Patent Description:
Gas turbine engines (used in propulsion and power applications and broadly inclusive of turbojets, turboprops, turbofans, turboshafts, industrial gas turbines, and the like) are being considered for use with cryogenic fuel, in particular hydrogen fuel. For example, <CIT> identifies various cryogenic fuels including hydrogen.

<CIT> discloses a prior art method of evaporating a low temperature liquid medium.

Cryogenic fuels must be heated, but in a controlled manner. The natural choice is to heat with the gaspath, but temperatures/temperature differentials are high and can lead to uncontrolled heating of the fuel. The use of an intermediary fluid, intervening between the gaspath and the fuel may limit the effects of the differences.

From one aspect, there is provided a method for operating a turbine engine as recited in claim <NUM>.

<FIG> shows a gas turbine engine <NUM>. As is discussed below, the engine is illustrated as a schematic modification of a baseline existing engine. It may be a baseline engine designed for cryogenic hydrogen fuel or a baseline engine designed for conventional liquid jet fuel further modified for use with cryogenic hydrogen fuel. <FIG> schematically shows the example gas turbine engine <NUM> as a turbofan engine having a centerline or central longitudinal axis <NUM> and extending from an upstream end at an inlet <NUM> to a downstream end at an outlet <NUM>. The example engine schematically includes a core flowpath or gaspath <NUM> passing a core flow <NUM> and a bypass flowpath <NUM> passing a bypass flow <NUM>. The core flow and bypass flow are initially formed by respective portions of a combined inlet airflow <NUM> divided at a splitter <NUM>. Thus, the example core flow starts out as air and downstream of the combustor comprises combustion products as combustion gas.

A core case (inner diameter (ID) case) or other structure <NUM> divides the core flowpath from the bypass flowpath. The bypass flowpath is, in turn, surrounded by an outer case (outer diameter (OD) case) <NUM> which, depending upon implementation, may be a fan case. A bypass duct <NUM> is configured radially between the ID case and OD case. From upstream to downstream, the engine includes a fan section <NUM> having one or more fan blade stages, a compressor <NUM> having one or more sections each having one or more blade stages, a combustor <NUM> (e.g., annular, can type, or reverse flow), and a turbine <NUM> again having one or more sections each having one or more blade stages. For example, many so called two-spool engines have two compressor sections and two turbine sections with each turbine section driving a respective associated compressor section and a lower pressure downstream turbine section also driving the fan (optionally via a gear reduction <NUM>). Yet other arrangements are possible.

Various illustrated and non-illustrated features of the engine may be otherwise conventional including basic control hardware, programming, and use and manufacture methods. The control hardware shown and discussed may be implemented merely via additional programming of and connections to baseline control hardware at engine level or aircraft/vehicle level (e.g., the full authority digital engine control (FADEC)) or may be further implemented with additional control components such as additional microcontrollers or the like.

<FIG> is a schematic view of a fuel system <NUM> including a fuel preheat heat exchanger <NUM>. One or more fuel flowpaths <NUM> start at one or more fuel vessels <NUM>. An example of fuel vessels are tanks, such as insulated metallic or composite overwrapped pressure vessels (COPV). These vessels may contain fuel at cryogenic conditions of ambient or near ambient pressure and -<NUM> to -<NUM> temperature (more narrowly -<NUM> to - <NUM>). These storage conditions may be maintained by insulation and pressure relief valve(s) allowing for off-gassing.

The fuel flowpath(s) <NUM> extend to outlets <NUM> (fuel discharge outlets) in the engine combustor(s) (e.g., to outlets <NUM> in one or more fuel nozzles <NUM> discharging fuel streams <NUM>). As such, the fuel flowpaths may have multiple branches at least to the nozzles. But, however, as discussed below, there are or may be multiple further branches upstream which may subsequently converge fully or partially.

For efficient combustion, the cryogenic fuel should be preheated to a supercritical state. For example, it may be heated from a temperature of -<NUM> to -<NUM> (more broadly - <NUM> to -<NUM>) to a temperature of -<NUM> to -<NUM> (more broadly -<NUM> to -<NUM>) while maintaining a pressure of 1700kPa to 1750kPa pressure (more broadly 1600kPa to 1800kPa).

To preheat the fuel, the engine includes the heat exchanger <NUM> for transferring heat from the gaspath <NUM> to the fuel flowpath(s) <NUM> (from combustion gas <NUM> flowing along/through the gaspath to fuel <NUM> flowing along/through the fuel flowpath(s)). The example heat exchanger <NUM> is positioned axially/streamwise downstream/aft of the turbine section(s) and upstream/forward of an exhaust nozzle. As is discussed further below, the heat exchanger <NUM> utilizes an intermediate/intermediary fluid (heat transfer fluid) intervening between the combustion gas and the fuel.

