GB2334087A

GB2334087A - Combustor restrictor

Info

Publication number: GB2334087A
Application number: GB9802471A
Authority: GB
Inventors: Eric Roy Norster; Pietro Simon De
Original assignee: COMBUSTION TECHNOLOGY INTERNAT
Current assignee: COMBUSTION TECHNOLOGY INTERNAT
Priority date: 1998-02-03
Filing date: 1998-02-03
Publication date: 1999-08-11
Also published as: GB9802471D0

Abstract

A combustor for a gas turbine comprising at least one restrictor 21 to restrict the flow of fluid through the combustor, the restrictor 21 comprising a member 4 movable between restricting and non-restricting positions, wherein a pressure differential exists across the member during operation of the combustor, and wherein the member 4 is arranged to move between the said positions in response to changes in the pressure differential across the said member 4.

Description

COMBUSTOR RESTRICTOR Field of the Invention This invention relates to gas turbines, and in particular to a restrictor to restrict the amount of air/fuel entering the combustor of a low emission gas turbine.

Background of the Invention Gas turbines are used widely, for example in power generation, and aircraft propulsion.

Generally, a gas turbine is provided with a combustor which produces hot combustion gases, and these gases turn a turbine and/or generate a propulsive force.

Environmental considerations have led to the introduction of legislation which aims to constrain smog and acid rain by restricting the permissible level of emission of oxides of nitrogen into the atmosphere. In order to meet the requirements of this legislation so called "lean burn" combustors have been developed. These "lean burn" combustors operate at lower flame temperatures than conventional combustors, and because of this emit oxides of nitrogen at a much lower level.

However, "lean burn" combustors present their own problems. Under certain conditions, "lean burn" combustors can emit large amounts of carbon monoxide, unburned fuel, and smoke. These conditions occur when the gas turbine is under partial load conditions, and during start up of the turbine. The emission of large amounts of carbon monoxide, unburned fuel, and smoke occurs in these conditions because the amount of fuel in the air/fuel mixture falls and hence the temperature in the combustor is lower than when the turbine is running under full load.

An additional problem found with "lean burn" combustors is that they can be very difficult to ignite due to the higher than conventional amounts of air entering the combustion zone.

Yet another problem found with "lean burn" combustors is that when a liquid fuel is used, the amount of unburned fuel and smoke emitted increases. The exhaust plume from liquid fuel fired "lean burn" combustors can smell, and can have a corrosive effect on objects, such as cars, on which it may settle. Attempts have been made to mitigate this problem by improved atomising of the liquid fuel before it is fed into the combustor, which results in an increased heat release rate and a greater proportion of the fuel being burned.

It is also known to restrict the air-flow through the combustor zone in order to increase the fuel/air ratio during start-up of the combustor, which has the effect of increasing the combustor flame temperature and decreasing the emission of carbon monk oxide, unburned fuel and smoke. However, the known systems are very complicated and use complex control mechanisms.

It would therefore be desirable to provide a combustor for a gas turbine which would permit "lean burn", but does not suffer from the disadvantages of known "lean burn" combustors.

Summary of the Invention According to the invention there is provided a combustor for a gas turbine comprising at least one restrictor to restrict the flow of fluid through the combustor, the restrictor comprising a member movable between restricting and non-restricting positions, wherein a pressure differential exists across the member during operation of the combustor, and wherein the member is arranged to move between the said positions in response to changes in the pressure differential across the said member.

Preferably, the member is arranged to switch between its restricting and nonrestricting positions in response to a change in pressure differential across the said member. Alternatively, the member is arranged to move continuously from its restricting position to its non-restricting position in response to changes in the pressure differential across the said member.

The said member may comprise a plug arranged to slide in a bore. The plug may be circular, semitircular, or rectangular in cross-section. Alternatively, the plug may be aerodynamically shaped. The said member may comprise a flap.

