GB2345957A - Gas flow restrictor for gas turbine combustor - Google Patents

Abstract

Vibrations generated by the reaction dynamics in a gas turbine combustion chamber (11) are modulated to reduce or nullify troublesome vibrations by using a gas flow restrictor (20) which has one or more restrictor holes (21, 121) and is positioned in a fuel gas passage (16) upstream of a fuel gas injector (17). The restrictor may be in the form of a disc (320) having a co-axial tubular member (23).

Description

GAS TURBINE COMBUSTOR Field of the Invention The invention is concerned with gas fuelled gas turbine combustors and with a method of modifying reaction dynamics within a gas turbine combustor.

Background to the Invention Gas fuelled gas turbine combustors comprise a combustion chamber provided with a combustion air inlet and with a fuel gas passage connected to a gas injector arranged to discharge fuel gas into the combustion chamber. With some designs of combustor, there may also be further inlets supplying other gases, for example steam, for injection into the burning gas. The flow of combustion air is produced by a compressor and enters the combustion chamber under pressure. The fuel gas flow is produced by a pump and is injected under pressure into the combustion chamber. The resultant combustion is essentially very fast and generates dynamic forces in the form of pressure fluc- tuations or shock waves which are manifested on the casing of the combustor as vibrations. In addition to the vibrations inherent in the combustion dynamics (that is those forces initiated by the combustion process), there are also vibrations emanating from the flows of fuel gas, air and other gases (where used).

The magnitude and frequency of the forces generated by combustion can by themselves cause serious mechanical damage to the structure of the combustor. When designing a combustor, it is difficult to predict the frequency, amplitude and wave form of the vibrations generated by the combustion dynamics, and this task is further compli- cated where the combustion dynamics are not constant-this of course occurs if the load on a gas turbine changes during operation and there is consequently a change in the combustion flame characteristics and their associated combustion dynamics. The prediction of the net effect of vibrations generated by the combustion dynamics over the full range of operation of a gas turbine is further complicated by the interaction of vibrations emanating from the flows of air, fuel gas and other gases within the combustion chamber. This interaction can cause resonant or beat frequencies of much larger amplitude. As a consequence it is particularly difficult to design a gas-fuelled combustor which is not subjected to undesirable vibration for part of its range of operation.

Apart from having a mechanical effect on the hardware of the combustor, the combustion gas dynamics also influence combustion stability and can cause combustion "flame-out"with the result that the engine dies.

This invention is concerned with the interaction between the dynamic forces caused by combustion and those caused by the flow of fuel gas, air and other gases (where used).

Summary of the Invention According to one aspect of the invention, a gas turbine combustor comprises a combustion chamber provided with a combustion air inlet and with a fuel gas passage connected to a gas injector arranged to discharge fuel gas into the combustion chamber, and a gas flow restrictor arranged within the fuel gas passage upstream of the gas injector and has a flow characteristic selected to modify combustion dynamics within the combustion chamber.

The flow characteristic of the gas flow restrictor may be selected to damp vibration generated by the combustion of the fuel gas with the combustion air. In this manner the dynamic forces caused by the gas flowing through the gas flow restrictor are adjusted to cause a vibration which will damp the vibration caused by combustion.

The flow characteristic of the gas flow restrictor may be selected to achieve combustion stability over a range of gas flows. In this manner the dynamic forces caused by the gas flowing through the gas flow restrictor are adjusted to modify the vibration caused by combustion such that the combustion will remain substantially stable.

The spacing between the gas flow restrictor and the gas injector is preferably selected to influence combustion dynamics within the combustion chamber. In this manner the resonance of the column of fuel gas between the gas flow restrictor and the gas injector can be tuned to enhance the transmission of dynamic forces, caused by the flow of fuel gas through the gas flow restrictor, into the combustion chamber. Alternatively, it may be tuned to absorb combustion waves entering the fuel gas passage.

The gas flow restrictor may define a single restrictor hole which may be aligned with the gas injector. The gas flow restrictor preferably has a cross-sectional area not less than the effective cross-sectional area of the gas injector.

Alternatively the gas flow restrictor may define a plurality of restrictor holes, each of which may either have substantially the same, or different, cross-sectional areas.

The combined cross-sectional areas of the restrictor holes is preferably not less than the effective cross-sectional area of the gas injector.

Alternatively, the gas flow restrictor may have a tubular portion positioned coaxially within the fuel gas passage, the downstream end of the tubular portion being sealed, and at least one restrictor hole being formed through the wall of the tubular portion.

