Abstract:
A transition duct assembly with a thermally free aft frame and mounting system for use in a gas turbine engine are disclosed. The aft frame is capable of adjusting to thermal gradients while the mounting system provides for at least transverse movement of the transition duct during engine assembly. The mounting system also provides a means for raising the natural frequency of the transition duct outside of the engine&#39;s dynamic excitation ranges.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application Ser. No. 61/012,636, filed on Dec. 10, 2007. 
    
    
     TECHNICAL FIELD 
     The present invention relates to gas turbine engines. More particularly, embodiments of the present invention relate to an apparatus and method for lowering thermal and mechanical stresses in a transition duct assembly while also providing a transition duct assembly with a natural frequencies outside of critical engine frequencies. 
     BACKGROUND OF THE INVENTION 
     Gas turbine engines operate to produce mechanical work or thrust. Specifically, land-based gas turbine engines typically have a generator coupled thereto for the purposes of generating electricity. A gas turbine engine comprises at least a compressor section having a series of rotating compressor blades. The compressor receives air from an engine inlet. The air passes through the compressor, which causes the pressure of the air to increase. 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 at least a radial profile. 
     A change in the radial profile can cause numerous assembly issues between the combustor and the turbine. Also, such a change in geometry for the transition duct assembly, which is operating at extremely high temperatures can create high thermal and mechanical stresses in the transition duct assembly. 
     By nature, the transition duct assembly has a natural operating frequency. Also, the gas turbine engine has a natural frequency, and orders of the natural frequency (i.e. 1E, 2E, 3E, etc). When a component has a natural frequency that coincides with an engine natural frequency or order thereof, the component can become dynamically excited and if care is not taken to avoid the crossings of these frequencies, or minimizing the time for the crossing, the component may experience excessive wear or failure due to the excessive vibrations that occur when operating at the natural frequency or order thereof. 
     SUMMARY 
     Embodiments of the present invention are directed towards a system and method for, among other things, improving movement at the aft frame of a transition duct assembly due to thermal gradients. A mounting system is disclosed that provides for at least lateral movement of the aft frame to adjust due to thermal growth while securing the transition duct assembly at both the inlet and outlet in order to raise the natural frequency of the transition duct assembly outside of the gas turbine engine natural frequency or order thereof. 
     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 assembly in accordance with an embodiment of the present invention; 
         FIG. 2  depicts an alternate perspective view of a transition duct assembly installed in a gas turbine engine in accordance with an embodiment of the present invention; 
         FIG. 3  depicts a cross section view of a transition duct assembly in accordance with an embodiment of the present invention; 
         FIG. 4  depicts an elevation view of the transition duct assembly of  FIG. 3  looking aft from an inlet of the transition duct assembly in accordance with an embodiment of the present invention; 
         FIG. 5  depicts an elevation view of the transition duct assembly of  FIG. 3  looking forward from an outlet of the transition duct assembly in accordance with an embodiment of the present invention; 
         FIG. 6  depicts a top elevation view of the transition duct assembly of  FIG. 3  in accordance with an embodiment of the present invention; 
         FIG. 7  depicts a cross section view of a mounting system of the transition duct assembly of  FIG. 3  in accordance with an embodiment of the present invention; 
         FIG. 8  depicts an exploded assembly view of the transition duct assembly in accordance with an embodiment of the present invention; and, 
         FIG. 9  depicts a perspective view of a portion of the mounting system of a transition duct assembly 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. 
