Abstract:
A device for guiding a stream of air entering a combustion chamber of a turbomachine is disclosed. The device includes a flow straightener followed by a diffuser. One of the flow straightener shrouds is formed as one with one wall of revolution of the diffuser. The other flow straightener shroud is added and attached to the other wall of revolution of the diffuser. The vanes of the flow straightener are secured by one end to one shroud of the flow straightener and separated by a small clearance from the other shroud at their other end.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a device for guiding a stream of air entering a combustion chamber of a turbomachine, such as an aircraft turbojet or turboprop engine. 
     A device of this type is positioned at the outlet of a compressor and comprises a flow straightener followed by a diffuser, the diffuser comprising two coaxial walls of revolution joined together by radial partitions and the flow straightener comprising two coaxial shrouds axially aligned with the two walls of revolution of the diffuser respectively, and between which substantially radial vanes extend. 
     DESCRIPTION OF THE PRIOR ART 
     In the state of the art, it is known practice to form the flow straightener and the diffuser independently of one another in order to optimize their shape, their size and their function, and then for the downstream end of the flow straightener to be fixed by appropriate means to the upstream end of the diffuser. However, the fixing means used are heavy and bulky and may create pressure drops in the flow of air through the device. 
     It is also known practice for the flow straightener and the diffuser to be formed as one. However, this technique is very restricted because it does not allow the functions to be optimized nor does it allow the production of complex flow straighteners, particularly flow straighteners comprising a great many vanes of small radial dimension (such as, for example, a flow straightener comprising over 100 vanes). In particular, it is extremely complicated to cast a flow straightener and a diffuser as one because the casting cores that need to be inserted between the vanes of the flow straightener and the radial partitions of the diffuser are not very accessible and are therefore very difficult if not impossible to extract. 
     SUMMARY OF THE INVENTION 
     It is a particular object of the invention to provide a simple, economical and effective solution to these problems of the prior art. 
     To this end, the invention proposes a device for guiding a stream of air entering a combustion chamber of a turbomachine, comprising a flow straightener followed by a diffuser, the flow straightener comprising two coaxial shrouds between which substantially radial vanes extend, and the diffuser comprising two coaxial walls of revolution joined together by radial partitions, wherein one of the shrouds of the flow straightener is formed as one with one wall of revolution of the diffuser, the other of the shrouds of the flow straightener being added and attached to the other wall of revolution of the diffuser, the vanes of the flow straightener being secured by one end to one shroud of the flow straightener and separated by a small clearance from the other shroud at their other end. 
     According to the invention, just one of the shrouds of the flow straightener is formed as one with the diffuser, and the other shroud of the flow straightener is attached and fixed, for example welded, to the diffuser so that the device becomes a one-piece unit. The device according to the invention therefore does not require any special heavy and bulky fixing means, thus allowing the mass of the device to be reduced and making it possible not to restrict the aerodynamic function whereby the stream of air is guided by the device. 
     The invention is therefore a compromise between the manufacturing techniques hitherto used and allows the functions to be optimized while at the same time maintaining relative ease of manufacture. In particular, the shapes and sizes of the flow straightener can be optimized independently of those of the diffuser and are not restricted by the method used to produce the device, it being possible for the flow straightener to be complicated and to have a small radial dimension and a great many vanes. For example, when one of the shrouds of the flow straightener and the diffuser are formed as a single casting with the vanes of the flow straightener and the radial partitions of the diffuser, the casting cores are readily accessible for extraction because the other of the shrouds of the flow straightener is not present. 
     The invention makes it possible, in general, to save on the need to produce one of the shrouds of the flow straightener, because this shroud is cast with the diffuser. It also makes it possible to save on having to produce the flow straightener vanes, when these are cast with the diffuser. 
     In addition, it is simpler to manufacture and to install and allows a mass saving that is not insignificant over current techniques. It also reduces the risks of non-conformity, reduces manufacturing times and improves reliability because the flow straightener-diffuser assembly is ultimately a one-piece monobloc unit. 
     According to another feature of the invention, the outer shroud of the flow straightener and the outer wall of revolution of the diffuser are formed as one, and the end of the inner shroud of the flow straightener is fixed, for example welded, to the upstream end of the inner wall of revolution of the diffuser. 
     In one embodiment of the invention, the vanes of the flow straightener are secured via their radially outer end to the outer shroud of the flow straightener and are separated by a small radial clearance from the inner shroud of the flow straightener. 
     As an alternative, the vanes of the flow straightener are secured via their radially inner end to the inner shroud of the flow straightener and are separated by a small radial clearance from the outer shroud of the flow straightener. 
