Abstract:
A device for filtering intake air of an internal combustion engine, including at least one filtering element including an inlet for air being treated, a first clean air outlet toward the engine, and a second air outlet toward the outside, and a ventilation mechanism driving the air from the second air outlet. The ventilation mechanism includes a fan wheel driven by a turbine set into motion by a gas flow taken from the engine.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of internal combustion engines and more particularly to that of gas turbine engines as used in aviation, especially on helicopters. It concerns the treatment of the intake air of such engines. 
     2. Description of the Related Art 
     A helicopter may be required to maneuver in an environment heavily charged with free particles, for example of sand or dust. When it is close to the ground, during takeoff or landing, it raises and causes to be suspended in the air a large quantity of particles which are ingested by the engines. The engines suffer from impaired performance and are subjected to serious corrosion by the particles passing through them. 
     In order to restrict this phenomenon, it is known to provide devices, referred to as sand filters, for treating the intake air of the engine, by means of which the particles suspended in the aspirated air are separated therefrom and evacuated. 
     A known sand filter is of the vortex type. It comprises a parallelepipedic or annular chamber in which are disposed tubular filtering elements crossing from one wall of the chamber to the other, in which filtering elements the air is subjected to a swirling motion. A first, axial, outlet communicates with the inlet channel of the engine and a second, lateral, outlet opens into the interior of the chamber. The interior of this chamber is ventilated to ensure that the air charged with particles is evacuated to the outside. 
     The ventilation, referred to as secondary, is effected by a device for impelling the air charged with particles, the throughput of which device may amount to as much as 10% of the throughput of the engine. 
     The known devices include a fan driven by an electric motor, the electrical energy being drawn from the engine itself or from the helicopter. The current supply requires cutoff, control and protection systems. Thus, although this device is very flexible, allowing the power demand to be managed according to the needs and phases of flight, it is relatively heavy and inconvenient to install from the point of view of wiring and electrical control. 
     Another known solution consists in providing ventilation by means of an ejector using a venturi. The flow required to operate the ejector is drawn from a high-pressure stage of the engine. This system has the advantage of being very reliable and having relatively low weight. However, the take-off from the high-pressure stage is very costly in terms of performance, and reduces the flight envelope of the engine. 
     BRIEF SUMMARY OF THE INVENTION 
     It is the object of the applicant to achieve the ventilation throughput of the vortex filtering system using a means which does not have the disadvantages of the prior art. 
     It is also the object to provide a ventilation device offering a better compromise between mass, impact on engine performance, cost and ease of repair, as compared to the prior art. 
     These objects are achieved with a device for filtering the intake air of an internal combustion engine comprising at least one filtering element having an inlet for air being treated, a first clean air outlet towards the engine and the second air outlet towards the outside, a ventilation means impelling the air from said second outlet, characterized in that the ventilation means includes a fan wheel driven by a turbine set in motion by a gas flow taken from the engine, the turbine and the fan wheel being arranged concentrically with respect to one another. 
     The use of the turbine allows the amount of air drawn off to be reduced in relation to systems using an ejector; the pressure level of the gas flow may be that of an intermediate compression level or even of a lower turbine stage. 
     In particular, the blades forming the turbine are arranged radially on the outside of a ring connecting the outer ends of the fan blades. 
     A one-piece assembly which is economical to maintain can be achieved in this way. 
     Advantageously, the turbine is of the action type. 
     In this way the need for gas-tightness between the two flows passing respectively through the turbine and the fan wheel is reduced. 
     According to a preferred embodiment, the fan wheel and the turbine form a rotor mounted in a casing in which are provided a central channel for ventilation air and a channel, in the form of at least one ring sector, for supplying the turbine. In particular, the fan wheel and the turbine form a rotor at the end of a central shaft mounted in a nose cone of said casing. 
     The gas may preferably be drawn off either at the compressor or between the power turbine and the high-pressure turbine, or between the stages of the power turbine. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Further characteristics and advantages will be apparent from the following description of a non-limiting embodiment with reference to the drawings, in which: 
         FIG. 1  shows schematically a vortex filtering device mounted in the air intake of a gas turbine engine; 
         FIG. 2  shows a filtering element of the device of  FIG. 1 ; 
         FIG. 3  shows an embodiment of the invention viewed in axial section; 
         FIG. 4  is a view along the section line A-A of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The diagram of  FIG. 1  shows an example of a gas turbine engine having, located successively along its main axis XX, a compression section  2  for the air admitted through the inlet channel  6 , a combustion chamber  3  and a turbine section  4 . The engine may be a single-spool engine or a multi-spool engine with a low-pressure spool and a high-pressure spool. The compression section may comprise one or more axial stages and/or one or more centrifugal stages. Likewise, the turbine section may comprise one or more in-line stages connected by a central shaft to a corresponding compression stage. The turbine section may comprise a free turbine connected to a power take-off shaft. 
     A sand filter  8  is mounted across the air intake channel  6 . The filter may be parallelepipedic or annular in shape depending on the shape of the inlet channel. 
