Abstract:
An annular combustion chamber for a turbomachine is disclosed. The combustion chamber includes radially internal and radially external cylindrical walls which are fixed by bolting at their upstream ends to an internal and an external annular flange of an annular chamber end wall, and an annular fairing extending in the upstream direction from the chamber end wall. The internal and external annular ends are fixed by bolting to the flanges of the chamber end wall in axial alignment with the annular ends of the walls of the chamber.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an annular combustion chamber for a turbomachine, such as an airplane turbojet or turboprop engine. 
     An annular combustion chamber of a turbomachine comprises two coaxial cylindrical walls connected at their upstream ends to a very rigid annular chamber end wall and comprising, at their downstream ends, flanges for fixing to casings of the turbomachine. It also comprises an upstream annular fairing fixed to the chamber end wall and intended to direct the stream of air entering or bypassing the combustion chamber. 
     DESCRIPTION OF THE PRIOR ART 
     In the known art, the upstream part of the combustion chamber is assembled by superposing the radially internal and external downstream ends of the fairing with, respectively, the radially internal and external upstream ends of the cylindrical walls of the chamber, the assembly being fixed by bolting or welding onto respectively radially internal and external annular flanges of the chamber end wall. A bolted connection is generally preferred because the maintenance operations performed on the combustion chamber are then simpler and less expensive than is the case with a welded connection. 
     Assembling the upstream part of the combustion chamber by radially superposing the ends of the fairing and the cylindrical walls with the chamber end wall flanges results in a build-up of manufacturing tolerances which are generally large in the case of parts exhibiting symmetry of revolution and in the stiffnesses of each of the parts being combined with one another. As a result, a screw/nut fastening system has to be tightened firmly enough to compensate for the sum of these tolerances and the sum of these forces, and such a level of tightening may exceed the acceptable limit for the screws and/or lead to plastic deformation of the fairing and of the cylindrical walls in particular, thus reducing the mechanical integrity and the life of the combustion chamber. Deformation of the parts may also cause gaps to appear between the fairing and the walls, thus creating air leaks. Furthermore, when the turbomachine is running, the chamber is subjected to high levels of vibration which may cause slippage of the parts (fairing, chamber walls and end wall) relative to one another if fasteners are lost. 
     When the tightening torque is unable to compensate for the stiffnesses and mounting tolerances of the parts, the parts cannot be mated together correctly, and this means that the necessary reactions between the parts will be insufficient to transfer, through friction, the forces that pass through the turbomachine when it is in operation. This means that the parts will be able to slip more easily. Turbomachine vibrations may then damage the bolted connection, particularly the screws, leading to an increased loss of fasteners and destruction of the parts starting from the joint. 
     In order to obtain the flexibility needed for a good mechanical joint in the combustion chamber upstream end parts, it has been proposed to produce axial slots in the ends of the fairing between the fixing bolts. However, these slots give rise to additional airflows around the chamber, disrupting the airflow and therefore the overall operation of the turbomachine. In addition, the ends of these slots are sensitive to turbomachine vibrations, thus weakening the fairing. 
     In another technique, the parts of the fairing, the superposed ends of the cylindrical walls and of the chamber end wall have complementary undulating surfaces, the fixings being fastened in the clefts of the undulations. This known solution makes it easier to mate the parts together but creates deformations upon tightening with a risk of air leaks. 
     SUMMARY OF THE INVENTION 
     A subject of the present invention is a combustion chamber for a turbomachine which avoids the aforementioned disadvantages of the prior art in a simple, effective and economical way. 
     To this end, the invention proposes an annular combustion chamber for a turbomachine, comprising two cylindrical walls these being respectively radially internal and radially external with respect to the axis of the turbomachine and fixed by bolting at their upstream ends to an internal annular flange and an external annular flange of an annular chamber end wall, and an annular fairing extending in the upstream direction from the chamber end wall, wherein the internal and external downstream annular ends of the fairing are fixed by bolting respectively to the internal and external annular flanges of the chamber end wall in axial alignment with the annular ends of the internal and external walls of the chamber. 
