Patent Publication Number: US-6334310-B1

Title: Fracture resistant support structure for a hula seal in a turbine combustor and related method

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to gas turbine combustors, and particularly to a fracture resistant support structure for a so-called “hula seal” between a combustion liner and a transition piece. The support structure is placed between the hula seal and combustion liner. 
     Current combustion liner cooling sleeves are attached at their forward ends to the radially inner combustor liner with a circumferential fillet weld (either intermittent or continuous). For purposes of this discussion, the “aft” end is that which is closer to the exit face of the liner, while the “forward” end is that which is closer to the inlet of the liner. Generally, the liner runs hotter than the outer sleeve by 300-500°F., because the liner is exposed directly to the hot combustion gases. More specifically, the liner temperature is typically in the 1200-1400° F. range, whereas the outer sleeve temperature is typically in the range of 700-900° F. If the initial radial gap between the sleeve and liner is set to zero, then the liner will expand more than the outer sleeve, and will therefore create compressive radial stresses at the interface, and tensile hoop stresses in the outer sleeve. The resulting thermally induced deformations cause hoop extension such that the outer sleeve diameter increases to the extent that the sleeve is permanently deformed. During the cooling cycle, however, the liner contracts but the outer sleeve cannot return to its original diameter due to the permanently set deformation. The inability of the outer sleeve to recover its original shape creates a radial gap which acts as a crack opening displacement, impinging on the fillet weld. This crack opening displacement may increase the stress intensity factor to the critical stress intensity factor (KIC) in order to drive the crack into the weld. 
     BRIEF SUMMARY OF THE INVENTION 
     In the present invention, the outer sleeve is made slightly oversized to produce a radial gap between the liner and the outer sleeve at ambient temperature. The gap is calculated by considering the operating temperatures of both components and their respective thermal expansion coefficients. The calculated value is the value that will create no thermal mismatch stresses. Once the gap is determined, the outer sleeve can be formed with the appropriate diameter. The aft end of the outer sleeve is swaged inwards an amount equal to the gap value to insure that the edge of the outer sleeve touches the liner. After welding prep is applied, the outer sleeve is welded over the liner. Because of the swaged end, the crack tip that impinges on the fillet weld is no longer infinitely sharp. Rather, a blunt crack tip is provided that reduces the stress intensity factor in the weld, and thus reduces the propensity for cracking. 
     To further reduce the crack driving energy, the outer sleeve may be separated into multiple segments at the welded end. Each segment is welded with an independent fillet weld so that the fracture energy in each segment is limited, and the segments are flexible during thermal growth. These segments are positioned with respect to axial slots in the liner and the in respective cooling holes in the outer sleeve. 
     In one embodiment, the axial channels in the liner are completely covered by the outer sleeve. The air inlet holes in the outer sleeve are placed over a circumferential channel which acts as a plenum and feeds air into the axial channels. 
     In a second embodiment, the axial channels extend beyond the length of the outer sleeve. The exposed length of the axial channels provides air inlet locations, thus replacing the inlet holes of the previous design. 
     The number or location of the segments can be independent of the number and location of the axial channels and the location of air inlet holes. 
     Accordingly, in its broader aspects, the present invention relates to a combustion liner and outer cooling sleeve assembly for a turbine combustor comprising a substantially cylindrical combustion liner having a forward end and an aft end; and a substantially cylindrical outer cooling sleeve surrounding at least an axial portion of the combustion liner; wherein the outer cooling sleeve is secured to the combustion liner by a weld at an end of the outer cooling sleeve, with a predetermined radial gap between the combustion liner and the outer cooling sleeve extending at least partially about the combustion liner, the radial gap determined by respective operating temperatures and thermal expansion coefficients of the combustion liner and the outer cooling sleeve. 
     In another aspect, the invention relates to a combustion liner and cooling sleeve assembly for a turbine combustor comprising a substantially cylindrical combustion liner; and a substantially cylindrical cooling sleeve surrounding at least an axial portion of the combustion liner; wherein the outer cooling sleeve is secured to the combustion liner by a weld at one end of the outer cooling sleeve, with a predetermined radial gap between the combustion liner and the cooling sleeve; wherein the end is circumferentially divided into segments and wherein the weld is continuous in each segment; and further wherein the end is swaged radially inwardly an amount equal to the radial gap such that the end engages an outer surface of the combustion liner. 
     In still another aspect, the invention provides a method of reducing crack propensity in a substantially cylindrical combustion liner and substantially cylindrical outer cooling sleeve assembly where one end of the outer cooling sleeve is welded to the combustion liner, the method comprising a) determining a radial gap between the combination liner and the outer cooling sleeve as a function of operating temperatures and thermal expansion coefficients of the combustion liner and the cooling sleeve; b) forming the outer cooling sleeve with a diameter sufficient to provide the radial gap; c) swaging the end of the outer cooling sleeve to bring the end into engagement with the combustion liner; and d) welding the outer cooling sleeve to the combustion liner about the end. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cross section illustrating a conventional interface between a combustor outer cooling sleeve and an inner combustor liner; 
     FIG. 2 is a partial cross section illustrating an interconnection between an outer cooling sleeve and an inner combustor liner in accordance with an exemplary embodiment of this invention; 
     FIG. 3 is a perspective view of the interface between the outer cooling sleeve and the inner combustor liner in accordance with an exemplary embodiment of the invention; 
     FIG. 4 is a partial perspective view of the interface between an outer cooling sleeve and an inner combustor liner in accordance with an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates, in partial section, the aft end of a current combustor liner  10  and a surrounding outer cooling sleeve  12 . The radially outer cooling sleeve  12  is provided with a circumferentially arranged row of cooling holes  14  (one shown but two or more rows can be utilized) that permits cooling air to impinge on the liner  10 . The liner  10  is provided with a circumferential groove  16  in axial alignment with the row of cooling holes  14 , and a plurality of axially extending, circumferentially spaced cooling channels  18  communicate at one end with the groove  16 . 
