Abstract:
A combustor includes a central fuel nozzle assembly and a plurality of outer fuel nozzle assemblies, each of the plurality of outer fuel nozzle assemblies having a center body and an outer shroud, the plurality of outer fuel nozzle assemblies being configured to abut one another in a surrounding relationship to the central cylinder such that no gaps are present between any two abutting ones of the plurality of outer fuel nozzle assemblies. One or more of the plurality of fuel nozzle assemblies may traverse axially back and forth according to embodiments of the invention.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/352,674 filed on Jan. 13, 2009. 
    
    
     BACKGROUND OF THE INVENTION 
     Premixed Dry Low NOx (DLN) combustion systems for heavy-duty gas turbines for both annular and can-annular designs are based on fuel staging, air staging, or a combination of the two. This enables operation across a relatively wide range of conditions. The window for premixed combustion is relatively narrow when compared to the duty cycle of a modern gas turbine. Thus, conditions within the combustion system are typically “staged” to create local zones of stable combustion despite the fact that bulk conditions may place the design outside its operational limits (i.e., emissions, flammability, etc.). 
     Additionally, staging affords an opportunity to “tune” the combustion system away from potentially damaging acoustic instabilities. Premixed systems may experience combustion “dynamics”. The ability to change the flame shape, provide damping, or stagger the convective time of the fuel to the flame front have all been employed as a means to attempt to control the onset of these events. However, these features tend to be either non-adjustable or can only be exercised at the expense of another fundamental boundary such as emissions. 
     Dynamics mitigation is a source of continuous investigation. Most combustor designs have a means of staging the fuel flow (commonly referred to as a “fuel split”) but this creates an emissions penalty. Other designs have multiple fuel injection planes to create a mixture of convective times. Again, here numerous approaches are possible, such as fuel forcing, resonators, quarter wave tubes, etc. 
     Acoustic instabilities are an indication of a coincidence of heat release fluctuations with one or more of the inherent acoustic modes of the combustion chamber. The manner in which these heat release fluctuations interact with the chamber is dictated to a large extent by the shape of the flame and the transport time of the fuel/air mixture to the flame front. Both parameters are commonly manipulated by changing the distribution of the fuel to the various nozzles within the combustor. If the nozzles are in a common axial plane, then the main effect is to change the flame shape. If instead the nozzles are in distinct axial locations, then the main effect is to change the convective times. Additionally, nozzles in a common plane may result in detrimental nozzle-to-nozzle flame front interactions unless one nozzle is “biased” to prevail from a stability standpoint over the adjacent nozzles. However, either adjustment leads to a reduction in operability. That is, non-uniform fuel distribution in a common plane leads to relatively higher NOx emissions through the well-established exponential dependency of NOx formation on local flame temperature. Also, non-uniform fuel distribution in distinct axial locations can create a potential flame holding location if one nozzle group is upstream of the other (e.g., the “quat” system). 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one aspect of the invention, a combustor includes a fuel nozzle assembly that has a center body, an inner shroud that surrounds at least a portion of the center body, an outer shroud that surrounds at least a portion of the inner shroud, and a plurality of cooling holes formed in a portion of the outer shroud, cooling air being introduced in a space between the inner and outer shrouds and exiting from the plurality of cooling holes. The combustor also includes an actuator that moves at least the center body in an axial direction. 
     According to another aspect of the invention, a combustor includes at least one fuel nozzle assembly having a center body, a shroud that surrounds at least a portion of the center body, and a vane disposed between the center body and the shroud. The combustor also includes an actuator that moves at least the center body in an axial direction. 
     According to yet another aspect of the invention, a combustor includes a central fuel nozzle assembly and a plurality of outer fuel nozzle assemblies, each of the plurality of outer fuel nozzle assemblies having a center body and an outer shroud, the plurality of outer fuel nozzle assemblies being configured to abut one another in a surrounding relationship to the central cylinder such that no gaps are present between any two abutting ones of the plurality of outer fuel nozzle assemblies. 
