DE19849300A1 - Acoustic energy reduction method for gas turbine plant - Google Patents

Abstract

The method involves providing acoustic isolation between adjacent combustion sources within a gas turbine plant by increasing their effective acoustic spacing, or providing destructive acoustic coupling between the adjacent combustion sources, so that the overall acoustic energy level is reduced. The spacing between the combustion sources may be determined in dependence on the wavelength of the lowest frequency of the acoustic energy, to prevent acoustic resonance between them.

Description

The invention relates to a method and arrangements for Reduction of the acoustic energy of neighboring fire sources in incinerators.

In incinerators, such as. B. boiler, stoves or gas turbine NEN, instabilities occur during the combustion process that in connection with low-frequency vibrations to mechani vibrations in the combustion chamber. The mechanical Cohesion of the components is endangered and the service life the plant is drastically reduced. Are in gas turbines these low frequency phenomena as "humming", "rumbling" and Known "buzz".

These phenomena are caused by thermoacoustic interactions depending on the feedback loop between the Combustion-induced noise from the combustion source (Burner) and the acoustics of the combustion chamber let write.

The pressure waves in the combustion chamber change important ones Flow characteristics, for example vortex formation and vortex interaction, and details of combustion, for example the Flame extension, periodic extinction / re-ignition ignite, which in turn causes the pressure / entropy fluctuations be changed, which is generated during (turbulence) combustion become. The reflection of those generated by the combustion Pressure fluctuations completed by the chamber acoustics the feedback, which is released by the combustion put energy that delivers system performance.

Because the feedback loop is created by the overall system and the coupling with the chamber acoustics is a crucial one  Role in feedback can be played in different ways Chambers of installed burners have very different behavior show at least. Therefore the burning sources and their Environment are considered together. Because the efficiency and the energy density of combustion systems will continue in the future thermoacoustic considerations are soaking aspect in the design of combustion systems.

The acoustic equations can be used to show that if two sound sources of similar frequency close together be classified, their total sound energy is the sum of one cell energies. However, if the sound sources have opposite phases, is due to the interaction the total energy is reduced.

The feedback of the combustion instability can be represented using the Rayleigh criterion (Lord Rayleigh: "On the Theory of Sound", sect.322g-j, 1878) as a formula:

If R is positive, the pressure fluctuations p and the heat release fluctuations q 'are in phase, there is positive feedback, and the system is unstable. This result can be directly related to the sound energy balance (Candel SM: "Combustion Instabilities Coupled by Pressure Waves and Their Active Control", 24 ^th Symposium (International) on Combustion, The Combustion Institute, 1277-1296, 1992; Pierce AD: " Acoustics: An Introduction to its Physical Principles and Applications ", Acoustical Society of America, 1989):

The heat release fluctuations correspond to the quantity h _k w _k (where h _k and w _{k represent} the specific enthalpy or the formation rate) in the source expression S of the Schallen energy. The dissipation D results from the viscosity and the heat conduction, whereby the absorption on surfaces, which becomes visible in the boundary conditions, also contributes to the dissipation of the sound energy.

Starting from the equation above, but with neglect dissipation, various experts have analyti cal models of thermoacoustic behavior for relative developed simple geometries, for example for a ram nozzle tilting burner (Abouseif GE et al. "Ramjet Rumble: The Low- Frequency Instability Mechanism in Coaxial Dump Combustors ", Comb.Sci.Tech. 36: 83-108, 1984; Award E et al. "On the Existence and Stabilityf Limit Cycles for Longitudinal Acoustic Modes in a Combustion Chamber ", Comb.Sci.Tech. 46: 195-222, 1986; Yang V et al. "Analysis of Low Frequency Combustion Instabilities in a Laboratory Ramjet Combustor ", Comb.Sci.Tech. 45: 1-25, 1986).

For more complex combustion systems that have multiple burners contain sources or have more complicated geometries, the analytical expansion of the models is not yet in sight bar. The effort to adapt complicated systems in practice on these models it seems more effective  Proposals to change the generation and dissipation of To develop sound energy than to continue trying that To describe processes theoretically.

