Patent ID: 12188413

DETAILED DESCRIPTION OF THE DISCLOSURE

Aspects and embodiments of the present disclosure will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art.

FIG.1is a plan view of an aircraft400comprising a gas turbine engine10. The gas turbine engine10may be a ducted fan gas turbine engine. The aircraft400may comprise any number of gas turbine engines10.

FIG.2illustrates a gas turbine engine10having a principal rotational axis9. The engine10comprises an air intake12and a propulsive fan23that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine10comprises a core11that receives the core airflow A. The engine core11comprises, in axial flow series, a low pressure compressor14, a high-pressure compressor15, combustion equipment16, a high-pressure turbine17, a low pressure turbine19and a core exhaust nozzle20. The engine core11has a central axis192that is substantially aligned with the principal rotational axis9. A nacelle21surrounds the gas turbine engine10and defines a bypass duct22and a bypass exhaust nozzle18. The bypass airflow B flows through the bypass duct22. The fan23is attached to and driven by the low pressure turbine19via a shaft26and an epicyclic gearbox30.

For clarity,FIGS.1to3,5to10,13and14each show directions of a cylindrical coordinate system180of the principal rotational axis9and of the central axis192. The cylindrical coordinate system180includes an axial direction (denoted as z)182, a radial direction (denoted as r)184and a tangential or circumferential direction (denoted as Θ)186. The terms axially, axial, radially, radial, circumferentially, circumferential, tangentially and tangential are defined with respect to the principal rotational axis9(and, thus, the central axis192). The cylindrical coordinate system180is shown as being displaced away from the principal rotational axis9and the central axis180in the abovementioned Figures, however it will be appreciated that the cylindrical coordinate system180is centred on the principal rotational axis9and the central axis192(i.e. the axial direction182is aligned with the principal rotational axis9).

In use, the core airflow A is accelerated and compressed by the low pressure compressor14and directed into the high pressure compressor15where further compression takes place. The compressed air exhausted from the high pressure compressor15is directed into the combustion equipment16where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines17,19before being exhausted through the core exhaust nozzle20to provide some propulsive thrust. The high pressure turbine17drives the high pressure compressor15by a suitable interconnecting shaft27. The fan23generally provides the majority of the propulsive thrust. The epicyclic gearbox30is a reduction gearbox.

The gas turbine engine10further comprises a heat exchanger assembly101disposed annularly around the core11and configured to transfer heat generated by the core11into the bypass air flow B. In particular, the heat exchanger assembly101is disposed radially outward of the core11and radially inward of a portion of the bypass duct22. As shown, the heat exchanger assembly101comprises an inlet region opening111and an outlet region opening121. The location of the heat exchanger assembly101inFIG.2is purely illustrative, and it will be appreciated that the heat exchanger assembly101may be disposed around the core11at any suitable location within the gas turbine engine10.

An exemplary arrangement for a geared fan gas turbine engine10is shown inFIG.3. The low pressure turbine19(seeFIG.2) drives the shaft26, which is coupled to a sun wheel, or sun gear,28of the epicyclic gear arrangement30. Radially outwardly of the sun gear28and intermeshing therewith is a plurality of planet gears32that are coupled together by a planet carrier34. The planet carrier34constrains the planet gears32to precess around the sun gear28in synchronicity whilst enabling each planet gear32to rotate about its own axis. The planet carrier34is coupled via linkages36to the fan23in order to drive its rotation about the engine axis9. Radially outwardly of the planet gears32and intermeshing therewith is an annulus or ring gear38that is coupled, via linkages40, to a stationary supporting structure24.

Note that the terms “low pressure turbine” and “low pressure compressor” as used herein may be taken to mean the lowest pressure turbine stages and lowest pressure compressor stages (i.e. not including the fan23) respectively and/or the turbine and compressor stages that are connected together by the interconnecting shaft26with the lowest rotational speed in the engine (i.e. not including the gearbox output shaft that drives the fan23). In some literature, the “low pressure turbine” and “low pressure compressor” referred to herein may alternatively be known as the “intermediate pressure turbine” and “intermediate pressure compressor”. Where such alternative nomenclature is used, the fan23may be referred to as a first, or lowest pressure, compression stage.

