Air biasing system in a gas turbine combustor

A combustor (10) including: a first premix main burner (14) comprising a first swirler airfoil section (38); a second premix main burner (15) comprising a second swirler airfoil section (40); and a supply air reversing region upstream of the premix burners (14), (15). The first swirler airfoil section (38) and the second swirler airfoil section (40) are effective to impart swirl to a first airflow and a second airflow characterized by a same swirl number as the airflows exit respective burners (14), (15). The combustor (10) is effective to generate a first airflow volume through the first premix main burner (14) that is different than a second airflow volume through the second premix main burner (15).

FIELD OF THE INVENTION

The invention relates to controlling combustion dynamics in a gas turbine engine. More particularly, this invention relates to controlling combustion dynamics by biasing airflow to a combustion flame in the gas turbine engine.

BACKGROUND OF THE INVENTION

Gas turbine engines are known to include a compressor for compressing air, a combustor for producing a hot gas by burning fuel in the presence of the compressed air produced by the compressor, and a turbine for expanding the hot gas to extract shaft power. Gas turbine engines using annular combustion systems typically include a plurality of individual burners disposed in a ring about an axial centerline for providing a mixture of fuel and air to an annular combustion chamber disposed upstream of the annular turbine inlet vanes. Other gas turbines use can-annular combustors wherein individual burner cans feed hot combustion gas into respective individual portions of the arc of the turbine inlet vanes. Each can includes a plurality of main burners disposed in a ring around a central pilot burner.

During operation, the combustion flame can generate combustion oscillations, also known as combustion dynamics. Combustion oscillations in general are acoustic oscillations which are excited by the combustion itself. The frequency of the combustion oscillations is influenced by an interaction of the combustion flame with the structure surrounding the combustion flame. Since the structure of the combustor surrounding the combustion flame is often complicated, and varies from one combustor to another, and because the combustion flame itself may vary over time, it is difficult to predict the frequency at which combustion oscillations occur. As a result, combustion oscillations may be monitored during operation and parameters may be adjusted in order to influence the interaction of the combustion flame with its environment.

A combustion flame emits sound energy during combustion. A more uniform flame will generate more uniform acoustics, but perhaps with higher peak amplitude at a particular frequency than a less uniform flame. When an emitted frequency of combustion coincides with a resonant frequency of the combustion chamber the system may operate in resonance, and the resulting combustion dynamics may damage the gas turbine components, or at least reduce their lifespan.

One known way to reduce the interaction of the combustion flame with the combustion acoustics is to reduce the coherence of the flame, i.e. reduce the spatio-temporal uniformity of the flame. A flame with less uniform combustion throughout its volume is likely to perturb the gas turbine less than a uniform flame because the energy released is spatially distributed and therefore decreases its coupling to the system resonant frequencies or acoustic modes. This is the well known Rayleigh criterion. As a result, combustion dynamics of flames with less uniform combustion throughout its volume are less likely to be exacerbated than by a more uniform flame.

One way that has been utilized to reduce flame coherence has been to vary the fuel/air ratio throughout the flame. Main premix burners often have a swirler that swirls an airflow flowing through the burner. Fuel outlets in the burner introduce a flow of fuel into the airflow to produce a fuel/air mixture of a certain ratio. The fuel/air ratio from main burners may be varied. For example, some of the main burners of a combustor may be controlled by one fuel stage, and the remaining burners of the combustor by another stage. Since the structure of the main burners and swirlers in them are uniform throughout the burners in the combustor, varying the fuel from burner to burner varies the fuel/air ratio. Since each fuel/airflow has a different amount of fuel when it reaches the combustion flame, the combustion/temperature of the combustion flame varies throughout its volume and the flame is less coherent.

Such a fuel biasing of the combustion flame has drawbacks. Separate fuel stages are very expensive to manufacture and complicated to operate. Further, localized regions of leaner and richer combustion within the combustion flame produce less than optimal emissions.

Another way that has been utilized to reduce flame coherence has been to vary portions of the combustion flame axially with respect to other portions of the combustion flame which results in a less uniform combustion flame, thereby reducing combustion dynamics. This has been accomplished, in one example, by increasing the volume of fuel/air flow through one burner with respect to another burner. This has also been accomplished by positioning burners in different locations axially with respect to other burners in a combustor. However, these configurations may not work under all situations, so there remains room in the art for combustor configurations to reduce flame coherence and associated combustion instabilities.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have devised an innovative way to configure a combustor utilizing premix main burners (i.e. burners) so that different burners will deliver fuel/air flows having a differing parameter which will, in turn, reduce flame coherence and associated combustion dynamics. The differing parameter need not be the fuel/air ratio, so that combustion dynamics may be controlled without sacrificing optimized emissions.

