FUEL SUPPLY SYSTEM

Abstract

A fuel supply system includes a metering and splitting arrangement receiving a fuel flow and controllably meters and splits the received fuel flow into metered pilot and mains flows for injecting at injector pilot and mains fuel discharge orifices to perform combustor staging control. The system includes mains isolation valves and a mains fuel distribution pipework has fuel lines connected to one of the orifices and extending therefrom to a respective one of the valves. Each valve has a closed position for removing the mains fuel from its injector through its line when the mains distribution pipework is deselected for pilot-only operation, and isolates its line from the metering and splitting arrangement. Each valve has an open position for refilling its injector with mains fuel through its line when the mains distribution pipework is selected for pilot and mains operation, and reconnects its line to the metering and splitting arrangement.

Claims

1. A fuel supply system for fuel injectors of a multi-stage combustor of a gas turbine engine, the fuel supply system including: a metering and splitting arrangement which receives a fuel flow and controllably meters and splits the received fuel flow into metered pilot and mains flows for injecting respectively at pilot and mains fuel discharge orifices of the injectors to perform staging control of the combustor; and pilot and mains fuel distribution pipeworks respectively distributing fuel from the metering and splitting arrangement to the pilot and mains discharge orifices; wherein the metering and splitting arrangement is operable to select the pilot distribution pipework and deselect the mains distribution pipework for pilot-only operation in which there is a pilot supply to the combustor but no mains supply to the combustor from the injectors, and is operable to select both the pilot and mains distribution pipeworks for pilot and mains operation in which there are pilot and mains supplies to the combustor from the injectors; wherein the fuel supply system further includes plural mains isolation valves and the mains fuel distribution pipework has plural fuel lines each of which is fluidly connected to a respective one of the mains fuel discharge orifices and extends therefrom to a respective one of the isolation valves; wherein each isolation valve has a closed position in which it removes the mains fuel from its injector through its fuel line when the mains distribution pipework is deselected for pilot-only operation, and fluidly isolates its fuel line from the metering and splitting arrangement; and wherein each isolation valve has an open position in which it refills its injector with mains fuel through its fuel line when the mains distribution pipework is selected for pilot and mains operation, and reconnects its fuel line to the metering and splitting arrangement.

2. A fuel supply system according to claim 1, wherein the pilot fuel distribution pipework includes a pilot fuel manifold distributing fuel from the metering and splitting arrangement to the pilot discharge orifices.

3. A fuel supply system according to claim 1, wherein the metering and splitting arrangement includes: a metering valve which receives and controllably meters the fuel flow, and a splitting unit which receives the metered flow from the metering valve and controllably splits the metered flow into the pilot and mains flows.

4. A fuel supply system according to claim 1, wherein each isolation valve in its closed position also removes mains fuel from a portion of its fuel line adjacent its injector, and in its open position refills said portion of its fuel line with mains fuel.

5. A fuel supply system according to claim 1, wherein each fuel line has a top portion at an end thereof and a bottom portion at an opposite end thereof, and is routed such that its injector is at the top end and its isolation valve is at the bottom end.

6. A fuel supply system according to claim 1, wherein each isolation valve has: a valve housing which forms an inlet to the mains distribution pipework between the isolation valve and the metering and splitting arrangement, and which forms an outlet to the fuel line of the isolation valve; and a piston which is slidably movable in the housing between first and second end positions which respectively correspond to the closed and open positions of the valve; wherein the housing and the piston are configured such that in the second end position of the piston the inlet and the outlet fluidly communicate with each other, and such that in the first end position of the piston a fluid tight seal is formed between the inlet and the outlet.

7. A fuel supply system according to claim 6, wherein: each isolation valve has a variable volume, fuel storage sink which is in fluid communication with the outlet, the volume of the sink being at its greatest when the piston is in its first end position, and being at its smallest when the piston is in its second end position; and the housing and the piston are configured such that, at an intermediate position of the piston, the inlet is substantially closed off by the piston, whereby on moving from its intermediate position to its first end position, the piston draws mains fuel in the fuel line into the sink through the outlet, thereby removing the mains fuel from the injector, and whereby on moving from its first end position to its intermediate position, the piston pushes fuel stored in the sink into the fuel line through the outlet, thereby refilling the injector with mains fuel.

