Patent Description:
Fuel management systems are conventionally used for providing fuel to a gas turbine engine and for management of thermal loads. Some gas turbine engines comprise a combustor and a reheat, also known as an afterburner, to provide different levels of engine thrust. Known fuel management systems for gas turbine engines comprising a reheat can be complex and inefficient. It is therefore desirable to provide an improved fuel management system for a gas turbine engine comprising a reheat.

<CIT> discloses fuel control system for an aircraft gas turbine engine that includes a thrust augmentation system. An augmentor fuel pump is arranged to provide pressurized fuel to an exhaust nozzle throat area actuation system to eliminate the need for a separate hydraulic pump to provide pressurized fluid for exhaust nozzle actuation. An augmentor fuel bypass arrangement is provided to enable the augmentor fuel pump to provide pressurized fuel to the main fuel pressurizing valve and to components operated by the main fuel system in the event of failure of the main fuel pump. The augmentor fuel pump pressure and output flow are controlled as a function of thrust augmentation demand and main fuel system operation. The system provides redundancy by enabling either the main fuel system or the augmentor fuel system to maintain engine operation if one of the fuel systems fails or provides inadequate fuel flow.

<CIT> discloses an aircraft gas turbine engine having a continuously driven centrifugai pump for supplying liquid fuel to supplementary burners. The supply of liquid fuel can be stopped by closing an inlet valve which is disposed be-tween the fuel source and the centrifugal pump while the liquid fuel within the pump is vented by connecting the delivery side of the pump to the induction chamber of an ejector pump. The ejector pump has an inlet nozzle connected to an independent source of liquid fuel under pressure and a diffuser passage connected to a low-pressure region of the fuel supply system.

<CIT> discloses a fuel delivery and control system for use with a gas turbine engine. The system includes a fuel-oil heat exchanger for reducing engine oil temperature wherein a minimum rate of fuel flow is maintained through the heat exchanger and main fuel control by including a second recirculating valve in parallel flow connection to the fuel control metering valve. The recirculating valve is arranged to automatically open and close as a function of fuel temperature, metering valve position and in-flight engine fuel shutoff.

<CIT> discloses a fuel supply device for a turbo jet engine with an afterburner having pipes bifurcating from a main fuel pipe for the supply of fuel to the afterburner and the combustion chamber. To deliver the fuel at the required rate of flow and at the required pressure, provision is made in the main fuel pipe for a first fuel delivery pump, and for a second fuel pump and respectively an afterburner pump in the fuel pipes. In order also to be able to supply the afterburner via the second fuel pump when a suitably free pump capacity exists, downstream of the afterburner pump and the second fuel pump, provision is made for a connecting pipe between the two fuel pipes. When operating the turbo jet engine with the afterburner, the afterburner can be supplied by the second fuel pump via the connecting pipe when the fuel requirement of the turbo-combustion chamber is reduced, so that its delivered quantity of fuel is high. An undesirably high fuel temperature downstream of the second fuel pump can thus be prevented. An oil cooler may be provided as well as a <NUM>/<NUM> directional control valve in the connecting pipe.

<CIT> discloses a fuel metering and actuation system that includes a main centrifugal pump supplying fuel to the combustor. A gear pump takes suction from the main pump discharge and supplies actuators at high pressure. A variable set point pressure relief valve operates to decrease the gear pump pressure level when high pressure is not required. A selection valve directs discharge from the gear pump to the combustor when the main pump supplied insufficient pressure for the fuel nozzles. A bypass line around the selector valve with a check valve permits main pump to discharge fuel to be used for the actuator if the gear pump becomes inoperative.

According to a first aspect of the present disclosure, there is provided a fuel management system for a gas turbine engine according to claim <NUM>.

The fuel management system comprises: a fuel tank configured to store fuel for the gas turbine engine; a fuel supply line configured to supply fuel from the fuel tank to a combustor of the gas turbine engine; a reheat fuel supply line configured to supply fuel from the fuel tank to a reheat of the gas turbine engine, the reheat fuel supply line extending from a reheat branching point on the fuel supply line to the reheat; a fuel supply pump disposed along the fuel supply line upstream of the reheat branching point; a reheat pump disposed along the reheat fuel supply line, the reheat pump configured to pressurise fuel to a reheat delivery pressure for delivery to the reheat; and a reheat recirculation line configured to recirculate fuel from the reheat fuel supply line to the fuel tank, or to the fuel supply line upstream of the fuel supply pump, the reheat recirculation line extending from a reheat recirculation branching point on the reheat fuel supply line downstream of the reheat pump; characterised in that the fuel management system further comprises a reheat recirculation control system configured to:.

recirculate fuel at the required recirculation rate via the reheat recirculation line to provide fuel to the reheat at the reheat fuel demand rate.

