Abstract:
The invention relates to a device for injecting a mixture of air and fuel into a combustion chamber of a turbomachine where the supply of air is improved. The invention relates more particularly to a new type of sliding bushing.

Description:
BACKGROUND OF THE INVENTION 
   The invention applies to the field of turbomachines and relates to a device for injecting a mixture of air and fuel into a combustion chamber of a turbomachine. 
   It relates more precisely to a novel type of sliding bushing in which the supply of air for the air/fuel mixture is improved. 
   In the remainder of the description, the terms “upstream” and “downstream” will be used to denote the positions of the structural elements in relation to one another in the axial direction, taking the gas flow direction as reference point. Likewise, the terms “internal” or “radially internal” and “external” or “radially external” will be used to denote the positions of the structural elements in relation to one another in the radial direction, taking the axis of rotation of the turbomachine as reference point. 
   A turbomachine comprises one or more compressors which deliver pressurized air to a combustion chamber where the air is mixed with fuel and ignited so as to generate hot combustion gases. These gases flow downstream 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 work required, for example, to power an aircraft. 
   DESCRIPTION OF THE PRIOR ART 
   Typically, as illustrated in  FIG. 1 , the combustion chambers  1  used in aeronautics comprise an internal wall  2  and an external wall  3  interconnected at their upstream end by a chamber endwall  4 . The chamber endwall  4  has, spaced circumferentially, a plurality of openings each accommodating an injection device  10  which allows the mixture of air and fuel to be fed into the chamber. 
     FIG. 2  shows a sectioned view through an injection device  10  according to the prior art. The injection device  10 , the axis of symmetry of revolution of which is denoted X, comprises, positioned from upstream to downstream, a sliding bushing  20  connected by an annular dish  30  to radial swirl inducers  40 . The radial swirl inducers  40  comprise a venturi  50  and are connected by their downstream end to a bowl  60  that has a divergent conical wall. The bowl  60  is itself connected to the chamber endwall  4  by means of a deflector  70 . The sliding bushing  20  comprises, positioned from upstream to downstream, an upstream wall  21  of convergent conical shape continuing into a cylindrical wall  24  which terminates downstream in a flange  23 . 
   The combustion chamber  1  is supplied with liquid fuel mixed with air from a compressor. The liquid fuel is conveyed as far as the chamber by injectors  5 . The downstream end  6 , also known as the head, of the injectors  5  is positioned in the injection device  10 , in the sliding bushing  20 , in such a way that the axis of symmetry of the injector head  6  corresponds to the axis of symmetry of the sliding bushing. Thus, one of the functions of the sliding bushing  20  is to guide the injector into position and to provide the seal between this injector and the injection device  10 . The function of guiding the injector is performed by the upstream wall  21  of the sliding bushing  20 . 
   The air and fuel are mixed at the injection device  10 , in several places. An initial mixing operation is performed at the sliding bushing  20 . To do this, pressurized air is carried via drillings  22 , also known as purge holes, into contact with the fuel leaving the injector  5 . Thus, a spray of fuel is initiated, its formation continuing thereafter at the radial swirl inducers  40  and the bowl  60 . The sliding bushing  20  therefore has as an additional function that of performing an initial mixing between the pressurized air and the fuel. The air which reaches the purge holes  22  allows control over the correct formation of the spray of fuel but also cools the injector head  6 . The purge holes  22  are made on the flange  23  of the sliding bushing  20 . 
   A third function of the sliding bushing  20  is to allow relative movement between the injector  5  and the combustion chamber  1  while at the same time maintaining sealing between the injector  5  and the injection device  10 . This movement is associated with the manufacturing tolerances and with the differential expansions there are between the injector and the chamber. To this end, the connection between the sliding bushing  20  and the radial swirl inducers  40  is by means of a dish  30  which allows the sliding bushing to move by several millimeters in all the directions of the plane containing the flange  23 . 