     Referring initially to  FIG. 1 , a transition duct assembly  100  in accordance with an embodiment of the present invention is shown. The transition duct assembly  100  includes a generally cylindrical inlet sleeve  102  and a panel assembly  104 . The inlet sleeve  102  has an inner diameter and an outer diameter, while the panel assembly  104  extends from the inlet sleeve  102  at the inner and outer diameter thereof via a first panel  106  and a second panel  108 , as can be seen with additional reference to  FIG. 3 . Each of the first panel  106  and second panel  108  is typically formed from a single sheet of metal. The panel assembly  104  is formed by the first panel  106  being fixedly joined to the second panel  108  along a plurality of axial seams  110  by means such as welding (as seen in  FIG. 2 ). Once assembled, the panel assembly  104  forms a duct having an inner wall  114   a , an outer wall  114   b , and a first thickness there between as shown in  FIG. 3 . The panel assembly  104  further contains a generally cylindrical inlet (forward) end  116  (adjoining the inlet sleeve  102 ) and a generally rectangular exit (aft) end  118 , with the exit or outlet end being generally defined by a pair of arcs of different diameters concentric about a center and connected by a pair of radial lines extending from the center. For instance, the arcs of the exit end  118  may be concentric about a center defined by a gas turbine engine  1000  coupled to the exit end  118  of the duct assembly  100 . In the construction of the duct assembly  100  described, the inlet sleeve  102  is coupled with an outlet of a combustor (e.g., a can-annular combustor), with the exit end  118  directing the combustion gases to the turbine  1000 . The duct also has a generally rectangular aft frame  120  having opposing sidewalls  122  and being fixed to the exit end  118  of the panel assembly  104 . The opposing sidewalls  122  are, in one configuration, generally perpendicular to the arcs of the panel assembly exit end  118 . Additionally, in one configuration, the aft frame  120  includes a plurality of retention lugs  124  formed on or proximate the arcs of the panel assembly exit end  118 . The retention lugs  124  each have a particular thickness and have formed therein a slot  126 . Further, the laterally outermost retention lugs  124   a  are generally located proximate ends of the arcs which define the panel assembly exit end  118 , and each possess a slot  126   a  having a first circumferential length and a first radial width with the first circumferential length greater than the corresponding first radial width. 
     It should be understood that the terms “axial”, “radial”, and “circumferential”, as used herein, generally are provided with reference to the turbine  1000  (e.g., a theoretical turbine) connected with the transition duct assembly  100 . Accordingly, “axial” generally means with reference to an axis identical to (or parallel with) an axis of the turbine  1000 , “radial” generally means along a radius extending from a center rotational axis of the turbine  1000 , and “circumferential” generally means along a circumference of a circular frame of the turbine  1000  with which a plurality of transition duct assemblies  100  with exit ends  118  are mounted. Further, the terms “fastener”, “bolt”, and “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). 
     With continued reference to  FIGS. 1-3 , and additional reference to  FIGS. 4 and 5 , embodiments of the present invention provide a mounting system for securing the transition duct assembly  100  to the gas turbine engine  1000  in order to provide an improved range of allowable movements at the transition duct exit end  118  due to thermal gradients as well as a stable mounting of the transition duct assembly  100  overall in order to raise the natural frequency of the duct assembly  100  outside of the turbine  1000  operational natural frequency and engine operational frequencies or order thereof. Accordingly, a forward mounting bracket  200  and an aft frame assembly  300  are provided. The forward mounting bracket  200  includes a central portion  202 , or base frame, with a pair of arms  204  extending generally radially from opposing sides of the central portion  202 . The central portion  202  and arms  204  have a depth in the axial direction and a thickness in the radial direction sufficient to rigidly and securely mount the forward end (e.g., at the inlet sleeve  102  or otherwise proximal to the inlet end  116 ) of the duct with a mounting block  1002  or other support structure of the turbine  1000  (e.g., via bolts extending through radial mounting holes  206  in the bracket central portion  202 ). Depending on manufacturing and assembly tolerances, it may be necessary to adjust the vertical location of the forward mounting bracket  200 . If the vertical location needs to be raised up, or outward radially, then one or more shim plates can be placed between the central portion  202  and mounting block  1002 . 
