Abstract:
A turbomachine combustion chamber including a chamber bottom with at least one opening designed to receive a combustion bowl in the axis of which an air and fuel injection device is mounted is disclosed. The downstream end of the flared bowl includes a divergent with a double partition delimiting an annular cavity. The first outer partition includes inlet orifices arranged to cool the second inner partition by impact and the second inner partition includes outlet orifices. The inlet orifices, distributed in at least two circular rows on the periphery of the divergent, are in staggered rows with the outlet orifices.

Description:
The present invention relates to the field of combustion chambers of aviation turbomachines. 
     BACKGROUND OF THE INVENTION 
     It relates more precisely to a turbomachine combustion chamber comprising a chamber bottom which has at least one opening designed to receive a bowl in the axis of which an air and fuel injection device is mounted, said bowl being flared in the direction of flow of the gases and comprising cooling means. The bowl  30  comprises a concentric cylindrical portion and a frustoconical portion, called a divergent. Such a combustion chamber is represented in  FIG. 1 . 
     In the combustion chambers of this type, notably the chambers of turbojets for military use, the bowls and the deflectors or cups fitted to the chamber bottoms are under particular stress. 
     Because of the evolution of turbojets, the chamber is subjected to very considerable heat and mechanical stresses to the chamber-bottom elements, more particularly the combustion bowl and the partition of the downstream collar of the bowl are subjected to high temperatures. 
     DESCRIPTION OF THE PRIOR ART 
     Through patent EP0821201B1, it is known practice to cool the divergent of the combustion bowl by convection by causing air to circulate in a cavity formed in the divergent. Flow disrupters are placed in the cavity in order to slow the air flow which is then expelled into the combustion chamber in order to participate in the spraying of the fuel. However, because of the slowing of the air in the cavity, the air flow tends to heat up and does not make it possible to cool the divergent effectively. 
     Through patent EP0182687B1, it is also known practice to cool a combustion bowl having a double-partition divergent. Since the outer partition of the divergent comprises inlet orifices to cool the downstream partition by impact, the air then escapes via an outlet channel arranged downstream of the divergent and designed to cool the latter by “blowing”. Cooling by impact, as is carried out in this instance, does not make it possible to cool the divergent effectively. Since the outlet channel is arranged at a distance from the inlet orifices, the air flow tends to heat up during its passage between the partitions, which adversely affects the cooling of the divergent. 
     SUMMARY OF THE INVENTION 
     In order to solve at least certain of these disadvantages, the applicant proposes a combustion chamber allowing an effective cooling of the divergent of the combustion bowl while promoting the spraying of the fuel-air mixture originating from the injector. 
     For this purpose, the invention relates to a turbomachine combustion chamber comprising a chamber bottom which comprises at least one opening designed to receive a combustion bowl in the axis of which an air and fuel injection device is mounted, said flared bowl comprising downstream a divergent consisting of a double partition delimiting an annular cavity,
         the first outer partition comprising inlet orifices arranged to cool the second inner partition by impact;   the second inner partition comprising outlet orifices;
 
a chamber in which the inlet orifices, distributed in at least two circular rows on the periphery of the divergent, are in staggered rows with the outlet orifices.
       

     The inner partition of the divergent is advantageously cooled by impact via the two circular rows of inlet orifices, which makes it possible to guide an air flow over the entire surface of the divergent while allowing an effective circulation of the flow due to the orifices being configured in a staggered manner. 
     Preferably, the tangential incidence of the outlet orifices is between 20° and 45°. 
     Again preferably, the tangential incidence of the inlet orifices is equal, and in the same direction, to that of the outlet orifices. 
     The cooling air, entering the annular cavity through the inlet orifices and leaving through the outlet orifices, is advantageously made to swirl which creates turbulence promoting the spraying and shearing of the fuel-air mixture. 
     Again preferably, the combustion chamber comprises at least one spiral arranged in order to swirl the air and the fuel injected into the chamber. 
     Again preferably, the tangential incidence of the plurality of outlet orifices is in the opposite direction to the direction of swirl of the spiral. 
