Method and apparatus of combustor dynamics mitigation   

BACKGROUND OF THE INVENTION

The subject matter disclosed herein generally relates to combustors. More specifically, the subject disclosure relates to mitigation of combustion dynamics in combustors.

As requirements for gas turbine emissions have become more stringent, one approach to meeting such requirements is to move from conventional diffusion flame combustors to combustors utilizing lean fuel and air mixtures (having equivalence ratios of approximately 0.58 to 0.65) during fully premixed operations mode to reduce emissions of, for example, NOx and CO. These combustors are known in the art as Dry Low NOx (DLN), Dry Low Emissions (DLE) or Lean Pre Mixed (LPM) combustion systems. Such combustors typically include multiple fuel nozzles housed in a barrel, also known as a cap cavity.

Because these combustors operate at such lean fuel/air ratios, small changes in velocity fluctuations can result in large changes in mass flow and fuel air fluctuations. These fluctuations result in a large variation in the rate of heat release and high pressure fluctuations in the cap cavity. Interaction of fuel/air fluctuation, vortex-flame interactions and unsteady heat release leads to a feed back loop mechanism resulting in dynamic pressure pulsations in the combustion system. The phenomenon of pressure pulsations is referred to as thermo-acoustic or combustion-dynamic instability, or plainly, combustion dynamics. High levels of combustion dynamics limit the operability envelope of the combustor which limits reductions in emissions and/or power. Further, combustion dynamics shortens hardware life and results in damage to combustor components which results in downtime of the combustor for repair and/or replacement of the components.

According to one aspect of the invention, a combustor includes an enclosed combustor cap volume and a plurality of fuel nozzles extending through the combustor cap volume. At least one baffle plate is located in the combustor cap volume dividing the combustor cap volume into at least two volumes. A plurality of feed tubes extend inwardly into at least one of the at least two volumes form a flow channel and are configured to reduce thermo acoustic dynamics of the combustor.

According to another aspect of the invention, a thermo acoustic damper for a combustor includes at least one resonator in flow communication with a flow channel of the combustor. The resonator includes an enclosed volume and at least one baffle plate positioned in the enclosed volume dividing the enclosed volume into at least two volumes. A plurality of feed tubes extend inwardly into at least one of the at least two volumes from the flow channel and are configured to reduce thermo acoustic dynamics of the combustor.

According to yet another aspect of the invention, a method for reducing thermo acoustic combustor dynamics includes locating at least one resonator in flow communication with a flow channel of the combustor. The resonator includes an enclosed volume, at least one baffle plate located in the enclosed volume dividing the enclosed volume into at least two volumes, and a plurality of feed tubes extending inwardly into at least one of the at least two volumes from the flow channel. The method further includes directing a fluid flow through the flow channel and across an open end of at least one feed tube of the plurality of feed tubes, and exciting a resonance frequency of the resonator thereby reducing the thermo acoustic combustor dynamics.

These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

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-sectional view of an embodiment of a combustor;

FIG. 2 is another cross-sectional view of an embodiment of a combustor;

FIG. 3 is a schematic cross-section of the combustor of FIG. 2; and

FIG. 4 is a cross-sectional view of another embodiment of a combustor.

The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.

DETAILED DESCRIPTION

Shown in FIG. 1 is an embodiment of a combustor 10. The combustor includes a cap assembly 12 at an upstream end 14 connected to a combustor liner 16. The cap assembly 12 includes a cap barrel 18, which in some embodiments is substantially tubular in shape. A back plate 20 is fixed to the cap barrel 18 and defines an upstream extent of the cap assembly 12. Similarly, an effusion plate 22 is fixed to the cap barrel 18 and defines a downstream extent of the cap assembly 12. I the embodiment shown in FIG. 1, the back plate 20 and the effusion plate 22 are circular in shape, but other configurations of the back plate 20 and the effusion plate 22 are contemplated within the scope of the present disclosure. The cap barrel 18, the back plate 20 and the effusion plate 22 define a cap volume 24 therebetween. A plurality of fuel nozzles 26 are arranged in the combustor 10. In the embodiment shown, the plurality of fuel nozzles 26 includes a plurality of primary fuel nozzles 26 arranged in a circular pattern around a secondary fuel nozzle 26 that is located at a center of the combustor 10. Each fuel nozzle 26 is surrounded by a burner tube 28 and extends into the cap assembly 12 through openings 30 in the back plate 20 of the cap assembly 12. The burner tube 28 surrounding each fuel nozzle 26 extends to the effusion plate 22.

