Abstract:
The present invention relates to a gas turbine engine diffuser ( 30 ) defined between an external casing ( 32 ) and an internal casing ( 34 ) of said engine and supplied with air via an upstream annular diffuser duct ( 36 ), comprising a combustion chamber ( 10 ) of the convergent type, forming an external annular duct ( 28 ) with the external casing ( 32 ) and an internal annular duct with the internal casing ( 34 ), which diffuser comprises a cowling partially closing off the external annular duct. More specifically, the cowling is positioned toward the closed end of the combustion chamber.

Description:
BACKGROUND OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART 
       [0001]    The present invention relates to the technical field of combustion chambers for gas turbine engines such as turbojet engines. It is aimed in particular at a diffuser comprising a cowling on the combustion chamber. 
         [0002]    In everything which follows, the terms “axial”, “radial” and “transverse” correspond respectively to an axial direction, a radial direction, and a transverse plane of the turbojet engine and the terms “upstream” and “downstream” correspond respectively to the direction in which the gases flow through the turbojet engine. 
         [0003]    A conventional combustion chamber known as a divergent combustion chamber is illustrated in  FIG. 10  which is an axial cross section showing half of the combustion chamber, the other half thereof being symmetric therewith respect to the axis (not depicted) of the engine. The combustion chamber  110  is contained in a diffuser  130  which is an annular space defined between an external casing  132  and an internal casing  134 , into which a compressed oxidant originating upstream from a compressor (not depicted) is introduced via an annular diffuser duct  136 . 
         [0004]    This conventional combustion chamber known as a divergent combustion chamber  110  has an external wall  112  and an internal wall  114  which are coaxial and substantially conical, and which widen in the direction from upstream to downstream at a cone angle α 1 . The external  112  and internal  114  walls of the combustion chamber  110  are connected to one another toward the upstream end of the combustion chamber via a chamber end wall  116 . 
         [0005]    The chamber end wall  116  is provided with injection systems  118  through which injectors  120  which introduce fuel into the combustion chamber  110  in which combustion reactions occur pass. 
         [0006]    These combustion reactions are intended to cause heat to radiate from the downstream to upstream direction toward the chamber end wall  116 . In order to prevent damage to this chamber end wall  116  as a result of the heat, heat shields also known as deflectors  122  are provided, these being positioned on an interior face of the chamber end wall  116 . They are cooled using jets of cooling air which enter the combustion chamber  110  through cooling orifices  124  pierced in the chamber end wall  116 . These air jets, which flow in the direction from upstream to downstream, are guided by a chamber cowling  126 , pass through the chamber end wall  116  through the cooling orifices  124  and impinge on an upstream face of the deflectors  122 . The cowling  126  is also used to guide the air supplied to the injection systems  118 . It has a substantially semi-toric shape and extends between two concentric edges for attachment to the edges of the chamber wall  116 . A central portion of the cowling  126  is open to allow the fuel injection pipes to run as far as the injectors  120 . The openings may be a substantially circular single slot. In this case, the cowling  126  is made up of two flanks known as fairings. As an alternative, the openings may consist of a collection of apertures each leading to a group of injectors. 
         [0007]    In more recent designs of combustion chamber known as convergent combustion chambers, the external and internal walls of the combustion chamber are inclined such that they widen in the direction from downstream to upstream rather than from upstream to downstream as was the case in the “divergent” conventional combustion chambers described hereinabove. 
         [0008]    A “convergent” combustion chamber  10  such as this is illustrated in part in  FIG. 11 , in axial section. This  FIG. 10  shows an axial direction  100  parallel to the axis of the turbojet engine, a generatrix direction  200  of the combustion chamber  10 , and a cone angle α 2  between these two axes  100 ,  200 . The combustion chamber  10  comprises an external wall  12  and an internal wall  14  which are coaxial and substantially frustoconical, and which widen in the direction from downstream to upstream at a cone angle α 2 . 
         [0009]    The external  12  and internal  14  walls of the combustion chamber  10  are connected to one another toward the upstream end of the combustion chamber by a chamber end wall  16  which is a substantially frustoconical part running between two substantially transverse planes and widening in the direction from upstream to downstream. The chamber end wall  16  is connected to each of the two, external  12  and internal  14 , walls of the combustion chamber  10 . It is provided with injection systems  18  through which injectors  20  pass these passing through the outer casing  32  and introducing fuel into the combustion chamber  10  where the combustion reactions take place. 
