Patent Publication Number: US-2009229270-A1

Title: Apparatus for controlling combustion device dynamics

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to combustion devices and, more particularly, to a method and apparatus for controlling combustion dynamics developed within combustion devices. 
     Gas turbine engines typically include a compressor section, a combustor section, and at least one turbine section. The compressor compresses air, which is mixed with fuel and channeled to the combustor. The mixture is then ignited to generate hot combustion gases. The combustion gases are channeled to the turbine which extracts energy from the combustion gases for powering the compressor, as well as producing useful work to power a load, such as to propel an aircraft in flight. 
     Gas turbine engines operate in many different operating conditions, and combustor performance facilitates engine operation over a wide range of engine operating conditions. Controlling combustor performance improves overall gas turbine engine operations. For example, at least some gas turbine low NO X  emissions combustion systems employ a process known as lean premixed combustion wherein fuel and combustion air are mixed upstream of the combustion zone to facilitate controlling NO X  production. Such systems often function well over a relatively narrow operating range. Outside of the range, combustion dynamics levels (noise due to oscillatory combustion process) may approach an amplitude that can shorten the maintenance intervals and/or ultimately cause component damage and failure. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, a method for controlling a combustion dynamics level within a combustion device is provided. A high dynamics operating state is defined at a first fuel split ratio. The first fuel split ratio is a ratio of an amount of fuel supplied to the combustion device through a first fuel line to a total amount of fuel supplied to the combustion device. A low dynamics operating state is defined at a second fuel split ratio different from the first fuel split ratio. The second fuel split ratio is a second ratio of an amount of fuel supplied to the combustion device through the first fuel line to a total amount of fuel supplied to the combustion device. Periodic switching between the first fuel split ratio and the second fuel split ratio controls the combustion dynamics level within the combustion device. 
     In another aspect, a method to facilitate controlling a combustion dynamics level within a combustion device is provided. The method includes defining a combustion dynamics level and a NO X  emissions level for a plurality of fuel split ratios for an operating range of the combustion device. The combustion dynamics level is a measurement of pressure within the combustion device during a combustion process. The NO X  emissions level is a measurement of NO X  emitted from the combustion device during the combustion process. The fuel split ratio is a ratio of an amount of fuel supplied to the combustion device through a first fuel line to a total amount of fuel supplied to the combustion device. A first operating state defined by a first fuel split ratio and a second operating state defined by a second fuel split ratio different from the first operating state is determined. The combustion dynamics level is controlled within the combustion device based on the first operating state and the second operating state. 
     In another aspect, a system for controlling combustion dynamics levels within a combustion device is provided. The system includes a first fuel line in flow communication with a first premix chamber formed within a combustion casing of the combustion device and a second fuel line in flow communication with a second chamber formed at least partially within the combustion casing. A fuel transfer circuit is in independent operational control communication with each of the first fuel line and the second fuel line. The fuel transfer circuit controls an amount of fuel flowing through the first fuel line into the first premix chamber and an amount of fuel flowing through the second fuel line into the second chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a fuel transfer circuit coupled to a combustion device, according to one embodiment of this invention; 
         FIG. 2  is a graphical representation of a combustion dynamics level and a NO X  emission level verses fuel split ratios within an operating range for the combustion device; 
         FIG. 3  is a graphical representation of a pressure wave developed within a combustion device operating at a high dynamic state verses time; 
         FIG. 4  is a graphical representation of a pressure wave developed within a combustion device operating at a low dynamic state versus time; and 
         FIG. 5  is a graphical representation of a pressure wave verses time for the combustion device switching between a high dynamic state and a low dynamic state to control dynamics within the combustion device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a method and apparatus for controlling combustion dynamics within a combustion device, such as a gas turbine engine, wherein a fuel transfer circuit controls a path of at least a portion of the fuel supplied to the gas turbine engine through a first fuel source and/or a second fuel source. By modulating or switching fuel split ratios between a high dynamics operating state and a relatively low dynamics operating state, undesirable combustion dynamics, including pressure oscillations and/or acoustical vibrations, developed within the gas turbine engine during the combustion process are prevented or minimized. In one embodiment, a firing temperature at which a first fuel split ratio is selected to define a first operating state is the same as a firing temperature at which a second fuel split ratio is selected to define a second operating state. 