Reheat should be understood as synonymous with "afterburner".

The term fuel tank should be understood as a bulk fuel storage system for the gas turbine engine. The fuel tank may be an airframe fuel tank. The fuel tank may be an engine-located fuel tank or fuel sump.

The reheat recirculation control system may further comprise a reheat controller configured to receive a reheat control signal and to determine the reheat fuel demand based on the reheat control signal.

"Determining" the fuel supply and demand rates may include measuring the rates using sensors, determining the rates by calculation or otherwise, and/or receiving inputs or values representing the rates from other systems, such as the main control systems of the aircraft or gas turbine engine.

For example, the reheat pump supply rate may be measured using a flow rate sensor downstream of the reheat pump, and the reheat fuel rate may be determined by receiving a reheat control signal from the aircraft control systems which might indicate or be determined to indicate the demand rate. For example, if a "no reheat" control signal is received by the reheat circulation control system, then this may be determined to require a reheat fuel demand rate of substantially zero. Therefore, the required recirculation rate may be determined to substantially all of, or equal to, the fuel supply rate. If a "full reheat" control signal is received by the reheat circulation control system, then this may be determined to require a maximum reheat fuel demand rate. Therefore, the required recirculation rate may be determined to substantially zero. Other intermediate recirculation rates can be envisaged, for example if an intermediate amount of reheat is required.

The reheat control signal may be a human-activated control signal, or an automatically generated control signal from an autopilot, an automated control system, or the like.

The reheat recirculation line may comprise a non-return valve for preventing backflow along the reheat recirculation line. The reheat recirculation line may comprise an orifice plate, fixed or adjustable, for maintaining a minimum predetermined pressure in the reheat recirculation line.

The reheat recirculation line may further comprise a recirculation heat exchanger. The reheat recirculation line may comprise a first recirculation branch line and a second recirculation branch line. In some examples, one of the recirculation branch lines may comprise the recirculation heat exchanger.

The first and second recirculation branch lines may be configured to combine upstream of the non-return valve and/or orifice plate. The first and second recirculation branch lines may be configured to begin at a branching point in the reheat recirculation line or may be configured as two separate recirculation lines each having its own reheat recirculation branching point on the reheat fuel supply line.

The recirculation heat exchanger may be configured to reject excess heat from fuel in the first recirculation line. The recirculation heat exchanger may reject heat from the fuel in the recirculation line to ram air, to fuel in the fuel supply line, or any other heat sink in the thermal management system of the gas turbine engine or aircraft.

The fuel management system may further comprise a heat exchanger disposed along the reheat fuel supply line and/or along the fuel supply line upstream of the reheat branching point.

The heat exchanger may be configured to reject excess heat into fuel in the reheat fuel supply line or the fuel supply line. The recirculation heat exchanger may receive rejected heat from the fuel in the recirculation line, from fuel in elsewhere in the fuel supply line, or from any other heat source in the thermal management system of the gas turbine engine or aircraft, such as from an oil thermal management line.

The fuel management system may further comprise a combustor pump disposed along the fuel supply line downstream of the reheat branching point and upstream of the combustor, configured to pressurise fuel to a combustor delivery pressure for the combustor or to provide a demanded combustor fuel delivery rate; and a combustor heat exchanger located on the fuel supply line downstream of the reheat branching point and upstream of the combustor configured to exchange heat into fuel in the fuel supply line between the reheat branching point and the combustor.

The combustor heat exchanger may be arranged downstream or upstream of the combustor pump.

A metering unit may be provided downstream of the combustor pump and upstream of the combustor, configured to meter fuel to the combustor. The metering unit may comprise a combustor recirculation line configured to recirculate fuel downstream of the combustor pump to upstream of the combustor pump.

The fuel management system may further comprise a reheat pump controller configured to control the speed, pressure, and/or flow rate of the reheat pump. The reheat pump controller may be configured to control the reheat pump independent of an auxiliary gearbox shaft speed.