8. A fuel supply system according to claim 6, wherein the piston of each isolation valve is spring biased towards its first end position.

9. A fuel supply system according to claim 6, wherein the movements of the pistons of the isolation valves are hydraulically controlled, and the fuel supply system further includes a solenoid or servo valve which sets the hydraulic fluid control pressure.

10. A fuel supply system according to claim 6, wherein the movements of the pistons of the isolation valves are hydraulically controlled by fuel pressure,

11. A gas turbine engine having a multi-stage combustor and the fuel supply system according to claim 1 for supplying fuel to and performing staging control in respect of pilot and mains fuel discharge orifices of fuel injectors of the combustor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

[0044] FIG. 1 shows schematically a combustion staging system for a gas turbine engine in pilot and mains operation mode;

[0045] FIG. 2 shows a longitudinal cross-section through a ducted fan gas turbine engine;

[0046] FIG. 3 shows schematically a fuel supply system for fuel injectors of a multi-stage combustor of the gas turbine engine;

[0047] FIG. 4 shows stages in the operation of an isolation valve of the fuel supply system of FIG. 3;

[0048] FIG. 5 shows schematically a variant e supply system for fuel injectors of the multi-stage combustor; and

[0049] FIG. 6 shows schematically a further variant fuel supply system for fuel injectors of the multi-stage combustor.

DETAILED DESCRIPTION AND FEATURES

[0050] With reference to FIG. 2, a ducted fan gas turbine engine incorporating the invention is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.

[0051] During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate-pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate-pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high-pressure compressor 14 where further compression takes place.

[0052] The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate-pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.

[0053] The engine has a pump system comprising typically a low pressure (LP) pumping stage which draws fuel from a fuel tank of the aircraft and supplies the fuel at boosted pressure to the inlet of a high pressure (HP) pumping stage. The LP stage typically comprises a centrifugal impeller pump while the HP pumping stage may comprise one or more positive displacement pumps, e.g. in the form of twin pinion gear pumps. The LP and HP stages are typically connected to a common drive input, which is driven by the engine HP or IP shaft via an engine accessory gearbox.

[0054] The combustion equipment 15 of the engine 10 includes a multi-stage combustor. A fuel supply system accepts fuel from the HP pumping stage for feeding to the combustor. This system typically has a hydro-mechanical unit (HMU) which performs total metering and comprises a fuel metering valve operable to control the rate at which fuel is allowed to flow to the combustor. The HMU further typically comprises: a pressure drop control arrangement (such as a spill valve and a pressure drop control valve) which is operable to maintain a substantially constant pressure drop across the metering valve, and a pressure raising and shut-off valve at the fuel exit of the HMU which ensures that a predetermined minimum pressure level is maintained upstream thereof for correct operation of any fuel pressure operated auxiliary devices (such as variable inlet guide vane or variable stator vane actuators) that receive fuel under pressure from the HMU. Further details of such an HMU are described in EP 2339147 A (hereby incorporated by reference).

[0055] An engine electronic controller (EEC) commands the HMU fuel metering valve to supply fuel to the combustor at a given flow rate. The metered fuel flow leaves the HMU and arrives at a staging system 30 of the fuel supply system.

[0056] The staging system 30 splits the fuel into two flows: one for a pilot flow along pilot fuel distribution pipework 34 to first 31a and second 31b segments of a pilot manifold and the other for a mains flow along mains fuel distribution pipework 32. Fuel injectors 33 (only two being shown in FIG. 3) of a combustor of the engine are split into two groups. Pilot (primary) discharge orifices of the fuel spray nozzles (FSNs) of the injectors of one group are connected to the first pilot manifold segment 31a, while pilot discharge orifices of the FSNs of the injectors of the other group are connected to the second pilot manifold segment 31b. The mains flow feeds mains (secondary) discharge orifices of the FSNs of the fuel injectors. The pilot and mains discharge orifices have respective weight distribution valves (WDVs) to reduce gravitational head effects between the injectors.