The epicyclic gearbox30is shown by way of example in greater detail inFIG.4. Each of the sun gear28, planet gears32and ring gear38comprise teeth about their periphery to intermesh with the other gears. However, for clarity only exemplary portions of the teeth are illustrated inFIG.4. There are four planet gears32illustrated, although it will be apparent to the skilled reader that more or fewer planet gears32may be provided within the scope of the claimed invention. Practical applications of a planetary epicyclic gearbox30generally comprise at least three planet gears32.

The epicyclic gearbox30illustrated by way of example inFIGS.3and4is of the planetary type, in that the planet carrier34is coupled to an output shaft via linkages36, with the ring gear38fixed. However, any other suitable type of epicyclic gearbox30may be used. By way of further example, the epicyclic gearbox30may be a star arrangement, in which the planet carrier34is held fixed, with the ring (or annulus) gear38allowed to rotate. In such an arrangement the fan23is driven by the ring gear38. By way of further alternative example, the gearbox30may be a differential gearbox in which the ring gear38and the planet carrier34are both allowed to rotate.

It will be appreciated that the arrangement shown inFIGS.3and4is by way of example only, and various alternatives are within the scope of the present disclosure. Purely by way of example, any suitable arrangement may be used for locating the gearbox30in the engine10and/or for connecting the gearbox30to the engine10. By way of further example, the connections (such as the linkages36,40in theFIG.3example) between the gearbox30and other parts of the engine10(such as the input shaft26, the output shaft and the fixed structure24) may have any desired degree of stiffness or flexibility. By way of further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts from the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement ofFIG.3. For example, where the gearbox30has a star arrangement (described above), the skilled person would readily understand that the arrangement of output and support linkages and bearing locations would typically be different to that shown by way of example inFIG.3.

Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox styles (for example star or planetary), support structures, input and output shaft arrangement, and bearing locations.

Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

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 inFIG.2has a split flow nozzle18,20meaning that the flow through the bypass duct22has its own nozzle18that is separate to and radially outside the core exhaust nozzle20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct22and the flow through the core11are 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 engine10may not comprise a gearbox30.

FIG.5is a perspective view of a conventional heat exchanger assembly301not in accordance with the present disclosure. The heat exchanger assembly301is shown as being located within an annular housing99, although this need not be the case. Hidden features (i.e. those not observable from the viewpoint ofFIG.5) are shown in dashed lines.

The conventional heat exchanger assembly301comprises a heat exchanger duct305and a heat exchanger330. The heat exchanger duct305has an inlet region310and an outlet region320. The heat exchanger330is disposed between the inlet region310and the outlet region320. The heat exchanger duct305further comprises an inlet duct opening311and an outlet duct opening321, both of which are in fluid communication with the bypass duct22. The heat exchanger330comprises an inlet face332, an outlet face334and an interior disposed between the inlet face332and the outlet face334. The inlet face332and the outlet face334are axially offset from each other with respect to the axis9. The centreline6of the heat exchanger duct305(i.e. the axis running along the midpoint of the heat exchanger duct305) is coplanar with the axis9. Accordingly, the heat exchanger duct305is linear and the direction of the centreline6of the heat exchanger duct305does not have a tangential component186(i.e. a component in direction Θ) or a circumferential component.

During operation, the inlet duct310receives a flow of air in a direction indicated by arrow311′ from the bypass duct22via the inlet duct opening311. The flow of air passes along the inlet duct opening311to the inlet face332of the heat exchanger330. The flow of air then passes through the interior of the heat exchanger330, where it is heated by a process fluid, and exits the heat exchanger330via the outlet face334. The flow of air then passes along the outlet region320and is discharged into the bypass duct22via the outlet duct opening321in a direction indicated by arrow321′.

FIG.6shows a perspective view of a first example heat exchanger assembly101in accordance with the present disclosure. The first example heat exchanger assembly101is shown as being located within an annular housing199, although this need not be the case.