The reheat pump controller may be configured to operate the reheat pump so as to substantially maximise an efficiency of the reheat pump.

The reheat pump has one or more efficiency curve, and it is desirable to operate the pump at, or close to, its maximum efficiency. However, the maximum or high efficiency settings for the reheat pump may not suit the required fuel flow rate for the reheat at any given time. Therefore, the reheat recirculation line may permit the reheat pump to be operated at maximum efficiency, even if this outputs too much fuel, as the excess fuel can simply be recirculated without being delivered to the reheat.

The fuel management system may further comprise a flow control valve disposed in the reheat fuel supply line downstream of the reheat pump and upstream of the reheat recirculation branching point.

According to a second aspect of the present disclosure, there is provided a gas turbine engine according to claim <NUM> comprising a fuel management system according to the first aspect above.

According to a third aspect of the present disclosure, there is provided a method of managing fuel in a gas turbine engine according to claim <NUM>, comprising: supplying fuel from a fuel tank to a combustor of the gas turbine engine via a fuel supply line comprising a first fuel supply pump; branching fuel from the fuel supply line into a reheat fuel supply line for supplying fuel to a reheat of the gas turbine engine at a reheat branching point, the reheat branching point being downstream of the first fuel supply pump; pressurising the fuel in the reheat fuel supply line to a reheat delivery pressure using a reheat pump disposed along the reheat fuel supply; recirculating fuel from the reheat fuel supply line via a reheat recirculation line to the fuel tank or to the fuel supply line upstream of the first fuel supply pump, the reheat recirculation line extending from a reheat recirculation branching point on the reheat fuel supply line downstream of the reheat pump, controlling recirculation of the fuel from the reheat fuel supply line via the reheat recirculation line; determining a reheat pump supply rate from the reheat pump; determining a reheat fuel demand rate required by the reheat; determining a required recirculation rate based upon the reheat pump supply rate and the reheat fuel demand rate; and recirculating fuel at the required recirculation rate via the reheat recirculation line to provide fuel to the reheat at the reheat fuel demand rate.

A portion of the fuel management system upstream of the fuel supply pump may be a low pressure portion of the system. A portion of the fuel management system downstream of the fuel supply pump and upstream of the combustor pump and the reheat pump may be a medium pressure portion of the system. A portion of the fuel management system downstream of the combustor pump and downstream of the reheat pump may be a high pressure portion of the system. The reheat recirculation line may extend from the high pressure portion, and in particular the high pressure portion of the reheat fuel supply line, to the low pressure portion of the system.

The gearbox may be a reduction gearbox (in that the output to the fan is a lower rotational rate than the input from the core shaft). Any type of gearbox may be used. For example, the gearbox may be a "planetary" or "star" gearbox, as described in more detail elsewhere herein.

According to an aspect, there is provided an aircraft comprising a cabin blower system or a gas turbine engine as described and/or claimed herein.

The engine core <NUM> comprises, in axial flow series, a low pressure compressor <NUM>, a high-pressure compressor <NUM>, combustor <NUM>, a high-pressure turbine <NUM>, a low pressure turbine <NUM> and a core exhaust nozzle <NUM>.

The compressed air exhausted from the high pressure compressor <NUM> is directed into the combustor <NUM> where it is mixed with fuel and the mixture is combusted.

Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in <FIG> has a split flow nozzle <NUM>, <NUM> meaning that the flow through the bypass duct <NUM> has its own nozzle <NUM> that is separate to and radially outside the core engine nozzle <NUM>. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct <NUM> and the flow through the core <NUM> are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine <NUM> may not comprise a gearbox <NUM>.

<FIG> shows a schematic view of a first example fuel management system 400A for a gas turbine engine according to the present disclosure. The fuel management system 400A comprises a fuel supply line <NUM> configured to supply fuel from a fuel tank <NUM> to a combustor <NUM> of the gas turbine engine. An inlet <NUM> is configured to receive fuel from the fuel tank <NUM> of the gas turbine engine and/or a fuel tank <NUM> of an aircraft. A boost pump <NUM> may be provided at or near the inlet of the fuel supply line <NUM> from the fuel tank <NUM> in order to pressurise the fuel to a suitable pressure for entering the fuel supply line. The fuel management system 400A further comprises a combustor pump <NUM> located upstream of a combustor metering unit <NUM> along the fuel supply line <NUM>. The combustor pump <NUM> is configured to increase a pressure of fuel within the fuel supply line <NUM> and thereby pressurise fuel to a delivery pressure for the combustor <NUM>.

