Abstract:
Combustion chamber for a turbomachine, comprising a chamber endwall equipped with fuel injection means and axisymmetric walls connecting the chamber endwall to internal and external flanges for fastening to internal and external casings, these flanges including annular portions pierced by orifices of triangular or approximately triangular shape for the flow of air for cooling downstream components.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a combustion chamber for a turbomachine, such as an aircraft turbojet or turboprop.  
       DESCRIPTION OF THE PRIOR ART  
       [0002]     Such a combustion chamber comprises a chamber endwall, which carries fuel injection systems and is formed with air intake orifices, this chamber endwall being joined by internal and external axisymmetric walls to internal and external annular flanges for fastening to internal and external casings of the turbomachine, via means of the bolt-nut type or the like.  
         [0003]     In operation, a portion of the air stream delivered by a feed compressor for the combustion chamber must bypass the combustion chamber in order to cool components located downstream of this chamber, and to do this it passes through orifices formed in the internal and external flanges of the chamber.  
         [0004]     These orifices are machined in annular portions of the flanges centered on the axis of the chamber and are usually rectangular or circular in shape.  
         [0005]     Orifices of rectangular shape make it possible to further reduce the mass and provide a larger air flow area. However, these flanges formed with rectangular orifices have an inferior resistance to vibrational stresses, while flanges having circular orifices have a better vibration resistance, but they provide a lower air flow area with an increase in pressure drop in the flow of air for cooling the components located downstream.  
         [0006]     The object of the present invention is in particular to provide a simple, effective and inexpensive solution to this problem, by combining the advantages of these two known solutions, while avoiding their drawbacks.  
       SUMMARY OF THE INVENTION  
       [0007]     The subject of the invention is, for this purpose, a combustion chamber for a turbomachine, such as an aircraft turbojet or turboprop, comprising a chamber endwall equipped with fuel injection means and axisymmetric walls connecting the chamber endwall to internal and external flanges for fastening to internal and external casings, these flanges having annular portions pierced by orifices for the flow of air for cooling downstream components, wherein the orifices of at least one of the flanges are of triangular or approximately triangular shape.  
         [0008]     The invention provides the or each flange formed with triangular orifices with a large air flow area and with a low mass, like a flange having rectangular orifices, while still having good vibration resistance and good circumferential stiffness, like a flange with circular orifices.  
         [0009]     The shapes and dimensions of the triangular orifices and their arrangement in the annular portions of the flanges are optimized in order for the following objectives to be effectively met: 
        a maximum air flow area and a minimum pressure drop;     good resistance to vibrational stresses and good circumferential stiffness;     a reduced flange mass; and     simple and easy machining of the orifices in the flange.        
 
         [0014]     The triangular orifices may be isosceles triangles, equilateral triangles or triangles of any shape, and they preferably each have rounded apexes so as to reduce the stress concentrations at the apexes of the triangular orifices.  
         [0015]     Advantageously, the successive triangular orifices are arranged in a staggered and head-to-tail configuration so as to optimize the air flow area and to reduce the mass. In this configuration, the triangular orifices are separated from one another by a strip of material that extends in a zig-zag fashion between the internal and external edges of the annular portion of the flange. This makes it possible to increase the circumferential stiffness of the flange relative to a flange pierced by rectangular orifices, in which flange the strips of material separating the orifices extend radially between the internal and external edges of the annular flange portion.  
         [0016]     In one embodiment of the invention meeting the aforementioned objectives, the triangular orifices of one flange comprise two rows of identical triangular orifices, in which the orifices each have a base located on a circumference centered on the axis of the chamber, the orifices being reversed from one row to another and the two rows being partially imbricated one in the other.  
         [0017]     In order for the flange formed with triangular orifices to have a sufficient circumferential stiffness, the circumferential distance between the bases of the triangular orifices of any one row is equal to or greater than 1.5 times the width of the strip of material that separates two successive orifices.  
         [0018]     The degree of partial imbrication of the two rows is determined by the ratio H 2 /H 1 , where H 1  is the distance between the base of a triangular orifice and its apex and H 2  is the radial distance between the apexes of two successive orifices, opposite their bases, this ratio H 2 /H 1  being equal to or greater than 0.6 in order to optimize the air flow area.  
