GB2288660A

GB2288660A - Apparatus for damping thermoacoustic vibrations in combustion chamber

Info

Publication number: GB2288660A
Application number: GB9506413A
Authority: GB
Inventors: Melch Fischer
Original assignee: ABB Management AG
Current assignee: ABB Management AG
Priority date: 1994-04-23
Filing date: 1995-03-29
Publication date: 1995-10-25
Also published as: DE4414232A1; GB9506413D0; JPH07293885A

Description

i

DESCRIPTION 2288660

APPARATUS FOR DAMPING THERMOACOUSTIC VIBRATIONS IN A COMBUSTION CHAMBER TECHNICAL FIELD

The present invention relates to the field of combustion technology. It relates to an apparatus f or damping thermoacoustic vibrations in a combustion cham- ber, in particular in the combustion chamber of a gas turbine, comprising a Helmholtz resonator having a resonator space and a connecting tube by means of which the resonator - space is connected to the combustion chamber.

Publication EP-Al-O 577 862, discloses such an apparatus.

PRIOR ART for example,

In combustion chambers, as used in particular in gas turbines, thermoacoustic vibrations are excited by an amplifying interaction between thermal and acoustic disturbances. Undesirably high vibration amplitudes can appear in the process if acoustic natural vibrations of the combustion chamber are excited. The adverse consequences are inadmissably high mechanical loading of the combustion chamber, a rise in the emissions through inhomogeneous -combustion and, in the extreme case. extinguishing of the flame. This problem is intensified in modern combustion chambers, since cooling-air openings in the combustion chamber which would dampen the pressure pulsations are as far as possible dispensed with.

In gas-turbine combustion chamberst depending on the size of the combustion chamber, a narrow-band excitation of high amplitude in the frequency range of 100 to 250 Hz typically occurs. This excitation can be dampened by means of so-called Helmholtz resonators,, the frequency of the Helmholtz resonator having to be accurately coordinated with the frequency of. the combustion-chamber vibration. In connection with an afterburner for a gas turbine, it has already been proposed in the publication mentioned at the beginning to use such a Helmholtz resonator for this purpose.

But experience now shows that, depending on the operating conditions (full load/partial load, ambient temperature. fuel/air ratio, gas or oil operation.. etc.),, the combustion-chamber frequency can vary by up to 20%. On the other hand, the frequency in the Helmholtz resonator is also dependent upon the operating conditions: experimental tests have shown that the Belmholtz frequency can be displaced during rising pulsation amplitudes by up to 19% towards lower values. But it is known that the damping performance is massively reduced by only slight differences between the two frequencies.

SUMMARY OF THE INVENTION

The object of the invention, then, is to specify a damping apparatus having a Helmholtz resonator, by means of which damping apparatus a uniform damping performan e can be achieved even under fluctuating operating conditions.

This object is achieved in an apparatus of the type mentioned at the beginning when the Helmholtz resonator is equipped with first means for controlling the resonator frequency as a function of the frequency of the combustion-chamber vibrations.

The essence of the invention therefore consists in designing the apparatus in such a way that the resonator frequency is appropriately readjusted during a change in the frequency of the vibration to be dampened.

The readjustment of the resonator can be effected in different ways. In a first preferred embodiment of the apparatus according to the invention, the first means comprise second means for controlling the density of the gases located in the connecting tube. This enables the frequency of the resonator to be changed in a simple manner without a (mechanically complicated) change in the resonator volume The control can be realized in an especially simple manner when, in a second preferred embodiment, the second means control the density of the gases located in the connecting tube by changing the temperature.

A first advantageous embodiment of the temperature control is characterized in that the second means comprise an electric heating element by which the gases located in the resonator space can be heated.

A second advantageous embodiment of the temperature control is distinguished by the fact that the second means comprise an electric heating element by which the gases located in the connecting tube can be heated.

A third advantageous embodiment of the temperature control is characterized in that the second means comprise a scavenging-air feed line which leads.

into the resonator space and by means of which scavenging air can be directed through the resonator space. The temperature in the resonator space can be specifically lowered relative to the combustion chamber by the (relatively cold) scavenging air in order to stabilize frequency differences.

A further embodiment of the apparatus according to the invention is distinguished by the fact that the second means comprise a gas feed line which leads into the resonator space and by means of which gases of different density can be directed alternatively into the resonator space. In this way, the resonance frequency can be changed by changing the average density in a gas mixture of different composition.