According to another aspect of the invention, a method of modifying reaction dynamics within a gas turbine combustor comprises inserting a gas flow restrictor within a fuel gas passage upstream of a gas injector. In this manner the method enables the existing reaction dynamics of a gas turbine combustor to be modified to reduce vibration or to reduce the danger of combustion"flame-out".

Brief Description of the Drawings The invention will now be described, by way of example only, with reference to the drawings, in which: Figure 1 is a schematic longitudinal section through a gas turbine combustor; Figure 2 is a transverse section, as if taken along the line 2-2 in Figure 1, but il lustrating a modified gas flow restrictor; Figure 3 illustrates a variation of the gas flow restrictor illustrated in Figure 2; Figure 4 is a scrap section illustrating a further modification of the gas flow restrictor illustrated in Figure 1; Figure 5 is a side elevation of a gas injector, drawn part in section to show the gas flow restrictor; Figure 6 shows the dynamic pressure generated during an engine test, and Figure 7 shows the comparative dynamic pressure after fitting 9mm restrictor in each gas injector.

Detailed Description of the Illustrated Embodiments With reference to Figure 1, a gas turbine combustor comprises a cylindrical casing 10 defining a combustion chamber 11, and an end plate 12 which interconnects the casing 10 and a fuel gas passage 13.

Combustion air inlets 14 are defined between the casing 10, the end plate 12 and inlet guide vanes 15 for receiving an airflow from an unshown compressor and for directing this airflow into the combustion chamber 11.

The fuel gas passage 13 defines a fuel gas inlet 16 for receiving a gas flow from an unshown pump and for directing this gas flow through a fuel gas injector 17 which is shown diagrammatically as comprising a member 18 secured in the fuel gas passage 13 at the downstream end of the gas inlet 16. The fuel gas injector 17 directs the fuel gas into the airflow entering the combustion chamber 10 to produce a combustion flame, which is indicated generally at 19.

Details of the compressor for producing the airflow, the pump for producing the gas flow, ignitors for initiating combustion, and the construction of the combustion chamber 10, are not given as they are well-known in the art and do not form part of the present invention. One or more further passages can be arranged, in well-known manner, to communicate with the combustion chamber 10 for the injection of other gases through further injectors or orifices.

The combustion dynamics generate vibrations which radiate in all directions from the flame 19 and some of these reflect off the casing 10 and the end plate 12 to give a complex interaction of the vibrations within the combustion chamber 11. The dynamics of the fuel gas injection also generate vibrations which issue from the region of the gas injector 17 and further interact with the vibrations within the combustion chamber 11. The vibrations generated by the fuel gas injection emanate from the pumping of the fuel gas and its flow along the fuel gas passage 13 and through the injector 17.

The dynamics of the airflow also introduce further vibrations which interact with the vibrations within the combustion chamber 11.

Where steam or other gases are also injected, further vibrations are generated and issue from the region of their injectors to interact with the vibrations within the combustion chamber 11.

Dependent on the characteristics of the vibrations generated by the combustion dynamics of the flame, by the flow dynamics of the fuel gas, by the flow dynamics of the combustion air, or by the flow dynamics of the injection of steam or other gases, any of these vibrations may react resonantly with the structure of the combustor assembly to detrimental effect. Any of the vibrations generated by the flow dynamics of the fuel gas, or the combustion air, or the injected steam or other gases, may combine with vibrations generated by the combustion dynamics to produce a vibration of greater amplitude which will be of increased detrimental effect. Apart from structural damage to the structure of the combustor, such vibration, or their interaction under particular operating conditions of the combustion chamber, can adversely affect combustion and can cause"flame-out".

The present invention provides a method and structure for manipulating the interaction of these vibrations to reduce the level of vibration within the combustion chamber 10 and to reduce any vibration which might cause"flame-out".

The vibrations emanating from the gas injector 17 are altered by the positioning of a gas flow restrictor 20 within the fuel gas passage 13 as shown in Figure 1. The gas flow restrictor 20 is a circular disc which has its periphery sealed to the inner wall of the gas passage 13 but defines a single restrictor hole 21 through which the gas can pass to the gas injector 17. Although the single restrictor hole 21 is shown coaxially aligned with the gas injector 17 and of substantially the same cross-sectional area, it can be enlarged or positioned to be non-aligned with the fuel jet orifice 17. We have found that the positioning and size of the single restrictor hole 21 will adjust the frequency and/or amplitude of vibrations passing through the gas injector 17 into the combustion chamber 11. In this manner the gas flow restrictor 20 can be used to tune, within limits, the vibrations emanating from the fuel gas inlet 16 so that they will have a beneficial effect on the vibrations within the combustion chamber 11. It is desirable that the crosssection area of the single restrictor hole 21 should not be less than that of the gas in jector 17 as this would increase the pumping pressure significantly. The fuel gas passage 13 can be further tuned by selecting the spacing between the gas flow restrictor 20 and the gas injector 17.

