Abstract:
A device for cooling a turbine casing in a turbomachine including a turbine is disclosed. The turbine includes several stages, at least one of the stages includes a nozzle assembly formed of an annular row of fixed vanes and an impeller mounted to rotate in a cylindrical shroud formed of ring sectors fixed to the casing, a cooling circuit including ducts carrying cooling air into cavities formed in the vanes of the nozzle assembly, and an air-carrying arrangement which carries air to casing upstream hooks for suspending the ring sectors.

Description:
BACKGROUND OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART 
     The present invention relates to a device for cooling a turbine casing of a turbomachine, particularly an aviation turbojet engine or turboprop. 
     A turbine of this type comprises several stages each including a distributor formed of an annular row of fixed vanes borne by the casing of the turbine and an impeller mounted to rotate downstream of the nozzle assembly in a cylindrical shroud formed by ring sectors fixed circumferentially on casing hooks of the turbine via C-shaped or U-shaped fasteners. 
     The vanes of the first-stage or upstream-stage nozzle assembly are exposed to high temperatures and comprise internal cavities for the flow of cooling air bled off upstream from the turbomachine compressor and carried by ducts to a volume formed in the casing around the turbine upstream nozzle assembly. Cylindrical connecting tubes are mounted in the volume and each connect the volume to an internal cavity of a vane of the upstream nozzle assembly. The cooling air leaves this cavity at the radially internal end of the vane, the trailing edge of which may also comprise orifices opening into the cavity so that the cooling air can leave. 
     The hooks that secure the ring sectors, and especially those located directly downstream of the vanes of the upstream-stage nozzle assembly are shielded from the heat by an annular sealing plate which is mounted between the ring sectors and the external ends of the vanes of the nozzle assembly in order to restrict the passage of gas from the airstream radially outward into an annular space that houses the casing hooks. 
     However, sealing is imperfect and leaks of hot gases from the turbine airstream may cause the temperature of the casing hooks to rise and cause cracking or fissuring liable to destroy the hooks. 
     Furthermore, it would not be possible to fit the turbine with an additional cooling circuit leading cool air bled off upstream of the combustion chamber onto these suspension hooks because of the complexity, limitation on space and costs involved. 
     SUMMARY OF THE INVENTION 
     It is a particular object of the invention to respond to this problem simply, effectively and economically. 
     To this end, the invention proposes a device for cooling a turbine casing in a turbomachine particularly in an aviation turbojet engine or turboprop, this turbine comprising several stages each including a nozzle assembly formed of an annular row of fixed vanes borne by the casing of the turbine and an impeller mounted to rotate inside the casing in a cylindrical shroud formed of ring sectors fixed circumferentially to the casing, and a cooling circuit for cooling the vanes of the nozzle assembly of the upstream stage, comprising ducts for carrying cooling air into cavities formed in the vanes of the nozzle assembly, and means of carrying air to casing upstream hooks for suspending the ring sectors surrounding the impeller of the upstream stage, these air-carrying means connecting the internal cavities of the vanes of the nozzle assembly of the upstream stage to the annular space in which the upstream hooks lie, wherein: the internal cavities of the vanes are closed, at their radially external ends, by plates attached to the nozzle assembly; and the air-carrying means comprise drillings formed in these plates and drillings formed in an external annular rim of the nozzle assembly which extends radially between the radially external walls of the vane cooling cavities and the upstream hooks for suspending the ring sectors. 
     The air bled from the cavities of the vanes of the casing upstream stage nozzle assembly is carried into the annular space housing the casing upstream hooks and allows their temperature to be brought down, something which results in an appreciable reduction in the risk of cracking or fissuring of the hooks without the need to add ducts carrying cool air to the turbine casing. 
     This air also makes it possible to keep the annular space in which the hooks are housed at a pressure higher than that of the combustion gases passing through the turbine, and this itself opposes the ingress of these gases into the annular space housing the hooks. 
     The airflow bled off for cooling the upstream hooks represents a small fraction of the total airflow used for cooling the vanes of the nozzle assembly, and so has very little influence on the cooling of the vanes of the nozzle assembly of the upstream stage and on the output of the turbomachine. 