     In the embodiment shown in  FIGS. 1 ,  2  and  4 , the inlet sleeve  102  is formed with a circumferentially mounted, radially flanged collar  128  interconnected with a forward side of the mounting bracket arms  204  via threaded mounting bolts or pins  130  received through axial holes in the collar  128  and into threaded axial bores of the mounting bracket arms  204 . Note that in order to alter the natural frequency for a swaying mode (60 hz), a plurality of pin/hole couplings (two in this case) are required. In one preferred, though not limiting, arrangement, the forward mounting bracket  200  is sized such that the height H 1  of the arms  204  is generally about 2 to 4 times the thickness T 1  of the arms  204 , and the lateral distance L 1  between the axis of the axial bores of the arms  204  is generally about 2 to 5 times the height H 1  of the arms  204 . The fit between the pins  130  and the axial holes in the collar  128  is designed so as to remain tight during operation and provide torsional rigidity at the forward mounting bracket  200 . This relatively tight fit occurs during operation due to changes in operating temperature of the transition duct assembly and helps to increase the natural frequency of the transition duct assembly  100 . In one embodiment of the present invention, the pins  130  are fabricated from a cobalt-based alloy such as L-605 and is coated with a Tungsten-Cobalt coating whereas the collar  128  is fabricated with an L-605 sleeve through which the pins  130  pass therethrough. 
     Continuing in reference to  FIGS. 1-5 , and with additional reference to  FIG. 8 , the aft frame assembly  300  is generally secured with the aft frame  120  at the panel assembly exit end  118  and preferably with a turbine inlet frame section  1004  (see  FIG. 2 ) by a single pair of bolts  132  or other mounting means. The frame assembly  300  includes, in broad terms, a mounting plate  302 , an inner bulkhead assembly  304 , an outer bulkhead assembly  306 , and an aft mounting brackets  308 , as well as various bushings  310  and mounting means (e.g., threaded nut and bolt combinations), as explained in detail below. 
     The mounting plate  302  preferably has a pair of axial holes  312  there through matching a pair of axial holes  314  formed in the aft mounting bracket  308 . Accordingly, in assembly, the pair of bolts  132  are inserted through the mounting plate axial holes  312  and the aft mounting bracket axial holes  314  to secure the mounting plate  302  and the aft mounting bracket  308  together in abutting relation and mount the aft frame assembly  300  to the turbine  1000  (e.g., via the frame section  1004 ). As explained in detail below, the remaining portions of the aft frame assembly  300  mount the transition duct exit end  118  with the turbine  1000  through a coupling with the aft mounting bracket  308 . 
     The inner bulkhead assembly  304  and the outer bulkhead assembly  306  are fixed to the aft frame  120  through the retention lugs  124  and  124   a . The inner bulkhead assembly  304  includes a first inner bulkhead  316  and a second inner bulkhead  318  positioned on opposite sides of the aft frame retention lugs  124  and  124   a . Each of the bulkheads  316  and  318  has a plurality of axial holes  320  there through positioned for alignment with the slots  126  of the aft frame retention lugs  124  and  124   a . In assembly, a fastener  322 , such as a bolt, is inserted through each axial hole  320  of the bulkheads  316 ,  318  and through the corresponding slots  126  of the aft frame retention lugs  124  from the exit side of the aft frame assembly  300 . A washer  324  and a threaded nut  326  capture each fastener  322  on the forward side of the assembly  300 . Additionally, bushings  310  are located on the particular fasteners  322  that extend through the slots  126   a  in the laterally outermost retention lugs  124   a . Each bushing  310  has a second axial length, a second circumferential length, a second radial width, and a through hole for receiving there through the fastener  322 . In this configuration, the bushings  310  reside within each slot  126   a  of the outermost retention lugs  124   a  and are preferably pressfit into the slots  126   a . The bushings  310  are sized such that the first circumferential length of the slots  126   a  is greater than the second circumferential length of each bushing  310 , thereby allowing for relative circumferential movement of each of the outermost retention lugs  124   a , and hence aft frame  120 , relative to the bushings received therein. This is due to thermal expansion between the retention lugs  124   a  and respective bulkhead assemblies. 