     The present invention is described below in reference to its application in connection with and operation of a stationary gas turbine engine. However, it will be obvious to those skilled in the art and guided by the teachings herein provided that the invention is likewise applicable to any combustion device including, without limitation, boilers, heaters and other gas turbine engines, and may be applied to systems consuming natural gas, fuel, coal, oil or any solid, liquid or gaseous fuel. 
     As used herein, references to “combustion” are to be understood to refer to a chemical process wherein oxygen, e.g., air, combines with the combustible elements of fuel, namely carbon, hydrogen and sulfur, at an elevated temperature sufficient to ignite the constituents. 
     As shown in  FIG. 1 , a combustion device, such as a gas turbine engine  10 , includes a first or primary fuel line  12  and a second or secondary fuel line  14  independently operatively connected to engine  10 . As shown in  FIG. 1 , first fuel line  12  is in flow communication at least one entry orifice  20  with a first region, such as a first premix chamber  16  formed within a combustion casing  18 . In one embodiment, first fuel line  12  extends through a first fuel nozzle cover  22  and into first premix chamber  16  at a plurality of entry orifices  20  positioned about first fuel nozzle cover  22 . Second fuel line  14  is in flow communication with a second region, such as a second chamber  26  at least partially formed within combustion casing  18 . As shown in  FIG. 1 , second fuel line  14  extends through a second nozzle assembly  28  at an entry orifice  30  and into a venturi assembly  32  positioned at a downstream portion of second chamber  26 . In one embodiment, first premix chamber  16  and second chamber  26  run in parallel. In an alternative embodiment, first premix chamber  16  and second chamber  26  combine or mix together downstream. 
     As shown in  FIG. 1 , engine  10  includes a fuel transfer circuit  40  that controls an amount of fuel that flows through first fuel line  12  and/or second fuel line  14  into respective first premix chamber  16  and second chamber  26 . In one embodiment, fuel transfer circuit  40  is in independent operational control communication with first fuel line  12  and/or second fuel line  14 . Fuel transfer circuit  40  is a fluidic oscillator that selectively controls the fuel flow through first fuel line  12  and/or second fuel line  14 . Alternatively, fuel transfer circuit  40  includes a suitable mechanical or fluidic valve assembly for controlling fuel flow through first fuel line  12  and/or second fuel line  14 . Other suitable oscillator assemblies and/or valve assemblies known to those skilled in the art and guided by the teachings herein provided can be used to control fuel flow through first fuel line  12  and/or second fuel line  14 . 
     A theoretical or ideal fuel split ratio results when each fuel line provides an equal amount or percentage of the total fuel consumed by the engine. A possible fuel split with two fuel supply lines, for example first fuel line  12  and second fuel line  14 , can be 50:50, wherein first fuel line supplies 50% of the total fuel consumed by the engine and second fuel line supplies 50% of the total fuel consumed by the engine. However, a 50:50 fuel split ratio is infeasible due to combustion dynamics levels, including pressure oscillations and/or acoustical vibrations, which develop within the engine during the combustion process. Such combustion dynamics levels ultimately result in engine component damage and/or engine failure. 
     To avoid such combustion dynamics levels, conventional engines run at a constant offset ratio to prevent engine failure. For example, a conventional engine including two fuel supply lines is configured such that a first fuel supply line constantly supplies about 10% to about 90% of the total fuel consumed by the engine and a second fuel supply line constantly supplies the remaining fuel. 
