Abstract:
The present invention discloses a novel and improved device and method for securing a combustion liner within a gas turbine combustor. The improved configuration comprises a plurality of equally spaced support tab assemblies secured about the combustion liner and positioned radially outward of the combustion zone. The support tab assemblies comprise parallel liner tabs which provide increased flexibility, greater structural support, and lower operating stresses.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/708,323 filed on Oct. 1, 2012. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention generally relates to a system and method for improving the mechanical and thermal stresses in a combustion liner mounting system. More specifically, improvements in a mounting system are provided that compensate for the mounting system being positioned adjacent the combustion zone. 
       BACKGROUND OF THE INVENTION 
       [0003]    In an effort to reduce the amount of pollution emissions from gas-powered turbines, governmental agencies have enacted numerous regulations requiring reductions in the amount of oxides of nitrogen (NOx) and carbon monoxide (CO). Lower combustion emissions can often be attributed to a more efficient combustion process, with specific regard to fuel injector location and mixing effectiveness. 
         [0004]    Early combustion systems utilized diffusion type nozzles, where fuel is mixed with air external to the fuel nozzle by diffusion, proximate the flame zone. Diffusion type nozzles produce high emissions due to the fact that the fuel and air burn stoichiometrically at high temperature to maintain adequate combustor stability and low combustion dynamics. 
         [0005]    An enhancement in combustion technology is the utilization of premixing, such that the fuel and air mix prior to combustion to form a homogeneous mixture that burns at a lower temperature than a diffusion type flame and produces lower NOx emissions. Premixing can occur either internal to the fuel nozzle or external thereto, as long as it is upstream of the combustion zone. An example of a combustion liner used in a premixing combustor of the prior art is shown in  FIG. 1 . The combustion liner  100  is generally cylindrical in shape and includes an inlet end  102  and an opposing outlet end  104 . A plurality of fuel nozzles (not shown) are positioned at the inlet end  102  for injecting fuel into the combustion liner  100  to mix with compressed air and form a fuel-air premixture. This premixture is then ignited and burns to form the hot combustion gases used to drive then engine&#39;s turbine and provide either thrust for propulsive power or mechanical energy for driving an electrical generator. 
         [0006]    Combustion systems for a gas turbine engine can come in a variety of configurations. Generally, for the purpose of discussion, the gas turbine engine may include low emission combustors such as those disclosed herein and may be arranged in a can-annular configuration about the gas turbine engine. One type of gas turbine engine (e.g., heavy duty gas turbine engines) may be typically provided with, but not limited to, six to eighteen individual combustors, each of the combustors having a casing, flow sleeve, combustion liner, fuel nozzles and an end cover. 
         [0007]    In order to regulate the amount of air provided to a low emissions combustor, a flow sleeve  110  is positioned around the combustion liner  100 , as shown in partial cross section in  FIG. 2 . More specifically, the combustion liner  100  is located radially within a flow sleeve  110  while the flow sleeve  110  is located radially within a combustor casing  112 . The flow sleeve  110  is secured in an axial position within the casing  112  by a forward flange  114  which is seated within a casing flange  116 . However, like the flow sleeve  110 , the combustion liner  100  also must be secured in the proper axial position such that the fuel nozzles and other mating hardware (not shown) are properly positioned within the gas turbine combustor. 
         [0008]    Combustion systems of the prior art have attempted to position and secure the combustion liner  100  in place in a variety of manners. For example, with reference to  FIGS. 1-3 , a common technique of securing a combustion liner in a flow sleeve is for the liner to include a plurality of T-shaped tabs  118  which extend radially outward from the combustion liner  100  and are received within slots of corresponding flow sleeve pegs  120 . However, as it can be seen in  FIG. 3 , this technique includes a thick T-shaped liner tab  118  secured to the outer wall of the combustion liner  100  and such a thick tab design results in high thermal and mechanical stresses due its geometry, the amount of load on each tab and welding techniques applied for the joint between the tab and the combustion liner. A similar T-shaped style liner tab  118  is shown on the outer surface of an alternate combustion liner in  FIG. 4 . 
