Patent Publication Number: US-2017370283-A1

Title: Exhaust frame of a gas turbine engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to European Patent Application No. EP16461528, filed on Jun. 23, 2016, which is hereby incorporated by reference in its entirety herein. 
     TECHNICAL FIELD 
     The present application and the resultant patent relate generally to gas turbine engines and more particularly relate to an exhaust frame for containing and directing combustion gases along a hot gas path of a gas turbine engine. 
     BACKGROUND OF THE INVENTION 
     In a gas turbine engine, hot combustion gases generated in one or more combustors generally may flow along a hot gas path extending through a turbine and an exhaust frame positioned downstream of the turbine. The exhaust frame may include an inner casing, an outer casing, and a number of struts extending between the inner casing and the outer casing. The inner casing may house a shaft bearing that supports a main shaft of the gas turbine engine therein. The combustion gases flowing through the exhaust frame may be contained between the inner casing and the outer casing and may flow over the struts. In this manner, the inner casing, the outer casing, and the struts may be subjected to high temperatures resulting from the flow of combustion gases along the hot gas path, which may result in the generation of high thermal stresses in these components and the interfaces therebetween. Because the efficiency of a gas turbine engine is dependent on its operating temperatures, there is an ongoing demand for components positioned along and within the hot gas path, such as the inner casing, the outer casing, and the struts of the exhaust frame, to be capable of withstanding increasingly higher temperatures without deterioration, failure, or decrease in useful life. 
     According to certain exhaust frame configurations, each strut may be welded at one end to the inner casing and at another end to the outer casing. During operation of the gas turbine engine, high stresses may be generated in the struts, particularly in the welded regions adjacent the inner casing and the outer casing, due to large temperature gradients produced in the exhaust frame. For example, during startup of the gas turbine engine, high stresses may be generated as the struts heat up faster than the inner casing and the outer casing. In a similar manner, high stresses may be generated during shut down of the gas turbine engine, as the struts cool down faster than the inner casing and the outer casing. During steady state operation of the gas turbine engine, high stresses may be generated due to cooling of the inner casing and/or the outer casing, such as via a cooling air system or external air, while the struts experience higher temperatures within the hot gas path. Additionally, when the inner casing is used to support the shaft bearing, high stresses may be generated in the struts due to imbalance of the main shaft, as may result from a “blade out” event or other causes. Ultimately, stress concentrations in the struts may lead to failure of the welds, which generally may have lower fatigue resistance than the base material (i.e., the inner casing or the outer casing) being welded. 
     There is thus a desire for an improved exhaust frame for containing and directing combustion gases along a hot gas path of a gas turbine engine at high operating temperatures. Such an improved exhaust frame should reduce stress concentrations in the struts thereof, particularly in the welded regions of the struts adjacent the inner casing and the outer casing of the exhaust frame. In this manner, such an improved exhaust frame should reduce the risk of failure of the welds, thereby increasing the life of the struts and the overall exhaust frame. 
     SUMMARY OF THE INVENTION 
     The present application and the resultant patent thus provide an exhaust frame for a gas turbine engine. The exhaust frame may include an inner casing extending along a longitudinal axis of the exhaust frame, an outer casing positioned radially outward from the inner casing, a strut extending radially from the inner casing to the outer casing, and a relief groove defined in the inner casing or the outer casing and positioned about a perimeter of the strut. 
     The present application and the resultant patent further provide a method for distributing stress concentrations in an exhaust frame of a gas turbine engine. The method may include the step of providing an exhaust frame including an inner casing extending along a longitudinal axis of the exhaust frame, an outer casing positioned radially outward from the inner casing, a strut extending radially from the inner casing to the outer casing, and a relief groove defined in the inner casing or the outer casing and positioned about a perimeter of the strut. The method also may include the step of directing a flow of combustion gases through the exhaust frame, wherein stresses generated in the relief groove are higher than stresses generated in the strut. 