     The outer bulkhead assembly  306  has a similar configuration to the inner bulkhead assembly  304 , and includes a first outer bulkhead  328  and a second outer bulkhead  330  positioned on opposite sides of the aft frame retention lugs  124  and  124   a . Each of the bulkheads  328 ,  330  likewise has a plurality of axial holes  332  there through positioned for alignment with the slots  126 ,  126   a  of the aft frame retention lugs  124 ,  124   a . As with the inner bulkhead assembly  304 , assembly is accomplished via placement of fastener  322  through each bulkhead axial hole  332  and through the corresponding slots  126  of the aft frame retention lugs  124  from the exit side of the aft frame assembly  300 . A washer  334  and a threaded nut  336  capture each fastener  322  on the forward side of the assembly  300 . Additionally, the bushings  310  are used in the same manner in the outer bulkhead assembly  306  as in the inner bulkhead assembly  304 . 
     The interconnection between the outer bulkhead assembly  306  and the aft mounting bracket  308  serves as the coupling point between the aft frame  320  (and thus the transition duct assembly  100 ) and the turbine frame section  1004 . Specifically, the second outer bulkhead  330  is formed with a main body section  338  where the axial holes  332  are disposed, and two or more towers  340  extending radially outward from the main body section  338  generally proximate the circumferential ends of the bulkhead  330 . Each tower  340  has a through hole  342  oriented generally perpendicularly to the axial holes  332 . The aft mounting bracket  308  is formed with a set of receiving channels  344  sized to receive therein the towers  340  of the bulkhead  330 . The channels  344  are each formed between an end flange  346  and a block member  348  of the bracket  308 , with both the end flange  346  and block member  348  extending generally in the axial direction. For the embodiment depicted in  FIGS. 6-9 , the tower  340  has a thickness that is approximately equal to the thickness of the adjacent block member  348 . Furthermore, the towers  340  have a radial height that is up to twice its thickness. The size aspects are necessary to raise the transition piece natural frequency to an acceptable level. Each end flange  346  is formed with a through hole  350  and each block member  348  is formed with a threaded counterbore  352  aligned with the through hole  350 . The through holes  350  and counterbores  352  are oriented generally perpendicular to the mounting axial holes  314  of the bracket  308 , thus being configured for alignment with the through holes  342  of the corresponding towers  340  of the second outer bulkhead  330 . In assembly, a fastener  354  is inserted through each end flange through hole  350  and tower through hole  342  to be preferably threadingly received within one of the threaded counterbores  352  of the respective block  348 , thereby securing the second outer bulkhead  330  and thus the transition duct aft frame  120  with the aft mounting bracket  308 . 
     With further reference to  FIGS. 6 ,  7  and  9 , in one embodiment of the frame assembly  300 , the receiving channels  344  of the aft mounting bracket  308  are formed with curved radii  345 , whereby the radius thereof originates about a center aligned with a radial axis of the turbine  1000  itself. This configuration provides a small amount of yaw adjustment, or movement in a transverse direction, for the transition duct aft frame  120  in mounting with the turbine  1000 . This can be advantageous if parts are not fabricated to exact tolerances, during assembly of the duct assembly to the turbine, or when thermal growth occurs during turbine operation. In particular, because each fastener  354  is merely slid through the end flange through hole  350  of the aft mounting bracket  308  and tower through hole  342  of the second outer bulkhead  330  (being threadingly received by the counterbore  352  of the aft mounting bracket  308 ), there is a small amount of “free play” between the interconnection between the aft mounting bracket  308  and the second outer bulkhead  330  (regulated by the diameter of the bolt  354 ). Due to the pivot location of the transition duct assembly  100  being located proximate the aft frame  120 , a small amount of movement (0.060″-0.080″) in the transverse direction can result in +/−0.200″ of movement near the transition duct assembly inlet end. 
     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.