GB2098718A - Gas turbine fuel injector - Google Patents

Abstract

In order to assist engine start-up at low temperatures and/or when using low grade fuels, an auxiliary flow from a separate compressor or the engine APU of air is supplied to airspray type fuel injectors through an air duct (56) to the main air passage (38) through an annular nozzle (58) and the shroud air passage (44) through nozzle (64). Swirl vanes (66) can be located in the nozzle (58) to enclose shear between the fuel and air and the air duct (56) can be upstream of the fuel duct (50) or the fuel and air ducts can be concentric to provide thermal insulation of the fuel duct. This arrangement avoids the need for a pressure jet pilot fuel nozzle. <IMAGE>

Description

SPECIFICATION Gas turbine fuel injector This invention relates to gas turbine fuel injectors and is concerned with improving lighting performance particularly at low temperatures and improving or retaining a reasonable lighting performance when using degraded or high viscosity fuels.

At low engine power conditions and during the engine start-up procedure over a reasonable temperature range, the flow of air from the engine compressor to the combustion chamber is at a velocity which is sufficient to cause acceptable atomisation of the fuel in the fuel injectors. If the ambient air temperature is low and/or if the fuel being used is degraded or of high viscosity, the velocity of the air will be insufficient to provide acceptable fuel atomisation and start-up will be difficult or impossible and idling will be difficult to maintain.

One method to improve performance under cold conditions and using the lower quality fuels would be to provide an airspray fuel injector which has a pressure jet pilot injector. The atomisation of the fuel by the pilot injector is unaffected by the velocity of the compressor delivery as it is dependent only on fuel pressure and nozzle size. The pilot injector would be used for starting the engine and would run continuously and the fuel to the airspray injector would be scheduled in when the engine had started.

Since the pilot injector would run continuously, the engine would be able to idle satisfactorily.

Such an arrangement whilst having adequate performance, requires double ducting of fuel, a high pressure fuel pump for the pressure jet fuel injector and a more complicated fuel control system as compared with a single fuel supply system. There is also the possibility that the fuel supply ducting may have to be double-skinned for safety reasons and fuel decomposition of the lower quality fuels can be a problem.

The present invention proposes the use of an airspray or airblast type of fuel injector which has been modified to provide the required performance in relation to engine starting at low temperatures and/orthe use of degraded or high viscosity fuels.

The modification comprises the application of an auxiliary supply of high velocity air to the fuel injector separate from the supply of air from the engine compressor, the fuel injector having a ducting to direct the separate air supply to the main and shroud air passages of the injector. When the auxiliary air supply is not being provided the air ducting acts as an insulator to prevent thermal degradation of the fuel flowing through the injector.

It has already been proposed in US patent no.

3866413 which shows a dual fuel airblast atomiser, to introduce boost air through the gas fuel ducting into the swirling shroud air, downstream of the liquid fuel injection into the main air passage, to facilitate engine starting when operating on heavy, low volatility liquid fuels.

U.K. patent application no. 2003552A also discloses a liquid fuel airblast atomiser having a central main air passage into which liquid fuel is injected and a surrounding shroud air passage through which swirling air flows, boost air being injected into the main air passage to facilitate fuel atomisation at engine start-up and low power conditions. The boost air can also be swirled.

In neither of these disclosures does the boost or auxiliary air, boost the air supply to both the main and shroud air passages and there is no disclosure of arranging the auxiliary air ducting to act as an insulating medium for the liquid fuel in the atomiser or injector.

Accordingly, the present invention provides a gas turbine engine airspray injector comprising a fuel feed arm terminating in a fuel injector, the injector including a central main air passage, having a control nozzle a surrounding shroud air passage, fuel ducting opening into the main air passage, and auxiliary air ducting opening into the main and shroud air passages, the auxiliary air ducting being connected to a source of compressed air independent of the air supply from the engine compressor, at least a part of the auxiliary air ducting being arranged to at least partially insulate a part of the fuel ducting.

The auxiliary air ducting may include air swirl vanes or some other means of swirling the auxiliary prior to the auxiliary air flowing into the main and/or the shroud air passage.

The auxiliary air ducting and the fuel ducting in the fuel feed arm is arranged so that the auxiliary air ducting thermally shields or insulates the fuel ducting against the relatively hot delivery air from the engine compressor.

The auxiliary air ducting and the fuel ducting in the fuel feed arm may be in the form of two ducts with the auxiliary air duct located upstream of the fuel duct, or the ducts can be arranged concentrically with the fuel duct inside the air duct creating an annular air space between the ducts.

The present invention will now be more particularly described with reference to the accompanying drawings in which: Figure 1 shows a gas turbine engine incorporating a fuel injector according to the present invention, Figure 2 is a view to a larger scale of a fuel injector without an auxiliary air supply, Figure 3 is a sectional view of the fuel injector shown in Figure 2.

Figure 4 is a sectional view of one form of fuel injector according to the present invention and, Figure 5 is a sectional view of another form of fuel injector according to the present invention.