Each fuel/air flow may be characterized by the same swirl number but a different mass flow rate. The swirl number (S) is defined as the ratio of the axial flux of the angular momentum (Gϕ) to the axial thrust (Gx) times the exit radius (R),

S≡GϕGx⁢R.
In an embodiment the fuel/air flows emanating from each burner may have the same fuel/air ratio. As a result of a uniform fuel/air ratio from burner to burner, localized areas of varying temperature within the combustion flame may be reduced or eliminated. By eliminating these localized areas, the less than optimal emissions associated with them are also eliminated.

A different flow from one burner to the next may result from directing differing flows to respective burners, or by varying the geometry within a burner to influence the airflow there through, or both. Maintaining the same fuel/air ratio may be accomplished by mechanically configuring each fuel outlet to produce this result, or by fuel control via staging, or a combination of both.

FIG. 1shows a cutaway of a combustor10of a gas turbine engine. Inside the combustor10is a pilot burner12, and a plurality of premix main burners14,15disposed around the pilot burner12. Inside each main burner14,15is a swirler (not visible) that imparts a swirl to a flow flowing through each burner. Also inside each burner is at least one fuel outlet (not shown) that directs fuel into the airflow flowing through the main burner14,15. The airflow is delivered from an upstream region18. A combustion flame (not shown) occurs in the combustion region16where the fuel/air flow from the pilot burner12and swirled fuel/air flows from the main burners14,15converge during operation. It can be seen that if each fuel/air flow from the main burners is uniform, then the combustion flame is likely to be more uniform. Thus, by varying the fuel/air flow from each burner the resulting combustion flame may be less uniform.

As can be seen inFIG. 2, supply air20originates outside the combustor. In this configuration supply air20flows into a reversing region22where it reverses direction and enters the upstream region,18of the combustor10. In this embodiment flow conditioning plate24is disposed in the reversing region22, transverse to the flow of supply air22, such that the supply air20must flow through circumferentially disposed openings in the flow conditioning plate24in the reversing region22before entering upstream region18of the combustor10. In order to direct portions of the supply air20to the main burners14,15, the flow conditioning plate24may have uniform holes of differing sizes and asymmetric positioning throughout the flow conditioning plate24. For example, there may be larger holes28, smaller holes30, and uniform holes32. Larger holes28may be disposed in the flow conditioning plate24where necessary to permit a relatively larger mass flow rate of airflow to a chosen main burner. This location may be wherever necessary in the supply air20flow to produce the desired airflow at the chosen main burner downstream. Likewise, smaller holes30may be disposed in the flow conditioning plate24where necessary to permit a relatively smaller mass flow rate of airflow to a specified main burner. The remainder of the flow conditioning plate may comprise uniform holes32or no holes at all. Any configuration of holes and hole sizes that results in a non-uniform axial cross section of supply air20flow inside the combustor10upstream of the burners14,15is envisioned, as this would enable different amounts of air flow to different burners14,15. In other words, a different percentage of the total supply air volume can be directed to different burners. In this manner, the flow delivered to respective main burners14,15can be different, which in turn will result in different flows from respective main burners14,15into the combustion flame. Different flows into the combustion flame will reduce flame coherence, which will reduce combustion dynamics.

When the flows into the main burners14,15are conditioned in this manner the swirlers (not shown) within the main burners14,15may be the same throughout all the main burners14,15. In this manner the respective flow of air that does make it to a particular burner will be subject to the same swirl as other flows. The only thing that will change is the mass flow rate of air flowing through the particular burner with respect to other burners. As a result this configuration for conditioning respective flows lends itself well to a retrofit application, where a flow conditioning plate24may be installed on existing combustors10. Adding a flow conditioning plate24to existing combustors10is a simple and relatively inexpensive way to condition the supply flow20into flows tailored for respective burners. Since most combustors10that could be retrofitted in this manner already have fuel staging, the fuel staging may be adjusted as necessary to produce the same fuel/air ratio from each burner, which would reduce or eliminate varying temperature within the combustion flame, thereby reducing emissions. It is also envisioned where the fuel/air ratio may still be varied in fuel/air flows from burner to burner. This provides an added degree of control and/or fine tuning. Similarly, the fuel/air ratio may be adjusted during operation such that at times the fuel/air ratios of all the respective flows are the same, and at other times, the fuel/air ratio of all the respective flows are different. This may be necessary when other factors are considered, such as transient operating conditions etc. It is also envisioned that the flow conditioning plate24may be used in conjunction with the teachings below.

