Abstract:
The invention relates to the field of turbomachines and concerns a device ( 20   a,    20   b ) for injecting a mixture of air and fuel into a combustion chamber ( 4 ) of a turbomachine ( 1 ). It relates more specifically to an injection device ( 20   b ) provided with a novel sliding bushing ( 30   b ) and with a novel annular cup ( 50   b ) for retaining the sliding bushing, making it possible to improve the air feed of the injection device.

Description:
BACKGROUND OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART 
       [0001]    The invention is concerned with the field of turbomachines and relates to a device for injecting a mixture of air and fuel into a combustion chamber of a turbomachine. 
         [0002]    It relates more specifically to an injection device provided with a novel sliding bushing and with a novel annular cup for retaining the sliding bushing, making it possible to improve the air feed of the injection device. 
         [0003]    Throughout the remainder of the description, the terms “upstream” or “downstream” will be used to denote the positions of the structural elements with respect to one another in the axial direction, taking the gas flow direction as reference. Likewise, the terms “internal” or “radially internal” and “external” or “radially external” will be used to denote the positions of the structural elements with respect to one another in the radial direction, taking the axis of rotation of the turbomachine or the axis of symmetry of the structure in question as reference. 
         [0004]    A turbomachine comprises one or more compressors delivering pressurized air to a combustion chamber in which the air is mixed with fuel and ignited so as to generate hot combustion gases. These gases flow in the downstream direction of the chamber toward one or more turbines which convert the energy thus received in order to rotate the compressor or compressors and provide the necessary work, for example, to power an aircraft. 
         [0005]    Typically, a combustion chamber used in aeronautics comprises an internal wall and an external wall interconnected at their upstream end by a chamber end wall. The chamber end wall has, spaced circumferentially, a plurality of openings each accommodating an injection device at the center of which an injector is positioned, the assembly allowing the mixture of air and fuel to be fed into the chamber. 
         [0006]    The combustion chamber is supplied by liquid fuel mixed with air from a compressor. The liquid fuel is fed right to the chamber by the injectors in which it is vaporized into fine droplets. This vaporization is initiated in the region of the injector by means of nozzles and is continued particularly in the region of the venturi and the bowl by the effect of pressurized air from a compressor. This pressurized air passes, on the one hand, through the radial swirlers of the injection device so as to cause the fuel sprayed by the injector to rotate, and, on the other hand, through orifices formed in various parts of the injection device, such as the bowl. 
         [0007]    As illustrated particularly in document FR 2 753 779, an injection device has a symmetry of revolution and comprises, arranged from upstream to downstream, a sliding bushing connected by an annular cup to radial swirlers. The radial swirlers comprise a venturi and are connected by their downstream end to a bowl with a divergent conical wall. The bowl is itself connected to the chamber end wall via a deflector. According to the prior art, the sliding bushing includes, arranged from upstream to downstream, an upstream wall of convergent conical shape extended by a cylindrical wall, which terminates downstream in an annular flange extending radially outward. 
         [0008]    Owing to the convergent conical shape of its upstream part, the sliding bushing constitutes an obstacle that the air coming from the compressor must negotiate. Likewise, since the outside diameter of the annular cup, which serves as radial guide for the sliding bushing, is large, the annular cup itself constitutes an obstacle to be negotiated. This is the origin of air recirculation and substantial pressure drops deleterious to the air feed of the injection device. The spraying quality is therefore degraded, resulting in a larger droplet size, and a coke deposit may appear on the venturi and the bowl. The carburetion and therefore the performance are thus degraded. 
       SUMMARY OF THE INVENTION 
       [0009]    The object of the invention is to produce an injection device for optimizing the air feed, while minimizing the pressure drops between the outlet of the compressor and the inside of the combustion chamber. This objective is achieved by specifying the profile and the shape of the sliding bushing and of the annular cup. 
         [0010]    The invention makes it possible to solve this problem by providing an injection device comprising a sliding bushing and an annular cup, the geometries of which are such that the air coming from the compressor no longer encounters an obstacle in the upstream part of the injection device. 
         [0011]    More specifically, the invention relates to a device for injecting a mixture of air and fuel into a combustion chamber of a turbomachine, the injection device having a symmetry of revolution about an axis and comprising, arranged from upstream to downstream in the gas flow direction, a sliding bushing at the center of which an injector is positioned, an annular cup retaining the sliding bushing axially, at least one radial swirler comprising a venturi, and connected by its downstream end to a bowl, which device is noteworthy in that the sliding bushing comprises, arranged from upstream to downstream, a cylindrical upstream part extended by a convergent conical intermediate part and terminating in an annular flange extending radially outward. 
         [0012]    Advantageously, the annular cup comprises, arranged from upstream to downstream, a profiled upstream fairing extended by an annular flange extending radially inward. 