[0057] A fuel flow splitting valve (FFSV) 35 receives the metered fuel flow from the HMU. Typically, the FFSV has a slidable spool under the control of a servo-valve 36, the position of the spool determining the outgoing flow split between two outlets forming respectively the pilot flow and the mains flow. The spool can be positioned so that the mains stage is completely deselected, with the entire metered flow going to the pilot stage. An LVDT can provide feedback on the position of the spool to the EEC, which in turn controls staging by control of the servo-valve.

[0058] The pilot fuel distribution pipework 34 splits the pilot flow between the first 31a and second 31b segments of the pilot manifold. A lean blow out protection valve 37 and a solenoid-operated control valve 38 may be located between the pilot fuel distribution pipework and the second pilot manifold segment 31b.

[0059] The mains fuel distribution pipework 32 splits the mains flow into sub-flows, one for each injector 33. More particularly, each sub-flow is directed to a respective isolation valve 39 and then through a respective fuel line 41 which extends to the given injector. The isolation valves perform de-prime and re-prime (discussed in more detail below) and isolation functions on their injectors. Each fuel line 41 can be routed vertically with its fuel injector 33 at the top and its isolation valve 39 at the bottom. This helps to ensure that, if the fuel line is not fully emptied, then fuel does not egress into the fuel injectors, causing coking of the injector nozzle.

[0060] The isolation function, as well as assisting with injector de-priming and re-priming, allows the isolation valves 39 to controllably isolate their injectors from the mains flow from the FFSV 35, so that the EEC can perform partial mains staging. Typically this involves staging in a subset of the injectors, the injectors of the subset being equally circumferentially spaced around the combustor.

[0061] The fuel supply system aims to improve on combustion staging systems of the type shown in FIG. 1 by removing a requirement for individual check valves (mains FSVs) at the mains injector heads. As discussed above, FSVs can inadvertently cause injector-to-injector fuel flow variation, which can potentially reduce the life of the combustor and turbine gas path components. The staging system 30 removes the requirement for such valves by de-priming the mains fuel passages of the injectors 33 (and preferably also the fuel lines 41) when the mains flame is staged-out, and then re-priming the mains fuel passages of the injectors (and the fuel lines if necessary) prior to the mains flame being staged back in.

[0062] In particular, the isolation valves 39 have a closed position for pilot-only operation in which the valves remove (de-prime) the mains fuel from their injector 33 through their fuel lines 41, and fluidly isolate their fuel line from the FFSV 35, and an open position for pilot and mains operation in which the valves refill (re-prime) their injectors with mains fuel through their fuel lines, and reconnect their fuel lines to the FFSV. The positional state of the isolation valves 39 is determined by an isolation control valve 43, which in turn is controlled by the EEC. The isolation control valve can be, for example, a solenoid valve, as shown, or a servo valve.

[0063] As shown in FIG. 4, each isolation valve 39 has a housing 44 and a piston 45 which is slidably movable in the housing. More particularly, FIG. 4 shows at left the piston in a first end position corresponding to the closed position of the valve, at right the piston in a second end position corresponding to the open position of the valve, and at centre the piston in an intermediate position between the two end positions. The housing defines an inlet 46 to the mains distribution pipework 32 between the isolation valve and the FFSV 35, and an outlet 47 to the valve's fuel line 41. In the first end (closed) position of the piston, a drip tight seal carried by the piston engages with housing to seal off the inlet from the outlet, and thereby isolate the fuel line from the FFSV, and indeed the fuel line 41 from the other fuel lines 41. In contrast, in the second end (open) position of the piston, the inlet and the outlet are able to fluidly communicate, reconnecting the fuel line 41 to the FFSV.

[0064] A spring 49 at one end of the piston 45 biases the piston towards the closed position. The isolation control valve 43 ports either high pressure or low pressure fuel to a servo chamber 50 at the other end of the piston 45. In the case of high pressure being ported to the servo chamber, the spring bias is overcome, allowing the isolation valve 39 to open. Conversely, porting low pressure to the servo chamber allows the spring bias to close the isolation valve. Each isolation valve 39 can have a restriction orifice 51 in the line between its servo chamber and the isolation control valve 43. These orifices improve the distribution of flow between the isolation valves to ensure that they move more synchronously. A dynamic seal 52 on the piston 45 can isolate the servo chamber pressure from the mains sub-flow delivered to the injector 33 through the isolation valve.