Preferably, a biasing means to bias the said member into the restricting position is provided. The biasing means may comprise a spring, which may be a coil spring. The biasing means may comprise a pressurised fluid system, wherein pressurised fluid exerts a force on the member to bias it into the restricting position. The pressurised fluid system may comprise control means to vary the pressure of the fluid exerted on the said member, and thereby increase or decrease the biasing force on the said member. The control means may typically comprise a pressure sensor arranged to sense the pressure in the combustor, or a sensor arranged to sense another parameter indicative of load, for instance rotational speed or temperature may be sensed. The pressure sensor may comprise a pressure tapping into the combustor. The control means may comprise a switch and the control means may be arranged to vary the biasing force on the said member in accordance with the position of the switch. Preferably, fluid passing through a pressure tapping is directed through a valve movable between a position in which the fluid pressurises the restrictor so that the pressure differential across the said member is negligible, and a position in which the valve directs the fluid passing through the bore to a region of low pressure, for example the exhaust system of the combustor.

Preferably, the combustor comprises a swirler, and the restrictor is arranged so that the said member restricts the flow of air through the swirler. The restrictor may be arranged so that the member restricts the flow of air through a slot or passage of the swirler. Advantageously, the swirler comprises a plurality of slots or passages. A restrictor may be provided for one or more of the said plurality of slots or passages. A restrictor may be arranged so that the said member moves into and out of a slot or passages to restrict the flow of air therethrough. A restrictor may be arranged so that the said member is outside the swirler adjacent the entrance to a slot or passage, and movement of the said member obscures or reveals the said entrance, to restrict the flow of air into the swirler. A restrictor may be arranged so that the said member is mounted in the body of the swirler and protrudes into a slot or passage thereof, and so that the said member may move into or out of the slot or passage between its restricting and non-restricting positions. Where more than one of the slots or passages is provided with a restrictor, the members of the restrictors may restrict the flow of fluid by different amounts when in their restricting positions.

In one embodiment of the invention the swirler comprises a plurality of slots or passages and a restrictor is provided for each of the slots or passages. The restrictors may each be arranged to move away from their restricting positions when subjected to the same change in fluid pressure. Alternatively, the restrictors may each be arranged to move away from their restricting positions when subjected to different changes in fluid pressure.

Restrictors may be grouped so that the said members of a first group of restrictors moves away from their restricting position when a first change in pressure different tial has occurred, and the said members of a second group of restrictors move away from their restricting positions when a second change in pressure differential has occurred. Subsequent groups of the restrictors thereof move away from their restricting positions when a subsequent change in fluid pressure has occurred.

In one embodiment of the invention, an eight-slot swirler is provided each slot having a restrictor, wherein the restrictors are grouped into two groups of four restrictors.

In another embodiment of the invention, a twelve-slot swirler is provided, each slot having a restrictor, wherein the restrictors are grouped into three groups of four restrictors, or alternatively four groups of three restrictors.

Fuel injectors may be mounted on the swirler, or adjacent thereto. At least one fuel injector may be provided for one or more of slots in the swirler.

The combustor may be annular or tubular in form, and may have a plurality of swirlers. At least one of the swirlers may be provided with restrictors, and the restrictors may be operated individually or in groups.

The invention also provides a gas turbine comprising a combustor according to the invention.

By restricting the flow of air through the combustor by reducing the effective cross-sectional area of the swirler slots, the gas turbine can be ignited and brought to full speed, no load, much more easily than gas turbines which do not restrict air flow.

Furthermore, the emission of carbon monoxide from start up to full speed full load is far less than is found with conventional gas turbines.

By arranging the restrictors so that the members thereof move away from their restricting positions when subjected to different changes of fluid pressure the level of emission of carbon monoxide can be controlled to a very fine degree.

Brief Description of the Drawings In the drawings, which illustrate exemplary embodiments in accordance with the invention: Figure 1 is an end view of a swirler having restrictors mounted therein to restrict the flow of air or air and fuel into a combustor; Figure 2 is a cross-section of a combustor for a gas turbine according to the invention; Figure 2a is a cross-section of a combustor as shown in Figure 2 and including a control circuit; Figure 3 shows in detail a cross-section of a restrictor used in a combustor according to the invention; Figure 4 is a cross-section of parts of the restrictor shown in Figure 3; Figure 5 shows a graph of carbon monoxide emissions against load for an eightslot swirler; Figure 6 shows a graph of carbon monoxide emissions against load for a twelve slot swirler; and Figure 7 illustrates a multi-tubular arrangement.