         [0013]    Preferably, the profiled upstream fairing of the annular cup has a divergent conical shape and a convex external wall. 
         [0014]    The cylindrical upstream part of the sliding bushing may comprise a beveled upstream face, the upstream face being inclined radially inward. 
         [0015]    Preferably, the upstream face of the sliding bushing is inclined at an angle of about 45° to the axis of symmetry of the injection device. 
         [0016]    The cylindrical upstream part of the sliding bushing may also be provided with an axial swirler. 
         [0017]    Advantageously, the annular flange of the sliding bushing has a radially internal end provided with at least one row of orifices. The orifices of the row may be distributed in a regular fashion around the entire circumference of the annular flange and their axis may be generally parallel to the convergent conical intermediate part of the sliding bushing. 
         [0018]    According to a preferred embodiment, when the bowl and the annular cup each have an external radius, the external radius of the bowl is between 1.1 and 1.2 times the value of the external radius of the annular cup. In this case, advantageously, the bowl has a divergent conical wall that makes an angle of between 70 and 130° with the axis of symmetry of the injection device. 
         [0019]    Moreover, the invention also relates to a combustion chamber comprising an internal wall, an external wall, and a chamber end wall, and provided with at least one such injection device. 
         [0020]    The invention finally relates to a turbomachine provided with such a combustion chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention will be better understood and other advantages thereof will become more clearly apparent in the light of the description of a preferred embodiment given by way of nonlimiting example and with reference to the appended drawings, in which: 
           [0022]      FIG. 1  is a schematic sectional view of a turbomachine, more specifically an aircraft jet engine; 
           [0023]      FIG. 2   a  is a schematic sectional view of the lower upstream part of a combustion chamber provided with an injection device according to the prior art; 
           [0024]      FIG. 2   b  is a schematic sectional view of the upper upstream part of a combustion chamber provided with an injection device according to the invention; and 
           [0025]      FIG. 3  is a schematic sectional view of the upper upstream part of a combustion chamber provided with an alternative embodiment of an injection device according to the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]      FIG. 1  shows in section an overall view of a turbomachine  1 , for example an aircraft jet engine, comprising a low-pressure compressor  2 , a high-pressure compressor  3 , a combustion chamber  4 , a low-pressure turbine  5  and a high-pressure turbine  6 . The combustion chamber  4  may be of the annular type and is defined by two annular walls  7  spaced radially to the axis X of rotation of the jet engine, these walls being connected at their upstream end to an annular chamber end wall  8 . The chamber end wall  8  has a plurality of openings (not shown) with a regular circumferential spacing. In each of these openings is mounted an injection device. The combustion gases flow downstream in the combustion chamber  4  and then supply the turbines  5  and  6  which respectively drive the compressors  2  and  3 , arranged upstream of the chamber end wall  8 , by way of two respective shafts  9  and  10 . The high-pressure compressor  3  supplies air to the injection devices and also to two annular spaces respectively arranged radially to the inside and outside of the combustion chamber  4 . The air introduced into the combustion chamber  4  is involved in vaporizing the fuel and in its combustion. The air circulating outside the walls of the combustion chamber  2  is involved in cooling these walls and enters the chamber through dilution holes (not shown) in order to cool the combustion gases transmitted to the turbine. 
         [0027]      FIG. 2   a  shows a schematic sectional view of the lower upstream part of a combustion chamber  4  provided with an injection device  20   a  according to the prior art, the axis of symmetry of revolution of which is identified by Y. 
         [0028]    According to the prior art, the injection device  20   a  includes a sliding bushing  30   a , at the center of which an injector  40  is positioned. The sliding bushing  30   a  is made up of a convergent conical upstream part  31   a  extended by a cylindrical part  32   a  of axis Y, this cylindrical part being provided at its downstream end with an annular flange  33   a  extending radially outward. 
         [0029]    The convergent conical part  31   a  is inclined at about 45° to the axis Y and the function of this part is to guide the injector  40  when it is being mounted on the injection device. At the annular flange  33   a , the sliding bushing  30   a  is retained axially by an annular cup  50   a . The annular cup  50   a  has an annular flange  51   a  extending radially outward. Its outer radial end terminates in a cylindrical annular lip of axis Y pointing in the upstream direction. The annular cup  50   a  is connected to radial swirlers  60   a , two in this instance, by way of its annular lip  52   a  which is connected, for example by brazing, to a lip  61   a  of corresponding shape, placed at the outer upstream end of the swirlers  60   a . A space is provided axially between the annular flange  51   a  and the radial swirlers  60   a  so as to allow the sliding bushing  30   a  to move radially. This movement makes it possible to compensate for the relative displacements between the injector  40  and the injection device  20   a . The swirlers  60   a  are provided with a venturi  62   a  and are connected via their inner downstream end to a bowl  70 . The bowl  70  is fastened by means of a deflector  80  to the chamber end wall  8 . 