The first example heat exchanger assembly101comprises a heat exchanger duct105and a heat exchanger130. The heat exchanger duct105has an inlet region110, an inflection region135and an outlet region120. The heat exchanger130is disposed within the inflection region135. The heat exchanger duct105further comprises an inlet region opening111and an outlet region opening121, both of which are in fluid communication with the bypass duct22. The heat exchanger130comprises an inlet face132, an outlet face134and an interior disposed between the inlet face132and the outlet face134. The inlet face132and the outlet face134are offset from each other in a tangential direction with respect to the axis9. The heat exchanger130is fluidically connected to an external fluid circuit disposed within the core11of the gas turbine engine10.

During operation, the inlet region110receives a flow of air in a direction indicated by arrow111′ from the bypass duct22via the inlet duct opening111. The inlet region opening111is oriented with respect to a ducted fan of the gas turbine engine10so as to optimise air entry from the bypass duct22of the gas turbine engine10into the inlet region110of the heat exchanger assembly101. The flow of air passes along the inlet region110, passes along a portion of the inflection region135upstream of the heat exchanger130, and passes to the inlet face132of the heat exchanger130. Process fluid passing through the external fluid circuit is heated and conveyed to the heat exchanger130. The process fluid may be lubricating oil for a bearing chamber or a gearbox of the core11, or a coolant for cooling an electrical generator, a motor, a power electronics device or another heat generating device within the core11of the gas turbine engine10. The flow of air within the heat exchanger duct105passes through the interior of the heat exchanger130and is heated by the process fluid, thereby cooling the process fluid, which returns to the remainder of the external fluid circuit. Accordingly, the heat exchanger130is configured to transfer heat generated by the core11of the gas turbine engine10into the flow of air as it passes through the inflection region135. The flow of air then passes along a portion of the inflection region135downstream of the heat exchanger130, passes along the outlet region120and is discharged into the bypass duct22via the outlet duct opening121in the direction indicated by arrow121′. Accordingly, the inflection region135is configured to transfer the flow of air from the inlet region110to the outlet region120and the heat exchanger duct105is configured to convey the flow of air from the inlet region opening111to the outlet region opening121via the heat exchanger130.

FIG.7is a cross-sectional schematic plan view of the first example heat exchanger101assembly shown inFIG.6through a plane concentric to the axes9,192.

As shown, the heat exchanger duct105changes direction between the inlet region110and the inflection region135, and between the inflection region135and the outlet region120. Accordingly, the direction of the centreline106of the heat exchanger duct105(i.e. the axis running along the midpoint of the heat exchanger duct105) has a tangential component (i.e. a component in the direction Θ186) with respect to the axis9, and, thus, a circumferential component. In addition, the centreline106of the heat exchanger duct105is substantially curved within or along the inflection region135and has a curvature which changes sign within the inflection region135. Accordingly, the centreline106of the heat exchanger duct105inflects within the inflection region135. The heat exchanger duct105is therefore substantially serpentine.

The centreline106of the heat exchanger duct105is disposed on a single concentric plane extending around the axis9. Accordingly, the direction of the centreline106of the heat exchanger duct105does not have a radial component with respect to the axis9(i.e. a component in the direction r184). However, in alternative arrangements this need not necessarily be the case.

The inlet region opening111is disposed at a first circumferential position about the axis9and the outlet region opening121is disposed at a second circumferential position121about the axis9. The first circumferential position is different to the second circumferential position. In the arrangement shown inFIG.7, the inlet region110and the outlet region120are circumferentially offset with respect to the axis9(i.e. offset in a tangential direction) such that the inlet region110and the outlet region120do not circumferentially overlap with respect to the axis9. However, in alternative arrangements the inlet region110and the outlet region120are circumferentially offset while also circumferentially overlapping.

The centreline106of the heat exchanger duct105at the inlet region110and the centreline106of the heat exchanger duct105at the outlet region120are each substantially coplanar with the axis9and substantially mutually parallel.