         [0009]    An alternate prior art design for securing a combustion liner in a flow sleeve is depicted in  FIGS. 5 and 6 . In  FIG. 5 , a combustion liner  500  of the prior art is shown in perspective view and includes four equally-spaced liner tabs  502 . In this combustion system, due to the configuration of the fuel-air mixing and combustion, the liner tabs  502  were subjected to high operating temperatures and high thermal stresses since the liner tabs  502  were located radially outward of the combustion zone. The liner tabs  502  are shown in greater detail in  FIG. 6 . The liner tab  502  has a general pitch-fork like shape to it, including an inverted generally U-shaped portion  504  and tab extension  506 . The generally U-shaped portion  504  is welded to the combustion liner  500  along joints  508 . This prior art design attempted to remove the thick liner tab portions from the combustion liner, as discussed above with respect to  FIGS. 1-4 . However, despite these design changes, the joints  508  experienced some of the highest operating stresses in the combustion liner  500 . Such high stresses were due at least in part to the stiffness of the liner tabs  502 . 
         [0010]    Therefore, it is necessary to identify a design alternative that meets the mechanical and thermal loading conditions of the combustion liner and flow sleeve interface such that the combustion liner can be secured in its proper position and not be subjected to failure. 
       SUMMARY 
       [0011]    The present invention discloses a combustion liner having an improved means for securing the combustion liner within the gas turbine combustor. The securing means disclosed herein results in lower operating stresses and increased durability for the combustion liner. 
         [0012]    In an embodiment of the present invention, a generally cylindrical combustion liner is disclosed having an inner wall, an outer wall, an inlet end and an opposing outlet end as well as a plurality of support tab assemblies positioned about the outer wall. Each of the support tab assemblies include two parallel liner tabs extending radially outward from the outer wall of the combustion liner. 
         [0013]    In an alternate embodiment of the present invention, a support system for securing a combustion liner in a gas turbine combustor is disclosed. The support system comprises a plurality of liner tab pairings extending radially outward from the outer wall of the combustion liner. The liner tabs are secured to the liner outer wall and spaced apart by a liner tab as well as position a distance axially from the combustion liner inlet such that the liner tabs are radially outward of the combustion zone in the combustion liner. 
         [0014]    In yet another embodiment of the present invention, a method of reducing the thermal stresses at an interface between the combustion liner and its liner support assembly is disclosed. The method provides a way of securing a plurality of liner support assemblies to an outer wall of the combustion liner, where each liner support assembly comprises two parallel liner tabs extending radially outward from the combustion liner and spaced a distance apart. 
         [0015]    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. The instant invention will now be described with particular reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0016]    The present invention is described in detail below with reference to the attached drawing figures, wherein: 
           [0017]      FIG. 1  is a perspective view of a combustion liner in accordance with the prior art. 
           [0018]      FIG. 2  is a partial cross section view of a gas turbine combustor of the prior art. 
           [0019]      FIG. 3  is an alternate view of the gas turbine combustor of  FIG. 2  of the prior art. 
           [0020]      FIG. 4  a perspective view of a combustion liner in accordance with an alternate embodiment of the prior art. 
           [0021]      FIG. 5  is a perspective view of a combustion liner in accordance with yet another alternate embodiment of the prior art. 
           [0022]      FIG. 6  is a detailed perspective view of a portion of the prior art combustion liner of  FIG. 5 . 
           [0023]      FIG. 7  is a cross section view of a low emissions gas turbine combustor in which an embodiment of the present invention is utilized. 
           [0024]      FIG. 8  is a cross section taken through the gas turbine combustor of  FIG. 7  in accordance with an embodiment of the present invention. 
           [0025]      FIG. 9  is a detailed cross section view of a portion of  FIG. 8  in accordance with an embodiment of the present invention. 
           [0026]      FIG. 10  is a perspective view of a combustion liner in accordance with an embodiment of the present invention. 
           [0027]      FIG. 11  is a detailed perspective view of a portion of the combustion liner of  FIG. 10  in accordance with an embodiment of the present invention. 