     According to another characteristic of the invention, the means for carrying air to the upstream casing hooks are distributed over the periphery of the nozzle assembly and are formed in each fixed vane. 
     The means of carrying air comprise drillings formed in the plates attached to the radially external ends of the vanes for hermetically closing off the vane cooling cavities of the nozzle assembly of the upstream stage, and drillings formed in the external annular rim of the nozzle assembly which extends radially between the radially external walls of the vane cooling cavities and the upstream hooks for suspending the ring sectors. 
     The drillings may be formed by electro-discharge machining and have a diameter of between about 0.1 and 5 mm. 
     In one embodiment of the invention, the drillings formed in the external annular rim of the nozzle assembly extend obliquely with respect to this rim and with respect to the axis of rotation. 
     These drillings may at their downstream ends open directly into the annular space in which the casing upstream hooks lie. 
     As an alternative, the drillings are formed at the internal periphery of the external annular rim and at their downstream ends open into an annular passage formed between the external annular rim of the nozzle assembly and an annular deflector attached and fixed to a downstream end part of the nozzle assembly. 
     The drillings may in this case be formed in the external rim of the nozzle assembly in the immediate vicinity of an external wall of revolution of the nozzle assembly, thus making it possible to avoid creating a thermal gradient in the external rim of the nozzle assembly as such a gradient would result in differential thermal expansion of this rim across its radial spread and in significant stresses in the vanes of the nozzle assembly. 
     The annular deflector is for example engaged and fixed in an external annular groove of the nozzle assembly and bears axially on the upstream ends of the ring sectors in order to limit the passage of gas from the turbine airstream radially outward into the annular passage that houses the casing hooks. 
     The annular deflector is advantageously split into sectors and made up of several parts assembled end to end via sealing strips. 
     In yet another alternative, the drillings formed in the external annular rim of the nozzle assembly are more or less perpendicular to this rim and are supplied with cooling air via slots formed in regions where this rim catches on the casing of the turbine. 
     The present invention also relates to a turbine for a turbomachine such as an aviation turbojet engine or turboprop and which comprises a cooling device as described hereinabove. 
     The present invention also relates to a turbomachine turbine upstream nozzle assembly comprising an annular row of vanes which are connected at their radially internal ends to an internal wall of revolution and at their radially external ends to an external wall of revolution, the vanes comprising internal cavities for the flow of cooling air and the external wall comprising an external annular rim at its downstream end which rim is formed with means for catching on a casing of the turbomachine, wherein: the internal cavities of the vanes are closed, at their radially external ends, by plates attached to the external wall of the nozzle assembly; and these plates and the annular rim of the nozzle assembly comprise drillings for the passage of cooling air. 
     The drillings may be formed at the internal periphery of the annular rim. They may also be formed obliquely or perpendicularly with respect to the annular rim. 
     An annular deflector may also be fixed to the external wall of revolution of the nozzle assembly downstream of its annular rim. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and other characteristics, details and advantages thereof will become more clearly apparent from reading the description which follows, given by way of nonlimiting example with reference to the attached drawings in which: 
         FIG. 1  is a schematic part-view in axial section of a turbomachine equipped with the device according to the invention; 
         FIG. 2  is a view on a larger scale of part of  FIG. 1  and depicts the nozzle assembly of the upstream stage of the turbine; 
         FIG. 2   a  is an enlarged view of detail I 2  of  FIG. 2 ; 
         FIG. 3  is a schematic part-view in perspective of the nozzle assembly of the upstream stage of the turbine, viewed in side view and from the upstream end; 
         FIG. 4  is a view corresponding to  FIG. 2  and depicts an alternative form of embodiment of the device according to the invention; 
         FIG. 4   a  is an enlarged view of detail I 4  of  FIG. 4 ; 
         FIG. 5  is a schematic part-view in axial section of another alternative form of embodiment of the device according to the invention; 
         FIGS. 6 and 7  are schematic part-views in perspective of the external annular rim of the nozzle assembly of  FIG. 5 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1 , the reference  10  denotes a turbine of a turbomachine consisting of a high-pressure module  12  arranged at the outlet of a combustion chamber  14  and of a low-pressure module  16  situated downstream of the high-pressure module  12  and comprising four stages each including a nozzle assembly  18  formed of an annular row of fixed vanes  12  borne by an external casing  22  of the turbine and an impeller  24  downstream of the nozzle assembly  18 . 