The cross-sectional area of the heat exchanger duct105defined on a plane perpendicular to the centreline106at the inflection region135is greater than the cross-sectional area of the heat exchanger duct105defined on a plane perpendicular to the centreline106of the heat exchanger duct105at the inlet region110and the cross-sectional area of the heat exchanger duct105defined on a plane perpendicular to the centreline106of the heat exchanger duct105at the outlet region120.

The inlet face132and the outlet face134of the heat exchanger are offset with respect to each other so as to define the interior portion of the heat exchanger130. The inlet face132and the outlet face134are offset in a tangential direction with respect to the axis9, and, thus, are circumferentially offset. The inlet face132and the outlet face134are substantially coplanar with the axis9. Accordingly, the heat exchanger130is aligned with the axis9.

The inflection region135upstream of the heat exchanger130is defined by a first convex surface116and a first concave surface118. The inflection region135downstream of the heat exchanger130is defined by a second convex surface126and a second concave surface128. The first convex surface116and the first concave surface118are offset in a tangential direction with respect to the axis9and, thus, are circumferentially offset. Likewise, the second convex surface126and the second concave surface128are offset in a tangential direction with respect to the axis9, and, thus, are circumferentially offset.

The geometry of the heat exchanger duct105, including the inlet region110, the inflection region135, the outlet region135, the first and second convex wall surfaces116,126and the first and second concave wall surface118,128, ensures that a velocity profile of the flow of air conveyed by the inlet region110is diffused (i.e. linearised) prior to the flow of air passing through the interior portion of the heat exchanger130. A linearised velocity profile of the flow of air through the interior portion of the heat exchanger130provides more uniform and more optimised convective cooling of the flow of process fluid within the heat exchanger130.

By providing a heat exchanger assembly101having a heat exchanger duct105with an inlet region110, an outlet region120and an inflection region135, in which the heat exchanger duct105has centreline106with a tangential component and in which a heat exchanger assembly101is disposed within the inflection region135, a relatively large heat exchanger130(i.e. a heat exchanger having a relatively large cooling capacity) can be provided without the heat exchanger duct105having to occupy a large circumferential or radial footprint within the gas turbine engine10. This reduces cost and weight, provides additional space for other components, improves performance and allows more heat exchangers to be provided in a single gas turbine engine10, for example.

FIG.8shows a perspective view of a second example heat exchanger assembly102. The second example heat exchanger assembly102is substantially identical in form and function to the first example heat exchanger assembly101, with like reference numerals indicating common or similar features. However, the second example heat exchanger assembly102differs from the first example heat exchanger assembly101insofar as the length of the heat exchanger130of the second example heat exchanger assembly102is greater than the length of the heat exchanger130of the first example heat exchanger assembly101. The heat exchanger duct105at the inflection region135has a greater axial dimension to accommodate the elongate heat exchanger130.

The elongate heat exchanger130of the second example heat exchanger assembly102may comprise a plurality of distinct fluid circuits131A,131B, rather than a single fluid circuit. For example, the elongate heat exchanger130may comprise one or more internal partitions for separating the elongate heat exchanger130into a plurality of distinct sections through which distinct flows of process fluid may be provided by respective (i.e. separate) external fluid circuits. This allows a single heat exchanger130to be shared by a plurality of external fluid circuits.

In such examples, the need for additional heat exchanger assemblies to cater for the plurality of flows of process fluid is eliminated, which provides simpler means for cooling the flows of process fluid within the gas turbine engine10. Each heat exchanger assembly102is associated with a pressure drop within the bypass duct22between the inlet region opening111and the outlet region opening121. Accordingly, the configuration ofFIG.8may provide a lower total pressure drop within the bypass duct22than would otherwise be present with the inclusion of a plurality of distinct heat exchanger assemblies.

The configuration of such a heat exchanger assembly102also allows a plurality of flows of process fluid to be cooled within the heat exchanger130without requiring mixing of the plurality of flows prior to being conveyed through the heat exchanger130. Consequently, each of the plurality of flows of process fluid may comprise different types of process fluid which are not suitable for mixing within the heat exchanger130.

FIG.9is a cross-sectional schematic plan view of a third example heat exchanger assembly103through a plane concentric to the axis9in a normal air supply mode. The third example heat exchanger assembly103is substantially identical in form and function to the first example heat exchanger assembly101, with like reference numerals denoting common or similar features.