     The present application and the resultant patent further provide a gas turbine engine. The gas turbine engine may include a compressor, a combustor in communication with the compressor, a turbine in communication with the combustor, and an exhaust frame in communication with the turbine. The exhaust frame may include an inner casing extending along a longitudinal axis of the exhaust frame, an outer casing positioned radially outward from the inner casing, a strut extending radially from the inner casing to the outer casing, and a relief groove defined in the inner casing or the outer casing and positioned about a perimeter of the strut. 
     These and other features and improvements of the present application and the resultant patent will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a gas turbine engine including a compressor, a combustor, a turbine, an exhaust frame, and an external load. 
         FIG. 2  is an end view of an embodiment of an exhaust frame as may be described herein and as may be used in the gas turbine engine of  FIG. 1 , the exhaust frame including an inner casing, an outer casing, and a number of struts. 
         FIG. 3  is a cross-sectional view of the exhaust frame of  FIG. 2 , taken along line  3 - 3 , showing the inner casing, the outer casing, and two of the struts. 
         FIG. 4A  is a detailed cross-sectional view of portions of the exhaust frame of  FIG. 2 , taken along line  4 - 4 , showing an embodiment of an interface between the inner casing and one of the struts. 
         FIG. 4B  is a detailed cross-sectional view of portions of the exhaust frame of  FIG. 2 , taken along line  4 - 4 , showing an embodiment of an interface between the inner casing and one of the struts. 
         FIG. 4C  is a detailed cross-sectional view of portions of the exhaust frame of  FIG. 2 , taken along line  4 - 4 , showing an embodiment of an interface between the inner casing and one of the struts. 
         FIG. 4D  is a detailed cross-sectional view of portions of the exhaust frame of  FIG. 2 , taken along line  4 - 4 , showing an embodiment of an interface between the inner casing and one of the struts. 
         FIG. 4E  is a detailed cross-sectional view of portions of the exhaust frame of  FIG. 2 , taken along line  4 - 4 , showing an embodiment of an interface between the inner casing and one of the struts. 
         FIG. 5A  is a detailed cross-sectional view of the portions of the exhaust frame of  FIG. 4A , taken along line  5 A- 5 A, showing the interface between the inner casing and the strut. 
         FIG. 5B  is a detailed cross-sectional view of the portions of the exhaust frame of  FIG. 4B , taken along line  5 B- 5 B, showing the interface between the inner casing and the strut. 
         FIG. 5C  is a detailed cross-sectional view of the portions of the exhaust frame of  FIG. 4E , taken along line  5 C- 5 C, showing the interface between the inner casing and the strut. 
         FIG. 6A  is a detailed cross-sectional view of portions of the exhaust frame of  FIG. 2 , taken along line  6 - 6 , showing an embodiment of an interface between the outer casing and one of the struts. 
         FIG. 6B  is a detailed cross-sectional view of portions of the exhaust frame of  FIG. 2 , taken along line  6 - 6 , showing an embodiment of an interface between the outer casing and one of the struts. 
         FIG. 6C  is a detailed cross-sectional view of portions of the exhaust frame of  FIG. 2 , taken along line  6 - 6 , showing an embodiment of an interface between the outer casing and one of the struts. 
         FIG. 6D  is a detailed cross-sectional view of portions of the exhaust frame of  FIG. 2 , taken along line  6 - 6 , showing an embodiment of an interface between the outer casing and one of the struts. 
         FIG. 6E  is a detailed cross-sectional view of portions of the exhaust frame of  FIG. 2 , taken along line  6 - 6 , showing an embodiment of an interface between the outer casing and one of the struts. 
         FIG. 7A  is a detailed cross-sectional view of the portions of the exhaust frame of  FIG. 6A , taken along line  7 A- 7 A, showing the interface between the outer casing and the strut. 
         FIG. 7B  is a detailed cross-sectional view of the portions of the exhaust frame of  FIG. 6B , taken along line  7 B- 7 B, showing the interface between the outer casing and the strut. 
