Abstract:
A combustor comprises an outer combustor casing defining a plurality of circumferentially adjoining combustion chambers. Each combustion chamber comprises a dome at an upstream end and an outlet at a downstream end. A plurality of pre-mixers are joined to the combustor dome of each respective combustion chamber. The pre-mixers comprise a duct having an inlet at one end for receiving compressed air, an outlet at an opposite end disposed in flow communication with the combustion chamber and a swirler disposed in the duct adjacent the duct inlet for swirling air channeled therethrough. A fuel injector is provided for injecting fuel into the pre-mixer ducts and for mixing with the air in the ducts for flow into the combustion chamber to generate a combustion flame at the duct outlets. A portion of the pre-mixers comprise an altered flameholding capability so as to distribute the resulting combustion flames from the respective portion of the pre-mixers axially downstream with respect to the non-altered pre-mixers so as to reduce the dynamic pressure amplitude of the combustion flames.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to industrial turbine engines, and more specifically, to combustors therein. 
     Industrial power generation gas turbine engines include a compressor for compressing air that is mixed with fuel and ignited in a combustor for generating combustion gases. The combustion gases flow to a turbine that extracts energy for driving a shaft to power the compressor and produces output power for powering an electrical generator, for example. The turbine is typically operated for extended periods of time at a relatively high base load for powering the generator to produce electrical power to a utility grid, for example. Exhaust emissions from the combustion gases are therefore a concern and are subjected to mandated limits. 
     More specifically, industrial gas turbine engines typically include a combustor design for low exhaust emissions operation, and in particular for low NOx operation. Low NOx combustors are typically in the form of a plurality of burner cans circumferentially adjoining each other around the circumference of the engine, each burner can having a plurality of premixers joined to the upstream end. 
     Lean-premixed low NOx combustors are more susceptible to combustion instability in the combustion chamber as represented by dynamic pressure oscillations in the combustion chamber. The pressure oscillations, if excited, can cause undesirably large acoustic noise and accelerated high cycle fatigue damage to the combustor. The pressure oscillations can occur at various fundamental or predominant resonant frequencies and other higher order harmonics. 
     Such combustion instabilities may be reduced by introducing asymmetry in the heat release or for example by axially distributing or spreading out the heat release. One current method commonly used to introduce asymmetry for reducing combustion oscillations is to bias fuel to one or more burners generating more local heat release. Although this fuel-biasing method has been shown to reduce combustion instabilities, NOx emissions are substantially increased by the higher temperatures generated. Distributing the flame axially has been accomplished by physically offsetting one or more fuel injectors within the combustion chamber. A drawback to this offset approach, however, is that the extended surface associated with the downstream injectors must be actively cooled to be protected from the upstream flame. This additional cooling air has a corresponding NOx emissions penalty for the system. 
     Therefore, it is apparent from the above that there is a need in the art for improvements in combustor dynamics. 
