Abstract:
Drained fluid evacuation stub ( 16 ) for a propulsion assembly ( 10 ), comprising a drained fluid storage cavity and at least one orifice ( 32 ) for evacuation of the fluids contained in said cavity, characterized in that it comprises means ( 36, 38 ) for detecting a pressure difference with the exterior of the stub and a component for purging the cavity which is movable between a first closed position of the evacuation orifice and a second release position of the orifice, the component being configured to move from the first to the second position when the pressure difference is greater than or equal to a predetermined value.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a stub for discharging drained fluids for an aircraft propulsion unit comprising in particular an engine (such as a turbojet engine or a turboprop engine) surrounded by a nacelle. 
       PRIOR ART 
       [0002]    An aircraft propulsion unit is in general equipped with means for draining fluids (oil, water and/or fuel) from the engine in order to prevent these fluids accumulating and interfering with the functioning of the engine. The oil and fuel are drained by means of dynamic seal technologies (pumps, AGB, metering devices, actuators, etc.) not affording a perfect seal. It is therefore necessary to drain the fluids that pass through the dynamic seals in order to prevent leakages in the engine. The water is drained in order to avoid retention zones, which often cause corrosion. 
         [0003]    In the prior art, the drained fluids may be discharged directly outside. The means for draining fluids from the engine may also be connected by conveying means, such as ducts, to a retention box comprising a drained-fluid storage cavity. This retention box is situated in the propulsion unit. It is fixed to the engine and is in general situated at the lower part of the propulsion unit so that the drained fluids flow by gravity in the conveying means as far as the storage cavity. 
         [0004]    The propulsion unit further comprises a drainage stub for discharging the drained fluids to the outside. This stub is carried by the nacelle and projects towards the outside of the nacelle. It is also situated at the lower part of the propulsion unit, opposite the retention box, and recovers fluids emerging from the box. The stub comprises a bottom end comprising an orifice for discharging fluids to the outside of the nacelle. When the storage cavity of the retention box is emptied, the fluids are discharged to the opening in the stub and then discharged to the outside of the propulsion unit. 
         [0005]    However, the volume of the retention box is limited (intentional limitation of the volume in order to limit the space requirement in the engine environment). The retention box cannot therefore indefinitely collect the drained fluids and some aircraft constructors require that fluids should not be released when the aircraft is on the ground and in normal operation. One solution therefore consists of draining the retention box when the aircraft is in flight, this drainage being able to be controlled automatically by an engine computer (FADEC) or particular pressure conditions in an oil or fuel system. 
         [0006]    However, this solution is not entirely satisfactory since, in the absence of a lack of command, the retention box is not emptied and fluids may be discharged from the box through its overflow when the aircraft is on the ground. 
         [0007]    The present invention proposes a simple, effective and economical solution to this problem by means of a system for retaining drained fluids of a propulsion unit, which is equipped with an autonomous drainage, that is to say which functions without the intervention of the engine computer or without the measuring of pressure on a system of the engine. 
       DISCLOSURE OF THE INVENTION 
       [0008]    To this end the invention proposes a stub for discharging drained fluids for a propulsion unit, this stub comprising a cavity for storing the drained fluids and at least one orifice for discharging the fluids contained in the storage cavity, characterised in that it comprises means for measuring a difference in pressure outside the stub and a member for draining the storage cavity, this member being able to move between a first position of closing the discharge orifice and a second position of releasing this orifice, the member being configured so as to move from the first to the second position when said difference in pressure is greater than a predetermined value. 
         [0009]    The invention is particularly advantageous since it makes it possible to use a pressure difference such as the external dynamic pressure and therefore the velocity of the aircraft to trigger the drainage of the retention stub and the discharge of fluids. This is because the dynamic pressure outside the stub varies according to the velocity of the aircraft. When the aircraft is at rest, the dynamic pressure is zero and the member is in its first position. The more the velocity of the aircraft increases, the more the dynamic pressure increases. The stub is designed so that its drainage is actuated when the dynamic pressure has reached a certain threshold, that is to say when the aircraft has reached a certain velocity in flight (the member is then in its second position). Thus the drainage of the stub is carried out in flight autonomously, which limits the risk of loss of drained fluids when the aircraft is on the ground. 