The fuel management system 400A comprises a combustor heat exchanger <NUM> located on the fuel supply line <NUM> upstream of the combustor pump <NUM>. The combustor heat exchanger <NUM> is configured to exchange heat from a heat source of the gas turbine engine (such as an oil cooling system, or other heat source) to fuel in the fuel supply line <NUM>. In this example, the combustor heat exchanger <NUM> is configured to receive a thermal load from the gearbox <NUM>. In other examples, the combustor heat exchanger may be located downstream of the combustor pump. Accordingly, the fuel supply line <NUM> is configured to supply fuel from the fuel tank <NUM> to the combustor <NUM> via the combustor metering unit <NUM> such that fuel passing through the combustor metering unit <NUM> has been subject to pressurisation to the delivery pressure by the combustor pump <NUM> and has also passed through the combustor heat exchanger <NUM>. The combustor metering unit <NUM> is configured to meter fuel from the fuel supply line <NUM> to the combustor <NUM>. A combustor recirculation line <NUM> is configured to recirculate fuel from the combustor metering unit <NUM> upstream on the fuel supply line <NUM> to upstream of the combustor pump <NUM> to provide a metered output flow from the combustor metering unit <NUM> to the combustor <NUM>.

The fuel supply line <NUM> comprises a fuel supply pump <NUM> located upstream of the combustor pump <NUM>. The fuel supply pump may be a boost pump or may be provided in addition to a boost pump, such as pump <NUM>. The fuel supply pump <NUM> is generally configured to receive fuel at a first pressure, in particular the storage pressure in the fuel tank <NUM> (or the output pressure provided by the boost pump <NUM>, if present) and pressurise the fuel to a second pressure for supply downstream along the fuel supply line <NUM>. The portion of the fuel management system 400A upstream of the fuel supply pump <NUM> may be a low pressure portion of the system, the portion of the fuel management system 400A downstream of the fuel supply pump <NUM> and the combustor pump <NUM> may be a medium pressure portion of the system, and the portion of the fuel management system downstream of the combustor pump <NUM> may be a high pressure portion of the system. A filter <NUM> is disposed in the fuel supply line <NUM> for filtering fuel.

The fuel management system 400A further comprises a reheat fuel supply line <NUM> which is configured to supply fuel from the fuel tank <NUM> to a reheat <NUM> (i.e., afterburner) of the gas turbine engine <NUM> from the fuel supply line <NUM>. In the example of <FIG>, the reheat fuel supply line <NUM> extends from a reheat branching point <NUM> on fuel supply line <NUM> to the reheat <NUM>. The reheat fuel supply line <NUM> comprises a reheat pump <NUM>. The reheat pump <NUM> is configured to increase a pressure of fuel within the reheat fuel supply line <NUM> and thereby pressurise fuel to a delivery pressure for the reheat <NUM>. The reheat pump <NUM> is powered by an independent pump controller <NUM> such that the pump <NUM> is fully- and independently-controllable, i.e., it can operate independently of the auxiliary gearbox shaft speed. In other examples, the reheat pump <NUM> may be driven by an engine gearbox rather than an independent controller, and such a configuration may also provide many of the benefits contemplated in this disclosure.

The reheat fuel supply line <NUM> further comprises a valve <NUM> downstream of the reheat pump <NUM>, for closing the reheat fuel supply line <NUM> if required.

The fuel management system 400A comprises a reheat heat exchanger <NUM> located on the reheat fuel supply line <NUM> downstream of the reheat pump <NUM>. The reheat heat exchanger <NUM> is configured to exchange heat from a heat source of the gas turbine engine (such as an oil cooling system, or other heat source) to fuel in the reheat fuel supply line <NUM>. In this example, the reheat heat exchanger <NUM> is configured to receive a thermal load from the gearbox <NUM>. In other examples, the reheat heat exchanger <NUM> may not be provided, or may be provided in another location on the reheat line <NUM> or upstream thereof, such as shown in <FIG> of this disclosure.

The fuel management system 400A further comprises a reheat recirculation line <NUM> configured to recirculate an excess portion of fuel from the reheat fuel supply line <NUM> to a location upstream for resupply to the fuel supply line <NUM>. The reheat recirculation line <NUM> extends from a reheat recirculation branching point <NUM> on the reheat fuel supply line <NUM>, which is located downstream of the reheat pump <NUM>. In this example, the reheat recirculation branching point <NUM> is also downstream of the reheat heat exchanger <NUM>.