     The clearance between the vanes and the shroud of the flow straightener ranges, for example, between about 0.1 and 0.5 mm in operation. 
     The vanes are secured to the shroud of the flow straightener by inserting and fixing, for example by brazing, one of the ends of the vanes in a corresponding slot made in the shroud. The vanes may thus be optimized and produced independently of the shroud to which they are fixed. 
     As an alternative, the vanes are formed as one with the shroud of the flow straightener. When this shroud is the outer shroud of the flow straightener, the vanes are formed at the same time as the diffuser. 
     The inner shroud of the flow straightener advantageously comprises an annular axial rim collaborating with the inner wall of revolution of the diffuser to make it easier for the shroud to be positioned and fixed on the diffuser. 
     The invention also relates to a diffuser for a turbomachine, comprising two coaxial walls of revolution joined together by radial partitions, wherein one of the walls of revolution of the diffuser is extended axially in the upstream direction beyond the other of the walls of revolution of the diffuser to form a shroud of a flow straightener intended to be positioned axially upstream of the diffuser. 
     According to one embodiment, the upstream extension of the wall of the flow straightener comprises a plurality of radial through-slots uniformly distributed about the axis of the diffuser. There are, for example, in excess of 100 of such slots. 
     As an alternative, the upstream extension of the wall of the flow straightener is formed as one with radial vanes which are uniformly distributed about the axis of the diffuser. There are, for example, in excess of 100 of such vanes. 
     The invention also relates to a shroud for a turbomachine flow straightener and which is formed as one with vanes extending radially from a cylindrical surface of the shroud and distributed uniformly about the axis of the shroud. 
     The invention also relates to a shroud for a turbomachine flow straightener and which comprises a plurality of radial through-slots uniformly distributed about the axis of the shroud. 
     In addition, the invention relates to a vane for a turbomachine flow straightener, comprising a leading edge and a trailing edge for the gases and which at one of its longitudinal ends comprises means for fitting into a corresponding slot in a flow straightener shroud, in a direction substantially parallel to the longitudinal axis of the vane. 
     Finally, the invention relates to a turbomachine, such as an aircraft turbojet or turboprop engine and which comprises a device as described hereinabove. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and other details, features and advantages of the present invention will become apparent from reading the following description, given by way of nonlimiting example and with reference to the attached drawings in which: 
         FIG. 1  is a partial schematic view in axial section of the device for guiding a stream of air according to the invention; 
         FIG. 2  is a view on a larger scale of part of  FIG. 1 ; 
         FIG. 3  is a schematic perspective view of a variant embodiment of the device according to the invention, viewed from the upstream direction and without the inner shroud of the flow straightener; 
         FIG. 4  is another schematic perspective view of the device of  FIG. 3 ; 
         FIG. 5  is a view corresponding to  FIG. 2  and illustrating another variant embodiment of the invention; 
         FIG. 5   a  is an enlarged view of an embodiment detail in a variant of the invention; 
         FIG. 6  is a schematic perspective view of the device of  FIG. 5 , viewed from the upstream direction and without the inner shroud and the vanes of the flow straightener; 
         FIG. 7  is a schematic perspective view of part of the flow straightener of  FIG. 6 ; 
         FIG. 8  is a view corresponding to  FIG. 7  and illustrating a variant embodiment of the flow straightener. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The device  10  according to the invention depicted in  FIG. 1  is mounted axially between a compressor (not depicted) located upstream and a combustion chamber  12  of the turbomachine in order to straighten and guide the stream of air leaving the compressor and supplying air to the chamber  12  which itself feeds a turbine (not depicted) positioned downstream of the chamber. 
     This device  10  comprises, from the upstream direction downstream, a flow straightener  14  and a diffuser  16  which are joined together and supported by an outer frustoconical web  18  which diverges outward in the upstream direction and is fixed by an outer annular flange  20  to an outer casing  22  of the chamber and by an inner frustoconical web  24  which converges inward in the downstream direction and is fixed by an inner annular flange  26  to an inner casing  28  of the chamber  12 . 
     The outer casing  22  of the chamber bears fuel injectors  30  uniformly distributed on a circumference about the longitudinal axis of the chamber and opening into the chamber at their radially inner end. The fuel injected into the chamber is intended to mix with air leaving the device  10  and be burnt then injected into the turbine to drive the rotation of a shaft of the turbomachine. 
     The diffuser  16  comprises an inner wall of revolution  32  connected to the inner frustoconical web  24  and an outer wall of revolution  34  connected to the outer frustoconical web  18 , the walls  32  and  34  being joined together by radial partitions  36 , for example 18 of these. 
