Abstract:
A turbomachine includes blade stages on the trajectory of a flow of air or gas delimited by a symmetrical wall along the axis of the turbomachine. The wall has orifices communicating with a cavity outside the flow of air of generally symmetrical structure. The cavity is connected to a pipe. Symmetry degrading arrangements are provided inside the cavity, for example in the form of a portion of tube mounted in the pipe to project partly into the cavity.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a turbomachine including a plurality of blade stages on the trajectory of a flow of air or gas. 
     2. Description of the Prior Art 
     In this kind of turbomachine, one or more cavities are provided on the outside of the flow of air and communicate with the flow of air via a plurality of orifices formed in a symmetrical wall along the axis of the turbomachine delimiting the flow of air. A pipe including a discharge valve is generally connected to the cavity to sample a portion of the flow of air to be rejected to the outside when the turbomachine is operating under partial load to improve stable operation of the turbomachine or to satisfy an auxiliary demand. The volume of the cavity must therefore be sufficient to enable regular sampling in use. 
     It is nevertheless found that the air or the gas flowing across the cavity can trigger acoustical resonance in the cavity in some speed ranges because of boundary layer shear. 
     Such resonance is encouraged by the structure of the cavity, which is generally symmetrical. The cavity can be symmetrical with respect to the axis of the turbomachine or incorporate patterns, bosses or other raised members regularly distributed over its periphery to produce cyclic symmetry. The break in the symmetry caused by the intake of the pipe for sampling air for the discharge valve or for cooling the discs and blades of the turbine of the turbomachine is insufficient for it to be certain that acoustical resonance in the cavity will be prevented. 
     Acoustical resonance has major drawbacks and can lead to the risk of blades breaking. 
     The invention therefore relates to a turbomachine including means for eliminating or preventing the generation of rotating acoustical waves in the previously mentioned cavity and therefore suppressing the drawbacks due to acoustical resonance in said cavity. 
     SUMMARY OF THE INVENTION 
     The turbomachine in accordance with the invention includes a plurality of blade stages on the trajectory of a flow of air or gas delimited by a symmetrical wall along the axis of the turbomachine, the wall having orifices communicating with a cavity outside the flow of air, of generally axially or cyclically symmetrical structure. Symmetry degrading means are provided inside said cavity. 
     The symmetry degrading means can take various forms. 
     In a preferred first embodiment of the invention the symmetry degrading means include a spacer mounted in a pipe connected to the cavity to project partly into the cavity. 
     The spacer is preferably force-fitted into said pipe to limit vibration in operation. 
     The spacer can advantageously be a portion of tube force-fitted into said pipe from the outside. 
     In a second embodiment the symmetry degrading means include a convex localized area of the cavity having its convex side facing towards the inside of the cavity. 
     The convex area can be obtained by localized stamping of the outside or inside wall of the cavity. 
     In another embodiment of the invention the symmetry degrading means include a member fixed to the inside face of the outside wall of the cavity at a particular location, for example a portion of sheet metal welded to the inside face of the outside wall of the cavity or to the inside wall of the cavity. 
     In another embodiment of the invention the symmetry degrading means include a screw passing through the outside wall of the cavity and projecting into the cavity. 
    
    
     The invention will be better understood after reading the following description of embodiments of the invention shown by way of non-limiting example only in the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an outside lateral view of a turbomachine. 
     FIG. 2 is a partial sectional view of the turbomachine shown in FIG. 1, showing a first embodiment of the invention. 
     FIG. 3 is a sectional view similar to FIG. 2, showing a second embodiment of the invention. 
     FIG. 4 is a sectional view similar to FIG. 2, showing a third embodiment of the invention. 
     FIG. 5 is a sectional view similar to FIG. 2, showing a fourth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIGS. 1 and 2, the turbomachine in accordance with the invention has an air intake  1  provided with a first set of rotary blades  2 . 
     The outside wall  3  of the turbomachine has a connecting spigot  4  for a pipe for rejecting some of the flow to the surrounding air. 
     The sectional view of FIG. 2 shows the rotating shaft  5  on which are mounted the rotating blades  6  of a first compressor stage of the turbomachine. The fixed hub  7  has fixed director blades  8 . The arrows  9  symbolize the flow of air. 
