GB2072827A - A tubo-annular combustion chamber - Google Patents

Abstract

In combustion equipment of the multiple chamber of tubo-annular type, the emission of NOx can be reduced by providing each cylindrical combustion chamber with a pre- mixing zone 22 in which a relatively weak fuel/air mixture is substantially vapourised before passing into a main mixing and combustion zone, (24) Fig. 1 (not shown) and a pilot zone 20 in which a relatively richer fuel/air mixture is mixed and partially burnt in stable combustion conditions bedore mixing with the fuel/air mixture from the pre-mixing zone in the main combustion zone. The pre-mixing and pilot zones are fuelled from a single fuel injector 34 for each combustion chamber, the injector having a nozzle (50) for injecting fuel into the pre-mixing zone and a number of other angularly directed nozzles 52 for injecting fuel into the pilot zone at circumferentially spaced locations. <IMAGE>

Description

SPECIFICATION Gas turbine engine combustion equipment This invention relates to combustion equipment for use in gas turbine engines, and is particularly, though not exclusively concerned with combustion equipment designed to reduce the emissions of nitrogen oxide (NOx) and smoke in stationary gas turbine engines.

A number of methods are available for reducing undesirable emissions from such combustion equipment, one of such methods being known as fuel staging or staged fuel injection. This method involves injecting fuel into two different zones of the combustion equipment, e.g. a central premixing zone which can be defined by a chamber and a pilot zone usually surrounding or adjacent the pre-mixing zone, so that the fuel to air ratio in each zone can be controlled. This control is advantageous in that it allows a relatively lean fuel and air mixture to mix and to be vapourised to a significant degree in the pre-mix zone, before the mixture issues into the main section of the combustion equipment.This relatively lean and vapourised mixture will burn at a relatively low temperature, thereby reducing the emission of NOx, since it is known that NOx emission during combustion of a mixture of fuel and air is a function of combustion temperature. The fuel and air mixture in the pilot zone can be stoichiometric and is thoroughly mixed and at least partially burnt before issuing into the main section of combustion equipment, and mixing with the gasses issuing from the pre-mixing zone.

A difficulty with staged fuel injection systems is the supply of fuel to the zones of the combustion equipment. A multiplicity of fuel pipes can be avoided by the use of a unitary fuel injector having a number of fuel nozzles, of the type shown in UK patent application no. 2013788. This patent specification also shows combustion equipment having a pre-mixing zone and a pilot zone, although the combustion equipment is of the annular type only.

The present invention seeks to provide combustion equipment of the multiple chamber type or the tubo-annular type having pilot and premix zones with associated unitary fuel injectors.

The present invention provides, in combustion equipment of the multiple chamber or tuboannular type, a combustion chamber of circular cross-section having a circular outer wall and at its upstream end, an inner cylindrical wall extending into the combustion chamber and partially defining a pre-mixing zone, the pre-mixing zone having an inlet and an outlet for the inflow of fuel and compressed air and the outlet of at least a partially vapourised fuel and air mixture, respectively, a pilot zone arranged to receive fuel and compressed air and to discharge a fuel and air mixture therefrom, the pilot zone being defined by the outer wall of the combustion chamber and the inner cylindrical wall, and a fuel injector having a fuel nozzle for injecting fuel into the pre-mixing zone and a plurality of other angularly directed fuel nozzles arranged to inject fuel into the pilot zone.

The pilot zone may have fuel and air inlets each comprising an external duct arranged to receive fuel from one of the angularly directed fuel nozzles and air from the compressor of the engine of which the combustion equipment form a part, the duct being in communication with a plenum chamber having an outlet into the pilot zone normal to the fuel and air inlet, there being an air inlet adjacent the external duct so that compressed air entering the pilot zone impinges on the fuel and air mixture issuing from the outlet of the plenum chamber.

The inner and outer cylindrical walls may include air flow inlet and guiding means of the type shown in our UK patent no. 1136543 so that a double vortex flow re-circulation system is generated in the pilot zone, the vortices generated being of opposite hand.

The pre-mixing zone may comprise the inner cylindrical wall of the combustion chamber and pre-mixing flow duct attached to the inner cylindrical wall, the pre-mixing flow duct terminating in a plurality of radially directed chutes.

The pre-mixing flow duct may include a venturi portion and an annular duct may be provided between the inner cylindrical wall and the premixing flow duct for the throughflow of cooling air.

The downstream end of the pre-mixing flow duct may include a flared member spaced away from the flow duct, cooling air flowing into the space between the flow duct and the flared member from the said annular space and flowing into the combustion chamber. The flared member may be provided with holes for the throughflow of cooling air.

