Abstract:
A low of compressed air originating from a compressor stream ( 3 ) is drawn off through first orifices ( 13 ) towards ducts ( 5, 6 ) leading outside the casing, creating a flow division through adjacent cavities ( 11, 12 ) in the casing ( 1 ). These arrangements make it possible to carry a large air flow even if the cavity cross sections ( 11, 12 ) are small.

Description:
DESCRIPTION  
         [0001]    The invention is related to a system for drawing off air from a compressor.  
           [0002]    Aircraft engines comprise gas turbines in which the air used in the composition of the fuel mix is compressed. A portion of the compressed air is drawn off from the compressor for various purposes, including pressurization of the cabin, de-icing and ventilation of the engine turbine to cool it.  
           [0003]    Compressed air is drawn off through the stator casing, which in particular is composed of an outer skin that makes the assembly rigid and an inner lining that delimits the air flow stream and is essentially composed of annular segments assembled to each other, some of which carry the fixed compressor blades and the others that alternate with the first are arranged in front of the mobile blades of the rotor. Orifices are drilled in this lining so that part of the air flowing into the stream enters the lining; if the flow to be drawn off is large, these orifices may be replaced by a complete circular slit. The orifices communicate with a cavity formed between the outer skin and the inner lining of the casing, and therefore it is possible that it is sufficient to drill orifices through the outer skin to access this cavity and drawing off air that flowed in it and allowing it to take a path leading to where it is wanted.  
           [0004]    However, problems sometimes arise when a large air flow is to be drawn off. Since the air flow speed is limited, the flow can be increased only by increasing the size of the cross-section available for the drawing off flow, at all positions along this flow. This requirement can be satisfied partly by increasing the size of the orifices passing through the inner lining and the outer skin of the casing, and the evaluation duct; but it is impossible to increase the size of the intermediate cavities between the outer skin and the inner lining, that the drawn off air passes through in the tangential direction of the engine to collect under the outer orifice, since the section of the annular cavities is determined by the shapes of the outer skin and the inner lining imposed for other reasons.  
           [0005]    Therefore, the section of the drawing off cavity is frequently too small to enable the requested air flow. It is also possible that the cavity is not suitable for convenient routing of drawn off air, if it is partitioned or for another reason. A conventional solution to this difficulty is that up to now, a circular tube called the manifold was placed around the cavity, the role of which was to replace this cavity by providing a sufficient cross section for annular air flow collecting towards the final drawing off duct. Air then passed approximately radially through the cavity, leaving through a group of a sufficient number of orifices leading to the inside of the manifold. However, the manifold is relatively expensive to make and to install, and the casing surround was sometimes too large to contain it. An equivalent design described in patent FR 2 616 890 A consisted of replacing the manifold by a large number of separate ducts, each connected to one of the orifices passing through the outer skin of the casing. Another design represented by U.S. Pat. No. 5,351,478 is compromise between the previous design and a design with a single drawing off duct; it is recommended that four drawing off ducts should be arranged crosswise around the outer skin in order to reduce the air movement in the tangential direction in the cavity.  
           [0006]    Therefore, prior art shows that the tangential movement in the cavity can be reduced at will by increasing the number of taking off orifices, such that the hindrance to flow caused by the cross sectional area or the conformation of the cavity disappears. However, the ramification of the flow may be considered to be a problem if it is excessive due to design constraints imposed by a manifold surrounding the orifices or a large number of separate drawing off ducts.  
           [0007]    The purpose of the invention is then to enable drawing off of compressed air through a casing comprising an outer skin and an inner lining and that only tolerates a limited air flow within its body in the tangential direction, but in which excessive ramification of the flow is avoided and the drawing off device around the casing is simple and compact. The ramification in the tangential direction is replaced by a combined tangential and longitudinal ramification that advantageously simplifies the system.  
           [0008]    The invention thus relates to a system for drawing off air from a compressor in an aircraft engine through a lining of a casing that delimits an air flow stream, comprising orifices passing through an outer skin of the casing that delimits several cavities with the lining, the take off air passing through the orifices after passing through the lining, and also comprising drawing off ducts connected to the outer skin around the orifices, characterized in that two of the cavities carry drawn off air, the drawing off ducts extend through openings around pairs of orifices, the orifices in each of the pairs leading to the said corresponding two cavities through which the drawn off air passes. Typically, there are two diametrically opposite ducts on the casing. Finally, it is advantageous that the lining should comprise a single slit in front of the upstands delimiting the two cavities through which the air flow passes and through which air inlet orifices are drilled in the said cavities; air drawn off from a particular location in the compressor is in the same homogeneous state in the two cavities and is combined in the ducts without turbulence. 
       