Further embodiments follow from the dependent claims.

BRIEF DESCRIPTION OF THE FIGURES The invention is to be described in more detail below with reference to exemplary embodiments in connection with the drawing, in which:

Fig. 1 shows in schematic representation afirst exemplary embodiment of an apparatus accor ding to the invention having a heatable resonator space; Fig. 2 shows in schematic representation a second exemplary embodiment of an apparatus accor ding to the invention having a heatable connecting tube; shows in schematic representation a third exemplary embodiment of an apparatus accor ding to the invention having a, device for feeding auxiliary gases of a different density; shows in schematic representation a fourth exemplary embodiment of an apparatus accor ding to the invention having a device for feeding scavenging air; and.

shows in schematic representation apparatus according to Fig. 1 in combination with a complete control system is Fig. 3 Fig. 4 Fig. 5 0.

1 The essence of the invention is to provide a Helmholtz resonator which can be controlled with simple means and the frequency of which can be accurately regulated to the combiustion-chamber vibration frequency in all operating states. The frequency of the Helmholtz resonator is described by the following generalized ecuation:

S, 0 =Cl, M-WplIV where: Helmholtz-resonator frequency ell_ - sound velocity in the resonator = crosssectional area of the connecting tube 41 1 = length of the connecting tube pjjj = air density in the connecting tube v = resonator volume p,, = air density in the resonator The Helmholtz frequency w can now be - detuned in various ways in accordance with the combustionchamber frequency. A first exemplary embodiment for an apparatus having controllable resonator frequency is schematically shown in Fig. 1. A Helmholtz resonator 1 is shown which comprises a resonator space 2 enclosed by a resonator wall 13 and having the resonator volume V. The resonator space 2 is connected via a connecting tube 3 (length 1; cross-sectional area s; see Fig. 3) to a combustion chamber 7 which in turn is defined by a combustion-chamber wall 8.

The detuning of the resonator is effected here by increasing the temperature in the resonator space 2 by means of a controlled (electric) heating element 5 which is either attached directly in the gas volume or heats up the gas volume via the resonator wall 13. An increase in the temperature by 10% from, for example. 6000K to 660K reduces the density pIII by 10% and thus (according to the abovementioned formula) increases the resonator frequency by 5%.

It is advantageous in this case to insulate thermally at least the resonator space 2 by means of thermal insulation 4. Furthermore, it is advantageous to blow shielding air 9 from outside (e.g. f rom the plenum 10 of a gas turbine) into the connecting tube 3 to prevent the ingress of hot gases from the combustion chamber 7 into the resonator space 2. The requisite heating output which has to be applied to the terminals 6a,b of the heating element 5 can thereby be reduced to a minimum.

A second possibility of detuning the resonator by temperature change is shown in the exemplary embodiment in Fig. 2. Here, the gas in the connecting tube 3 to the combustion chamber 7 is heated with a heating element 12 and thus the gas density is k A --- reduced directly in the connecting tube 3. The heating element 12 can be, for example, a heater winding wound around the connecting tube 3. Here, tooi the requisite heating output can be minimized by the connecting tube 3 being surrounded in a primary manner with thermal insulation 11 and by the resonator space 2 also being surrounded in a secondary manner with thermal insulation 4. The feeding of shielding air 9 can also be provided as in Fig. 1.

A third possibility of detuning the resonator by temperature change is reproduced in the exemplary embodiment in Fig. 4. Via a scavenging-air feed line 26 equipped with a control valve 19. (cool) scavenging air is sent here in a controlled manner through the Belmholtz resonator 1 and thus the temperature in the resonator is influenced. At a small scavenging-air quantity,, the air is heated in the resonator space 2 from the hot-gas side (i.e. from the combustion chamber 7). At a high scavenging-air quantity, the air in the resonator space 2 cools down accordingly. It is also advantageous in this case if the Belmholtz resonator 1 together with connecting tube 3 is surrounded with thermal insulation 4 and 11 respectively. The air in the resonator space 2 can thereby be heated to a higher temperature if this is necessary. Furthermore,, it is advantageous if the connecting tube -3 projects with a tube piece 20 slightly into the combustion chamber 7 so that the thermal coupling of the resonator to the combustion chamber 7 is improved. Thit assists the heating of the air in the resonator space.

