Abstract:
An oil tank and scavenge pipe assembly of a gas turbine engine comprises a tank, and a scavenge pipe having a discharge portion disposed inside the tank. The discharge portion comprises a first portion having first and second ends. The first end is adapted to connect to an oil return line for receiving a mixture of oil and air. A bend extends from the second end downstream thereof relative to a flow of the mixture of oil and air through the scavenge pipe. The bend is configured to cause stratification of the mixture of oil and air as the mixture of oil and air flows through it. An outlet downstream of the bend delivers the mixture of oil and air to the tank. A method of delivering an oil and air mixture to a rotating oil volume of a tank of a gas turbine engine is also presented.

Description:
TECHNICAL FIELD 
       [0001]    The application relates generally to oil tanks in gas turbine engines and, more particularly, to scavenge pipes and oil tanks. 
       BACKGROUND OF THE ART 
       [0002]    In a gas turbine engine, different parts of the engine need to be lubricated. An oil system provides the lubrication oil to these different parts. The oil system includes a tank containing the oil to be supplied to the engine by an oil supply line. A scavenge pipe delivers that oil that has been used in the engine back to the tank. During lubrication of the engine, the oil becomes mixed with air, and when the oil is brought back to the tank air bubbles are also carried in. These bubbles can induce the production of foam in the tank, which may clog the air vent of the tank and as a consequence increase the pressure in the tank. 
       SUMMARY 
       [0003]    In one aspect, there is provided an oil tank and scavenge pipe assembly of a gas turbine engine, the assembly comprising: a tank; and a scavenge pipe having a discharge portion, the discharge portion being disposed inside the tank, the discharge portion comprising: a first portion having a first end and a second end, the first end being adapted to connect to an oil supply line for receiving a mixture of oil and air; a bend extending from the second end of the first portion downstream thereof, the bend being configured to cause stratification of the mixture of oil and air as the mixture of oil and air flows through it; and an outlet downstream of the bend delivering the mixture of oil and air from the bend to the tank. 
         [0004]    In another aspect, there is provided an oil system of a gas turbine engine, the oil system comprising: an oil supply line carrying oil to the engine; an oil return line carrying a mixture of oil and air from the engine; and an oil tank connected to the oil return line and the oil supply line, an inside of the oil tank receiving a discharge portion of a scavenge pipe of the oil return line, the discharge portion including: first portion connected to the oil return line and receiving the mixture of oil and air from the engine; a bend extending from the first portion downstream thereof, the bend being configured to cause stratification of the mixture of oil and air as the mixture of oil and air flows through it; and an outlet downstream of the bend delivering the mixture of oil and air from the bend to the tank. 
         [0005]    In a further aspect, there is provided a method of delivering an oil and air mixture to a rotating oil volume of a tank of a gas turbine engine, the method comprising: carrying the oil and air mixture in a discharge portion of a scavenge pipe disposed inside the tank; at least partially separating the oil from the air by carrying the oil and air mixture in a bend of the discharge portion; and discharging the oil and air mixture to the rotating oil volume through an outlet of the discharge portion downstream of the bend disposed adjacent to a concave curved sidewall of the tank. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]    Reference is now made to the accompanying figures in which: 
           [0007]      FIG. 1  is a schematic cross-sectional view of a gas turbine engine; 
           [0008]      FIG. 2  is a schematic of an oil system for the gas turbine engine of  FIG. 1 ; 
           [0009]      FIG. 3  is a schematic of an oil tank according to a first embodiment for use in the oil system of  FIG. 2 ; 
           [0010]      FIG. 4  is a picture of a top of the oil tank of  FIG. 3  shown transparent to reveal a portion of a scavenge pipe extending within the oil tank; 
           [0011]      FIG. 5  is a bottom plan view of the portion of the scavenge pipe of  FIG. 4 ; 
           [0012]      FIG. 6  is a bottom perspective view of the top of the oil tank of  FIG. 3 ; 
           [0013]      FIG. 7  is a side elevation view of the top of the oil tank of  FIG. 3 ; 
           [0014]      FIG. 8  is a schematic of a portion of the scavenge pipe of  FIG. 4 ; and 
           [0015]      FIG. 9  is a portion of an oil tank according to a second embodiment for use in the oil system of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  illustrates a gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan  12  through which ambient air is propelled, a compressor section  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. 
