WO2008059045A1

WO2008059045A1 - Improvements in or relating to combustion apparatus

Info

Publication number: WO2008059045A1
Application number: PCT/EP2007/062435
Authority: WO
Inventors: Nigel Wilbraham
Original assignee: Siemens Aktiengesellschaft
Priority date: 2006-11-17
Filing date: 2007-11-16
Publication date: 2008-05-22
Also published as: GB0622963D0; GB2443839A

Abstract

A combustion apparatus includes a first combustor (A) connected to a second combustor (B) by way of a passage (30, 34), in which is disposed a swirler (32). The swirler may be of the axial (32) or radial (70, 100) type. The passage is orientated approximately perpendicularly to a direction of flow of the combustion products produced, during normal use, by the first (A) and second (B) combustors. In an optional initial stage of operation, a flame ignited in the first combustor is accelerated by a pressure drop, which is created by the swirler (32, 70, 100), through the passage into the second combustor, where it ignites a flame from fuel and air supplied to the second combustor. Thereafter, in a main stage of operation, the combustion products produced by the first and second combustors exit the combustors to drive a turbine, and a portion of the combustion products of the first combustor is fed along the passage and is accelerated by the pressure drop into the second combustor where it penetrates deep into the second combustor, thereby assisting in proper mixing of the fuel and air in the second combustor.

Description

IMPROVEMENTS IN OR RELATING TO COMBUSTION APPARATUS

The present invention relates to a combustion apparatus and especially to a combustion apparatus for use in a gas turbine engine .

So-called "reburn" systems are known in connection with gas turbine engines, in which the combustion products of a first combustor are arranged to be fed into a second combustor, where they are burned a second time. The point of this is that various harmful emissions (e.g. NOx) can be reduced by this means.

An example of a known reburn system is shown in Fig. 1, which is taken from United States patent 6,050,078, issued on 18^th April 2000 to ABB Research Ltd as assignee. In Fig. 1 a first combustion stage 1, which may be annular in shape, is associated with one or more burners 2, which are fed with air 3 and fuel 4. The hot gases 5 produced in the first combustion stage 1 are fed into a second combustion stage 6, where they are reburned. The resulting working gases 7 of the second combustion stage are then passed to a downstream turbine stage 8, which consequently performs work. This arrangement may be termed a sequential reburn system, since the input of the second combustion stage 6 is formed by the output of the first combustion stage 1.

A further example of a known reburn system is illustrated in Fig. 2, which is derived from US 6,192,669, issued on 27^th February 2001 to assignee ABB AG. In this system a combustion chamber is configured as a toroid 9, which extends around a rotor (not shown) . A number of premix combustors 10 are disposed in the peripheral direction of the toroidal chamber 9. The premix combustors 10 are fed from a plenum with combustion air from a compressor and the hot gases produced inside the combustors 10 enter the toroid 9, where they turn into a vortex flow of gases. This vortex flow exits into a hot-gas duct, the end of which is preferably fitted with guide vanes in a peripheral direction. The appropriately guided flow in the hot-gas duct (not shown) is then admitted to turbine blades, which perform the desired work.

It is also known for combustion systems, and this includes Dry Low Emission (DLE) systems, to employ devices designated as cross-fire tubes or interconnectors . Cross-fire tubes are ducts which connect together a combustor with one or more other combustors in parallel. These combustors are arranged in usually equidistantly spaced manner around the periphery of a gas turbine engine. Each combustor may or may not have its own igniter. In a system in which not every combustor has an igniter, one combustor alone may have an igniter and produce a flame, when ignited. This flame is then propagated through the cross-fire tubes to each of the other combustors in series, igniting a further flame in each of these other combustors as it goes. These other combustors, which are "secondarily" ignited, may thus be called "slave" combustors in contrast to the "master" combustor, which is subject to "primary" ignition. The flames are then maintained by air and fuel supplied to each combustor. Examples of known cross-fire tube arrangements are described in EP 0503018, published on 27^th December 1995, and GB 2339468, published on 26^th January 2000.

