Abstract:
A fuel nozzle sheath has a lateral opening for admitting air about a nozzle stern. The stress distribution along the perimeter of the window is smoothed out by increasing the corner radius of the window corner presenting the highest stress concentration. The different corner radii of the window opening in the sheath allows to reduce stresses resulting from the load transferred from the combustor liner to the sheath.

Description:
TECHNICAL FIELD 
     The invention relates generally to a fuel nozzle for gas turbine engines and, more particularly, addresses stress concentration in fuel nozzle sheaths. 
     BACKGROUND OF THE ART 
     In use, fuel nozzle sheaths are submitted to relatively severe stresses. This significantly impedes the service life of the nozzle sheaths. Stress concentration zones in the sheath may lead to sheath deformations. Large sheath deformation should be avoided to prevent load transfer from the combustion shell to the fuel nozzle stem via the nozzle sheath. Sheath deformations can also result in fretting damage on the fuel nozzle stem. 
     Accordingly, there is a need to provide a solution to the above mentioned problems. 
     SUMMARY 
     In one aspect, there is provided a fuel nozzle sheath adapted to be mounted about a gas turbine engine fuel nozzle stem having a spray tip, the sheath comprising a tubular body having a perimeter and extending longitudinally from a first end to an opposite second end, the first end being adapted to surround an inlet portion of the fuel nozzle stem while the second end surrounds the spray tip, and a lateral opening defined through the tubular body and extending longitudinally along at least a portion of said perimeter, said lateral opening having four corners, the radius of at least one of said corners being larger than the radii of the other corners. 
     In another aspect, there is provided a gas turbine engine fuel nozzle comprising: a fuel conveying member defining at least one fuel passage, a spray tip connected in fluid flow communication with said at least one fuel passage, said spray tip having an air discharged openings, a sheath provided about said fuel conveying member, an air passage defined between said fuel conveying member and said sheath, said air passage leading to said air discharged openings, a window defined in said sheath for supplying air to said air passage, said window being circumscribed by an edge having at least one corner presenting a stress concentration, and wherein said stress concentration is smoothed out by increasing a radius of curvature of said corner. 
     In a still further aspect, there is provided a method of smoothing out a stress distribution in a fuel nozzle sheath mounted about a fuel conveying member of a fuel nozzle, the fuel nozzle sheath defining a lateral window for supplying air about the fuel conveying member, the method comprising: reducing a stress concentration at a first corner of said window by increasing a corner radius of said first corner. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a gas turbine engine; 
         FIG. 2  is an axial cross-sectional view of a reverse flow combustor of the gas turbine engine showing a fuel nozzle; 
         FIG. 3  is a front elevation view of a tubular sheath of the fuel nozzle, the sheath having a window with different corner radii; and 
         FIG. 4  is a side view of the fuel nozzle illustrating the radius difference between a top corner and a bottom corner of the window defined in the sheath. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       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 multistage compressor  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. The resulting high temperature combustion gases are used to turn the turbine section  18  and produce thrust when passed through a nozzle. 
     Reference is now made to  FIG. 2  of the drawings which illustrates one exemplary embodiment of the combustor  16 . The combustor  16  shown is a reverse flow combustor  16 , however it should be understood that other types of combustor, such as an axial flow combustor, may have also been exemplified. The combustor  16  is fixedly mounted by suitable means in an air flow path, designated generally by arrows  20 , and receiving air from the compressor  14  or any other source of air. More particularly, the combustor  16  is mounted within the engine casing  22  which defines an annular or cylindrical flow path. The combustor  16  comprises an annular or cylindrical shell  24  which defines a primary combustion zone  26  and a dilution zone  28 . Mounted to the engine casing walls  22  is a plurality of fuel nozzles  30 , only one of which is shown in  FIG. 2 . The fuel nozzle  30  extends through the engine casing  22  and the combustor shell  24  such that it is in fluid flow communication with the primary combustion zone  26 . 
     The fuel nozzle  30  exemplified in  FIG. 2  comprises a fluid conveying member or stem  32  having a mounting flange  34 . The stem  32  is adapted to be coupled at its inlet end to a fuel manifold adapter  36  and at its outlet end  38  to a spray tip assembly  40 . Accordingly, the spray tip assembly  40  is coupled through the stem  32  to the fuel manifold adapter  36  which is connected to a fuel injector (not shown). The configuration of the stem  32  allows for the fuel supplied by the fuel injector to be directed from the fuel manifold  36  to the spray tip assembly  40 . The fuel is then atomized by the spray tip assembly  40  for ignition in the primary combustion zone  26 , as is well known in the art. 