Referring to the Figures, Figure 1 shows a gas turbine engine 10 comprises a fan 12 driven by a turbine 14, an intermediate pressure compressor 16 driven by an intermediate pressure turbine 18, a high pressure compressor 20 driven by a high pressure turbine 22, and a combustion system 24. The air propelled by the fan 12 leaves the engine through an annular propulsion nozzle 26 and the hot gases from the turbines 14, 18,22 exhaust thorough a propulsion nozzle 28.

The combustion system 24 comprises an annular combustion chamber 30 and a number of equispaced fuel injectors 32 which are located around the circumference of the combustion chamber at its upstream end so that the chamber 30 receives a flow of compressed air from the high presure compressor 20 and a flow of fuel from the fuel injectors 32. An igniter or igniters (not shown) are provided to initiate combustion of the fuel.

Referring to Figure 2 and 3 each fuel injector 32 comprises a fuel feed arm 34, and an injector 36. The fuel injector has air ducting which includes a main air passage 38 which terminates in a control nozzle 40 defined by the wall of the passage and a central pintle 42 and a surrounding shroud air passage 44 having an annular outlet 46 and air inlets 48.

The fuel feed arm has a fuel duct 50 supplying fuel to a manifold 52, the fuel being injected into the main air passage 38 through inclined passages 54.

With this type of fuel injector problems are sometimes experienced on light-up, particularly on cold days because the low air velocities through the injector are insufficient to atomise the fuel for ignition. These problems will be made more difficult when using degraded or high viscosity fuels.

Referring now to Figures 4 and 5 in which corresponding components have been given the same reference numerals as in Figure 3. Figure 4 shows a fuel injector in which the feed arm 34 includes an air duct 56 which supplies high velocity air to the main air passage 38 through an annular nozzle 58 close to the fuel passages 54, and the shroud passage 44 via ducts 60, a manifold 62 and an annular nozzle 66. The air supply to the duct is from an auxiliary compressor 64 or the engine auxiliary power unit. In addition air swirl vanes 64 can be positioned in the nozzle 58 to enhance the shear between the fuel leaving the passages 54 and the air flowing through the passage 38, and to control the final fuel droplet placement.

In use, at engine start-up the compressor 64 is operated to provide a flow of compressed air to the air duct 58 in each fuel injector 32, and a high velocity flow of air enters the main and shroud air passages through nozzles 58 and 64 respectively, to boost the relatively low velocity, low momentum air from the high pressure compressor 20 of the engine 10. The boost flow of air materially assists in the fuel atomisation and enables the engine to be started under adverse conditions e.g. low temperatures and low grade fuels.

The compressor 64 or the engine auxiliary power unit need only supply a very small mass flow and would only be operated at engine start-up, or at engine idle if required.

The air duct 56 is located upstream of the fuel duct 50 and so at least partially shields the fuel duct from the heating effect of the flow of compressed air from the compressor 20, the temperature of which can be in the region of 600"C in high pressure ratio engines.

The insulating effect of the air duct is particularly important when using low grade fuels as fuel gumming can take place in the fuel ducts due to thermal degradation of the fuel.

Figure 5 shows a modified form of fuel injector in which the thermal insulation of the fuel duct is improved by placing the fuel duct, concentric inside the air duct with a small gap, say up to 0.1" between the ducts to allow for the air to be supplied to the nozzles 58 and 64, when required.

The advantages of the air assistance to the fuel injectors as shown in Figures 4 and 5 are that the fuel system would remain simple, the extra air pipes need only be single skinned, no additional gumming problems would be encountered, and that there is no pilot spray to interact and spoil the main fuel spray under normal running conditions.

Claims (6)

1. A gas turbine engine airspray fuel injector comprising a fuel feed arm terminating in a fuel injector, the injector including a central main air passage having a control nozzle, a surrounding air passage, fuel ducting opening into the main air passage and auxiliary air ducting opening into the main and shroud air passages, the auxiliary air ducting opening into main air passage terminating in a nozzle located upstream of and adjacent the fuel ducting opening into the main air passage, the auxiliary air ducting being connected to a source of compressed air independent of the air supply from the engine compressor, at least a part of the auxiliary air ducting being connected to a source of compressed air independent of the air supply from the engine compressor, at least a part of the auxiliary air ducting being arranged to at least partially insulate a part of the fuel ducting.

2. A fuel injector as claimed in claim 1 in which the nozzle includes air swirling means.

3. A fuel injector as claimed in claim 1 or claim 2 in which the fuel feed arm includes a fuel duct and an auxiliary air duct, the auxiliary air duct being positioned upstream of the fuel duct

4. A fuel injector as claimed in claim 1 or claim 2 in which the fuel feed arm includes a fuel duct and an auxiliary air duct, the fuel duct being located concen tricallywithin the air duct.

5. A fuel injector constructed and arranged for use and operation substantially as herein described and with reference to Figures 4 and 5 of the accompanying drawings.

6. A gas turbine engine including one or more fuel injectors as claimed in any one of the preceding claims.