GB2100852A - Fuel and air injectors for use in gas turbine engines - Google Patents

Abstract

A fuel and air injector (27) for combustion chambers (23) of gas turbines using catalytic ignition, comprises two plates (28 and 29) bonded together, each plate having an arrangement of holes, which combine to form apertures (30), and there is also an internal arrangement of passages (32) for conveying fuel from a fuel supply pipe (31) to each aperture (30). Air enters the combustion chamber (23) through an aperture (26) in a head (25) and flows into the aperatures (30) of the fuel and air injector (27), fuel is then injected from passages (32) into the apertures (30) to give efficient mixing with the air. Further mixing of the fuel and air takes place in mixing zone (33). The mixing efficiency for each aperture (30) is proportional to the l/d ratio, where l is the length of the combustion chamber (23) available for mixing, and d is the diameter of the aperture. The l/d ratio and mixing efficiency for the combustion chamber (23) becomes that for a single aperture (30). <IMAGE>

Description

SPECIFICATION Fuel and air injectors for use in gas turbine engines This invention concerns fuel and air injectors for the combustion chambers of gas turbine engines, particularly for combustion chambers using catalytic ignition.

In order to meet anti-pollution regulations it is becoming necessary to control the exhaust emission of gas turbines. Excessive levels of undesirable emissions indicate inefficient combustion, and also have unwanted effects on the atmosphere.

The emissions from gas turbines comprise unburnt hydrocarbons (UHC), carbon monoxide (CO), oxides of nitrogen (NOx) and smoke.

Carbon monoxide and unburnt hydrocarbons are generally emitted when the engine is running at low power conditions when the air pressure and air temperature are too low, resulting in efficient combustion.

Oxides of nitrogen and smoke occur in significant quantities at high power conditions of the engine, due particularly to insufficient mixing between the fuel and air.

It is desirable for engine combustion systems to operate so as to minimise the emissions of these products in all operating conditions and to burn fuel efficiently.

Low emission levels of carbon monoxide and unburnt hydrocarbons can be achieved by burning rich mixtures of fuel and air but cause higher emissions of NOx and smoke, and low emission levels of nitrogen and smoke can be achieved by burning lean fuel/air mixtures, but cause higher emissions of CO and UHC, and also poor flame stability.

A potential solution to this apparent conflict is to burn a relatively lean fuel/air mixture at all operating conditions to avoid excessive emissions of NOx and smoke at the high power condition, and thoroughly pre-mix the fuel and air to provide an homogenous mixture and burn this mixture at as near 100% combustion efficiency as possible to reduce CO and UHC, and improve weak flame stability.

A catalytic ignition combustion chamber can achieve lower emission levels if lean-mixtures of fuel and air are thoroughly premixed to disperse local rich pockets of fuel, prior to ignition and burning of the mixture in a platinum catalyst.

The platinum catalyst, promotes reaction between the fuel and the air, while keeping the flame stable and giving very high combustion efficiency which solves the problem of carbon monoxide and unburnt hydrocarbon emission.

The difficulties associated with the production of a uniform or homogeneous mixture of fuel and air must be overcome.

The mixing efficiency of the fuel and air is proportional to l/d where 1 is the length of chamber available for mixing and d is the diameter of the chamber. For good mixing l/d must be large, but for the space available in most gas turbine engines for the combustion chamber or chambers, I/d is relatively small e.g. 2.5.

The ratio l/d can be increased by providing a number of air inlets each having a fuel supply, so that whilst "i" remains the same "d" becomes the diameter of each air inlet and l/d can reach a value of, for example 5 or 6.

It is already known to use a number of such separate air injectors, for each chamber and to inject fuel into the upstream end of each air injector, from an arrangement of fuel pipes at the upstream end of the air injectors. The fuel pipes experience forces due to the airflow, and interfere with the airflow, and as such are not satisfactory.

Further such arrangements of air injectors and fuel pipes are relatively complex and awkward to manufacture.

It is an object of the invention to provide a means of obtaining a thorough premixing of fuel and air prior to burning promoted by the catalyst, by providing a fuel and air injector with internal ducting for supplying fuel to each air injector.

According to the present invention a fuel and air injector for a gas turbine engine comprises two conjoined plates each having an arrangement of axially aligned holes, which in combination form apertures for the through flow of compressed air, and passage means for supplying fuel to the apertures internally of the injector.