Further, for sake of simplicity it has been assumed that the supply air20may have an essentially uniform pressure throughout its volume before being conditioned when a flow conditioner24is used. The same assumption is made about the region into which the airflows leaving the burners flow. This simplification contributes to a more ready understanding of the invention because the pressure drop from before the conditioning plate24to the region downstream of the burners would be the same regardless of what path the supply air takes between the conditioning plate24and the region downstream of the burners. Thus it is easier to envision how different burner/swirler geometries may influence the flow through the respective burner. Similarly, in embodiments where no conditioning plate24is used, it is assumed that the supply air20may have an essentially uniform pressure throughout its volume before entering respective burners, and after leaving the burners. Here again it is easier to envision how different burner/swirler geometries may influence the flow through the respective burner. However, the inventors understand that pressure variations may occur throughout the volumes of each of these areas of assumed uniform pressure, and these pressures and locations of pressure variations may change during operation. In embodiments where all main burner fuel outlets are controlled by a single stage and uniform fuel/air ratios among all flows are desired, it is understood that perfect uniformity for fuel/air ratios may not always be achieved. Such operating variations are envisioned and may be tolerable, depending on the design. Such variations are likely to be less than variations present in existing fuel biasing combustors, and so combustors as disclosed herein are still likely to have improved emissions when compared to fuel biasing combustors. Minor lack of uniformity may be tolerable if, for instance, the cost saving associated with a single stage controlling the fuel to all the main burners14,15is preferred. When more uniformity is desired then staging the control the fuel among the main burners may be preferred, despite the added cost.

FIG. 3is a partial cross section of the main burners14,15as they would be positioned in a combustor10. Visible are swirlers34,35. Each swirler has airfoils36which swirl air flowing through the burner, and therefore through the swirler34. In an embodiment, the swirlers34,35may have different diameters, D1, D2, but be aerodynamically proportional so that although there will be different mass flow rates of air flow through respective swirlers, each will be characterized by the same swirl number. Due to the design of combustors10, supply air20must flow through one of the main burners14,15or the pilot burner12. Thus, a different swirler diameter will permit a different percentage of the total supply air20to pass through the swirler34,35. Each fuel/air flow produced will be characterized by the same swirl number, but the diameter of the fuel/air flow, and therefore the total mass flow rate of fuel/air flow exiting a main burner swirler will be different from the fuel/air flow exiting from another main burner swirler. As a result, different sized fuel/air flows will be entering the combustion flame at different locations of the combustion flame, and the combustion flame coherence will be reduced. This reduced coherence will reduce combustion dynamics. There may be two different diameters, and these may be staggered or otherwise grouped, or there may be a different diameter for each swirler34,35. For example, in an embodiment a first premix main burners14may comprise a larger diameter (D1) swirler34and form a first swirler airfoil section38, and second premix main burner15may comprise a smaller diameter (D2) swirler35and form a second swirler airfoil section40. These may be arranged in an alternating pattern, or grouped together in other patterns, though these examples are not meant to be limiting.

When the diameters of respective swirlers differ, but the swirlers are aerodynamically proportional, the fuel/air ratio of the flows from respective burners can be varied or can be the same. In an embodiment where the same fuel/air ratio is desired for all flows, this can be accomplished by mechanically configuring the respective fuel outlets without the need for staging among the main burners14,15, or by utilizing staging among the main burners14,15, or both. In an embodiment where the fuel/air ratio is to be the same from burner to burner, and the fuel outlets are mechanically configured to produce consistent fuel/air ratios throughout, multiple stages of fuel to control fuel to the main burners14,15may not be needed. This is particularly advantageous because fuel staging is expensive to manufacture, operate and maintain. Eliminating a fuel stage for the main burners14,15would result in a significant cost savings, without sacrificing the needed control over the combustion dynamics, and may even improve emissions over staged/fuel biasing schemes. Nonetheless, it is envisioned that staging among main burners14,15may still be desired, and may afford a greater degree of control over combustion dynamics and emissions. The balance of cost versus desired control may determine which ultimate configuration is chosen, and this flexibility is the result of this innovative approach.

