GB2523126A - Fuel supply system - Google Patents

Abstract

A fuel supply system for a fuel injector of a multi-stage combustor includes a splitter valve 32 delivering, and controlling the split between, fuel flows to a pilot manifold 40 and a mains manifold 42. A pilot feed 44 and a mains feed 46, each optionally comprising a pigtail pipe assembly to absorb vibration, extend respectively from manifolds 40, 42 to pilot and mains discharge orifices of the fuel injector. A cooling flow extraction path 52 extends from mains feed 46 to the reduced static pressure region of pilot feed 44 formed by a venturi 48, and a cooling flow supply path 58 extends from pilot manifold 40 to mains manifold 42, so that when splitter valve 32 supplies fuel only to pilot manifold 40, a cooling fuel flow flows through mains manifold 42 via supply path 58 and extraction path 52. The system allows a balanced cooling flow through mains manifold 42, avoiding coking, while avoiding the need for a recirculation manifold.

Description

I

FUEL SUPPLY SYSTEM

Fi&d of the lnventbn The present invention relates to a fuel supply system for a fuel injector of multi-stage combustor, in particular multi-stage combustor of a gas turbine engine.

Background of the Invention

Multi-stage combustors are used particularly in lean burn fuel systems of gas turbine engines to reduce unwanted emissions while maintaining thermal effidency and flame stahiUty. For example, duplex fu& injectors have pilot and mains fuel manifolds feeding pilot and mans discharge orifices of the injectors. At low power conditions only the pilot stage a activated, while at higher power conditions both pilot and mains stages are activated. The fuel for the manifolds typically derives from a pumped and metered supply. A splitter valve can then be provided to selectively split the metered supply between the manifolds as required for a gwen stagng.

A typical annular combustor has a circumferential arrangement of fuel injectors, each associated with respective pilot and mains feeds extending from the circumferentially extending pilot and mains manifolds. Each injector generally has a nozzle forming the discharge orifices which discharge fuel inLo the combustion chamber of the combustor, a feed arm for the transport oF Fuel to the nozzle, and a head at the outside of the combustor at which the pilot and mains feeds enter the feed arm. The pilot and mains feeds may include pigtail pipe assemblies between the manifolds and the injector heads in order to accommodate vibration and movement between the combustor and the manifolds.

Multi-stage combustors may have further stages and/or manifolds. For example, the pilot manifold may be split into two manifolds for lean blow-out prevention.

During pilot-only operation, the splitter valve directs fuel for burning flows only through the pilot fuel circuit (i.e. pilot manifold and feeds). It is therefore conventional to control temperatures in the stagnant (i.e. mains) fuel circuit to prevent coking due to heat pick up from the hot engine casing. One known approach, for example, is to provide a separate re-circulation manifold which is used to keep the fuel in the mains manifold cool when it is staged. It does this by keeping the fuel in the mains manifold moving. However, a disadvantage of such an approach is that it requires an additional manifold, which increases overafl cost, weight and maintenance overheads. Another disadvantage is that a cooUng flow has to be maintained in the re<irculahon manifold during mains operaton to avoid coking. Especiafly at low fuel flows, this can result in an error n fuel split against demand, requiring complex calibrations to compensate.

arofthelnvention A first aspect of the present invention provides a fuel supply system for a fUel injector of a mulfl-stage combustor, the system including: a splitter valve which delivers respective fuel flows to pilot and mains manifolds, the splitter valve being operable to control the fuel flow split between the manifolds; a pilot feed extending from the pilot manifold to direct fuel flow to a pilot discharge orifice of the fuel injector, and a mains feed extending from the mains manifold to direct fuel flow to a mains discharge orifice of the fuel injector; a venturi located on the pilot feed to create a reduced static pressure region of the pilot feed; and a cooling flow extraction path extending from the mains feed to the reduced static pressure region of the pilot feed, and a cooling flow supply path extending from the pilot manifold to the mains manifold; wherein the extraction and supply paths are configured such that, when the splitter valve is operated to provide a pilot-only fuel flow split in which fuel flows from the splitter valve to the pilot manifold but substantially no fuel flows from the splitter valve to the mains manifold, a cooling fuel flow flows through the mains manifold, the cooling fuel flow being supplied from the pilot manifold to the mains manifold through the supply path, and being extracted from the mains feed to the reduced static pressure region of the pilot feed through the extraction path.