         [0017]      FIG. 2  illustrates an oil system  20  that conveys oil to the engine  10 . The oil system  20  shown in  FIG. 2  is schematic and simplified, and is only one example of oil system that can be used to convey oil to the engine  10 . The oil system  20  includes a tank  30  containing the oil to be supplied to the engine  10  by an oil supply line  22 . The oil lubricates different parts of the engine  10 . Examples of such parts include bearings and gearboxes. During lubrication of the engine  10 , oil becomes mixed with air. As a result, the fluid leaving the engine  10  is not only oil but oil mixed with air bubbles. This fluid is carried from the engine  10  to a scavenge pump  26  by fluid line  24 . The scavenge pump  26  then imparts momentum to the fluid in order to direct it back to the tank  30  via an oil return line  28 . 
         [0018]    Referring to  FIG. 3 , the tank  30  will be described in greater details. The tank  30  is located in the gas turbine engine  10 . The tank  30  has an elongated cylindrical body  32 . The body  32  defines a vertical V in a direction of elongation of the body  32  and a horizontal H in a direction perpendicular to the direction of elongation of the body  32 . The body  32  has a top  33 , a bottom  35  and a cylindrical straight sidewall  34  extending between the top  33  and the bottom  35 . The body  32  shown in  FIG. 3  is cylindrical, but it is contemplated that the body  32  could have other shapes, as long as the sidewall  34  is somewhat curved in cross-section. For example,  FIG. 9  shows a top portion of a body  32 ′ having a non-cylindrical cross-section. The body  32 ′ has a top  33 ′ and a sidewall  34 ′ having a generally curved square cross-section  35 ′. Furthermore, the sidewall  34 ′ is not straight like the sidewall  34  is, but curved along its direction of elongation. A shape of the body  32  is dictated at least in part by the space available for the tank  30  in the gas turbine engine  10 . Other factors influencing the shape of the body  32  include some or all of weight, internal pressure, temperature, fire resistance, cost, fabrication method. These factors may for example influence a shape of the sidewall  34 , straight or curved. 
         [0019]    The body  32  contains an oil volume  27  of oil, which supplies the oil supply line  24 . The body  32  also contains an air volume  29  of air incoming from the oil return line  28 , which as discussed above carries a mixture of oil and air. The oil volume  27  is disposed toward the bottom  35  of the body  32 , while the air volume  29  is disposed toward the top  33  of the body  32 . The oil volume  27  is rotating inside the tank  30  as illustrated by arrow  31 . The rotation of the oil volume  27  is not due to a rotation of the tank  30 , but rather by a momentum that is imparted by the oil as released from the oil return line  28  into the tank  34 . The rotating oil volume  27  is a free vortex having a dip  27   a  at its center and elevated sides  27   b  at the sidewall  34 .  FIG. 3  is schematic and the vortex may differ from shown therein. The air volume  29  may vary from 2 to 20 times the air volume  27  in the tank  30 . 
         [0020]    The body  32  includes three apertures, namely an air vent  36  and a scavenge pipe  40  connection  39  at the top  33  of the body  32 , and an oil supply pipe  41  connection  43  at the bottom  35  of the body  32 . The air vent  36  is disposed at a center of the top  33 . The air vent  36  is positioned above the oil level. According to the embodiment of tank shown in  FIG. 3 , the air vent  36  is disposed above the dip  27   a  of the oil volume  27  which is a lowest point of the oil volume  27  for minimizing oil that would involuntarily enter the vent  36 . It is contemplated however, that the air vent  36  could be disposed elsewhere on the body  32 . For example, the air vent  36  could be on a side of the top  33  or on a top portion of the sidewall  34 , as long as the air vent  36  is disposed in the air volume  29  so as to effectively extract air and not oil form the tank  30 . The air vent  36  maintains a minimum pressure in the tank  30  and releases air if the pressure in the tank  30  exceeds a predetermined pressure. The air vent  36  shown in the Figures is a calibrated orifice. It is contemplated that the air vent  36  could be a pressure regulator other than a calibrated orifice. For example, the air vent  36  could be a pressure regulating valve or a pressure relief valve. 
