Abstract:
The invention relates to a twin-spool gas turbine turbojet comprising a high-pressure rotor ( 1 ) and a low-pressure rotor, the low-pressure rotor shaft ( 2 ) being connected, at its upstream end, to a fan ( 10 ) housed in a fan casing ( 14 ), which turbojet comprises, upstream of the fan ( 10 ), a fixed cowl element ( 12 ) centered on the axis (X 3 ) of the engine and on which there is mounted an electric current generator ( 20 ) designed to take mechanical power off the low-pressure rotor shaft ( 2 ) and convert it into electrical power.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a gas turbine turbojet with an upstream fan and to a method of mounting an electric current generator in the turbojet. 
         [0002]    Some of the power generated by an aeronautical turbojet engine is used to power various parts both of the turbojet and of the aircraft propelled in full or in part by that turbojet. 
         [0003]    Some of this power is currently taken off the high-pressure (HP) compressor, the compressed air of which is used, particularly for pressurizing and conditioning the cabin of the aircraft, or alternatively for de-icing. Some of this power is taken mechanically off the shaft of the HP stage of the turbojet to drive the input shaft of an accessories gearbox positioned on a casing of the turbojet. This input shaft is rotationally driven by a transmission shaft extending through a structural arm of the casing and itself driven by a pinion secured to the HP shaft. 
         [0004]    There is a current trend toward increasing the installed electrical power so tapping mechanical power from the engine is anticipated. 
         [0005]    However, drawing too much mechanical power has a detrimental effect on the operation of the HP spool because it is liable to adversely affect engine operability, particularly when the engine is running at low speed. 
       DESCRIPTION OF THE PRIOR ART 
       [0006]    Patent Application FR 2882096 discloses taking some of the mechanical power off the low-pressure (LP) spool to drive the rotation of the input shaft of an accessories gearbox. A solution such as this entails structural modifications to the LP shaft  2  by adding a power transmission pinion to it. A system such as this is difficult to assemble because it involves moving around metallic parts that are bulky and heavy. 
         [0007]    Patent Application WO2007/036202 also discloses mounting an electric current generator in the turbojet spool. The generator is made up of a stator element positioned circumferentially in the compression casing of the turbojet, and of rotor elements fixed to the ends of blades secured to the HP shaft and rotationally driven in the compression casing of the turbojet. 
         [0008]    The rotational movement of the rotor elements induces a current in the stator element, which current is transmitted to the various pieces of equipment that require power. A current generator such as this is difficult to access and entails partial disassembly of the turbojet when it needs to be replaced or serviced. The compressor casing is of a small size, making it complicated to route the generated current to the various pieces of equipment. 
       SUMMARY OF THE INVENTION 
       [0009]    In order to alleviate at least some of these disadvantages, the applicant company has proposed a twin-spool gas turbine turbojet comprising a high-pressure rotor and a low-pressure rotor, the low-pressure rotor shaft being connected, at its upstream end, to a fan housed in a fan casing, which turbojet comprises, upstream of the fan, a fixed cowl element centered on the axis of the engine and on which there is mounted an electric current generator designed to take mechanical power off the low-pressure rotor shaft and convert it into electrical power. 
         [0010]    The turbojet advantageously allows power not to be taken from the HP rotor shaft. The current generator is simple to access, allowing it to be replaced in a limited length of time by dismantling a minimum number of turbojet components. 
         [0011]    The current generator is positioned upstream of the fan, in a cool region of the turbojet, thus reducing its need for cooling and therefore its mass. 
         [0012]    The current generator comprises a stator element connected to the fixed cowl element, and a rotor element driven by the upstream end of the low-pressure rotor shaft. 
         [0013]    According to one embodiment, the turbojet comprises a fan disk on which the fan blades are mounted. A rotor element of the current generator is rotationally driven by a journal secured to said fan disk. 
         [0014]    The cowl element is advantageously connected to the fan casing by radial retaining arms. 