     Advantageously, the swirling generated by the spiral is disrupted by the swirling, rotated in the reverse direction, generated by the outlet orifices which improves the spraying and shearing of the fuel-air mixture. 
     Preferably, the outlet orifices and the inlet orifices are distributed in circular rows, the orifices of each row being evenly distributed over the periphery of the bowl. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood with the aid of the appended drawings in which: 
         FIG. 1  represents a view in radial section of a combustion chamber bottom according to the prior art; 
         FIG. 2  represents a view in radial section of a combustion chamber bottom with a combustion bowl according to a first embodiment of the invention; 
         FIG. 3  represents a schematic arrangement of the inlet and outlet orifices arranged in a staggered manner in the partitions of the divergent of the combustion bowl; 
         FIG. 4  represents a view in radial section of a combustion chamber bottom with a combustion bowl according to a second embodiment of the invention; and 
         FIG. 5  represents a view in radial section of a combustion chamber bottom with a combustion bowl according to a third embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  shows the upstream end of a combustion chamber  1  for a turbojet comprising an air and fuel injection system  22 . A fraction of the upstream air originating from the compressor is guided through the injection system  22  for the formation of a fuel-air mixture injected along an axis X; the latter enters the primary zone where the combustion reactions take place, then the gases produced are diluted and cooled in the downstream secondary zone, not shown, and are distributed toward the turbine that they drive. 
     The injection system  22  comprises a fuel injector  2  with aerodynamic spraying for example as described in patent FR-A-2 206 796. 
     This injector  2  comprises a profiled central fuel-delivery body extended downstream by swirling fins  23  with radial flow forming an internal centripetal spiral; an annular cap  25  is provided with an inner channel connecting to the inner spiral  23 . A row of outer fins  24  forming an outer spiral with substantially radial flow is mounted on this cap  25 . 
     The thin layer of fuel is thus sprayed by shearing effect between the air flow made to swirl by the inner spiral and the air flow made to swirl by the outer spiral. 
     The injector  2  is connected to the combustion chamber  1  by means of a part of circular section, called a combustion bowl  30 , which is, for its part, frustoconical widening out in the downstream direction. The bowl  30  comprises a cylindrial portion concentric with the inner spiral and a frustoconical portion, called a divergent  31 , forming with the cap  25  an annular channel for the swirling air flow originating from the outer spiral. 
     The bowl  30  is connected to the wall  12  of the chamber bottom at its downstream edge, the chamber being delimited by an outer wall  13 . 
     The divergent  31  of the combustion bowl consists of a double partition delimiting an annular cavity  35  with a thickness of between 0.5 and 0.8 mm. This double partition comprises a first outer partition  33  and a second inner partition  34  comprising respectively inlet orifices  331  and outlet orifices  332  for the cooling air flow originating from the compressor. 
     With reference to  FIG. 2 , representing a first embodiment of the invention, the inlet orifices  331  form three circular peripheral rows  331 A,  331 B,  331 C in the outer partition  33 . The inlet orifices  331  are, for each row, evenly distributed over the periphery of the bowl  30 . These inlet orifices  331  are arranged to guide the air flow originating from the compressor and cool the inner partition  34  of the divergent  31  by impact. The jets of cooled air strike the inner partition  34  of the divergent  31  at high speed which makes it possible to lower its temperature and limit the formation of hot spots in the bowl  30 . 
     The outlet orifices  332 , in a manner similar to the inlet orifices  331 , form three circular peripheral rows  332 A,  332 B,  332 C in the inner partition  34 . The outlet orifices  332  are, for each row, evenly distributed over the periphery of the bowl  30 . The inlet orifices  331  are in this instance placed in a staggered manner with the outlet orifices  332  as shown in  FIG. 3  in order to even out the cooling of the inner partition  34  of the bowl  30 . 
     With reference more particularly to  FIG. 3 , the inlet orifices  331  have a small diameter, of between 0.8 mm and 1 mm, in order to increase the speed of the air flow in the annular cavity  35 . As an example, the inlet orifice  331  of the row  331 C leads to four outlet orifices  332  whose diameter, greater than that of the inlet orifices  331 , is between 1.5 mm and 2.5 mm. 