The efficiency of the known reburn systems is limited by various factors. One such factor is the degree to which the various elements - the incoming combustion products from the first-stage (pre-mix) combustors, the air and fuel entering the second-stage (re-burn) combustor and the combustion products generated in the re-burn combustor - can mix properly. Inadequate mixing can mean, among other things, the existence of pockets of fuel-rich mixture in the re-burn combustor, which in turn increases the production of harmful emission products such as NOx and CO. It is therefore desirable to be able to increase this efficiency. It is also desirable to ensure that adequate reburn takes place in all operational states of the machine (typically a gas turbine engine) , in which the reburn system is being used.

In accordance with the present invention there is provided a combustion apparatus including a first combustor which, in use, generates first combustion products flowing in a first general direction, a second combustor which, in use, generates second combustion products flowing in a second general direction, and a passage for transporting a portion of the first combustion products from the first combustor to the second combustor, such that the first combustion products leave the first combustor in a general direction transverse to the first general direction and enter the second combustor in a general direction transverse to the second general direction, the passage including a device for increasing the velocity of flow of the first combustion products from the first to the second combustor, the device imparting a swirling motion to the flow along the passage.

The device may be a swirler, and more particularly, an axial swirler .

In this case, a portion of the passage is disposed between the axial swirler and the first combustor and has a reduced cross-sectional area relative to a cross-sectional area of the axial swirler including air-inlet ports thereof, thereby to allow the axial flow of air into said air-inlet ports.

The passage may be arranged such as to allow an axial flow of at least a portion of the first combustion products into air- inlet ports of the axial swirler.

A portion of the passage may be disposed between the axial swirler and the first combustor and have a cross-sectional area at least as great as a cross-sectional area of the axial swirler including a portion or the whole of said air-inlet ports, thereby to allow the axial flow of the portion of the first combustion products into the air-inlet ports.

The passage may comprise air-inlet means for allowing the axial flow of air into the air-inlet ports of the axial swirler along with the portion of the first combustion products .

The passage may comprise fuel-inlet means for allowing the flow of fuel along with the first combustion products.

The fuel-inlet means may comprise a fuel manifold supplying fuel ducts located at leading edges of swirl vanes of the axial swirler, the fuel ducts having injection holes for allowing the flow of fuel from the fuel manifold into the axial swirler.

In an alternative arrangement, the device may be a radial swirler .

In this case, a portion of the passage may be disposed between the radial swirler and the first combustor and have a cross-sectional area greater than a cross-sectional area of the radial swirler, such as to form a manifold in communication with air-inlet ports of the radial swirler, thereby to allow the radial flow of the portion of the first combustion products into the air-inlet ports.

The passage may comprise air-inlet means for allowing a radial flow of air into the air-inlet ports of the radial swirler along with the portion of the first combustion products.

Whichever type of swirler is employed, the apparatus may be arranged such that, during a start-up phase of the apparatus, a flame in the first combustor is accelerated by the device into the second combustor, thereby to ignite the second combustor .

The apparatus may comprise a plurality of the first combustors connected to the second combustor by way of respective said passages, each of the passages including the velocity-increasing device.

Embodiments of the invention will now be described, by way of example only, with the aid of the drawings, of which:

Fig. 1 is a simplified representation of a known sequential reburn system in a gas-turbine engine;

Fig. 2 is a perspective diagram of a further known reburn system for use in a gas-turbine engine;

Fig. 3 is a sectional drawing of a typical combustor for a gas turbine engine; Figs 4A and 4B are side and perspective views, respectively, of a combustion apparatus in accordance with an embodiment of the invention;

Figs 5A-5C are various perspective views of an axial swirler arrangement as employed in a first embodiment of a combustion apparatus according to the invention; Figs 6A and 6B are side- and end-views, respectively, of the axial swirler arrangement of Figs 5A-5C in a variant thereof; Figs 7A and 7B are perspective views of a radial swirler arrangement as employed in a second embodiment of a combustion apparatus according to the invention; Fig. 8 is a side-view of the radial swirler arrangement of