Thus, by careful selection of the size of the restrictor hole 21 and its positioning, it is possible to achieve a fuel wave dynamic which acts either to absorb or to oppose the combustion dynamics, thereby having a damping effect on the combustion dynamics.

Two distinct damping techniques are possible by selectively sizing and/or positioning the restrictor hole 21. Firstly, it is possible to produce a substantially neutral waveform in the fuel gas between the gas flow restrictor 20 and the gas injector 17 to create a fluid cushion to absorb vibrations caused by the combustion dynamics. Secondly the gas flow restrictor 20 may be used as a wave tuning device and, by experimentation or calculation (when all parameters are known), the fuel inlet 16 can be arranged to emit anti-phase vibrations which have the same frequency as a particularly troublesome vibration generated by the combustion dynamics, whereby the troublesome vibration is either reduced or cancelled by the anti-phase vibration. In this manner the gas flow restrictor 20 can be tuned to dampen combustor dynamics and to reduce cyclic vibrations or the occurrence of beat frequencies.

In order to achieve a desired damping effect it may be necessary for the single restrictor hole 21 to have a cross-sectional area that is smaller than the gas injector 17.

This incurs the disadvantage of an increased pressure drop along the fuel gas passage 13 which therefore requires greater pumping effort to sustain the gas flow and gives a consequent loss of efficiency. However, this problem can be met by the alternative gas flow restrictor 120 illustrated in Figure 2. The gas flow restrictor 120 is also a circular disc which has its periphery sealed to the inner wall of the fuel gas passage 13, but is provided with three restrictor holes 121 which are each of the same cross-sectional area, but smaller than the cross-sectional area of the gas injector 17 so that the desired damping effect can be achieved. However, the combined cross-sectional area of the three restrictor holes 121 is not less than the cross-sectional area of the gas injector 17, thereby avoiding increased pumping effort and loss of efficiency.

The gas flow restrictor 220 illustrated in Figure 3 is similar to that described with reference to Figure 2, except that the three restrictor holes 221 are of different cross-sectional areas to produce the desired damping effect. Again the combined crosssection areas of the three restrictor holes 221 is not less than the cross-sectional area of the gas injector 17 for the same reason.

The number, size and positioning of the restrictor holes may be varied as desired to provide a required characteristic, and also the spacing between the gas flow restrictor 20,120 or 220 and its associated gas injector 17 may be varied to suit.

The gas flow restrictors 20,120 or 220 can be of different design and their restrictor holes 21, 121 or 221 may, for instance, be positioned at an angle to their associated gas injector 17. Figure 4 illustrates an example of an alternative construction in which the gas flow restrictor 320 is also a circular disc having its periphery sealed to the inner wall of the fuel gas passage 13, but has a central aperture 22 leading to a coaxial tubular member 23 of which the downstream is sealed by a plate 24. Two restrictor holes 321 are formed through the wall of the tubular member 23 and are accordingly directed normal to the axis of the associated gas injector 17. The internal crosssection area of the tubular member 23 is not less than the cross-sectional area of the associated gas injector 17. Although the two restrictor holes 321 are shown as having the same cross-sectional area and are oriented in the same direction, their orientation may differ and also their cross-sectional areas.

Although all of the restrictor holes 21, 121,221 and 321 have been illustrated as being circular, different profiles may be used if necessary to provide a desired charac teristic.

Although the description and illustration of the gas flow restrictors 20, 120,220 and 320 have been in respect of their installation in the fuel gas passage 13, they could also be positioned within either an air inlet or a steam or other gas inlet.

It will be understood that the size and position of the gas flow restrictors will need to be varied to suit individual systems, since factors such as engine size and type, fuel used and typical loading cycle will influence the combustion dynamics.