     SUMMARY OF THE INVENTION 
     A combustor comprises an outer combustor casing defining a plurality of circumferentially adjoining combustion chambers. Each combustion chamber comprises a dome at an upstream end and an outlet at a downstream end. A plurality of pre-mixers are joined to the combustor dome of each respective combustion chamber. The pre-mixers comprise a duct having an inlet at one end for receiving compressed air, an outlet at an opposite end disposed in flow communication with the combustion chamber and a swirler disposed in the duct adjacent the duct inlet for swirling air channeled therethrough. A fuel injector is provided for injecting fuel into the pre-mixer ducts for mixing with the air in the ducts for flow into the combustion chamber to generate a combustion flame at the duct outlets. A portion of the pre-mixers comprise an altered flameholding capability so as to distribute the resulting combustion flames from the respective portion of the pre-mixers axially downstream with respect to the non-altered pre-mixers so as to reduce the dynamic pressure amplitude of the combustion flames. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a representative industrial gas turbine engine having a low NOx combustor joined in flow communication with a compressor and turbine; 
     FIG. 2 is a schematic representation of a portion of an industrial gas turbine engine having a low NOx combustor in accordance with one embodiment of the present invention; 
     FIG. 3 is a schematic representation of a portion of an industrial gas turbine engine having a low NOx combustor in accordance with another embodiment of the present invention; 
     FIG. 4 is a schematic representation of a portion of an industrial gas turbine engine having a low NOx combustor in accordance with one embodiment of the present invention; and 
     FIG. 5 is a schematic representation of a portion of an industrial gas turbine engine having a low Nox combustor in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An industrial turbine engine  10  having a compressor  12  disposed in serial flow communication with a low NOx combustor  14  and a single or multistage turbine  16  is shown in FIG.  1 . Turbine  16  is coupled to compressor  12  by a drive shaft  18 , a portion of which drive shaft  18  extends therefrom for powering an electrical generator (not shown) for generating electrical power, for example. Compressor  12  charges compressed air  20  into combustor  14  wherein compressed air  20  is mixed with fuel  22  and ignited for generating combustion gases or flame  24  from which energy is extracted by turbine  16  for rotating shaft  18  to power compressor  12 , as well as producing output power for driving the generator or other external load. 
     In this exemplary embodiment combustor  14  includes a plurality of circumferentially adjoining combustion chambers  26  each defined by a tubular combustion casing  28 . Each combustion chamber  26  further includes a generally flat dome  30  at an upstream end thereof and an outlet  32  at a downstream end thereof. A conventional transition piece (not shown) joins the several outlets  32  to effect a common discharge to turbine  16 . 
     Coupled to each combustion dome  30  are a plurality of premixers  34 . Each premixer  34  includes a tubular duct  36  having an inlet  38  at an upstream end for receiving compressed air  20  from compressor  12  and an outlet  40  at an opposite, downstream end disposed in flow communication with combustion chamber  26  through a corresponding hole in dome  30 . Dome  30  is typically larger in radial extent than the collective radial extent of the several premixers which allows premixer  34  to discharge into the larger volume defined by combustion chamber  26 . Further, dome  30  provides a bluff body which acts as a flameholder from which combustion flame  24  typically extends downstream from during operation. 
     Each of premixers  34  preferably includes a swirler  42 , which swirler  42  includes a plurality of circumferentially spaced apart vanes exposed in duct  36  adjacent to duct inlet  38  for swirling compressed air  20 . A fuel injector  44  is provided for injecting fuel  22  such as a natural gas, into the several ducts  36  for mixing with swirled air  20  in ducts  36  for flow into combustion chamber  26  to generate combustion flame  24  at duct outlets  40 . 
     In the exemplary embodiment illustrated in FIG. 1, each of premixers  34  further includes an elongate center body  46  disposed coaxially in duct  36 , and having an upstream end  48  at duct inlet  38  joined to and extending through the center of swirler  42 , and a bluff or flat downstream end  50  disposed at duct outlet  40 . The center body  46  is spaced radially inwardly from duct  36  to define a cylindrical load channel  52  therebetween. 
     Fuel injector  44  may include conventional components such as a fuel reservoir, conduits, valves and any required pumps for channeling fuel  22  into the several center bodies  46 . 
     In order to maintain suitable dynamic stability of combustor  14  during operation, the various frequencies of pressure oscillation should remain at relatively low pressure amplitudes to avoid resonance at unsuitably large pressure amplitudes leading to combustor instability expressed in a high level of acoustic noise or high cycle fatigue damage, or both. Combustor stability is conventionally effected by adding damping using a perforated combustion liner for absorbing the acoustic energy. This method, however, is undesirable in a low emissions combustor since the perforations channel film cooling air which locally quench the combustion gases thereby increasing the CO levels. Moreover, it is preferable to maximize the amount of air reaching the premixer for reduced NOx emissions. 
