Abstract:
A fuel nozzle for a gas turbine engine includes an engine mount end and a discharge end that discharges an air/fuel mixture into a combustion chamber. The fuel nozzle includes a centerbody and a heat shield. The heat shield is fixed to the centerbody at a mid-mount position that is centrally located between first and second ends of the heat shield to allow the heat shield to remain thermally isolated from radially adjacent components to reduce the adverse effects of thermal stresses.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application relates to a fuel nozzle with a heat shield mounted within a centerbody to reduce stress levels at an inner air swirler attachment interface. 
         [0002]    A fuel injector or fuel nozzle is utilized in a combustion section of a gas turbine engine to discharge an air/fuel mixture into a combustion chamber. The fuel nozzle includes a centerbody that defines a portion of a fuel passage. A heat shield is mounted to the centerbody within an air passage. Air is pre-swirled prior to being mixed with fuel via an inner air swirler (IAS) having a plurality of vanes, with the IAS being fixed to an inner surface of the heat shield. The IAS is fixed to the heat shield by brazing at radially outward tips of the vanes. 
         [0003]    In a current attachment configuration between the heat shield and centerbody, the heat shield has a first end that is press fit into the centerbody and a second end that is not attached to the centerbody. The heat shield is directly attached to the centerbody by brazing at a location immediately adjacent the press fit at the first end. Another known mounting configuration has the press fit and heat shield attachment joint fixed to the second end, while the first end is not attached to the centerbody. In either of these configurations, an outer surface of the heat shield is in close proximity or contacts an inner surface of the centerbody along a substantial portion of the axial length of the heat shield such that there is minimal to no gap between the centerbody and the heat shield. 
         [0004]    One disadvantage with these known mounting configurations is that at elevated temperatures there is insufficient ability for the heat shield to remain thermally isolated from radially adjacent components. This can result in higher thermal stress levels at attachment interfaces for components attached to the heat shield. For example, these higher thermal stress levels can cause undesirable cracking at the attachment interface between the IAS and the heat shield leading to premature component wear, which is not desirable. Thus, there is a need for a heat shield mounting configuration that reduces the adverse effects of thermal stresses. 
       SUMMARY OF THE INVENTION 
       [0005]    A heat shield is fixed to a fuel nozzle centerbody at a mid-mount position that is centrally located between first and second ends of the heat shield. The first and second ends of the heat shield are received as slip fits within the centerbody. This allows the heat shield to expand freely in opposing axial directions to reduce the effects of thermal stresses in radially adjacent components. 
         [0006]    In a disclosed embodiment of this invention, the fuel nozzle is used in a gas turbine engine and includes an engine mount end and a discharge end that discharges an air/fuel mixture into a combustion chamber. A fuel nozzle support extends from the engine mount end to support the centerbody. The centerbody defines a portion of a fuel passage with an inlet end and a discharge or outlet end. The first end of the heat shield is located at the inlet end and the second end of the heat shield is located at the outlet end. An inner air swirler is fixed to an inner surface of the heat shield near the inlet end. The heat shield is fixed to the centerbody at a position that is axially between the inner air swirler and the outlet end. In one example, the outer surface of the heat shield is spaced from an inner surface of the centerbody to form an air gap. This air gap extends along a substantial axial length of the heat shield. 
         [0007]    The subject invention provides a heat shield mounting configuration that significantly reduces the effects of thermal stresses on adjacent components when compared to previous designs. These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic of a gas turbine engine incorporating the present invention. 
           [0009]      FIG. 2  is a perspective view of a fuel nozzle as used in the gas turbine engine of  FIG. 1 . 
           [0010]      FIG. 3  is an enlarged view of an inner air swirler and heat shield interface. 
           [0011]      FIG. 4  is a cross-sectional view of a fuel nozzle with heat shield and inner air swirler designed according to the prior art. 
           [0012]      FIG. 5  is a cross-sectional view of a fuel nozzle with heat shield and inner air swirler incorporating the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0013]      FIG. 1  shows a gas turbine engine  10 , such as a gas turbine used for power generation or propulsion, circumferentially disposed about an engine centerline, or axial centerline axis  12 . The engine  10  includes a fan  14 , a compressor  16 , a combustion section  18 , and a turbine  20 . As is well known in the art, air compressed in the compressor  16  is mixed with fuel which is burned in the combustion section  18  and expanded in turbine  20 . The air compressed in the compressor  16  and the fuel mixture expanded in the turbine  20  can both be referred to as a hot gas stream flow. The turbine  20  includes rotors  22  and  24  that, in response to the expansion, rotate, driving the compressor  16  and fan  14 . The turbine  20  comprises alternating rows of rotary blades  26  and static airfoils or vanes  28 .  FIG. 1  is a somewhat schematic representation, for illustrative purposes only, and is not a limitation of the instant invention, that may be employed on gas turbines used for electrical power generation and aircraft. 