Sound transmission mechanisms can be changed Geometry and / or material to change the Hall properties of the chamber, d. H. Reduction of the echo effect, or by changing the flow geometry to the disturbance the homogeneity and suppression of the spread of Pressure fluctuations, for example due to soundproofing on built in exhaust systems, improve. The reinforcement of the In homogeneity directly in the combustion chamber, to suppress the Spread of pressure fluctuations, comes into conflict with the for the maintenance of favorable combustion properties required good homogeneity. Radical changes in the Chamber geometries are complex, unsafe and are caused by Combustion requirements, for example low NOx content, as well as by the flow requirements, for example Pressure drop, limited.

The combustion acts as a volume expansion, with the under different heat releases sound. The combustion corresponds to an independent sound source, so that in the sem sense instead of "source of fire" the term "Sound source" can be used.

The object of the invention is a generic method and to develop arrangements with which the disadvantages of the State of the art by taking advantage of existing ones Interactions between adjacent burners during the combustion process, especially in gas turbines, ver avoided and a reliable reduction in acoustic Energy from neighboring sound sources and thus an effective one Noise reduction is guaranteed for selected frequencies the.  

The object is achieved with the features of claims 1, 6 and 9 solved.

The neighboring burners acting as sound sources Incinerators are acoustically decoupled by the Burner designed and arranged to each other that the effective acoustic distance between the sound sources increases and the total sound output of the sound sources ver is reduced or the neighboring burners become audible destructively coupled by designing the burners that the acoustic interaction between neighboring Burners (sound sources) to weaken the entire Sound energy leads.

According to a preferred embodiment, burners with un different acoustic properties staggered felt arranged so that the effective acoustic distance between the sound sources increases and the whole Sound emission of the sound sources is reduced.

The distance between the adjacent burners is preferred in each case chosen according to the lowest relevant frequency, at least at a distance of one sixth of that for the Sound intensity responsible wavelength.

In a further preferred embodiment, the be neighboring burners operated out of phase with each other, in which the burners are arranged at such a distance from each other be net that a phase coupling between the sound sources len is achieved and their interaction the entire scarf lenergy weakens. This "phase link mechanism" can be realized with active or with semi-passive means become.

Further expedient embodiments of the method and Arrangements are described in the subclaims.

The invention is described below with reference to a drawing in two Exemplary embodiments in gas turbines explained in more detail. It demonstrate:

Fig. 1 shows the graphical representation of acoustically decoupled burners by separation of the burner,

Fig. 2 shows the graphical representation of acoustically decoupled burners by staggered arrangement of the burners and

Fig. 3 shows the graphical representation of phase-shifted burners.

The interaction between sound sources is in theory the acoustics as follows:

"If the sound sources have the same frequency and arranged close to each other, they come in together Interaction, and the two sources can be more sound generate energy as if they were far apart would be. "(Skudrzyk, E .:" The Foundations of Acoustics: Basic Mathematics and Basic Acoustics ", Chapter 18.17, Springer- Verlag, 1971).

This interaction between sound sources can be explained by the fact that each source must act against its own sound pressure (reaction of the medium to its movement) and against the sound pressure generated by the nearby source. Since the basic sound field equations are linear, the resulting pressure equals a superposition of the pressures. However, since the total sound energy is obtained by integrating the square of the pressure over a spherical surface with a sufficiently large radius, the sound energy is not a mere superposition, but also includes the interaction between the two sound sources:

To determine the extent of the interaction between independent sound sources, the wave equation for the speed potential Φ must be set up in spherical coordinates:

where the complex periodic solution is:

where k = ω / c, k = 2π / λ. The reluctance of only the divergent wave results in pressure, speed and acoustic impedance:

The constant Q was strategically chosen so that it corresponds to the volume flow connected to the source at kr <1:

being the extension

was used.