         [0010]    The combustion chamber  10  is contained in a diffuser  30  which is an annular space defined between an external casing  32  and an internal casing  34  and into which a compressed oxidant originating upstream from a centrifugal compressor (not depicted) is introduced via an annular diffuser duct  36 . The oxidant is generally air. The combustion chamber  10  is positioned right into the diffuser  30  between an external part  28  and an internal part  29  of this diffuser  30 . The external part  28  of the diffuser  30  constitutes an annular and conical space contained between the external casing  32  and the external wall  12  of the combustion chamber  10 . The internal part  29  of the diffuser  30  constitutes an annular and conical shape contained between the internal casing  34  and the internal wall  14  of the combustion chamber  10 . 
         [0011]    Some of the oxidant, generally air, enters the diffuser  30  followed by the combustion chamber  10  to participate in the combustion reactions taking place therein. The entry of oxidant to the combustion chamber  10  is guided by the cowling  226 . Some more of the oxidant flows into the diffuser  30 , bypassing the combustion chamber  10 , on the one hand through an external part  28  of the diffuser  30  which is contained between the external casing  32  and the external wall  12  of the combustion chamber and, on the other hand, through an internal part  29  of the diffuser  30  which is contained between the internal casing  14  and the internal wall  34  of the combustion chamber. 
         [0012]    With a configuration such as this, an imbalance arising between the bypass flow bypassing the combustion chamber  10  around the outside, in the external part  28  of the diffuser  30 , and the bypass flow bypassing this same combustion chamber  10  on the inside, through the internal part  29  of the diffuser  30 . It then follows that the pressure drops available across the external wall  12 , and which correspond to the difference in pressure between the external part  28  of the diffuser  30  and the inside of the combustion chamber  10  exceed the pressure drops available across the internal wall  14 , which correspond to the difference in pressure between the internal part  29  of the diffuser  30  and the inside of the combustion chamber  10 . 
         [0013]    This imbalance in the pressure drops between the external wall  12  and the internal wall  14  is detrimental to the correct operation of the combustion chamber  10  because the primary jets enter and are diluted better in the region of the external wall  12  than in the region of the internal wall  14 . Furthermore, because the pressure drops available are lower across the internal wall  14 , this wall is more difficult to cool. 
         [0014]    What is more, the pressure drops available for supplying air to the injection systems  18  is reduced because the diffuser duct  36  does not lie directly facing the injection systems  18 . 
       SUMMARY OF THE INVENTION 
       [0015]    The invention proposes to remedy these disadvantages and proposes a design which appreciably reduces this imbalance. 
         [0016]    In a first aspect, the invention relates to a gas turbine engine diffuser defined between an external casing and an internal casing of said engine and supplied with air via an upstream annular diffuser duct, comprising a combustion chamber of the convergent type, forming an external annular duct with the external casing and an internal annular duct with the internal casing, which diffuser comprises a cowling partially closing off the external annular duct. More specifically, the cowling is positioned toward the closed end of the combustion chamber. 
         [0017]    The cowling preferably comprises a body substantially in the form of a part exhibiting symmetry of revolution about a cowling axis, said body extending between two planes which are substantially transverse with respect to said cowling axis. 
         [0018]    According to one embodiment, said two planes coincide and said body is an annular portion of a disk. 
         [0019]    According to another embodiment, said two planes are distinct from one another and said body is frustoconical. 
         [0020]    As a preference, said body has a substantially flat cross section and has a radially external and a radially internal end, and said cowling comprises an external edge extending from said radially external end and an internal edge extending from said radially internal end. 
         [0021]    Furthermore, the cowling has at least one aperture formed in said body. As a preference, said aperture is an aperture with turned-down edges. 
         [0022]    The cowling further comprises fixing means for fixing it to the combustion chamber. As a preference, said fixing means are positioned on said internal edge. 
         [0023]    In a second aspect, the invention relates to a combustion chamber positioned in a diffuser according to the first aspect. 
         [0024]    When the combustion chamber is of the type comprising an external wall, an internal wall and a chamber end wall connecting the aforesaid two walls, the cowling is fixed to the chamber end wall. In particular, the cowling is fixed to the combustion chamber toward the connection between the chamber end wall and said external wall. To fix the cowling to the chamber end wall when the cowling is of the type comprising an internal edge extending from a radially internal end of the body of the cowling, said internal edge is fixed to an upstream face of the chamber end wall. 
