Abstract:
The present invention relates to a method for treating an airflow, laden with oil particles, flowing in a tube ( 9 ) communicating with a rolling bearing enclosure of a gas turbine engine, wherein said airflow is made to travel into a coking box ( 20 ) associated with a heating means, in which the air is heated to a sufficient temperature to coke the oil particles contained in the airflow. Preferably, the solid residues produced by the coking are collected in the coking box ( 20 ).

Description:
[0001]    The present invention relates to the field of aviation gas turbine engines and is aimed more particularly at a method for treating bearing lubrication oil waste. 
       BACKGROUND OF THE INVENTION 
       [0002]    A gas turbine engine consists basically of an air compression assembly that supplies a combustion chamber in which the air is mixed with a fuel to produce hot gases whose energy is recovered in a turbine assembly driving the compression means. The shafts connecting the various rotor bodies are supported in the statoric portions by bearings mounted in a pressurized enclosure. The enclosures make it possible to contain the oil that is injected onto the rolling bearings to ensure their lubrication and comprise sealing members, most frequently of the labyrinth type, that the containment air passes through. This air is laden with oil particles and in order to keep the oil consumption to as low a level as possible, it is a known practice to use deoiling equipment that separates the oil from the air that has flowed into the rolling bearing enclosures. In current engines, this equipment is incorporated either into the engine near the bearings themselves, or in the accessory gearbox, also called the AGB. The deoilers however are not 100% efficient. The exhausted air after it has passed through the deoilers still contains oil residues in the form of droplets that are ejected into the atmosphere. They are therefore the source of pollution and harm the environment. 
         [0003]    The applicant has set itself the objective of reducing the polluting effect of the oil waste in the atmosphere. 
         [0004]    According to the invention, the applicant has perfected a method for treating an airflow, laden with oil particles, flowing in a tube in communication with a rolling bearing enclosure of a gas turbine engine, wherein said airflow is made to travel into a coking box associated with a heating means, in which the air is heated to a sufficient temperature to coke the oil particles that it contains. Preferably, the solid residues produced by the coking are collected in the coking box. 
         [0005]    Therefore the transformation by coking of the oil into gaseous and solid residues makes it possible to reduce the toxicity of the gases ejected into the atmosphere. On the one hand the coking makes it possible to reduce the volume of the oil waste, on the other hand, the residues are less toxic in themselves. 
       DESCRIPTION OF THE PRIOR ART 
       [0006]    Document GB 2 374 026 is known, according to which the flow is made to travel into a box in which the air is heated to a sufficient temperature to vaporize the oil particles contained in the airflow. However, the oil particles are heated for the purpose of making the emissions in the atmosphere invisible and not to reduce their toxicity. There is no chemical transformation of the coking. 
       SUMMARY OF THE INVENTION 
       [0007]    According to one embodiment of the method, the box is attached to said tube so that the air travels from the tube into the box. More particularly, in a gas turbine engine comprising an exhaust cone downstream of the turbine, the box is placed in said cone so that it is heated by the gases in the exhaust cone. 
         [0008]    According to a preferred embodiment, the air is heated to a sufficient temperature to pyrolize the oil particles particularly by taking hot gases immediately downstream of the turbine. 
         [0009]    A coking box for the treatment of an airflow containing oil particles flowing in a tube communicating with a rolling bearing enclosure of a gas turbine engine comprises a cylindrical casing with an opening on one side to receive the airflow from the tube and means forming chicanes. Preferably the box comprises a means for collecting the coked oil and more particularly the casing comprises an attachment means at the end of the central ventilation tube of the engine and an internal grid through which the oil particles are sprayed onto the internal wall of the casing and forming a means for collecting the coked oil. 
         [0010]    The invention also relates to a gas turbine engine comprising a central ventilation tube and a gas exhaust cone, said tube opening into the exhaust channel, wherein the coking box is mounted on the central tube so as to be heated by the engine gases from the exhaust channel. 
         [0011]    Preferably, said coking box is heated by gases taken downstream of the turbine at a sufficient temperature to pyrolize the oil particles. For example, the coking box is placed on the central ventilation tube close to a gas offtake connection immediately downstream of the turbine where the temperature is of the order of 500° C. This solution is of value because it leads to the elimination of any solid residue. In this case, it is therefore no longer necessary to intervene repeatedly on the coking box. 
         [0012]    According to another embodiment for a gas turbine engine comprising an AGB with gears driving accessories, said box is mounted on said AGB. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    An embodiment of the invention will now be described in greater detail with reference to the drawings in which 
           [0014]      FIG. 1  represents a gas turbine engine to which the invention applies, 
           [0015]      FIG. 2  shows the rear portion of an engine with a central ventilation tube fitted with a coking box of the invention, 
           [0016]      FIG. 3  shows in greater detail a coking box according to the invention, 
           [0017]      FIG. 4  shows a means for heating the coking box according to the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    The gas turbine engine of  FIG. 1  is a turbojet with a turbofan. Such an engine comprises, in this example, a high-pressure body with an HP compressor  2  driven by an HP turbine  4  both mounted on one and the same shaft  6 . A shaft  5  concentric with the shaft  6  connects an upstream fan  3 , associated with a low-pressure LP compressor  3 ′, to a low-pressure LP turbine  7  downstream of the HP turbine  4 . The air compressed by the compressors  3 ′ then  2  travels into an annular combustion chamber  8  where it is mixed with the fuel to produce combustion gases. The latter are guided toward the HP turbine  4  then the LP turbine  7  and finally exhausted via the downstream nozzle comprising an exhaust cone. The propulsion force is supplied mostly for this type of engine by the airflow bypassing the combustion chamber and exhausted either directly into the atmosphere via an annular nozzle or mixed with the gases originating from the turbine and forming the main exhaust flow. The shafts are supported by intershaft bearings for the concentric rotating portions and by bearings mounted on the fixed, statoric, structures for the shaft  5 . 