In a multi-engine aircraft, there may be separate fuel flowpath(s) <NUM> for each engine. For example, as discussed below, there may be multiple fuel flowpaths for a given engine, allowing shut-off of flow along each flowpath individually, such as in case of a leak.

<FIG> schematically shows a control system or controller <NUM>. Various control and/or power and/or data lines are shown as broken lines extending to controlled components, sensors, and the like. These lines may represent digital communication (wires, fiber, or radio frequency (RF)) or electrical/power (e.g., wires). Although a single box is schematically shown, multiple components in distributed locations may be used.

further shows a shutoff valve <NUM> and a low pressure pump <NUM> in a fuel flowpath <NUM> upstream of the heat exchanger <NUM>. Example valves <NUM> are solenoid valves. Example pumps <NUM> are centrifugal pumps. further shows a high pressure pump <NUM> and pressure regulating valve <NUM> in that fuel flowpath <NUM> downstream of the heat exchanger <NUM>. Example valves <NUM> are solenoid valves. Example pumps <NUM> are centrifugal pumps. In an example of a wing pylon-mounted engine, the vessel(s) <NUM>, valve(s) <NUM>, and pump(s) <NUM> may be located inside the airframe/fuselage and the pump(s) <NUM> and valve(s) <NUM> may be in/on the engine.

<FIG> shows a flow <NUM> of the heat transfer fluid entering a chamber <NUM> of the heat exchanger. The fuel flowpath <NUM> passes in branches through tubes (discussed below) in the heat exchanger chamber <NUM>. Discussed below, an example heat transfer fluid is nitrogen (N<NUM>). It may be commercially pure nitrogen or at least substantially more pure and oxygen-free than air (e.g., at least <NUM> percent nitrogen by weight or at least <NUM>% or at least <NUM>% with an example at most <NUM>% or at most <NUM>% or at most <NUM>% or at most <NUM>% oxygen or combustible gases by weight). The nitrogen in the heat transfer loop may be maintained in a pressurized state or, at higher pressures specifically a supercritical state. For example, at above about 5000kPa and temperature over about <NUM> it may be supercritical. Thus example nitrogen pressures maintained in the heat transfer fluid loop may be in the range of 3000kPa to <NUM>,000kPa including supercritical examples at the higher end.

The heat transfer fluid is heated by the combustion gas in the gaspath <NUM> and, in turn, heats the fuel in the tubes. <FIG> shows a fuel temperature sensor <NUM> (e.g., thermocouple) along the fuel flowpath <NUM> positioned to measure the temperature of fuel exiting or downstream of the heat exchanger <NUM>. This may essentially represent the temperature of fuel entering the fuel nozzles <NUM> for combustion.

The heat transfer fluid passes as a recirculating flow along a heat transfer fluid flowpath <NUM> (<FIG>) shown comprising or consisting of a loop <NUM>. The loop passes through the heat exchanger <NUM>. More particularly, the loop passes through the chamber <NUM> of the heat exchanger <NUM>. A plurality of branches of the fuel flowpath <NUM> pass within the chamber. For example, the plurality of tubes may pass within the chamber carrying the fuel. The heat transfer fluid in the chamber <NUM> may surround the tubes within the chamber. Thus, in this example, the fuel flowpath <NUM> branches out in a fuel inlet plenum <NUM> (<FIG>) of a fuel inlet manifold <NUM> of the heat exchanger <NUM> and the branches converge in a fuel outlet plenum <NUM> of a fuel outlet manifold <NUM>. The example fuel inlet manifold and fuel outlet manifold are part of a single combined manifold structure <NUM> at one axial end <NUM> of the heat exchanger. A return manifold <NUM> (<FIG>) and its plenum <NUM> are located at the opposite axial end <NUM>. A plurality of first tubes (or tube legs) <NUM> (<FIG>) thus convey fuel from the fuel inlet manifold/plenum to the return manifold and a plurality of second tubes (or tube legs) <NUM> thus convey fuel from the return manifold back to the fuel outlet manifold/plenum.

In this example, the combined manifold <NUM> and return manifold <NUM> are each annular structures (or may be annular segments (segments of an annulus)). Such annular segments may be used in the case of a fully segmented heat exchanger (e.g., where the chamber <NUM> is a segment bounded by circumferential end walls so that both the fuel flow and heat exchange fluid flow may be independently controlled for each segment). Or, such annular segments may be used in a situation of a full annulus chamber <NUM> (e.g., where fuel flow is independently controllable over a sector or segment while heat transfer fluid flow is not).

