Abstract:
A combustor liner for a gas turbine includes a substantially cylindrical body having a plurality of raised circular ribs arranged in an array on an outside surface of the combustor liner, each rib defining an enclosed are on the outside surface of the liner, forming a dimple or bowl that is sufficient to form vortices for fluid mixing in order to bring about heat transfer enhancement by both turbulated effect and dimpled effect.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to turbine components and more particularly to a combustor liner that surrounds the combustor in land based gas turbines having can annular combustion systems. 
     Traditional gas turbine combustors use diffusion (i.e., non-premixed) combustion in which fuel and air enter the combustion chamber separately. The process of mixing and burning produces flame temperatures exceeding 3900 degrees F. Since conventional combustors and/or transition pieces having liners are generally capable of withstanding for about ten thousand hours (10,000 hrs.), a maximum temperature on the order of only about 1500 degrees F., steps to protect the combustor and/or transition piece must be taken. This has typically been done by film-cooling which involves introducing relatively cool compressor air into a plenum formed by the combustor liner surrounding the outside of the combustor. In this prior arrangement, the air from the plenum passes through louvers in the combustor liner and then passes as a film over the inner surface of the liner, thereby maintaining combustor liner integrity. 
     Because diatomic nitrogen rapidly disassociates at temperatures exceeding about 3000° F. (about 1650° C.), the high temperatures of diffusion combustion result in relatively large NOx emissions. One approach to reducing NOx emissions has been to premix the maximum possible amount of compressor air with fuel. The resulting lean premixed combustion produces cooler flame temperatures and thus lower NOx emissions. Although lean premixed combustion is cooler than diffusion combustion, the flame temperature is still too hot for prior conventional combustor components to withstand. 
     Furthermore, because the advanced combustors premix the maximum possible amount of air with the fuel for NOx reduction, little or no cooling air is available, making film-cooling of the combustor liner and transition piece premature at best. Nevertheless, combustor liners require active cooling to maintain material temperatures below limits. In dry low NOx (DLN) emission systems, this cooling can only be supplied as cold side convection. Such cooling must be performed within the requirements of thermal gradients and pressure loss. Thus, means such as thermal barrier coatings in conjunction with “backside” cooling have been considered to protect the combustor liner and transition piece from destruction by such high heat. Backside cooling involved passing the compressor air over the outer surface of the combustor liner and transition piece prior to premixing the air with the fuel. 
     With respect to the combustor liner, one current practice is to impingement cool the liner, or to provide linear turbulators on the exterior surface of the liner. Another more recent practice is to provide an array of concavities on the exterior or outside surface of the liner (see U.S. Pat. No. 6,098,397). The various known techniques enhance heat transfer but with varying effects on thermal gradients and pressure losses. Turbulation strips work by providing a blunt body in the flow which disrupts the flow creating shear layers and high turbulence to enhance heat transfer on the surface. Dimple concavities function by providing organized vortices that enhance flow mixing and scrub the surface to improve heat transfer. 
     There remains a need for enhanced levels of active cooling with minimal pressure losses and for a capability to arrange enhancements as required locally. 
     SUMMARY OF INVENTION 
     This invention provides convection cooling for a combustor liner by means of cold side (i.e., outside) surface features that result in reduced pressure loss. 
     In the exemplary embodiment of this invention, discrete ring turbulators are provided on the cold side of the combustor liner, each ring defined by a circular raised tubular shaped rib enclosing an interior area or hollow interior region. The ring turbulators are preferably provided as a uniform staggered array over substantially the entire cold side surface of the liner. In one arrangement, the ribs have a square cross-section, but the cross-sectional shape may vary to include, for example, rectangular and tapered inside and/or outside edge surfaces. The edge surfaces may also vary about the periphery of the ring, dependent on the direction of cooling air flow. In addition, the inside and outside corners of the ribs may be sharp or smooth. Ring type turbulators maintain many of the positive effects of known linear turbulators, but the rounded shape and the “concave” areas enclosed by the ring will produce lower pressure loss. The round shape of the turbulators still disrupts the flow, but does so in a manner which is more distributed, especially if the rings are patterned in a staggered fashion. At the same time, the “dimple” or “bowl” shaped interiors form the vortices for fluid mixing. Thus, heat transfer enhancement is by both turbulated effect and dimpled effect. 
     The height and width of the ribs may also vary, and the “floor” of the enclosed area may be raised above the outer non-ring surface area of the liner. 
     Accordingly, in its broader aspects, the invention relates to a combustor liner for a gas turbine comprising a substantially cylindrical body having a plurality of raised ribs arranged on an outside surface of the combustor liner, each rib defining an enclosed area on said outside surface. 
     In another aspect, the invention relates to a combustor liner for a gas turbine comprising a substantially cylindrical body having a plurality of raised circular turbulator rings arranged on an outside surface of the combustor liner; and wherein the rings have height and width dimensions of between about 0.020 and 0.120 inches and inside diameters of between 2 and 5 times the height dimension. 
     In still another aspect, the invention relates to a method of cooling a combustor liner in a gas turbine combustor comprising establishing a flow path for compressor discharge air along an outer surface of combustor liner; and forming a plurality of discrete ring turbulators on the outer surface of the combustor to enhance heat transfer, each ring turbulator comprising a raised peripheral rib of substantially round or oval shape. 
