21 . This downstream wall  26  extends from the upstream edge  24 , fixedly with the latter, in this instance in a single piece therewith. More precisely, from the upstream edge  24  folded in the downstream direction there extends a planar portion  27  and then a second, outwardly folded edge  28 , from which the downstream wall  26  extends outwardly and in the downstream direction, downstream of the upstream wall  21 . This downstream wall  26  comprises a planar downstream fastening portion  29  folded in the downstream direction, which is in this case parallel to the downstream fastening portion  22  of the cap  20  fixed to the upstream wall  21  and situated on the inside with respect to this upstream wall. The downstream fastening portion  29  of the downstream wall  26  comprises holes  30  for the insertion of a bolt (not shown) so that it can be fastened to the external wall  31  of the combustion chamber  1 , each hole being coaxial with a corresponding hole  23  of the downstream fastening portion  22  of the upstream wall  21 . 
   With reference to  FIG. 3 , the downstream fastening portion  29  of the downstream wall  26  is fastened to the external wall  31  of the combustion chamber  1 , on its inner side. More precisely, it is fastened to a flange  34  of the chamber end section  2 , on its inner side, which flange is itself fastened directly to the inner surface of the external wall  31  of the combustion chamber  1 . This flange  34  comprises corresponding holes  35  for the insertion of the fastening bolts. Each fastening bolt passes through from the outside to the inside and therefore plays a part in fastening the downstream fastening portion  22  of the upstream wall  21  of the cap  20 , the external wall  31  of the combustion chamber  1 , the flange  34  of the chamber end section  2 , and the downstream fastening portion  29  of the downstream wall  26  of the cap  20 . 
   The external cap  20  is intended to be fastened here, on the one hand, on the outer side of the external wall  31  of the combustion chamber  1 , as regards the downstream fastening portion  22  of the upstream wall  21 , and, on the other hand, on the inner side of the external wall  31  of the combustion chamber  1 , as regards the downstream fastening portion  29  of the downstream wall  26 . It goes without saying that any other arrangement can be contemplated in which the external cap  20  is fastened to the external wall  31  of the chamber  1 , on the one hand, at the downstream fastening portion  22  of its upstream wall  21 , and, on the other hand, at its additional downstream fastening portion  25  continuing its upstream edge  24 . It is in particular not necessary for the fastening of these parts also to participate in the fastening of the flange  34  of the chamber end section  2 . Fastening is performed here by means of bolts, but any other fastening method may be contemplated, for example by welding, riveting, etc. 
   In the specific case in question, the downstream wall  26  has cutouts  36  distributed along its circumference so as to reduce its mass. However, the downstream wall  26  may also be solid. In this case, the rigidity and mechanical strength of the external cap  20  are increased, while in the event of a foreign body being ingested and striking the upstream wall  21  and causing a fracture there, the downstream wall  26  can act as a safety wall. 
   By virtue of the additional downstream fastening portion  25  of the external cap  20 , fastened to the external wall  31  of the chamber  1 , the rigidity of the external cap  20  is increased, which involves shifting the frequency values of its resonance modes, which are thus distanced from the vibrational frequencies to which the external cap  20  is subjected. The external cap  20  is therefore subjected to smaller vibrational forces and moreover has greater overall strength. Its dynamic response is greater. The cantilever effects are attenuated. The aerodynamic function of separating the primary gas stream is also preserved, since the surface encountered by this stream—the upstream surface of the upstream wall  21 —is the same as for the external caps  20  of the prior art. The fairing  9  is, moreover, formed by two caps  20 ,  11 , which allows an optimum flow cross section for the combustion stream  6 . 
   In the second embodiment of  FIGS. 4 and 5 , the additional downstream fastening portion  25  comprises a plurality of fastening tabs  37 , also forming reinforcements, which extend from the upstream edge  24  of the external cap  20 ′, fixedly with the latter, in this instance in a single piece therewith. More precisely, from the upstream edge  24  folded in the downstream direction there extends a planar portion  27  and then a second edge  28  folded symmetrically to the upstream edge  24 , from which the tabs  37  extend outwardly and in the downstream direction, downstream of the upstream wall  21  of the external cap  20 ′. 