In another embodiment, the airfoils36of one swirler may be a different thickness than airfoils36of another swirler. If the remainder of the geometry is the same among swirlers, then the thicker blades of one swirler36will restrict the air flowing through that swirler. The mass flow rate of the air through the swirler is thus reduced, but the flow is characterized by the same swirl number as a flow emanating from a burner where the swirler airfoils36are relatively thinner. This can be seen inFIG. 4, which is a schematic representation of airfoils36. Relatively thinner airfoils42of one swirler result in a larger flow path width46between airfoils42. Relatively thicker airfoils44of another swirler result in a narrower flow path width48between airfoils44. Thus, the mass flow rate of air flowing through a swirler with thinner airfoils42will be greater than a mass flow rate of air flowing through a swirler with thicker airfoils44. There may be only two different airfoil thicknesses, or there may be as many airfoil thicknesses as there are swirlers.

This configuration may likewise be designed to produce the same fuel/air ratio in all fuel/air flows, or different fuel/air ratios. If the same fuel/air ratio is desired, the fuel outlets can be configured mechanically do produce the desired fuel/air ratios, without staging among the main burners14,15. The fuel may also be controlled with staging among the main burners14,15. Both techniques may also be used together to control fuel/air ratios.

Also shown schematically inFIG. 4are fuel outlets50,52, and respective stages54,56for controlling a flow of fuel to each fuel outlet50,52from a fuel supply58. In an embodiment fuel may be injected into an airflow60via pegs62which are separate from the airfoils42,44. However, fuel can be injected into the airflow60in any number of ways, including outlets incorporated into the airfoil, and/or outlets upstream or downstream of the swirler.

In another embodiment individual airfoils within one swirler may differ in geometry from other airfoils in the same swirler. Only one swirler may have airfoils of differing geometry, or as many as all of the swirlers may have airfoils of differing geometry. For exampleFIG. 5schematically depicts air flow paths between a plurality of airfoils36. It can be seen that there may be thinner airfoils64and thicker airfoils66. Thinner airfoils64and thicker airfoils66may be grouped as shown, or in any configuration to achieve a desire effect. As shown, placing two thicker airfoils66next to each other will result in a smaller opening68between them than an opening70between a thinner airfoil64and a thicker airfoil66. This will result in a reduced flow through the swirler, but the flow will be characterized by the same swirl number. The blade thicknesses can be varied in any number of ways to tailor the swirl as desired. Within a swirler there may be one common airfoil thickness, or there may be as many differing airfoil thicknesses as there are airfoils in that swirler.

In another embodiment the shape of the airfoil within the swirler differs from blade to blade within the swirler. For example, in the previous embodiments the discrete flow paths between adjacent airfoils in a swirler may have a rectangular cross section. As seen inFIG. 6, which is a schematic view of the flow paths between airfoil blades as seen by the air flowing through them, (i.e. the flow is flowing into the page), the shapes of the airfoils can be different in order to contour the discrete flow paths between airfoils. A cross section of discrete flow path72would be more rounded than a rectangular cross section of a traditional flow path. Similarly flow path74would be more arched, and flow path76would be more traditionally rectangular, and all these shapes can exist within the same swirler. Any combination is envisioned. Further, the shapes can vary in other ways than that shown inFIG. 6. The airfoils can vary along their length, width, and height. What matters is that the fuel/air flow exiting the swirler be characterized by the same flow number as the fuel/air flows exiting from other swirlers. Within a swirler there may be one common airfoil shape, or there may be as many differing airfoil shapes as there are airfoils in that swirler.

It can be seen that the inventors have devised an air biasing structure capable of reducing flame coherence, and associated combustion dynamics, in a manner not yet seen in the art. This structure provides greater design flexibility without sacrificing necessary control over combustion dynamics. Further, when the fuel/air ratio of all fuel/air flows flowing into the combustion flame are kept the same an entire stage of fuel controls for the main burners may be removed, saving substantial manufacturing and operating costs, while reducing emissions over fuel biasing schemes of the prior art.