     The upstream end of the combustion chamber is thus assembled by radially superposing two parts rather than three parts, thus reducing the combined stiffness and the build-up of manufacturing tolerances. The tightening torque that needs to be applied to the bolts can be optimized and the radial deformations of the chamber when the fairing and the walls respectively are fixed to the end wall are reduced. 
     According to another feature of the invention, the aligned ends of the fairing and of the cylindrical walls of the combustion chamber have complementary indentations or undulations which fit into one another and through which fixing bolts for connecting to the chamber end wall pass. 
     These indentations or undulations give the fairing and the cylindrical walls a certain radial flexibility making them easier to fix to the end wall. Furthermore, the risk of the parts sliding relative to one another if the fixing bolts break is greatly reduced by the use of fairing and wall shapes that complement each other and by the independent fixings of the fairing and of the walls to the chamber end wall. 
     The indentations or undulations of the ends of the fairing and of the cylindrical walls comprise an alternation of solid parts and hollow parts, the fixing bolts passing through the solid parts and being distributed in an annular row on the external annular end of the fairing and on the corresponding end of the external wall of the chamber, and an annular row on the internal annular end of the fairing and on the corresponding end of the internal wall of the chamber. 
     Arranging the bolts in an internal annular row and an external annular row makes it possible to reduce the axial space occupied. 
     Advantageously, the fixing bolts which connect the external annular end of the fairing and the annular end of the external wall are angularly offset with respect to the fixing bolts which connect the internal annular end of the fairing and the annular end of the internal wall. 
     Thus, the configuration is such that a fixing bolt for the external downstream end of the fairing is not radially aligned with a fixing bolt for the internal downstream end of the fairing. An offset such as this makes it possible to avoid forming lines of radial deformation between the internal and external fixing bolts, thus contributing toward improving the rigidity of the combustion chamber and toward limiting the risk of resonance which could cause cracks to spread under the effect of vibration. 
     In one embodiment of the invention, each solid part of the indentations or undulations comprises a single fixing bolt through-hole. 
     In another embodiment of the invention, the solid parts of the indentations or undulations of the ends of the fairing comprise the same number of fixing bolts as the solid parts of the indentations or undulations of the ends of the walls of the chamber. 
     In an alternative form of embodiment, the solid parts of the indentations or undulations of the ends of the fairing comprise a number of fixing bolts that differs from that of the solid parts of the indentations or undulations of the ends of the walls of the chamber. 
     The annular fairing may be made of a single piece or of two annular pieces these respectively being a radially internal and a radially external piece. 
     The invention also relates to a turbomachine such as an airplane turbojet or turboprop engine and which comprises an annular combustion chamber of the type described hereinabove. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and features of the invention will become apparent from reading the following description which is given by way of nonlimiting example and with reference to the attached drawings in which: 
         FIG. 1  is a partial schematic half view in axial section of a turbojet engine combustion chamber according to the prior art; 
         FIG. 2  is a partial schematic view in axial section illustrating the assembly of the upstream end of the combustion chamber according to the prior art; 
         FIG. 3  is a partial schematic view in axial section illustrating the assembly according to the invention; 
         FIG. 4  is a partial perspective view of one embodiment of a combustion chamber according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference is made first of all to  FIG. 1  which is a schematic half view of an annular combustion chamber  10  according to the prior art of the invention, viewed in section on the axis of rotation  12  of the turbomachine. 
     The combustion chamber  10  is supplied with air by a diffuser  14  mounted at the exit of a high-pressure compressor  16 . It comprises a radially internal cylindrical wall  18  and a radially external cylindrical wall  20  which are connected upstream to an annular chamber end wall  22  and downstream to casings  24  and  26  by means of an internal annular flange  28  and an external annular flange  30 , respectively. 
     The chamber end wall  22  comprises holes  36  through which air from the diffuser  14  and fuel sprayed by injectors  34  borne by the external casing  26  can pass. Each injector  34  comprises a head  38  mounted on the chamber end wall and aligned with the axis  40  of a hole  36 . An annular fairing  60  which extends in the upstream direction and comprises through-holes  44  for the passage of air and of the injectors is fixed to chamber end wall flanges  22  with the ends of the cylindrical walls  18  and  20  of the combustion chamber. 