     Dynamic uncoupling may be better understood by understanding the apparent theory of operation of combustor dynamics as discussed in co-pending, commonly assigned, application Ser. No. 08/812,894 U.S. Pat. No. 5,943,866, entitled “Dynamically Uncoupled Low NOx Combustor,” filed on Mar. 10, 1997, which application is herein incorporated by reference. 
     It has been shown that Rayleigh&#39;s criteria must be met for strong oscillations to grow in a pre-mixed combustion system. This criteria suggests that instabilities grow if fluctuations in heat release are in phase with the fluctuating acoustic pressure. Accordingly, combustion instabilities can be reduced if the heat release is controlled with respect to the acoustic pressures. 
     As shown in FIG. 1, premixer  34  includes a relatively narrow passage at duct outlet  40  to accelerate the flow of fuel  22  and air  20  into combustion chamber  26  so as to prevent flame propagation back into pre-mixer  34  (i.e., flashback). This relatively narrow duct outlet  40  of premixer  34  in combination with the choked turbine nozzle (not shown) at the exit of combustor  14  approximates an acoustic chamber having both ends nearly closed. For an acoustic chamber having both ends very nearly closed the fundamental longitudinal acoustic standing wave mode is a half wavelength. Accordingly, applying this approximation to combustion chamber  26 , the half wavelength acoustic standing wave  58 , as depicted in graph  60  has maximum fluctuations in pressure at dome end  30  of combustion chamber  26  and at outlet  32 . Additionally, standing wave  58  further comprises a pressure node  62  having about zero fluctuating pressure at about the center of combustion chamber  26  as identified by reference line  64 . 
     As shown in FIG. 1, flame structure  24  is typically stabilized and anchored at dome end  30  of combustion chamber  26 . In this conventional configuration, flame structures  24  are all essentially concentrated in one axial position at dome end  30  of combustion chamber  26  in a region of maximum fluctuations in pressure (see graph  60 ). Accordingly, both the heat release (flame  24 ) and the maximum pressure fluctuation exist in dome end  30  of combustion chamber  26  maximizing Rayleigh&#39;s criteria and consequently maximizing the opportunity for coupling between the heat release and the pressure oscillation. 
     In accordance with the instant invention, combustor  14  is configured such that at least a portion of flame structures  24  are axially positioned at or near pressure node  62  where pressure fluctuations are significantly reduced. Because the pressure fluctuations are reduced with respect to at least a portion of the flame structures  24 , Rayleigh&#39;s criteria is minimized and coupling between the pressure wave and the combustion wave is lessened. 
     In accordance with one embodiment of the instant invention, combustor  110  is shown in FIG.  2 . As shown in FIG. 2, flame structure asymmetry is introduced within combustor  110  by axial distribution of at least a portion of flame structures  124 . Through this asymmetric distribution of flame structures  124 , at least a portion of the combustion taking place within combustion chamber  26  will be axially positioned closer to pressure node  62  so as to decouple the heat release from flame structures  124  from the maximum pressure located at dome end  30 . 
     In one embodiment of the instant invention, center body  46  further comprises at least one and typically a plurality of orifices  112  disposed within the downstream end  50  of a portion of pre-mixers  136  having axially distributed flame structures  124 . High velocity air  130  is directed through orifices  112  so as to impinge upon a root portion  116  of the axially distributed flame structures  124  so as to lift flame structures  124  from the conventional anchoring location at downstream end  50  of center body  46  and at dome end  30  of combustion chamber  26  to an axial location downstream towards pressure node  62 . The velocity of high velocity air  130  should be great enough to overcome the flame propagation speed. In one embodiment of the instant invention, high velocity air  130  is supplied directly to orifices  112  from a high pressure air source  120 . In another embodiment of the instant invention, high velocity air  130  is supplied passively to orifices  112  by providing fluid communication between at least one orifice  112  and a high pressure region of turbine engine  10 . 
