GB2297830A - Radially staged annular combustor - Google Patents

Abstract

A radially staged annular combustor has a hollow annular member 32 between the main 40 and pilot 38 combustor regions which divided internally by wall 48 into separate chambers which discharge air into the main and pilot combustion regions. The separate chambers within the hollow member receive air from a common source but communication between the exit ports (44, 46) Fig. 3 is prevented by the internal divider wall in order to damp out mutually interfering oscillations of the air flows into the main and pilot combustion zones. The annular member is formed in segments that are bolted to end wall 28. Ajoining segment have a seal therebetween.

Description

RADIALLY STAGED ANNULAR COMBUSTOR The invention concerns radially-staged annular combustors for gas turbine engines.

Radially-staged combustors having pilot and main burner sections at different radial spacings within an annular combustor have been proposed as a way of achieving reduced emission levels, especially oxides of nitrogen, compared with conventional single annular combustors.

A radially staged combustion of this type is known, for example, from published European Patent Application No 0,476,927. In this arrangement the radially staged combustion zones are defined by radially outer and inner annular combustor domes which are disposed coaxially about a combustor centreline axis together with a hollow annular centrebody disposed between the inner and outer domes. Although not described in said published document it is well known in the art to cool said annular centrebody by promoting a flow of air through its hollow interior. The base or upstream portion of the centrebody which lies between adjacent circumferences of the annular combustor domes is in open communication with a source of pressurised air at the combustion chamber inlet.Outlet apertures in the walls of the centrebody form a passageway from the higher pressure source at the inlet to the interior of the annular combustors thus fulfilling a dual function of providing a metered quantity of air directly into the combustion region and serving to cool the centrebody itself.

It has been found in practice, notwithstanding stable conditions of flow through the centrebody during established burning in one or both of the combustion regions, that resonance can occur in the centrebody during changeover periods. This can lead to unstable burning conditions and possibly extinction of one or other of the burners. The present invention is intended to overcome this drawback.

According to one aspect of the invention there is provided a radially-staged annular combustor having pilot and main burner stages at different radial spacings separated by an annular divider at a nominal intermediate radius, said annular divider being formed as a hollow body partitioned into separate chambers from which air issues into the radially inner and outer burner stages.

Preferably the separate chambers of the annular divider are in communication with a common source of air.

The hollow body of the annular divider may be partitioned into separate radially inner and outer chambers, and preferably the air flows through the radially inner and outer chambers of the divider are unequal in accordance with the flow division between the main and pilot burners.

An example of the invention and how it may be carried into practice will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a part-sectional view of an annular combustor module, Figure 2 is a detail view on a radial plane of one limb of the combustor of Figure 1, and Figure 3 is a detail view of a part circumferential segment of the annular centre splitter of the combustor of Figures 1 and 2 showing an inter-segment seal.

Referring now to the drawing,s Figure 1 shows an annular combustor module of a gas turbine aeroengine designated generally by reference 2. This module 2 consists of generally co-axial inner and outer annular casings 4,6 respectively which define therebetween an annular combustion section volume 8. An air inlet 10 to this annular volume 8 is provided by an annular array of circumferentially spaced guide vanes 12, towards the left side of the drawing of Figure 1. The inlet 10 constitutes the delivery outlet of a high pressure (HP) compressor section and the vanes 12 constitute HP compressor outlet guide vanes although they may be replaced by the outlet of a diffuser.At the downstream side of the combustor module, to the right of the drawings, there is a combustor outlet nozzle 14 defined by a second annular array of circumferentially spaced guide vanes 16 which, in the complete engine, constitute HP turbine inlet nozzle guide vanes.

Within the combustion volume 8 there is mounted an annular combustion chamber generally indicated by reference 18. In Figure 1 a portion of the annular chamber is cut-away in order to reveal more clearly details of its internal construction. Figure 2 also shows more detail of the combustor in a radial section view.