     In the known art depicted in  FIG. 2 , the upstream part of the combustion chamber is assembled by inserting the internal  46  and external  48  ends of the cylindrical walls between, on the one hand, the internal  50  and external  52  annular ends of the fairing and, on the other hand, the internal  54  and external  56  annular flanges of the chamber end wall. These three parts thus superposed are fixed together using bolts  42 , which results in a build-up of manufacturing tolerances and causes the stiffnesses to be combined with one another. 
     According to the invention, these disadvantages are avoided by virtue of the fact that, as depicted in  FIG. 3 , the internal  50  and external  52  downstream ends of the fairing are aligned respectively with the annular ends of the internal  46  and external  48  walls of the combustion chamber and are bolted to the flanges of the end wall independently of the ends of the walls  46  and  48 . 
     The upstream part of the chamber is thus assembled by a radial superposition of two parts rather than three parts. As a result, the impact that the build-up of manufacturing tolerances and the combination of respective stiffnesses of the fairing, of the walls and of the end wall has is lower thus making it easier to assemble the chamber and improving the mechanical integrity of the assembly. The tightening torque that has to be applied to the fixing bolts that connect the fairing to the flanges of the end wall can be optimized to account solely for the stiffnesses and manufacturing tolerances of the fairing and of the end wall. Likewise, in the fixing of the cylindrical walls to the chamber end wall flanges, only the stiffnesses and manufacturing tolerances of the walls and of the end wall of the chamber are taken into consideration. This assembly makes it possible to limit the radial deformations of the fairing and of the cylindrical walls and avoid the formation of additional air leaks which disturb combustion and airflow. 
     In the exemplary embodiment depicted in  FIGS. 3 and 4 , the upstream ends of the internal  18  and external  20  cylindrical walls have undulations or indentations formed by an alternation of hollow parts  62  and solid parts  48  which run in line with these walls. Likewise, the internal  50  and external  52  downstream ends of the fairing have undulations formed by an alternation of hollow parts  64  and solid parts  50 . The hollow parts  62  and the solid parts  48  of the cylindrical walls engage in the solid parts  50  and hollow parts  64 , respectively, of the annular fairing. These undulations give the cylindrical walls and the fairing some radial flexibility making them easier to fix to the end wall. The use of complementary shapes at the ends of the fairing and on the cylindrical walls and nesting them together allows the chamber better to withstand the vibrations of the turbomachine. 
     The fixing bolts on the chamber end wall pass through the solid parts of the undulations and are distributed in an external annular row and an internal annular row. The external annular row is formed by an alternation of fixing bolts  66  connecting the external cylindrical wall to the chamber end wall flange  56  and of fixing bolts  68  connecting the external upstream annular end of the fairing to this flange. Likewise, the internal annular row of bolts is formed by an alternation of fixing bolts  70  connecting the internal cylindrical wall and of fixing bolts  72  connecting the internal upstream annular end of the fairing to the chamber end wall flange  54 . 
     Advantageously, the fixing bolts  68  for connecting the external annular end  52  of the fairing are angularly offset by one spacing with respect to the fixing bolts  72  for fixing the internal annular end of the fairing, and the bolts  66  and  72 , and the bolts  68  and  70 , are radially aligned. This method of attachment with an angular offset has the advantage of stiffening the combustion chamber as a whole while at the same time preventing the formation of lines of deformation between an internal bolt  72  and an external bolt  68  if these were diametrically opposed. The natural frequencies of vibration are thus higher making it possible to eliminate the risks of cracks spreading under the effect of vibration. 
     In the embodiment depicted in  FIG. 3 , each solid part of the indentations or undulations has a single through-hole for a fixing bolt. 
     In alternative forms of embodiment which have not been depicted, the solid parts of the undulations at the ends of the fairing have either the same number, for example 2, or a different number of fixing bolts, as the solid parts of the undulations at the ends of the walls of the chamber. 
     The annular fairing may be made as a single piece or alternatively be made as two annular pieces, these being a radially internal and a radially external piece. 
     The invention is not restricted to the combustion chambers described hereinabove and can be applied in general to all types of combustion chamber such as, for example, those designed to accept a number of injector heads arranged in concentric rings.