WO2000034715A1

WO2000034715A1 - Modification of combustion reaction dynamics

Info

Publication number: WO2000034715A1
Application number: PCT/GB1999/004174
Authority: WO
Inventors: Theodore Ishaq Mina
Original assignee: ABB Alstom Power UK Ltd
Current assignee: Alstom Power UK Holdings Ltd
Priority date: 1998-12-09
Filing date: 1999-12-09
Publication date: 2000-06-15
Anticipated expiration: 2001-06-09

Abstract

The combustion reaction dynamics in a gas turbine combustion chamber (11) are modulated to improve combustion stability and/or reduce or nullify troublesome vibrations by using a gas flow restrictor (20) which has one or more restrictor holes (21) and is positioned in a gas passage (16) upstream of a gas injector orifice (17) and preferably closely spaced thereto.

Description

MODIFICATION OF COMBUSTION REACTION DYNAMICS

Field of the Invention

The present invention is concerned with gas turbine engines and particularly relates to modification of combustion reaction dynamics to improve combustion stability and reduce combustion noise and vibration.

Background to the Invention

Gas turbine combustors comprise a combustion chamber provided with a combustion air inlet and with a fuel passage connected to a fuel injector arranged to discharge fuel into the combustion chamber. The disclosure of the present invention is particularly directed to combustors supplied with gaseous fuels, but of course liquid fuels may also be supplied to combustors. 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 through the combustion air inlet. The fuel 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 fluctuations 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 complicated 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 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 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 extinguishing of the combustion flame - so-called "flame-out" - with the result that the engine stops producing power.

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

Summary of the Invention

According to one aspect of the invention, a gas supply system for a gas turbine engine comprises; a plurality of gas injectors, each injector being arranged to discharge gas into a combustion chamber through injector orifice means, for each gas injector, a gas supply passage extending to the injector orifice means, wherein for each gas injector, a gas flow restrictor is located within the gas supply passage upstream of the gas injector orifice means, the gas flow restrictor having at least one flow characteristic selected to modify combustion reaction dynamics within the combustion chamber, all said gas flow restrictors in said gas supply system having essentially identical flow characteristics. Preferably, the gas supply system is a fuel supply system for supplying fuel gas to the combustor, but it is envisaged that the gas supply system could alternatively be an air or steam supply system.

In accordance with gas injection requirements for the combustion process, the gas injector orifice means may comprise a single hole or a group of holes. It should be understood that where a gas supply system is fitted with gas flow equalisers, such as those commonly used to equalise fuel gas flows from a fuel gas supply source to each of a number of fuel gas injectors which inject fuel gas into a combustion chamber, the gas flow restrictors of the present invention are fitted in the gas passages downstream of the gas flow equalisers. This is necessary to avoid the problem of such gas flow equalisers interfering with the proper functioning of the gas flow restrictors. Furthermore, it should be noted that whereas the gas flow restrictors of the present invention are designed to have essentially identical flow characteristics in respect of each gas supply system for which they are designed, known gas flow equalisers fitted in the same gas supply system would not necessarily all have the same flow characteristics, because the degree of fuel flow rate equalisation needed at each injector will depend upon the position of the injector with respect to the common fuel source and the resistances of the various flow paths to the injectors.

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 selected flow characteristic of the gas flow restrictor may comprise the open area of at least one restrictor hole in the restrictor. Alternatively, or additionally, the selected flow characteristic of the gas flow restrictor may comprise transverse positioning of the at least one restrictor hole with respect to the fuel gas passage.

Furthermore, the distance between the gas flow restrictor and the gas injector orifice means is preferably selected to influence combustion dynamics within the combustion chamber. The combustion dynamics are influenced because variation of the aforesaid distance can be used to tune the resonance of the column of gas between the gas flow restrictor and the gas orifice means so as to enhance the transmission of dynamic forces, caused by the flow of gas through the gas flow restrictor, into the combustion chamber. Alternatively, the column of gas may be tuned to absorb combustion waves entering the gas passage. The gas flow restrictor may define a single restrictor hole which may be aligned with a gas injector orifice. 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. In a further embodiment, the gas flow restrictor has a tubular portion positioned co-axially within the 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. However, whichever variation of the invention is adopted, the effective total open area of the gas flow restrictor is preferably not less than the effective total open area of the gas injector orifice means. Conveniently, and to ensure close-coupling of the gas flow restrictor to the gas injection orifice means, thereby avoiding excessive attenuation of the combustion modifying waveforms generated by the restrictor. the combustor is provided with a gas injector assembly incorporating the gas injector orifice means and the gas flow restrictor.