     The impellers  24  comprise disks  26  assembled axially with one another by annular flanges  28  and bearing radial vanes  30 . The impellers  24  are connected to a turbine shaft (not depicted) by means of a drive cone  32  fixed to annular flanges  28  of the disks  26 . 
     Each impeller  24  is surrounded externally, with a small clearance, by a cylindrical shroud formed of ring sectors  34  fixed circumferentially to the casing  22  of the turbine by means of C-shaped or U-shaped locking pieces as will be described in greater detail hereinafter. 
     The nozzle assemblies  18  comprise internal and external walls of revolution  36  and  38 , respectively, which between them delimit the airstream for the flow of the gases through the turbine and between which the vanes  20  extend radially. 
     The external wall  38  of the nozzle assembly  18  of the upstream stage best visible in  FIG. 2  comprises upstream  40  and downstream  42  radially external annular rims including axial annular lugs  44  directed in the upstream direction and intended to be engaged in corresponding axial annular grooves  45  in the casing  22  of the turbine. 
     The vanes  20  of this nozzle assembly  18  comprise internal cavities  46  for the circulation of cooling air originating from a supply volume  48  (as depicted by the arrows  43 ) radially external to the wall  38  of the nozzle assembly, this air being partially removed in the airflow of gases of the turbine through orifices  50  formed near the trailing edge of the vanes  20  and opening into their internal cavities  46  (arrows  51 ) and partially removed into a volume  52  radially internal to the wall  36  of the nozzle assembly (arrows  53 ). The cooling air is bled off upstream from a compressor of the turbomachine and carried to the supply volume by ducts which have not been depicted. 
     The vane cavities  46  are connected to the external  48  and internal  52  volumes by cylindrical tubes  54  and  55  respectively. Each tube  54  for the passage of air between the external volume  48  and the cavity  46  of a vane has one end engaged airtightly in a bushing  56  fixed into an orifice formed in the wall  38  of the nozzle assembly between the external annular rims  40 ,  42  and opening into the internal cavity  46  of a vane. The other of its ends is engaged airtightly in a bushing  57  fixed in an orifice formed in the casing  22  of the turbine. The tubes  55  for the passage of air between the cavities  46  of the vanes and the internal volume  52  have their ends engaged airtightly in orifices  58 ,  59  in the wall  36  of the nozzle assembly and of an annular rim of a casing  60  of the volume  52 , respectively. 
     The cavity  46  of each vane of the nozzle assembly  18  comprises an opening formed in the external wall  38  of the nozzle assembly near the orifice in which the bushing  56  is fixed. A plate  64  is attached and fixed to the wall  38  as can be seen in  FIG. 3  in order to hermetically close off the vane cavity  46 . 
     The ring sectors  34  situated directly downstream of the nozzle assembly  18  of the upstream stage ( FIGS. 2 and 2   a ) each comprise, at their upstream ends, a circumferential hook  70  in the form of a portion of a cylinder which is pressed against a corresponding circumferential hook  72  in the form of a portion of a cylinder belonging to the casing  22  and is held in place by a C-shaped or U-shaped fastener  74  engaged via the upstream side over the circumferential hooks  70  and  72 . 
     The fasteners  74  and the hooks  70 ,  72  are housed in an annular space  76  which extends around the ring sectors  34  between the casing and the nozzle assembly  18 , the fasteners  74  bearing at their upstream ends against a downstream face of the downstream annular rim  42  of the external wall  38  of the nozzle assembly. 
     The fasteners  74  and the circumferential hooks  70  and  72  of the ring sectors  34  and of the casing  22  are shielded from the heat by an annular sealing sheet  78  which is mounted between the ring sectors  34  and the downstream face of the annular rim  42  of the nozzle assembly in order to restrict the passage of gas from the turbine airflow radially outward into the annular space  76  that houses the casing hooks  72 . 