         FIG. 7C  is a detailed cross-sectional view of the portions of the exhaust frame of  FIG. 6E , taken along line  7 C- 7 C, showing the interface between the outer casing and the strut. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, in which like numerals refer to like elements throughout the several views,  FIG. 1  shows a schematic diagram of a gas turbine engine  10  as may be used herein. The gas turbine engine  10  may include a compressor  15 . The compressor  15  compresses an incoming flow of air  20 . The compressor  15  delivers the compressed flow of air  20  to a combustor  25 . The combustor  25  mixes the compressed flow of air  20  with a pressurized flow of fuel  30  and ignites the mixture to create a flow of combustion gases  35 . Although only a single combustor  25  is shown, the gas turbine engine  10  may include any number of combustors  25 . The flow of combustion gases  35  is in turn delivered to a turbine  40 . The flow of combustion gases  35  drives the turbine  40  so as to produce mechanical work. The mechanical work produced in the turbine  40  drives the compressor  15  and an external load  50 , such as an electrical generator and the like, via a shaft  45 . The flow of combustion gases  35  is delivered from the turbine  40  to an exhaust frame  55  positioned downstream thereof. The exhaust frame  55  may contain and direct the flow of combustion gases  35  to other components of the gas turbine engine  10 . For example, the exhaust frame  55  may direct the flow of combustion gases  35  to an exhaust plenum or an exhaust diffuser. Other configurations and other components may be used herein. 
     The gas turbine engine  10  may use natural gas, various types of syngas, and/or other types of fuels. The gas turbine engine  10  may be any one of a number of different gas turbine engines offered by General Electric Company of Schenectady, N.Y., including, but not limited to, those such as a 7 or a 9 series heavy duty gas turbine engine and the like. The gas turbine engine  10  may have different configurations and may use other types of components. Other types of gas turbine engines also may be used herein. Multiple gas turbine engines, other types of turbines, and other types of power generation equipment also may be used herein together. 
       FIGS. 2 and 3  show an embodiment of an exhaust frame  100  as may be described herein. The exhaust frame  100  may be used in the gas turbine engine  10  and generally may be configured and arranged in a manner similar to the exhaust frame  55  described above. In particular, the exhaust frame  100  may be positioned downstream of the turbine  40  and may be configured to receive the flow of combustion gases  35  flowing along a hot gas path  102  of the gas turbine engine  10 . The hot gas path  102  may extend through the turbine  40  and the exhaust frame  100 , as described below. The exhaust frame  100  may have a leading or upstream end  104  and a trailing or downstream end  106 , as shown. The exhaust frame  100  may be configured to contain and direct the flow of combustion gases  35  to other components of the gas turbine engine  10 , such as an exhaust plenum or an exhaust diffuser positioned downstream of the exhaust frame  100 . 
     As shown, the exhaust frame  100  may include an inner casing  110 , an outer casing  112 , and a number of struts  114  extending between the inner casing  110  and the outer casing  112 . The inner casing  110  may be formed as a tube shaped body extending axially along and coaxial with a longitudinal axis  116  of the exhaust frame  100 . The inner casing  110  may house a shaft bearing  120  that supports the shaft  45  of the gas turbine engine  10  for rotation therein. The outer casing  112  may be formed as a tube shaped body extending along and coaxial with the longitudinal axis  116  of the exhaust frame  100 . As shown, the outer casing  112  may be spaced apart from and positioned radially outward from the inner casing  110 . In this manner, the inner casing  110  and the outer casing  112  may define a portion of the hot gas path  102  therebetween (i.e., the annular space between the inner casing  110  and the outer casing  112 ). In some embodiments, as described below with respect to  FIGS. 8A-8C , the exhaust frame  100  also may include a liner and/or insulation disposed along the inner casing  110 , the outer casing  112 , and/or the struts  114 . In such embodiments, the liner may define a portion of the hot gas path  102  extending through the exhaust frame  100 . 