         [0010]    Advantageously, the stub comprises a first means for measuring the total pressure outside the stub and a second means for measuring the static or pseudo-static pressure outside the stub. In aeronautics, the dynamic pressure is added to the static pressure to give the total pressure. The dynamic pressure is equal to the difference between the total pressure and the static pressure. The difference between the static pressure and the pseudo-static pressure will be detailed hereinafter. The “pseudo-static” pressure is the total pressure at the site of maximum negative pressure around an aerodynamic profile, which corresponds to the minimum total pressure, which is lower than the static pressure. This negative pressure increases with the velocity of the aircraft. 
         [0011]    According to one embodiment of the invention, the member is able to move in translation in a recess in the stub and comprises or carries a transverse element separating two chambers inside the recess, a first chamber being connected to said first pressure-measuring means and a second chamber being connected to said second pressure-measuring means. The first chamber is thus subjected to the total pressure and the second chamber is subjected to the static or pseudo-static pressure. 
         [0012]    The member may be acted on in its first position by resilient return means. In a particular embodiment of the invention, the dynamic pressure exerts on the transverse element a force that must be greater than the return force of the resilient means so that the member moves from its first to its second position. 
         [0013]    The member and its element may be designed so that the member is held in its first, closed position when the pressure difference between the two chambers is less than 6 kPa (which corresponds to a speed of air flow around the stub of approximately 80 m·s −1 ), and is held in its second, open position when the pressure difference between the two chambers is greater than 12 kPa (approximately 120 m·s −1 ). Between these values, the member can move from the first to the second position, and vice versa. 
         [0014]    The first and second pressure-measuring means may each comprise an air-passage orifice emerging outside the stub. 
         [0015]    The first measuring means is preferably connected to the first chamber by a duct comprising at least two parts, one of which is inclined or perpendicular with respect to the other. This limits the risk of particles liable to enter the orifice in the first pressure-measuring means reaching the first cavity and interfering with the measurement of dynamic pressure. These particles are intended to be trapped in the duct. 
         [0016]    The stub according to the invention preferably has an aerodynamic profile such as an NACA profile of the biconvex symmetrical type. 
         [0017]    The NACA profile accelerates the fluid very locally, which causes a local negative pressure. Likewise, the NACA profile slows down the fluid very locally on the stop surface, which causes local overpressure. The difference in pressure between the minimum pressure point and the maximum pressure point on the profile therefore becomes greater than the dynamic pressure. For example, in the case of a theoretical NACA0018 profile, this pressure difference can be calculated as being greater than or equal to 1.6 times the dynamic pressure. This pressure difference serves to control the opening mechanism by means of application surfaces. This has the advantage of increasing the available control pressure, and therefore gaining in compactness for the same resulting force. 
         [0018]    The first measuring means may be situated on a leading edge of the profile, where the maximum total pressure is found, equal to the sum of the static pressure and dynamic pressure. The second measuring means may be situated on one side of the profile. The second measuring means is advantageously situated in a region of the profile where the pressure coefficient Cp has a substantially maximum value where the negative pressure is at its greatest, and therefore the measured pressure is at a minimum. The pressure measured at this level is then referred to as the pseudo-static pressure. The pseudo-static pressure may thus be considered to be the pressure on the region of the profile where the negative pressure is at its greatest, and therefore the total pressure is at a minimum. 
         [0019]    The present invention also relates to a propulsion unit, characterised in that it comprises a stub as described above. 
         [0020]    Preferably, the stub has a substantially radial orientation with respect to the longitudinal axis of the propulsion unit and at least partly projects on an external surface of the nacelle of the propulsion unit. The measuring means may be situated in the vicinity of the radially external end of the stub. 
         [0021]    The present invention also relates to a method for designing a stub as described above, characterised in that it comprises the steps consisting of:
       determining an aerodynamic profile of the stub, such as an NACA profile of the biconvex symmetrical type,   positioning a first means of measuring a maximum pressure on the leading edge of the profile,   determining by calculation the distribution of the pressure coefficient Cp along the profile and deducing therefrom the region of the profile where this coefficient has a substantially maximum value, and   positioning a second means for measuring a minimum pressure in said region.       