In another aspect, the invention comprises a combustion chamber provided with a combustion air inlet, and a gas injector assembly including gas injector orifice means arranged to discharge gas into the combustion chamber and a gas supply passage connected to the gas injector orifice means, wherein a gas flow restrictor is located in the gas supply passage, the gas flow restrictor having a flow characteristic selected to modify combustion reaction dynamics within the combustion chamber.

According to a further aspect of the invention, for a gas turbine engine having combustor means and a gas supply system comprising a plurality of gas supply passages and a corresponding plurality of gas injectors connected to the gas supply passages for discharging gas into combustor means through injector orifice means: a method of modifying combustion reaction dynamics within the combustor means comprising; providing a gas flow restrictor for insertion in at least one gas supply passage in the engine at a known distance upstream of a corresponding gas injector orifice means, the gas flow restrictor having at least one predetermined flow characteristic, repeatedly varying at least one of the distance of the gas flow restrictor from the gas injector orifice means and at least one flow characteristic of the gas flow restrictor and observing the effect of such variation on combustion reaction dynamics, selecting values of at least one flow characteristic and the distance of the gas flow restrictor from the gas injector orifice means which maximise at least one of reduction of combustion noise in the test rig and combustion stability without excessive restriction of gas flow through the gas injector orifice means, and providing each gas supply passage in the gas supply system of the gas turbine engine with a gas flow restrictor having said selected values. In this manner the method enables the existing reaction dynamics of a gas turbine combustor to be modified to reduce noise and vibration and/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 shows a schematic longitudinal section through a gas turbine combustor, the combustor being supplied with fuel gas through a fuel system incorporating a gas flow restrictor in accordance with the invention;

Figure 2 is a transverse section, as if taken along the line 2-2 in Figure 1, but illustrating 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 partly 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 a 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 supply passage 13. The fuel gas passage is part of a fuel gas supply system 1 supplying a plurality of such combustors from a common fuel source 3, such as a fuel pump feeding a fuel supply manifold. Each combustor is individually supplied with fuel from the fuel source 3 through its own gas supply passaεe 13. To supply air for burning with the fuel, an array of combustion air inlets 14 extends circumferentially around a reduced diameter portion of the combustion chamber 11. The air inlets are defined between the casing 10, the end plate 12 and the radially outer ends of an array of angularly spaced apart inlet guide vanes 15 for receiving an airflow from a compressor (not shown) and for directing this airflow into the combustion chamber 11. Adjacent vanes 15 define passages therebetween which are oriented to give the incoming air swirling motion, i.e., radial and tangential components of velocity, as it enters the reduced diameter part of the combustion chamber 11.

The fuel system 1 pumps the fuel gas through the passage 13 into a fuel gas inlet 16 and thence through a fuel gas injector orifice 17. Orifice 17 directs the fuel gas into the centre of the swirling airflow entering the combustion chamber 10 to produce a combustion flame, indicated generally at 19. Gas injector orifice 17 is provided in an injector shown diagrammatically as comprising a member 18 secured in the fuel gas passage 13 at the downstream end of the gas inlet 16.

Details of the compressor for producing the airflow, the fuel system for supplying the gas fuel, igniters for initiating combustion, and the construction of the combustion chamber 10, are not detailed as they are well-known in the art and do not form part of the present invention. As necessary, one or more further fuel passages can be arranged, in well-known manner, to communicate with the combustion chamber 10 for the injection of further fuels or other fluids 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 orifice 17 and further interact with the vibrations within the combustion chamber 1 1. 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 orifice 17. The dynamics of the incoming swirling airflow also introduce further vibrations which interact with the vibrations within the combustion chamber 11.

Where steam or other fluids are also injected, further vibrations are generated and issue from the region of their injector orifices 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 fluids, 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 fluids, 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 vibrations, 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". We have found that the vibrations emanating from the gas injector orifice 17 are altered by variation of certain flow characteristics of the gas flow restrictor 20, namely the size of the restrictor hole 21 defined by the restrictor .and the position of the hole 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 orifice 17. Although the single restrictor hole 21 is shown coaxially aligned with the gas injector orifice 17 and of substantially the same open area, it can be enlarged or positioned to be non-aligned with the fuel jet orifice 17. In other words, the size and the transverse position of the restrictor hole with respect to the fuel gas passage can be selected to influence combustion dynamics within the combustion chamber. 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 orifice 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 cross-sectional area of the single restrictor hole 21 should not be less than that of the gas injector orifice 17 as this would increase the pumping pressure significantly.