The present invention will now be more particularly described with reference to the accompanying drawings in which; Figure 1 shows a side elevation of a part of one form of combustion equipment according to the present invention, Figure 2 is a part view on arrow A in Figure 1, excluding the fuel injector, Figure 3 is a section on line 3-3 in Figure 2, Figure 4 is a section on line 4-4 in Figure 2, Figure 5 is a part view on arrow B in Figure 1, Figure 6 is a section on line 6-6 in Figure 1, Figure 7 is a part view on arrow C in Figures 6, Figure 8 is a part view on arrow D in Figure 1, and Figure 9 is a section on line 9-9 in Figure 1.

Referring to the Figures, combustion equipment 10, is only part of which is shown, of a gas turbine engine (not shown) comprises a plurality of equispaced combustion chambers 1 2 located within an annular housing defined by inner and outer walls, 14 and 1 6 respectively. Each combustion chamber 12 is defined by an outer cylindrical wall 18 and includes a pilot zone 20, a pre-mixing zone 22 and a main mixing and combustion zone 24, only part of which is shown. Also included but not shown is a downstream dilution zone.

The pre-mixing zone extends from the upstream end of the combustion chamber into the combustion chamber and comprises an inner cylindrical wall 25 to which is attached a venturishaped flow duct 26, an annular space 27 for the through flow of cooling air, being provided between the wall 25 and the duct 26. The duct 26 terminates in a plurality of radially directed chutes 28, the flow of cooling air from the space 27 entering the zone 24 between adjacent chutes. A flared cone 29 having holes 30 for the throughflow of cooling air, is attached to the downstream end of the duct 26.

The pilot zone 20 is annular in section and is defined by part of the cylindrical wall 1 8 and the wall 26 of the pre-mixing zone. The pilot zone 20 has six inlet ducts 32 (see particularly Figure 2) which receive compressed air from the compressor of the engine and fuel from a fuel injector 34, which will be described in detail below. Referring particularly to Figures 1, 2, 6 and 7 the air and fuel flowing through each duct 32, flows into a chamber 36 and leaves the chamber through a slot 38 in the chamber side-wall.

Adjacent each chamber 36 is a slot 40 in the head of the combustion chamber, through which compressed air flows and impinges on the flow of fuel and air issuing from the slot 38. This fuel injection arrangement is described and claimed in ourcopending application no. 79.17335 entitled "Combustion apparatus for gas turbine engines", filed 18.5.79.

The flow pattern in the pilot zone comprises two opposite handed vortices 42, 44 and the arrangement is generally of the type disclosed in our UK patent specification no. 1 136543. The vortices 42, 44 are generated by flows of compressed air through sets of apertures 46, 48 respectively.

The fuel injector 34, which is similar to the fuel injector disclosed in our co-pending UK patent application no. 2013788, although that fuel injector is concerned with fuelling an annular combustion chamber, comprises an arm having a central fuel nozzle 50 supplying fuel to the premixing zone 22, and six other fuel injectors 52, each one supplying fuel to one of the inlet ducts 32. Fuel is supplied to the injectors through supply ducts 54 and an arrangement of pipes in the fuel injector 34.

In operation fuel from the injector and compressed air from the engine compressor flow into the pre-mixing and pilot zones of the combustion chamber. The arrangement of the pilot and pre-mixing zones is intended to provide combustion equipment in which the emission of NOx is suppressed whilst also ensuring stable combustion. The supression of NOx emission is achieved by the preparation of a lean, substantially vapourised, combustible mixture in the pre-mixing zone 22. Such a mixture has the low combustion temperature required for low NOx concentration Auto-ignition of the fuel and air mixture in the premixing zone is avoided by providing conditions of laminar flow in the pre-mixing zone.Flame from the pilot zone and fresh mixture from the premixing zone mix in the main mixing and combustion zone 24 for ignition of the fresh mixture and completion of burning of the pilot mixture. The pilot zone, where relatively richer mixtures is burnt in conditions of recirculatory flow, provides the stability of combustion which the pre-mixed mixture does not have because of its lean composition. The perforated end plate 28 is designed to ensure a uniform distribution of the flow from the pre-mixing zone across the flow from the pilot zone.

Fuel is introduced into the pre-mixing zone from the nozzle 50 of the fuel injector which also receives compressed air from the compressor of the engine. The fuel and air mixture in the zone 22 is of combustible proportions and is intended to vapourise to a significant extent in the zone 22, so as to eliminate elements of liquid fuel. The vapourisation is achieved by generating a very fine spray from the nozzle 50 and by making the length of the zone 22 sufficiently long for substantial vapourisation to take place under the relatively high temperature of the compressed air.

In order to avoid premature ignition in the zone 22 which may be due to the high temperature of the air or flame migrating from the pilot zone through the chutes 28 and along the slowly moving boundary layer at the wall of the duct 26, it is arranged that the flow in the zone 22 is as nearly laminar as possible and that the flow velocity is higher than the speed of flame propagation.