Referring now to FIG. 2, a baffle plate 32 is disposed in the cap volume 24 between the back plate 20 and the effusion plate 22. The baffle plate 32 extends entirely across the cap volume 24 to divide the cap volume 24 into a baffled volume 34 between the baffle plate 32 and the effusion plate 22, and a separated volume 36 between the baffle plate 32 and the back plate 20. A plurality of feed tubes 38 extend through the cap barrel 18 from the baffled volume 34 into a flow channel 40 defined by the cap barrel 18 and a combustor flow sleeve 42. In some embodiments, the feed tubes 38 are circular in cross-section, but it is to be appreciated that other cross-sectional shapes are contemplated within the present scope.

As shown in FIG. 3, the baffled volume 34, together with the plurality of feed tubes 38, act as an acoustic damper by which acoustic pressure and velocity at the feed tube 38 location is altered and results in overall system acoustics change. A resonator connected to a flow channel 40 without a steady flow through it may resonate at a frequency (f) which is determined by a cross-sectional area (S) of the plurality of feed tubes 38, a length (L) of the plurality of feed tubes 38, and a volume (V) of the baffled volume 34. The frequency is given by equation 1:

f=(c/(2*π))*sqrt(S/(V*L))  (1)

where “c” is the speed of sound. A desired frequency can be achieved by changing a position of the baffle plate 32, thus increasing or decreasing the volume (V) of the baffled volume 34 and/or by changing the length (L) or cross-sectional area (S) of the plurality of feed tubes 38. However, a baffled volume 34 into which there is a steady flow does not necessarily act as a resonator, such as where there is steady flow through the feed tubes 38, but succeeds in acting as a thermo acoustic damper. To mitigate a natural frequency of the combustor 10, a matching frequency is chosen, and the characteristics of V, L, and S are set to attain the desired frequency. To achieve the desired L, the feed tubes 38 may extend into the baffled volume 34 as shown in FIG. 2. During operation of the combustor 10, the chosen frequency is effectively “tuned out” thus preventing combustion dynamics problems associated with that natural frequency from occurring.

In some embodiments, the plurality of feed tubes 38 are circumferentially equally-spaced around the cap barrel 18. In other embodiments, however, it may be desired to alter the spacing of the plurality of feed tubes 38 circumferentially to asymmetrically tune the combustor 10.

In some embodiments, it may be desired to tune out more than one natural frequency of the combustor 10. Referring to FIG. 4, the tuning out of two natural frequencies is accomplished by providing a plurality of feed tubes 38 extending through the cap barrel 18 from the separated volume 36 into the flow channel 40 thus defining a second resonator utilizing the separated volume 36. As with the resonator utilizing baffled volume 34, a particular L and S may be chosen to produce the desired resonance frequency f. While resonators and a single baffle plate 32 are shown in FIG. 4, it is to be appreciated that additional baffle plates 32 may be installed in the cap volume 24 thus creating additional volumes therein. By installing pluralities of feed tubes 38 in the additional volumes, more natural frequencies of the combustor 10 can be tuned out.

Utilizing the baffle plate 32 and the plurality of feed tubes 38 mitigates combustion dynamics effectively with out effecting the flow through the flow channel 40. Further, the baffle plate 32 may be installed into existing combustors 10 in a desired position without the need to modify other combustor 10 components and the position of the baffle plate is customizable to specific combustor 10 natural frequencies.

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.