         [0021]    The scavenge pipe  40  connects perpendicularly to the top  33  of the body  32  at the connection  39 . The scavenge pipe  40  carries fluid, i.e. the mixture of oil and air, from the oil return line  28  from the scavenge pump  26  to the tank  30  (arrow  45 ). The scavenge pipe  40  includes an external pipe portion  42  disposed outside of the body  32  and an internal discharge portion  44  disposed within the body  32 , in the air volume  29  of the tank  30 . In addition of carrying the fluid back to the tank  30 , the discharge portion  44  of the scavenge pipe  40 , which will be described below, is designed to separate at least partially the air from the oil in the mixture, and to reduce a hydraulic jump when oil is returned to the oil volume  27 . The connection  39  to the scavenge pipe  40  is disposed at the top  33  of the body  32 , off-set from the air vent  36 . It is contemplated that the connection  39  could be disposed elsewhere on the body  32 . For example, the connection  39  could be on the sidewall  34 . It is also contemplated that the scavenge pipe  40  could connect with the top  33  at an angle other than 90 degrees. Although the discharge portion  44  is shown in  FIGS. 4 to 8  to be connected to the tank body  32  only by the connection  39 , it is contemplated that additional connections could support the discharge portion  44  inside the tank  40 . The embodiment of the tank body  32 ′ shown in  FIG. 9  shows such example with the use of a support  31 ′ extending from the top  33 ′ to the internal pipe portion  44 . 
         [0022]    The oil supply pipe&#39;s  41  connection  43  is disposed at the bottom  35  of the body  32 . The oil supply line  43  evacuates oil from the oil volume  27  and directs it to the engine  10  (arrow  47 ). 
         [0023]    Turning now to  FIGS. 4 to 8 , the discharge portion  44  of the scavenge pipe  40  will now be described in detail. 
         [0024]    The fluid entering the discharge portion  44  of the scavenge pipe  40  being oil with air bubbles, it constitutes a foam precursor if released directly into the tank  30 . In order to control the foam, which may cause the air vent  36  to clog and/or to release oil instead of air only which in turn will increase pressure in the tank  30 , the discharge portion  44  is designed to limit factors that would lead to foam formation. In particular, the discharge portion  44  includes a first bend which stratifies the flow and separates the air from the oil for the fluid, and a second optional bend which insures that the flow remains stratified until it is discharged to the oil volume  27 . 
         [0025]    As best shown in  FIG. 4 , the discharge portion  44  includes a first portion  46  extending from the top  33  of the body  32  of the tank  30  to a first bend  48 . As best shown in  FIG. 5 , the discharge portion  44  includes a second portion  50  extending from the first bend  48  to a second bend  52 . The first  46  and second 50 portions are generally straight and form together an angle a 1  of about 90 degrees. It is contemplated that the angle a 1  could be greater than 90 degrees but lower than 180 degrees. As best shown in  FIG. 8 , the first bend  48  starts at an inception  51  and finishes at an end  53 . The inception  51  and the end  53  of the bend  48  are locations where a radius of curvature C of the discharge portion  44  changes. The inception  51  of the bend  48  corresponds to an end of the first portion  46 , while the end  53  of the bend  48  corresponds to an end of the second portion  50 . In the embodiment described herein, the first portion  46  and the second portion  50  being straight, they have an infinite radius of curvature, while the bend  48  has a definite radius of curvature. It is contemplated however, that the first portion  46  and the second portion  50  could have some radius of curvature. Another way to characterise the bend  48  is to define it by an angle a 1  between a perpendicular to the first bend  48  at the inception  51  and a perpendicular to bend  48  at the end  53 . In the embodiment shown in the Figures, the angle a 1  of the bend  48  is 90 degrees. It is contemplated that the angle a 1  could be at least 70 degrees. When the first bend  48  has a constant radius of curvature C, as does the example shown in the Figures, the perpendiculars of the first bend  48  at the inception  51  and end  53  correspond to radiuses of the first bend  48 . The radius of curvature C of the first bend  48  may be 2 to 3 times a diameter D of the first portion  46 . 