         [0025]    In a third aspect, the invention relates to a gas turbine engine such as a turbojet engine which comprises a diffuser according to the first aspect with a combustion chamber according to the second aspect. When the gas turbine engine is of the type comprising a combustion chamber and an external casing and an internal casing between which casings said combustion chamber lies, it preferably comprises a cowling having an external edge which rests against said external casing. In particular, said cowling resting on said external casing allows there to be some axial clearance between these two parts. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The invention will be better understood from reading the detailed description which follows, of some particular embodiments of the invention, which are given by way of entirely nonlimiting indication and illustrated by means of the attached drawings, in which: 
           [0027]      FIG. 1  is a view in axial section of part of a gas turbine engine having a combustion chamber of the convergent type, showing half of the combustion chamber and showing half of a cowling according to the invention, the other half being axially symmetric therewith; 
           [0028]      FIG. 2  is a view on a larger scale of a detail of  FIG. 1 , showing a first embodiment of the cowling according to the invention; 
           [0029]      FIG. 3  depicts a cowling according to the invention, viewed from upstream in the direction of arrow III in  FIG. 2 , and showing apertures in the cowling according to the invention; 
           [0030]      FIGS. 4 to 7  schematically depict other shapes of aperture according to the invention; 
           [0031]      FIG. 8  is a schematic view of a cowling according to a first embodiment, in section on its axis; 
           [0032]      FIG. 9  is a view similar to  FIG. 8  for a second embodiment of the cowling; 
           [0033]      FIG. 10 , which has already been described, is a view in axial section of a divergent combustion chamber of the prior art, provided with a cowling of the prior art; and 
           [0034]      FIG. 11 , which has already been described, is a view in axial section of a convergent combustion chamber provided with another cowling of the prior art. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]    Reference is made first of all to  FIG. 1  which, in axial section, depicts half of a combustion chamber of the convergent type. This combustion chamber  10  is substantially similar to that of the prior art illustrated in  FIG. 11  and comprises an external wall  12  and an internal wall  14  which are coaxial and substantially frustoconical and which widen in the direction from downstream to upstream at a cone angle α 2 . 
         [0036]    The combustion chamber  10  is contained in a diffuser  30  which is an annular space defined between an external casing  32  and an internal casing  34  into which an annular diffuser duct  36  opens. The diffuser  30  comprises an external part  28  delimited between the external casing  32  and the external wall  12  of the combustion chamber and an internal part  29  delimited between the internal casing  34  and the internal wall  14  of the combustion chamber  10 . 
         [0037]    The external  12  and internal  14  walls are connected, toward the upstream end of the combustion chamber, via a chamber end wall  16  substantially similar to that of  FIG. 11  and provided with injection systems  18  through which injectors  20  which pass through the outer casing  32  pass. 
         [0038]    This combustion chamber  10  according to the invention differs from that of the prior art illustrated in  FIG. 11  through its cowling  26  and the connection between this cowling  26  and the chamber end wall  16 . 
         [0039]    As illustrated in  FIGS. 1 ,  2 ,  8  and  9 , the cowling  26  according to the invention is an annular part exhibiting symmetry of revolution about a cowling axis  260  and which is positioned between the external casing  32  and the combustion chamber  10  in such a way that it closes off the external part  28  of the diffuser  30 . It has a cowling body  40  with a substantially flat cross section and a radially external end  42  and a radially internal end  44 . The cowling  26  is provided with an external edge  46  extending from the radially external end  42  and with an internal edge  48  extending from the radially internal end  44 . 
         [0040]    According to the first embodiment illustrated more specifically in  FIG. 8 , the body  40  is in the form of a cone frustum lying between two planes P 1  and P 2  which are transverse with respect to the cowling axis  260 . When the cowling  26  is in place in the diffuser  30 , the external edge  46  of the cowling  26  extends substantially toward the upstream end of the diffuser  30 , and its internal edge  48  extends substantially toward the cowling axis  260  which then coincides with the axis of the turbojet engine  100 . 
         [0041]    According to the second embodiment illustrated more specifically in  FIG. 9 , the body  40  is in the form of a portion of a disk contained in a plane P 3  that is transverse with respect to the cowling axis  260 . When the cowling  26  is in place in the diffuser  30 , the external edge  46  and the internal edge  48  of the cowling  26  extend substantially toward the upstream end of the diffuser  30 . In addition, when the cowling  26  is in use, the cowling axis  260  and the axis  100  of the turbojet engine coincide. 