The energy of a sound source results from the product of the surface, the speed and the pressure, which is in phase with the speed:

The acoustic energy for overcoming the pressure of another force source Q _j = a _ij -Q _i at a distance dis and a phase ϕij is given by:

The total energy associated with a source is given by:

where by definition a _ii = 1 and ϕ _ii = 0. The own energy of a source (kd _ii <1) is defined by:

and that with the interaction of another source of the same strength and phase, which is at a distance d, is defined by:

Thus, the total energy from two closely spaced sources is given by:

the sign is positive for in-phase sources and negative for in-phase sources. If the sources are arranged close to each other (kd _ii <1) or spatially separated (d <λ / 2it), the following applies:

and

N = 2N ₁₁ [1 ± 1]

The total power of in-phase sources is thus twice as high large as the sum of the individual elements, while the total force is zero for phase-shifted sources.

This interaction is shown graphically in FIG. 1 for different frequencies.

If the frequencies of the sound sources are sufficiently below are different or the sound sources are physically wide are not enough apart, is the interaction between the two Sources as positive as negative as often, so that the be written interaction between sound sources no longer takes place.

If, on the other hand, in the case of adjacent sound sources, both sources are operated with the same frequency but out of phase, the overall interaction is reduced. There are therefore three possible methods for reducing the acoustic energy of adjacent sound sources:

• - spatial separation
• - frequency separation
• - phase shift

The physical (spatial) separation of the sound sources is usually not in combustion plants such as gas turbines possible because u. a. Force density and emission requirements and conflict with the existing design of the firing system. There remain the to reduce the acoustic energy Frequency separation and the phase shift of the sound sources left.

The noises occurring during a combustion process set zen are composed of a variety of frequencies and phases men. Regardless, the linear character of the Aku means mathematical equations that there is an overlay and each individual frequency or each frequency band independently can be sought.  

A first exemplary embodiment of the method according to the invention provides for the acoustic decoupling of the combustion sources arranged adjacent to one another in a combustion system such as a gas turbine, as shown in FIG. 1.

The low frequencies are preferred for the acoustic interactions between neighboring sound sources (burners). The effective acoustic distance to be observed corresponds to the lowest frequency of interest in FIG. 1.

FIG. 2 shows how the staging of the firing sources len a, b, c, whose acoustic interaction is to be minimized to avoid acoustic resonance, can be carried out. The combustion sources a, b, c should have different acoustic properties in the interesting frequency ranges f1, f2.

In practice, a staggered arrangement of different burner types or with differently set burners should be selected. Or the burners are operated in different modes of operation, such as diffusion and premixing. In Fig. 2 is the staggered arrangement of combustion sources a, b, c, for which the effective acoustic neighbor is farther away for a certain frequency f1, f2, the interaction between the sources a, b, c and thus the to reduce acoustic energy. A sixth of the wavelength (corresponds to kd <1) was used as the criterion for the separation distance between sources a, b, c that are sufficiently apart. The combustion sources (sound sources) a, b, c in FIG. 2 each have different acoustic characteristics for the frequencies of interest. Sound sources of the same name also have the same acoustic properties. This means, for example, in FIG. 2 that the two sources a for the frequency f1 have a separation distance of λ / 3 from one another, while the separation distance between the sources a and b is λ / 6.

A second embodiment of the method according to the invention is the destructive acoustic coupling between sound sources len in an annular combustion chamber of a gas turbine.

In FIG. 3, the phase-shifted operation of adjacent sound sources a, b shown, an annular combustion chamber of diameter D having an even number of burners (sound sources) a, b is adopted. The sound sources len a, b are to be considered in pairs.

The acoustic interaction between neighboring burners can be used to the effect that they are out of phase work. When operating neighboring brenes out of phase the interaction between them lowers the entire acu energy under the sum of the energies of the individual Swell. In the case of two nearby sound sources (kd <1), which are operated out of phase by 180 °, approaches the total sound emission is zero. At further from distant sources the coupling is less strong, but the combined sound output of the sources remains wide less than if the sources were completely isolated.