Figs 7A and 7B in a variant thereof; Figs 9A and 9B are a simplified sectional view and a simplified perspective view, respectively, of an angled-jet device for use in an embodiment of a combustion apparatus according to the invention; Figs 1OA and 1OB are end- and top-views, respectively, of a third embodiment of a combustion apparatus in accordance with the invention; Figs HA and HB are a top view and a perspective view, respectively, of a more practical version of the embodiment of Figs 1OA and 1OB; Fig. 12 is an end-view of a fourth embodiment of a combustion apparatus in accordance with the invention, and Fig. 13 is a side view of the third embodiment and illustrating a preferred method of ensuring the flow of combustion products from master to slave combustor.

Fig. 3 shows a typical combustor for a gas turbine engine. The combustor comprises a combustor head section 10, through which fuel lines 12 and 14 are run. Fuel line 12 is a main fuel line, while fuel line 14 is a so-called pilot fuel line. A flame 16 is ignited in the combustor and is kept lit by the supply of fuel in the fuel lines and air through air-inlet ports 18. Combustion products arising from the combustion of the fuel by the flame are propagated down a combustion- chamber portion 20 of the combustor and introduced into a turbine housing downstream of the combustion chamber, thereby to drive the turbine and produce power.

As mentioned at the start, gas turbine engines sometimes employ a system of cross-firing, in which case the flame initially ignited in the combustor shown in Fig. 3 will be propagated down a cross-fire tube (not shown) to another combustor, which may or may not have its own ignition capability. The present invention is based on an adaptation of the cross-fire tube and such an adaptation is shown, as part of an embodiment of the invention, in Figs 4A and 4B.

Fig. 4A includes a pair of combustors: a master combustor (combustor A) and a slave combustor (combustor B) . Combustor A may be configured as shown in Fig. 3, for example, and therefore has its own fuel and air supplies. Combustor B may be similarly configured, having its own fuel and air supplies, but without any means of igniting its own flame. As shown, combustors A and B would correspond to the combustor of Fig. 3 turned through 90 degrees, so that the fuel lines enter the combustor head from the top in Fig. 4A. The combustion products from both combustor A and combustor B therefore flow downwards in Fig. 4A. Connecting combustors A and B is a passage 22, which communicates with the side portions of the two combustors. The passage 22 comprises at an upstream end thereof a passage portion 30, which is connected to combustor A at an intermediate point along its length. Passage 30 connects at its other end to a swirler 32, which in the example shown in Figs 4A and 4B is an axial swirler. A further passage portion 34 connects the swirler 32 with combustor B, likewise at an intermediate point along its length.

In use, a flame 35 is ignited in combustor A by suitable means and the combustion products from this ignition flow downward out of the combustor toward the turbine part of the engine (see arrow 36 associated with combustor A) . It is assumed in this scenario that combustor B does not have its own ignition capability. Two alternative, subsequent secondary-ignition procedures are then possible. In the first procedure, no air is fed into the air-inlet ports 40 of the swirler 32. Since there is a difference between the pressure in combustor A and that in combustor B (this pressure difference takes the form of a higher pressure in combustor A due to the ignition of that combustor) , part of the flame in combustor A will be drawn through passage 22 into combustor B, where, if combustor B is supplied with its own fuel and air, a flame will be ignited and sustained. In the second procedure, air is introduced into the air-inlet ports 40 of the swirler 32, and this air is imparted a rotational component of flow by the orientation of the swirler vanes at an angle to the longitudinal axis of the passage 22. This rotational component establishes a swirling action, hence the name "swirler". As a result of the swirling action, a further pressure drop, additional to that already existing between the combustors, is created in the direction shown by the arrow 42. This further pressure drop acts to accelerate the flame present in combustor A and a portion of the hot combustion products being produced in combustor A out of combustor A through the passage portion 30, through the radially inner part of the swirler 32 and along the passage portion 34 into the side of combustor B. At the same time, as with the first procedure, fuel and air are fed into the top of combustor B. The fuel is ignited by the incoming flame, thereby providing combustor B with its own flame, which is maintained by this combustor' s own air and fuel supplies. Consequently combustor B produces its own combustion products, which flow in a downward direction (see arrow 36 associated with combustor B) into the turbine part of the engine. In a system in which combustor B has its own ignition facilities, the cross-firing capability of the arrangement just described acts as a useful backup, ensuring that secondary ignition can take place in combustor B, should combustor B' s own primary ignition process fail.