A further possibility of detuning the resonator is shown in the exemplary embodiment in Fig. 3. The frequency is detuned here by adding a gas of higher density (e.g. C02) or lower density (e.g. helium) into the resonator space 2. Here. too,, the sound velocity is varied via a density change; but the density change is not based on the temperature change in a gas but on the change in the mixture ratio of gases of various density. For this purpose, at least - 7 one gas feed line 14 is attached to the resonator space 2, through which gas f eed line 14 the auxiliary gases can be f ed. If two branches 15, 16 having one control valve 17, 18 each are provided at the gas feed line 14,' via which branches 15, 16 a gas of higher and lower density can be added simultaneously in a controlled manner, frequency changes at the resonator in both directions can easily be achieved.

The phase angle (the phase difference) 10 between the pressure vibration in the combustion chamber 7 and that in the resonator space 2 is the most suitable control variable for the frequency control. in which case:

= -900 for fBY, = fRE < -900 for fBY,. > fRE > -900 for fBY, < fRE where fBK = combustion-chamber frequency, fRE = resona tor frequency, and = phase angle.

The phase angle reacts very sensitively to frequency differences and is therefore the most suitable control variable for the heating output or scavenging-air or auxiliary-gas supply. A corresponding complete control system for an arrangement from Fig. 1 is reproduced in Fig. 5. To record the pressure vibrations, in' each case at least one pressure sensor 21a and 21b respectively is arranged at a suitable point in the combustion chamber 7 and in the resonator space 2. The measuring signals from the pressure sensors 21a,b are processed in downstream measuring transducers 22, 23 and transmitted to the two inputs of a phase comparator 24 which derives a control signal from the phase difference and delivers it to a following activating unit 25. The activating unit 25 contains a power part which controls the heating output in the heating element 5. In the examples from Figs. 2 to 4, the heating element 12 and the control valves 17 to 19 respectively are accordingly connected to. the activating unit 25.

h Aw- The invention results overall in a simple and functionally reliable apparatus for damping the thermoacoustic vibrations in combustion chambers even under fluctuating operating conditions.

Claims

PATENT CLAIMS

1. Apparatus for damping thermoacoustic vibrations in a combustion chamber in particular in the combustion chamber of a gas turbine, comprising a Belmholtz resonator having a resonator space and a connecting tube by means of which the resonator space is connected to thecombustion chamber characterized in that the Helmholtz resonator is equipped with first means for controlling the resonator frequency as a function of the frequency of the combustion-chamber vibrations.

2. Apparatus according to Claim 1, characterized in that the first means comprise second means for controlling the density of the gases located in the connecting tube 0 3. Apparatus according to Claim 2# characterized in that the second means control the density of the gases located in the connecting tube by changing the temperature.

4. Apparatus according to Claim 3,, characterized in that the second means comprise an electric heating element by which the gases located in the rdsonator space can be heated.

5. Apparatus according to Claim 3, characterized in that the second means comprise a scavenging-air feed line which leads into the resonator space and by means of which scavenging air can be directed through the resonator space 0 6. Apparatus a6cording to Claim 5. characterized in that a control valve is provided for controlling the scavenging-air feed in the scavenging air feed line.

Apparatui according to either of Claims 5 or 6, characterized in that, to improve the thermal coupling of the resonator space to the combustion chamber the connecting tube projects with a tube piece into the combustion chamber h Apparatus according to Claim 3. characterized in that the second means comprise an electric heating element by which the gases located in the connecting tube can be heated.

9. Apparatus according to one of Claims. 3 to 8, characterized in that the resonator space and/or the connecting tube is surrounded x,ith thermal insulation -- 0 1.0. Apparatus according to Claim 2, characterized in that the second means comprise a gas feed line which leads into the resonator space and by means of which gases of different density can be directed alternatively into the resonator space 11. Apparatus according to either of Claims 4 and 8, characterized in that,, to prevent the ingress of hot gases from the combustion chamber into the resonator space shielding air is. blown from outside into the connecting tube 12. Apparatus according to Claim 2, characterized 20 in that the first means comprise a control circuit which controls the density of the gases located in the connecting tube. - in accordance with the phase difference between the combustion-chamber vibrations and the resonator vibrations.

13. Apparatus according to Claim 12, characterized in that the control circuit comprises at least. one pressure sensor each in the combustion chamber and the resonator space downstream measuring transducers comparator -, and ah activating unit it 01 a phase 0 11.

14. Apparatus for damping thermoacoustic vibrations in a combustion chamber substantially as herein described with reference to any of figures 1 to 4 of the accompanying drawings.