We have also found that the fuel gas passage 13 can be further tuned by varying the distance between the gas flow restrictor 20 and the gas injector orifice 17 and selecting the distance which gives the best result in terms of combustion stability or noise reduction.

Thus, by careful selection of the size of the restrictor hole 21, its positioning with respect to its distance from the gas passage wall, and its spacing from the injector orifice, it is possible to achieve a gas 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 and/or spacing it from the injector orifice. Firstly, it is possible to produce a substantially neutral waveform in the fuel gas between the gas flow restrictor 20 and the gas injector orifice 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 orifice 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 orifice 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 orifice 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 cross- sectional areas of the three restrictor holes 221 is not less than the cross-sectional area of the gas injector orifice 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 orifice 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 orifice 17. The internal cross-sectional area of the tubular member 23 is not less than the cross-sectional area of the associated gas injector orifice 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 characteristic. 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.

Although in Figure 1, the gas injector orifice 17 has been shown as a single round hole, other configurations are possible. For example, a group of several round holes could be used instead of a single hole, or orifice 17 could take the form of one or more slots, particularly two or more arcuate slots arranged to form a circle.

Figure 5 illustrates a known type of gas fuel injector assembly 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. A gas flow restrictor 20 is inserted within the fuel gas inlet passage 26. The pipe union fitting 13 defining the fuel gas passage 26 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. In accordance with the invention, 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.

It is important for proper working of the invention that the restrictor 20 and the gas injector orifice(s) 17 (Fig. 1) are close-coupled, i.e. that the restrictor is not positioned excessively far upstream of the gas injector. Such gas injector orifices are not shown in Fig. 5, but are of course at the far right of the injector assembly 27. For any particular type of injector assembly, the optimum distance between the injector orifice(s) and the restrictor 20 along the passage 13 (Fig. 1) or 34 (Fig. 5) will be determined empirically. However, qualitatively, the distance must not be so great as to excessively attenuate the waveform(s) produced in the passage 13 or 34 by the flow of gas through the restrictor. Hence, as shown in Fig. 5, the restrictor 20 is relatively closely spaced to the gas injector orifice(s) by virtue of being conveniently located in the injector assembly 27 - in this case in the fuel gas inlet pipe union 13.

It is known to fit gas fuel supply systems with gas flow equalisers to equalise fuel flows through the injectors, thereby ensuring that all injectors inject fuel into the combustion chambers at substantially the same rate. Such equalisers, like the gas flow restrictors of the present invention, are fitted in the fuel gas passages upstream of the injector orifices. Referring back to Fig. 1, one of these equalisers is indicated diagrammatically by dashed line 5. However, it will be noted that the gas flow restrictor 20 is fitted in the fuel gas passage downstream of the gas flow equaliser 5. This is necessary to avoid the gas flow equaliser 5 interfering with the proper functioning of the gas flow restrictor 20. It will be remembered that the fuel system 1 comprises a plurality of (e.g., six) injectors 18. Each injector will be in a different position with respect to the manifold and a different distance from the fuel pump. Hence, the gas flow equalisers 5 will probably not all have the same flow characteristics as each other (primarily, the relevant flow characteristic is orifice open area) and will be designed to different criteria from the gas flow restrictors 20. In contrast, in respect of each different type or variation of fuel system or combustor for which they are designed, the gas flow restrictors of the invention will all have essentially identical flow characteristics.

In Figure 6 an engine under test on a test rig has been provided with injector assemblies like that shown in Fig. 5, each injector being fitted with a gas flow restrictor 20 having a 12.7mm diameter hole. 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 (PSIrms), 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 PSIrms 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 PSIrms 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 the injectors 27 with restrictors 20 having 9mm diameter holes. It will be noted that the dynamic pressure in the gas manifold has dropped from 0.347 PSIrms to 0.103 PSIrms at 18Hz, whilst the dynamic pressure in the steam port 30 has dropped from 1.28 PSIrms to 0.618 PSIrms 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:

The reduction in dynamic pressure by reducing the restrictor size from 9mm to 8mm is particularly notable.