Subsequent to the just-described secondary ignition process, the portion of the combustion products in the combustion chamber of combustor A already being drawn off along the duct continues to be drawn off as flow 38 along the passage portion 30 and into the axial swirler 32. To achieve this, air continues to be fed into the air-inlets 40 of the swirler 32, so that the swirler 32 imparts the above-mentioned rotational component of flow to the already axial component of flow of the combustion products flowing down the passage from combustor A. These products therefore continue to enter the side of combustor B with such a rotational component. This would not be the case in the conventional cross-flow situation, since the pressures in the two ignited combustors would be arranged to be equal. In the invention, however, the swirler creates a rotational component of flow, thereby increasing the velocity of flow of the combustion products from combustor A due to the pressure drop shown by the arrow 42. Consequently, the combustion products in the passage enter combustor B at relatively high speed and thereby penetrate more deeply into combustor B than if the swirler were not present. This, together with the already imposed rotational component, gives rise to better mixing of the air and fuel in combustor B and thereby increases the efficiency of combustor B. In the process, the combustion products from combustor A are "re-burned" in combustor B in a manner, and with the concomitant advantages, described earlier in connection with the known re-burn systems.

It should be noted that the above-described initial secondary-ignition process is not an essential part of the invention. Thus, a cross-fire type tube can be used in this respect purely as a re-burn facilitating device, without any secondary ignition taking place. This will be the case when combustor B is ignited simultaneously with combustor A. Where secondary ignition is necessary, however, the duct can usefully act as both a cross-fire tube and a re-burn duct.

An example of a suitable axial swirler is illustrated, by way of example only, in various views in Figs 5A, 5B and 5C. These figures show the passage portion 30, which passes through the centre of the swirler 32. The swirler 32 has an outer wall 50 of greater diameter than the passage 30, and between this outer wall and the passage are arranged a series of air-inlet ports 40, which are defined by adjacent vanes 52. As already mentioned, the vanes 52 are orientated at an angle to the longitudinal axis of the passage portion 30.

Also shown in these figures is a fuel manifold 54, which is fed from a fuel line (not shown) . The manifold 54 communicates with fuel ducts 56 located at the leading edges of the vanes 52 and holes 58 are provided at an intermediate point of the ducts 56 to allow fuel to pass into the swirler from the manifold. The holes 58 are orientated approximately perpendicularly to the longitudinal axis of the passage portion 30. By this means fuel can be introduced into the swirler along with air and this can assist the combustion in combustor B .

In a variant of the first embodiment just described, the combustion products from combustor A are allowed to flow not only through the inner part of the swirler, but through the air-inlet ports 40 also. This may be achieved by increasing the diameter of the passage portion 30 so that it includes the air-inlet ports 40. An example of this is shown in Fig. 6A, in which the passage portion 30 is seen to flare outwards as a flared portion 31 near to the swirler. The fuel-manifold 54 and ducts 56 may be retained in this variant. In this arrangement the swirling action is created by the imposition of a rotational flow component on the combustion products, rather than on air introduced from outside. This has the advantage that the flow out of the apparatus into the slave combustor will disperse and mix more easily. In addition, less unburned air is used. Alternatively, both combustion products and air may be allowed to flow into the swirler. This may be achieved, for example, by configuring the passage portion 30 as shown in Fig. 6B . In Fig. 6B the flared portion 31 only meets the manifold 54 at the ducts 56. Between the ducts the passage 30 is provided with a recess 60 in a radial direction referred to the longitudinal axis of the passage 30. This creates an opening 62, through which air can flow in from the outside. The pressure of the incoming air should be greater than the pressure of the combustion products within the passage. Thus the vanes 52 impart a rotational flow component on both the incoming air and the combustion products already flowing through passage portion 30 from combustor A.