         [0019]     The length of the straight portion of the strip of material between the rounded apexes of two successive orifices must be equal to or less than three times the width of this strip so that the flange has good mechanical strength in bending.  
         [0020]     The angle at the apex of the triangular orifices, opposite their bases, must be equal to or greater than 20°.  
         [0021]     The bases of the orifices are separated from the internal and external edges of the annular flange portion by strips of material having a circumferential orientation, the radial widths of which are defined by a=d and b=H 2 , where a is the radial width of the strip of material between the base of a triangular orifice of the outer row and the external edge of the annular flange portion, d is the width of the strip of material separating successive orifices, b is the radial width of the strip of material between the base of a triangular orifice of the inner row and the internal edge of the annular flange portion, and H 2  is the radial distance between the apexes of two successive orifices, opposite their bases.  
         [0022]     The invention also relates to a turbomachine, such as an aircraft turbojet or turboprop, which includes a combustion chamber of the type described above. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The invention will be more clearly understood and other features, details and advantages thereof will become more clearly apparent on reading the following description, given by way of nonlimiting example and with reference to the appended drawings in which:  
         [0024]      FIG. 1  is a schematic half-view, in axial section, of a diffuser and of a combustion chamber of a turbomachine;  
         [0025]      FIG. 2  is a partial schematic view, in perspective, of a combustion chamber of a turbomachine according to one embodiment of the invention;  
         [0026]      FIG. 3  is a partial front view of the combustion chamber of  FIG. 2 ; and  
         [0027]      FIG. 4  is a partial front view of the internal annular flange of the combustion chamber of  FIG. 2 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     In  FIG. 1 , the combustion chamber  10  is placed at the outlet of a diffuser  12 , which is itself located at the outlet of a compressor (not shown), and comprises an axisymmetric internal wall  14  and an axisymmetric external wall  16  connected upstream to an annular chamber endwall  17  and fastened downstream by internal and external annular flanges  18 , and  20  respectively to an internal frustoconical shell  22  of the diffuser and to a downstream end of an external casing  24  of the chamber, the upstream end of this casing  24  being fastened to an external frustoconical shell  26  of the diffuser.  
         [0029]     The chamber endwall  17  includes orifices  30  through which air from the diffuser  12  and fuel fed by injectors  32  passes, said injectors being fastened to the external casing  24  and uniformly distributed around a circumference about the longitudinal axis A of the chamber. Each injector  32  comprises a fuel injection head  34  mounted on the chamber endwall  17  and aligned with the axis  36  of one of the orifices  30  of this endwall. An annular shroud  38 , curved toward the upstream end, is fastened to the upstream ends of the walls  14 ,  16  and  17  of the chamber and includes air flow orifices  40  aligned with the orifices  30  in the chamber endwall  17 .  
         [0030]     One portion of the air stream delivered by the compressor and exiting the diffuser  12  passes through the orifices  40  and  30  and feeds the combustion chamber  10 , the other portion of the air stream feeding the internal  42  and external  44  annular channels for bypassing the combustion chamber  10 .  
         [0031]     The internal channel  42  is formed between the internal shell  22  of the diffuser  12  and the internal wall  14  of the chamber, and the air that passes into this channel is divided into a stream  46 , which penetrates the chamber  10  via holes  48  in the internal wall  14 , and into a stream  50 , which passes through the orifices  52  in a frustoconical annular portion of the internal flange  18  so as to cool components (not shown) located downstream of this chamber.  
         [0032]     The external channel  44  is formed between the external casing  24  and the external wall  16  of the chamber, and the air that passes into this channel is divided into a stream  56 , which penetrates the chamber  10  via holes  54  in the external wall  16 , and into a stream  58  that passes through the orifices  60  in a U-shaped curved annular portion of the external flange  20  so as to cool components downstream.  
         [0033]     The air flow holes  48  and  54  in the chamber are dilution holes, called primary holes, and holes formed by multiperforations.  