The example heat exchanger has tubes at only a single radial position (a single radial stage) over a majority of their lengths with first tubes <NUM> circumferentially alternating with second tubes <NUM>. However, other configurations including multiple radial stages are possible. Additionally, whereas the return manifold <NUM> may contain a plenum <NUM> shared by all its associated tubes <NUM> and <NUM>, alternative manifold structures may have single passageways for each adjacent pair of a first tube <NUM> and a second tube <NUM>. Additionally, to take the place of a return manifold and separate first and second tubes, tubes may be bent to have first and second legs and a turn, with the turn replacing the return manifold <NUM>.

In the example combined manifold <NUM>, to avoid fuel inlet plenum <NUM> and fuel outlet plenum <NUM> interfering, one may be radially shifted relative to the other. For example, the illustrated fuel inlet plenum <NUM> is shifted radially inward and the fuel outlet plenum <NUM> is shifted radially outward in <FIG>. Thus, end portions of the tubes <NUM>, <NUM> mating with the combined manifold are bent to provide the shift/offset. Also, in the example, the heat exchanger or sector or segment has a single fuel inlet conduit <NUM> extending to the fuel inlet plenum <NUM> (e.g., to a fitting <NUM> on the fuel inlet manifold forming a fuel inlet of the heat exchanger) and a single fuel outlet conduit <NUM> extending from the fuel outlet plenum (e.g., from a fitting <NUM> on the fuel outlet manifold forming a fuel outlet of the heat exchanger).

As noted above, the heat transfer fluid flowpath <NUM> (<FIG>) comprises a recirculating loop <NUM>. Along the loop, are a pump <NUM> for pumping the heat transfer fluid and a pressure sensor <NUM> (discussed further below) for measuring pressure in the loop. An example pump <NUM> is a gear pump. An example pressure sensor <NUM> is pressure transducer such as a remote electrical pressure transducer. Example heat transfer fluid pressure in the loop <NUM> is about 4000kPa, more broadly 3000kPa to 5000kPa. Example heat transfer fluid temperature entering the heat exchanger is about <NUM>, more broadly <NUM> to <NUM>. Example heat transfer fluid temperature leaving the heat exchanger is about <NUM>, more broadly <NUM> to <NUM>. An example temperature increase of the heat transfer fluid across the heat exchanger is about <NUM>, more broadly <NUM> to <NUM>. Within the heat exchanger, the heat transfer fluid has greater thermal gradients between portions of the heat transfer fluid adjacent the inner wall and portions adjacent the tubes.

The engine further includes a heat transfer fluid reservoir <NUM> coupled to the loop <NUM> via a regulating valve <NUM>. Depending upon implementation, if present, the reservoir <NUM> may act as a supply reservoir to add heat transfer fluid to the loop and/or an accumulator or storage reservoir to receive heat transfer fluid from the loop. An example reservoir <NUM> is a piston-type reservoir or a bladder-type reservoir. In such reservoirs, a pressurant gas (e.g., nitrogen) is isolated from the pressurized fluid (e.g., the pressurant gas is on the opposite side of the piston from the pressurized fluid or is inside the bladder or on an opposite side of the bladder or membrane). Additional means may be provided for maintaining pressure of the pressurant gas such as a pump or a compressor (not shown). An example, regulating valve <NUM> is a spring-biased pressure regulating valve or a controlled throttling or modulated valve.

<FIG> show the example accumulators as having a housing 226A, 226B with a port <NUM> and a volume <NUM> in communication with the heat transfer fluid flowpath for containing heat transfer fluid. The piston of the accumulator 210A has a range of motion between a maximally extended condition bottomed against a shoulder <NUM> in a range of compressed conditions. A maximally compressed condition may be determined by another shoulder (not shown) or by bottoming against an opposite end of the housing. Depending on implementation, as a practical matter, the piston <NUM> may never reach the maximally compressed condition due to pressure of the gas <NUM>.

Similarly, the bladder-type accumulator 210B has a maximally expanded condition of the bladder <NUM> in which it presses against and closes the poppet valve <NUM> (which may serve in lieu of the valve <NUM>). Again, the bladder may compress through a range of conditions to a maximally compressed condition which potentially may never be reached due to the properties of the pressurant gas <NUM>.

In the example, the loop <NUM> proceeds in a downstream direction: from the heat transfer fluid pump <NUM> to a heat transfer fluid inlet <NUM> of the heat exchanger <NUM> and its chamber <NUM> (e.g., at the fitting <NUM>); through the heat exchanger chamber to a heat transfer fluid outlet <NUM> (e.g., at the fitting <NUM>) of the heat exchanger and its chamber; through or past the heat transfer fluid pressure sensor <NUM>; and back to the heat transfer fluid pump <NUM>. Outside of the heat exchanger and pump, the heat transfer fluid flowpath <NUM> may be generally defined and bounded by appropriate conduit (e.g., metallic piping) as may be conventional or yet-developed.