    
    
     The invention will now be described in detail in conjunction with the following drawings. 
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic representation of a known gas turbine combustor; 
     FIG. 2 is a schematic view of a cylindrical combustor liner with linear turbulators; 
     FIG. 3 is a schematic view of a known cylindrical combustor liner with an array of concavities on the exterior surface thereof; 
     FIG. 4 is a schematic side elevation view of a cylindrical combustor liner with discrete ring turbulators in accordance with the invention; 
     FIG. 5 is a cross-section taken along the line  5 — 5  of FIG. 4; 
     FIG. 6 is a cross-section through a ring turbulator in accordance with another embodiment of the invention; and 
     FIG. 7 is a cross-section through a ring turbulator in accordance with still another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 schematically illustrates a typical can annular reverse-flow combustor  10  driven by the combustion gases from a fuel where a flowing medium with a high energy content, i.e., the combustion gases, produces a rotary motion as a result of being deflected by rings of blading mounted on a rotor. In operation, discharge air from the compressor  12  (compressed to a pressure on the order of about 250-400 lb/in 2 ) reverses direction as it passes over the outside of the combustors (one shown at  14 ) and again as it enters the combustor en route to the turbine (first stage indicated at  16 ). Compressed air and fuel are burned in the combustion chamber  18 , producing gases with a temperature of about 1500° C. or about 2730° F. These combustion gases flow at a high velocity into turbine section  16  via transition piece  20 . The transition piece connects to the combustor liner  24  at  22 , but in some applications, a discrete connector segment may be located between the transition piece  20  and the combustor liner. 
     In the construction of combustors and transition pieces, where the temperature of the combustion gases is about or exceeds about 1500° C., there are known materials which can survive such a high intensity heat environment without some form of cooling, but only for limited periods of time. Such materials are also expensive. 
     FIG. 2 shows in schematic form a generally cylindrical combustor liner  24  of conventional construction, forming a combustion chamber  25 . The combustor liner  24  has a combustor head end  26  to which the combustors (not shown) are attached, and an opposite or forward end to which a transition piece assembly  28  is attached. The transition piece assembly includes the transition piece  27  and a surrounding sleeve  29 . The transition piece assembly  28  may be connected to the combustor liner  24  and its respective flow sleeve  32  by connecting double-wall segments (not shown). 
     The combustor liner  24  is provided with a plurality of upstanding, annular (or part-annular) ribs or turbulators  30  in a region adjacent the head end  26 . These ribs are elongated or “linear” in shape, arranged transversely to the direction of cooling air flow. A cylindrical flow sleeve  32  surrounds the combustor liner in radially spaced relationship, forming a plenum  34  between the liner and flow sleeve that communicates with a plenum  36  formed by the transition piece  27  and its own surrounding flow sleeve  29 . Impingement cooling holes or apertures  39  are provided in the flow sleeve  32  in a region axially between the transition piece assembly  28  and the turbulators  30  in the liner  24 . 
     FIG. 3 illustrates in schematic form another known heat enhancement technique. In this instance, the exterior surface  40  of the combustor liner  42  is formed over an extended area thereof with a plurality of circular concavities or dimples  44  (see U.S. Pat. No. 6,098,397). 
     Turning to FIG. 4, a combustor liner  46  in accordance with an exemplary embodiment of this invention is formed with a plurality of circular ring turbulators  48 . Each ring turbulator  48  comprises a discrete or individual circular ring defined by a raised peripheral rib  50  that creates an enclosed area  52  within the ring. The ring turbulators are preferably arranged in an orderly staggered array axially along the length of the liner  46  with the rings located on the cold side surface of the liner, facing radially outwardly toward a surrounding flow sleeve (not shown but similar to flow sleeve  32  in FIG.  2 ). The ring turbulators may also be arranged randomly (or patterned in a non-uniform but geometric manner) but generally uniformly across the surface of the liner. 
     As best seen in FIG. 5, the rib  50  is substantially square in cross-section. FIG. 6 illustrates an alternative cross-section for a ring turbulator  53  where the inside edge surface  56  and outside edge surface  54  are tapered. In FIG. 7, the ring turbulator  58  is formed with a raised rib  60 , the cross-sectional shape of which varies about the circumference thereof. More specifically, those edge portions  62 ,  64  that face the cooling flow (indicated by the flow arrows) are blunt, while on the trailing side, the inside and outside edge surfaces  66 ,  68  are tapered. If desired, this arrangement may be reversed with the blunt edges away from the flow direction, resulting in lower pressure losses. In addition, the edges and the lower corners can each or all be rounded/fillets if desired. 
     In presently preferred configurations, the height and width of the ring turbulators range from about 0.020 to 0.120 inches. The inner diameter of each ring turbulator is related to height, and is no more than 5 times the height and no less than 2 times the height. In addition, the “floor” within the round enclosure formed by the raised rib may be raised relative to the liner surface outside the ring turbulator. While circular ring turbulators are illustrated in FIGS. 4-7, it will be appreciated that the turbulators may be oval or other suitable shapes, recognizing that the dimensions and shape must establish an inner dimple or bowl that is sufficient to form vortices for fluid mixing. The combined enhancement aspects of turbulation and vortex mixing serve to improve heat transfer and thermal uniformity, and result in lower pressure loss than conventional turbulators. 
     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.