The third example heat exchanger assembly103differs from the first example heat exchanger assembly101in that it comprises a supplementary air supply opening140positioned downstream of the heat exchanger130at or adjacent to the second convex surface126and an additional air supply opening141positioned upstream of the heat exchanger130at or adjacent to the first concave surface118. The supplementary air supply opening140is fluidically connected to a supply of fluid from the core11of the gas turbine engine10via a supplementary air supply line142. The additional air supply opening141is fluidically connected to a supply of fluid from the core11of the gas turbine engine10via an additional air supply line143. A supplementary air control valve144is disposed along the supplementary air supply line142for modifying a flow rate of the supplementary flow of air. An additional air control valve149is disposed along the additional air supply line143for modifying a flow rate of the additional flow of air.

It will be appreciated that in other examples, the supplementary air supply opening140and/or the additional air supply opening141may be configured to receive their respective flows of air from another source, such as a cabin blower compressor or an auxiliary air supply. In other configurations, the sources of air may be different for each of the supplementary air supply opening140and/or the additional air supply opening141.

As mentioned above, the heat exchanger assembly101is associated with a pressure drop within the bypass duct22between the inlet region opening111and the outlet region opening121. In order for the flow of air to be conveyed through the heat exchanger assembly101, the pressure of air within the bypass duct22upstream of the inlet region opening111must be sufficiently large so as to be able to overcome the pressure drop between the inlet region opening111and the outlet region opening121. The pressure drop is dependent on, among other things, an effective exit area of the flow of air being conveyed by the outlet region120through the outlet region opening121and into the bypass duct22of the gas turbine engine10. The effective exit area may be regarded as an area through which the flow of air is laminar. A smaller effective exit area is associated with an increased pressure drop, whereas a larger effective exit area is associated with a decreased pressure drop.

In the normal air supply mode shown inFIG.9, the supplementary air control valve144is closed and the supplementary flow of air is not provided to the supplementary air supply opening140or to the additional air supply opening141. For illustrative purposes only,FIG.9shows a plurality of streamlines129approximating the flow of air within the inflection region135and the outlet duct120in the normal air supply mode (corresponding streamlines also exist upstream of the heat exchanger130, but are not shown).

As the flow of air passes from the heat exchanger130through the inflection region135, an adverse pressure gradient tends to develop on a region proximal to the second convex surface126. The adverse pressure gradient results in internal flow separation with respect to the second convex surface126, which in turn leads to the development of a region of recirculating flow as shown by streamline129A. The size of the region of recirculating flow129A reduces the effective exit area Ae1and therefore adversely increases the pressure drop between the inlet region opening111and the outlet region opening121.

FIG.10shows the third example heat exchanger assembly103in a supplementary air supply mode. In the supplementary air supply mode, the supplementary air control valve144is at least partially open and the supplementary flow of air is directed into the outlet duct120and onto or near the second convex surface126of the inflection region135.

Due to the positioning of the supplementary air supply opening140, the supplementary air control valve144directs the supplementary flow of air onto or near the second convex surface126. Consequently, the supplementary flow of air has a tendency to attach to the second convex surface126as a result of the Coandă effect. Accordingly, the supplementary flow of air discourages internal flow separation with respect to the second convex surface126, which reduces the size of a region of recirculating flow approximated by streamline129A′. The reduced size of the region of recirculating flow129A′ increases the effective exit area Ae2in the supplementary air supply mode compared to the effective exit area Ae1in the normal air supply mode and therefore decreases the pressure drop between the inlet region opening111and the outlet region opening121in the supplementary air supply mode compared to the normal air supply mode.

The decreased pressure drop in the supplementary air supply mode has the effect of promoting the airflow through the heat exchanger assembly103, and so a mass flow rate of the flow of air from the bypass duct22through the heat exchanger assembly103(and therefore through the heat exchanger130) may be greater in the supplementary air supply mode than in the normal air supply mode, which in turn increases a rate of convective heat transfer between the flow of air and the or each flow of process fluid within the heat exchanger130. In other words, a rate of cooling of the or each flow of process fluid within the heat exchanger130is higher in the supplementary air supply mode than in the normal air supply mode.