Such a system does not require a separate re-circulation manifold, avoiding the disadvantages mentioned above, in. addition, as the venturi does not require moving parts, the system can be made mechanically simple. Moreover, in relation to a combustor which has a plurality of fuel injectors, locating a venturi on the pilot feed of each injector can ensure that the cooling flow is drawn through the parts of the fuel circuit most susceptible to coking (i.e. generally those parts of the pilot and mains feeds such as pigtail pipe assemblies, closest to the injector). That is, a dedicated venturi associated with each injector can prevent excess cooling flow being directed to the feeds of one injector while insufficient cooling flow is directed to the feeds of another.

A second aspect of the present invention provides a gas turbine engine having a multi-stage combustor and the fuel supply system of the first aspect.

Optional features of the invention will now be set out. These are applicable singly or in any combination with any aspect of the invention.

Conveniently, the extraction and supply paths may be further configured such that, when the splitter valve is operated to provide a mains+pilot fuel flow split in which fuel flows from the splitter valve to the pilot manifold and to the mains manifold, substantially no fuel flow is extracted from the mains feed to the reduced static pressure region of the pilot feed through the extraction path.

The fuel supply system may further include a metering valve which delivers a metered fuel flow to the splitter valve. The fuel supply system may further include a pressure-raising valve in flow series between the metering valve and the splitter valve.

The cooling flow supply path may be located adjacent to or integrated with the splitter valve.

In this way, the cooling flow can be made to travel through the full extent of the mains manifold.

The mains feed may include a pigtail pipe assembly, the extraction path extending from the main feeds at a location downstream of the pigtail pipe assembly. This can force the cooling flow to travel through the pigtail pipe assembly -a part of the mains feed that may be particulaily susceptible to coking.

Generally, the pilot feed also includes a pigtail pipe assembly. Conveniently, the extraction path can then extend to the pilot feed at a location downstream of its pigtail pipe assembly.

The extraction path and the venturi may be located at or within a head of the fuel injector.

The supply path may contain a restrictor to limit the amount of cooling fuel flow provided. In this way, excessive cooling flows can be avoided.

One option is for the extraction path to be a simple pipe, dimensioned to extract fuel from the mains feed as required. In such an arrangement, a small leakage flow to the pilot feed through the extraction path can be tolerated even when the splitter valve is operated to provide the mains+pilot split, as this will not affect the overall metered fuel flow.

Another option, however, is for the extraction path to include an extraction path vSve (such as a spool valve) which opens the extraction path when the splitter valve is operated to provide the pilot-only spht, and closes the extraction path when the sphtter valve s operated to provide the mains+pilot spht. In this way, leakage flow to the pUot feed through the extraction path can be avoided. The extraction path valve may be configured such that, when it opens the extraction path during the pilot-only split, it blocks fuel from flowing further along the mains feed towards the mains discharge orifice. This results in fuel being retained in the mains manifold, thereby avoiding large fuel dips when staging iii the mains. The extraction path valve can operate under the control of the differential pressure between the pilot feed and the mains feed to open and close the extraction path as needed.

Alternatively, the extraction path valve can be operated under the control of an electronic controller of the fuel supply system. Such a controller can be pan of a more general engine electronic controller (EEC).

One option for the supply path is for it to include a shut-off valve which is operable to open the supply path when the splitter valve is operated to provide the pilot-only split and to close the supply path when the splitter valve is operated to provide the mains+pilot split. For example, the shut-off valve can be a solenoid valve. It can be controlled by an electronic controller of the fuel supply system. The splitter valve may also be controlled by the controller. In this way, the shut-off valve can be opened when the controller operates the splitter valve to provide the pilot-only split.

Another option, however, is for the shut-off valve to be a passive valve, such as a check valve. Such a valve can prevent reverse flow through the supply path from the mains to the pilot manifold, and does not require scheduling by an electronic controller.

The combustor may have a plurality of fuel injectors, the fuel supply system having, for each injector, a respective pilot feed extending from the pilot manifold, a respective mains feed extending from the mains manifold; a respective venturi, and a respective extraction path.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in whicft Figure 1 shows a longitudinal section through a ducted fan gas turbine engine; Figure 2 shows schematicafly elements of a fuel supply system for the engine in pflot--only operation; Figure 3 shows schematicafly the fu& supply system of Figure 2 in mains+pflot operation; Figure 4 shows schematicafly elements of a variant of the fuel St ppiy system of Figure 2 in pot-oniy operation; and Figure 5 shows schematically the variant fuel supply system of Figure 4 in mains+pflot operation.