     The fuel nozzle  30  also comprises an open ended tubular sheath  42  having a sidewall  44  that surrounds the stem  32  defining an annular flow passage  46  therebetween. In addition of protecting the stein  32  from the hot combustion gases, the sheath  42  provides support to the combustor shell  24  axially and circumferentially while allowing relative radial movement to occur therebetween. As shown in  FIGS. 3 and 4 , the sheath sidewall  44  extends from an inlet end  48  to an outlet end  50 . A mounting flange  52  is provided at the upper end of the sheath  42  for securing the sheath  42  to the undersurface of flange  34  of stem  32  by any appropriate means, such as by brazing or welding. Clipping means could also be used to detachably attach the sheath  42  in position about the stem  32 . The sheath  42  is preferably of unitary construction and has a generally cylindrical shape which is angularly truncated at the outlet end  50  to define a slanted opening configured to accommodate the spray tip  40 , as shown in  FIG. 2 . A lateral air supply window or opening  58  is defined in the sidewall  54  at the inlet end  48  of the sheath  42 . As shown in  FIG. 2 , the opening  58  is disposed in the air flow path  20  in facing relationship with the incoming discharged compressor air. The opening  58  connects the annular air flow passage  46  in fluid flow communication with the air flow path  20 . According to the embodiment illustrated in  FIGS. 3 and 4 , the opening  58  has a generally elongated rectangular shape and extends about 50% of the circumference of the sheath  52 . The window width is generally comprised in a range of about 35% to about 41% of the circumference of the sheath  42 . The window  58  has a width to height ratio in the range of 2.1 to 2.5. 
     The presence of such a relatively large window in the sheath  42  makes it vulnerable to high stress and might result in large sheath deflection. Large sheath deformations are to be avoided since they can potentially result in load transfer from the combustor shell  24  to the stem  32 , thereby reducing the fatigue life of the stem  32 . Sheath deflection should also be avoided in order to minimize contact stress and prevent fretting damages between the sheath  42  and the stem  32 . Accordingly, stress concentration in the sheath  42  is to be avoided. 
     Applicants have found through analytical methods, such as finite elements, and testing procedures that the window top corner  42   b  is subject to higher stresses than the other corners  42   a ,  42   c  and  42   d  and as such is more likely to give rise to sheath deflection. It is herein proposed to reduce the stresses in the top corner  42   b  by increasing stresses in the other corners  42   a ,  42   c  and  42   d  where the level of stress has been identified as being lower. This can be achieved by increasing the corner radius in corner  42   b  and reducing the radii of the other corners  42   a ,  42   c  and  42   d . Reducing the corner radius at corners  42   a ,  42   c  and  42   d  has for effect of increasing the level of stress thereat. Conversely, by increasing the corner radius of corner  42   b , the stress thereat is reduced. This provides for a more uniform distribution of the stress along the window perimeter. 
     According to one embodiment, the corners  42   a ,  42   c  and  42   d  have a corner radius r1 equal to 0.090″, whereas corner  42   b  has a corner radius r2 equal to 0.180″ that is two times greater than radius r1. It is understood that other r1/r2 ratios could be used as well to smooth out the stress distribution about the window  58 . For instance, the ratio r2/r1 could be comprised between about 1.5 to about 2.0. 
     In use, the sheath  42  supports the combustor shell  24  axially and circumferentially while providing freedom of movement in the radial direction. As shown in  FIG. 2  the aperture  58  in the tubular sheath  52  faces the air flow path  20  so as to intake oncoming compressor discharged air. The sheath  52  with its window  58  captures the dynamic head that is imposed by the incoming compressor air. The captured air flows along the annular air passage  46  towards the spray tip  40  coupled to the outlet end  50  of the sheath  52 . The air is ejected into the primary combustion zone  26  through air openings defined in the spray tip  40  in order to atomize the fuel delivered through the stem  32 . The selected increased and reduced corner radius r2 and r1 ensure proper stress distribution in the sheath  42 , thereby preventing combustor load transfer on the nozzle stem  32  through the sheath  42  during normal engine operations. 
     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 department from the scope of the invention disclosed. For example, the sheath  42  could have a different configuration than the one shown and herein described. The shape of the sheath is not limited to cylindrical and the term “cylindrical” should be herein broadly construed. It should also be understood that the tubular sheath may be attached to the fuel adapter and spray tip assembly in many different ways. The window does not necessarily have to be rectangular. Other shapes are contemplated as well as long as they provide adequate air supply to the fuel nozzle. 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.