Power plants – Combustion products used as motive fluid – Process
Reexamination Certificate
1999-09-09
2002-08-13
Freay, Charles G. (Department: 3746)
Power plants
Combustion products used as motive fluid
Process
C060S725000, C060S737000, C060S760000, C431S115000
Reexamination Certificate
active
06430933
ABSTRACT:
TECHNICAL FIELD
The invention relates to devices and methods for attenuating acoustic and/or thermoacoustic oscillations in combustors, in particular in combustors of gas turbines.
BACKGROUND OF THE INVENTION
Combustors today are designed primarily with an eye toward the lowest possible formation of noxious substances, and thus with the lowest possible discharge of noxious substances during the operation of the combustor. Significant noxious substances formed during combustion are nitrogen oxides which, depending upon the atmospheric altitude in which they are discharged, can cause either a decrease or increase in ozone. Nitrogen oxides (NOx) form at very high temperatures. Such high temperatures occur during combustion with, in particular, a slight excess of air, i.e. a rich combustion. Such conditions exist, for example, in case of an insufficient atomization and gasification of a liquid fuel in the immediate area around fuel droplets. In order to prevent the formation of nitrogen oxides, today's combustors are mostly designed as premixing combustors. In this case, the fuel, which is mostly gaseous in stationary gas turbines, is first mixed in a premixing device with air prior to the actual combustion. The premixing device often consists of one or more burners, such as are described, for example, in publication DE 43 04213 A1. Furthermore, the admixture of secondary air during the combustion process is now either absent or almost absent in modern combustors. The air supplied for combustion therefore flows completely or almost completely through one or more burners into the combustion chamber at its inlet. This causes a highly homogeneous gas/air mixture to form in the combustion chamber. Thus, a local fuel/air mixture that is too rich can be substantially prevented. As a result, the nitrogen oxide formation can be reduced.
The construction of such a so-called Low-NO
x
combustor differs from standard combustors in particular in the air supply. As was already mentioned, no secondary air, or almost none, is mixed in with the internal flow of the combustion chamber downstream from the combustion chamber inlet. In standard combustors, secondary air is supplied via bores in the combustion chamber wall, in particular for cooling the wall housing of the internal combustion chamber flow. The secondary air flowing into the combustion chamber also resulted in a stabilization of the combustion flow. In addition to an aerodynamic stabilization of the flame, the inflowing secondary air also resulted in a strong acoustic attenuation inside the combustor. Wall pressure fluctuations in the combustor undergo a particularly strong attenuation due to the incoming secondary air flow, particularly if the secondary air mass flux is large and the entry speed is low. Because of this high acoustic pressure level, the combustor in return had a high attenuation capacity with respect to the acoustic and/or thermoacoustic oscillations of the combustor which were attenuated by dissipation. The absence of a secondary air supply in the combustion flow in modern combustors, in contrast, has led to a low acoustic attenuation of the combustors. Acoustic and/or thermoacoustic oscillations occur in combustors as a result of different causes. Inhomogeneous temperature distributions in the combustion flow when passing through the turbine result, for example, in inhomogeneous pressure and therefore thermoacoustic oscillations because of a spatially or temporarily inhomogeneous enthalpy conversion. These oscillations, in principle, cannot be prevented. In the presence of a too low attenuation and in relation to the acoustic behavior of the combustor, for example of the natural frequencies, these oscillations may, however, result in undesired high pressure amplitudes. In addition to a high mechanical strain of the combustor due to the pressure change amplitudes, this results in increased emissions of noxious substances as a result of inhomogeneous combustion and, in the extreme case, in an extinguishing of the flame.
To attenuate such acoustic and/or thermoacoustic oscillations, Helmholtz resonators, as described in the publication by J. J. Keller and E. Zauner, “On The Use Of Helmholtz Resonators As Sound Attenuators”, Zangew Math Phys 46, 1995, p. 297-327, were used in the past. These Helmholtz resonators are hereby connected at least on the inlet side to the combustion chamber. But Helmholtz resonators function only in a narrow frequency band around a base frequency. This does not therefore provide any broad-band attenuation of different oscillation frequencies.
SUMMARY OF THE INVENTION
The invention therefore is based on the objective of effectively attenuating acoustic and/or thermoacoustic oscillations in a combustor of a turbo machine, in particular a gas turbine, over the largest possible frequency range.
According to the invention this objective is realized in that the combustor has at least one fluid supply device and one combustion chamber, and that the combustion chamber furthermore has at least one recirculation opening for attenuating acoustic and/or thermoacoustic oscillations. The recirculation opening provides a local pressure compensation for acoustic and/or thermoacoustic oscillations, resulting in a destructive interference of acoustic waves and their reflections. Depending on the pressure conditions in front of and behind the recirculation opening, an inflow or outflow of fluid through the recirculation opening occurs with acoustic and/or thermoacoustic oscillations. Naturally, a perfect pressure compensation would require that the flow speed would just disappear. It is useful that the recirculation opening merges into the fluid inflow to the combustion chamber, i.e. it flows in a useful manner into the fluid supply device, But the recirculation opening also may additionally merge with another volume. If it merges with the fluid inflow, the fluid flowing from the combustion chamber is transported along with the fluid flowing into the combustion chamber. This results in a reentry of the flow into the combustor, and thus in a recirculation of the fluid flowing out of the combustion chamber. But given the appropriate pressure conditions, the fluid from the fluid infeed can also flow through the recirculation opening into the combustion chamber. Without restricting either of the two possible flow directions through the recirculation opening, however, as a rule, the present invention is concerned primarily with the outflow of fluid from the combustion chamber. With a suitable, preferable design of the combustor, a useful, primarily very small outflow of fluid through the recirculation opening from the combustion chamber occurs. Furthermore, while not limiting the general application, only the recirculation of the fluid will be considered for reasons of simplification. It was found that acoustic and/or thermoacoustic oscillations of the combustor are attenuated in a sustained manner as a result of the pressure compensation near the recirculation openings.
At least part of the fluid supply device preferably extends so as to immediately adjoin the outside of the combustion chamber wall. Along with the supply of a fluid, in most cases air, to the combustion chamber of the combustor, this arrangement of the fluid supply device causes the combustion chamber wall on the outside of the combustion chamber to be cooled convectively. The fluid in the fluid supply device in this case therefore flows in reverse direction to the flow in the combustion chamber. It is useful that the fluid supply device merges into an antechamber, and from there into the combustion chamber. It is hereby desired that the most homogeneous flow status of the fluid that is possible develops in this antechamber. The flow status of the fluid relates to the static pressure, temperature, and flow speed of the fluid. An inhomogeneous flow status would lead to an inhomogeneous inflow into the combustion chamber of the combustor, and finally to an inhomogeneous combustion occurring in the combustion chamber. A more simple version of the
Alstom
Burns Doane Swecker & Mathis L.L.P.
Freay Charles G.
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