     The flow straightener  14  comprises an inner shroud  38  which is axially aligned with the inner wall of revolution  32  of the diffuser, an outer shroud  40  which is axially aligned with the outer wall of revolution  34  of the diffuser, and vanes  42  which extend radially between the inner  38  and outer  40  shrouds of the flow straightener. There are, for example, 126 vanes  42  that are independent of, and axially offset in the upstream direction from, the radial partitions  36  of the diffuser. 
     In the example depicted in  FIGS. 1 and 2 , the outer shroud  40  and the vanes  42  of the flow straightener  14  are formed as one with the outer wall  34  of the diffuser  16 . The inner shroud  38  of the flow straightener is fixed at its downstream end, for example by welding  44 , to the upstream end of the inner wall  32  of the diffuser, and is separated from the radially inner end of each vane  42  by a small radial clearance  46 . The vanes  42  and the outer shroud  40  of the flow straightener are, for example, cast as one with the diffuser  16 . The inside diameter of the shroud  38  at its downstream end is equal to the inside diameter of the wall  32  of the diffuser at its upstream end. 
     The radial clearance  46  between the radially inner ends of the vanes and the inner shroud may vary from 0.1 to 0.5 mm in operation because of thermal expansions and because of the centrifugal forces to which the vanes  42  and the shrouds  38 ,  40  of the flow straightener are subjected. 
     In the variant embodiment depicted in  FIGS. 3 and 4 , the vanes  42  are fixed at their radially outer end  48  to the outer shroud  40  of the flow straightener which is still formed as one with the diffuser  16 . The radially inner ends  50  of the vanes are separated by a small radial clearance  46  from the inner shroud (not depicted) of the flow straightener which is welded to the upstream end  51  of the inner wall  32  of the diffuser. 
     The vanes  42  comprise, in the conventional way, an intrados  52  or concave interior surface and an extrados  54  or convex exterior surface which are joined at their upstream end at a leading edge  56  and at their downstream end at a trailing edge  58 . The vanes are for example produced by ECM (Electro Chemical Machining). 
     The radially outer end part of each vane forms means  48  of insertion into a corresponding slot  60  made in the outer shroud  40  of the flow straightener. The slots  60  are substantially radial and through-slots and are uniformly distributed about the axis of the shroud. The means of insertion  48  of the vane are engaged in a slot  60  in a substantially radial direction and are held therein for example by brazing the radially outer end of the vane to the outer cylindrical surface of the shroud  40 . The slots  60  may be cast in or formed by laser cutting of the shroud. 
     In one particular embodiment, the vanes  42  are held in position in the slots  60  of the shroud by means of manually aimed spot flash welding (with spots 1 mm in diameter for example) or electric arc welding between the radially outer ends of the vanes and the outer cylindrical surface of the shroud. 
     In another variant depicted in  FIGS. 5 to 8 , the vanes  42  of the flow straightener are secured by their radially inner end to the inner shroud  38  of the flow straightener which is attached and fixed at its downstream end to the upstream end of the inner wall  32  of the diffuser. The outer shroud  40  of the diffuser is formed as one with the diffuser  16  and is separated by a small radial clearance from the radially outer end of each vane. 
     In  FIG. 7 , the vanes  42  and the inner shroud  38  of the flow straightener are formed as one by an appropriate technique such as casting, EDM (Electro Discharge Machining), high speed milling, etc. 
     As an alternative and as depicted in  FIG. 8 , the radially inner ends  62  of the vanes  42  are inserted and fixed in corresponding radial slots  64  of the inner shroud. These vanes are similar to those described with reference to  FIGS. 3 and 4  and the slots  64  in the shroud are through-slots uniformly distributed about the axis of the shroud. The radially inner ends of the vanes may be brazed to the interior cylindrical surface of the shroud  38 . 
     The inner shroud  38  of the flow straightener advantageously comprises a centering axial annular rim  66  ( FIG. 5   a ) intended to be engaged radially inside the inner wall of the diffuser to make it easier to fit the shroud. The shroud  38  in this case is fixed to the wall  32  of the diffuser by welding the downstream end of this rim  66  to the interior cylindrical surface of the wall  32  in order to prevent the spots or bead of weld material from projecting toward the inside and disturbing the flow of air through the device. The inner shroud  38  of each aforementioned device may also be equipped with this axial rim  66 . 
     In the examples of  FIGS. 4 ,  5  and  6 , the diffuser  16  comprises a divider  70  which is borne by the radial partitions  36  of the diffuser between its inner  32  and outer  34  walls, this divider having the purpose of splitting the stream of gas leaving the flow straightener into two coaxial annular flows. 
     The flow straightener  14  and the diffuser  16  are, for example, made of an alloy based on nickel and chromium.