Abstract:
A fuel nozzle configured to channel fluid toward a combustion chamber within a gas turbine engine and a method for assembling the same are provided. The fuel nozzle includes a first hollow tube fabricated from a first material that has a first coefficient of thermal expansion and a second tube fabricated from a second material that has a second coefficient of thermal expansion that is different from the first coefficient of thermal expansion. The second tube is coupled within the first tube such that the first tube substantially circumscribes the second tube, and the second tube thermally expands approximately at a same rate as the first tube during fuel nozzle operation.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to gas turbine engines and, more particularly, to fuel nozzle assemblies for use with gas turbine engines. 
     At least some known fuel nozzle assemblies used with gas turbine engines include tubular components that define internal passages. The variety of fluids that may flow through the internal passages may cause each tubular component to operate at different temperatures. The different temperatures may cause disparate thermal growths of the various tubular components, which may induce thermal strains. To relieve such thermal strains, at least some known fuel nozzle assemblies include a bellows assembly that facilitates compensating disparate thermal growths. A bellows assembly, however, generally increases the cost and complexity of manufacturing, handling, and maintaining the gas turbine engine. 
     BRIEF SUMMARY OF THE INVENTION 
     In one embodiment, a method for assembling a fuel nozzle for use with a gas turbine engine is provided. The method includes providing a first hollow tube fabricated from a first material that has a first coefficient of thermal expansion, providing a second hollow tube fabricated from a second material that has a second coefficient of thermal expansion that is different from the first coefficient of thermal expansion, and coupling the second tube within the first tube such that the first and second tube are oriented to channel fluids toward a combustion chamber. The first tube substantially circumscribes the second tube, and the second tube thermally expands approximately at a same rate as the first tube during fuel nozzle operation. 
     In another embodiment, a fuel nozzle configured to channel fluid towards a combustion chamber defined within a gas turbine engine is provided. The fuel nozzle includes a first hollow tube fabricated from a first material that has a first coefficient of thermal expansion and a second tube fabricated from a second material that has a second coefficient of thermal expansion that is different from the first coefficient of thermal expansion. The second tube is coupled within the first tube such that the first tube substantially circumscribes the second tube, and the second tube thermally expands approximately at a same rate as the first tube during fuel nozzle operation. 
     In yet another embodiment, a gas turbine engine is provided. The gas turbine engine includes a combustion chamber and a fuel nozzle configured to channel fluid toward the combustion chamber. The fuel nozzle includes a first tube and a second tube coupled within the first tube such that the first tube substantially circumscribes the second tube. The first tube is fabricated from a first material that has a first coefficient of thermal expansion, and the second tube is fabricated from a second material that has a second coefficient of thermal expansion that is different from the first coefficient of thermal expansion such that the second tube thermally expands approximately at a same rate as the first tube during fuel nozzle operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an exemplary gas turbine engine; 
         FIG. 2  is a cross-sectional schematic illustration of an exemplary combustor that may be used with the gas turbine engine shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional schematic illustration of a known fuel nozzle assembly; and 
         FIG. 4  is a cross-sectional schematic illustration of a fuel nozzle assembly that may be used with the combustor shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A fuel nozzle assembly with a simple and inexpensive alternative to a bellows assembly is desired. The present invention facilitates the relief of axial and radial thermal strains while limiting the number of parts and joints necessary to facilitate such relief. 
       FIG. 1  is a schematic illustration of an exemplary gas turbine engine. Gas turbine engine  100  includes a compressor  102  and a combustor  104 , which includes a fuel nozzle assembly  106 . Gas turbine engine  100  also includes a turbine  108  and a common compressor/turbine shaft  110 . In one embodiment, gas turbine engine  100  is a PG9371 9FBA Heavy Duty Gas Turbine Engine commercially available from General Electric Company, Greenville, S.C. Notably, the present invention is not limited to any one particular engine and may be used in connection with other gas turbine engines. 
     During operation, air flows through compressor  102  and compressed air is supplied to combustor  104  and, more specifically, to fuel nozzle assembly  106 . Fuel is channeled to a combustion region defined within combustor  104 , wherein the fuel is mixed with the compressed air and the mixture is ignited. Combustion gases generated are channeled to turbine  108 , wherein gas stream thermal energy is converted to mechanical rotational energy. Turbine  108  is rotatably coupled to, and drives, shaft  110 . 