Figure 5 illustrates a known injector 27 provided with a main fuel gas inlet 26, a pilot inlet 28, a main liquid fuel inlet 29 and a steam inlet 30. The various inlets 26,28, 29 and 30 are connected, by respective passages within the body of the injector 27, to deliver their supplies to the right-hand end of the injector which would be positioned at the upstream end of the combustion chamber. This known injector 27 is modified by inserting a gas flow restrictor 20 within the fuel gas inlet 26. The fitting defining the fuel gas passage 13 has been machined to define an abutment 31 and a groove 32. The gas flow restrictor 20 is located axially against the abutment 31 by a spring clip 33 fitted into the groove 32. The size of the single restrictor hole in the restrictor 20 is chosen and positioned to modify the gas flow characteristics in the passage 34 so that it will modify the reaction dynamics within the combustion chamber 11.

In Figure 6 the engine under test has 12.7mm gas restrictors and the dynamic pressure has been recorded when the engine was under full load at a turbine operating temperature (TOP) of 533 C. The graphs plot the dynamic pressure, measured as pounds per square inch root mean square (PSlrms), against frequency within the range of 0-800 Hertz (Hz). The upper graph records the dynamic pressure within the gas manifold as being 0.347 PSlrms at 20Hz, whilst the lower graph records the dynamic pressure within the steam port (that is the steam inlet 30 in Figure 5) as being 1.28 PSlrms at 20Hz. Under these high dynamic conditions a rumbling noise was generated.

Figure 7 shows comparative graphs for the same engine under full load at a turbine operating temperature of 535 C, but after fitting 9mm restrictors 20 in the injectors 27. It will be noted that the dynamic pressure in the gas manifold has dropped from 0.347 PSlrms to 0.103 PSlrms at 18Hz, whilst the dynamic pressure in the steam port 30 has dropped from 1.28 PSlrms to 0.618 PSlrms also at 18Hz. For the purpose of this test, the steam port 30 was disconnected from the steam supply and was used as a convenient way to measure the dynamic pressure within the combustion chamber 11.

The test therefore establishes that the use of 9mm restrictors halved the dynamic pressure under full load and also reduced its frequency.

Similar tests of a different engine produced the following results :

RESTRICTOR TOP COMBUSTOR CASING Frequency Amplitude Frequency Amplitude mm C Hz mpsirms Hz mpsirms 9 520 18 964 18 1040 8 510 16 769 16 797 75201672016702 The reduction in dynamic pressure by reducing the restrictor size from 9mm to 8mm is particularly notable.

Claims (15)

1. A gas turbine combustor comprising a combustion chamber provided with a combustion air inlet and with a fuel gas passage connected to a gas injector arranged to discharge fuel gas into the combustion chamber, and a gas flow restrictor arranged within the fuel gas passage upstream of the gas injector and having a flow characteristic selected to modify reaction dynamics within the combustion chamber.

2. A gas turbine combustor, according to Claim 1, in which the flow characteristic of the gas flow restrictor is selected to damp vibration generated by combustion of the fuel gas with the combustion air.

3. A gas turbine combustor, according to Claim 1 or 2, in which the flow characteristic of the gas flow restrictor is selected to achieve combustion stability over a range of gas flows.

4. A gas turbine combustor, according to any preceding claim, in which the spacing between the gas flow restrictor and the gas injector is selected to influence combustion dynamics within the combustion chamber.

5. A gas turbine combustor, according to any preceding claim, in which the gas flow restrictor defines a single restrictor hole.

6. A gas turbine combustor, according to Claim 5, in which the single restrictor hole is aligned with the gas injector.

7. A gas turbine combustor, according to Claim 5 or 6, in which the gas flow restrictor has a cross-sectional area not less than the effective cross-sectional area of the gas injector.

8. A gas turbine combustor, according to any of Claims 1 to 4, in which the gas flow restrictor defines a plurality of restrictor holes.

9. A gas turbine combustor, according to Claim 8, in which each of the restrictor holes has substantially the same cross-sectional area.

10. A gas turbine combustor, according to Claim 8, in which the restrictor holes have different cross-sectional areas.

11. A gas turbine combustor, according to any of Claims 8 to 10, in which the combined cross-sectional areas of the restrictor holes is not less than the effective cross-sectional area of the gas injector.

12. A gas turbine combustor, according to any of Claims 1 to 4, in which the gas flow restrictor has a tubular portion positioned coaxially within the fuel gas passage, the downstream end of the tubular portion is sealed, and at least one restrictor hole is formed through the wall of the tubular portion.

13. A gas turbine combustor substantially as described herein with reference to the accompanying drawings.

14. A method of modifying reaction dynamics within a gas turbine combustor, comprising inserting a gas flow restrictor within a fuel gas passage upstream of a gas injector.

15. A gas turbine engine comprising a gas turbine combustor according to any of Claims 1 to 13.