     During operation of the gas turbine engine  10 , the combustion gases  35  flowing along the hot gas path  102  may be contained between the inner casing  110  and the outer casing  112  and may flow over the struts  114 . The inner casing  110  may be formed as a single component or may include a number of segments joined together to form the inner casing  110 . Similarly, the outer casing  112  may be formed as a single component or may include a number of segments joined together to form the outer casing  112 . Although the inner casing  110  and the outer casing  112  are shown as having circular cross-sectional shapes, other shapes may be used in other configurations. 
     The struts  114  may extend radially from the inner casing  110  to the outer casing  112  with respect to the longitudinal axis  116  of the exhaust frame  100 . As shown, the struts  114  may be arranged in a circumferential array about the longitudinal axis  116 . Although eight struts  114  are shown in  FIG. 2 , the exhaust frame  100  may include any number of struts  114  extending between the inner casing  110  and the outer casing  112 . Each strut  114  may be attached at a radially inner end thereof to the inner casing  110  and may be attached at a radially outer end thereof to the outer casing  112 . In particular, each strut  114  may be welded at the radially inner end thereof to the inner casing  110  via a first weld  124  and may be welded at the radially outer end thereof to the outer casing  112  via a second weld  126 . The first weld  124  and the second weld  126  each may be a continuous weld extending around a perimeter of the strut  114  along the respective casing  110 ,  112 , as shown. Alternatively, the first weld  124  and/or the second weld  126  may be an intermittent weld extending around the perimeter of the strut  114  along the respective casing  110 ,  112 . In some embodiments, the first weld  124  and the second weld  126  each may be a fillet weld, although other types of welds may be used in other embodiments. 
     The inner casing  110  and/or the outer casing  112  may include a number of relief grooves defined therein and configured to reduce stress concentrations in the struts  114  during operation of the gas turbine engine  10 . In particular, as shown in  FIG. 3 , the inner casing  110  may include a first relief groove  134  positioned about each of the struts  114 . In a similar manner, the outer casing  112  may include a second relief groove  136  positioned about each of the struts  114 . Although both the first relief grooves  134  and the second relief grooves  136  are shown in  FIG. 3 , it will be appreciated that, in some embodiments, the exhaust frame  100  may include only the first relief grooves  134  (while the second relief grooves  136  are omitted), and in other embodiments, the exhaust frame  100  may include only the second relief grooves  136  (while the first relief grooves  134  are omitted). 
       FIGS. 4A and 5A  are detailed cross-sectional views showing an embodiment of an interface between the inner casing  110  and one of the struts  114 . The radially inner end of the strut  114  may be attached to a radially outer surface  140  of the inner casing  110  via the first weld  124 , which may encircle the perimeter of the strut  114 . As shown, the first relief groove  134  may be defined in the radially outer surface  140  of the inner casing  110 . The first relief groove  134  may have a depth d 1  that is less than a wall thickness wt 1  of the inner casing  110 . In other words, the first relief groove  134  does not extend through the inner casing  110  to the radially inner surface  142  thereof. In some embodiments, the depth d 1  may be constant along the path of the first relief groove  134 . In other embodiments, the depth d 1  may vary along the path of the first relief groove  134 . In such embodiments, a maximum value of the depth d 1  along the path of the first relief groove  134  may be less than the wall thickness wt 1  of the inner casing  110 , such that the first relief groove  134  does not extend through the inner casing  110  to the radially inner surface  142  thereof. 
     As shown, the first relief groove  134  may extend along the entire perimeter of the strut  114 , thereby forming a complete loop around the strut  114 . The path of the first relief groove  134  generally may be contoured to correspond to the shape of the perimeter of the strut  114 . As shown, the first relief groove  134  may be spaced apart from the first weld  124 . In other words, a portion of the radially outer surface  140  of the inner casing  110  may be disposed between the first relief groove  134  and the first weld  124  along the entire path of the first relief groove  134 . In particular, the first relief groove  134  may be spaced apart from the first weld  124  by an offset distance od 1  along the path of the first relief groove  134 . In some embodiments, the offset distance od 1  may be constant along the path of the first relief groove  134 . In other embodiments, the offset distance od 1  may vary along the path of the first relief groove  134 . In such embodiments, a minimum value of the offset distance od 1  along the path of the first relief groove  134  may be greater than zero, such that a portion of the radially outer surface  140  of the inner casing  110  is disposed between the first relief groove  134  and the first weld  124  along the entire path of the first relief groove  134 . 