The annular combustion chamber 18 is, as previously mentioned, of the kind referred to as being radially staged. That is, it has separate combustion zones pitched at different radii known as pilot and main zones which may be fuelled and burned individually. The present example has two combustion zones and is commonly referred to as a double annular combustor, it is to be understood, however, that the invention will be found useful in any number of multiple annular combustors.

The construction of the combustion chamber 18 consists of: an outer annular chamber wall 20 which is roughly cylindrically but tapers in a diameter towards its downstream end, that is towards the right hand side in Figure 2, so that the wall profile provides a smooth flow surface onto a radially outer shroud 22 of the exit guide vanes 16 in outlet 14, an inner annular chamber wall 24 mounted co-axially with outer wall 20 and which also has a diameter increasingly progressively towards the downstream outlet to provide a smooth profile onto a radially inner shroud 26 of the exit guide vanes 16. The upstream edges of the concentric annular chamber walls 20,24 are joined to an annular metering panel 28 which, in turn, carried an annular, domed section combustor head 30 on its upstream side. In Figure 2 upstream is towards the left side of the drawing.

A hollow annular member 32 is mounted on the downstream face of the panel 28 at an intermediate radius. This annular member 32 projects in a downstream direction into the combustion chamber internal volume and effectively divides it into two radially staged combustion zones 34,36. Fuel/air mixture is introduced into these two combustion zones by pilot and main airspray burners 38,40 respectively mounted in the panel 28. There are a plurality of pilot burners 38 spaced apart circumferentially around the annulus of the outer combustion zone, and similarly a plurality of main burners 40 spaced apart circumferentially around the annulus of the inner combustion zone. The pilot and main burners being on opposite sides of the annular splitter 32, as will be apparent from the drawings.

In the particular example illustrated the pilot and main burners 38,40 are paired and each pair is joined to a common burner stem 42 but each burner is supplied individually with fuel via individual fuel galleries (not shown) formed in the interior of a stem and in communication with the burner fuel outlet. The burner stems 42 are mounted in the wall of the combustion chamber outer casing 6 and the pilot and main burner fuel galleries are fed from respective fuel supply pipes, also not shown. Combustion air enters the burner mixing passages and the combustion chamber through apertures in the domed head 30 and opening in the upstream sides of the burners.As is well known in the field a proportion of the air is inducted through the burners to be mixed with fuel while the remainder to support the combustion process enters through apertures formed in the metering panel 28 and also through the annular splitter member 32.

A further quantity of air may be allowed through apertures in the chamber walls to cool the walls. In addition these walls may be lined on the inside by a layer of refractory tiles. In another arrangement the combustion chamber walls are formed of a ceramic material.

The present invention is concerned with improvements which may be brought about by and with respect to the annular splitter member 32. The purpose of this member is to separate the pilot and main burner stages and, in particular, to confine fuel/air mixture from the pilot burner to the pilot combustion zone during pilot stage operation when the main burner is extinguished. The annular member 32 functions as a physical barrier between the pilot and main combustion 34,36 immediately downstream of the burners 38,40 and extends downstream in a substantially axial direction for up to about half the axial length of the combustor. As previously mentioned, and as can be seen more clearly by reference t Figures 2 and 3 of the drawings, the member 32 is also hollow and is formed with a multiplicity of apertures 44,46 in its radially outward and inward facing surfaces respectively.

The interior of the hollow annular member 32 is in communication with a source of compressed air. Thus streams of pressurised air are directed into the flame propagation zones from the apertures 44,46. It has been found that this kind of arrangement is susceptible to instability, especially when the main burners are ignited, and there is a tendency for the airflow through the hollow member 32 to resonate. A solution to this problem is to decouple the two arrays of apertures 44,46 from each other by means of an internal dividing wall 48 which extends around the entire annulus of the member 32.