       FIG. 2  is a cross-sectional schematic view of combustor  104 . Combustor  104  is coupled in flow communication with compressor  102  and turbine  108 . Compressor  102  includes a diffuser  112  and a compressor discharge plenum  114  that are coupled in flow communication with each other. Combustor  104  includes an end cover  120  that provides structural support to a plurality of fuel nozzle assemblies  122 . End cover  120  is coupled to combustor casing  124  with retention hardware (not shown in  FIG. 2 ). A combustor liner  126  is positioned radially inward from combustor casing  124  such that combustor liner  126  defines a combustion chamber  128  within combustor  104 . An annular combustion chamber cooling passage  129  is defined between combustor casing  124  and combustor liner  126 . A transition piece  130  is coupled to combustion chamber  128  to facilitate channeling combustion gases generated in combustion chamber  128  downstream towards turbine nozzle  132 . In the exemplary embodiment, transition piece  130  includes a plurality of openings  134  defined in an outer wall  136 . Transition piece  130  also includes an annular passage  138  that is defined between an inner wall  140  and outer wall  136 . Inner wall  140  defines a guide cavity  142 . In the exemplary embodiment, fuel nozzle assembly  122  is coupled to end cover  120  via a fuel nozzle flange (not numbered). 
     During operation, turbine  108  drives compressor  102  via shaft  110  (shown in  FIG. 1 ). As compressor  102  rotates, compressed air is discharged into diffuser  112  as the associated arrows illustrate. In the exemplary embodiment, the majority of air discharged from compressor  102  is channeled through compressor discharge plenum  114  towards combustor  104 , and the remaining compressed air is channeled for use in cooling engine components. More specifically, pressurized compressed air within discharge plenum  114  is channeled into transition piece  130  via outer wall openings  134  and into annular passage  138 . Air is then channeled from annular passage  138  through annular combustion chamber cooling passage  129  and to fuel nozzle assemblies  122 . Fuel and air are mixed, and the mixture is ignited within combustion chamber  128 . Combustor casing  124  facilitates shielding combustion chamber  128  and its associated combustion processes from the outside environment, such as, for example, surrounding turbine components. Combustion gases generated are channeled from combustion chamber  128  through guide cavity  142  and towards turbine nozzle  132 . 
       FIG. 3  is a cross-sectional schematic view of a known fuel nozzle assembly  300 . Fuel nozzle assembly  300  includes two concentrically-aligned tubular components, inner tubular component  310  and outer tubular component  320 , that define internal passages. In the exemplary embodiment, tubular components  310  and  320  are fabricated from the same material composition. The variety of fluids that may flow through the internal passages may cause tubular components  310  and  320  to operate at different temperatures. 
     Operating tubular components  310  and  320  at different temperatures may cause differential thermal growth, which may eventually induce fatigue cracks or joint failures. Such thermal strains are typically induced between tubular components  310  and  320  and their attachment joints. To relieve such thermal strains, fuel nozzle assembly  300  includes bellows assembly  330  that is fabricated from a different material composition than that of the tubular components  310  and  320 . 
     In the exemplary embodiment, bellows assembly  330  includes a plurality of components (not shown) and a plurality of joints (not shown). The number of components and joints associated with bellows assembly  330  increases the cost and complexity of manufacturing, handling, and maintaining gas turbine engine  100  as compared to fuel nozzle assemblies without bellows assembly  330 . For example, in one embodiment, bellows assembly  330  includes a thin bellows tubular component (not shown) that is coupled to end caps (not shown) via fillet welds (not shown). Moreover, thin bellows tubular component is coupled to at least one of tubular components  310  and  320  via brazing or welding. In one embodiment, shields and adaptors (not shown) are also coupled to bellows assembly  330  via welding to facilitate shielding bellows assembly  330  from mating parts. 
     Moreover, bellows assembly  330  accommodates axial thermal growth within fuel nozzle assembly  300 , but accommodates radial thermal growth only at an attachment joint. To accommodate radial thermal growth within fuel nozzle assembly  300 , additional bellows assemblies  330  are required. For example, in one embodiment, bellows assembly  330  is coupled to each axial end of at least one of tubular components  310  and  320 . Incorporating additional bellows assemblies  330 , however, increases the costs and complexity of manufacturing, handling, and maintaining gas turbine engine  100  as compared to fuel nozzle assemblies with fewer, or no, bellows assembly  330 . Furthermore, the inclusion of additional materials when incorporating additional bellows assemblies  330  creates additional loading at the attachment joints. 
       FIG. 4  is a cross-sectional schematic view of an exemplary fuel nozzle assembly  400  that may be used with combustor  104  (shown above). As described in more detail below, fuel nozzle assembly  400  may reduce or eliminate the need for bellows assembly  330  (shown in  FIG. 3 ) with a simple, low-load design. 
     In the exemplary embodiment, fuel nozzle assembly  400  includes two substantially concentrically-aligned tubular components, inner tubular component  410  and outer tubular component  420 , with a distinct strength, durability, and coefficient of thermal expansion, as described in more detail below. Notably, while the exemplary embodiment includes two tubular components, it should be understood that the number of tubular components is not intended to limit the invention in any manner. 