     The casing hooks  72  are, in service, subjected to high temperatures which may cause cracking or fissuring liable to destroy them. 
     The invention provides a simple solution to this problem by virtue of means for carrying cooling air to these hooks. 
     In a first embodiment of the invention as depicted in  FIGS. 2 and 3 , these means comprise drillings  80  formed in the plates  64  of each vane and drillings  82  formed obliquely in the downstream external rim  42  of the external wall  38  of the nozzle assembly to connect the internal cavities  46  of the vanes to the annular space  76  housing the hooks  70 ,  72 , the drillings  80  and  82  being uniformly distributed about the axis of the turbine. 
     In the example depicted, each plate  64  comprises, more or less in the middle, a cylindrical drilling  80  ( FIG. 3 ) directed more or less radially with respect to the axis of the turbine and opening at one end into the cavity  46  of the corresponding vane and at its other end into an annular passage  79  situated radially outside the wall  38  of the nozzle assembly and bounded axially by the external annular rims  40 ,  42  of the nozzle assembly. As an alternative, just some of the plates may have drillings  80  or the plates may comprise two drillings  80  or more. The drillings could equally be inclined with respect to the axis of the turbine and, for example, directed downstream and outward. 
     The drillings  82  formed in the external annular rim  42  of the nozzle assembly  18  are oblique with respect to the axis of the turbine and directed downstream and outward. At their upstream end they open into the annular passage  79  and at their downstream ends they open onto an internal cylindrical face of the fasteners  74  fitted over the hooks  70 ,  72 . 
     A small fraction of the airflow circulating through the cavities  46  of the vanes of the nozzle assembly  18  enters the annular passage  79  through the drillings  80  in the plates  64 , then enters the annular space  76  housing the hooks  70 ,  72  through the drillings  82  in the annular rim  42  of the nozzle assembly as depicted by the arrows  84 . The hooks  72  are thus cooled sufficiently to eliminate the risk of cracking or fissuring of the hooks. 
     This supply of air also makes it possible to keep the annular space  76  housing the hooks at a pressure higher than that of the hot gases flowing through the turbine, thus opposing the passage of these gases between the ring sectors  34  and the annular rim  42  of the nozzle assembly  18  at the annular sealing sheet  78 . 
     The number of drillings  80  formed in the plates  64  in the example depicted is greater than the number of drillings  82  formed in the annular rim  42  of the nozzle assembly  18 . The number of drillings  80  is, for example, about 96, and the number of drillings  82  is, for example, about 72. 
     As an alternative, the number of drillings  80  formed in the plates  64  may be equal to or lower than the number of drillings  82  formed in the annular rim  42  of the nozzle assembly  18 . 
     In the alternative form of embodiment of the invention depicted in  FIGS. 4 and 4   a , the drillings  80  formed in the plates  64  of the nozzle assembly are identical to those described with reference to  FIGS. 2 ,  2   a  and  3  and the annular passage  79  is connected to the annular space  76  housing the hooks by way of axial drillings  90  formed in the external annular rim  42  of the nozzle assembly and of axial slots  92  formed in the annular lugs  44  of this external rim  42 . The drillings  90  and the slots  92  are uniformly distributed about the axis of the turbine. 
     The drillings  90  formed in the external annular rim  42  of the nozzle assembly  18  are more or less parallel to the axis of the turbine and perpendicular to the rim  42  and at their upstream ends open onto an upstream face of the annular rim  42  which face lies radially on the outside of the annular catching lug  44  and at their downstream ends they open onto the downstream face of the annular rim  42  in the annular space  76  housing the hooks  70 ,  72 . 
     The slots  92  are formed in internal  94  and external  96  cylindrical surfaces of the annular lug engaged in the annular groove  45  of the casing  22 . 
     The slots  92  on the external cylindrical surface  96  at their downstream ends open in the vicinity of the upstream ends of the drillings  90  and at their upstream ends open into the bottom of the groove  45 , and the slots on the internal cylindrical surface  94  at their upstream ends open into the bottom of the groove  45  and at their downstream ends open into the annular passage  79 . 