Detailed Descriotion and Further Optional Features of the Invention With reference to Figure 1 a ducted fan gas turbine engine incorporating the invention is generally indicated at 10 an.d has a principal and rotational axis X-X. The engine comprises, in axal flow senes, 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 ow-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generaUy surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.

During operation, air entering the intake 11 is accelerated by the fan 12 to produce o 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.

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 comnbusted. The resultant hot combusUon 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 ow-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.

The combustion equipment 15 is a multi-stage combustor having a plurality of fuel injectors, each injector having a fuel spray nozzle with a pilot discharge orifice and a concentric mains discharge orifice.

The injectors are supplied with fuel by a fuel supply system which is under the control of an electronic controller. For example, the system may comprise a hydro-mechanical unit (HMU) containing a metering valve operable by the controller to control the rate at which fuel from a pumping unit is supplied to a supply line, a pressure raising and shut-off valve on the supply line which ensures that the pressure of the fuel upstream thereof is maintained at above a predetermined minimum level, and a pressure drop control arrangement which maintains a substantiafly constant pressure drop across the metering valve.

Figures 2 and 3 show schematically further elements of the fuel supply system downstream of the HMU. The metered fuel on supply line 30 arrives at a splitter valve 32. A servo valve 34 under the control of the electronic controller 36 determines the position of the splitter valve, and a linear variable differential transformer (LVDT) 38 provides position feedback on the splitter valve to the controller.

A pilot manifold 40 and a mains manifold 42 extend from the splitter valve 32. The splitter valve is operable to provide either a pilot-only fuel flow split (shown in Figure 2) in wftch the metered fuel from the supply line 30 flows from the sphtter valve to the p;lot manifold hut not to the mains manifold, or a mains±pilot fuel flow spht (shown in Figure 3) in which the metered fuel flows from the splitter valve to both the pilot manifold and the mains manifold.

The manifolds in turn supply the fuel to respective pilot 44 and mains 46 feeds of each injector. Only one pair of such feeds is shown in Figures 2 and 3. The solid arrowed lines on the supply line, manifolds and feeds in Figures 2 and 3 indicate the flow of metered fuel to the injectors.

The pilot 44 and mains 46 feeds comprise pigtail pipe assemblies to accommodate relative motion and vibration between the combustor and the manifolds 40, 42. Each pair of feeds then enters the head of the respective injector and extends along a feed arm thereof to terminate at the injectors fuel spray nozzle. In piIotonly operation, it is necessary to prevent stagnation of the fuel in the mains manifold 42 and in the mains feed 46 (e.g. in its pigtail pipe assembly) in order to avoid coking.

Accordingly, the fuel supply system has: A venturi 48 located on each pilot feed 44, e.g. in the injector head, to create a reduced static pressure region of the pilot feed; A coofing flow extraction path 52 extending from each mains feed 46 to the reduced stabc pressure region of the corresponding pilot feed, the extraction path typically also being located in the injector head; and A coofing flow supply path 50 extending from the pot manifold 40 to the mains manifold 42, preferably at a location adjacent to the splitter valve 32.

The system also has: * A spool valve 54 on each extraction path 52; * A solenoid controlled shut-off valve 56 on the supply path 50; and * A restrictor 58 on the supply path 50.

In pilot-only operation (shown in Figure 2), the splitter valve 32 is positioned to supply the metered fuel only to the pilot manifold 40, and the solenoid controlled shut-off valve 56 connecting the pilot and mains manifolds is commanded by the controller 36 to open the supply path 50. The fuel pressure in the pilot feed 44 is higher than the fuel pressure in the mains feed 46 Which results in the spool valves 54 moving to a position that opens the extraction paths 52 and blocks fuel from flowing further along the mains feeds 46 to the mains discharge orifices. In this state, the pilot and mains manifolds are therefore connected together via the restrictor 58, which is sized to limit a cooling flow from the pilot 40 to the mains 42 manifold. Only a relatively low cooUng flow, indicated by dashed arrows in Figure 2. is needed to en. sure the mains manifold and mains pigtail pipe assemblies are kept cool.

The motive force for generating the coohng flow in the mains circuit is provided by the venturis 48, which can be located within the respective injector heads. Each venturi uses a converging-diverging nozzle to convert the pressure energy of the respective pilot flow into velocity energy. This generates a ow pressure region in the extraction path 52 that draws fuel in via suction from the corresponding mains feed 46. After passing through the throat of the converging-diverging nozzle, the mixed fluid expands and reduces in velocity, which has the effect of re-pressurising the fluid The spool valves 54 have a dual purpose as they also shut-of? the fuel flow to the mains discharge orifices of the injectors in pilot-only operation. This results in the fuel in the mains manifold 42 and upstream parts of the mains feeds 46 (e.g. the pigtail pipe assemblies) being retained in pHot-only operation. This fuel retention helps to avoid large fuel dips when staging in the mains circuit.