     In contrast to conventional engine configurations, the method and apparatus of the present invention provides a fuel split ratio that is periodically modulated or switched between a first fuel split ratio and a second fuel split ratio to actively control the combustion dynamics levels developed within engine  10  and prevent undesired pressure oscillations and/or acoustical vibrations. In one embodiment, during a first operating state  50  having a first fuel split ratio, engine  10  operates within a high dynamic state, as shown in  FIG. 2 . The high dynamic state is within an unstable operating region and is defined by a relatively low NO X  emissions level (along curve  52 ) but an increased combustion dynamics level (along curve  54 ). In one embodiment, first fuel line  12  supplies about 10% to about 90%, for example 45%, of the total fuel consumed by engine  10  during first operating state  50 . Second fuel line  14  supplies the remaining percentage, for example about 55%, of the total fuel consumed by engine  10 . First operating state  50  has a set time duration of about 10 msec to about 100 msec. In alternative embodiments, first operating state  50  has a time duration less than about 10 msec or greater than about 100 msec, as desired. 
       FIG. 3  graphically represents the pressure oscillations measured in pounds per square inch within engine  10  during engine operation at first operating state  50  (high dynamics operating state). Over a time duration, an amplitude of the pressure increases and approaches an undesirable maximum amplitude, which represents an undesirable, relatively high combustion dynamics level. At an expiration of the time duration, before the maximum amplitude is reached, fuel transfer circuit  40  is activated to initiate a change in engine operation to a second operating state  55  to reduce the combustion dynamics experienced during operation at first operating state  50 . Fuel transfer circuit  40  controls the switching from the first fuel split ratio to a second fuel split ratio to actively control the combustion dynamics level developed within engine  10 . The switching from first operating state  50  to second operating state  55  is represented by reversible arrow  60  in  FIG. 2 . 
     During second operating state  55  having a second fuel split ratio, engine  10  operates within a low dynamic state, as shown in  FIG. 2 . The low dynamic state is within a stable operating region and is defined by a relatively low combustion dynamics level but an increased NO X  emissions level. During second operating state  55 , first fuel line  12  supplies about 10% to about 90%, for example about 55%, of the total fuel consumed by engine  10  and second fuel line  14  supplies the remaining percentage, for example about 45%, of the total fuel consumed by engine  10 . First fuel line  12  and second fuel line  14  each supplies the selected amount of fuel during second operating state  55 , over a suitable time period of about 10 msec to about 100 msec. In alternative embodiments, second operating state  55  has a set time duration less than about 10 msec or greater than about 100 msec, as desired. 
       FIG. 4  graphically represents the pressure oscillations measured in pounds per square inch within engine  10  during engine operation at second operating state  55  (low dynamics operating state). In contrast to the pressure oscillations measured during first operating state  50 , the pressure oscillations measured over time for second operating state  55  maintain a generally random wave without approaching an undesirable maximum amplitude, which result in engine component damage and/or engine failure as discussed above. However, it is not desirable to maintain engine operation at second operating state  55  because such engine operation generates undesirable NO X  emissions levels. Therefore, at expiration of a set time duration, fuel transfer circuit  40  is activated to initiate a change in engine operation from second operating state  55  to first operating state  50 , as represented by reversible arrow  60  in  FIG. 2 . Fuel transfer circuit  40  controls the switching between the first fuel split ratio and the second fuel split ratio to actively control the combustion dynamics developed within engine  10  and prevent combustion dynamics levels sufficient to damage engine components and/or failure of engine  10 . 
       FIG. 5  graphically represents the pressure developed within engine  10  during engine operation. Engine  10  operates at first operating state  50  within an unstable operating region for an adjustable time duration. During the time duration, the amplitude of the pressure wave increases and approaches an undesirable amplitude representing component damage and/or engine failure, requiring activation of fuel transfer circuit  40  to initiate switching to a second fuel split ratio, represented by line  65  in  FIG. 5 . Upon activation of fuel transfer circuit  40 , engine  10  operates at second operating state  55  within the stable operating region. Within second operating state  55 , the amplitude of the pressure wave remains generally steady within an acceptable pressure range, represented graphically by a general sinusoidal wave as shown in  FIGS. 4 and 5 . At a suitable time, represented by line  70  in  FIG. 5 , fuel transfer circuit  40  is activated to initiate engine operation within first operating state  50 . Upon expiration of the set time duration, represented by line  75  in  FIG. 5 , fuel transfer circuit  40  is again activated to initiate engine operation within second operating state  55 . Thus, the present invention provides an apparatus to actively control the combustion dynamics level developed within a combustion device, such as engine  10 , by alternating periodically at a low frequency between first operating state  50 , representing a high dynamics operating state, and second operating state  55 , representing a low dynamics operating state. 