     These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a cross section view of a combustor having a traversing fuel nozzle assembly according to an embodiment of the invention; 
         FIG. 2  is a more detailed cross section view of the combustor with the traversing fuel nozzle assembly of  FIG. 1 ; 
         FIG. 3  is a perspective view of a combustor having a plurality of traversing fuel nozzles according to another embodiment of the invention; and 
         FIG. 4  is a cross section view of a combustor having a traversing fuel nozzle assembly according to yet another embodiment of the invention. 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 and 2 , a combustor  100  for a gas turbine includes a plurality of fuel nozzle assemblies  104 , one of which is shown in the embodiment of  FIGS. 1 and 2 . One or more of the plurality of fuel nozzle assemblies  104  may traverse axially back and forth according to embodiments of the invention. As shown in  FIG. 1 , the combustor  100  also includes a combustor case  108  and an end cover  112 . Each of the fuel nozzle assemblies  104  may include a vane  116 , an inner shroud  120 , a center body  124 , a liner  128 , a seal assembly  132 , a bulkhead/cap assembly  136 , a seal  140 , an outer shroud  144 , and an actuator mechanism  148 . 
     In accordance with one embodiment of the invention, the entire fuel nozzle assembly  104  may be moved or traversed axially. In accordance with another embodiment, only the center body  124  of the fuel nozzle assembly  104  may be moved axially. In addition, only one of the fuel nozzle assemblies  104  may be moved axially at any one time, or some combination of two or more of the fuel nozzle assemblies  104  may be moved axially at any one time. Movement of a portion or all of one or more of the fuel nozzle assemblies  104  is typically carried out to tune the performance of the combustor  100  as desired. Regardless of the type of movement of the fuel nozzle assemblies  104 , such movement is achieved by one or more of the actuator mechanisms  148 . The actuator mechanism  148  may comprise any type of suitable actuator, such as electric, hydraulic, pneumatic, etc., that is controlled by a controller (not shown). The output of the actuator mechanism  148  connects by suitable mechanical linkages to the center body  124  of the corresponding fuel nozzle assembly  104 . The actuator mechanism  148  is operable to move only the center body  124  or, where desired, may move the fuel nozzle assembly  104  that includes not only the center body  124  but also the vane  116  and the inner and outer shrouds  120 ,  144 . Such movement is in an axial direction (i.e., back and forth in  FIGS. 1 and 2 ). Each fuel nozzle assembly  104  may have a dedicated actuator mechanism  148 , or one or more fuel nozzle assemblies may be “ganged” or connected together and moved in unison by a single actuator mechanism  148 . 
     This type of movement sets the depth of emersion of the center body  124  into a combustion “hot zone”, which is that portion of the combustor  100  to the right of the bulkhead/cap assembly  136  as viewed in  FIGS. 1 and 2 . The “emersion zone” is indicated in  FIG. 2  by the reference number  152 . As can be seen from  FIGS. 1 and 2 , the center body  124  of the fuel nozzle assembly shown there protrudes somewhat past (i.e., to the right of) the bulkhead/cap assembly  136  and into the combustion “hot zone”. Typical temperatures in this “hot zone” may be approximately 3000 degrees Fahrenheit. As a result, it is necessary to cool the inner shroud  120 , which also protrudes past the bulkhead/cap assembly  136  and into the combustion “hot zone”. In the embodiment of  FIGS. 1 and 2 , the inner and outer shrouds  120 ,  144  are configured to go beyond the right end of the center body  124  as viewed in these figures. However, an alternative embodiment may have the right end of the center body  124  be even with the ends of the inner and outer shrouds  120 ,  144 . 
     This type of cooling of the inner shroud  120  may be achieved by forming a number of cooling holes  156  in the outer shroud  144  and forcing relatively cooler air in the space between the inner and outer shrouds  120 ,  144  from the left side in  FIGS. 1 and 2 . The cooling air then exits through the cooling holes  156  in the outer shroud  144 . This type of film cooling is suitable to cool the inner shroud  120  and prevent its destruction by melting in the combustion “hot zone”. 
     In the fuel nozzle assemblies  104  illustrated in  FIGS. 1 and 2 , the shrouds  120 ,  144  may have a round or circular cross section when viewed at their exit (i.e., as viewed from right to left in  FIGS. 1 and 2 ). As such, this necessitates the use of a cap as part of the bulkhead/cap assembly  136 . The cap is typically a relatively thin cooled plate that fills in the spaces between the circular cross section fuel nozzle assemblies  104 , thus isolating the zone of heat release from the upstream components. Referring to  FIG. 3 , there illustrated is an embodiment of a combustor  300  of the invention in which the nozzles  304 ,  308  are shaped to completely fill in any inter-nozzle gaps (i.e., “closely packed nozzles”). As such, this embodiment eliminates the need for the combustion cap as part of the bulkhead/cap assembly  136  of  FIGS. 1 and 2  (i.e., a “cap-less combustor assembly”), which removes a recurring reliability issue for the thin cooled plate. In  FIG. 3 , a center fuel nozzle assembly  304  may be of circular or cylindrical shape and may contain a centrally located fuel nozzle  306 . 
