Burner

Details Burner Burner

1999-11-05

2001-03-06

Yeung, James C. (Department: 3743)

Combustion

Flame holder having protective flame enclosing or flame...

C431S114000, C431S185000, C431S351000

Reexamination Certificate

active

06196835

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a burner for operating an aggregate for generating a hot gas.
BACKGROUND OF THE INVENTION
Thermoacoustic oscillations pose a risk for any type of combustion applications. They cause pressure fluctuations with a high amplitude, restrict the operating range, and may increase the emissions associated with combustion. These problems occur in particular in combustion systems with low acoustic attenuation, as often represented by modern gas turbines.
In standard combustors, the coolant air flowing into the combustor acts in a sound-absorbing manner and in this way contributes in the attenuation of thermoacoustic oscillations. In order to achieve low NO
x
emissions, an increasing share of the air is conducted through the burners themselves in modern gas turbines, and the coolant air stream is reduced. The sound absorption associated with this causes the initially mentioned problems to occur more often in modern combustors.
One method of absorbing the sound consists of connecting Helmholtz dampers inside the combustor hood or near the coolant air supply. But given the tight space conditions typically found with modern combustors built in a compact manner, the arrangement of such dampers may be difficult, however, and may be associated with high constructive expenditure.
Another possibility is the control of thermoacoustic oscillations with active acoustic excitation. The shear layer that develops near the burner is hereby stimulated acoustically. With a suitable phase position between the thermoacoustic oscillations and the excitation, an absorption of the combustor oscillations can be achieved. Such a solution requires the attachment of additional elements near the combustor, however.
SUMMARY OF THE INVENTION
The invention is based on the objective of creating a device that permits an effective suppression of thermoacoustic oscillations and is associated with the lowest possible constructive expenditure.
Coherent structures play a critical role in mixing processes between air and fuel. The dynamics of these structures as related to space and time influence the combustion and heat release. The invention is based on the idea of interfering with the formation of coherent turbulence structures in order to reduce the periodic heat release fluctuation and therefore the amplitude of the thermoacoustic oscillations.
A burner according to the invention for operating an aggregate for generating a hot gas consists essentially of at least two hollow partial bodies stacked inside each other in the direction of the flow, the center axes of said hollow bodies extending offset to each other in such a way that at the burner slits, adjoining walls of the partial bodies form tangential air inlet channels for the inflow of combustion air into an inside chamber defined by the partial bodies. To add an axial turbulence force to the flow, the burner is provided according to the invention with a number of baffles projecting into the flow.
In a preferred embodiment, the baffles are arranged at the burner outlet. It was also found to be particularly advantageous if the burners are arranged both at the burner outlet and along the burner slits.
The baffles can have any conceivable shape. They may be either flat or a have a distinctly three-dimensional shape. They are advantageously constructed, for example, in a saw tooth structure, sinus shape or rectangular shape. It is particularly advantageous if the baffles are designed in the shape of turbulence generators. “Turbulence generator” hereby means a device that adds the axial turbulence force into a flow without generating a recirculation zone in a wake area.
The flow instabilities in the combustor usually have a dominant mode. The dampening of this dominant mode is necessary for suppressing thermoacoustic oscillations. The wavelength of the dominant mode of the instability is obtained from its frequency ƒ and the convection speed u
c
via &lgr;=u
c
/ƒ. The relevant frequencies are between several 10 Hz and several kHz. The convection speed depends on the burner and typically is several 10 m/s, for example 30 m/s.
It was now found that the dominant mode is suppressed especially effectively if the distances s of adjoining baffle elements are smaller or about equal to half the wavelength of the dominant mode. This is true for the distance of baffles located along the burner outlet and also for elements located along the burner slits.
The invented introduction of turbulence force in axial direction in order to interfere with coherent turbulence structures by means of baffles projecting into the flow not only can be used for the double-cone burner described here but also with other types of burners.


REFERENCES:
patent: 5129226 (1992-07-01), Bigelow et al.
patent: 5433596 (1995-07-01), Dobbeling et al.
patent: 5487274 (1996-01-01), Gulati et al.
patent: 5513982 (1996-05-01), Athaus
patent: 5676538 (1997-10-01), Lovett
patent: 5784889 (1998-07-01), Joos et al.
patent: 2262256 (1975-09-01), None
patent: WO98/11383 (1998-03-01), None

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