Contrasted with direct heat exchange from combustion gas to fuel, the heat transfer fluid may serve as a thermal buffer mitigating temperature spikes. As is discussed below, via control over flow in the heat transfer loop <NUM>, the heat imparted to the fuel may be controlled to control fuel temperature.

The heat transfer fluid in the chamber <NUM> may also help contain the fuel in the event of a fuel leak. Depending on the nature of the heat transfer fluid, the heat transfer fluid may serve to inert leakage (e.g., if an actual inert gas such as argon or a relatively inert gas such as nitrogen or carbon dioxide (inert in that it does not react with the fuel)). Where the heat transfer fluid pressure is higher than that of the fuel, any breach of a tube, connection, etc. (e.g., due to thermal stresses) within the heat exchanger will vent heat transfer fluid into the fuel flowpath. This may be detected via a drop in pressure in the loop <NUM> measured by the pressure sensor <NUM>. For example, the controller <NUM> (e.g., FADEC or other controller) may continuously monitor pressure from the sensor <NUM>. Upon detection of a pressure drop (e.g., a threshold decrease over a threshold time), the controller may stop fuel flow through the associated heat exchanger chamber by shutting down the pumps <NUM> and <NUM> and closing the valves <NUM> and <NUM> to shut off fuel flow (e.g., through just a sector or as part of shutting down the entire engine).

Contrasted with a series loop (an intermediate fluid loop absorbing/receiving heat from the gaspath at a first heat exchanger and passing to a remote second heat exchanger to, in turn, reject heat to the fuel flowpath), the system <NUM> may save mechanical complexity and cost associated with having separate heat exchangers between the combustion gas and the heat transfer fluid on the one hand and between the heat transfer fluid and the fuel on the other hand.

In the context of an example generally annular gaspath having an inner wall/boundary (e.g., an outer wall of a centerbody <NUM> (<FIG>) and an outer wall/boundary, the example heat exchanger <NUM> has an inner wall <NUM> (<FIG>) locally forming the outer boundary of the gaspath. An alternative such gaspath may be simply non-annular circular in transverse section (e.g., lacking a centerbody). The heat exchanger also has an outer wall <NUM>. The chamber <NUM> is thus configured radially between the inner wall and the outer wall. In the example engine, the chamber <NUM> may be full annulus or may be a segment of an annulus. For example, there may be a circumferentially-arrayed plurality of heat exchanger segments fluidically in parallel along the fuel flowpath or on separate branches of the fuel flowpath. Such segmentation may allow for ease of assembly and manufacture and for independent operation. For example, one aspect of independent operation allows shut-off of fuel through an individual segment responsive to a leak or other problem with that segment. An example number of segments is two (e.g., each <NUM>° or slightly less) to twelve (e.g., each <NUM>° or slightly less).

As noted above a sectorized heat exchanger may offer similar construction/assembly benefits to a segmented one where separate chambers are formed in separate segments. The example sectorized heat exchanger has a full annulus chamber <NUM>. The outer wall <NUM> may be single-piece or segmented such as by bolting at axially-extending, radially-protruding flanges (not shown). Similarly, the inner wall <NUM> may be single-piece or segmented such as by bolting at axially-extending, radially-protruding flanges (not shown). Separate groups of tubes <NUM>, <NUM> and their associated manifolds and plenums may occupy separate sectors of the full annulus chamber. An example number of such groups and sectors is two to twelve as with the segments noted above.

Depending upon implementation, the heat exchanger may form a principal structural component of the engine or may be ancillary. For example, the example heat exchanger is shown mounted between a turbine exhaust case (TEC) <NUM> (<FIG>) and a nozzle <NUM>. If implemented as a structural heat exchanger, the heat exchanger <NUM> provides the structural mechanical mounting coupling the nozzle to the TEC to maintain their relative position. Alternatively, there may be separate structure (e.g., axial struts) with the heat exchanger mounted to such structure and not adding substantial strength to the connection between TEC and nozzle. Additionally, there may be radial struts supporting the heat exchanger <NUM> relative to the engine centerbody along the inner boundary of the gaspath. In yet further variations, radial struts may extend through the chamber <NUM> to transfer loads and/or prevent ballooning of the chamber under pressure. To accommodate such radial struts, there may be non-uniform spacing between some adjacent tubes.