     The wall  10  delimits the flow of air on the outside and has a symmetrical configuration with respect to the axis of the turbomachine. A cavity  11  whose structure is also generally symmetrical with respect to the axis of the turbomachine is defined between the wall  10  and an outside wall  12  and substantially at the location of the rotary blades  6 . The wall  10  has at its periphery a plurality of orifices  13  establishing communication between the cavity  11  and the flow of air. The orifices  13  can be slots, half-moon shapes or circular grooves. Of course, in different embodiments, the cavity  11  could feature cyclic symmetry, i.e. include a plurality of patterns or other elements regularly disposed inside the cavity and therefore susceptible to cause acoustical resonance in the cavity. 
     A discharge valve  14  controlled by means that are not shown in the figure can be seen in the FIG. 2 cross-section. The valve  14  is downstream of the spigot  4  which is itself attached to the wall  12  at a particular location in the cavity  11 . 
     When it is open, the discharge valve  14  samples a portion of the flow of air in order to reject it to the exterior so as to improve the operation of the turbomachine under certain loads. When the discharge valve  14  is closed, the flow of air can cause resonance in the cavity  11  because of boundary layer shear on passing over the orifices  13  at speeds in certain ranges. 
     In accordance with the invention, the generation of rotating acoustical waves in the cavity  11  is prevented by intentionally and greatly degrading the symmetry of the cavity  11  with respect to the axis, over and above the degraded symmetry which already exists because of the presence of the spigot  4 . 
     In the embodiment shown in FIG. 2, the symmetry degrading means include a portion of tube  15  force-fitted into the pipe  4 . It is preferably fitted from the outside, the tube portion  15  being pushed in until a radial shoulder  16  on the outside edge of the tube portion  15  abuts against a conical portion  17  of the pipe  4  to define the final position of the tube portion  15 . 
     In this final assembly position, the tube portion  15  partly projects into the cavity  11 , beyond the wall  12 , but without coming into contact with the inside wall  10 . This is to prevent unduly disturbing the flow of air in the cavity  11  when the discharge valve  14  is open. 
     The embodiment shown in FIG. 3 differs from the FIG. 2 embodiment only in that the outside wall  12  has a localized convex area  18  whose convex side faces towards the inside of the cavity  11 . This area is preferably obtained simply by localized stamping of the sheet metal constituting the outside wall  12 . The stamped area  18  therefore degrades symmetry further, over and above the existing degraded symmetry when a pipe  4  is provided at another location in the cavity  11 , as shown in FIG.  2 . 
     The embodiment shown in FIG. 4 differs from the FIG. 3 embodiment in that a portion of sheet metal  19  disposed radially is welded to the inside face of the outside wall  12  of the cavity  11 . The welded sheet metal portion  19  therefore projects into the cavity  11  and prevents the generation of rotating acoustical waves in the cavity  11 . The pipe  4  can be at some other location in the cavity  11 , of course. Note that the dimensions of the welded sheet metal portion  19 , which is square in the embodiment shown by way of example in FIG. 4, are such that the welded sheet metal portion  19  extends from the outside wall  12  towards the inside wall  10  but without coming into contact with the latter. The welded sheet metal portion  19  can instead be fixed to the inside wall  10  and extend towards the outside wall  12 . 
     The embodiment shown in FIG. 5 differs from the FIG. 4 embodiment in that a screw  20  passes through the outside wall  12  of the cavity  11  and projects a particular distance into said cavity  11 . To facilitate mounting it, the outside wall  12  has an area  13  with a screwthread which can cooperate with the screwthread of the screw  20 , whose head  21  remains outside the outside wall  12 . 
     The dimensions of the screw  20  projecting into the cavity  11  are such that said screw extends towards the inside wall  10  in a radial plane without coming into contact with the inside wall  10 . 
     In all the embodiments that have just been described by way of example, symmetry degrading means are therefore introduced into the symmetrical cavity to prevent the generation of rotating acoustical waves and thereby prevent resonance in the cavity, regardless of the flow speed in the turbomachine.