     When the air circulates in the annular cavity  35 , the air flow enters through this inlet orifice  331  of small diameter and escapes rapidly through the four outlet orifices  332  placed in a staggered manner in its vicinity, in order to participate in the spraying of the fuel-air mixture and in the cooling of the walls of the combustion chamber. Therefore, thanks to this staggered arrangement, the air flow travels with a considerable speed in the cavity  35 . The air flow does not have the time to heat up which allows an effective cooling of the divergent  31 . 
     With reference to  FIG. 2 , the row  332 C of outlet orifices, placed furthest downstream of the divergent  31 , actively participates in the cooling of the walls of the combustion chamber  1 , the intermediate row  332 B participating in the spraying of the fuel-air mixture and the row  332 A of outlet orifices, placed furthest upstream, participating in the shearing of the fuel-air mixture in cooperation with the outer spiral  24  placed in its vicinity. 
     The inlet orifices  331  have a tangential incidence of between 20° and 45°, which makes it possible to increase the time that the cooling air spends in the annular cavity  35  and to prevent the latter from circulating between the partitions  33 ,  34  at too high a speed without taking heat from the divergent  31 . 
     In a similar manner, the outlet orifices  332  have a tangential incidence in the same direction and of the same value as the tangential incidence of the inlet orifices  331 . Therefore, the cooling air is swirled in the combustion chamber  1  in order to form a spiral air flow making it possible to spray rapidly and effectively the fuel-air mixture and to cool the walls of the combustion chamber  1 . 
     The tangential incidence of the outlet orifices  332  is adapted so as to be in the opposite direction from the orientation of the second outer radial spiral  24 . Therefore, in operation, the cooling air flow coming out of the outlet orifices  332  is made to swirl in the rotation direction contrary to that of the outer radial spiral  24 . This contra-rotating swirl promotes the shearing and spraying of the fuel-air mixture. 
     Each row of inlet orifices  331  and outlet orifices  332  comprises the same number of orifices which are placed in a staggered manner relative to one another. It is possible to modify the number of rows of orifices and their positioning on the divergent  31  according to the effect that it is desired to promote (shearing of the layer of fuel, spraying of the fuel-air mixture or cooling of the walls of the combustion chamber). 
     As an example, with reference to a second embodiment, the downstream partition  34 , shown in  FIG. 4 , comprises a single row of outlet orifices  332 C whose orifices  332  are placed in a staggered manner with the inlet orifices  331  arranged in the outer partition  33 , the inlet orifices  331  being divided into five rows. In this example, the inlet orifices  331  have a smaller diameter and are more numerous in comparison with the first embodiment of  FIG. 2 , the cooled air flow nevertheless remaining substantially equal. 
     Still with reference to  FIG. 4 , the row of outlet orifices  332 C is arranged downstream of the inner partition  34  of the divergent  31 . After the air flow has cooled by impact the inner partition  34 , the latter is guided into the annular cavity  35  before being expelled axially downstream of the divergent  31  in order to participate in the cooling of the walls of the combustion chamber  1 , thereby preventing the heat generated by the combustion from causing the creation of hot spots on the walls of the combustion chamber  1 . 
     With reference to a third embodiment represented in  FIG. 5 , the inner partition  34  comprises a single row of outlet orifices  332 A whose orifices  332  are placed in a staggered manner with the inlet orifices  331  arranged in the outer partition  33 , the inlet orifices  331  being divided into five rows in a manner similar to the second embodiment of the invention. 
     Still with reference to  FIG. 5 , the row of outlet orifices  332 A is arranged upstream of the inner partition  34  of the divergent  31 . After the air flow has cooled by impact the inner partition  34 , the row  332 A of outlet orifices radially shears the layer of fuel-air mixture in the immediate vicinity of the injector  2 . The tangential incidence of the outlet orifices  332  opposite to that of the second outer spiral  24  improves still more the shearing of the layer of fuel-air mixture and allows an even spraying without the creation of hot spots on the divergent  31  of the combustion bowl  30 .