In the example of <FIG>, the reheat recirculation branching point <NUM> is located within a reheat recirculation control system <NUM>. The reheat recirculation control system <NUM> is configured to control the recirculation of fuel from the reheat fuel supply line <NUM> via the reheat recirculation line <NUM>. By controlling the recirculation of fuel from the reheat fuel supply line <NUM>, the reheat recirculation control system <NUM> is configured to meter fuel flow to the reheat <NUM> by selectively recirculating a portion of fuel from the reheat fuel supply line <NUM> via the reheat recirculation line <NUM>.

The portion of the reheat fuel supply line <NUM> downstream of the reheat recirculation branching point <NUM> may be referred to as the reheat delivery line <NUM>. Any fuel entering the reheat delivery line <NUM> will be delivered to the reheat <NUM>.

In this example, the reheat recirculation line <NUM> extends from the reheat recirculation branching point <NUM> to the fuel tank <NUM>, such that fuel in the recirculation line <NUM> is delivered back to the fuel tank <NUM>. In other examples, the reheat recirculation line may extend to a location on the fuel supply line which is upstream of the fuel supply pump <NUM>. In other words, the reheat recirculation line may recirculate fuel from a high pressure portion of the reheat fuel supply line downstream of the reheat pump <NUM> to a low pressure portion of the system upstream of the fuel supply pump <NUM>.

The fuel supply pump <NUM> and the filter <NUM> are both located upstream of the reheat branching point <NUM> such that fuel travelling in both the downstream portion of the fuel supply line <NUM> and in the reheat fuel supply line <NUM> are filtered and pressurised before reaching the combustor pump <NUM> or reheat pump <NUM> respectively.

The reheat recirculation line <NUM> also comprises pressure reducing element <NUM>, such as a pressure reducing valve or orifice plate, a non-return valve <NUM> and a fuel outlet <NUM> from upstream to downstream. The pressure reducing element <NUM> may be configured to maintain a minimum upstream pressure in the reheat recirculation line <NUM> and may assist in avoiding sudden expansion of fuel when entering the fuel tank <NUM>. This may ensure that there is a sufficient pressure in the reheat recirculation control system <NUM> when fuel is recirculating along the recirculation line <NUM>. The non-return valve <NUM> is provided to mitigate the risk of reverse flow into system from the reheat recirculation line <NUM>. The fuel outlet <NUM> is configured to release fuel into the fuel tank <NUM> from the reheat recirculation line <NUM>. In other examples, the fuel outlet of the reheat recirculation line <NUM> may be configured to release fuel into the fuel supply line <NUM> upstream of the fuel supply pump <NUM>.

In view of the above, the operation of the fuel management system 400A will now be described in both reheat and non-reheat modes.

When reheat is desired, a reheat signal may be provided by a reheat control system <NUM>. The reheat signal may be derived from or related to a quantity of thrust demanded from gas turbine engine by, for example, an electronic fly-by-wire control system. The quantity of thrust demanded (i.e., the thrust demand) may vary continuously and/or discretely while the fuel management system 400A is in use, and so the fuel demand of the reheat <NUM> may vary continuously and/or discretely while the fuel management system 400A is in use.

The reheat control system <NUM> may indicate, include, or define a reheat fuel demand rate R which is required to be delivered to the reheat <NUM> in order to achieve the desired thrust from the engine.

The reheat control system <NUM> is configured to communicate with the reheat pump controller <NUM> and the reheat recirculation control system <NUM> to provide the required reheat fuel demand rate to the reheat <NUM> via the reheat delivery line <NUM>.

To achieve this, the fuel management system 400A further comprises a reheat pump flow sensor <NUM> configured to monitor a reheat pump supply rate of the fuel which is expelled from the reheat pump <NUM>. The reheat pump flow sensor <NUM> is therefore configured to monitor the flow rate upstream of the reheat recirculation branching point <NUM>. In other examples, the reheat pump supply rate may be inferred or otherwise calculated using other methods, such as using pressure-based throttle valves.

