Method and apparatus for controlling thermoacoustic...

Combustion – Combustion bursts or flare-ups in pulses or serial pattern

Reexamination Certificate

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C431S019000, C431S079000, C431S114000, C060S725000

Reexamination Certificate

active

06464489

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method and an apparatus for controlling thermoacoustic vibrations in a combustion system having a combustion chamber and a burner.
2. Background Art
Thermoacoustic vibrations represent a risk to every type of combustion application. They lead to pressure fluctuations of high amplitude and to a restriction in the operating range and may increase the emissions associated with the combustion. These problems occur in particular in combustion systems having low acoustic damping, as often represented by modern gas turbines.
In conventional combustion chambers, the cooling air flowing into the combustion chamber has a sound-damping effect and thus helps to dampen thermoacoustic vibrations. In order to achieve low NO
x
emissions, in modern gas turbines an increasing proportion of the air directed through the burners themselves and the cooling-air flow is reduced. Due to the accompanying lower sound damping, the problems referred to at the beginning in such modern combustion chambers accordingly occur to an increased extent.
One possibility of sound damping includes coupling Helmholtz dampers in the combustion-chamber dome or in the region of the cooling-air feed. However, if the space conditions are restricted, as are typical of modem combustion chambers of compact construction, the accommodation of such dampers may present difficulties and involves a high design cost.
SUMMARY OF THE INVENTION
Accordingly, one object of the invention is to provide a novel apparatus for controlling thermoacoustic vibrations, which apparatus effectively suppresses the thermoacoustic vibrations and involves as low a design cost as possible. Furthermore, an effective method of controlling thermoacoustic vibrations is to be provided.
In accordance with exemplary embodiments of the invention, this object is achieved by providing a method for controlling thermoacoustic vibrations in a combustion system having a combustion chamber and a burner and a working gas flowing through the combustion system. The vibrations are controlled by acoustically exciting a shear layer that forms in the working gas as the working gas flows through the combustion system. In accordance with exemplary embodiments of the invention, a mechanism is also provided for acoustically exciting the working gas in the combustion system to control the thermoacoustic vibrations. In accordance with exemplary embodiments of the invention, the mechanism is arranged in a region of the burner.
Coherent structures are of crucial importance during mixing actions between air and fuel. The spatial and time dynamics of these structures influence the combustion and heat release. The invention, then, is based on the idea of counteracting the formation of coherent structures. If the development of vortex structures at the burner outlet is reduced or prevented, the periodic heat-release fluctuation is also reduced as a result. Since the periodic heat-release fluctuations are the basis for the occurrence of thermoacoustic vibrations, the amplitude of the thermoacoustic vibrations is thereby reduced.
According to the invention, in the method of controlling thermoacoustic vibrations in a combustion system, the shear layer forming in the region of the burner is acoustically excited. Here, shear layer refers to the mixture layer which forms between two fluid flows of different velocity. Thus, shear layers are present in the mixing zones of two different fluid flows but also within one fluid flow when there are regions with changing or different velocities adjacent to each other, as e.g. in the center of a swirl flow or in the boundary layers of a fluid flow adjacent to a wall. The most relevant shear layers in the context of this invention are the boundary layers between two air-fuel-flows or between an air-fuel-flow and a (recirculating) exhaust gas flow.
FIG. 1
a
is a schematic drawing of a combustion chamber showing the existence of typical shear layers within a combustion chamber. However,
FIG. 1
a
does not—by far—show all shear layers which exist in a combustion chamber. Four types of shear layers are shown which are effected by a swirl-induced pipe flow (I), the deflection of the flow in front of the burner (II), separated flow regions in the corners of the combustion chamber with the corners of the combustion chamber being carried out as a shock diffuser (III) and the outlet flow of the recirculation zone of the combustor (IV). Accordingly, shear layers can result from velocity changes of the axial flow and from velocity changes of the azimuthal flow or from combinations thereof.
Influencing the shear layer has the advantage that excitations which are introduced are amplified in the shear layer. Therefore only a small amount of excitation energy is required in order to extinguish an existing sound field. Basic investigations have been conducted by the inventor and published e.g. in Paschereit C. O., et. al. ‘Experimental investigation of subharmonic resonance in an axisymmetric jet’, Journal of Fluid Mechanics, Volume 283, January 1995 which is incorporated by reference herewith. In these investigations a resonant subharmonic interaction between two axisymmetric traveling waves was induced in the shear layer of an axisymmetric jet by controlled sinusoidal perturbations of two frequencies. The measured results clearly indicate that both the fundamental wave and the subharmonic wave are greatly amplified as shown in
FIG. 1
b.
FIG. 1
b
shows the development of the kinetic (in the direction of the x-axis) energy content of four frequency waves which were induced into the shear layers. The filled squares of
FIG. 1
b
indicate the respective behavior of the subharmonic along the axial distance (direction of the x-axis) while the blank squares indicate the respective behavior of the fundamental frequency wave. The triangles show the behavior of further frequency waves (for 3/2 f and for the first harmonic 2 f with f being the fundamental frequency). It is clearly visible that the subharmonic rises most along the axis but also the fundamental frequency is greatly amplified. Furthermore, these investigations show that most of the energy for the resonant growth of the subharmonic originates from the mean flow. Contrasting with this is the principle of the antisound, in which an existing sound field is extinguished by a phase-displaced sound field of the same energy. In preliminary tests it was determined that it is very effective to control a combustion instability by affecting the evolution of the shear layer rather than to rely on anti-sound principles. The direct excitation of a shear layer benefits from the natural amplification of the flow within this shear layer and, thus, requires less energy to obtain the same effect as noise cancellation (with the latter also referred to as the anti-sound principle). By means of a simplifying calculation of the acoustic intensity it can be shown that the acoustic power in a combustor can be set to
P=
((
p
RMS
)
2
·A
)/(
&rgr;·c
)
where A is the area of the combustor cross section. With the following values for the nominal operating conditions of a typical combustor
p
RMS
=600 Pa
&rgr;=0.21 kg/m
3
c=800 m/s
the acoustic power results to P~2.1 kW (Kilowatts). Thus, for a 50% suppression in pressure fluctuation amplitude, the loudspeaker should supply a power of 75% of the acoustic power measured in the combustion chamber, if one would rely on anti-sound principles. Driving the loudspeakers at a power of P=100 W (Watts) and assuming a 10% efficiency of the loudspeaker, the power fed into the combustion chamber is only 0.6% of the suppressed power. As a further driving mechanism, flame and fluid flow dynamics in the combustion chamber, in particular thermoacoustic instabilities, can also be induced by changes in equivalence ratio. However, a comparison between the estimated OH change during one cycle of oscillation and the measured value showed that the driving mechanism which is initiated by the equiv

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