In a second embodiment of the invention a radial swirler is used instead of an axial swirler. An example of such a radial swirler is shown in Figs 7A and 7B. The radial swirler 70 comprises a series of air-inlet ports 72 defined by adjacent vanes 74. Air is introduced in a direction perpendicular to the longitudinal axis of the passage 30. This imparts a rotational component to the axial flow of combustion products flowing from combustor A through the passage portion 30, into the swirler and out through the passage 34 to combustor B. The result, as with the already described axial swirler, is a pressure drop in a direction from combustor A to combustor B, resulting in an acceleration of the combustion-product flow toward combustor B.

As with the axial swirler, the air-inlet ports of the radial swirler may be supplied with combustion products instead of air. This can similarly be achieved by enlarging the diameter of passage portion 30 at least in a region proximate to the swirler. This time, however, the enlarged-diameter portion should be greater in diameter than the outside diameter of the swirler, including the air-inlet ports. A possible configuration is shown in Fig. 8. In Fig. 8 the passage 30 is shown flaring out as portion 76 and surrounding the swirler 70, so as to leave a space between the air-inlet ports and the inside of the flared portion. This space allows a portion of the products flowing down the passage portion 30 to flow into the air-inlet ports and out into passage portion 34. As with the arrangement shown in Figs 7A and 7B, the swirler vanes are bounded on both sides with an end-wall 78.

Also as with the axial-swirler embodiment, it is envisaged that a mixture of air and combustion products may be fed through the radial swirler. In that case means (not shown) would be provided for feeding air into a suitable point of the passage portion 30 or swirler 70.

Other means for producing the pressure drop in the duct joining the two combustors may be employed. One such means is an angled-jet device, as illustrated in Figs 9A and 9B . In Figs 9A and 9B the passage 30 has attached to it at intervals about its circumference a series of jets 77, which are disposed at an angle to the longitudinal axis of the passage 30 in two orthogonal planes. Air under pressure is introduced into the jets, to create the equivalent of a swirling action.

A further re-burn configuration, in which the present invention may be employed, is shown in Fig. 1OA. In Fig. 1OA there are disposed around a gas-turbine engine (not shown) two groups of three combustors. Each group comprises a pair of so-called primary (or "master") combustors 80, which are connected by way of a passage 81 to a so-called re-burn (or "slave") combustor 82. In each passage there is provided a device for increasing the velocity of flow along the passage by imparting a swirling motion to the flow, such as the swirler described in connection with the first or second embodiment. Thus, referring to Fig. 1OB, the passage 81 comprises a first passage portion 84 (corresponding to the passage portion 30 described in connection with the first and second embodiments) , which connects the primary combustor 80 to a swirler 86, and a second passage portion 88, which connects the outlet side of the swirler to the re-burn combustor 82. Combustion products from the flames inside the combustors flow generally downward in the direction of the arrows 90, while a portion of the combustion products from the primary combustors is tapped off and fed along passages 84, through swirlers 86 and passages 88 to the sides of the re-burn combustor 82. The action of these tapped-off combustion products is essentially the same as in the first and second embodiments already described.