The mechanism to create a phase-shifted kopp between adjacent burners becomes clear when a external modulation can be imprinted, similar to it at an active control system. However, it is also possible to use a passive agent, for example Acoustic elements between the burner inlets for generation to arrange the phase shift. By disturbing one sensitive parameters, as it is based on pilot gas disturbances in Premix burners has been demonstrated (Seume JR et al: "Application of Active Combustion Instability Control to a Heavy Duty Gas Turbine ", ASME-97-AA-119, 1997), it is possible Lich, the combustion process a certain frequency / phase to impress.  

The azimuth resonance wavelengths of such a chamber are determined by λ πD / n. The acoustic separation between N burners arranged at equal distances from one another kd = 2πn / N. If kd <1 as a criterion for sufficient close arranged sound sources are used, would reduce the effectiveness of sound removal at azimuth decrease frequencies greater than n <N / 2π.

There is a phase-shifted operation of adjacent burners the possibility to excite the mode according to λ = 2πD / N. Because it however, because of influencing the more important flow features a frequency limit, is higher at this Mode is not the same degree of thermoacoustic change effects occur as with the original lower Frequency. In addition, the acoustic properties of the Brenn chamber, namely the spread of the frequencies across running resonance frequency, which hinder plane propagation (Skudryk 1971; Pierce 1989).

A system in which the neighboring burners are out of phase is operated with one frequency or one frequency band to operate, for which noise reduction is required is. The purpose of operating such a system is in effectively eliminating combustion noise for out selected frequencies. That is, there are burner constellations too create that work silently at certain frequencies, or the supply of sound at a frequency is temporarily too eliminate to time for the dissipation present in the system to re-stabilize the system.

The invention is not limited to those shown here Embodiments. Rather, it is possible through Kombina tion and modification of the means and features mentioned Realize other design variants without the scope of Leaving invention.

Claims (10)

1. A method for reducing the acoustic energy adjacent hard combustion sources in combustion plants, in particular in gas turbines, characterized in that the adjacent combustion sources acting as sound sources of the combustion system are acoustically decoupled by designing the combustion sources and arranging them to one another so that the effective acoustic distance between the sound sources is increased and the total sound output of the sound sources is reduced or that the adjacent sound sources are acoustically destructively coupled by designing the combustion sources so that the acoustic interaction between adjacent combustion sources leads to a weakening of the total sound energy. 2. The method according to claim 1, characterized, that the neighboring burning sources in the incinerator at a maximum distance of one sixth of that for the Sound energy responsible wavelength of the lowest relevant frequency can be arranged away from each other, an acoustic resonance due to the interaction between the neighboring Brenn acting as sound sources to avoid sources. 3. The method according to claim 2, characterized, that burning sources with different acoustic properties like fire sources of different types, under differently set burning sources, burning sources in under operated differently staggered be so that the effective acoustic distance between the Sound sources enlarged and the total sound energy of the Burn sources can be reduced.   4. The method according to claim 1, characterized, that the neighboring burning sources in the incinerator be operated out of phase by the burning sources in such a distance from each other that a Phase coupling between the sound sources is achieved leads to the reduction of sound energy. 5. The method according to claim 4, characterized, that burning sources of different types in groups, ins special pairs, over the circumference of an annular ver incinerator arranged at the same distance from each other become. 6. Arrangement to reduce the acoustic energy adj Combustible sources in combustion plants, especially in Gas turbines, characterized, that the combustion sources in the incinerator in an Ab stood facing each other, which is an acoustic ent Coupling of the neighboring burning sources guaranteed. 7. Arrangement according to claim 6, characterized, that the combustion sources in the incinerator in each a maximum distance of one sixth that for the sound energy responsible wavelength of lowest relevan th frequency are arranged from each other. 8. Arrangement according to claim 6, characterized, that burning sources with different acoustic properties shafts are staggered to each other. 9. Arrangement for reducing the acoustic energy adj burnt sources in incinerators,  especially in gas turbines, characterized, that working in opposite phase sources at a distance are arranged one another, which is a destructive acoustic Coupling of the neighboring burning sources guaranteed. 10. Arrangement according to claim 9, characterized, that burning sources of different types in groups, ins especially in pairs, at the same distance from each other above the Scope of an annular incinerator arranged are.