Abstract:
A transition duct having a thermally free aft frame and being capable of adjusting the natural frequency is disclosed. The aft frame is capable of permitting movement due to thermal gradients with the transition duct. The transition duct utilizes a spring plate located adjacent to an aft mounting bracket, where the spring plate, based on its thickness can either increase or decrease a frequency of the transition duct. Such an arrangement ensures that the transition duct natural frequency does not coincide with or cross other critical engine and/or combustor frequencies.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority to U.S. Provisional Patent Application Ser. No. 61/024,315 filed on Jan. 29, 2008. 
    
    
     TECHNICAL FIELD 
     The present invention relates to gas turbine engines. More particularly, embodiments of the present invention relate to an apparatus and method for altering the natural frequencies of a transition duct. 
     BACKGROUND OF THE INVENTION 
     Gas turbine engines operate to produce mechanical work or thrust. One type of gas turbine engine is a land-based engine that has a generator coupled thereto which harnesses the mechanical work for the purposes of generating electricity. A gas turbine engine comprises at least a compressor section having a series of rotating compressor blades. Air enters the engine through an inlet and then passes through the compressor, where the rotating blades compress the air and raise its pressure. The compressed air is then directed into one or more combustors where fuel is injected into the compressed air and the mixture is ignited. The hot combustion gases are then directed from the combustion section to a turbine section by a transition duct. Depending on the geometry of the gas turbine engine, often times the combustion section is located radially outward of the inlet to the turbine section, and therefore the transition duct must change in radial profile. However, a change in geometry for the transition duct, which is operating at extremely high temperatures, can create high thermal and mechanical stresses in the transition duct. 
     By nature, the transition duct has a series of natural operating frequencies and bending modes. The gas turbine engine and combustion system also have a natural frequency, and orders of the natural frequency (i.e. 1E, 2E, 3E, etc). When a component, such as a transition duct, has a natural frequency or mode that coincides with or approaches an engine natural frequency or order thereof, the component can become dynamically excited. If care is not taken to avoid the crossings of these frequencies, operating at these frequencies, or minimizing the time for the crossing, the component may experience excessive wear or failure due to the vibratory stress that occurs when operating at or near the natural frequency of the gas turbine engine or combustion system. 
     SUMMARY 
     Embodiments of the present invention are directed towards a system and method for, among other things, providing a way of altering a natural frequency of a transition duct such that the natural frequency is outside of other frequencies of at least the combustion system or order thereof. The natural frequency can be altered by incorporating a spring plate of various thicknesses into the transition duct. 
     The present invention also provides an embodiment directed towards a system and method for compensating for thermal and mechanical stresses that are imparted into the transition duct while also providing structural support against pressure loads applied to the transition duct. 
     Additional advantages and features of the present invention will be set forth in part in a description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned from practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The present invention is described in detail below with reference to the attached drawing figures, wherein: 
         FIG. 1  depicts a perspective view of a transition duct in accordance with an embodiment of the present invention; 
         FIG. 2  depicts an alternate perspective view of the transition duct of  FIG. 1  in accordance with an embodiment of the present invention; 
         FIG. 3  depicts an elevation view of the transition duct of  FIGS. 1 and 2  looking forward from an outlet of the transition duct in accordance with an embodiment of the present invention; 
         FIG. 4  depicts a top view of the transition duct of  FIGS. 1 and 2  in accordance with an embodiment of the present invention 
         FIG. 5  depicts an elevation view of the transition duct of  FIGS. 1 and 2  looking aft from an inlet of the transition duct in accordance with an embodiment of the present invention; 
         FIG. 6  depicts a cross section view of a transition duct of  FIGS. 1 and 2  in accordance with an embodiment of the present invention; 
         FIG. 7  depicts a cross section view of a portion of a gas turbine engine in which a transition duct in accordance with an embodiment of the present invention is installed; 
         FIG. 8  depicts a perspective view of a portion of a gas turbine engine in which a transition duct in accordance with an embodiment of the present invention is installed; 
         FIG. 9  depicts an exploded view of a transition duct in accordance with an embodiment of the present invention; 
         FIG. 10  depicts a detail exploded view of a spring plate, mounting system, and portion of the aft frame assembly of a transition duct in accordance with an embodiment of the present invention; 
         FIG. 11  depicts an exploded view of the mounting system and spring plate of a transition duct in accordance with an embodiment of the present invention; 
         FIG. 12  depicts a perspective view of the spring plate and portion of the bulkhead assembly of a transition duct in accordance with an embodiment of the present invention; and, 
         FIGS. 13A and 13B  depict top and front elevation of views of the spring plate and portion of the bulkhead assembly of a transition duct in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The subject matter of the present invention is described with specificity herein to meet statutory requirements. However, the description itself is not intended to limit the scope of this patent. Rather, the inventors have contemplated that the claimed subject matter might also be embodied in other ways, to include different components, combinations of components, steps, or combinations of steps similar to the ones described in this document, in conjunction with other present or future technologies. 