A practical realisation of such a re-burn arrangement is illustrated in Fig. HA. In Fig. HA the master combustors 80 and slave combustor 82 are connected at their lower ends to an annular transition duct 90. The duct 90 is, in turn, connected at its lower end to a nozzle guide-vane assembly 92, which is a part of a typical gas-turbine engine, while the guide-vane assembly is connected in turn to the first- stage turbine assembly 94. In practice the combustors 80, 82, which are fed at their top end, as shown, by fuel and air and are ignited to provide a flame, produce combustion products which enter the annular duct 90 and thereby enter the nozzle guide-vane assembly. The use of such an annular duct is advantageous in a situation where the temperature of the exhaust gas (combustion products) exiting the slave combustor may not be the same as the temperature of the exhaust gas leaving the master combustors, since it allows the three exhaust streams to mix first, and thereby achieve a more uniform temperature, before entering the turbine section. This can result in longer turbine life and can also reduce harmful emissions.

Fig. HB is a perspective view of part of Fig. HA in a more representative version thereof. In this version each of the combustors 80, 82 is connected to the annular duct by way of a connecting duct 94. The ducts 94 flare out to merge into the annular duct 90.

Instead of the six-chamber arrangement shown in Fig. 1OA, it is possible to employ an eight-chamber arrangement, as shown in Fig. 12. This is the similar to the Fig. 1OA arrangement, except that the neighbouring master combustors 80 are fed to a common additional slave combustor 82 in each case. Bearing in mind the difference in temperature at which the combustion products will, in practice, leave the master combustors in comparison with the slave combustors, this provides a more balanced temperature distribution all the way around the rotor, since master alternates with slave in strict rotation. However, an annular duct may be employed, as in the Fig. 10 arrangement, in order to further enhance the uniformity of the temperature distribution.

In a scenario in which radial swirlers are employed in the passages 81, it may be necessary to provide some means for ensuring that the flow of combustion products through the passages is in the correct direction, i.e. from master to slave. This can be achieved by an arrangement such as shown in Fig. 13. In Fig. 13, radial swirlers 100 are employed in the passages 81. In order to ensure that the flow of combustion products from the master combustors 80 will be toward the slave combustor 82, the swirl number, Sni and Sn₃, of swirlers 102 and 106 associated with the master combustors is arranged to be less than that, Sn2, of the swirler 104, which is associated with the slave combustor. Since the swirl number is defined as the ratio between the tangential and axial momentum of the fluid flowing through these swirlers, the tangential velocity of the fluid flowing through swirler 104 will be higher than that through swirlers 102 and 106, resulting in a greater pressure drop across the swirler 104 than that across swirlers 102 and 106. Consequently, the static pressure at point b2, which is an area normally occupied by the flame, is lower than that at points bl and b3 in the master combustors. (The pressures at bl and b3 may, for example, be arranged to be equal by making Sni₌Sn₃, as shown in Fig. 13. Likewise, the passages 81 either side of the slave combustor 82 may be equally sized, along with the swirlers 100, though this is not essential) . This establishes the pressure differential necessary to ensure that combustion products flow from master to slave combustor. As regards the pressures at the outlets of the three combustors, the use of transition members 108 and a set of downstream disposed guide vanes enables an equalization of the pressures at points cl, c2 and c3.

Since the passage or duct joining the adjacent combustors will carry hot gases from one to the other during normal running of a gas-turbine engine, it will be necessary to employ some form of cooling for the duct. Indeed, it is common for a standard cross-fire tube to employ some means of protection against overheating, even though it is only expected to transmit a flame from one combustor to the next over a short period of time, until secondary ignition has been achieved. Where an axial swirler is used in the present invention, this may provide some cooling of its own, since the incoming air flows close to the inside wall of the duct. By contrast, when a radial swirler or an angled-jet device is employed, the incoming air will be generally radial, and not axial, so that it will then definitely be necessary to introduce additional cooling, though even with an axial swirler additional cooling is to be preferred. One way of doing this is to use a double-walled duct, with air forced between the two walls. Alternatively, or additionally, the duct may be composed of a ceramic (or similar) heat-resistant material .