     In the exemplary embodiment, tubular components  410  and  420  are substantially cylindrical and have a substantially circular cross-sectional profile. In alternative embodiments, at least one of tubular components  410  and/or  420  has a non-uniform profile configured to accommodate additional thermal displacement. For example, in one embodiment, at least one of tubular component  410  and/or  420  is a corrugated cylinder. Tubular components  410  and  420  are welded to end pieces, including a fuel nozzle tip (not numbered) and a flange  430 , that provide robust braze joints. In one embodiment, tubular components  410  and  420  are electron beam welded to the fuel nozzle tip and flange  430 . Tubular components  410  and  420  are oriented in the exemplary embodiment such that a single braze operation may be completed. Flange  430  provides additional support and robustness to fuel nozzle assembly  400 . In one embodiment, flange  430  is fabricated from the same material composition as that of outer tubular component  420 . In one embodiment, short sections of material adapters are welded at the axial ends of tubular components  410  and  420  to provide robustness at high temperatures or to enhance braze joint robustness. 
     Tubular components  410  and  420  define internal passages for a variety of fluids, including liquid, gas, and any mixture thereof. More specifically, inner tubular component  410  defines inner flow channel  440  and outer tubular component  420  defines outer flow channel  450 . Generally, inner flow channel  440  is at a lower temperature than outer flow channel  450 . For example, in one embodiment, inner tubular component  410  is used to channel fuel, and outer tubular component  420  is used to channel air. Because inner flow channel  440  is generally at a lower temperature than outer flow channel  450 , each tubular component  410  and  420  generally operates at a different operating temperature. Thus, the material used in fabricating inner tubular component  410  is generally cooler than the material used in fabricating outer tubular component  420 . 
     To facilitate reducing thermal strains caused by the differential temperatures of tubular components  410  and  420 , outer tubular component  420  is fabricated from a material having a lower coefficient of thermal expansion than that of the material used in fabricating inner tubular component  410 . The materials used in fabricating tubular components  410  and  420  are selected based at least in part on operating conditions of tubular components  410  and  420 , including what fluids will channeled through tubular components  410  and  420 . Specifically, the respective coefficient of thermal expansions of tubular components  410  and  420  are proportional such that tubular components  410  and  420  would expand a substantially similar axial distance at the respective operating temperatures. In one embodiment, outer tubular component  420  is fabricated from a 400 series stainless steel, such as a martensitic stainless steel or a ferritic stainless steel, and inner tubular component  410  is fabricated from a 300 series stainless steel, such as an austenitic stainless steel. 
     For example, in one embodiment, outer tubular component  420  has twice the increase in temperature than that of inner tubular component  410 , but has approximately half of the rate of thermal expansion than that of inner tubular component  410 . For a more specific example, in one embodiment, a resting temperature for tubular components  410  and  420  is approximately 70 degrees Fahrenheit, an operating temperature for inner tubular component  410  is approximately 360 degrees Fahrenheit, and an operating temperature for outer tubular component  420  is approximately 640 degrees Fahrenheit. In this embodiment, if inner tubular component  410  is fabricated from a material having a coefficient of thermal expansion of approximately 10 e-6 in/in/F, outer tubular component  420  would be fabricated from a material having a coefficient of thermal expansion of approximately 6.4 e-6 in/in/F to expand a substantially similar axial distance at the respective operating temperatures. 
     Considering the material compositions of the materials used in fabricating tubular components  410  and  420  and their respective strain ranges against low cycle fatigues ratios, the wall thicknesses of tubular components  410  and  420  are selected to be structurally strong enough to contain the internal pressure and vibration loads that may be induced by the fluids channeling therethrough. Moreover, the wall thicknesses of tubular components  410  and  420  are also selected to facilitate the necessary thermal differential growth without inducing additional loading to the attachment joints. For example, in one embodiment, outer tubular component  420  is typically two to four times thicker than inner tubular component  410 . For a more specific example, in one embodiment, inner tubular component  410  is approximately 1/16 inch thick, and outer tubular component  420  is ⅛ to ¼ inch thick or more. 
     The methods, apparatus, and systems for a fuel nozzle assembly described herein facilitate the operation of a gas turbine engine. More specifically, the fuel nozzle assembly described herein facilitates reducing thermal strains induced within the fuel nozzle assembly, while reducing the parts and joints necessary for assembly while maintaining structural robustness of the associated fuel nozzle assembly. Practice of the methods, apparatus, or systems described or illustrated herein is neither limited to a fuel nozzle assembly nor to gas turbine engines generally. Rather, the methods, apparatus, and systems described or illustrated herein may be utilized independently and separately from other components and/or steps described herein. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.