In the illustrated example, the heat exchanger inner wall <NUM> (<FIG>), forward/upstream end wall <NUM>, and aft/downstream end wall <NUM> (<FIG>) are part of a single structural metallic piece or assembly formed of a robust high temperature alloy (e.g., a nickel-based superalloy) such as IN-<NUM>. At least along the gaspath-facing inner diameter (ID) surface of the inner wall <NUM>, the alloy may be a substrate coated with a thermal and/or environmental barrier coating (TBC/EBC) such as a sprayed or vapor-deposited ceramic (e.g., a yttria-stabilized zirconia (YSZ) and/or gadolinia-stabilized zirconia (GSZ or GZO)) optionally with an underlying bondcoat.

The metallic piece may be formed by casting, additive manufacturing, or fabrication. The forward and aft walls <NUM> and <NUM> may serve as respective mounting flanges mounted to adjacent flanges <NUM> and <NUM> of the TEC and nozzle, respectively (e.g., via fasteners such as bolts <NUM>). In the illustrated example, the heat transfer fluid flow <NUM> and flowpath enter and exit the chamber at ports in the outer wall. In alternative implementations, the heat transfer fluid may pass through the flanges (e.g., via appropriate plumbing fittings such as in <FIG>).

As noted above, fuel temperature may be controlled (e.g., by controller <NUM> of <FIG>) via controlling the flow rate of heat transfer fluid. In one group of examples, the heat transfer fluid will tend to lose heat in regions of the heat transfer fluid flowpath <NUM> (and its loop <NUM>) away from the heat exchanger. In some implementations, this may merely be via exposure of simple conduit/piping to external or other internal engine conditions. In other implementations, there may be specific heat transfer means such as an additional heat exchanger (not shown) discharging heat from the heat transfer fluid to air (e.g., bypass air).

The flow rate of heat transfer fluid will inversely correlate with the heat transferred to the fuel and thus to the fuel outlet temperature measured by fuel temperature sensor <NUM>. At no or low heat transfer fluid flow rate, there will be a substantial temperature gradient along the heat transfer fluid flowpath between the heat exchanger <NUM> and remote regions of the heat transfer fluid flowpath <NUM> (and its loop <NUM>). At increased flow rates, there will be less gradient and more heat discharged away from the heat exchanger.

In a simple control example, there is a closed loop feedback control of measured fuel outlet temperature (measured by sensor <NUM>) via heat transfer fluid flow rate (or corresponding parameters such as heat transfer fluid pump motor current). The feedback control may be to a specific single fuel outlet temperature or to a range. If fuel outlet temperature exceeds the single value or the range upper limit, then the controller <NUM> (<FIG>) increases the heat transfer fluid flow rate (or corresponding parameter) such as via a predetermined increment (e.g., in pump <NUM> speed or current). Similarly, if the measured fuel outlet temperature falls below the single value or range lower limit, the controller may decrease the heat transfer fluid flow rate (e.g., by decrementing directly or via the associated parameter). In sectorized or segmented systems, such control may be segment-by-segment.

The example accumulator 210A or 210B is purely passively pressure actuated (e.g., as opposed to having a controlled valves and/or pressurizing pumps or compressors). As discussed above, the accumulator has a range of conditions between a condition where the pressurant gas <NUM> is maximally expanded (e.g., a bladder is maximally expanded or a piston is at the end of its stroke near the port to the loop); and a condition where the pressurant is maximally compressed.

Table I below shows hypothetical parameters for a number of conditions for the example accumulator and heat transfer loop inlet to the heat exchanger. The example pressures may be relatively low compared with the supercritical range discussed above. The example volumes of the accumulator and amount of gas are chosen so that, in a shutdown/cooled down condition, the piston or bladder is in an intermediate condition. Thus, there is pressure equalization across the piston or bladder. For the bladder-type accumulator 210B, the poppet valve <NUM> is open. Thus, there is pressure equalization between the volume <NUM> and the port <NUM> (and thus adjacent section of the branch from/to the heat transfer loop).

In an example idle condition, due to heating of the fuel, heat transfer loop pressure has increased and thus vented into the accumulator, shifting the piston or bladder to a more compressed condition while pressure is still equalized. At an example take-off condition, the temperature is less so that the equalized pressure is lower but still above the shutdown condition pressure.

A leak from the heat transfer fluid loop or the chamber <NUM> to atmosphere will be detected via the sensor <NUM> as a drop in pressure. In response, the control system may shut off the fuel pumps and close the fuel valves as described above. However, the pressure may continue to drop because the leak has not been isolated. With continued pressure drop, eventually, the accumulator will reach its maximally expanded condition, after which the accumulator pressure may remain essentially constant/unaffected (e.g., other than by temperature) (Table I shows this as an example 500psi, <NUM> kPa) as the heat transfer loop pressure may drop to ambient.

