Combustion – Process of combustion or burner operation
Reexamination Certificate
2000-05-05
2002-10-08
Clarke, Sara (Department: 3743)
Combustion
Process of combustion or burner operation
C431S114000, C060S725000, C381S071200
Reexamination Certificate
active
06461144
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of combustion technology, as is of importance, in particular, for gas turbines. The invention relates to a method of suppressing or controlling thermoacoustic vibrations in a combustion system.
The invention also relates to a combustion system for carrying out the above method.
BACKGROUND OF THE INVENTION
Such a method or combustion system has been disclosed, for example, by the article by Paschereit, C. O., Gutmark, E., and Weisenstein, W., “Structure and Control of Thermoacoustic Instabilities in a Gas-Turbine Combustor”, 36
th
AIAA Aerospace Science Meeting and Exhibit, Reno, Nev., Jan. 12-15, 1998.
Thermoacoustic vibrations represent a risk to every type of combustion application or system. They lead to pressure fluctuations of high amplitude and to a restriction in the operating range and may increase the undesirable pollutant emissions. This applies in particular to combustion systems having low acoustic damping, as is normally the case in gas turbines. In order to permit a high power conversion with regard to pulsations and emissions over a wide operating range, active control or suppression of the combustion vibrations may be necessary.
Various active control systems have already been proposed in the past, these control systems working according to the principle of the “antisound”, i.e. the thermoacoustic vibrations are detected, displaced in phase by 180 degrees and induced in the system in a correspondingly amplified form in order to then lead to an extinction during superimposition with the thermoacoustic vibrations on account of the phase opposition. The antisound solutions have proved to be useful in combustion systems of low output. However, in combustion systems of high output with correspondingly pronounced pressure fluctuations, it becomes increasingly difficult to generate and induce corresponding acoustic vibrations at a justifiable cost.
In order to permit an active control even at high outputs, it has therefore been proposed to either modulate the burner flame itself via the fuel feed as a function of the detected instabilities (U.S. Pat. No. 5,145,355) or to introduce a vibration generator in the form of an auxiliary burner operating in a pulsating manner (U.S. Pat. No. 5,428,951). The desired acoustic vibrations of high power can thus be generated in both cases via deliberately generated fluctuations in the heat release. A disadvantage in this context, however, is that this type of vibration generation requires considerable intervention in the combustion system and therefore cannot readily be retrofitted, for example, in existing designs. In addition, such a system, on account of the complexity of the combustion actions coming into play in the process, can be influenced and controlled in a deliberate and stable manner only with difficulty over a larger operating range.
In the publication mentioned at the beginning, an active control of the thermoacoustic vibrations has now been proposed, and this active control is not based on the extinction of sound but intervenes in the development of the vibrations and can be described as follows: coherent structures are of crucial importance during mixing actions between air and fuel. The dynamics of these structures therefore influence the combustion and thus the heat release. Control of the combustion instabilities is possible by influencing the shear layer between the fresh-gas mixture and the recirculating exhaust gas. One possibility of influencing the shear layer is the acoustic excitation described in the publication mentioned at the beginning. The acoustic excitation permits suppression of the combustion-driven vibrations by preventing the formation of coherent structures. Periodic heat release and thus the basis for the occurrence of thermoacoustic vibrations are prevented by preventing the development of vortex structures at the burner outlet.
Unlike the principle of the antisound, in which an existing sound field is extinguished by introducing a phase-shifted sound field of the same energy, this method is based on directly influencing the shear layer. This direct influencing of the shear layer has the advantage that the disturbances which are introduced from outside are amplified in the shear layer itself, and therefore less energy is required for generating the disturbances than in the case of the direct extinction of a sound field by antisound. In this case, the shear layer may be excited both downstream and upstream of the burner. Since only low power is necessary, the sound energy may be introduced into the flow, for example, by acoustic drivers, in particular loudspeakers or the like. By selection of the correct phase difference between pulsation and acoustic excitation signal, the coherence of the developing instability waves can be disturbed and the pulsation amplitudes can be reduced.
An exemplary combustion system as has been used in the publication mentioned at the beginning and as is also suitable for the present invention is reproduced schematically in FIG.
1
. The combustion system
10
comprises a (swirl-stabilized) burner
11
, which works in a combustion chamber
12
. The burner
11
receives the requisite combustion air via an air feed
13
. A corresponding fuel feed
14
is provided for the fuel supply. Sensors
20
-
22
, which may be arranged on the air feed (sensors
20
) and/or on the combustion chamber (sensors
21
,
22
), are provided for detecting the thermoacoustic vibrations which develop in the region of the flame
15
. The sensors
20
-
22
may be designed for the direct detection of the pressure fluctuations or vibrations as (water-cooled) microphones or other dynamic pressure transducers. However, the sensors
20
-
22
may also be designed entirely or partly as optical sensors, with which the fluctuations in the heat release which are directly associated with the thermoacoustic vibrations may be detected directly via the chemiluminescence, e.g. of the OH molecules.
The sensors
20
-
22
are connected to a controller
23
, which on the output side activates various loudspeakers
16
-
19
, which are arranged symmetrically to the axis of the combustion system
10
alternatively in the region of the air feed
13
and/or the combustion chamber
12
. In accordance with the controller
23
, the loudspeakers
16
-
19
generate acoustic vibrations, which are then induced in the combustion system
10
and influence the described shear layers there. The combustion system
10
according to the prior art with the sensors
20
-
22
and the loudspeakers
16
-
19
—if the vibrations are detected at the combustion chamber
12
—forms the closed control loop
24
shown in FIG.
2
. The vibrations in the combustion chamber
12
which are detected by the sensors
21
and/or
22
are filtered in a following filter
25
and if need be amplified and are then shifted in phase by a desired amount by means of a phase shifter
26
with predeterminable phase setting
29
. The phase-shifted signal then triggers a signal generator
27
, the output signal of which is amplified in a power amplifier
28
with predeterminable amplitude setting
30
and is used to activate the loudspeakers
16
-
19
. With this known control, in which the acoustic vibrations are generated synthetically and the amplitude of these vibrations is firmly set, suppression (attenuation) of the combustion-driven vibrations by up to 6 dB has already been achieved in the system used.
However, it would also be desirable to achieve even better suppression with an arrangement according to FIG.
1
.
SUMMARY OF THE INVENTION
The object of the invention is therefore to specify a method of acoustically controlling thermoacoustic vibrations, which, while using the principle of the acoustic excitation of the shear layer, permits markedly improved suppression, and to specify a combustion system for carrying out such a method.
An aspect of the invention includes providing proportional control within the closed control loop which is formed by the combustion system with the sensors an
Gutmark Ephraim
Paschereit Christian Oliver
Weisenstein Wolfgang
Alstom (Switzerland Ltd
Burns Doane Swecker & Mathis L.L.P.
Clarke Sara
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