 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0026]    The invention will be better understood and other details, features and advantages of the invention will emerge from a reading of the following description by way of non-limitative example and with reference to the accompanying drawings, in which: 
           [0027]      FIG. 1  is a schematic perspective view of an aircraft propulsion unit according to the invention; 
           [0028]      FIG. 2  is a partial schematic view in perspective and axial section of the propulsion unit of  FIG. 1 , to a larger scale; 
           [0029]      FIGS. 3 and 4  are schematic perspective views of a discharge stub according to the invention; 
           [0030]      FIG. 5  is a schematic view in axial section of the discharge stub of  FIGS. 3 and 4 ; 
           [0031]      FIGS. 6 and 7  are highly schematic views in axial section of a discharge stub according to the invention, and illustrate two different positions of its drainage member; and 
           [0032]      FIG. 8  is a graph showing the change in the pressure coefficient Cp along a discharge stub having an NACA profile reproduced under the graph, a positive Cp corresponding to a negative pressure region and a negative Cp corresponding to an overpressure. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    Reference is made first of all to  FIG. 1 , which depicts an aircraft propulsion unit  10  comprising an engine  12  (such as a bypass turbojet engine, depicted schematically by broken lines) mounted inside a nacelle  14 . 
         [0034]    The engine  12  comprises, from upstream to downstream in the direction of flow of the gases (from left to right in the drawing), an air inlet, a fan, at least one compressor, a combustion chamber, at least one turbine and a duct for ejecting the combustion gases. The nacelle  14  comprises cowls that define the external surface of the propulsion unit. 
         [0035]    The turbine engine  10  comprises a stub  16  for discharging drained fluids, this stub has a substantially radial orientation (with respect to the longitudinal axis of the propulsion unit) and projects on the external surface of the nacelle  14 . It is situated at the bottom part of the propulsion unit, at 6 o&#39;clock by analogy with the dial of a clock. 
         [0036]    A plurality of types of fluid circulate in the turbine engine  10  and in particular fuel for supplying the chamber and the combustion of the gases, oil for lubricating the bearings of the rotating parts, and water, which may be aspirated by the ventilation scoops or formed by condensation on the engine. 
         [0037]    In operation, these fluids are drained in order to prevent them accumulating and interfering with the functioning of the propulsion unit. The turbine engine comprises means for draining these fluids (such as drains) that are connected by fluid-conveying means such as ducts  18  to a box for retaining these fluids, which is here integrated in the discharge stub. 
         [0038]    The discharge stub  16 , more clearly visible in  FIGS. 2 to 4 , comprises, at its radially external end, a plate  22  for fixing to the nacelle  14 . 
         [0039]    The plate  22  has a substantially parallelepipidal shape and is fixed to cowls of the nacelle  14 . It comprises orifices  24  aligned with orifices  26  on the nacelle for means of the screw and nut type to pass. The stub  16  has an aerodynamic profile and passes through a radial opening  28  in the nacelle. The stub  16  is mounted in this opening by radial translation towards the outside, from the inside of the nacelle, until the plate  22  bears on the internal surface of the nacelle. A seal may be provided, intended to be compressed between the plate and the nacelle. 
         [0040]    The stub  16  further comprises an internal cavity  30  for storing the drained fluids. 
         [0041]    The plate  22  of the stub  16  comprises means for connecting outlets of the ducts  18  bringing the drained fluids to the cavity  30 . These connection means comprise channels in which the outlets of the ducts  18  are fitted, the radially external ends of these channels emerging on the radially external surface of the plate  22 , and their radially internal ends emerging in the cavity  30  ( FIG. 5 ). 
         [0042]    The cavity  30  may be put in fluid communication with an orifice  32  for discharging the fluids contained in this cavity, with a view to draining said cavity. 
         [0043]    According to the invention, the drainage of the cavity  30  is carried out autonomously by means of a movable member, the actuation of which depends on the dynamic pressure outside the stub and therefore the velocity of the aircraft equipped with the propulsion unit. 
         [0044]    The cavity  30  comprises a recess  34  for housing the member, which is able to move between a first position of closing the orifice  32  and a second position of releasing this orifice. 
         [0045]    The stub  16  comprises means  36 ,  38  for measuring the dynamic pressure outside the stub, the member being intended to move from the first to the second position when the dynamic pressure is greater than, or equal to, a predetermined value. 
         [0046]    In any fluid flow, a pressure difference is created between the stagnation pressure and the static pressure as soon as the fluid is in movement. In this case, this difference in pressure is applied to a surface of the member in order to generate sufficient force to activate the drainage of the stub as soon as the velocity of the aircraft exceeds a given threshold. 