         [0015]    The radial arms allow the stator element of the generator to be held securely without requiring substantial structural modifications to the turbojet. 
         [0016]    Ducts for lubricating the current generator and electric cables are formed in the radial retaining arms. 
         [0017]    According to another embodiment, power transmission pinions are formed respectively on the rotor element of the current generator and on the upstream end of the low-pressure rotor shaft, the pinions meshing with one another in order to transmit the rotational movement of the low-pressure rotor shaft to the rotor element of the current generator. 
         [0018]    The invention also relates to a method of mounting a current generator in the fan of a gas turbojet, in which:
       the current generator is mounted on the fixed cowl element;   the fixed cowl element is positioned on the fan in such a way that the rotor element of the generator is in register with the upstream end of the low-pressure rotor shaft; and   the cowl element is immobilized on the fan casing with the radial retaining arms.       
 
         [0022]    The generator is simple to mount and this mounting is performed upstream of the turbojet. The components that need to be handled are small in size and light in weight thus allowing the current generator to be replaced quickly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    Other features and advantages will emerge from the following description of the turbojet of the invention with reference to the figures in which: 
           [0024]      FIG. 1  depicts a sectioned view of the upstream part of a turbojet according to the invention with a current generator positioned in the fan of the turbojet; 
           [0025]      FIG. 2  depicts a close-up view of the current generator of  FIG. 1 ; 
           [0026]      FIG. 3  depicts another embodiment of the invention; and 
           [0027]      FIG. 4  depicts a general arrangement of the turbojet of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    With reference to  FIG. 1 , the turbojet of the invention is a twin-spool gas turbojet  100  comprising a low-pressure (LP) rotor and a high-pressure (HP) rotor  1 , which are mounted so that they can rotate about the axis X 3  of the turbojet. This type of turbojet is well known to those skilled in the art. The terms internal or external, inner or outer, when used in the description, will be understood to mean radially internal or external or on the inside or the outside of the turbojet, with respect to the axis X 3  thereof. 
         [0029]    More specifically, with reference to  FIG. 4 , the turbojet functionally comprises, from the upstream direction downstream in the direction in which the gases flow, a fan  10 , a compressor, a combustion chamber, a turbine and a jetpipe. As it is a twin-spool engine, it comprises an LP compressor upstream of an HP compressor, and an HP turbine upstream of an LP turbine. 
         [0030]    The fan  10  in  FIG. 1  comprises a fan disk  19  attached by a flange to a journal  15 , itself supported by a bearing secured to the intermediate casing, the journal  15  here being secured to the low-pressure shaft  2 . 
         [0031]    The fan  10  further comprises a moving cowl element  11  which is fixed to the fan disk  19 . The moving cowl element  11  is of frustoconical shape and guides the incoming air stream. A fixed cowl element  12  is positioned upstream of the moving cowl element  11 . 
         [0032]    The fan  10  is rotationally driven inside the fan casing  14  by the LP rotor shaft  2  which rotates as one with the moving cowl element  11 . A housing  13  is formed in the moving cowl element  11 . 
         [0033]    With reference to  FIG. 1  and more specifically to  FIG. 2 , an electric current generator  20  is mounted on the fixed cowl element  12 , the generator  20  comprising a stator element  21 , firmly attached to the fixed cowl element  12 , and a rotor element  22  free to rotate with respect to the fixed cowl element  12 . 
         [0034]    In this instance, the rotor element  22  is an electromagnet extending axially along the axis X 3 . The stator element  21  is made up of windings which extend coaxially with respect to, and on the outside of, the rotor element  22 . A bearing  23  supports the rotor element  22  as it rotates in the stator element  21 . 
         [0035]    As the electromagnet  22  rotates about the axis X 3 , a magnetic field is created and induces an electric current in the windings  21 . 