Further, the rate of cooling of the or each flow of process fluid within the heat exchanger130in the supplementary air supply mode may be modulated by controlling the mass flow rate of the supplementary flow of air. Increasing the mass flow rate of the supplementary flow of air increases the strength of the Coandă effect adjacent to the second convex surface126, thereby discouraging internal flow separation with respect to the second convex surface126. Accordingly, an increased mass flow rate of the outlet supplementary flow of air is associated with an increased effective exit area Ae2and a reduced pressure drop in the supplementary air supply mode, thereby increasing the rate of cooling of the or each flow of process fluid within the heat exchanger130.

In the example ofFIGS.9and10, the third example heat exchanger assembly102comprises a sensor arrangement145and a controller148in communication with the sensor arrangement145and the supplementary air control valve144. In the illustrated examples, the sensor arrangement145comprises a process fluid temperature sensor146and a bypass duct pressure sensor147. However, it will be appreciated that in other examples, the sensor arrangement145may comprise only a process fluid temperature sensor146or a bypass duct pressure sensor147.

The process fluid temperature sensor146is configured to generate a signal indicative of a temperature of the process fluid within the heat exchanger130or within the at least one external fluid circuit. Preferably, the process fluid temperature sensor146may be configured to generate a signal indicative of a temperature of the process fluid as it exits the heat exchanger130. The bypass duct pressure sensor147is configured to generate a signal indicative of a pressure of air within the bypass duct22of the gas turbine engine10(e.g. a pressure upstream of the inlet duct opening).

The controller148is configured to control a state of the supplementary air control valve144based on the signal generated by the process fluid temperature sensor146and/or the bypass duct pressure sensor147in accordance with a method900, and thereby control the mass flow rate of the supplementary flow of air based on the signal generated by the process fluid temperature sensor146and/or the signal generated by the bypass duct pressure sensor147.

FIG.11is a flowchart of a such a first method900of operating the third example heat exchanger assembly103.

In a first step910of the first method900, a parameter of the gas turbine engine10is determined. In a second step920of the first method900, a parameter of the supplementary flow of air into the heat exchanger duct105is varied based on the parameter of the gas turbine engine10. The parameter of the gas turbine engine10may be a pressure of the flow of air within the bypass duct of the gas turbine engine10and/or the temperature of a process fluid exiting the heat exchanger130. The parameter of the supplementary flow of air may be a flow rate of the supplementary flow of air into the heat exchanger duct105.

The pressure of the flow of air within the bypass duct22may be determined by the controller148based on a signal received from the bypass duct pressure sensor147indicative of a pressure of air within the bypass duct22of the gas turbine engine10upstream of the inlet duct opening. The temperature of the process fluid exiting the heat exchanger130may be determined by the controller148based on a signal received from the process fluid temperature sensor146indicative of a temperature of the process fluid within the heat exchanger130or within the at least one external fluid circuit.

By way of a first example of the method900, the controller148may be configured to open the supplementary air control valve144in response to a determination that the temperature of the process fluid is greater than an upper temperature threshold, and thereby place the heat exchanger assembly103in the supplementary air supply mode. As mentioned above, this has the effect of promoting the airflow through the heat exchanger assembly103, which increases a rate of convective heat transfer between the flow of air and the or each flow of process fluid within the heat exchanger130.

Conversely, the controller148may be configured to close the supplementary air control valve144in response to a determination that the temperature of the process fluid is lower than a lower temperature threshold, and thereby place the heat exchanger assembly103in the normal air supply mode. This has the effect of reducing the airflow through the heat exchanger assembly103, which reduces a rate of convective heat transfer between the flow of air and the or each flow of process fluid within the heat exchanger130.

In this manner, the controller is configured to maintain a temperature of the process fluid within the heat exchanger130or within the or each external fluid circuit between the lower temperature threshold and the upper temperature threshold. The lower temperature threshold and/or the upper temperature threshold may be chosen based on a cooling requirement of the component or components with which the external fluid circuit or circuits are associated.