         [0047]    The dynamic pressure is equal to the difference between the total pressure and the static pressure. The means for measuring the pressure difference comprise a first means  36  for measuring the total pressure and a second means  38  for measuring the static (here pseudo-static) pressure. 
         [0048]    These measuring means  36 ,  38  comprise here orifices or ducts emerging on the external surface of the stub  16 . In the example embodiment in  FIG. 5 , the measuring means  36  comprises a duct comprising a first part, one end of which emerges on the external surface of the stub  16 , this first part being connected to another part of the duct that is inclined with respect to the first part. 
         [0049]      FIGS. 6 and 7  show highly schematically the functioning of the movable member, referenced  40 . 
         [0050]    The member  40  is here in the form of a piston able to move in the aforementioned recess  34 , the piston comprising a rod  42  connected by one end to a transverse element such as a disc  44 . The disc  44  separates the recess into two adjacent chambers  46 ,  48 . The first measuring means  36  puts the first chamber  46  in fluid communication with the outside of the stub, and the second measuring means  38  puts the second chamber  48  in fluid communication with the outside of the stub. The first and second chambers  46  are thus subjected to the total and pseudo-static pressures respectively. 
         [0051]    The rod  42 , in moving, releases a passage for draining the fluids. By way of example, the rod  42  of the member  40  passes through a duct  50  connecting the cavity  30  to the discharge orifice  32 , and comprises a through aperture  52  that must be positioned level with this duct so that the fluids contained in the cavity can flow as far as the orifice  32  and be discharged. 
         [0052]    In the first position of the member  40  depicted in  FIG. 6 , its rod  42  closes off the duct  50  and thus prevents drainage of the cavity  30 . In the second position of the member  40  depicted in  FIG. 7 , the aperture  52  of the rod  42  allows the fluids contained in the cavity  30  to pass until they are discharged through the orifice  32 . 
         [0053]    Resilient return means  54  are mounted in the recess and act on the member  40  in its first position. These means  54  here bear on the disc  44  of the member. 
         [0054]    The expression of the dynamic pressure P d  is P d =ρ*v 2 /2, in which:
       ρ is the mass per unit volume of the air, which decreases as the altitude increases; thus, at high altitude, drainage of the cavity  30  will occur at a higher velocity than at low altitude;   v is the velocity of the aircraft.       
 
         [0057]    This dynamic pressure is equal to the difference between the total pressure and the static pressure, P d =P t −P s . 
         [0058]    The force F d  that the dynamic pressure exerts on the member  40 , and in particular on its disc  44 , has the equation F d =S*ρ*v 2 /2, in which D is the surface area of application of the pressure, which is here the surface area of the disc  44 . 
         [0059]    The force F d  represents the resistance to be chosen for the drainage and is sized according to the velocity threshold as from which the drainage is intended to occur, the aim being for the drainage to occur when the aircraft reaches a given velocity, for example when it takes off. 
         [0060]    According to a preferred embodiment of the invention, the stub  16  has an aerodynamic profile of the NACA type, as can be seen in  FIGS. 3 and 4  and at the bottom of  FIG. 8 , which shows a cross section of the stub. The profile of this stub is here symmetrical and biconvex, and comprises a leading edge  60  and a trailing edge  62  for the air, and convex profiled sides  64 . 
         [0061]    According to the invention and as can be seen in  FIGS. 3 to 5 , the first means  36  for measuring the total pressure is situated on the leading edge  60  of the stub, in the vicinity of its radially external end, in order to be at a distance from the limit layer on the external surface of the nacelle  14 . The second means  38  for measuring the static pressure is situated on a side  64  of the stub, also in the vicinity of this radially external end. 
         [0062]    Preferably, the second means  38  is used for measuring a pseudo-static pressure, this pseudo-static pressure being the pressure in a region of the profile where the negative pressure is at its greatest, that is to say where the pressure coefficient Cp has a maximum value Cpmax. 
         [0063]    To determine this region and therefore the position of the second means  38 , it is therefore necessary to know the distribution of this coefficient Cp along the profile of the stub. This can be done by calculation in order to obtain a curve as shown in the top part of  FIG. 8 . The second measuring means  38  is positioned at an abscissa X 1  corresponding to the abscissa for which the pressure coefficient has a maximum value Cpmax.