Claims

1. A gas supply system (1) for a gas turbine engine comprising; a plurality of gas injectors (18), each injector being arranged to discharge gas into a combustion chamber (11) through injector orifice means (17), for each gas injector, a gas supply passage (13) extending to the injector orifice means (17), wherein for each gas injector (18), a gas flow restrictor (20) is located within the fuel gas passage upstream of the gas injector orifice means (17), the gas flow restrictor

(20) having at least one flow characteristic selected to modify combustion reaction dynamics within the combustion chamber (11), all said gas flow restrictors in said gas supply system having essentially identical flow characteristics.

2. A gas supply system according to Claim 1, in which the flow characteristic of the gas flow restrictor is selected to damp vibration generated by combustion in the combustion chamber.

3. A gas supply system 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 supply system according to any preceding claim, in which the selected flow characteristic of the gas flow restrictor comprises the open area of at least one restrictor hole in the restrictor.

5. A gas supply system according to claim 4, in which the selected flow characteristic of the gas flow restrictor comprises transverse positioning of the at least one restrictor hole with respect to the fuel gas passage.

6. A gas supply system according to any preceding claim, in which a distance between the gas flow restrictor and the gas injector orifice means is selected to influence combustion dynamics within the combustion chamber.

7. A gas supply system according to any preceding claim, in which the gas flow restrictor defines a single restrictor hole and the gas injector orifice means comprises a single orifice.

8. A gas supply system according to Claim 7, in which the single restrictor hole is aligned with the single gas injector orifice.

9. A gas supply system according to any preceding claim, in which the gas flow restrictor has a cross-sectional flow area not less than the effective cross-sectional flow area of the gas injector orifice means.

10. A gas supply system according to any of Claims 1 to 4, in which the gas flow restrictor defines a plurality of restrictor holes.

11. A gas supply system according to Claim 10, in which each of the restrictor holes has substantially the same cross-sectional area.

12. A gas supply system according to Claim 10, in which the restrictor holes have different cross-sectional areas.

13. A gas supply system according to any of Claims 10 to 12, in which the combined cross-sectional areas of the restrictor holes is not less than the effective cross- sectional area of the gas injector orifice means.

14. A gas supply system 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.

15. A gas supply system according to any preceding claim, having a gas injector assembly incorporating the gas injector orifice means and the gas flow restrictor.

16. A gas supply system according to any preceding claim, in which the gas is one of fuel gas, air and steam.

17. A gas supply system according to claim 16, in which the gas flow restrictor is located in a fuel gas inlet pipe union forming part of the gas fuel injector assembly.

18. A gas turbine engine having a gas supply system according to any of Claims 1 to 17.

19. A gas turbine combustor comprising a combustion chamber (11) provided with a combustion air inlet (14), and a gas injector assembly (27, Fig.5) including gas injector orifice means arranged to discharge gas into the combustion chamber and a gas supply passage (26,34) connected to the gas injector orifice means, wherein a gas flow restrictor (20) is located in the gas supply passage, the gas flow restrictor having a flow characteristic selected to modify combustion reaction dynamics within the combustion chamber.

20. A gas supply system according to claim 19, in which the gas is one of fuel gas, air and steam.

21. A gas turbine combustor according to claim 20, in which the gas flow restrictor is located in a fuel gas inlet pipe union (26) forming part of the gas fuel injector assembly (27).

22. A gas turbine engine having a combustor according to any one of claims 19 to 21.

23. For a gas turbine engine having combustor means and a gas supply system comprising a plurality of gas supply passages and a corresponding plurality of gas injectors connected to the gas supply passages for discharging gas into combustor means through injector orifice means: a method of modifying combustion reaction dynamics within the combustor means comprising; providing a gas flow restrictor for insertion in at least one gas supply passage in the engine at a known distance upstream of a corresponding gas injector orifice means, the gas flow restrictor having at least one predetermined flow characteristic, repeatedly varying at least one of the distance of the gas flow restrictor from the gas injector orifice means and at least one flow characteristic of the gas flow restrictor and observing the effect of such variation on combustion reaction dynamics, selecting values of at least one flow characteristic and the distance of the gas flow restrictor from the gas injector orifice means which maximise at least one of reduction of combustion noise in the test rig and combustion stability without excessive restriction of gas flow through the gas injector orifice means, and providing each gas supply passage in the gas supply system of the gas turbine engine with a gas flow restrictor having said selected values.