   The tabs  37  are uniformly angularly distributed along the circumference of the second edge  28 , or downstream inner edge  28 , in line with the holes  23  in the downstream fastening portion  22  of the external cap  20 ′ fixed to its upstream wall  21 . Each fastening tab  37  comprises a planar downstream fastening portion  38  folded in the upstream direction, which in this case is parallel to the downstream fastening portion  22  of the upstream wall  21  of the cap  20 ′ and situated on the inside with respect to this upstream wall. The downstream fastening wall  38  of each fastening tab  37  comprises a hole  39  for the insertion of a bolt (not shown) so that the tab  37  can be fastened to the external wall  31  of the combustion chamber  1 , this hole being coaxial with a corresponding hole  23  in the downstream fastening portion  22  of the upstream wall  21 . 
   With reference to  FIG. 5 , the fastening tabs  37  are fastened, at their downstream fastening portion  38 , to the external wall  31  of the combustion chamber  1 , on its inner side. More precisely, they are fastened to the flange  34  of the chamber end section  2 , on its inner side, which flange is itself fastened directly to the inner surface of the external wall  31  of the combustion chamber  1 . This flange  34  comprises corresponding holes  35  for the insertion of the fastening bolts. Each fastening bolt passes through from the outside to the inside and therefore plays a part in fastening the downstream fastening portion  22  of the upstream wall  21  of the cap  20 ′, the external wall  31  of the combustion chamber  1 , the flange  34  of the chamber end section  2 , and the downstream fastening portion  38  of the fastening tabs  37  of the cap  20 ′. 
   The external cap  20 ′ is intended to be fastened here, on the one hand, on the outer side of the external wall  31  of the combustion chamber  1 , as regards the downstream fastening portion  22  of the upstream wall  21 , and, on the other hand, on the inner side of the external wall  31  of the combustion chamber  1 , as regards the downstream fastening portion  38  of the fastening tabs  37 . It goes without saying that any other arrangement can be contemplated in which the external cap  20 ′ is fastened to the external wall  31  of the chamber  1 , on the one hand, at the downstream fastening portion  22  of its upstream wall  21 , and, on the other hand, at its additional downstream fastening portion  25  continuing its upstream edge  24 . It is in particular not necessary for the fastening of these parts also to participate in the fastening of the flange  34  of the chamber end section  2 . Fastening is performed here by means of bolts, but any other fastening method can be contemplated, for example by welding, riveting, etc. 
   Again, by virtue of the additional downstream fastening portion  25  of the external cap  20 ′, fastened to the external wall  31  of the chamber  1 , the rigidity of the external cap  20 ′ is increased and the cap  20 ′ is less subjected to the vibrational stresses. Moreover, it has greater strength and its dynamic response is greater. The cantilever effects are attenuated. The aerodynamic function of separating the primary gas stream is also preserved, with an optimum flow cross section for the combustion stream  6 . It will be noted that the discrete distribution of the fastening tabs  37  makes it possible for the external cap  201  to be fitted more simply by comparison with the first embodiment in which the downstream fastening portion  29  is continuous. However, the rigidity of the additional downstream fastening portion  25  is less than in the first embodiment. 