         [0019]    Because of the temperatures, the bearings are lubricated and cooled permanently by oil that is contained in an enclosure called the rolling bearing enclosure. Pressurized air is injected at the seals to form a barrier and prevent oil from traveling toward the hot portions of the engine and causing a fire. After it has traveled into the deoiler for oil collection, the containment air is usually exhausted to the atmosphere via the central ventilation tube  9 . This is the case when the deoilers are placed near the bearing enclosures. In the rest of the description, the invention applies to this case, but it is also valid for the case where the deoilers are placed on the AGB. 
         [0020]    The air channeled via the tube  9  is exhausted downstream via a central duct placed in the frustoconical or substantially frustoconical part  10 , defining the internal surface of the stream of gases originating from the turbine stages. This is called the exhaust cone. In the solutions of the prior art, the residual oil, even in a small quantity, is sent into the atmosphere through the exhaust cone. 
         [0021]    According to the invention, all the residual oil in the air is removed, before the latter is exhausted into the atmosphere, by trapping it and oxidizing it to transform it into gaseous species and into coke with a lower toxicity than oil. 
         [0022]    One way of achieving this is illustrated in  FIG. 2  which shows in greater detail the rear portion of an engine like that of  FIG. 1 . The rear bearing  12  supporting the trunnion  14  of the LP turbine can be seen. The rolling bearings  12   a  of the bearing  12  are mounted in a cage placed between the fixed bearing support  13  and the trunnion  14 . The assembly is contained in an enclosure  15 . Labyrinth seals  15   a ,  15   b  are arranged between the fixed structure of the enclosure and the LP rotor disk. Labyrinth seals  15   c  and  15   d  are also arranged between the fixed structure and the central ventilation tube that is fixedly attached to the LP shaft. The air of the enclosure is taken by the deoiler  16  and then is discharged centrally via the tube  9 . The elements that have just been described do not form part of the invention and are known per se. 
         [0023]    The air of the tube  9  is driven downstream inside the exhaust cone and is then mixed with the engine gases. 
         [0024]    According to the invention, a box, which will hereafter be called the coking box  20 , has been placed on the downstream end of the central ventilation tube  9 . This box is shown in detail in  FIG. 3 . 
         [0025]    It comprises a cylindrical casing  21  of slightly greater diameter than that of the tube  9 . The casing is attached at the end of the tube by any appropriate removable means. It is mounted so as to be open onto the tube. Inside the casing a cylindrical grid  22  makes an annular space  22   a  with the casing. Over the length of the casing  21 , regularly spaced transverse plates  24  are attached in the volume delimited by the grid  22 , in an alternating manner in the form of chicanes. The casing is closed downstream by a grid with axial holes and defining a flame-arrester  26 . The casing  21  is therefore mounted downstream of the tube  9  so as to collect therefrom the air that passes through it. This air is forced into the casing by the plates  24  in a path alternately radially outward and radially inward. It follows that the oil particles that it contains are centrifuged through the grid  22 . They are collected in the annular space  22   a . By being placed at the end of the tube  9 , the casing is heated by the ambient gases inside the exhaust cone which thereby, when the engine is operating, keep the wall of the casing  21  at a temperature of approximately 300° C. At this temperature, the oil in the space  22   a  sustains a thermal oxidation transformation. It is transformed partly into coked oil in the vapor phase CV and partly into coke CS, solid residue. 
         [0026]    A means of eliminating the solid residue is to remove the box and change it. It is an operation that is carried out easily on the ground during the maintenance operations of the engine beneath the aircraft wing. According to an embodiment that is more economically advantageous, the box may be made in the form of a removable and replaceable cartridge. However, it must be recognized that, in certain turbojet operating conditions or in the case of a fault (excessive oil consumption), the quantity of coke formed may not be negligible. This means either relatively frequent or unscheduled interventions, harming profitability, or an increase in the volume of the box resulting in a space requirement and a weight that is just as harmful. 
         [0027]    An advantageous means of eliminating the solid residue more rapidly is to pyrolize it as it forms by taking it to high temperature. Specifically, above 500° C., the coke is burnt without leaving any residue. Therefore, according to a particular embodiment, the device comprises a means of heating the box to 500° C.  FIG. 4  shows an installation of the box inside the exhaust cone as in the previous case but adding a tube  18  to bring a determined quantity of gas from the stream of the main flow to the walls of the box.