     The velocity of high velocity air  130  supplied from high pressure air source  120  can be manipulating so as to “tune” combustion chamber  26  for minimum combustion instabilities. As the velocity of high velocity air  130  is manipulated, the corresponding flame structures  124  will be axially manipulated such that flame structures  124  are positioned closer to outlet  32  or alternatively closer to dome end  30  depending on which direction will stabilize combustor  110 . 
     In accordance with another embodiment of the instant invention, a combustor  210  is shown in FIG.  3 . As shown in FIG. 3, flame structure asymmetry is introduced within combustor  210  by axial distribution of at least a portion of flame structures  224 . Through this asymmetric distribution of flame structures  224 , at least a portion of the combustion taking place within combustion chamber  26  will be axially positioned closer to pressure node  62  so as to decouple the heat release from flame structures  224  from the maximum pressures located at dome end  30 . 
     Asymmetry introduced within the flame structures  224  is created by manipulating the angle and profile of the swirl blades to have a smaller swirl angle within swirler  42 . The result of manipulating the angle profile of swirler  42  is that flame structures  224  will be exposed to a significantly different aerodynamic flow pattern of the entering combustion air  20  then the premixers supporting non-manipulated flame structures  24  are exposed to. The smaller swirl angles of manipulated swirlers  242  support longer narrower flame structures  224  when compared with non-manipulated flame structures. In one embodiment of the instant invention swirlers  242  comprise a swirl angle that is in the range between about 15% to 50% smaller than the swirl angle of non-manipulated swirlers  42 . 
     In accordance with another embodiment of the instant invention, combustor  310  is shown in FIG.  4 . As shown in FIG. 4, flame structure  324  of each premixer  334  is anchored downstream of dome end  30 . Through this axial distribution of flame structures  324  the combustion taking place within combustor  310  will be axially positioned proximate pressure node  62  so as to minimize Rayleigh&#39;s criteria so as to decouple the heat release from flame structures  324  with the maximum pressure fluctuations located with dome end  30 . 
     In one embodiment of the instant invention, combustor  310  further comprises a plurality of flameholders  312  positioned axially downstream from dome end  30  proximate pressure node  62 . Flameholders  312  may comprise any type of suitable flameholders including but not limited to gutters, v-gutters, rounded-nose gutters or jet curtain flameholders. Flame structures  324  anchor at flameholders  312  and accordingly flame structures  324  are axially positioned at or near pressure node  62  where pressure fluctuations are significantly reduced. Because the pressure fluctuations are reduced with respect to flame structures  324 , Rayleigh&#39;s criteria is minimized and coupling between the pressure wave and the combustion wave is reduced. 
     In one embodiment of the instant invention, combustor  310  may further comprise at least one, and typically a plurality, of orifices  314  disposed within the downstream end  50  of each premixer  334 . High velocity air  316  is directed through orifices  314  so as to quench the conventional anchoring location at downstream end  50  of center body  46  and at domd end  30  to ensure anchoring of flame structures  324  on flameholders  312  and not at dome end  30 . 
     Another acoustic mode which has been observed in pre-mixed combustors is the fundamental transverse radial standing wave resonance, as shown in FIG.  5 . Radial wave structures produce maximum pressure fluctuations at the center and outside diameter of combustion chamber  26 , with a pressure node  462  of zero fluctuation at an intermediate radius. In one embodiment of the instant invention combustor  410  is configured such that the reaction zone  424  is concentrated at a toroidal shape centered about nodal circle  462 . Because the pressure fluctuations are reduced with respect to flame structures  424 , Rayleigh&#39;s criteria is minimized and coupling between the pressure wave and the combustion wave is reduced. If toroidal reaction zones  424  are also positioned to correspond to longitudinal pressure node  62 , then each acoustic mode can be suppressed. Accordingly, flame  424  is both radially and longitudinally distributed for the suppression of these two nodes. 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.