           [0028]      FIG. 12  is a detailed top elevation view of a portion of the combustion liner of  FIG. 10  in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    By way of reference, this application incorporates the subject matter of U.S. Pat. Nos. 6,935,116, 6,986,254, 7,137,256, 7,237,384, 7,513,115, 7,677,025, and 7,308,793. 
         [0030]    The preferred embodiment of the present invention will now be described in detail with specific reference to  FIGS. 7-12 . Referring now to  FIG. 7 , a combustion liner  700  for use in a low emissions gas turbine combustor  750  is depicted. Referring to  FIG. 10 , the combustion liner  700  is generally cylindrical in shape and has an inner wall  702 , an outer wall  704  separated from the inner wall  702  by a liner wall thickness. To compensate for elevated operating temperatures of the combustion liner, a portion of the inner wall  702  can be coated with a thermal barrier coating generally comprising a bond coating and a ceramic top coating. 
         [0031]    The combustion liner  700  also has an inlet end  706  and an opposing outlet end  708 . Located along the outer wall  704  of the combustion liner, and extending radially outward, is a plurality of raised surfaces  710 . The raised surfaces  710 , which are more clearly shown in  FIGS. 10-12 , provide an increase in the total surface area of the outer wall  704  so as to enhance the available surface area of the combustion liner available to be cooled by the compressed air passing along the outer wall  704 . An increase in the overall surface area capable of being cooled improves the heat transfer across the thickness of the combustion liner  700 . As one skilled in the art understands, the raised surfaces  710  also serve to “trip” the passing airflow or disturb the airflow pattern in order to improve cooling effectiveness. The size and shape of the raised surfaces  710  can vary depending on the cooling requirements for the combustion liner  700 . For the embodiment of the present invention shown in  FIGS. 10-12 , the raised surfaces  710  are generally triangular in shape such that the radially outermost point of the raised surface  710  has a diameter greater than the outer wall  704 . However, the raised surfaces  710  can be of alternate shapes including circular, square or rectangular. The raised surfaces extend along a length of the combustion liner  700  requiring maximum cooling efficiency. 
         [0032]    The combustion liner  700  also comprises a plurality of support tab assemblies  712 . The support tab assemblies  712 , which are shown in more detail in  FIG. 8 , are positioned about the outer wall  704  of the combustion liner  700 . Each of the support tab assemblies include two parallel liner tabs  714 , spaced a tangential distance apart, secured to the outer wall  704  and extend radially outward from the outer wall  704 . Preferably, three support tab assemblies  712  are spaced at approximately 120 degree intervals around the outer wall  704  of the combustion liner  700 . 
         [0033]    As depicted in  FIG. 10 , the plurality of raised surfaces  710  extends along a majority of the axial length of the combustion liner  700 . However, as shown in  FIGS. 11 and 12 , in the area of the combustion liner  700  of the support tab assemblies  712 , the plurality of raised surfaces is interrupted to create a smooth region  705  of the outer wall  704 , or a region of the combustion liner outer wall that is free from the raised surfaces  710 . 
         [0034]    The combustion liner provides a location for the combustion process to occur. A combustion zone  752  is located within the combustion liner  700 , thereby heating the surrounding portion of the combustion liner  700 . Due to the geometry of the gas turbine combustor  750  it is necessary to locate the support tab assemblies  712  a distance axially downstream from the inlet end  706  such that the support tab assemblies  712  are radially outward of the combustion zone  752 . As such, the support tab assemblies  712  are subjected to higher operating temperatures than liner tabs of the prior art. Therefore, because of their elevated operating temperatures, it is necessary to minimize any heat sink associated with support tab assemblies  712  or other structures capable of absorbing the heat, and resulting thermal stresses, from the combustion zone  752 . 