     The present invention will now be described with reference to the accompanying  FIGS. 1-13B . Referring initially to  FIGS. 1 and 2 , a transition duct  100  in accordance with an embodiment of the present invention is shown. The transition duct  100  includes a generally cylindrical inlet region  102 , a panel assembly region  104 , and an aft frame region  106 . Elevation views of an embodiment of the present invention are shown in  FIGS. 3-5 . Specifically,  FIG. 3  shows a view from an outlet end of the transition duct  100  looking forward towards an inlet end,  FIG. 4  shows a top view of the transition duct  100 , and  FIG. 5  shows a view from the inlet end of the transition duct  100  looking aft towards the outlet. As it can be seen from  FIG. 7 , a combustion liner  300  inserts into the transition duct  100  at the inlet end, while the aft end of the transition duct  100  mates to a turbine vane ring  200 . 
     Referring now to  FIG. 6 , which is a cross section of the transition duct  100 , further details of the present invention can be seen. The transition duct  100  comprises a first panel assembly  110  having a first inner panel  112  fixed to a second inner panel  114 , such that the transition duct  100  has a first inner surface  116 , a first outer surface  118 , and a first thickness  120  therebetween. The transition duct  100  also comprises a first generally cylindrical inlet end  122  and a first generally rectangular exit end  124 , proximate the outlet of the transition duct  100 . The exit end  124 , as is better depicted in  FIG. 3 , is defined by a pair of arcs  126  of different diameters that are concentric about a center and are connected by a pair of radial lines  128  that extend from a center. 
     For the embodiment of the present invention depicted in the FIGS., the first panel assembly  110  may be surrounded by a second panel assembly  130 . Features of the second panel assembly  130  will be discussed in more detail below. 
     Referring now to  FIGS. 3 ,  6 , and  9 , a generally rectangular aft frame  132  is fixed to the exit end  124  and has a plurality of retention lugs  134  located along the aft frame  132 , proximate the arcs  126 . The retention lugs  134 , each have a second thickness and contain a slot  135  having a first circumferential length and a first radial width. For the embodiment shown in  FIG. 9 , there are four outermost retention lugs  134  each having a slot  135 , which are located proximate ends of the arcs  126 . 
     The present invention also comprises inner and outer bulkhead assemblies, which are shown in an exploded view state in  FIG. 9 . A first inner and generally arc-shaped bulkhead  136  has a plurality of first through holes  138  and a first outer and generally arc-shaped bulkhead  140  also has a plurality of first through holes  138 . The inner and outer bulkhead assemblies also comprise a second inner and generally arc-shaped bulkhead  142  having a plurality of second through holes  144  and a second outer and generally arc-shaped bulkhead  146  that also has a plurality of second through holes  144 . The second outer bulkhead  146  further comprises, in the embodiment shown in  FIG. 9 , two attachment portions  148  that extend radially outward and have a portion that is generally perpendicular to the second outer bulkhead  146 . The attachment portions  148  also have a through hole  150  that, due to the orientation of the attachment portions  148 , is oriented generally perpendicular to the plurality of second holes  144 . 
     A plurality of bushings  152  are sized so as to fit generally within the slots  135  of the retention lugs  134 . Each of the bushings  152  has a second axial length, a second circumferential length, a second radial length, and a third through hole. The inner bulkheads  136  and  142  are fastened to the retention lugs  134  and bushings  152  by a plurality of fasteners  154 . Specifically, a fastener  154  passes through the first and second holes,  138  and  144 , of the inner bulkheads  136  and  142 . Also, the fasteners  154  pass through the first and second holes,  138  and  144 , of the outer bulkheads  140  and  146  and through the bushings  152  in the retention lugs  134 . The fasteners  154  can be a variety of locking means. For the embodiment of the present invention, one form of fasteners  154  used is a threaded bolt and nut arrangement. 
     The transition duct  100  also comprises a leaf spring or spring plate  156  that is coupled to the second outer bulkhead  146 . The spring plate  156  has a flat portion  158  and one or more curved portions  160  that extend a distance so as to be adjacent to the attachment portions  148  of the second outer bulkhead  146 . The one or more curved portions  160  of the spring plate  156  also include holes  162 . The spring plate  156  is fixed to the attachment portions  148  of the second outer bulkhead  146  by a plurality of fasteners  154 . 
     An aft mounting bracket  164  is used to mount the transition duct  100  to a turbine vane ring  200  at the inlet of a turbine  202 , as shown in  FIGS. 7 and 8 . The aft mounting bracket  164  has a pin that passes through an opening in the spring plate  156  and is placed into the turbine vane ring. 