   The amount of air passing through the purge holes  22  is dependent in particular on their number and their diameter. The sizing of the purge holes  22  has an influence both on the total air flow passing through these holes and on the aerodynamic lock of the sliding bushing  20 . Aerodynamic lock is defined by the following ratio: 
           aerodynamic_lock   =       numbe   ⁢           ⁢     r     _   _       ⁢   _of   ⁢   _holes   ⁢   _   ⁢   22   ⁢           ×   diameter_of   ⁢   _the   ⁢   _holes   ⁢   _   ⁢   22       perimeter_of   ⁢   _the   ⁢   _circle   ⁢   _passing   ⁢   _through   ⁢   _the   ⁢   _center   ⁢   _of   ⁢   _the   ⁢   _holes   ⁢   _   ⁢   22             
For the same flow rate it is possible to obtain different values of aerodynamic lock and thus influence the formation of the spray of fuel. Specifically, this parameter has an influence on the penetration of the purge flow and on the level of interaction between the streams of air from the purge and from the swirl inducers. It plays a part in governing the size of the droplets and their distribution in the spray and the initial angle of the cone of fuel leaving the injector.
 
Since aerodynamic lock is proportional to the diameter of the purge holes  22  and since the air flow rate is proportional to the square of their diameter, it is possible to alter these two parameters differently to suit the specifics of the particular combustion chamber concerned.
 
   If there is a desire to increase the air flow rate passing through the purge holes  22  then it is necessary to increase either their diameter or the number of them. Now, the increase in diameter of the holes  22  is limited by the travel of the sliding bushing  20  relative to the radial swirl inducers  40 . In addition, increasing the number of holes  22  influences the aerodynamic lock, something which is not always desirable. 
   Another way to increase the flow rate is to increase the pressure of the air reaching the purge holes  22 . However, in order to supply the holes  22 , the air has to flow around the convergent wall  21  of the sliding bushing, and this gives rise to pressure drops and recirculation and therefore causes a loss of pressure and poor supply conditions. 
   One known way of addressing this problem, as illustrated in document FR 2753779, is to make the purge holes  22  in the cylindrical wall  24  of the sliding bushing so that their axis is parallel to the axis X of the injection device. The disadvantages of this solution are that the sliding bushing is then larger and that the holes are longer for the same diameter. This gives rise to a loss in energy experienced by the air as it passes through the purge holes, and gives rise to recirculation which is detrimental to the air flow rate. 
   SUMMARY OF THE INVENTION 
   The invention allows these problems to be solved by proposing an injection device comprising a sliding bushing arranged in such a way as to increase the air flow rate passing through the purge holes without being limited by the travel of the sliding bushing or by the impact on aerodynamic lock. 
   The invention also makes it possible to make the air flow rate passing through the purge holes independent of the geometry of the sliding bushing, for example independent of the length of the convergent upstream wall. 
   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 symmetry of revolution about an axis X and comprising, arranged from upstream to downstream in the direction in which the gases flow, a sliding bushing of axis of revolution Y connected to radial swirl inducers by a dish, an aerodynamic bowl spaced axially from the radial swirl inducers, the sliding bushing comprising a convergent conical upstream wall continuing into a cylindrical wall of axis X and a downstream flange extending radially at the downstream end of the cylindrical wall, the downstream flange being equipped with pressurized-air supply holes also known as purge holes, which device is notable in that the convergent upstream wall of the sliding bushing is equipped with at least one row of circumferentially-spaced additional pressurized-air supply orifices. 
   Advantageously, the additional supply orifices have a total passage cross section greater than or equal to the total passage cross section of the purge holes. 
   According to a number of embodiments, the additional supply orifices are of cylindrical shape with a directrix that is circular or oblong in shape or shaped as a quadrilateral. 
   The axis of the additional supply orifices may be parallel to the axis of the sliding bushing or alternatively may be orthogonal to the wall of the convergent upstream wall. 
   The invention finally relates to a turbomachine equipped with such a combustion chamber. 
   According to another embodiment of the invention, the additional supply orifices are positioned at the upstream end of the convergent upstream wall, have an open side and form scallops. 