The passage means can comprise passages formed in either or both of the plates for supplying fuel to the air apertures.

The passages have a layer of heat insulating material.

Fuel is injected into the apertures substantially at 900 to the direction of air flow through the apertures.

The air apertures can be parallel walled or venturi-shaped in cross-section.

The invention will be further illustrated with reference to the drawings in which: Figure 1 shows a schematic diagram of a gas turbine engine, Figure 2 shows a cross-sectional view of one of the combustion chambers in the gas turbine engine shown in Figure 1, and incorporates one form of fuel and air injector according to the present invention, Figure 3 shows a cross-sectional view of the combustion chamber which incorporates another form of fuel and air injector according to the present invention, Figure 4 shows arrangement of internal fuel passages of the injector shown in Figure 3, Figure 5 is a section on line V-V in Figure 4, illustrating the parallel walled air apertures, and Figure 6 illustrates a fuel and air injector similar to the one in Figure 4 but having venturi-shaped air apertures.

Referring to Figure 1, there is shown a gas turbine engine 10, comprising an air intake 11, a low pressure compressors 12, a high pressure compressor 13, a bypass duct 14, a combustion system 1 5, a high pressure turbine 1 6, a low pressure turbine 17, nozzle 18 and shafts 19 and 20.

In operation air enters the gas turbine 10 through the intake 11 and is then initially compressed by the low pressure compressor 1 2.

The air flows then splits into two portions. The first portion passes through the high pressure compressors 13, where the air is further compressed, before being injected into the combustion system 1 5. The air is mixed with fuel in the combustion system 15, before being burnt, and the hot gases of combustions expand through the high pressure turbine 1 6 and the low pressure turbine 17, which drive the high and low pressure compressors 13 and 12 respectively via shafts 1 9 and 20 respectively, before passing out of the gas turbine 10 through the nozzle 1 8.

The second portion of air passes along the bypass duct 1 4 and exits from the gas turbine 10 through the nozzle 1 8.

Figure 2 shows a cross-sectional view of a combustion system 1 5, which is of the canannular type and comprises an inner and an outer annular wall 21 and 22 respectively, which contain a number of equi-spaced tubular combustion chambers 23, and an annular air duct 24 is formed between the walls 21 and 22. Each tubular combustion chamber 23 has a domed head 25 at its upstream end, and the head 25 has an aperture 26. A fuel and air injector 27 is positioned in the tubular combustion chamber 23 downstream of the head 25 and extends radially across the tubular combustion chamber 23 and is secured to the walls thereof.

The fuel and air injector 27 comprises two plates 28 and 29 respectively which are brazed together, and there are a number of apertures 30 through the injector 27 formed by a plurality of coaxial holes in the plates 28 and 29. The injector 27 also has an internal arrangement of passages 32 for conveying fuel from a fuel supply pipe 31 to each of the apertures 30. The passages 32 can be machined in either or both of the plates 28 and 29 before they are brazed together. Hyperdermic tubes may be inserted into the passages 32 to prevent the braze material from contaminating the passages 32 during brazing, and the hyperdermic tubes may then be ground flush to the walls of the apertures 30.

A honeycomb catalyst 34 extends radially across the tubular combustion chamber 23 downstream of the fuel and air injector 27, and a mixing zone 33 is formed downstream of the injector 27 and upstream of the honeycomb catalyst 34, and an exit 35 is positioned at the downstream end of the tubular combustion chamber 23.

In operation air flows from the high pressure compressor 13 into the combustion system 1 5. A first portion of the air flows through the annular air duct 24 defined between the inner and outer annular walls 21 and 22 respectively, and is used to cool the walls of the combustion chamber 23.

A second portion of air flows through the aperture 26 in the head 25 of the combustion chamber 23, and flows through the apertures 30 in the fuel and air injector 27 into the mixing zone 33. The air mixes with fuel injected into the apertures 30 from the passages 32 in the fuel and air injector 27. and mixes further in the mixing zone 33 to become a uniform mixture before it comes into contact with the honeycomb catalyst 34.

The fuel and air mixture passes into the honeycomb catalyst 34 whore it is ignited and burnt. The honeycomb catalyst 34 promotes reaction between the suel and 9ir to give very high combustion efficiency and ensures the flame remains stable to reduce the chance of flame out.