Although in Figs 6A and 8 the passage portion 30 is seen to increase in diameter at a point near the swirler, the passage portion 30 may instead have such an increase in diameter either over more of its length or over its entire length up to the combustor A.

Claims

1. A combustion apparatus including a first combustor (80) which, in use, generates first combustion products flowing in a first general direction, a second combustor (82) which, in use, generates second combustion products flowing in a second general direction, and a passage (81) for transporting a portion of the first combustion products from the first combustor (80) to the second combustor (82), such that the first combustion products leave the first combustor (80) in a general direction transverse to the first general direction and enter the second combustor (82) in a general direction transverse to the second general direction, the passage (81) including a device for increasing the velocity of flow of the first combustion products from the first (80) to the second combustor (82), the device imparting a swirling motion to the flow along the passage (81) .

2. Combustion apparatus as claimed in claim 1, wherein the device is a swirler (32,70,86,100).

3. Combustion apparatus as claimed in claim 2, wherein the device is an axial swirler (32) .

4. Combustion apparatus as claimed in claim 3, wherein a portion of the passage (30,84) is disposed between the axial swirler (32) and the first combustor (80) and has a reduced cross-sectional area relative to a cross-sectional area of the axial swirler (32) including air-inlet ports (18,40,72) thereof, thereby to allow the axial flow of air into said air-inlet ports (18,40,72).

5. Combustion apparatus as claimed in claim 3, wherein the passage (81) is arranged such as to allow an axial flow of at least a portion of the first combustion products into air- inlet ports (18,40,72) of the axial swirler (32).

6. Combustion apparatus as claimed in claim 5, wherein a portion of the passage (76) is disposed between the axial swirler (32) and the first combustor (80) and has a cross- sectional area at least as great as a cross-sectional area of the axial swirler (32) including a portion or the whole of said air-inlet ports (72), thereby to allow the axial flow of the portion of the first combustion products into the air- inlet ports (72) .

7. Combustion apparatus as claimed in claim 6, wherein the passage (81) comprises air-inlet means for allowing the axial flow of air into the air-inlet ports (18,40,72) of the axial swirler along with the portion of the first combustion products .

8. Combustion apparatus as claimed in any one of the claims 4 to 7, wherein the passage (81) comprises fuel-inlet means for allowing the flow of fuel along with the first combustion products .

9. Combustion apparatus as claimed in claim 8, wherein the fuel-inlet means comprises a fuel manifold (54) supplying fuel ducts (56) located at leading edges of swirl vanes (52) of the axial swirler (32), the fuel ducts (56) having injection holes (58) for allowing the flow of fuel from the fuel manifold (54) into the axial swirler (32) .

10. Combustion apparatus as claimed in claim 2, wherein the device is a radial swirler (70,100).

11. Combustion apparatus as claimed in claim 10, wherein a portion of the passage (30) is disposed between the radial swirler (70) and the first combustor (80) and has a cross- sectional area greater than a cross-sectional area of the radial swirler (70), such as to form a manifold in communication with air-inlet ports (72) of the radial swirler (70), thereby to allow the radial flow of the portion of the first combustion products into the air-inlet ports (72).

12. Combustion apparatus as claimed in claim 11, wherein the passage (30,81) comprises air-inlet means for allowing a radial flow of air into the air-inlet ports (72) of the radial swirler (70) along with the portion of the first combustion products.

13. Combustion apparatus as claimed in any one of claims 10 to 12, wherein the passage (30,81) comprises fuel-inlet means for allowing the flow of fuel along with the first combustion products .

14. Combustion apparatus as claimed in any one of the preceding claims, wherein the apparatus is arranged such that, during a start-up phase of the apparatus, a flame in the first combustor (80) is accelerated by the device into the second combustor (82), thereby to ignite the second combustor (82) .

15. Combustion apparatus as claimed in any one of the preceding claims, wherein the apparatus comprises a plurality of the first combustors (80) connected to the second combustor (82) by way of respective said passages (30,81) , each of the passages (30,81) including the velocity- increasing device.