     In one embodiment, first fuel line  12  is initially configured to supply about 40% of the total fuel consumed by engine  10  and second fuel line  14  is initially configured to supply about 60% of the total fuel consumed by engine  10 . It is apparent to those skilled in the art and guided by the teachings herein provided that first fuel line  12  may be configured to supply any suitable amount of fuel to engine  10  either less than about 40% or greater than about 40%, with second fuel line  14  configured to supply the remaining required fuel to engine  10 . During engine operation, fuel transfer circuit  40  is activated to increase or decrease the amount of fuel supplied through first fuel line  12  by a desired percentage not greater than about 10%, for example about 2%. The amount of fuel supplied by second fuel line  14  is correspondingly adjusted. For example, fuel transfer circuit  40  is activated to reduce the amount of fuel supplied through first fuel line  12  to about 38% of the total fuel consumed by engine  10 . The amount of fuel supplied through second fuel line  14  is correspondingly increased to about 62% of the total fuel consumed by engine  10 . As an alternative example, fuel transfer circuit  40  may be activated to increase the amount of fuel supplied through first fuel line  12  to about 44% of the total fuel consumed by engine  10 . The amount of fuel supplied through second fuel line  14  is correspondingly decreased to about 56% of the total fuel consumed by engine  10 . 
     In the exemplary embodiment, a method for controlling combustion dynamics levels within a chamber of a combustion device, such as gas turbine engine  10  is provided. As shown in  FIG. 2 , the method includes determining a combustion dynamics level and NO X  emissions level for each fuel split ratio within an operating range of engine  10 . In one embodiment, a combustion dynamics level and/or a NO X  emissions level for a plurality of fuel split ratios are defined for an operating range of engine  10 . The combustion dynamics level is a measurement of pressure within engine  10  during a combustion process. The NO X  emissions level is a measurement of NO X  emitted from engine  10  during the combustion process. In the exemplary embodiment, fuel split ratio is a ratio of fuel supplied to engine  10  through first fuel line  12  to a total amount of fuel supplied to engine  10 , equal to the amount of fuel supplied through first fuel line  12  and the amount of fuel supplied through second fuel line  14 . The first operating state defined by a first fuel split ratio and the second operating state defined by a second fuel split ratio is then determined. 
     A graphical representation of high dynamics operating states and low dynamics operating states are defined by plotting combustion dynamics levels and associated NO X  emissions levels verses fuel split ratios within the operating range of engine  10 , as shown in  FIG. 2 . The stable operating region in which engine  10  operates in a stable condition is represented by a low dynamics operating state wherein the combustion process generates a relatively low level of combustion dynamics. However, the low dynamics operating state undesirably generates a relatively high level of NO X  emissions. Conversely, the unstable operating region in which engine  10  operates in an unstable condition is represented by an undesirably high dynamics operating state wherein the combustion process generates a relatively high level of combustion dynamics but advantageously generates a relatively low level of NO X  emissions. 
     Thus, with engine  10  operating within low dynamics operating state  55 , combustion dynamics, including pressure oscillations and/or acoustical vibrations, is relatively quite. However, it is generally not feasible to operate entirely within the stable operating condition due to the undesirable high NO X  emissions level. In contrast, with engine  10  operating within the unstable operating region, NO X  emissions are advantageously low. However, it is generally not feasible to operate entirely within the unstable operating condition due to the undesirable pressure oscillation and/or acoustical vibrations, which ultimately result in engine component damage and/or engine failure. 
     From the graph plotted for combustion dynamics levels and associated NO X  emissions levels verses fuel split ratios within the operating range of engine  10 , as shown in  FIG. 2 , a first operating state  50  outside a stable operating region, e.g. within an unstable operating region, is defined by a first fuel split ratio. A second operating state  55  within a stable operating region is defined by a second fuel split ratio. In one embodiment, a firing temperature at which the first fuel split ratio is selected to define first operating state  50  is the same as a firing temperature at which the second fuel split ratio is selected to define second operating state  55 . 