[0065] When moving each isolation valve 39 from its open position to its closed position to de-stage mains, the piston 45 has to first pass through its intermediate position. The piston and housing 44 are configured such that in this position a portion of the piston blocks the inlet 46. Consequently, subsequent movement of the piston towards its dosed position draws fuel from the fuel line 41 into a sink 54 formed in the housing and having a variable volume determined by the position the piston. In doing so, the isolation valve empties its injector of mains fuel, stopping fuel delivery to the combustion zone from the mains discharge orifice and stopping fuel from dribbling into the FSN where it can degrade and block the nozzles.

[0066] Conversely, when moving each isolation valve 39 from its closed position to its open position to stage in mains, the piston 45 has to move to its intermediate position before the inlet 46 becomes unblocked. This initial movement reduces the volume of the sink 54, thereby pushing fuel stored in the sink back into the fuel line 41 to refill the injector 33 with mains fuel.

[0067] The combination of piston stroke and diameter determines the volume of fuel withdrawn and pushed back into the fuel line 41, and a combination of the characteristics of the isolation control valve 43 and the restriction orifice 51 provide control of the piston's slew velocity. Sufficient volume should be withdrawn from the fuel line 41 to ensure that no mains fuel is delivered to the mains discharge orifice of the FSN during aircraft manoeuvres or when any fuel remaining in the fuel line 41 expands due to temperature increases. Thus in general, as well as withdrawing a volume which is enough to empty the injector 33 of mains fuel, typically also a further volume is withdrawn to remove mains fuel from at least that part of the fuel line 41 closest to the injector.

[0068] Advantageously, providing each isolation valve 39 with its own sink 54 can reduce the time needed to refill in re-priming, and also helps to avoid under- and over-fuelling the pilot and mains flames respectively.

[0069] In order that the bulk of the volume displaced by the isolation valve 39 is into and out of the fuel line 41, the stroke of the piston between the intermediate and open positions can be reduced. To achieve this, the inlet 46 can combine a short axial length with a large circumferential width to provide an acceptably low pressure loss when a maximum mains sub-flow is passing through the isolation valve.

[0070] When mains is staged out, any fuel remaining in the fuel lines 41 is stagnant and may require thermal management (e.g. by external air flow) to keep its temperature below an acceptable level.

[0071] As mentioned above, the fuel supply system permits the removal of mains FSVs and hence mitigates associated issues/risks (e.g. mal-scheduling due to a failed open FSV; nozzle-to-nozzle fuel distribution variation due to FSV-to-FSV component variation; and lifing issues such as seal wear/degradation leading to fuel dribbling and consequent nozzle coking).

[0072] However, a further advantage of the fuel supply system is that it can enable individual flow stream control for re-prime and subsequent staging, which can help to reduce/eliminate transient dips in pilots flow. It also allows valves to be moved away from the burner head into a more benign environment, providing for improved control of component temperatures, which in turn reduces the risk of degradation in component/system performance due to fuel coking.

[0073] FIG. 5 shows schematically a variant of the fuel supply system. In the variant a first subset of the isolation valves 39 is controlled by one isolation control valve 43, and a second subset of the isolation valves 39 is controlled by a second isolation control valve 43. This allows the mains sub-flow delivered to each injector 33 in one subset for a given total mains flow to be increased by staging out mains for the injectors of the other subset. This is advantageous as FSNs generally have a minimum required level of mains flow for satisfactory operation.

[0074] As the number of such isolation control valves 43 is increased the possibility for individual flow stream control is enhanced.

[0075] FIG. 6 shows schematically a further variant of the fuel supply system. In the further variant, each isolation valve 39 is operated by an electro-mechanical device 53, such as an electric motor and ball screw actuator, rather than a hydraulic or electro-hydraulic device.

[0076] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.