In the event of a leak in one of the fuel tubes, high pressure heat transfer fluid may flow through the leak into the tube. As noted above, this may be evidenced by a drop in heat transfer loop pressure measured by the pressure sensor <NUM> causing shutdown. The heat transfer loop pressure may stabilize at a level that depends on the nature of the nature of the shut-down of fuel flow. The nature of shutdown may depend on robustness of fuel conduits. For example, if the fuel conduit(s) between the valves <NUM> and <NUM> have a particular pressure rating, the valve <NUM> may be kept open until the pressure measured by sensor <NUM> drops to that rating and may be opened if the pressure again exceeds that rating.

The heat exchanger <NUM> is subject to a radial thermal gradient with the inner wall at much higher temperature than the outer wall. This potentially can cause differential thermal expansion of the inner wall <NUM> relative to the outer wall <NUM> (if all other factors are equal). To compensate for such potential differential thermal expansion, any of several mechanisms may be provided.

One group of compensation mechanisms or means involve differences in materials between inner wall <NUM> and outer wall <NUM> wherein the materials have different coefficients of thermal expansion (CTE). A lower CTE of the inner wall relative to the CTE of the outer wall will reduce differential thermal expansion compared with materials of the same CTE. For example, in the illustrated example heat exchanger, the outer wall or a section thereof may be formed of stainless steel which has a higher CTE than the nickel-base superalloy of the inner wall or portion thereof. Such higher CTE material may form an example at least <NUM>% of the length of the heat exchanger.

Even when such means are present, they may be imperfect due to transient behavior when one of the inner wall and outer wall heats or cools faster than the other between steady-state conditions. Thus, additionally or alternatively, the differential thermal expansion may be accommodated by mechanisms or means such as a sliding sealed interaction or a compliant section.

In the seal example, the seal may be axially captured relative to one component and axially sliding relative to the other. In the illustrated example, an annular (or annular segment) seal <NUM> (<FIG>) is captured in a radially outwardly open channel <NUM> of the aft wall <NUM>/flange <NUM> integral with the inner wall. Thus, in the illustrated example, an axial flange <NUM> extends forward from a radial flange portion of the aft wall <NUM>. The seal <NUM> is in sliding engagement with an inner diameter (ID) surface portion of the outer wall <NUM>. Example seals <NUM> are carbon seals (e.g., electro-graphitic carbon (EGC)). In various implementations, such a seal may be spring biased (not shown) into engagement with the mating surface of the outer wall.

<FIG> shows a compliant means in the form of a single annular (or annular segment) diaphragm <NUM> extending radially between the outer wall and the axial flange <NUM> (spaced radially outward of the <FIG> axial flange <NUM> by having the end wall/flange <NUM> extend further radially outward). Given the mounting function of the aft wall/flange, the aft wall/flange may protrude radially outward beyond the outer wall so that the diaphragm extends radially inward to the outer wall. The diaphragm may be formed of a relatively thin (compared to the flange) metallic element such as stainless steel or nickel-based superalloy. The diaphragm may be secured via fasteners (e.g., bolts - not shown) to the outer wall and the aft wall/flange or may be welded or brazed, particularly when formed of a compatible material to one of the two.

Although a single annular (or annular segment) diaphragm is shown in <FIG>, alternatives involve bellows cross-sections. Similarly, although the illustrated diaphragm is at one axial end, alternatives may involve rigid couplings at the axial ends and a diaphragm or bellows section at an intermediate location along the outer wall.

In a situation such as a full annulus chamber and outer wall (whether single-piece or segmented outer wall) the sliding seal may exist only at one end and the bellows may exist only at a single axial location and no other sliding seal may be needed. However, in the case of fully segmented heat exchangers wherein there are separate isolated chambers or sub-chambers for the heat transfer fluid, differential thermal expansion at circumferential ends of the segment may also be relevant.

In one example (not shown) of a fully-segmented heat exchanger, there may be somewhat similar axially-extending seals to the circumferentially-extending seals described above but along flanges at the outer diameter extremes of the circumferential end walls. In such a situation, the relative sliding motion due to differential thermal expansion will be principally axial (parallel to the elongate direction of the seal rather than transverse).

An additional thermal expansion consideration involves axial thermal expansion of the tubes relative to one or both of the inner wall and outer wall. In the example heat exchanger, the combined manifold, inner wall, and outer wall may be essentially relatively immovably secured to each other such as via bolting, welding, and the like. The return manifold may have indirect movable connection to the inner wall and the outer wall.

To maintain tube radial and circumferential position while permitting the axial movement, wear/guide blocks may be positioned at one or more axial locations. In the illustrated example, there are two axial stages of wear blocks (e.g., carbon blocks). An upstream wear block stage <NUM> (<FIG>) engages intermediate portions of the tubes and has an inner diameter (ID) surface <NUM> engaging the outer diameter (OD) surface of the inner wall <NUM> and an outer diameter (OD) surface <NUM> engaging the inner diameter (ID) surface of the outer wall <NUM>.