Detailed Description of the Preferred Embodiments In Figure 1, there is shown a swirler 1 having eight slots 2. A fuel injector 3 may be provided for each slot, and may be mounted externally of the swirler 1, or on the swirler 1. Six of the eight slots 2 of the swirler 1 are shown provided with restrictor plugs 4 to 9, of different shape, and at various positions with respect to the slots 2.

Restrictor plug 4 is circular in crosisection and is mounted within a slot 2 so that air, or an air/fuel mix can pass above and below the restrictor plug 4. Restrictor plug 5 is semiircular in cross-section, and is mounted in a slot 2 to one side thereof.

Air, or an air/fuel mix may only flow over the curved surface of the restrictor plug 5.

Restrictor plug 6 is circular in cross-section, and is mounted outside a slot 2. Restrictor plug 7 is rectangular in cross-section and is mounted within a slot 2. Air, or an air/fuel mix passes to one side of the restrictor plug 7. Restrictor plug 8 is mounted partially within a slot 2, and partially within the body of the swirler 1. Restrictor plug 9 is mounted within a slot 2, and is aerodynamically shaped. Air, or an air/fuel mix can pass to both sides of the restrictor plug 9.

Restrictor plugs having different shapes may be used on the same swirler. Also, on the same swirler, restrictor plugs may be mounted within or outwith the swirler.

The different shapes of restrictor plug each perform better in some ways than the other shapes of restrictor plug, and in other ways worse. For instance, the rectangular restrictor plug 7 is easy to manufacture, but its aerodynamic performance is poor.

The rectangular shape may cause dead zones, i.e. zones in which the airflow is unstable.

This can be dangerous because the airflow speed may fall below the flame speed.

When this occurs, a phenomenon known as flashback may result.

The other extreme of shape is the restrictor plug 9 which is aerodynamically shaped. This shape of restrictor plug avoids the generation of dead zones, but the shape of the restrictor plug is more costly to produce. The circular and semitircular shaped restrictor plugs 4 to 6, and 8 have better aerodynamic performance than the rectangular shaped restrictor plug 7 but not as good as that of the aerodynamically shaped restrictor plug 9. However, the restrictor plugs 4 to 6, and 8 are less costly to produce than restrictor plug 9.

Referring now to Figure 2, the swirler 1 is mounted between a combustor liner 10 and a injector head 11, which has mounted within it a flame stabilising block 20. Restrictors 21, which are described in greater detail with reference to Figures 3 and 4, extend through the injector head 11, the plugs 4 thereof being slidable into and out of the slots of the swirler 1.

In use, compressed air is fed through the swirler 1 and into the combustor liner 10. The passage of air through the slots is diminished by the restrictor plugs 4 which extend into the said slots. As the turbine (not shown) speed and/or load increases, there is an increase in pressure in the slots of the swirler, and when the pressure differential experienced by the restrictor plug overcomes the spring force, the plugs 4 are pushed into the apertures in which they slide, thereby allowing more air to flow through slots.

Figure 2a illustrates a combustor as shown in Figure 2, like numerals being used to represent like parts. In the upper portion of the injector head there is a pressure tapping 25 which allows the combustor air pressure to be sensed. A fitting 26 is screwed into the pressure tapping 25, and a pipe 30 is connected to a three-way valve 27 that is operated by a solenoid 28 when a switch 29 is closed. Two further pipes are connected to the three-way valve 27. The pipes 31 is connected to each of the swirlers 21, and the pipe 32 vents to a pressure lower than the combustor air pressure. By opening or closing the switch, the pressure in pipes 31 can be switched from a pressure equal to the combustor pressure to a pressure lower than the combustor pressure.

When the pressure change occurs, the restrictor plugs are subjected to a differential pressure causing them to be withdrawn from the swirler 1. The differential pressure is sufficient to overcome the force exerted by the springs 1 9 on the restrictor plugs, and the restrictor plugs move rapidly from the restricting position shown to a nonrestricting position. The switch 29 is opened and closed in response to sensed load on the turbine, and typically the load is indicated by combustor pressure. However, the switch may be opened and closed in response to another load function such as turbine speed or exhaust temperature. Where a swirler has many slots, and it is desirable to restrict different numbers of slots at different load conditions, a plurality of the control circuits shown in Figure 2a may be provided. For example two circuits may be provided for an eight-slot swirler arranged so that four or eight of the slots may be restricted.