By way of a second example of the method900, the controller148may be configured to open the supplementary air control valve144in response to a determination that the pressure of air within the bypass duct22is lower than a lower pressure threshold and thereby place the heat exchanger assembly103in the supplementary air supply mode. This has the effect of increasing the pressure of air within the bypass duct22and promoting the airflow through the heat exchanger assembly103, which increases a rate of convective heat transfer between the flow of air and the or each flow of process fluid within the heat exchanger130.

Conversely, the controller148may be configured to close the supplementary air control valve144in response to a determination that the pressure of air within the bypass duct22is greater than an upper pressure threshold and thereby place the heat exchanger assembly103in the normal air supply mode. This has the effect of reducing the pressure of air within the bypass duct22and reducing the airflow through the heat exchanger assembly103, which reduces a rate of convective heat transfer between the flow of air and the or each flow of process fluid within the heat exchanger130.

The upper pressure threshold and/or the lower pressure threshold may be chosen based on a driving pressure requirement of the heat exchanger assembly103. The driving pressure requirement may be related to the pressure drop between the inlet region opening111and the outlet region opening121.

For example, it may be that when the gas turbine engine10is stationary (that is, when the gas turbine engine10has no forward speed) and/or when a ducted fan of the gas turbine engine10only causes a small pressure difference between air in the bypass duct22and ambient air, the pressure of air within the bypass duct22is not sufficiently high so as to adequately drive the flow of air through the heat exchanger assembly103due to the pressure drop (that is, the driving pressure requirement) associated with the heat exchanger assembly103. Placing the heat exchanger assembly103in the supplementary air supply mode reduces the pressure drop associated with the heat exchanger assembly103, which promotes airflow through the heat exchanger assembly103(and thereby promotes cooling of the or each flow of process fluid within the heat exchanger130) even when the pressure of air within the bypass duct22is low. On the other hand, when the pressure of air within the bypass duct22is high, the heat exchanger assembly103may be placed in the normal air supply mode since the pressure of air within the bypass duct22is sufficiently high so as to adequately drive the flow of air through the heat exchanger assembly103without requiring the supplementary flow or air to be provided through the supplementary air supply line142.

The additional air supply opening141may instead be utilised to supply the additional flow of air to the heat exchanger duct105in the supplementary air supply mode instead of or in addition to the supplementary air supply opening140.

FIG.12is a flowchart of a second method1000of operating the third example heat exchanger assembly103.

In a first step1010of the method1000, a parameter of the gas turbine engine10in which the heat exchanger assembly103is incorporated is determined. In a second step1020of the method1000, a parameter of the additional flow of air into the heat exchanger duct105is varied based on the parameter of the gas turbine engine10. The parameter of the gas turbine engine10may be a pressure of the flow of air within a bypass duct22of the gas turbine engine10and/or a temperature of a process fluid exiting the heat exchanger130. The parameter of the additional flow of air may be a flow rate of the additional flow of air into the heat exchanger duct105and/or the temperature of the additional flow of air into the heat exchanger duct105.

The flow rate of the additional flow of air into the heat exchanger duct105may be varied, for example, by the controller148opening or closing the additional air control valve149. The additional air supply opening141is configured to supply an additional flow of air into the heat exchanger duct105and onto or adjacent the second concave surface118of the inflection region135.

The temperature of the additional flow of air into the heat exchanger duct105may be varied, for example, by the controller148controlling the composition of the additional flow of air. For example, in order to provide a relatively hot additional flow of air into the heat exchanger duct105, the controller148could control the core11to supply the further supplementary air supply line143with an additional flow of air originating from a relatively hot part of the core11(e.g. a compressor). Further, in order to provide a relatively cool additional flow of air into the heat exchanger duct105, the controller148could control the core11to supply the further supplementary air supply line143with an additional flow of air originating from a relatively cool part of the core11.