         [0026]    The first portion  46 , the first bend  48 , and the second portion  50  are contained in a common plane P 1 . In the discharge portion  44  shown in  FIGS. 4 to 8 , the plane P 1  is a vertical plane. It is contemplated that the plane P 1  could not be vertical should the discharge portion  44  be connected to the top  33  of the body  32  of the tank  30  at an angle other than 90 degrees. It is contemplated that the second portion  50  could be omitted and that the first bend  48  could be directly connected to the second bend  52 . 
         [0027]    The second bend  52  extends between the second portion  50  and a third portion  54 , and is fairly similar to the first bend  48  except that it is located in a plane different from the one of the bend  48 . The third portion  54  is at an angle a 2  of 90 degrees with the second portion  50 . It is contemplated that the angle a 2  could be greater than 90 degrees but lower than 180 degrees. An angle a 2  of the second bend  52 , defined to be the angle between a perpendicular to the second bend  52  at its inception and a perpendicular to the second bend  52  at its end (not shown in the Figures but similar to what has been described for a 1 ), is 80 degrees in the embodiment shown in the Figures. It is contemplated that the angle a 2  could be within 80 to 120 degrees range. A sum of the angles a 1  and a 2  is 180 degrees. It is contemplated that the sum of the angles a 1  and a 2  could be any value above 90 degrees. It could even be 400 degrees, should the discharge portion  44  swirls on itself. It is contemplated that the second bend  52  could be omitted, and that the discharge portion  44  would only have the bend  48 . 
         [0028]    The third portion  54  is straight and extends from the second bend  52  to a free end, or outlet,  56  of the internal portion  44 . It is contemplated that the third portion  54  could be curved. It is also contemplated that the third portion  54  could be omitted and that the first bend  48  or the second bend  52  connect directly with the outlet  56 . The second portion  50 , the second bend  52 , and the third portion  54  are contained in a common plane P 2 . In the embodiment shown in the Figures, the third portion  54  is inclined at an angle a 3  of 10 degrees relative to the horizontal H, and oriented toward the oil volume  29 . As such, an angle between the first plane P 1  and the second plane P 2  is 100 degrees. It is also contemplated that the angle a 3  could be within a 10 to 20 degrees range from the horizontal, and that as a result, the angle between the first plane P 1  and the second plane P 2  could be between 100 and 110 degrees. It is contemplated that the plane P 2  could be at a different angle with respect to the plane P 1 . For example, the plane P 2  could form with the plane P 1  an angle of 70 or 100 degrees. It is also contemplated that the planes P 1  and P 2  could be coplanar and that the bends  48 ,  52  could form a U-turn. 
         [0029]    The third portion  54  is disposed tangential a concave curved portion of the sidewall  34 , and is oriented such that the outlet  56  is disposed near the sidewall  34 , in the direction of rotation  31  of the oil volume  27 . It is contemplated that the outlet  56  could abut the sidewall  34 , or that the outlet  56  could be disposed elsewhere in the tank  30 . 
         [0030]    Although the embodiment shown in  FIGS. 4 to 8  shows that the discharge portion  44  has two bends  48 ,  52 , it is contemplated that the discharge portion  44  of the scavenge pipe  40  could have only one or more than two bends, as long as the bend(s) ensure some stratification of the flowing fluid. 
         [0031]    With reference to  FIG. 8  where the bend  48  is schematically illustrated, the bends  48 ,  52  will now be described in details. The bend  52  being similar to the bend  48 , reference will be made to the bend  48  only. 
         [0032]    The bends  48  (resp.  52 ) include a plurality of perforations  60  (resp.  62 ). The perforations  60  are disposed on the outside  48   b  of the bend  48 . An outside of a bend in a portion of the bend the farthest away from a center of the turn at a given radius of the bend. The perforations  60  allow a portion of the oil of the fluid delivered by the scavenge pipe  40  to be ejected from the scavenge pipe  40  (arrow  63 ) (i.e. bleeding). An area of the discharge portion  44  having the perforations  60  starts at 45 degrees downstream the bend&#39;s  48  inception  51  and continues until one diameter D after the end  53  of the bend  48 . It is contemplated that the perforations  60  could be contained in an area smaller than described above. For example, the perforations  60  could be contained only between the inception  51  and the end  53  of the bend  48 . 