   According to a variant of the invention, this invention relates to a device for injecting a mixture of air and fuel into a combustion chamber of a turbomachine, the injection device having symmetry of revolution about an axis X and comprising, arranged from upstream to downstream in the direction in which the gases flow, a sliding bushing of axis of revolution Y connected to radial swirl inducers by a dish, an aerodynamic bowl spaced axially from the radial swirl inducers, the sliding bushing comprising a convergent upstream wall continuing into a cylindrical wall of axis X and a downstream flange, the downstream flange being equipped with pressurized-air supply holes also known as purge holes, the device being notable in that the purge holes have different inlet and outlet diameters, the inlet diameter being greater than the outlet diameter. 
   Advantageously, the invention also relates to a combination of the different embodiment variants, in particular, the sliding bushing may have both additional supply orifices or scallops and purge holes with different inlet ( 26 ) and outlet ( 27 ) diameters, the inlet diameter ( 26 ) being greater than the outlet diameter ( 27 ). 
   The invention also relates to a combustion chamber and a turbomachine equipped with an injection device according to the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood and other advantages thereof will become more clearly apparent in the light of the description of some preferred embodiments which is given by way of nonlimiting example and makes reference to the attached drawings in which:
           FIG. 1  is a schematic sectioned view of a combustion chamber according to the prior art;     FIG. 2  is a schematic sectioned view of an injection device according to the prior art;     FIG. 3  is a schematic sectioned view of a turbomachine and, more specifically, of an aircraft jet engine;     FIG. 4  is a schematic sectioned view of a sliding bushing according to the invention;     FIG. 5  is a schematic sectioned view of a second embodiment of a sliding bushing according to the invention;     FIG. 6  is a schematic sectioned view of a third embodiment of a sliding bushing according to the invention;     FIG. 7  is a schematic sectioned view of a fourth embodiment of a sliding bushing according to the invention;     FIG. 8  is a schematic sectioned view of a fifth embodiment of a sliding bushing according to the invention;     FIG. 9  is a schematic sectioned view of a sixth embodiment of a sliding bushing according to the invention;     FIG. 10  is a schematic sectioned view of a first embodiment variant of a sliding bushing according to the invention;     FIG. 11  is a schematic sectioned view of a second embodiment variant of a sliding bushing according to the invention;     FIG. 12  is a schematic sectioned view of a combination of two embodiment variants of a sliding bushing according to the invention.       

   

   The same references will be kept throughout the description to denote parts or details which are similar from one figure to another. 
     FIG. 1  and  FIG. 2 , already described, show, in section, a combustion chamber and an injection device according to the prior art. 
   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3  shows, in section, an overall view of a turbomachine  100 , for example an aircraft jet engine, comprising a low-pressure compressor  200 , a high-pressure compressor  300 , a combustion chamber  1 , a low-pressure turbine  500  and a high-pressure turbine  600 . The combustion gases flow in the downstream direction in the combustion chamber  1  and thereafter feed into the turbines  500  and  600  which respectively drive the compressors  200  and  300  positioned upstream of the chamber endwall  4 , via two shafts  900  and  1000  respectively. The high-pressure compressor  300  supplies air to the injection devices together with two annular spaces positioned radially inside and outside the combustion chamber  1  respectively. The air introduced into the combustion chamber  1  contributes toward vaporizing the fuel and burning it. The air flowing on the outside of the walls of the combustion chamber  2  contributes toward cooling its walls and enters the chamber via dilution holes (not depicted) so as to cool the combustion gases forwarded to the turbine. 
     FIG. 4  shows, in section, an exemplary embodiment of a sliding bushing  20  of an injection device  10  according to the invention. The sliding bushing  20  is made up of a convergent conical upstream wall  21  continuing downstream in the form of a cylindrical wall  24 , the axis Y of which is parallel to the axis of symmetry of the injection device. The cylindrical wall  24  terminates in a flange  23  extending radially outwards. The flange  23  is equipped with purge holes  22  preferably positioned in its part closest to the cylindrical wall  24  so as not to be obstructed, even partially, by the radial swirl inducers  40  should there be any relative movement between the sliding bushing  20  and the injection device  10 . Additional orifices  25  for supplying the purge holes with pressurized air are made on the convergent upstream wall  21  of the sliding bushing  20 . In the example described here, the convergent upstream wall  21  is equipped with just one row of circumferentially spaced additional supply orifices  25 , but several rows of orifices could be produced. The axis Z of these additional orifices  25  may be orthogonal to the wall of the convergent upstream wall  21 , as illustrated in  FIG. 4 . It may also be parallel to the axis Y as illustrated in  FIG. 5  or alternatively form any angle with the convergent upstream wall  21 . 