         [0035]    As discussed above, the support tab assemblies  712  comprise a pair of parallel liner tabs  714 . The liner tabs  714  extend radially outward from the outer wall  704  of the combustion liner  700 , where each of the liner tabs  714  have an axial length L, a radial height H, and a thickness T and are secured to the combustion liner. For the embodiment of the present invention depicted in  FIGS. 10-12 , the axial length L of the tab is greater than the radial height H and the height H is greater than the thickness T. The liner tabs  714  are secured to the outer wall  704  of the combustion liner  700  by a means such as welding and forming a full penetration weld joint  716 . Since the liner tabs  714  are secured to the outer wall  704 , it is preferred that the liner tabs  714  be fabricated from a material comparable to that of the combustion liner  700 . In an alternate embodiment of the present invention, the liner tabs  714  and combustion liner  700  can have an alternate construction in which openings are cut in the combustion liner and the liner tabs extend through the openings and are then secured to the combustion liner. In yet another alternate embodiment, a machined plate with a liner tab can be welded into a pocket in the liner, such that the machined plate forms part of the liner wall, instead of resting on top of the liner wall as in the prior art of  FIGS. 1-4 . Alternate means of securing the liner tabs, such as brazing, can be used in lieu of welding. 
         [0036]    The liner tabs  714  are generally parallel and spaced apart by a liner tab gap G. The specific gap may vary depending on the specific design requirements of the combustion liner and flow sleeve, but for the embodiment depicted in  FIGS. 10-12 , the gap G between liner tabs  714  is approximately 0.5 inches. By placing a gap G between the liner tabs  714 , compressed air is permitted to flow between the liner tabs thereby providing cooling air to all sides of the liner tabs  714 . 
         [0037]    Referring back to  FIGS. 8 and 9 , the engagement of the combustion liner  700  in a flow sleeve  701  is shown. Each of the liner tabs  714  are located within slots formed in pegs  703  of the flow sleeve  701 . The pegs  703  provide a fixed location in which the liner is installed and ensures proper axial positioning of the combustion liner  700  within the gas turbine combustor. Because the material of the flow sleeve is generally harder than the combustion liner tabs  714 , the liner tabs tend to become worn. To prevent wear to the combustion liner tabs  714 , the flow sleeve slots are coated with a wear coating that is softer than the liner tabs  714 , such that any wear that does occur at this interface occurs to the wear coating. 
         [0038]    As discussed above, each of the liner tabs  714  has an axial length, a radial height and a thickness. While the specific size and shape of the liner tab  714  will vary depending on the combustor design, it is important to note that the thickness T of the liner tabs  714  for the embodiment disclosed in  FIGS. 7-12  is thinner than the single T-shaped liner tab of the prior art. Through various testing and analysis, it has been determined that the use of multiple, but thinner, liner tabs, provides an advantage over the prior art in terms of reduced thermal stresses due to disconnecting the adjacent tabs and the thinner liner tabs forming less of a heat sink and smaller thermal gradient. Furthermore, with the liner tabs  714  secured directly to the liner, they generate a minimal “footprint” on the combustion liner, compared to prior art designs, such as the T-shaped lug in  FIGS. 1-4 . More specifically, by only securing the liner tabs to the liner, and not a T-shaped plate, there is less material to absorb heat from the combustion liner. The present invention incorporates other stress-reducing design features such as chamfered edges, fillet radii, and improved flexibility to further reduce stresses. For example, utilizing the liner tab  714  of the present invention over the liner tab 402 has shown a reduction in maximum concentrated stress upwards of approximately 40%. 
         [0039]    In addition to the flexibility and improved heat transfer capability, utilizing multiple liner tabs  714  provides a stronger combustion liner support because of the multiple contact points with the mating flow sleeve. Also, the multiple liner tabs  714  provides for a corresponding increase in wear surfaces when engaged with the flow sleeve peg  703 . As one skilled in the art understands, when the liner tabs  714  are engaged in the flow sleeve and the combustor is operational, the combustion liner is subjected to high levels of vibrations. As such, the combustion liner tabs  714  vibrate within the flow sleeve pegs  703 , typically resulting in wear to the liner tabs  714 . Therefore, providing additional surfaces capable of sustaining the wear reduces any concentrated wear experienced by liner tabs of the prior art. 
         [0040]    While the invention has been described in what is known as presently the 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 within the scope of the following claims. 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 and required operations, such as machining of shroud faces other than the hardface surfaces and operation-induced wear of the hardfaces, will become apparent to those of ordinary skill in the art to which the present invention pertains without departing from its scope. 
         [0041]    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.