The reheat recirculation control system <NUM> is configured to determine the reheat pump supply rate from the reheat pump <NUM> using the reheat pump flow sensor <NUM> and determine a reheat fuel demand rate required by the reheat <NUM> based upon the reheat signal. Based upon the reheat pump supply rate and the reheat fuel demand rate, the reheat recirculation control system <NUM> will determine a required recirculation rate of fuel to be recirculated by via the reheat recirculation line <NUM> in order to provide the reheat fuel demand rate to the reheat delivery line <NUM>. For example, if the reheat fuel demand rate is lower than the reheat pump supply rate, then the reheat recirculation control system <NUM> will determine a required recirculation rate of fuel which must be recirculated via the reheat recirculation line such that fuel is supplied to the reheat delivery line <NUM> at the reheat fuel demand rate indicated by the reheat control system <NUM>. In some examples, the reheat control system <NUM> may additionally adjust the operation of the reheat pump <NUM> using the reheat pump controller <NUM> to adjust the reheat pump supply rate in addition to recirculating fuel to meet the reheat fuel demand rate.

If the reheat signal from the reheat control system <NUM> is adjusted such that the reheat fuel demand rate changes, the reheat recirculation control system <NUM> may adjust the recirculation rate accordingly. If more fuel is required by the reheat <NUM>, then the reheat recirculation control system <NUM> may recirculate less fuel so that more fuel is delivered to the reheat delivery line <NUM>, or if less fuel is required by the reheat <NUM>, then the reheat recirculation control system <NUM> may recirculate more fuel, such that less fuel is delivered to the reheat delivery line <NUM>. Again, the reheat control system <NUM> may additionally adjust the reheat pump controller <NUM> to adjust the reheat pump's supply rate to better suit the reheat fuel demand rate. This may be appropriate if, for example, the required recirculation rate was greater than a maximum possible recirculation rate for the reheat recirculation line <NUM>. It will be understood that the reheat recirculation line <NUM> permits fuel to be recirculated to account for any discrepancy between the required fuel rate for the reheat <NUM> and the fuel supply rate from the reheat pump <NUM>.

Turning now to a non-reheat mode, in which no or minimal reheat is required, the function of the fuel management system 400A will be further described.

In previously known reheat systems, when no reheat was required, a valve downstream of the reheat pump would simply be closed such that the reheat pump would continue to run 'dry' (i.e., the pump remains running without expelling fuel - fuel may or may not be retained in the pump chamber during 'dry' running). This is undesirable for many reasons. For example, dry running a pump is typically inefficient and also causes excessive heating of the pump and fuel in the reheat line, which in turn can cause fuel coking, reduce service life, and increase maintenance costs and failure rates. Further, the operation of the valve to turn the reheat supply on or off could also cause high hammer shock pressures that can damage the fuel system.

When reheat is inactive, all fuel flow in the reheat fuel supply line <NUM> may recirculated via the recirculation line <NUM> and no fuel may be delivered to the reheat burners via the reheat delivery line <NUM>.

In some examples, the reheat delivery line <NUM> may comprise a shut-off valve (not shown, but similar to valve <NUM>) so that the reheat <NUM> can be physically isolated from the reheat fuel supply line <NUM> when reheat is inactive. In this reheat-off mode, the pump controller <NUM> may control the reheat pump to provide a relatively low outlet pressure, and a flow rate based on the cooling requirements of the reheat heat exchanger <NUM>. This may minimise the pump power demand and the wasted heat generated but keep the fuel flowing for thermal management purposes.

In some examples, the reheat recirculation flow rate in reheat recirculation line <NUM> may be set based on, or based in part on, satisfying the requirements of a thermal management system of the engine.

Returning now to the exemplary fuel management system of <FIG>, the provision of a reheat recirculation line <NUM> in accordance with the present disclosure can mitigate some or all of these issues, as follows.

When reheat is not desired, the reheat signal of the reheat control system <NUM> may indicate, include, or define a reheat fuel demand rate R of substantially zero. In other words, when no reheat is required, then no fuel should be delivered to the reheat <NUM> via the reheat delivery line <NUM>.

Based upon the reheat pump supply rate and the reheat fuel demand rate of zero, the reheat recirculation control system <NUM> will determine a required recirculation rate of fuel for all of the fuel expelled by the reheat pump <NUM> to be recirculated by via the reheat recirculation line <NUM>, such that no fuel is provided to the reheat delivery line <NUM>.