         [0030]    The shape of the upstream part  31   a  of the sliding bushing constitutes an obstacle to the flow of air F coming from the compressor and creates an air recirculation  100  resulting in pressure drops. Likewise, because of its shape, and in particular because of the presence of the annular lip  52   a , the annular cup itself constitutes an obstacle, resulting in a recirculation of air  101  and pressure drops. This is the origin of poor air feed of the injection device  20   a  and results in particular in coke being deposited on the venturi  62   a  and on the bowl  70 . 
         [0031]    As illustrated in  FIG. 2   b , according to the invention the injection device  20   b  still has a sliding bushing  30   b  retained axially by an annular cup  50   b , but their shapes are adapted so as to improve the air feed of the injection device  20   b.    
         [0032]    For this purpose, the sliding bushing  30   b  is made up, from the upstream end to the downstream end, of a cylindrical upstream part  31   b  extended by a convergent conical intermediate part  32   b  and terminating in an annular flange  33   b  extending radially outward. The cylindrical upstream part  31   b  has a beveled upstream part  34   b , this upstream face being inclined radially inward at an angle of about 45°. This angle of inclination allows the sliding bushing according to the invention to fulfill its guide function when the injector  40  is being mounted. The cylindrical upstream part  31   b  may be provided with an axial swirler  35   b  for improving the air feed of the injection device by introducing air coming from the compressor at the downstream end of the injector  40 . The conical intermediate part  32   b  has an axial dimension and an angle of inclination that are adapted so as to connect the upstream part  31   b  and the annular flange  33   b , while still guiding the air output by the axial swirler  35   b.    
         [0033]    The sliding bushing  30   b  is retained axially on its annular flange  33   b  by the annular cup  50   b . The annular cup  50   b  includes a profiled upstream fairing  51   b . This upstream fairing  51   b  has a generally divergent conical shape, its wall being slightly convex. The upstream fairing  51   b  is extended by an annular flange  52   b  extending radially inward. The annular cup  50   b  is connected, at the connection between the upstream fairing  51   b  and the annular flange  52   b , to one or more radial swirlers  60   b . In the example shown here, the injection device  20   b  is provided with a single radial swirler  60   b  which is provided with a venturi  62   b . A space is provided axially between the annular flange  52   b  and the radial swirler  60   b  so as to allow the sliding bushing  30   b  to move radially. The radial swirler  60   b  is connected via its inner downstream end to a bowl  70 , which is itself attached to the chamber end wall  8  via a deflector  80 . 
         [0034]    Thanks to the convergent upper part on the sliding bushing  30   b  being replaced with a cylindrical upstream part  31   b , and thanks to the divergent convex shape of the upstream fairing  51   b  of the annular cup  50   b , the obstacles to the flow of air F are eliminated and the air coming from the compressor is guided into the radial swirler  60   b  and the bowl  70  without any shape dislocation, and therefore without any pressure drop. In addition, when the sliding bushing  30   b  is provided with an axial swirler  35   b , the axial swirler  35   b  has the same effect on the carburetion as the radial swirler placed upstream of the venturi  62   b  of the injection device according to the prior art. It also makes it possible to create a film of air along the internal wall of the venturi  62   b , which prevents coke from forming on the venturi. The combination of improvements made to the injection device  20   b  makes it possible to improve its air feed and in addition prevents coke formation. 
         [0035]    The anticoke film coming from the axial swirler  35   b  may be replaced, or its effect may be enhanced, by providing, as illustrated in  FIG. 3 , a row of orifices  36   b  on the radially internal end of the annular flange  33   b  of the sliding bushing  30   b . These orifices  36   b  may or may not be distributed in a regular fashion over the entire circumference of the annular flange  33   b . Preferably, their axis is parallel to the convergent conical intermediate part  32   b , both being arranged so that the air coming from the axial swirler  35   b  and from the orifices  36   b  flows along the internal surface of the venturi  62   b.    
         [0036]    Preferably, but not solely, the invention applies to injection devices for which the outer radius  71  of the bowl  70  is large enough to allow its optimum implantation. More precisely, the invention is particularly suitable for injection devices having an outer radius  71  of the bowl  70  between 0.5 and 0.7 times the internal radius  72  of the chamber taken directly downstream of the injection device. 
         [0037]    Moreover, in order for the air feed of the injection device to be optimum, the outer radius  71  of the bowl must be between 1.1 and 1.2 times the outer radius  73  of the annular cup  50   b . When this criterion is met, if it is desired to limit the axial extent of the injection device, the angle  74  between the divergent part of the bowl  70  and the axis of symmetry Y of the injection device  20   b  must be between 70° and 130°.