         [0033]    A diameter of the perforations  60  is equal to a thickness T of the internal pipe portion  44 . While other dimensions of the perforations  60  are contemplated, smaller perforations tend to create jets of oil while larger perforations release air in addition to oil, as opposed to oil only. The perforations  60  are a means to provide energy dissipation to the fluid before it reaches the outlet  56 . A preferred number of the perforations  60  can be found by trial and error. Too few perforations will not provide enough energy dissipation, while a too large number of perforations would release air in addition to oil, as opposed to oil only. 
         [0034]    Although the embodiment shown in  FIGS. 4 to 8  shows that each bend  48 ,  52  has perforations, it is contemplated that only one of the bends  48 ,  52  or none of the bends  48 ,  52  could have perforations. 
         [0035]    Flow in the discharge portion  44  is as follow. The mixture of air and oil enters the discharge portion  44  and flows substantially unaltered through the first portion  46 . As the fluid enters the first bend  48 , it is accelerated by the curvature of bend  48 . The relatively low centrifugal acceleration (2 to 3 g in the embodiment shown herein) provided by a geometry of the bend  48  forces the air trapped in bubbles to separate from the oil. Oil being heavier than air, oil accumulates on an outside  48   b  of the bend  48 , while the air bubbles have tendency to accumulate on an inside  48   a  of the bend  48 . As the mixture travels on the outside  48   b  of the bend  48 , it is being released by the perforations  60  so as to decrease the amount of oil exiting through the outlet  56  and in turn decrease an impact of the hydraulic jump. The fluid then flows in the second portion  50  substantially unaltered from its states when leaving the first bend  48 . The fluid reaches the second bend  52  which has a substantially same effect on the fluid than the first bend  48  does. In short second portions  50 , the fluid does not have time to reform as a mixture, remains somehow stratifies, and the second bend  52  ensures that the fluid keeps the stratification until it reaches the outlet  56 . In long second portions  50 , the fluid may have time to reform as a mixture, may loose of its stratification, and the second bend  52  ensures that the fluid regain the stratification until it reaches the outlet  56 . The fluid flows from the second bend  52  into the third portion  54  and the remaining of the mixture is expelled from the outlet  56  to flow along the sidewall  34  (as illustrated by reference numeral  61  in  FIG. 3 ). The oil expelled by the outlet  56  feeds the vortex of the oil volume  27 , while the air feeds the air volume  29 . Some foam may appear in the oil volume  27 . The somewhat smooth transition from the outlet  56  to the oil volume  27  using the sidewall  34  reduces the hydraulic jump. The hydraulic jump contributes to foam formation, which as discussed above could impart to an optimal functionality of the tank  30 . A tangential discharge of the fluid as described herein could even eliminate the hydraulic jump. 
         [0036]    With the oil return system described herein having a scavenge pipe having a portion extending in the tank being shaped and positioned within the tank to separate flow and reduce hydraulic jump, there is no need of a cyclonic de-aerator. The bubbly oil of the oil return line is separated into air and oil by the bends. Perforations in the pipe release oil and dissipate energy of the oil before it reaches the oil volume, thereby reducing form formation. With a reduce foam presence, air can be released by the air vent without the air vent being chocked by the foam. Using the scavenge pipe described above may provide a simpler design compared to traditional de-aerators. It may be more compact, lighter and more economical than having a de-aerator associated with the tank. Performance may be equivalent or even better than traditional de-aerators. For example, the above described tank may be 20% lighter of a similar performance cyclonic de-aerator and may fit in a smaller envelope. Further, with the scavenge pipe described above, only limited modifications of existing system need to be performed. Such modifications include replacing current scavenge pipes with the scavenge pipe described herein. In addition to reducing foam formation, the combined use of the perforations and the disposition of the outlet may avoid feeding the vortex of oil and avoid the dip of the oil volume to coincide with the oil supply line, in which case the oil supply line would carry air to the engine instead of purely oil. 
         [0037]    The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.