     The spring plate  156  is incorporated into the transition duct  100  so as to be able to alter its natural frequency. A prior art embodiment of a transition duct without a spring plate  156  had a natural frequency of approximately 140 Hz for the inlet and aft frame region. The combustion acoustic tones generated by the combustor that is coupled to the transition duct  100 , as shown in  FIG. 7 , operates in a range of approximately 120 Hz-145 Hz. As such, a natural frequency mode associated with the generally rectangular aft end  132 , as known to those skilled in the art of vibratory analysis, couples with an inlet ovalization mode, producing a transition duct natural frequency of approximately 140 Hz, which is within the range of combustor acoustic tones. Excessive wear and fatigue of has been known to occur in this embodiment of the transition duct that operates at or near the combustor frequency range due to resonance. When a spring plate, an end frame, and the mounting system, are incorporated into the transition duct  100 , as discussed in the present invention, the natural frequency for the mode described above is lowered to under approximately 100 Hz for the aft end modes, well outside of the natural frequency of the combustor. By using the spring plate  156 , the modes present in the aft frame and inlet (inlet ovalization) can be decoupled. Where the spring plate causes the frequency at the aft end to decrease, it raises the frequency at the inlet end from approximately 140 Hz to approximately 160 Hz. In this embodiment, by incorporating a spring plate  156  the natural frequency of the aft frame was lowered, while the natural frequency of the inlet was raised. The spring plate  156  used in this embodiment of the present invention is but one example of a style and size of a leaf spring. The thickness and mounting arrangement of the leaf spring can vary depending upon the transition duct geometry and desired shift in frequency level for the transition duct. 
     Due to the configuration of the retention lugs  134  of the aft frame  132 , the inner and outer bulkheads  136 ,  140 ,  142 , and  146  are secured to the aft frame  132  of the transition duct  100  in such a way that the aft frame  132  can expand thermally so as to minimize any thermal and/or mechanical stresses in the frame. That is, by the retention lugs  134  having elongated slots  135 , the transition duct  100  can expand in a generally circumferential direction, i.e. along the arcs  126  so as to dissipate any stress that accumulates in the aft frame region during operation. 
     In operation, the transition duct  100  is surrounded by a cooling fluid, such as air, that is supplied by the compressor. As previously discussed, the transition duct  100  contains hot combustion gases that are directed from the combustor to the turbine. However, these hot combustion gases are at a lower pressure than the surrounding air. As such, the aft frame  132  and transition duct  100  are exposed to a compressive pressure load by the surrounding air. In order to ensure that the aft frame  132  does not buckle or collapse under such applied pressure loads, sidewalls of the aft frame  132  that run along the radial lines  128  as well as the inner and outer bulkheads  136 ,  140 ,  142 , and  146  have a sufficient thickness to counteract this applied load and provide the necessary structural stiffness to prevent the aft frame  132  from collapsing under the applied pressure. 
     As previously discussed, an embodiment of the present invention incorporates a second panel assembly  130  that surrounds the first panel assembly  110 . The second panel assembly  130  comprises a first outer panel  170  and a second outer panel  172  that are fixed together along a plurality of generally axial seams. The second panel assembly  130  also includes a plurality of cooling holes  174  and plurality of cooling tubes  176 . The second panel assembly  130  is positioned so as to provide dedicated cooling to the first panel assembly  110  of the transition duct  100 . A cooling fluid, such as air, is passed through the cooling holes  174  and/or the cooling tubes  176  and impinges on the first outer surface  118  of the first panel assembly  110 . 
     The process by which the natural frequency of the transition duct  100  is determined and the size of the spring plate  156  is identified depends on a number of factors. Once the transition duct is assembled, except for the aft mounting bracket  164 , the transition duct  100  is ping-tested to determine the natural frequencies of the transition duct. This test data is compared to other test data and analytical models for at least the combustion system of the particular engine in which the transition duct will be installed to determine where potential overlaps in frequencies will occur. Based on these comparisons, a thickness for the spring plate  156  can be determined. The spring plate, having the desired thickness, is then installed on the transition duct, and the transition duct can be installed in the engine. 
     It should be understood that the terms “axial”, “radial”, and “circumferential”, as used herein, generally are provided with reference to the turbine  200  (e.g., a theoretical turbine) connected with the transition duct  100 . Accordingly, “axial” generally means with reference to an axis identical to (or parallel with) an axis of the turbine  200 , “radial” generally means along a radius extending from a center rotational axis of the turbine  200 , and “circumferential” generally means along a circumference of a circular frame of the turbine  200  with which a plurality of ducts  100  are mounted. Further, the terms “fastener”, “bolt”, “pin” are used interchangeably herein to denote a component for mechanically coupling adjacent structures together (e.g., through a threaded interconnection, an interference fit, etc). 
     The present invention has been described in relation to particular embodiments, which are intended in all respects to be illustrative rather than restrictive. Alternative embodiments will become apparent to those of ordinary skill in the art to which the present invention pertains without departing from its scope. 
     From the foregoing, it will be seen that this invention is one well adapted to attain all the ends and objects set forth above, together with other advantages which are obvious and inherent to the system and method. It will be understood that certain features and sub-combinations are of utility and may be employed without reference to other features and sub-combinations. This is contemplated by and within the scope of the claims.