         [0042]    According to the first or second embodiments of the cowling  26 , this cowling is fixed to the combustion chamber  10 . 
         [0043]    As illustrated in  FIG. 1 , the chamber end wall  16  and the external wall  12  are fixed to one another in an airtight manner. In the example illustrated in  FIGS. 1 and 2 , this fixing is performed using a screwed or bolted connection  15  between a flange  102  of the external wall  12  and a flange  106  of the chamber end wall  16 , these two flanges extending radially outward. These flanges may be annular about the axis of the turbojet engine  110  (see  FIGS. 1 and 2 ) or frustoconical about this same axis. 
         [0044]    As a preference, the cowling  26  is fixed to the combustion chamber  10  via fixing means positioned on its internal edge  48 . In the example illustrated in  FIGS. 1 and 2 , these fixing means comprise holes (not visible in the figures) and screws and/or bolts  45  passing through these holes and fixing onto a wall of the combustion chamber. As a preference, the cowling  26  is fixed to an upstream face  166  of the chamber end wall  16 . In the example illustrated, said screws and/or bolts  45  coincide with the screwed or bolted connection  15  already described, fixing being performed where the external wall  12  of the combustion chamber  10  meets the chamber end wall  16 . The holes and the screws and/or bolts  45  are, for example, distributed over the periphery of the internal edge  48  of the cowling  26 . Likewise, the fixing flanges  102  and  106  are provided with fixing holes uniformly distributed about their periphery. 
         [0045]    According to the invention, the external edge  46  of the cowling  26  is not fixed, but simply rests against an interior face of the external casing  32  delimiting the diffuser  30 . A non-fixed connection such as this has the advantage of allowing relative slippage of the cowling  26  with respect to said external casing  32  in a direction substantially parallel to the axis  100  of the turbojet engine. Because of the orientation of this external edge  46  with respect to the body  40  of the cowling  26  (see  FIG. 2 ), the connection between said cowling  26  and said external casing  32  is an airtight or almost airtight connection, give or take the translational clearance. 
         [0046]    As illustrated in  FIG. 3  which is a view from the upstream end of the body  40  of the cowling  26 , the cowling  26  according to the first embodiment or the second embodiment is preferably provided with at least one aperture  50  passing through said body  40 . This then means that the external part  28  of the diffuser  30  is not closed off by said cowling  26  in an entirely airtight fashion but, on the other hand, allows some oxidant, which is air, to pass through. 
         [0047]    In practice, the cowling  26  comprises a plurality of apertures  50  which are positioned on its body  40  and circumferentially distributed thereon. For example, there are the same number of apertures  50  as there are injection systems passing through the chamber end wall  16  and the apertures lie on extensions of the corresponding injection axes  52 . According to one particular embodiment, said apertures  50  are apertures  50  with turned-down edges, the edge of each aperture  50  extending in the downstream direction of the turbojet engine  2  when the cowling  26  is installed in said turbojet engine  2 . Apertures with turned-down edges guide the flow better than apertures that do not have turned-down edges. 
         [0048]    The shapes and sizes of the apertures  50  are determined according to the amount of oxidant that is to be allowed to pass through said apertures  50 . To simplify the process of manufacturing a cowling  26  such as this, the apertures  50  may be chosen to be identical to one another on one and the same cowling  26  body  40 . In the example illustrated in  FIG. 3 , said apertures  50  are substantially circular. According to other embodiments, said apertures are substantially oval or elliptical ( FIG. 4 ) or substantially square ( FIG. 5 ) or substantially rectangular ( FIG. 6 ) or may even be in the form of a slot ( FIG. 7 ). Of course, other shapes may also be chosen. 
         [0049]    Thus, the presence of the apertures  50 , their shape(s) and their sizes allow the relative pressure drops to be tailored to suit the flow of bypass air bypassing the combustion chamber  10  and passing via the external part  28  of the diffuser. It is thus possible to balance the pressure drops of this external bypass air flow with the pressure drops of the internal bypass flow passing through the internal part  29  of the diffuser and supplied to the injection systems  18  and the internal wall  14  of the combustion chamber  10 . 
         [0050]    One advantage of the invention lies in the fact that the internal bypass air flow bypassing the combustion chamber  10  is improved by the shape of the cowling  26 . This is because the internal bypass air is guided toward the injection systems  18  and toward the internal part  29  of the diffuser  30  by the web situated between the apertures  50  in the cowling  26 .