Fuel is supplied from a fuel supply (not shown) through a fuel supply pipe 31 to a number of passages 32 in the fuel and air injector 27. The fuel is injected into the apertures 30 from the passages 32 so that the fuel jet effectively covers the cross-sectional area of the aperture 30 at its downstream end to give efficient mixing with the air. The fuel is injected into the aperture 32 substantially at 900 to the direction of the air flow through the aperture 32. The burnt fuel and air passes through the exit 35 to the high and low pressure turbines 1 6 and 1 7 respectively.

The honeycomb catalyst 34 may comprise a platinum catalyst mounted on a ceramic monolith.

Each of the apertures 30 act independently mixing fuel injected from the respective passages 32 with air flowing through the aperture 30. The mixing efficiency for each aperture 30 is proportional to the l/d ratio, where I is the length of the combustion chamber 23 available for mixing, and d is the diameter of the aperture 30.

The diameters of the apertures 30 are similar and so their l/d ratio and mixing efficiencies will be similar. The mixing efficiency for the combustion chamber 23 is that for a single aperture 30 and the l/d ratio for the combustion chamber 23 is that for an aperture 30.

The fuel and air injector 27 effectively increases the l/d ratio and mixing efficiency of the combustion chamber 23 without dimensionally changing the combustion chamber 23.

Figure 3 shows a cross-sectional view of a combustion system 1 5 which has apertures 30 of different sizes, to take into account the difference in the mass flow of air at different radii from the central axis of the combustion chamber 23. This is to ensure the fuel/air ratio in each aperture 30 is substantially the same, and to give uniform mixing of fuel and air for each aperture 30.

Figure 4 shows an arrangement of fuel passages 32 of the injector 27 shown in Figure 3.

The arrangement of passages 32 in the injector 27 may vary depending upon the number, size and arrangement of the apertures 30 in the injector 27.

Figures 5 and 6 illustrate the shapes of the walls of the apertures 30. Figure 5 shows a parallel walled aperture 30 and Figure 6 shows a venturi shape aperture 30 and fuel is injected into the aperture 30 at the narrowest part of the venturi. Apertures 30 of other shapes may be used for example conical apertures converging at the downstream end.

The fuel passages and the air apertures can be formed by conventional machining techniques, but a convenient method is the form of electrochemical machining described in our UK Patent No. 1339544 (corresponding to US Patent No. 3990959).

The invention described as applied to a can-annular combustion system but may be applied to a tubular combustion systems, and to annular combustion systems.

It may be necessary to insulate the fuel in the passages 32 from the relatively high temperatures of the compressed air, entering the tubular combustion chamber 23 these high temperatures could result in the fuel being broken down and gumming, which may lead to blockages in the passages 32. This may be achieved by machining the passages 32 oversize and depositing a layer of heat insulating material on the inside of the passage 32.

The drawings and description have shown and described only one fuel supply pipe 31, it may be desirable or necessary to use more than one pipe 31 for instance if the injector 27 is adapted for use in an annular combustion chamber.

Claims (9)

1. A fuel and air injector for use in a combustion chamber of a gas turbine engine comprising two conjoined plates, each having an arrangement of axially aligned holes, which in combination form apertures for the through flow of compressed air, and passage means for supplying fuel to said apertures internally of the injector.

2. A fuel and air injector according to claim 1 wherein the passage means comprises passages formed in either or both of said plates.

3. A fuel and air injector as claimed in claim 2 in which the passages have a layer of heat insulating material.

4. A fuel and air injector according to any of claims 1 to 3 wherein fuel is injected into the apertures at 900 to the direction of air flow through the apertures.

5. A fuel and air injector according to any of claims 1 to 4 wherein the walls of the apertures are parallel.

6. A fuel and air injector according to any of claims 1 to 4 wherein the walls of the apertures form venturi.

7. A fuel and air injector as claimed in claim 6 in which the fuel is injected into the air apertures at the narrowest part of the venturi.

8. A fuel and air injector constructed and arranged for use and operation substantially as herein described, and with reference to the accompanying drawings.

9. A gas turbine engine including a fuel and air injector as claimed in any one of the preceding claims.