The laminar flow is achieved by the duct 26 being venturi-shaped and having a suitable rate of diffusion.

The venturi shape also increases the flow velocity in the duct and thus the shear between the fuel and air. The radially directed chutes 28 force the fuel and air mixture from the pre-mix zone to penetrate the fuel and air mixture from the pilot zone so that the contact area between the two mixtures is as high as possible to improve the heat flux and to maximise the mixing between the two mixtures.

Cooling air flows through the annular duct 27 to cool both the wall 25 and the duct 26 and thence flows into spaces defined by the walls of adjacent chutes 28. The cooling air flows into the space between the flared member 29 and the downstream end 31 of the duct 27 and thence into the zone 24, either between the end 31 and the member 29 or through the hole 30. These flows of cooling air wash the downstream faces of the duct 27 and the member 29 to minimise the deposition of carbon on these faces.

The pilot zone 20 receives fuel from the nozzles 52 of the fuel injector and the fuel and compressed air from the engine compressor flow into the chambers 26 through ducts 32. Partial vapourisation takes place in the chambers 36 and the partially mixed and vapourised fuel and air mixtures enters the pilot zone through the slots 38, when the mixtures are impinged upon by a flow of compressed air entering the pilot zone through the slots 40. This impingement assists in the mixing together of the fuel and air. The fuel/air mixtures is then substantially completely mixed in the two vortices 42, 44 generated by air flows through the sets of apertures 46, 48 respectively.

These re-circulations of flow provides the burning mixtures with a sheitered residence necessary for stable combustion of a wide range of fuel flow.

An igniter (not shown) is provided for initiating combustion in the pilot zone 20 when the engine is started.

The relative strength of the mixtures in the premixing and pilot zones is such that the mixture eventually established in the main zone 24 is sufficiently lean, say 3040% of the stoichiometric to have a burning temperature low enough to significantly reduce the emission of NOx. A certain proportion of the fuel will reach the main zone in droplet form from both zones 20 and 22 and will tend to burn with a locally high NOx emission, but overall such emission is reduced.

The pre-mix mixture may use about 50% of the compressor delivery air and itself have a mixture strength of 50% of stoichiometric while the pilot zone may have a mixture strength of 70-1 00% of stoichiometric.

Claims (8)

1. In combustion of the multiple chamber or tubo-annular type, a combustion chamber of circular cross-section having a circular outer wall and at its upstream end an inner cylindrical wall extending into the combustion chamber and partially defining a pre-mixing zone, the pre-mixing zone having an inlet and an outlet for the inflow of fuel and compressed air and the outlet of at least a partially vapourised fuel and air-mixture respectively, a pilot zone arranged to receive fuel and compressed air and to discharge a fuel and air mixture therefrom, the pilot zone being defined by the outer wall of the combustion chamber and the inner cylindrical wall, and a fuel injector having a fuel nozzle for injecting fuel into the pre-mixing zone and a plurality of other angularly directed fuel nozzles arranged to inject fuel into the pilot zone.

2. Combustion equipment as claimed in claim 1 in which the pilot zone includes a plurality of plenum chambers each being arranged to receive a supply of compressed air and fuel from one of the said angularly directed fuel nozzles through a duct aligned with the respective fuel nozzle, each plenum chamber having an outlet for the discharge of fuel and air therefrom, the pilot zone having a plurality of other air inlets, each said other air inlet being located adjacent a respective plenum chamber so that air entering the pilot zone through each said other air inlet impinges upon the fuel and air mixture issuing from the outlet of each respective plenum chamber.

3. Combustion equipment as claimed in claim 1 or claim 2 in which the pilot zone has a plurality of further air inlets in the outer and inner walls arranged to generate a double vortex flow pattern in the pilot zone, the vortices being opposite handed.

4. Combustion equipment as claimed in any one of the previous claims in which the pre-mixing zone comprises a pre-mixing duct secured to the inner cylindrical wall of the combustion chamber and defining therebetween an annular duct for the throughflow of cooling air, the pre-mixing duct including a venturi portion.

5. Combustion equipment as claimed in claim 4 in which the pre-mixing duct terminates in a plurality of radially directed chutes, through which a fuel and air mixture flows.

6. Combustion equipment as claimed in claim 5 in which the radially directed chutes are spaced apart circumferentially, the cooling air from the annular duct between the pre-mixing duct and the inner cylindrical wall flowing through the spaces defined by the walls of adjacent chutes.

7. Combustion equipment as claimed in any one of the preceding claims 4 to 6 in which the premixing duct includes a flared member attached to its downstream end, the flared member having a plurality of apertures for the throughflow of cooling air from the annular duct.

8. Combustion equipment constructed and arranged for use and operation substantially as herein described, and with reference to the accompanying drawings.