     The filter  8  comprises two walls,  8 A and  8 B, spaced from one another. Air enters through orifices formed in the upstream wall  8 A and exits, cleansed of any particles with which it may be charged, through the orifices in the downstream wall  8 B. “Downstream” and “upstream” are defined in relation to the circulation of air through the engine. 
     To achieve this cleaning, filtering elements  80  are arranged between the two walls, opening into the orifices of the walls  8 A and  8 B.  FIG. 2  shows the operating principle of these elements. An element  80  comprises a tube  81  open to an orifice of the wall  8 A. It is provided with vanes  82  suitable for imparting a swirling motion to the air entering through an orifice  8 A 1  of the wall  8 A. At the wall  8 B the tube  81  forms an opening, annular in this case, with a frustoconical tube  83 . The latter opens on to an orifice formed in the downstream wall  8 B. In operation, the air, having passed through the wall  8 A, is set in swirling motion in the upstream portion of the tube  81 . The particles with which the air may be charged are centrifuged along the internal face of the wall of the tubes  81  and are evacuated with the flow S 2  into the space between the tubes  81  and the walls  8 A and  8 B. At the center of the tube  81 , the unpolluted air flow S 1  enters the tube  83  and is evacuated through the wall  8 B. It is admitted to the channel  6  and is then guided towards the compression section. 
     In order to ensure ventilation and evacuation of the polluted air to the outside, a fan  10  aspirates the air from inside the chamber of the filter  8  and ejects it. 
     The ventilation module  10  according to the invention is represented in axial section in  FIG. 3 . 
     The ventilation module  10  comprises a fan wheel  20  mounted rotatably in a casing  30 . The wheel comprises a hub  21  prolonged on one side by a central shaft  22 . Fan blades  23 , shaped to be traversed axially by air parallel to the axis of the central shaft  22 , are fixed radially on the hub  21 . Axial-flow turbine blades  24  are arranged radially on the periphery of this wheel formed by the fan blades  23 . A ring  25  separates the flow of ventilation air from the gas flow passing the turbine blades in the axial direction. The wheel may advantageously be implemented as an integral structure by machining from a single block. 
     The central shaft  22  is mounted rotatably in a cantilevered manner in a nose cone  31  of the casing  30 . The casing includes a first cylindrical shell  33  which defines, with the nose cone  31 , a first annular channel for an axial air flow F 1 . Guide vanes  34  connect the surface of the nose cone  31  radially to the shell  33 . The air stream for the flow F 1  has an opening corresponding to that of the air stream traversed by the fan blades  23 . The shell is prolonged downstream by the flow separation ring  25 . 
     Outside the shell  33 , the casing defines a second channel concentric with the first channel and having two wall elements  35  and  36 . Viewed in axial section, the wall  35  forms an elbow with a radial inlet, which elbow joins the cylindrical wall tangentially. The wall  36 , spaced radially from the wall  35 , also forms an elbow with a radial inlet, which elbow joins tangentially a cylindrical wall  37  having a radius greater than that of the cylindrical wall  33 . This second channel therefore has a radial inlet and an axial outlet. The second channel, for the gas flow F 2 , has the shape of a ring sector, or is annular. The cylindrical wall forms a sealing ring around the rotor  1 . More particularly, it forms a sealing ring for the turbine blades  24 , having a minimum clearance from the tips thereof. Guide vanes  38  form gas injectors in the direction of the turbine blades  24 . The turbine is preferably an action turbine with gas pressure reduction in the injectors  38 . In this way the problems of sealing between the flows F 1  and F 2  passing through the fan rotor  20  are avoided.  FIG. 4  shows, in a section along the line A-A of  FIG. 3 , the shape of the turbine blades  24  and that of the guide vanes  38  forming injectors. 
     Upstream flanges  33 A allow the device to be mounted on a conduit from the sand filter. Flanges  35 A and  36 A allow mounting to a conduit communicating with a take-off on the motor, and flanges  37 A enable attachment to an evacuation conduit. 
     During operation of the engine, the ventilation device  10  receives gases F 2  taken from the engine which are guided by the second channel towards the turbine blades which they set in motion, driving the fan wheel. Through rotation of the fan wheel, the ventilation blades  23  aspirate the air F 1  from inside the filter  8 . The two flows F 1  and F 2  are combined downstream of the wheel  20  and directed to the outside. 
     In summary, by means of its architecture the device according to the invention makes available a wheel which is easily removed, thus simplifying maintenance procedures, and makes possible a machine having a small number of parts. For example, replacement of the rotor  21  can be easily effected from the rear. 
     The use of an action turbine in turn allows the distributor to be simplified, tolerating partial injection, the nozzle of which may be produced by simple drilling; it also limits the impact of leaks through the low-pressure delta on the wheel. 
     Advantageously, gas take-off points may be located either:
         between the high-pressure turbine(s) and the power turbine(s),   in the inter-stage zone of the power turbine (in the case of a multi-stage turbine),   at different stages on the compressor.