In some examples, the reheat control system <NUM> may additionally adjust the reheat pump controller <NUM> to adjust the reheat pump's supply rate to better suit the reheat fuel demand rate. In other examples, the reheat control system <NUM> may receive a signal indicating that more cooling capacity is required in the engine and may increase the reheat pump supply rate to increase the flow of fuel through the reheat heat exchanger <NUM> independently of the fuel requirements at the reheat. In the previously contemplated systems in which the pump was run 'dry' no such additional cooling capacity was achievable while the reheat was switched off.

A further benefit of the reheat recirculation line <NUM> is that the reheat pump <NUM> can be controlled to operate at high or maximum efficiency regardless of the reheat requirements of the engine. In previously considered systems, running the pump dry could be highly inefficient but the presently considered arrangement permits the pump to remain in operation flowing fuel even when fuel is not required at the reheat <NUM>. Accordingly, the pump efficiency and, therefore, the overall engine efficiency may be improved by recirculation of fuel from the reheat fuel supply line.

Additionally, as the reheat pump <NUM> is operating and flowing fuel downstream even when reheat is not required, the system latency for providing reheat on demand may be reduced compared to other systems. Once the reheat signal indicates that reheat is required, the reheat recirculation control system <NUM> can quickly divert the already flowing fuel to the reheat delivery line <NUM> (or rather stop recirculating fuel via the recirculation line <NUM>) in order to provide near-instant response time for reheat.

<FIG> shows a schematic view of a second example fuel management system 400B for a gas turbine engine according to the present disclosure. The second example fuel management system 400B is generally similar to the first example fuel management system 400A, with like reference numerals indicating common or similar features. In contrast to the first example fuel management system 400A, the second fuel management system 400B comprises an alternative heat exchanger arrangement and an alternative reheat recirculation line arrangement, as follows.

In fuel management system 400B, a heat exchanger is not provided on the reheat fuel supply line <NUM> itself and instead an additional upstream heat exchanger <NUM> is provided on the fuel supply line <NUM> upstream of the reheat branching point <NUM>. The heat exchanger <NUM> is configured to exchange heat from a heat source of the gas turbine engine (such as an oil cooling system, electrical cooling system, or other heat source) to fuel in the fuel supply line <NUM>. In this example, the combustor heat exchanger <NUM> is configured to receive a thermal load from the gearbox <NUM>. The heat exchanger <NUM> therefore transfers a thermal load into fuel in the upstream portion of the fuel supply line <NUM> from another location.

Providing the additional heat exchanger downstream of the reheat pump as per fuel management system 400A is less thermally efficient than providing it upstream of the reheat branching point per fuel management system 400B because it will receive less fuel flow; however, the former configuration has the additional advantages of lower temperatures and higher pressures at the reheat pump's inlet. Accordingly, the location of the additional heat exchanger can be selected according to the particular system requirements and priorities.

In some examples, the additional heat exchanger in the reheat line/ in system 400A and upstream of the reheat branching point <NUM> in system 400B may not interface directly with the oil system and may instead exchange heat with an intermediate thermal management loop, which incorporates multiple heat sinks (i.e., fuel and air). Such a configuration may mitigate against overcooling of the reheat fuel flow during particularly cold operational temperatures and, consequently, overcooling the oil system.

Another difference between the two exemplary fuel management systems 400A and 400B is the configuration of the reheat recirculation line <NUM>.

In the fuel management system 400B, the reheat recirculation line <NUM> further comprises a first recirculation branch line 480A and a second recirculation branch line 480B. The reheat recirculation line <NUM> branches from the reheat fuel supply line <NUM> at the reheat recirculation branching point <NUM> and extends some distance downstream as an upstream recirculation line portion, denoted as 480C. The upstream recirculation line portion 480C itself then branches into the first recirculation branch line 480A and the second recirculation branch line 480B at a branching point <NUM>. Fuel travelling along the upstream recirculation line portion 480C can therefore be directed via either of the recirculation branch lines 480A, 480B at the branching point <NUM>. The proportion of fuel directed along each of the recirculation branch lines 480A, 480B is controlled by the reheat recirculation control system <NUM>. In this example, the recirculation branch lines 480A, <NUM> extend downstream to a convergence point <NUM>, at which the recirculation branch lines 480A, 480B recombine and continue downstream as a downstream recirculation line portion 480D once again. The pressure reducing element <NUM> and the non-return valve <NUM> are provided on the downstream recirculation line portion 480D, and the downstream recirculation line portion 480D extends into a single fuel outlet <NUM> to supply recirculated fuel back into the fuel tank <NUM>. In other examples, each of the recirculation branch lines 480A, 480B may extend directly back to the fuel tank <NUM> at its own fuel outlet and may comprise its own pressure reducing element and/or non-return valve. In some other examples, the reheat recirculation line may comprise more than two recirculation branch lines.