Referring now to Figures 3 and 4, the restrictor 21 comprises a guide member 13, and a restrictor plug 4, both being mounted in a bore 22 in the injector head 11.

The guide member 1 3 is removably mounted in the bore 22, and a seal 23 is provided between the guide 1 3 and the injector head 11. Extending from the restrictor plug 4 is a tube 1 6, the external diameter of which is such that the tube may slide in a bore 1 5 in guide member 1 3. A helical spring 1 9 extends between the opposing ends of the guide member 1 3 and the restrictor plug 4 and is arranged to bias the restrictor plug into the position shown in Figure 3. Further movement of the restrictor plug 4 into the swirler slot is prevented by the reduction in diameter of bore 22 at 24. The tube 1 6 passes inside the spring 1 9. Seals 12 pass around the restrictor plug 4 which is provided with grooves to receive the seals. The tube 1 6 which slides in the bore 1 5 of the guide member 1 3 ensures that the restrictor plug 4 travels in a straight path. This could also be achieved without the tube 1 6 or guide member 1 3 by extending the portion of the plug which runs in bore 22 and providing a suitable retaining means to hold the spring 1 9 in the bore 22.

It can be seen that the tube 16 is provided with a bore 17 and apertures 18.

The bore 17 and apertures 1 8 allow air to be vented to a lower pressure, for example the exhaust system, as the restrictor plug 4 is forced back into bore 22 against the biasing force of spring 1 9.

The guide member 13 is provided with a coupling 14, to which a pipe can be attached in order that the vented air may be directed out of the gas turbine of which the combustor forms a part, or to a zone within the gas turbine that is at a lower pressure than the combustion zone. Also, a pressurised fluid may be introduced into the bore 1 5 through the pipe connected to the coupling 14, and act in conjunction with the spring 19 to bias the restrictor plug 4 into the position shown in Figure 3. Suitably control means may be provided so that the fluid pressure in one restrictor may differ from the fluid pressure in another restrictor, thereby providing for the restrictor plugs 4 of different restrictors to move away from the position shown in Figures 2 and 3 at different fluid pressures.

Referring now to the graph shown in Figure 5, when the load on the turbine is reduced the emission of carbon monoxide increases. By inserting into the swirler the first four restrictor plugs of the eight provided, the air/fuel ratio is reduced and the level of carbon monoxide emission drops in a step change. As the load on the gas turbine decreases further, the carbon monoxide emission level increases again. When the second four restrictors are inserted, so that all eight restrictors are inserted in the swirler, the air/fuel mixture is reduced further and the level of carbon monoxide emission drops again in a step change. If the load on the gas turbine is decreased further, the emission of carbon monoxide increases again. The level of carbon monoxide emission, from the low NOx gas turbine can therefore be controlled by reducing the air/fuel ratio as the load on the turbine falls.

The invention also enables carbon monoxide and unburnt fuel emission to be controlled during start-up and during the interval in which the turbine is brought from full speed no load to full speed full load. Referring again to Figure 5, it can be seen from the graph that the level of emission of carbon monoxide can be kept low by gradually withdrawing restrictor plugs from the swirler. At start up all eight restrictor plugs are in the position shown in Figure 2. During start up the emission level rises, peaks, and then falls. When the load has reached a desired part load, for example one third full load, four of the restrictor plugs are withdrawn from the swirler. The carbon monoxide level immediately increases, and then falls again as the load is further increased, until a second higher desired part load is reached. The remaining four restrictor plugs are then withdrawn from the swirler and again the carbon monoxide level immediately increases, and the falls again as the load increases to full load, at which point the emission of carbon monoxide is negligible.

The performance of a twelve-slot swirler is shown in Figure 6. The restrictor plugs are inserted into, or withdrawn from, the swirler slots in groups of three in the same manner as described with reference to Figure 5.

As well as, or instead of controlling carbon monoxide emissions, the restrictors may be used to control NOx emissions, and can also be used to control the air/fuel ratio to allow different fuels having different stoichiometry to be used in the same combustor.