   In the third embodiment of  FIGS. 6 and 7 , the additional downstream fastening portion  25  comprises a plurality of tabs  40 , forming reinforcements, which extend from the upstream edge  24  of the external cap  20 ″, fixedly with the latter, in this instance in a single piece therewith, these tabs being interconnected at their downstream outer end by an annular rim  41  bearing a plurality of fastening tabs  42 , or scallops, extending in the upstream direction. More precisely, from the upstream edge  24  folded in the downstream direction there extends a planar portion  27  and then a second edge  28  folded symmetrically to the upstream edge  24 , from which the tabs  40  extend outwardly and in the downstream direction, downstream of the upstream wall  21  of the external cap  20 ″. At their downstream end, the tabs  40  bear, and are connected by, an annular rim  41  folded in the upstream direction. This annular rim  41  bears the plurality of fastening tabs  42 , which are planar and extend in the upstream direction, these tabs in this case being parallel to the downstream fastening portion  22  of the upstream wall  21  of the cap  20 ″ and being situated on the inside with respect to this portion. 
   The reinforcing tabs  40  are uniformly angularly distributed along the circumference of the second edge  28 . Each fastening tab  42  is situated angularly between two reinforcing tabs  40 , in this instance equidistantly from these reinforcing tabs  40 , and is situated in line with a hole  23  in the downstream fastening portion  22  of the upstream wall  21 . Each fastening tab  42  comprises a hole  43  for the insertion of a bolt (not shown) so that the tab  42  can be fastened to the external wall  31  of the combustion chamber  1 , this hole being coaxial with a corresponding hole  23  in the downstream fastening portion  22  of the upstream wall  21 . 
   With reference to  FIG. 7 , the fastening tabs  42  are fastened to the external wall  31  of the combustion chamber  1 , on its inner side. More precisely, they are fastened to the flange  34  of the chamber end section  2 , on its inner side, which flange is itself fastened directly to the inner surface of the external wall  31  of the combustion chamber  1 . This flange  34  comprises corresponding holes  35  for the insertion of the fastening bolts. Each fastening bolt passes through from the outside to the inside and therefore plays a part in fastening the downstream fastening portion  22  of the upstream wall  21  of the cap  20 ″, the external wall  31  of the combustion chamber  1 , the flange  34  of the chamber end section  2 , and the fastening tabs  42  of the cap  20 ″. 
   The external cap  20 ″ is intended to be fastened here, on the one hand, on the outer side of the external wall  31  of the combustion chamber  1 , as regards the downstream fastening portion  22  of the upstream wall  21 , and, on the other hand, on the inner side of the external wall  31  of the combustion chamber  1 , as regards the fastening tabs  42 . It goes without saying that any other arrangement may be contemplated in which the external cap  20 ″ is fastened to the external wall  31  of the chamber  1 , on the one hand, at the downstream fastening portion  22  of its upstream wall  21 , and, on the other hand, at its additional downstream fastening portion  25  continuing its upstream edge  24 . It is in particular not necessary for the fastening of these parts also to participate in the fastening of the flange  34  of the chamber end section  2 . Fastening is performed here by means of bolts, but any other fastening method can be contemplated, for example by welding, riveting, etc. 
   Again, by virtue of the additional downstream fastening portion  25  of the external cap  20 ″, fastened to the external wall  31  of the chamber  1 , the rigidity of the external cap  20 ″ is increased and the cap  20 ″ is less subjected to the vibrational stresses. Moreover, it has greater strength and its dynamic response is greater. The cantilever effects are attenuated. The aerodynamic function of separating the primary gas stream is also preserved, with an optimum flow cross section for the combustion stream  6 . 
   It will be noted that this third embodiment is, as it were, intermediate between the first two embodiments, with fastening provided by discretely distributed tabs  42 , which facilitates fitting of the cap  20 ″, but with a more rigid structure than in the case of the second embodiment on account of the rim  41  connecting the reinforcing tabs  40 . The alternating angular arrangement of the reinforcing tabs  40  and the fastening tabs  42  affords better distribution of the forces. 
   It is possible to envision other embodiments in which the external cap comprises a downstream portion  22 , fixed to its upstream wall  21 , for fastening to the external wall  31  of the combustion chamber  1 , and an additional downstream fastening portion  25 , fixedly continuing the upstream edge  24  of its upstream wall  21 , for fastening to this same external wall  31 .