         [0014]    A fuel injector or fuel nozzle is shown generally at  30  in  FIG. 1 , and is shown in greater detail in  FIG. 2 . The fuel nozzle  30  is positioned within the combustion section  18  and includes an engine mount end  32  and a discharge end  34  from which an air/fuel mixture is discharged into a combustion chamber  36  ( FIG. 1 ). The fuel nozzle  30  includes a fuel nozzle support portion  38  that supports a centerbody  40 . Fuel enters through an opening  41 . Air enters through opening  42  to an air flow passage, and cooperates with an inner air swirler (IAS)  44 , which is shown in  FIG. 3 . 
         [0015]    The IAS  44  includes a plurality of vanes  46  that extend radially outwardly from a center hub  48 . An outer circumference of the vanes  46  is attached to an inner surface of a heat shield  50  by brazing. The structure and operation of the IAS is known and will not be discussed in further detail. 
         [0016]    A known attachment configuration between the heat shield  50  and centerbody  40  is shown in  FIG. 4 . In this configuration, the heat shield  50  has a first end  52  that is press fit into the centerbody  40  and a second end  54  that is not attached to the centerbody  40 . The heat shield  50  is directly attached by brazing to the centerbody  40  at a location  58  immediately adjacent the press fit at the first end  52 . An outer surface of the heat shield  50  is in close proximity or contacts an inner surface of the centerbody  40  along a substantial portion of the axial length of the heat shield  50  such that there is minimal to no gap between the centerbody  40  and the heat shield  50 . 
         [0017]    One disadvantage with this mounting configuration is that the heat shield does not have sufficient thermal isolation from radially adjacent components in response to increased heat levels. This can result in higher stress levels at attachment interfaces. 
         [0018]    Further, as discussed above, the heat shield  50  is directly attached by brazing to the centerbody  40  at a location  58  immediately adjacent the press fit at the first end  52 . This traditional mounting configuration can generate high thermal stress levels at this attachment interface. The high thermal stress levels can cause undesirable cracking at the attachment interface between the heat shield  50  and the centerbody  40  leading to premature component wear. 
         [0019]    The present invention in part addresses this problem by providing a fuel nozzle  30  with an improved mounting configuration for a heat shield  70 . The heat shield  70  is mounted to the centerbody  40 , with the inner surface of the heat shield defining an air passage  72 . The centerbody  40  defines a central axis A and has an inlet end  76  and a discharge or outlet end  78 . The heat shield  70  has a first end  80  positioned at the inlet end  76  and a second end  82  positioned at the outlet end  78 . 
         [0020]    An outer surface  84  of the heat shield  70  is fixed to the inner surface  74  of the centerbody  40  at a first attachment interface  86 . The IAS  44  is attached to an inner surface  88  of the heat shield  70  at a second attachment interface  90  that is spaced apart from the first attachment interface  86  in a direction along the central axis A. In one example, the first and second attachment interfaces comprise brazed joints. The first attachment interface  86  is positioned axially between the second attachment interface  90  and the second end  82  of the heat shield  70 . Thus, the first attachment interface  86  comprises a mid-mount location that is positioned generally centrally between the first  80  and second  82  ends of the heat shield  70 . 
         [0021]    The first  80  and second  82  ends of the heat shield  70  are mounted as slip fits within the centerbody  40 . The outer surface  84  of the heat shield is spaced radially apart from the inner surface  74  of the centerbody  40  to form a gap  92 . This gap  92  extends along a substantial length of the heat shield  70 . 
         [0022]    A raised mounting boss  94  is formed on the outer surface  84  of the heat shield  70  and extends in a radial direction toward the inner surface  74  of the centerbody  40 . The first attachment interface  86  is located at this raised mounting boss  94 . 
         [0023]    The IAS  44  and the heat shield  70  are preferably made from Inconel 625 material that provides reduced thermal growth compared to 347SS material used in prior designs. 
         [0024]    The present invention utilizes a unique configuration for a heat shield that reduces the effects of thermal stresses to less than 20% of the stresses from existing designs. This significantly reduces the possibility for cracking to occur at an IAS and heat shield attachment interface, increasing the overall wear life of the fuel nozzle. 
         [0025]    Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.