     In some embodiments, the first relief groove  134  may have a semi-circular cross-sectional shape, as shown. In other embodiments, the first relief groove  134  may have a semi-elliptical, semi-ovular, rectangular, square, or other polygonal or partial polygonal cross-sectional shape. In some embodiments, the cross-sectional shape of the first relief groove  134  may be constant along the path of the first relief groove  134 . In other embodiments, the cross-sectional shape of the first relief groove  134  may vary along the path of the first relief groove  134 . 
       FIGS. 4B and 5B  are detailed cross-sectional views showing another embodiment of an interface between the inner casing  110  and one of the struts  114 . The first relief groove  134  may be defined in the radially outer surface  140  of the inner casing  110  and may extend along the entire perimeter of the strut  114 , thereby forming a complete loop around the strut  114 . The path of the first relief groove  134  generally may be contoured to correspond to the shape of the perimeter of the strut  114 . As shown, at least a portion of the first relief groove  134  may be positioned adjacent (i.e., not spaced apart from) the first weld  124 , thereby forming a smooth transition from the first weld  124  to the first relief groove  134 . For example, lateral portions  146  of the first relief groove  134  may be positioned adjacent lateral portions  148  of the first weld  124 , as shown. In some embodiments, a leading end portion  152  of the first relief groove  134  may be spaced apart from a leading end portion  154  of the first weld  124 , and a trailing end portion  156  of the first relief groove  134  may be spaced apart from a trailing end portion  158  of the first weld  124 , as shown. In other embodiments, the first relief groove  134  may be positioned adjacent the first weld  124  along the entire path of the first relief groove  134 . Additional features of the first relief groove  134  shown may be similar to those described above. 
       FIG. 4C  is a detailed cross-sectional view showing another embodiment of an interface between the inner casing  110  and one of the struts  114 . The first relief groove  134  may be defined in the radially outer surface  140  of the inner casing  110 . As shown, the first relief groove  134  may extend along only a portion of the perimeter of the strut  114 . In particular, the first relief groove  134  may extend along a leading end  162  of the strut  114  and partially along lateral sides  164  of the strut  114 , as shown. The first relief groove  134  may have a generally U-shaped path, although other shapes of the path of the first relief groove  134  may be used. In some embodiments, the first relief groove  134  may be spaced apart from the first weld  124  along the entire path of the first relief groove  134 . In other embodiments, at least a portion of the first relief groove  134  may be positioned adjacent the first weld  124 . Additional features of the first relief groove  134  shown may be similar to those described above. 
       FIG. 4D  is a detailed cross-sectional view showing another embodiment of an interface between the inner casing  110  and one of the struts  114 . The first relief groove  134  may be defined in the radially outer surface  140  of the inner casing  110 . As shown, the first relief groove  134  may extend along only a portion of the perimeter of the strut  114 . In particular, the first relief groove  134  may extend along a trailing end  166  of the strut  114  and partially along the lateral sides  164  of the strut  114 , as shown. The first relief groove  134  may have a generally U-shaped path, although other shapes of the path of the first relief groove  134  may be used. In some embodiments, the first relief groove  134  may be spaced apart from the first weld  124  along the entire path of the first relief groove  134 . In other embodiments, at least a portion of the first relief groove  134  may be positioned adjacent the first weld  124 . Additional features of the first relief groove  134  shown may be similar to those described above. 
       FIGS. 4E and 5C  are detailed cross-sectional views showing another embodiment of an interface between the inner casing  110  and one of the struts  114 . The first relief groove  134  may be defined in the radially inner surface  142  of the inner casing  110 . The depth d 1  of the first relief groove  134  may be less than the wall thickness wt 1  of the inner casing  110 . In other words, the first relief groove  134  does not extend through the inner casing  110  to the radially outer surface  140  thereof. As shown, the first relief groove  134  may extend along a radial projection of the entire perimeter of the strut  114 , thereby forming a complete loop around the projection of the strut  114 . The path of the first relief groove  134  generally may be contoured to correspond to the shape of the perimeter of the strut  114 . Additional features of the first relief groove  134  shown may be similar to those described above. 