         [0036]    Radial retaining arms  16  structurally connect the fixed cowl element  12  to the fan casing  14 , the stator element  21  of the current generator  20  thus remaining immobile as the fan blades  18  rotate. The retaining arms  16  are attached by a flange to the fan casing  14 . 
         [0037]    The retaining arms  16  are advantageously hollow and can house ducts  41  for lubricating the current generator  20  and electric cables  42  depicted in  FIG. 2 . The upper retaining arm  16  is depicted showing hidden detail over part of its length so that the ducts  41  for lubricating the current generator  20  and the electric cables  42  may be seen. 
         [0038]    The ducts  41  for lubricating the current generator  20  allow a lubricant, such as oil, to be carried from an oil tank, positioned downstream of the fan, to the current generator  20  to cool and lubricate the current generator  20 . 
         [0039]    Once the current generator  20  has been cooled, hot oil flows through the retaining arms  16  thus de-icing the arms and cooling the oil. Such lubricating ducts  41  make it possible to reduce the size of the heat exchangers needed for cooling said oil. 
         [0040]    The electric cables  42  allow the current generated in the windings  21  to be led away to electrical equipment positioned downstream of the engine. 
         [0041]    The retaining arms  16  of the fixed cowl element  12  in this instance are shaped so as to conduct the incoming air stream toward the fan blades  18 . The retaining arms  16  are three in number here, spaced 120° apart. It goes without saying that this number may change according to the configuration of the engine. 
         [0042]    In this exemplary embodiment, the rotor element  22  is connected directly to the upstream end of the low-pressure rotor shaft  2 . Power transmitting bevel gear pinions  25 ,  26  are formed respectively on the rotor element  22  of the current generator  20  and on the upstream end of the low-pressure shaft  2 , the pinions  25 ,  26  meshing with one another in order to transmit the rotational movement of the low-pressure shaft  2  to the rotor element  22  of the current generator  20 . 
         [0043]    As the turbojet engine  100  runs, the low-pressure rotor shaft  2  is rotationally driven by the low-pressure turbine of the turbojet  100 . 
         [0044]    The low-pressure rotor shaft  2  rotates the electromagnet  22  about the axis X 3  and induces an electric current in the windings  21  of the current generator  20 . The current is carried by the retaining arms  16  of the fixed cowl element  12  via the electric cables  42  positioned in the arms  16 , the equipment situated mainly downstream of the fan therefore being supplied with electric current. 
         [0045]    In another form of embodiment, with reference to  FIG. 3 , an additional journal  17  is mounted between the fan disk  19  and the rotor element  22  of the current generator  20 , the fan disk  19  supporting the fan blades  18 . The journal  17 , which rotates as one with the LP rotor shaft  2 , drives the rotation of the rotor element  22 . 
         [0046]    The journal  17  is connected by a screw-nut connection to the fan disk  19  and to the rotor element  22 . The way in which the current generator  20  is fixed with the fixed cowl element  12  remains the same; it is only the way in which the rotor element  22  is driven that differs from the previous embodiment. 
         [0047]    The invention also relates to the method of mounting the current generator  20  in the turbojet  100 . 
         [0048]    The current generator  20  is mounted on the fixed cowl element  12 . The current generator  20  is screwed to the cowl in this instance. 
         [0049]    The fixed cowl element  12  is positioned on the fan  10  in such a way that the rotor element  22  of the generator  20  is in register with the upstream end of the low-pressure rotor shaft  2 . 
         [0050]    The power transmitting bevel gear pinion  25  of the stator element  22  is brought into register with the power transmission bevel gear pinion  26  of the low-pressure rotor shaft  2 . 
         [0051]    The fixed cowl element  12  is then immobilized on the fan casing  14  with the radial retaining arms  16 . 
         [0052]    The ducts  41  for lubricating the current generator  20  are connected to the current generator  20  in order to supply the current generator  20  with oil, the electric cables  42  being connected to the windings of the stator element  21  of the current generator  20  so as to carry the current to the various pieces of equipment of the aircraft.