   Thus, the purge holes  22  are no longer supplied with pressurized air that has to flow around the convergent upstream wall  21  of the sliding bushing  20  but are supplied directly with air from the additional supply orifices  25 . This makes it possible to do away with the pressure drops due to flow around the upstream wall  21 . In order for this type of supply to the purge holes  22  to be effective, the total passage cross section for the air at the additional supply orifices  25  needs to be greater than or equal to the total passage cross section for the air in the purge holes. The total passage cross section of the additional supply orifices  25  corresponds to the passage cross-sectional area of one orifice multiplied by the number of orifices. The same holds true for the passage cross section of the purge holes  22 . This being the case, the purge holes  22  are supplied with air at a higher pressure making it possible, for the same purge hole geometry and the same number of purge holes as in the prior art, to pass a higher flow rate of air through these holes. 
   At the same time, if the desired flow rate remains the same as in the prior art, the use of additional supply orifices  25  allows the use of purge holes  22  of a smaller diameter than in the prior art, thus making it possible to achieve a corresponding reduction in the inside diameter of the flange  23  and therefore to reduce the size of the radial swirl inducers  40  and therefore achieve a more compact injection device. 
   The higher the cross section of the additional supply orifices, the greater will be the tolerance on irregularities in shape and roughness of the purge holes  22 , and this will allow manufacturing costs to be lowered. 
   Although the additional supply orifices  25  are made on the convergent upstream wall  21  of the sliding bushing, inasmuch as the total passage cross section for air at these orifices is maintained, the supply to the purge holes  22  is now no longer dependent on the geometry of the sliding bushing, particularly of the convergent upstream wall. 
   The number of additional supply orifices  25  may be equal to or different from the number of purge holes  22 , the important factor being the total passage cross section. 
   Furthermore, the additional supply orifices  25  may be circular in shape, as illustrated in  FIGS. 4 and 5 , or alternatively oblong, parallelepipedal or trapezoidal, as illustrated in  FIGS. 6 to 9 . In the case of an oblong shape, the major length of the orifices  25  may be positioned along the circumference of the convergent upstream wall  21  as illustrated in  FIG. 6 . It may also be positioned parallel to the axis Y of the sliding bushing  20  as illustrated in  FIG. 7 , or at any angle ranging between these two positions, as illustrated in  FIG. 8 . 
   According to another embodiment of the invention, illustrated in  FIG. 10 , the additional supply orifices  25  may be positioned at the upstream end of the convergent wall  21 , produced in such a way that they have open sides and thus form scallops  28 . The scallops  28  have the advantage of being more simple to manufacture because, when the sliding bushing  20  is obtained using a casting method, the scallops  28  can be obtained directly in the casting, without requiring special machining. 
   According to a variant of the invention, the increase in pressure needed to increase the amount of air passing through the purge holes  22  can be obtained by altering the geometry of the purge holes  22 .  FIG. 11  shows a sectioned view of a sliding bushing  20  in which the purge holes  22  are conical, their inlet diameter  26  being larger than their outlet diameter  27 . Thus, as the flow rate remains constant throughout the length of the purge holes  22 , the air is accelerated and the amount of air passing through the purge holes  22  in a given space of time is higher. 
   It is of course possible to mix the variants of the invention by combining the effects associated with the addition of additional supply orifices  25 , or scallops  28 , and the effects associated with the use of conical purge holes  22 . An example of such a combination is illustrated in  FIG. 12 . 
   By way of example, three-dimensional numerical simulations have established that, for a sliding bushing equipped with 14 purge holes 1.4 mm in diameter, arranging  20  circular additional supply orifices as described in  FIG. 4 , with a diameter of 1.8 mm, yields a 15% increase in the air flow rate passing through the purge holes  22 .