    
    
       [0009]    These aspects of the invention and others will now be described in more detail with reference to the following figures:  
         [0010]    [0010]FIG. 1 is a general view showing the layout of the invention,  
         [0011]    [0011]FIG. 2 shows the detail of the drawing off method, and  
         [0012]    [0012]FIG. 3 illustrates the layout of orifices in the taking off ducts. 
     
    
       [0013]    [0013]FIG. 1 shows a compressor casing  1  separated from a concentric rotor  2  by a compressed air flow stream  3 . Compressed air is drawn off from the compressor through two ducts  5  and  6  that are used in common, for example for pressurization of the aircraft cabin, and are connected to the casing  1  through two diametrically opposite openings  7  and  8 . The ducts  5  and  6  are not joined together at all, or if they are joined together the junction is at a distance from the casing  1 .  
         [0014]    The remainder of the description is equally applicable to the region of the opening  8  and to the region of the opening  7 , but only the opening  7  will be described. With reference to FIG. 2, it can be seen that the casing  1  is composed of an outer skin  9  and a lining  10  separated by cavities, two of which are shown on the drawings and are marked with references  11  and  12 . The opening  7  is fixed to the outer skin  9 , while the lining  10  delimits the air stream  3 . The lining  10  comprises orifices, in this case a single circular slit  13  to draw off compressed air passing along the stream. The slit  13  is formed between a straightening ring  14  and a shell ring  15  which form part of the lining  10  and are connected to the outer skin  9  through approximately radial upstands marked with references  16  and  17  respectively and that extend behind the slit  13 , the cavity  11  extends behind the upstand  16  and the cavity  12  behind the upstand  17 .  
         [0015]    Orifices  18  and  19  are drilled in upstand  16  and  17  to enable the compressed air trapped in slit  13  common to cavities  11  and  12  to enter into it. Air passes through cavities  11  and  12  and half of it leaves through orifices  20  and  21  passing through the outer skin  9  to reach the opening  7 , the other half leading to opening  8 .  
         [0016]    According to the design described in document U.S. Pat. No. 5,351,478 A, four drawing off ducts would have communicated with the cavity  11 , for example, through the corresponding number of orifices with a sufficiently large cross section uniformly distributed at right angles. In this case the orifices were grouped differently, in two rows each comprising an orifice  20  and an orifice  21  oriented in the longitudinal direction of the compressor, each of the rows being surrounded by one of the openings  7  and  8 . The area of the orifices  20  and  21  may be the same as the area that would have been provided with a manifold. Oblong orifices  20  and  21  can be provided extending in the tangential direction of the compressor, so that the openings  7  and  8  with a rectangular (almost square) transition cross-section can be used, that connect to ducts  5  and  6  with a smaller cross-section, by gradually modifying the section so that they are in the general shape of a bell.  
         [0017]    The use of several drawing off ducts  5  and  6  reduces the air flow passing tangentially through cavities  11  and  12  to reach openings  7  and  8 , and sharing the drawn off flow between the two cavities  11  and  12  further reduces this tangential flow. Thus, the drawn off flow is no longer limited by the size of the sectional cavities  11  and  12  being too small. Another major advantage of this invention is that orifices  20  and  21  are grouped in adjacent pairs under openings that are large enough so that half of the drawing off ducts in previous designs can be eliminated.  
         [0018]    [0018]FIG. 3 also shows other orifices  22  passing through the outer skin  9  and located on the line of the orifices  21 , in other words leading to the same cavity  12 . These are drawing off orifices for other uses of a proportion of the compressed air drawn off through the slit  13  and that ended up in cavity  12 . Thus, the joint contribution of cavities  11  and  12  for the air supply to be used for other specific purposes (in this case the cabin) does not exclude the use of these cavities to supply an additional air flow to be used for other purposes.