Power plants – Combustion products used as motive fluid – Combustion products generator
Reexamination Certificate
1998-07-08
2001-01-09
Thorpe, Timothy S. (Department: 3746)
Power plants
Combustion products used as motive fluid
Combustion products generator
C431S114000
Reexamination Certificate
active
06170265
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a gas turbine, comprising a device for the fuel injection, which injects fuel into a mixing device, the injected fuel being mixed with combustion air in the mixing device. The gas turbine also has a combustion chamber arranged downstream of the mixing device, the length of the combustion chamber being L
BK
and the length of the mixing device being L
Mix
.
2. Discussion of Background
Undesirable thermoacoustic vibrations often occur in combustion chambers of gas turbines. In this case, thermoacoustic vibrations denote thermal and acoustic disturbances which amplify one another. In the process, high vibration amplitudes may occur and these may lead to undesirable effects, such as, for instance, high mechanical loading of the combustion chamber, increased emissions due to inhomogeneous combustion, and even extinction of the flame.
The cooling air which flows into the combustion chamber has an important function in the case of conventional combustion chambers, since the cooling-air film on the combustion-chamber wall has a sound-damping effect. In modern gas turbines, however, virtually the entire portion of the air is directed through the burner itself in order to achieve the lowest possible NO
x
emissions, and therefore the portion for the film cooling of the combustion chamber is reduced. As a result, the cooling air largely does not function as a damper of acoustic and thermoacoustic vibrations.
A further possibility of sound damping consists in coupling Helmholtz dampers in the region of the cooling-air feed, as described, for instance, in EP-A10576717. However, this is not always possible for reasons of space. In addition, this method often requires considerable expenditure in terms of design.
SUMMARY OF THE INVENTION
Accordingly, one object of the invention is to provide a novel method which is as simple as possible and involves the lowest possible expenditure in terms of design and the least possible additional space requirement and with which undesirable thermoacoustic vibrations in gas-turbine combustion chambers can be minimized.
This object is achieved according to the invention by suitable tuning of mixing device, burner and/or combustion chamber in such a way that entropy waves produced by fluctuations of the gas velocity at the location of the fuel injection induce pressure fluctuations at the combustion-chamber outlet which are superimposed in phase opposition on the pressure fluctuations prevailing in the combustion chamber and thus bring about an overall reduction in the fluctuation amplitudes. According to the invention, this is achieved by a suitable selection of a series of parameters of the combustion chamber, the mixing device and the combustion variables themselves.
Experience shows that fluctuations in the flow velocity at the location of the fuel injection lead to fluctuations in the fuel concentration at the location of the heat release and thus to temperature fluctuations in the hot gas. These temperature fluctuations, more generally designated as entropy fluctuations, are transferred convectively to the combustion-chamber outlet. Due to the narrowing cross section at the combustion-chamber outlet or in the first turbine row, these entropy fluctuations induce pressure fluctuations at a critical cross section, at which the gas velocity virtually reaches or fully reaches the sound velocity. The phase of these pressure fluctuations relative to the phase of the acoustic pressure fluctuations of the combustion chamber is determined by a series of parameters of the combustion chamber, such as, for instance, the length of the combustion chamber, the length of the mixing device and the temperatures of hot gas and fresh gas (and thus the sound velocities in the hot and fresh gas).
According to the invention, these parameters are now selected in such a way that the entropy-wave-induced pressure fluctuations are in phase opposition to certain acoustic pressure fluctuations at the combustion-chamber outlet. In this case, in phase opposition means that there is a phase difference of &pgr;, 3&pgr;, 5&pgr;, etc., that is, an odd-numbered multiple of &pgr;, between the two phases at this point. The entropy-wave-induced pressure fluctuations cannot in general be selected in phase opposition to the acoustic pressure fluctuations at all frequencies. According to the invention, the entropy-wave-induced pressure fluctuations are then selected in phase opposition to the acoustic pressure fluctuations at such a frequency &ohgr; at which the combustion chamber tends to produce considerable pressure fluctuations on account of its geometry and its mechanical properties. In this case, the most frequently occurring forms of acoustic pressure fluctuations are the acoustic natural modes.
This tuning in phase opposition is preferably achieved by a corresponding selection of the length of the combustion chamber and/or the length of the mixing section. Setting via the mass flow in the mixing device, for instance by a change in the inlet-vane row setting of the compressor, may also be advantageous. Furthermore, the mass flow in the combustion chamber or the hot-gas temperature may also be suitably selected in an advantageous manner.
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patent: 4122674 (1978-10-01), Anderson et al.
patent: 4199295 (1980-04-01), Raffy et al.
patent: 4409787 (1983-10-01), David et al.
patent: 4760695 (1988-08-01), Brown et al.
patent: 5092425 (1992-03-01), Shaw, Jr.
patent: 5428951 (1995-07-01), Wilson et al.
patent: 0576717A1 (1994-01-01), None
Dobbeling Klaus
Paschereit Christian Oliver
Polifke Wolfgang
ABB Search Ltd.
Burns Doane Swecker & Mathis L.L.P.
Thorpe Timothy S.
Torrente David J.
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