In this example, the first recirculation branch line 480A comprises a recirculation heat exchanger <NUM>. The recirculation heat exchanger <NUM> is configured to exchange heat from the fuel in the line 480A to a heat sink of the gas turbine engine (such as a ram air heat exchanger <NUM>, or a refrigeration system). Depending on the fuel temperature and the thermal management system capacity, the recirculated fuel flow may be transported back to the fuel tank <NUM> via the first recirculation branch line 480A (and therefore via the recirculation heat exchanger <NUM> to cool the fuel) or via the second recirculation branch line 480B without cooling. Further, in some examples, such as where precise fuel temperature control is required, the fuel flow in the upstream recirculation line portion 480C may be bifurcated so that a proportion of the fuel flow passes via each of the recirculation branch lines 480A, 480B, and so that the re-combined fuel flow in the downstream recirculation line portion 480D entering the fuel tank <NUM> at a carefully controlled temperature.

Generally, when the thermal management system requires substantial additional thermal capacity in the fuel, the fuel will be recirculated along the first recirculation branch line 480A. For example, when a fuel tank temperature limit has been reached or exceeded, the fuel would be preferentially recirculated via the first recirculation branch line 480A and the recirculation heat exchanger <NUM> to bring down the bulk fuel temperature in the fuel tank <NUM>. The configuration of fuel management system 400B is particularly advantageous because allowing fuel to recirculate via the reheat fuel supply line <NUM> and the recirculation branch lines 480A, 480B, additional cooling capacity can be provided even when the reheat is not in use.

Control of the fuel flow along the recirculation branch lines 480A, 480B is controlled by the reheat recirculation control system <NUM>. The reheat recirculation control system <NUM> may be configured to receive instructions from a thermal management control system (not shown) to divert fuel along one or both recirculation branch lines 480A, 480B in accordance with the requirements of the thermal management control system. It should be understood that various controllable valves may be provided in the reheat recirculation line <NUM> for control by reheat recirculation control system <NUM> in order to direct the fuel flow as required.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. The scope of protection is defined in the appended claims.

It should be understood that features of the fuel management systems 400A and 400B of <FIG> and <FIG> may be combined within the principles of this disclosure, except where mutually exclusive.

Claim 1:
A fuel management system (400A) for a gas turbine engine, the fuel management system comprising:
a fuel tank (<NUM>) configured to store fuel for the gas turbine engine;
a fuel supply line (<NUM>) configured to supply fuel from the fuel tank (<NUM>) to a combustor (<NUM>) of the gas turbine engine;
a reheat fuel supply line (<NUM>) configured to supply fuel from the fuel tank (<NUM>) to a reheat of the gas turbine engine, the reheat fuel supply line (<NUM>) extending from a reheat branching point (<NUM>) on the fuel supply line (<NUM>) to the reheat;
a fuel supply pump (<NUM>) disposed along the fuel supply line (<NUM>) upstream of the reheat branching point (<NUM>);
a reheat pump (<NUM>) disposed along the reheat fuel supply line (<NUM>), the reheat pump (<NUM>) configured to pressurise fuel to a reheat delivery pressure for delivery to the reheat; and
a reheat recirculation line (<NUM>) configured to recirculate fuel from the reheat fuel supply line (<NUM>) to the fuel tank (<NUM>), or to the fuel supply line (<NUM>)upstream of the fuel supply pump (<NUM>), the reheat recirculation line (<NUM>) extending from a reheat recirculation branching point (<NUM>) on the reheat fuel supply line (<NUM>) downstream of the reheat pump (<NUM>);
characterised in that the fuel management system further comprises a reheat recirculation control system (<NUM>) configured to:
determine a reheat pump supply rate from the reheat pump (<NUM>);
determine a reheat fuel demand rate required by the reheat;
determine a required recirculation rate based upon the reheat pump supply rate and the reheat fuel demand rate; and
recirculate fuel at the required recirculation rate via the reheat recirculation line (<NUM>) to provide fuel to the reheat at the reheat fuel demand rate.