     The combustion dynamics level of engine  10  is actively controlled by modulating or switching between first operating state  50  and second operating state  55 . In one embodiment, fuel transfer circuit  40  controls an amount of fuel that flows through first fuel line  12  and/or second fuel line  14 . In one embodiment, a high dynamics operating state is defined at a first fuel split ratio. In this embodiment, the first fuel split ratio is a ratio of an amount of fuel supplied to engine  10  through first fuel line  12  to a total amount of fuel supplied to engine  10 . A low dynamics operating state is defined at a second fuel split ratio different from the first fuel split ratio. The second fuel split ratio is a second ratio of an amount of fuel supplied to engine  10  through first fuel line  12  to a total amount of fuel supplied to engine  10 . The combustion dynamics level within engine  10  is controlled by periodically switching between the first fuel split ratio and the second fuel split ratio at a set time duration of about 10 msec to about 100 msec. 
     At the high dynamics operating state and/or at the low dynamics operating state, a first amount of fuel supplied to engine  10  through first fuel line  12  and a second amount of fuel supplied to engine  10  through second fuel line  14  is actively controlled. A first amount of fuel is supplied through first fuel line  12  equal to about 10% to about 90% of a total amount of fuel supplied to the combustion device and a second amount of fuel supplied through second fuel line  14  is equal to the remaining percentage of the total fuel supplied to the combustion device. In one embodiment, the first amount of fuel supplied by first fuel line  12  or the second amount of fuel supplied by second fuel line  14  is increased and the other of the first amount of fuel or the second amount of fuel is correspondingly decreased to periodically modulate or switch between the first fuel split ratio and the second fuel split ratio. For example, the first amount of fuel is increased by a percentage value not greater than about 10% of the total amount of fuel supplied to the combustion device and the second amount of fuel is correspondingly decreased by the percentage value. Alternatively, the first amount of fuel is decreased by a percentage value not greater than about 10% of the total amount of fuel supplied to the combustion device and the second amount of fuel is correspondingly increased by the percentage value. 
     In this embodiment, fuel transfer circuit  40  is activated to adjust a first amount of fuel that flows through first fuel line  12  and a second amount of fuel that flows through second fuel line  14 . For example, fuel transfer circuit  40  is activated to increase or decrease the first amount of fuel by a fuel input adjustment value not greater than about 10% of a total amount of fuel supplied to the combustion device and correspondingly decrease or increase, respectively, the second amount of fuel by the fuel input adjustment value. 
     In an alternative embodiment, gas turbine engine  10  includes any suitable number of fuel lines. For example, in this alternative embodiment, in addition to first fuel line  12  and second fuel line  14 , a tertiary or third fuel line and a quaternary or fourth fuel line each is independently operatively connected to engine  10 . Fuel transfer circuit  40  controls an amount of fuel that flows through first fuel line  12 , second fuel line  14 , third fuel line and/or fourth fuel line. 
     The above-described method and apparatus of the present invention actively controls combustion dynamics levels developed within a gas turbine engine during engine operation. More specifically, a fuel transfer circuit periodically adjusts a path of at least a portion of the fuel supplied to the gas turbine engine through a first fuel line and/or a second fuel line to effectively mitigate the combustion dynamics levels within the gas turbine engine. The periodic modulation or switching at low frequency between a high dynamic state and a low dynamic state effectively controls the combustion dynamics levels developed within the gas turbine engine to mitigate the combustion dynamics levels over time. 
     Exemplary embodiments of a method and an apparatus for actively controlling combustion dynamics levels developed within a gas turbine engine during engine operation are described above in detail. The method and apparatus are not limited to the specific embodiments described herein, but rather, steps of the method and/or elements or components of the apparatus may be utilized independently and separately from others described herein. Further, the described method steps and/or apparatus elements or components can also be defined in, or used in combination with, other methods, apparatus and/or systems and are not limited to practice only as described herein. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.