Where multiple restrictors are to be used, it may be advantageous to arrange the restrictors so that some restrictor plugs protrude into the swirler slots further than others do. This is because different fuel/air ratios in different slots give non-uniform burning which helps to improve carbon monoxide emissions and combustion stability.

Referring now to 7, there is shown a multi-tubular combustor arrangement of six combustors 40, each combustor having a plurality of swirlers 41. Two of the combustors are provided with ignitors 42, and cross-light tubes 43 provide for ignition in the other combustors. Each swirler is provided with a plurality of restrictors similar in construction to the restrictors 21 described with reference to Figures 3 and 4.

Claims

1. A combustor for a gas turbine comprising at least one restrictor to restrict the flow of fluid through the combustor, the restrictor comprising a member movable between restricting and non-restricting positions, wherein a pres sure differential exists across the member during operation of the combus tor, and wherein the member is arranged to move between the said posi tions in response to changes in the pressure differential across the said member.

2. A combustor according to Claim 1, wherein the said member is arranged to switch between its restricting and non-restricting positions in response to a change in pressure differential across the said member.

3. A combustor according to Claim 1, wherein the said member is arranged to move continuously from its restricting position to its non-restricting position in response to changes in the pressure differential across the said member.

4. A combustor according to any preceding claim, wherein the said member comprises a plug arranged to slide in a bore.

5. A combustor according to Claim 4, wherein the plug is substantially circular, semitircular, or rectangular in cross-section.

6. A combustor according to Claim 4 or 5, wherein the plug is aerodynamically shaped.

7. A combustor according to any preceding claim, further comprising a biasing means to bias the said member into the restricting position is provided.

8. A combustor according to Claim 7, wherein the biasing means comprises a spring.

9. A combustor according to Claim 7 or 8, wherein the biasing means com prises a pressurised fluid system, wherein pressurised fluid exerts a force on the member to bias it into the restricting position.

1 0. A combustor according to any of Claims 7 to 9, wherein the biasing means comprises control means to vary the biasing force exerted on the said member.

11. A combustor according to Claim 10, wherein the control means comprises a pressure sensor arranged to sense the pressure in the combustor.

12. A combustor according to Claim 11, wherein the pressure sensor comprises a pressure tapping in the combustor.

1 3. A combustor according to Claim 11 or 12, wherein the control means com prises a switch and the control means is arranged to vary the biasing force on the said member in accordance with the position of the switch.

14. A combustor according to Claim 1 3 when dependent on Claim 9, wherein fluid passing through the pressure tapping is directed through a valve mov able between a position in which the fluid pressurises the restrictor so that the pressure differential across the said member is negligible, and a position in which the valve directs the fluid passing through the bore to a region of low pressure.

1 5. A combustor according to any preceding claim, wherein the combustor comprises a swirler, and the restrictor is arranged so that the said member restricts the flow of air through the swirler.

1 6. A combustor according to Claim 15, wherein the restrictor is arranged so that the member restricts the flow of air through a slot or passage of the swirler.

17. A combustor according to Claim 16, wherein the restrictor is arranged so that the said member moves into and out of a slot or passage to restrict the flow of air therethrough.

1 8. A combustor according to Claim 16, wherein the restrictor is so mounted that the said member is outside the swirler adjacent the entrance to a slot or passage, and movement of the said member obscures or reveals the said en trance, to restrict the flow of air into the swirler.

1 9. A combustor according to according to Claim 1 6 to 18, wherein the swirler comprises a plurality of slots or passages and a restrictor is provided for one or more of the said plurality of slots or passages.

20. A combustor according to Claim 19, wherein more than one of the slots is provided with a restrictor, and wherein the members of the restrictors re strict the flow of fluid by different amounts when in their restricting posi tions.

21. A combustor according to Claim 19 or 20, wherein the restrictors are grouped so that the said members of a first group of restrictors move away from their restricting position when a first change in pressure differential has occurred, and the said members of a second group of restrictors move away from their restricting position when a second change in pressure differential has occurred.

22. A combustor according to any preceding claim, wherein fuel injectors are mounted on or adjacent to the swirler.

23. A gas turbine comprising a combustor according to any of Claims 1 to 22.

24. A combustor substantially as described with reference to, and as shown in, Figures 1 to 4, and Figure 7.