The example upstream wear block stage <NUM> comprises an inner piece and an outer piece (<FIG>) meeting at a junction <NUM> and sandwiching the tubes between the pieces with the tubes being accommodated in grooves <NUM> in the mating surfaces. The pieces also include grooves <NUM> (or holes) for passing the heat transfer fluid as portions of the heat transfer fluid flowpath <NUM>. Depending upon implementation, the wear block stage <NUM> may be in sliding relation with all three of the inner wall, tubes, and outer wall or may be rigidly secured to one or captured by of those three. For example, they may be captured by the walls (e.g., protrusions from the walls on opposite axial sides of the blocks or protrusions captured in recesses in the blocks).

The example downstream second wear block stage <NUM> (<FIG>) is mounted to the return manifold and may comprise a separate inner ring and outer ring. This may be secured to the return manifold <NUM> via fasteners (not shown) or simply via mechanical interfitting of projections from one element being received in compartments or recesses of another element.

For further enhancing heat transfer from the combustion gas to the heat transfer fluid, the example heat exchanger includes a first plurality of fins <NUM> (<FIG>) (first fins or inner diameter (ID) fins) protruding into the gaspath <NUM> and a second plurality of fins <NUM> (second fins or outer diameter (OD) fins) protruding into the heat transfer fluid. The first fins <NUM> protrude from the inner wall <NUM> ID surface of the heat exchanger and may be unitarily formed with the inner wall (e.g., via casting as a unit or additively manufacturing) or be separately formed and mounted thereto (e.g., by welding). Similarly, the second fins <NUM> may protrude from the OD surface of the inner wall <NUM>. In the illustrated embodiment, each first fin is circumferentially in registry with an associated second fin. Such a configuration may be particularly useful if it is desired that each first fin and the associated second fin be formed as a single piece and mounted to the inner wall such as via a slot in the inner wall substrate (followed by welding or brazing to seal and secure). Or, even if separate pieces, the alignment may improve heat transfer and may improve manufacture via allowing co-welding or co-brazing.

In the illustrated example heat exchanger, the second fins <NUM> are interdigitated with the tubes with each second fin <NUM> protruding radially between two tubes (or, for terminal tubes in a sector only circumferentially outboard of that terminal tube). This protrusion may cause the second fins to radially extend at least as far out as the inner diameter boundary of the adjacent tubes and, more preferably, at least radially past the centerlines of the adjacent tubes.

Various means may be provided to accommodate the wear blocks to the fins and/or vice-versa. For example, the second fins may be segmented, interrupted along the length of the heat exchanger to provide gaps <NUM> (<FIG>) accommodating the wear blocks. Or, the wear blocks may be grooved to accommodate the fins.

<FIG> and <FIG> show an alternate heat exchanger <NUM> that, except for differences discussed, may be of similar configuration, manufacture, and use to those described above. The heat exchanger <NUM> provides one example of two stages (groups) of fuel tube legs or segments offset from the gaspath normal to the direction of heat transfer from the gaspath (offset in the radial direction in the annular example). The example heat exchanger <NUM> is schematically shown as having tube banks <NUM> sectors positioned end-to end in close facing or contacting relation in an annular chamber <NUM>. The outer wall is shown segmented with inter-segment boundaries <NUM> (e.g., at mating axially-extending and radially protruding bolting flanges (not shown)).

The example heat exchanger is schematically shown flattened. Although it may represent an actual flat heat exchanger such as might be used in non-annular ducts, it may alternatively be arcuate. <FIG> is simplified in not showing various material thicknesses and details including the wear/guide blocks (schematically shown in <FIG>).

The example heat exchanger <NUM> radially spaces the two axial legs/segments. The outboard (away from the gaspath)/upstream (along the fuel flowpath) segments <NUM> proceed aftward from the inlet manifold <NUM> and plenum to an aft turn <NUM>. The inboard/downstream segments <NUM> pass forward from the aft turn to the outlet manifold <NUM> and plenum. The aft turn <NUM> may take the form of an individual turn connecting an individual associated upstream leg to an individual associated downstream leg. Alternatively, the aft turn may take the form of a plenum/manifold common to multiple legs <NUM>, <NUM> of each stage.

With the example radial stagger of the legs, to provide generally counterflow heat exchange between the heat exchange fluid and the fuel, the chamber <NUM> (<FIG>) is divided by a wall (divider) <NUM> between the inboard segments and the outboard segments forming an outboard section <NUM> of the chamber and an inboard section <NUM> of the chamber. The heat exchange fluid inlet is relatively forward on the outboard section and the heat exchange fluid outlet is relatively forward on the inboard section. Thus, the heat exchange fluid flowpath is generally aftward in the outboard section <NUM> and forward in the inboard section <NUM>, turning at an aft edge <NUM> of the divider.