Thus, in pilot-only operation, a coong flow is drawn from the pilot manifold 40 at the respective cufiet of the splitter valve 32. The cooling flow passes through. the shut-off valve 56 and the restrictor 58 of the supply path 50, along the mains manifold 42, and then through the mains pigtaU pipe assemblies of the mains feeds 46 before being mixed with the pilot flow 01 each injector. Hence, the flow used for keeping the mains manifold and pigtaiis cool is sourced from the total metered flowS Turning to mainspilot operation (shown in Figure 3), the splitter valve 32 is now positioned to supply the metered fuel to both the pilot 40 and mains 42 manifolds, and the shut-off valve 56 is commanded by the controller 36 to close the supply path 50. The elevated fuel pressure in the mains circuit results in the spool valves 54 closing the extraction paths 52 and unblocking the mains feeds 46, allowing fuel to he delivered to the mains discharge orifices. In this state, therefore, the pilot and mains circuits are isolated, and all the pilot spht of the metered fuel is delivered to the pilot feeds 44 via the pilot manifold 40 and all the mains split of the metered fuel is delivered to the mains feeds 4$ via the mains manifold 42.

The extraction paths can be configured so that they minimise the presence of stagnant fuel in rnains+pilot operation.

For the spool valve 54 in each injector head to close th.e respective extraction path 52, the fuel pressure in the mains feed 46 has to be higher than the total of the fuel pressure in the pilot feed 44 and the spool valve cracking pressure. The cracking pressure for the spool valves 54 can. have a relatively low value e.g. about 10 paid (007 MPa). Thus; if the pilot feed 44 fuel pressure is 500 psi (344 MPa). the fuel pressure in the mains feed 46 only needs to be greater than 510 psi (3.52 MPa) in order for the spool valves to unblock the mains feeds 46 in mains4-pilot operation and supply the mains discharge orifices.

Alternatively, however, the spool valves could be scheduled valves under the control of the controller 36.

The fuel supply system provides a number of advantages: By providing a venturi 48 for each injector, it can prevent imbalances beeen the cooling flows of the mains feeds 46.

It does not require a re-circulation manifold, therefore a simpler manifold configuration can be implemented, providing significant cost saving.

The venturis 48 do not have moving parts, which allows them to operate in a hot region of the engine. Further, the venturis can be integrated into the injector heads along with the spool valves 54.

* In pilot-only operation, aU the metered fuel is burnt, reducing errors in the total flow meter reading. In particular, any pHot flow that leaks past a spool valve piston is drawn back into the pilot feed 44 by the venturi 48.

It avoids a low pressure cooling flow return back to Ifle inlet of the pump unit.

Therefore; the risk of combustor inlet air being drawn into the pump unit inlet is lowered, helping to reduce the risk of cavitation which can cause damage in the pump unit and instabilities in the fuel control system. Also fuel temperature increases caused by coding flow return can be avoided.

* The thermal management of the mains circuit does not compromise fuel split accuracy. A fuel system with a re-circulation manifold also has to maintain a cooling flow in mains operation to prevent the re-circulation manifold from coking when not in use. This results in an error in fuel split versus demand, which can be particularly pronounced at low fuel flows and may require the use of complex cahbration techniques.

In the system of Figures 2 and 3, operation of the shut-off valve 56, scheduled with operation of the splitter valve 32, opens or closes the supply path 50 linking the pilot 40 and mains 42 manifolds; as required to supply or shut off the cooling flow. However, a passive form of control is possible as discussed below with reference to Figures 4 and 5, which show schematically a variant of the fuel supply system of Figures 2 and 3. In Figure 4 the variant is in pilot-only and in Figure 5 it is in mains÷pilot operation. Corresponding features bear the same reference numbers in Figures 2 to 4.

The system of Figures 4 and 5 is identical to that of Figures 2 and 3 except that instead of the solenoid controlled shut-off valve 56; a shut-off valve in the form of a simple check valve 56' is provided on the supply path 50.

Wth the splitter valve 32 positioned for pilot-only operation (Figure 4), the venturis 48 on each pilot feed 44 lower the fuel pressure in the mains manifold 42, resulting in the check valve 56 opening. Fuel is therefore drawn from the pilot manifold 40 at the pilot outlet of the splitter valve into the mains manifold via the check valve.

