Acoustics – Sound-modifying means – Muffler – fluid conducting type
Reexamination Certificate
2001-05-25
2003-10-21
Nappi, Robert E. (Department: 2837)
Acoustics
Sound-modifying means
Muffler, fluid conducting type
C181S219000, C181S241000, C181S277000, C381S071500
Reexamination Certificate
active
06634457
ABSTRACT:
This application claims priority under 35 U.S.C. §§119 and/or 365 to Appln. No. 100 26 121.3 filed in Germany on May 26, 2000; the entire content of which is hereby incorporated by reference.
FIELD OF APPLICATION
The present invention relates to an apparatus for damping acoustic vibrations in a combustor, as well as a combustor arrangement, in particular of a gas or steam turbine, that contains the apparatus.
The main field of application of the present invention is the field of industrial gas turbines. Worldwide, increasingly higher requirements with respect to readiness, life span, and waste gas quality are placed on industrial gas turbines, especially when used in power plants. An increasing consciousness of environmental protection and environmental compatibility requires compliance with the lowest possible values for noxious emissions.
Low emissions can only be achieved economically in industrial gas turbines by using premix burners. However, in closed combustors, because of the creation of coherent structures and resulting variable release of heat, this type of combustion tends to generate thermoacoustic vibrations in the combustor. These thermoacoustic vibrations do not only adversely affect the combustion quality, but also may drastically reduce the life span of the highly stressed components.
STATE OF THE ART
The principle of the so-called Helmholtz resonator has been used for a long time to dampen such thermoacoustic vibrations. This principle is explained in more detail below in reference to FIG.
1
. The figure shows the principal structure of a Helmholtz resonator
4
comprising a resonance volume
3
and a connecting channel
2
to chamber
1
, in which the thermoacoustic vibrations are occurring. Such an apparatus can be seen as analogous to a mechanical spring/mass system. The volume V of the Helmholtz resonator
4
hereby acts as a spring, and the gas present in the connecting channel
2
acts as the mass. The resonance frequency f
0
of the system can be calculated using the volume dimensions:
whereby:
V=volume of Helmholtz resonator
4
R=radius of connecting channel
2
l=length of connecting channel
2
S=area of opening through which stimulation occurs
At this resonance frequency f
0
, a Helmholtz resonator behaves acoustically as an opening of infinite size, i.e., it prevents the creation of a standing wave at this frequency.
This technique of damping thermoacoustic vibrations with a Helmholtz resonator is also already used to dampen the vibrations in combustors of gas or steam turbines. However, when used in gas or steam turbines, the problem occurs that the frequency to be damped is not determined by intermittent combustion but by fulfilling the Rayleigh criterion in the combustor and by the acoustic response of the surrounding system comprising the supply line, burner, combustor, and acoustic terminus.
In these systems, the frequency to be damped therefore cannot be predetermined with the required accuracy by using the mathematical tools currently available. But this predetermination is the precondition for being able to take into consideration the exact dimensions of the resonance volume when building the gas turbine. Furthermore, the acoustical behavior of the system and thus the frequencies of the vibrations to be damped may critically change when the operating point is changed, so that it may become necessary to use additional resonators that are adapted to additional frequencies.
Such an arrangement with several Helmholtz resonators is described, for example, in DE 33 24 805 A1. This document concerns an apparatus for preventing pressure vibrations in combustors, in which apparatus several Helmholtz resonators with different resonance volumes are arranged along the gas conduit path towards the burner. The different resonator volumes in this system are able to dampen vibrations with different frequencies. However, here again the optimal sizing of the various Helmholtz resonators requires knowledge regarding the frequencies occurring during the operation of the system, where again exact frequencies cannot be provided when the system is being built. Furthermore, the arrangement of several Helmholtz resonators is disadvantageous due to the additional space needed for this purpose.
DE 196 40 980 A1 describes another known apparatus for damping thermoacoustic vibrations in a combustor. In this apparatus, the side wall of the resonance volume of the Helmholtz resonator is constructed as a mechanical spring. An additional mass has been secured to the wall of the front face of the resonance volume, said wall vibrating due to the action of the spring. This arrangement influences the virtual volume of the Helmholtz resonator and achieves greater damping power. By changing the mechanical mass at the resonator, a fine-tuning to the resonance frequency can be performed at a later time. This also requires a subsequent modification of the construction of the gas turbine system.
In the past, Helmholtz resonators were also used for damping vibrations in the field of exhaust gas systems of combustion engines. From this field, the use of adjustable resonators for changing the resonance frequencies is known. Even during World War I, for example, the two-cycle diesel engines for the Maybach company's Zeppelin dirigible were adjusted to the necessary operating point with adjustable resonators located in the exhaust pipe. For this purpose, mechanical gears moved cylinders inside each other and in this way changed the resonance volume. In said exhaust systems, this technology was found to be practical because of the good accessibility of these systems and the relatively low pressure and temperature ratio present there. But such a solution is completely unsuitable for use in the pressure range found in modern industrial gas turbines. The passing of a mechanical gear through the pressure container of a gas turbine would inevitably cause leaks and result in intolerable losses. The temperature influences associated with industrial gas turbines also could only be compensated for with a very complex gear.
The present invention describes an apparatus for damping thermoacoustic vibrations as well as a combustor arrangement comprising this apparatus that enables continuous adaptation to the frequencies of the vibrations to be damped even under high pressure conditions as occur, for example, in gas turbines.
DESCRIPTION OF THE INVENTION
The apparatus includes a Helmholtz resonator with a connecting channel that is connected to the combustor, for example, the combustor of a gas turbine. In contrast to the known damping devices, the present apparatus is provided with a hollow body, the volume of which can be changed by adding or draining a fluid via a supply line, and which is arranged either within the Helmholtz resonator or is located adjacent to it in such a way that the resonance volume of the Helmholtz resonator changes when the volume of the hollow body changes.
When the adjustable-volume hollow body is located in the Helmholtz resonator, the resonance volume decreases when the hollow body is inflated via the supply line, for example with a gas. Correspondingly, the resonance volume of the Helmholtz resonator increases, when a certain amount of the gas is drained from the hollow body. The change in resonance volume in the known manner causes a change in the resonance frequency.
In this way, the resonance frequency of the Helmholtz resonator can be adapted at any time to the thermoacoustic vibration frequencies occurring in the chamber volume by a simple inflation or deflation of the hollow body. For this reason, an exact knowledge of the frequencies occurring during operation is no longer necessary when the system is built. The vibrations can be damped by means of a broad spectrum of individually set frequencies. In practical use, the resonance frequency of the built-in resonators can be changed at any time during the operation of the system in accordance with the current operating point by changing the resonance volume.
As a special advantage, t
Flohr Peter
Paschereit Christian Oliver
Polifke Wolfgang
Weisenstein Wolfgang
Alstom (Switzerland Ltd
Martin Edgardo San
Nappi Robert E.
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