In the example, the outboard legs <NUM> are circumferentially staggered relative to the inboard legs <NUM> (e.g., the two groups are exactly out of phase circumferentially). This may be particularly useful where a single bent tube forms an outboard leg, an inboard leg and the joining turn to allow a greater radius of curvature at the turn for a given radial offset. This also may expose the outboard stage to slightly greater heat transfer.

The example divider <NUM> may be secured at its forward end. The securing may be directly to the body of the heat exchanger (e.g., shown secured to the outer wall <NUM>). This may be via welding, brazing, or the like. Alternative securing may be to one or both of the fuel manifolds <NUM>, <NUM>. Additionally, the divider may be retained radially such as via the wear/guide blocks. In this situation, there may be wear/guide blocks guiding the outboard tube legs and wear/guide blocks guiding the inboard tube legs. For example, at each axial position, the block stages may be formed by a pair of outboard pieces and a pair of inboard pieces such as those pairs shown for the <FIG> embodiment of a single tube stage. One aspect of the divider implementation is that such blocks may be mounted to the divider (e.g., via threaded fasteners or projections of divider received in recesses of the blocks (or vice-versa)). In such a situation of mounting the blocks to the divider, the blocks may freely slide relative to their respective associated tubes and associated inboard wall or outboard wall.

In the illustrated <FIG> example, the fins <NUM> interdigitate with the inboard stage of tube legs. Also, in this example, both the first fins <NUM> and the second fins <NUM> are segmented.

Further variations may have the inner wear blocks (engaging the tube legs <NUM>) and the outer wear blocks (engaging the tube legs <NUM>) axially offset from each other. Additionally, they be of unequal number (e.g., <FIG> shows an orphan outer block stage near the aft end).

Further variations may have other manifold/plenum configurations. For example, the fuel tubes may penetrate the outer wall to connect to an external inlet or outlet manifold. Similarly, there may be an external inlet or outlet manifold for heat transfer fluid to better circumferentially distribute through a circumferential array of ports.

Further variations may have other than axially-extending tube legs. For example, the tube legs may extend transverse to the engine centerline and/or downstream gaspath direction. Such transverse orientation may be circumferential in the case of an annular (whether full annular or segmented annular or other) heat exchanger. Yet alternative configurations may involve spiral tubes (spiraling radially) or helical tubes or combinations thereof as in a frustoconical helix/spiral.

Where a measure is given in English units followed by a parenthetical containing SI or other units, the parenthetical's units are a conversion and should not imply a degree of precision not found in the English units. Except where explicitly or implicitly described otherwise, all pressures are absolute rather than gage.

Claim 1:
A method for operating a turbine engine (<NUM>), wherein the turbine engine (<NUM>) comprises:
a compressor (<NUM>);
a combustor (<NUM>);
a turbine (<NUM>);
a gaspath (<NUM>) passing downstream from the compressor (<NUM>) through the combustor (<NUM>) and then through the turbine (<NUM>);
a fuel source (<NUM>);
a fuel flowpath (<NUM>) from the fuel source (<NUM>) to the combustor (<NUM>); and
a heat exchanger (<NUM>; <NUM>) for transferring heat from the gaspath (<NUM>) to the fuel flowpath (<NUM>),
wherein the heat exchanger (<NUM>; <NUM>) comprises:
an inner wall (<NUM>) in heat transfer relation with the gaspath (<NUM>);
an outer wall (<NUM>);
tubes (<NUM>, <NUM>) between the inner wall (<NUM>) and the outer wall (<NUM>) bounding respective segments of the fuel flowpath (<NUM>); and
a heat transfer fluid (<NUM>) between the inner wall (<NUM>) and the outer wall (<NUM>) and in heat transfer relation with the tubes (<NUM>, <NUM>) and the inner wall (<NUM>),
wherein the method comprises:
passing fuel along the fuel flowpath (<NUM>) from the fuel source (<NUM>) to the combustor (<NUM>);
combusting the passed fuel in the combustor (<NUM>) to generate combustion gas; and
transferring heat from the combustion gas to the fuel via the heat transfer fluid (<NUM>) prior to the passing of the fuel to the combustor (<NUM>) to heat the fuel, wherein the method further comprises: measuring a pressure of the heat transfer fluid (<NUM>); and responsive to an increase in pressure of the heat transfer fluid (<NUM>), venting heat transfer fluid (<NUM>) to an accumulator (210A; 210B); and/or responsive to a decrease in the measured pressure of the heat transfer fluid (<NUM>), shutting down the turbine engine (<NUM>) and/or shutting off at least portion of the passing fuel.