     The center fuel nozzle assembly  304  may be completely surrounded by a plurality (e.g., six) of the outer fuel nozzle assemblies  308 . Each outer fuel nozzle assembly  308  may have a center body  310  and a trapezoidal shaped double walled cooled shroud  312 . However, a trapezoidal shape for the shrouds  312  is purely exemplary; other shapes may be used so long as when the outer fuel nozzle assemblies  308  are placed near or adjacent one another there are no gaps between such assemblies  308  and no cap is needed to cover any gaps between such assemblies  308 . The back end  314  of each outer fuel nozzle assembly  308  may have a circular shaped vane or swirler. Also, a compliant seal  316  may be provided at each junction between adjacent outer fuel nozzle assemblies  308 , or between the center fuel nozzle assembly  304  and any one or more of the outer fuel nozzle assemblies  308 , to eliminate any gaps therebetween. In this embodiment, the center body  310  and the vane  314  of the outer fuel nozzle assemblies  308 , along with the center body  306  and vane  314  of the center fuel nozzle assembly, are moved in an axial back and forth direction. The plurality of fuel nozzle assemblies  304 ,  308  may be moved in an axial direction by the actuator mechanism  148  of  FIG. 1 . That is, the configuration of fuel nozzle assemblies  304 ,  308  illustrated in  FIG. 3  may replace the circular or cylindrical fuel nozzle assemblies  104  in the embodiments of  FIGS. 1 and 2  or the embodiment of  FIG. 4  described hereinafter. As in the embodiments of  FIGS. 1 and 2 , a certain one or more of the fuel nozzle assemblies  304 ,  308  may be moved as desired to tune the combustor performance. 
     Referring to  FIG. 4 , a combustor  400  according to another embodiment of the invention is somewhat similar to the combustor  100  of the embodiment of  FIGS. 1 and 2 . Like reference numerals in  FIG. 4  are used to denote like components in  FIGS. 1 and 2 . In the embodiment of  FIG. 4 , only the center body  124  and the vane  116  are moved or traversed axially in a back and forth direction by the actuator mechanism  148 . A pair of fuel feed holes  160  is shown in the vane  116 . The inner shroud  120  is fixed or attached to the bulkhead  136 , which prevents any movement of the inner shroud  120 . As such, there is no need for the outer shroud  144  of  FIGS. 1  and  2  along with the cooling holes  156 . This is due to the fact that the inner shroud  120  does not enter the “hot zone”, thereby eliminating the need for any cooling of the inner shroud  120 , in contrast to the embodiment of  FIGS. 1 and 2 . 
     Embodiments of the invention provide for an adjustable feature to target flame shape and convective times by allowing for the axial displacement of certain one or more of the fuel nozzle assemblies within the combustion chamber. By allowing for one or more fuel nozzle assemblies to traverse axially within the combustion chamber, both flame shape and convective time are affected without impacting NOx emissions or operability. More specifically, axial displacement of the nozzles changes the flame shape and the convective times to the flame front, thus affecting two of the most fundamental dynamics drivers in the combustor of a gas turbine. Also, the axial displacement of the nozzles can be leveraged to achieve improved (greater) turndown by delaying the quenching effect that under-fueled neighboring nozzles have on the “anchor” nozzles (i.e., preventing premature quenching of the anchor nozzles). 
     In addition, embodiments of the invention eliminate the need for a combustion “cap”, which is a relatively thin cooled plate that fills in the space between the nozzles  104 , thus isolating the zone of heat release from the upstream components. Instead, embodiments of the invention shape the nozzles to completely fill in the inter-nozzle gaps, resulting in “closely packed nozzles”. The elimination of the combustion cap (i.e., a “cap-less combustor assembly”) removes a recurring reliability issue for the thin cooled plate. 
     Further, each fuel nozzle assembly  104  has a burner tube or shroud that is cooled to allow the nozzle to protrude into the combustion “hot zone” of the combustion chamber. Cooling the nozzle burner tubes to allow the tubes to protrude into the “hot zone” is synergistic with the flame holding tolerant concepts (i.e. nozzles that can withstand flame holding long enough to detect and correct the event). Thus, cooling of nozzle burner tubes fits into the growing demand for fuel flexible designs. 
     Therefore, embodiments of the invention provide for a dynamics “knob” that does not impact emissions or flame holding and is synergistic with fuel flexibility improvements as well as increased turndown effects. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.