The construction of the annular member 32 is apparent from Figure 3 in particular. The annular member 32 comprises a plurality of part-annular segments mounted on the panel 28 in end-to-end relationship. In Figure 3 two such segments 32a, 32b are illustrated but all such segments are constructed identically. Each segment is formed with an outer wall 50 U-shaped in section which extends in a circumferentially direction thereby defining an internal volume which is divided by the internal wall 48 into radially inner and outer chambers. The two limbs of the U-shaped outer wall 50 extend parallel to the combustor axis and terminate in a plane which lies transverse to the combustor axis. The internal wall 48 is formed integrally with the outer wall 50 and extends from the curved tip of the segment, at the curved base of the U, as far as the same transverse plane.In the assembled combustor the distal ends of U-shaped wall 50 and internal wall 48 abut the downstream face of metering panel 28.

Each of the part-annular segments 32 is mounted on the panel 28 by means of a threaded stud 52. This stud 52 is formed integrally with the internal wall 48 towards one end of the segment in a circumferential direction. For mounting the stud is inserted through a clearance hole 54 formed in panel 28 and secured by a lock nut 56. The adjacent part-annular segment 32 is located by means of a dual function U-shaped sealing strip 58 inserted into similarly shaped location grooves 60 in the abutting end faces 62a, 62b of the segments 32a, 32b. The strips 58 serve to locate the "floating" end of segment 32 a relative to the "secured" end of segment 32b and to control leakage of pressurised air from the interior of the segments. Communication with the interior of the segments 32 is through apertures 64 formed in the panel 28 in register with the open mouths of the U-shaped segments.Pressurised air is thereby free to flow from a source of such air in the cavity formed between the domed combustor head 30 and the metering panel 28. The communicating apertures 64 in metering panel 28 are preferably formed as a circumferential array of discrete apertures spaced apart around the panel, thus portions of the panel remain between apertures to carry loads in the panel and, in some cases, to receive the mounting studs 52. These communicating apertures may be arranged in a single circumferential array or in two concentric arrays matching the inner and outer chambers of the hollow annular member 32 on opposite sides of the dividing wall 48.

The airflow requirements of the main and pilot zones of the staged combustor are not necessarily equal. In general the main combustor stage has a greater requirement and it may be expected that a corresponding division of airflows between the radially inner and outer, or main and pilot, chambers of the annular member 32 will be required. The main chamber may be expected to pass a greater proportion of the airflow through the member and, therefore, the total area of the entry and exit apertures may be greater than for the pilot chamber.

This may be achieved in several ways. For example: the dividing wall 48 may be biased towards one side or the other, ie it may be offset to one side and therefore not bisect the internal volume of the annular member 32. The exit apertures may be sized accordingly to be relatively smaller on the pilot side or fewer in number. The entry throats at the upstream ends of the member segments adjacent the panel 28 may be sized as required such as by inwardly turned lips which reduce throat depth more on the pilot side than on the main side. The apertures 64 in the metering panel may be sized or reduced/increased in number to provide the desired split. The measures may be adapted individually or in any combination to produce the desired result.

Claims (9)

1A radially-staged annular combustor having pilot and main burner stages at different radial spacings separated by an annular divider at a nominal intermediate radius, said annular divider being formed as a hollow body partitioned into separate chambers from which air issues into the radially inner and outer burner stages.

2 A radially-staged annular combustor as claimed in claim 1 wherein the separate chambers of the annular divider are in communication with a common source of air.

3 A radially-staged annular combustor as claimed in claim 1 or claim 2 wherein the hollow body of the annular divider is partitioned into separate radially inner and outer chambers.

4 A radially-staged annular combustor as claimed in any preceding claim wherein the annular divider is partitioned into separate radially inner and outer chambers by an annular dividing wall.

5 A radially-staged annular combustor as claimed in claim 4 wherein the radially inner outer surfaces of the annular divider are formed with inwardly facing and outwardly facing ports.

6 A radially-staged annular combustor as claimed in any preceding claim wherein the air flows through the separate chambers of the divider are unequal in accordance with the flow division between the main and pilot burners.

7 A radially-staged annular combustor as claimed in claim 6 wherein the air flows through the separate chambers of the divider enter via entry ports in the base of the divider.

8 A radially-staged annular combustor as claimed in claim 7 wherein the entry ports are sized according to the division of air flows between the separate chambers.

9 A radially-staged annular combustor substantially as hereinbefore described with reference to the accompanying drawings.