         [0034]     In the known technique, the orifices  52 ,  60  of the flanges  18 ,  20  are of rectangular or circular shape and do not allow the cooling air flow area and the mass of the flanges to be optimized, while still maintaining good resistance to vibrational stresses and good circumferential stiffness and limiting the pressure drops of the air flow through the orifices in the flanges.  
         [0035]     In the combustion chamber according to the invention shown in FIGS.  2  to  4 , the internal flange  18  includes, in its frustoconical annular portion, orifices  62 ,  64  of triangular shape that are arranged in two mutually imbricated coaxial annular rows.  
         [0036]     The triangular orifices  62  of the inner row are isosceles triangles and each has a base  66 , located on a circumference centered on the axis A of the chamber, and rounded apexes  68 , their apex opposite their base being directed toward the outside of the chamber.  
         [0037]     The triangular orifices  64  of the outer row are identical to the orifices  62  and are arranged in a staggered and inverted or head-to-tail configuration with the orifices  62 , that is to say the base  66  of each orifice  64  is located on an outer circumference centered on the axis A of the chamber and their apex opposite their base is directed toward the inside of the chamber.  
         [0038]     The two rows of triangular orifices  62 ,  64  are at least partly imbricated in one another and the orifices  62 ,  64  are separated from one another by a strip of material  70  that extends in a zig-zag manner between the internal edges  72  and external edges  74  of the annular flange portion.  
         [0039]     Parameters have been defined that characterize the shapes and the dimensions of the orifices  62 ,  64  and also their relative arrangement, which make the axial and circumferential stiffnesses of the flange  18  vary. These parameters have been optimized for the flange  18  so that the latter can provide a large air flow area while still maintaining good vibratory strength and good circumferential stiffness and limiting the pressure drops as the air flows through the orifices  62 ,  64 .  
         [0040]     The angle α at the apex of the triangular orifices opposite their bases must preferably be equal to or greater than 20°.  
         [0041]     The strip of material  70  that extends in a zig-zag manner between the internal  72  and external  74  edges of the annular flange wall includes straight portions that separate the parallel sides of the successive orifices  62 ,  64  and connect the rounded apexes  66  of the orifices, opposite their bases. Denoting the width and the length of these straight portions by d and L, respectively, it is preferable for the length L to be equal to or less than three times the distance d so that the flange has good mechanical strength in bending.  
         [0042]     The circumferential distance between the bases of the successive triangular orifices of any one row is denoted by c. In the example shown, this distance is identical for the inner and outer rows of triangular orifices. This distance c must preferably be equal to or greater than 1.5 times the aforementioned distance d so that the flange retains sufficient circumferential stiffness.  
         [0043]     H 1  denotes the height of a triangular orifice  62 ,  64 , that is to say the distance between the base  66  of an orifice and the apex  68  opposite this base, and H 2  denotes the radial distance between the apexes  68  of two successive orifices  62 ,  64  opposite their bases. To optimize the air flow area, it is necessary for the degree of partial imbrication of the two rows, determined by the ratio H 2 /H 1 , to be equal to or greater than 0.6.  
         [0044]     The inner and outer rows of orifices are separated from the internal  72  and external  74  edges of the annular portion of the flange by circumferentially oriented strips of material  76 ,  78 .  
         [0045]     The radial widths of the strips of material  76  and  78  located between the base  66  of an orifice  64  of the outer row and the external edge  74  of the annular portion, and between the base  66  of an orifice  62  of the inner row and the internal edge  72  of the annular portion, are denoted by a and b respectively. The width a must be equal to or greater than the aforementioned distance d and the width b must be equal to or less than the aforementioned distance H 2  so that the flange  18  has sufficient flexibility to damp the thermal expansions and deformations in operation.  
         [0046]     The external flange  20  may also be pierced by triangular orifices  62 ,  64  like the internal flange, or it may be the only flange to include triangular orifices.  
         [0047]     Moreover, the triangular orifices in the flanges may be of any shape, but still being arranged in a staggered and head-to-tail configuration in order to reduce the mass and the pressure drop.  
         [0048]     The triangular orifices in the flanges can be produced by milling cutting or by laser cutting.