By way of example of the second method1010, the controller148may be configured to supply the additional air supply opening141with a relatively hot supply of air (or a relatively large flow rate of the relatively hot supply of air) when the temperature of the process fluid exiting the heat exchanger130is lower than a lower temperature threshold, thereby reducing the relative temperature difference between the process fluid in the heat exchanger130and the flow of air passing through the heat exchanger duct105and reducing the amount of cooling of the process fluid within the heat exchanger130. Conversely, the controller148may be configured to supply the additional air supply opening141with a relatively cool supply of air (or a relatively small flow rate of the relatively hot supply of air) when the temperature of the process fluid exiting the heat exchanger130is above an upper temperature threshold, thereby increasing the relative temperature difference between the process fluid in the heat exchanger130and the flow of air passing through the heat exchanger duct105and increasing the amount of cooling of the process fluid within the heat exchanger130.

In alternative arrangements, the heat exchanger assembly may comprise only one of the supplementary air supply opening140and the additional air supply opening141.

FIG.13shows a perspective view of a first example heat exchanger arrangement.201comprising a plurality of heat exchanger assemblies, wherein each heat exchanger assembly is in accordance with any of the examples described above, with like reference numerals indicating common or similar features. In the example ofFIG.12, the plurality of heat exchanger assemblies includes a first heat exchanger assembly101and a second heat exchanger assembly101′. It will, however, be appreciated that the heat exchanger arrangement201may comprise additional heat exchanger assemblies in accordance with example heat exchanger assemblies described above.

The first heat exchanger assembly101is circumferentially offset from the second heat exchanger assembly101′ with respect to the axis9. Accordingly, the first heat exchanger130of the first heat exchanger assembly101is circumferentially offset with respect to the second heat exchanger130′ of the second heat exchanger assembly101′ with respect to the axis9. The heat exchanger duct105of the first heat exchanger assembly101and the heat exchanger duct105of the second heat exchanger assembly102are configured to convey a respective flow of air from respective inlet region openings111to respective outlet region openings121.

The first heat exchanger assembly101and the second heat exchanger assembly101′ are separated by a septum wedge150. The septum wedge150partially defines the first heat exchanger duct105and the second heat exchanger duct105′. In other words, the septum wedge150partially defines the internal geometry of the first heat exchanger duct105and the internal geometry of the second heat exchanger duct105′. The septum wedge150may be integrally formed by a casing of the gas turbine engine10, such as an inner bypass casing of the gas turbine engine10. The provision of the septum wedge150between each heat assembly allows the internal geometry of each heat exchanger duct to be defined without significantly increasing a part count (and therefore a complexity) of the heat exchanger arrangement201.

FIG.14shows a perspective view of a second example heat exchanger arrangement202. The second example heat exchanger arrangement202is generally similar to the first example heat exchanger arrangement201, with like reference numerals indicating common or similar features. However, the inflection region of the first heat exchanger assembly101is axially offset from the inflection region of the second heat exchanger assembly101′ with respect to the axis9. Further, the heat exchanger130of the first heat exchanger assembly101and the heat exchanger130′ of the second heat exchanger assembly101′ are axially offset with respect to the axis9of the core11of the gas turbine engine10(i.e. offset with respect to each other along the axial direction182). In addition, the heat exchanger130of the first heat exchanger assembly101and the heat exchanger130′ of the second heat exchanger assembly101′ are circumferentially aligned with respect to the axis9. The heat exchanger duct105of the first heat exchanger assembly101and the heat exchanger duct105of the second heat exchanger assembly102are adapted to convey a flow of air from respective inlet region openings111to respective outlet region openings121.

The first heat exchanger duct105abuts the second heat exchanger duct105′ to provide a compact heat exchanger arrangement202. However, in other examples, the first heat exchanger duct105may not abut the second heat exchanger duct105. In such examples, the first heat exchanger duct105and the second heat exchanger duct105may be defined by a septum wedge150, as described above.

The example heat exchanger arrangements described above allow a plurality of heat exchanger assemblies to be easily disposed around the core11of the gas turbine engine10, without requiring an increase in the height of each heat exchanger assembly.

It will be understood that the invention is not limited to the examples above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein. The scope of protection is defined in the appended claims.