     The outer cooling sleeve  12  is attached to the liner with a circumferential fillet weld  20  which may be an intermittent or “stitch” weld, or a continuous 360° weld. 
     Notice that there is essentially no radial gap between the liner  10  and outer sleeve  12 , and also note the sharp crack tip at  22 . With this design, the first heated liner  10  pushes the outer cooling sleeve  12  radially outwardly, causing plastic deformation in the outer sleeve. When cooled, the liner shrinks inwardly away from the permanently deformed sleeve, pulling away at the weld  20  causing a crack to develop, made worse by the sharp crack tip at  22 . As the liner shrinks away, the entire length of the outer sleeve develops a resisting spring force which creates elastic energy in the body. This elastic “spring” energy is available for crack propagation at the weld. 
     Turning to FIGS. 2 and 3, an exemplary embodiment of this invention is illustrated and, for convenience, certain reference numerals similar to those in FIG. 1, but with the prefix “1” added, are used to identify corresponding components. The combustion liner  110  is surrounded by an outer cooling sleeve  112 . A circumferential row of cooling holes  114  supply cooling air to the liner, the air impinging on a circumferential cooling groove  116  that supplies air to the axially extending cooling channels  118 . In this design, however, the outer sleeve  112  is made slightly oversize, creating a radial air gap  124  between the liner and the sleeve. The aft end of the sleeve  112  must then be swaged inwards an amount equal to the gap to ensure that the edge of the sleeve engages the liner. Welding prep is applied, based on the fillet weld size, and the outer sleeve  112  is welded over the liner, with weld  120  either a continuous 360° weld, or an intermittent stitch weld as best seen in FIG.  3 . 
     Because of the swaged end of the outer sleeve  112 , the crack tip  122  that impinges on the fillet weld is blunt, reducing the stress intensity factor in the weld, and thus reducing the propensity for cracking. 
     The radial gap  124  between the combustion liner  110  and the outer cooling sleeve  112  is calculated by considering the operating temperatures of both components and their respective thermal expansion coefficients (the latter may be the same or different). 
     An example of the thermal gap calculation is provided below: 
     Assumptions 
     Sleeve Material=Nimonic 263 
     Sleeve Temperature=850 deg. F. 
     Thermal Expansion at Temp=7.4e−6 in/in 
     Sleeve Young&#39;s Mod=28 e6 psi 
     Sleeve Thickness=0.040″ for 7FA, 
     Liner Material=Nimonic 263 
     Liner Temperature=1350 deg. F. 
     Thermal Expansion at Temp=8.4e−6 in/in 
     Liner Young&#39;s Mod=24e6 psi 
     Liner Thickness (effective)=0.125″ for 7FA, 
     Liner Outer Diam=14.−010″ for 7FA, 13.895″ for 9H 
     Crack Opening Displacement (COD), Radial Gap=(14/2)*(8.4e−6*(1400-70)−7.4e-6*(850-70))=0.0378 in. 
     As already noted, during operation, the combustion liner  110  expands more than the outer cooling sleeve  112 . This is so even if the thermal expansion coefficients are the same, because the liner  110  is considerably hotter (e.g., 1400° F. vs. 900° F.). In any event, the radial gap  124  provides room for thermal growth. As the combustion liner  110  expands, the gap will close, but not entirely, leaving a residual gap. As a result, the outer cooling sleeve  112  is not deformed and both components regain substantially their original shapes upon cooling. This factor, along with the smooth bend at the weld  120  and the blunt crack tip geometry at  122 , significantly reduces the likelihood of cracking. 
     It will be appreciated that the radial gap  124  need not extend a full 360° between the liner  110  and sleeve  112 . The liner  110  and sleeve could be configured to create for example, a radial gap that extends only 180° (or any other suitable extent). 
     With specific reference to FIG. 3, the stitch weld  120  is interrupted by axial slots  125  originating in certain of the cooling holes  114 , and defining the segments  126 . The weld  120  is continuous within each segment, and the number of segments may vary (preferably four or more). Separating the forward end of the outer cooling sleeve  112  into multiple segments increases the flexibility of the weld connection. Separation also decreases the tendency for weld cracking because less elastic strain energy becomes available to the crack tip. By providing a circumferential groove  116 , it will be appreciated that it is not necessary to align the cooling holes  114  with the axially extending channels  118 . 
     FIG. 4 illustrates a similar arrangement, but where the segments  226  of the outer cooling sleeve  212  are defined by notches or cut-outs  225 . Radially inward of the segment cut-outs  225  are axial cooling channels  218  which extend axially forward and rearward of the stitch weld  220 . These channels may communicate with a circumferential cooling groove  216  in the combustion liner  210 . 
     Returning to FIG. 2, a preferably segmented centering ridge  128  may be machined in the outer surface of the combustion liner  110  or, alternatively, machined on the inner surface of the outer cooling sleeve  112 . While there may be some localized deformation of the outer cooling sleeve  112  as the combustion liner  110  expands, it will not directly affect the remote weld  120 . The ridge can also have an optional stop portion  130  that will prevent excessive axial movement of the outer cooling sleeve in the event of weld failure. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.