       FIGS. 6A and 7A  are detailed cross-sectional views showing an embodiment of an interface between the outer casing  112  and one of the struts  114 . The radially outer end of the strut  114  may be attached to a radially inner surface  170  of the outer casing  112  via the second weld  126 , which may encircle the perimeter of the strut  114 . As shown, the second relief groove  136  may be defined in the radially inner surface  170  of the outer casing  112 . The second relief groove  136  may have a depth d 2  that is less than a wall thickness wt 2  of the outer casing  112 . In other words, the second relief groove  136  does not extend through the outer casing  112  to the radially outer surface  172  thereof. In some embodiments, the depth d 2  may be constant along the path of the second relief groove  136 . In other embodiments, the depth d 2  may vary along the path of the second relief groove  136 . In such embodiments, a maximum value of the depth d 2  along the path of the second relief groove  136  may be less than the wall thickness wt 2  of the outer casing  112 , such that the second relief groove  136  does not extend through the outer casing  112  to the radially outer surface  172  thereof. 
     As shown, the second relief groove  136  may extend along the entire perimeter of the strut  114 , thereby forming a complete loop around the strut  114 . The path of the second relief groove  136  generally may be contoured to correspond to the shape of the perimeter of the strut  114 . As shown, the second relief groove  136  may be spaced apart from the second weld  126 . In other words, a portion of the radially inner surface  170  of the outer casing  112  may be disposed between the second relief groove  136  and the second weld  126  along the entire path of the second relief groove  136 . In particular, the second relief groove  136  may be spaced apart from the second weld  126  by an offset distance od 2  along the path of the second relief groove  136 . In some embodiments, the offset distance od 2  may be constant along the path of the second relief groove  136 . In other embodiments, the offset distance od 2  may vary along the path of the second relief groove  136 . In such embodiments, a minimum value of the offset distance od 2  along the path of the second relief groove  136  may be greater than zero, such that a portion of the radially inner surface  170  of the outer casing  112  is disposed between the second relief groove  136  and the second weld  126  along the entire path of the second relief groove  136 . 
     In some embodiments, the second relief groove  136  may have a semi-circular cross-sectional shape, as shown. In other embodiments, the second relief groove  136  may have a semi-elliptical, semi-ovular, rectangular, square, or other polygonal or partial polygonal cross-sectional shape. In some embodiments, the cross-sectional shape of the second relief groove  136  may be constant along the path of the second relief groove  136 . In other embodiments, the cross-sectional shape of the second relief groove  136  may vary along the path of the second relief groove  136 . 
       FIGS. 6B and 7B  are detailed cross-sectional views showing another embodiment of an interface between the outer casing  112  and one of the struts  114 . The second relief groove  136  may be defined in the radially inner surface  170  of the outer casing  112  and may extend along the entire perimeter of the strut  114 , thereby forming a complete loop around the strut  114 . The path of the second relief groove  136  generally may be contoured to correspond to the shape of the perimeter of the strut  114 . As shown, at least a portion of the second relief groove  136  may be positioned adjacent (i.e., not spaced apart from) the second weld  126 , thereby forming a smooth transition from the second weld  126  to the second relief groove  136 . For example, lateral portions  176  of the second relief groove  136  may be positioned adjacent lateral portions  178  of the second weld  126 , as shown. In some embodiments, a leading end portion  182  of the second relief groove  136  may be spaced apart from a leading end portion  184  of the second weld  126 , and a trailing end portion  186  of the second relief groove  136  may be spaced apart from a trailing end portion  188  of the second weld  126 , as shown. In other embodiments, the second relief groove  136  may be positioned adjacent the second weld  126  along the entire path of the second relief groove  136 . Additional features of the second relief groove  136  shown may be similar to those described above. 