Wth the spUtter valve 32 positioned for mains+pflot operation (Figure 5), the check valve 56' connecting the pilot 40 and mains 42 manifolds doses under the increased mains manifold pressure, preventing mains Mow from entering the pilot manifold.

The functional operation of the system is thus the same as for the active control approach of Figures 2 and 3, except that the electronic controfler 36 does not need to schedule the operation of a solenoid controfled sh.utoff valve as the check valve 56' provides automatic control of the fuel flow connection between the manifolds 40, 42.

Other variants are also possible. For example, the spool valve 54 can be replaced with a passive arrangement, such as a simple pipe connecting the low static pressure connection point of the venturi 48 to the mains 46 feeds. A check valve in the mains feed 46 downstream of the connecting pipe can then prevent the mains manifold from emptying in pilotoniy operation. This arrangement relies on the relative pressures in the feeds and the relaUve ppe/feed dimensons to pull fuel from the mans feed nto the pilot feed dunng plot*-only operation but not during mains+pilot operation. The connecting pipe is preferably as small as possible to reduce amounts of any stagnafing fuel n the pipe during rnains+pilot operation. A leakage flow through the pipe can be tolerated during mains+piio operation, providing it is small enough not to significantly affect the fuel split between pilot and mains circuits.

The shut-off valve 56 shown in Figures 2 and 3 is a single stage solenoid However, other options are possible such as a two stage solenoid-controlled hydraulic valve. The shut-off valve may be integrated into the splitter valve 32.

The LVDT 38 on the splitter valve 32 can be replaced by any type of position sensor.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will he 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 deschbed embod!ments may be made without departing from the spirit and scope of the invention. ii

Claims (10)

1. C LA MS1. A fu& supply system for a fuel injector of a multi-stage combustor, the system including: a sphtter valve (32) which dehvers respective fuel flows to pilot (40) and mains (42) manifolds, the spUtter valve being operable to control the fuel flow spht between the manifolds; a pot feed (44) extending from the pilot manifold to direct fuel flow to a pilot discharge orifice of the fuel injector, and a mains feed (46) extending from the mains manifold to direct fuel flow to a mains discharge orifice of the fuel injector; a venturi (48) located on the pilot feed to create a reduced static pressure region of the pilot feed; and a cooling flow extraction path (52) extending from the mains feed to the reduced static pressure region of the pilot feed, and a coding flow supply path (50) extending from the pilot manifold to the mains manifold; wherein the extraction and supply paths are configured such that, when the splitter valve is operated to provide a pilot-only fuel flow split in which fuel flows from the splitter valve to the pilot manifold but substantially no fuel flows from the splitter valve to the mains manifold, a cooling fuel flow flows through the mains manifold, the cooling fuel flow being supplied from the pilot manifold to the mains manifold through the supply path, and being extracted from the mains feed to the reduced static pressure region of the pilot feed through the extraction path.
2. 2. A fuel supply system according to claim I wherein the supply path is located adjacent to or integrated with the splitter valve.
3. 3. A fuel supply system according to claim 1 or 2, wherein the mains feed includes a pigtail pipe assembly, the extraction path extending from the main feeds at a location downstream of the pigtail pipe assembly.
4. 4. A fuel supply system according to any one of the previous claims, wherein the supply path contains a restrictor (58) to limit the cooling fuel flow.
5. 5, A fuel supply system according to any one of the previous claims, wherein the extraction path includes an extraction path valve (54) which opens the extraction path when the splitter valve is operated to provide the pilot-only split, and closes the extraction path when the splitter valve is operated to provide the mainsi-pilot split.
6. 6. A fuel supply system according to claim 5, wherein the extraction path valve is configured such that, when it opens the extraction path it blocks fuel from flowing further along the mains feed towards the mains discharge orifice.
7. 7. A fuel supply system according to any one of the previous claims, wherein the supply path includes a shut-off valve (56, 56') which is operable to open the supply path when the splitter valve is operated to provide the pilot-only split and to dose the supply path when the splitter valve is operated to provide the mains+pilot split
8. 8. A fuel supply system according to any one of the previous claims, wherein the combustor has a plurality of fuel injectors, the fuel supply system having, for each injector, a respective pilot feed extending from the pilot manihld, a respective mains feed extending from the mains manifold, a respective venturi, and a respective extraction path.
9. 9. A gas turbine engine 10 having a multi-stage combustor and the fuel supply system of any one of the previous claims.
10. 10. A fuel supply system as any one herein described with reference to and as shown in Figures 2 to 5.