     In the example depicted, each drilling  90  is associated with two slots  92  formed in the internal  94  and external  96  cylindrical surfaces of the annular lug  44 , respectively, which may or may not lie in the same radial plane as the drilling  90 . 
     The air in the annular passage  79  originating from the internal cavities  46  of the vanes is carried into the annular space  76  housing the hooks by the slots  92  on the internal then external surfaces of the annular lug  44  of the external rim  42  of the nozzle assembly, then by the drillings  90  in the external rim  42 , as depicted by the arrows  98 . 
     As an alternative, it is possible for the slots  92  not to be parallel to the axis of the turbine. These slots  92  could also be formed on the cylindrical surfaces of the groove  45  against which the cylindrical surfaces  94 ,  96  of the annular lug  44  rest, these slots opening into the annular passage  79  and in the vicinity of the drillings  90  as described previously. 
     In the alternative form depicted in  FIGS. 5 to 7 , the drillings  100  of the external angular rim  42  of the nozzle assembly  18  are not formed in the central or radially external part of the rim  42  but are formed in the immediate vicinity of the external wall  38  of the nozzle assembly and extend more or less parallel to this wall. 
     The drillings  100  at their upstream ends open into the annular passage  79  and at the downstream ends open into a second annular passage  102  running transversely with respect to the axis of the turbine and communicating at its external periphery with the annular space  76  that houses the hooks  72 . 
     The annular passage  102  surrounds the external wall  38  of the nozzle assembly and is axially bounded by the rim  42  of the nozzle assembly and by a deflector  104  attached and fixed to the external wall  38  of the nozzle assembly, downstream of the external rim  42 . 
     In the example depicted, the drillings  100  at their downstream ends open into an annular groove  106  opening outward and formed in the external wall  38  of the nozzle assembly, downstream of the rim  42 , and also comprising a radial wall  108  to which a radially internal end part of the deflector  104  is pressed and fixed by brazing or welding. 
     The deflector  104  is axially preloaded through the pressing of its radially external end part against the annular sealing sheet  78  mounted on the upstream ends of the ring sectors  34 , so as to limit the passage of gas from the turbine airflow radially outward into the annular space  76  housing the hooks  70 ,  72 . 
     As an alternative, the deflector  104  can bare axially directly on the downstream ends of the ring sectors  34 . 
     Air from the first annular passage  79  enters the second annular passage  102  through the drillings  100  and is then carried into the annular space  76  housing the hooks as depicted by the arrows  110 . 
     In the example depicted in  FIG. 6  the number of drillings  100  is greater than the number of drillings  80  formed in the plates  64  ( FIG. 3 ). The number of drillings  100  lies for example between 360 and 504. 
     The deflector  104  is preferably split into sectors and formed of a plurality of parts  112  assembled end to end by means of sealing strips. 
     In the example depicted in  FIG. 7 , the parts  112  are associated at each of their ends with means  114  into which a sealing strip can fit (although this is not depicted), each strip being engaged at one end in the means  114  of one part  112  and at an opposite end in the means  112  of an adjacent part  114 . 
     The fasteners  74  and the hooks  70  on the ring sectors  34  may also comprise drillings  116  and  118  for the passage of air in order to cool the hooks  72  of the casing  22  ( FIG. 5 ). 
     The drillings  80 ,  82 ,  90 ,  100 ,  116  and  118  have a diameter ranging between about 0.1 and 5 mm and may be formed by electro-discharge machining or by any other appropriate technique. 
     The embodiment of  FIGS. 5 to 7  makes it possible to avoid the creation of a thermal gradient in the external annular rim  42  of the nozzle assembly, something which would result in differential thermal expansion of this rim across its radial spread and in stresses in the vanes of the nozzle assembly  18 . The high number of drillings  100  allows the temperature over the internal periphery of the rim  42  to be evened out and allows this temperature to be lowered considerably. 
     The deflectors  104  allow the air used to cool the rim  42  to be recovered for cooling the casing hooks  72 . A slight increase in the cooling air flow rate compensates for the fact that the air is warmed a little by cooling the annular rim  42 , without detracting from engine performance.