       FIG. 6C  is a detailed cross-sectional view showing another embodiment of an interface between the outer casing  112  and one of the struts  114 . The second relief groove  136  may be defined in the radially inner surface  170  of the outer casing  112 . As shown, the second relief groove  136  may extend along only a portion of the perimeter of the strut  114 . In particular, the second relief groove  136  may extend along the leading end  162  of the strut  114  and partially along the lateral sides  164  of the strut  114 , as shown. The second relief groove  136  may have a generally U-shaped path, although other shapes of the path of the second relief groove  136  may be used. In some embodiments, the second relief groove  136  may be spaced apart from the second weld  126  along the entire path of the second relief groove  136 . In other embodiments, at least a portion of the second relief groove  136  may be positioned adjacent the second weld  126 . Additional features of the second relief groove  136  shown may be similar to those described above. 
       FIG. 6D  is a detailed cross-sectional view showing another embodiment of an interface between the outer casing  112  and one of the struts  114 . The second relief groove  136  may be defined in the radially inner surface  170  of the outer casing  112 . As shown, the second relief groove  136  may extend along only a portion of the perimeter of the strut  114 . In particular, the second relief groove  136  may extend along the trailing end  166  of the strut  114  and partially along the lateral sides  164  of the strut  114 , as shown. The second relief groove  136  may have a generally U-shaped path, although other shapes of the path of the second relief groove  136  may be used. In some embodiments, the second relief groove  136  may be spaced apart from the second weld  126  along the entire path of the second relief groove  136 . In other embodiments, at least a portion of the second relief groove  136  may be positioned adjacent the second weld  126 . Additional features of the second relief groove  136  shown may be similar to those described above. 
       FIGS. 6E and 7C  are detailed cross-sectional views showing another embodiment of an interface between the outer casing  112  and one of the struts  114 . The second relief groove  136  may be defined in the radially outer surface  172  of the outer casing  112 . The depth d 2  of the second relief groove  136  may be less than the wall thickness wt 2  of the outer casing  112 . In other words, the second relief groove  136  does not extend through the outer casing  112  to the radially inner surface  170  thereof. As shown, the second relief groove  136  may extend along a radial projection of the entire perimeter of the strut  114 , thereby forming a complete loop around the projection of the strut  114 . The path of the second relief groove  136  generally may be contoured to correspond to the shape of the perimeter of the strut  114 . Additional features of the second relief groove  136  shown may be similar to those described above. 
     During operation of the gas turbine engine  10 , the first relief grooves  134  of the inner casing  110  and/or the second relief grooves  136  of the outer casing  112  may reduce stress concentrations in the struts  114 . In particular, the first relief grooves  134  and/or the second relief grooves  136  may locally reduce the stiffness of the respective casing  110 ,  112 , such that the highest stresses generated in the exhaust frame  100  are in the first relief grooves  134  and/or the second relief grooves  136  instead of the struts  114  or the welds  124 ,  126 . 
     The embodiments described herein thus provide an improved exhaust frame for containing and directing combustion gases along a hot gas path of a gas turbine engine at high operating temperatures. As described above, the exhaust frame may include relief grooves defined in the inner casing and/or the outer casing and positioned about each of the struts. The relief grooves may reduce stress concentrations in the struts by locally reducing the stiffness of the respective casing, such that the highest stresses generated in the exhaust frame are in the relief grooves instead of the struts or the welds. In this manner, the relief grooves may reduce the risk of failure at the welds, thereby increasing the life of the struts and the overall exhaust frame. The exhaust frame also may include one or more liners that protect the inner casing, the outer casing, and/or the struts from direct exposure to the combustion gases, and one or more layers of insulation that insulate the inner casing, the outer casing, and/or the struts from the high temperatures resulting from the combustion gases. 
     It should be apparent that the foregoing relates only to certain embodiments of the present application and the resultant patent. Numerous changes and modifications may be made herein by one of ordinary skill in the art without departing from the general spirit and scope of the invention as defined by the following claims and the equivalents thereof.