Patent Description:
In order to limit emissions of carbon dioxide, use of hydrogen as an alternative to hydrocarbon fuel in gas turbine engines has historically only been practical in land-based installations. However, cryogenic hydrogen fuelled airliners have recently been proposed. The fuel for such aircraft however must be heated prior to combustion. Doing so in a manner which is efficient from an overall propulsion system perspective is a significant challenge.

United States patent application <CIT> discloses a gas turbine engine having a waste heat recovery system. The gas turbine engine includes a compressor section, a combustion section, a turbine section, and an exhaust section in serial flow order and together defining a core air flowpath, the exhaust section including a primary exhaust flowpath and a waste heat recovery flowpath parallel to the primary exhaust flowpath; and the waste heat recovery system includes a heat source exchanger positioned in thermal communication with a first portion of the waste heat recovery flowpath.

United States patent application <CIT> discloses a gas turbine engine system which includes a gas turbine engine and a fuel turbine system. The gas turbine engine includes a heat exchange system configured to transfer thermal energy from a first compressed air flow and an exhaust gas flow to a fuel to produce a gaseous fuel. The fuel turbine system includes a fuel turbine fluidly coupled to the heat exchange system and a combustor of the gas turbine engine, and a fuel pump fluidly coupled to the heat exchange system and configured to be driven by the fuel turbine. The fuel turbine is configured to extract energy from expansion of the gaseous fuel to produce the gaseous fuel at a lower pressure for delivery to the combustor.

United Kingdom patent application <CIT> discloses an engine comprising a turbojet having an air compressor, a combustor, a first turbine which drives the compressor and a jet pipe which terminates in a variable area nozzle. A source of liquid hydrogen is provided and the engine has a first heat exchanger cooled by the fuel for cooling the air entering the intake. A bypass valve is provided so as to bypass the heat exchanger to prevent icing in high humidity air. The hydrogen is vaporised in a second heat exchanger in the jet pipe and the gaseous fuel is used to drive the liquid fuel turbo pump and a second turbine which also drives the air compressor. The exhaust from the second turbine is fed to the main combustor to a reheat burner in the jet pipe. Liquid oxygen can be used to cool further the intake air or to cool the turbine components.

The invention is directed towards a propulsive aircraft gas turbine engine comprising:.

Advantageously, a portion of gas turbine engine core flow is unimpeded by the recuperator, allowing for improved core flow thrust recovery, while providing for recovery of exhaust heat from the gas turbine engine core to heat the fuel, thereby improving thermal efficiency of the gas turbine engine core, while minimising loss of propulsive efficiency.

The gas turbine engine may be configured to burn hydrogen fuel.

The recuperator is installed adjacent a radially inner wall of a gas turbine core exhaust nozzle, and may be generally annular. The gas turbine engine exhaust nozzle comprises a generally radially inner recuperator flow and a generally radially outer core bypass flow. Advantageously, the relatively slower moving core air adjacent the inner wall of the exhaust nozzle is transferred through the heat exchanger, rather than the higher velocity air at the outer wall. Consequently, heat exchange effectiveness is improved due to the longer dwell time within the heat exchanger matrix, and core exhaust thrust recovery is improved relative to where the higher velocity exhaust flow is used.

The recuperator is provided within a core centre body of the gas turbine engine, aft of the turbine. Advantageously, space that is normally empty within the gas turbine engine is utilised for the recuperator.

The gas turbine engine may comprise a recuperator channel arranged to guide core flow from the turbine exit, through the recuperator heat exchanger, and out a recuperator exhaust.

The recuperator channel may be configured to redirect flow from a generally axial direction from the turbine exhaust to a generally radially inward flow through a heat exchange matrix of the recuperator heat exchanger. Advantageously, flow the heat exchange matrix can be oriented in a generally radial direction, allowing for a planar heat exchanger extending generally axially in its largest dimension. Consequently, a large surface area of heat exchange matrix can be provided, without occupying excessive space within the gas turbine engine, and without restricting main core gas flow.

The recuperator channel may be configured to redirect flow from a generally radially inward direction from the heat exchange matrix to a generally axial direction out the recuperator exhaust. Advantageously, heat exchanger exhaust flow can be used to generate core thrust.

The gas turbine engine comprises an axially translating turbine centre body configured to vary an outlet area of the recuperator. Advantageously, exhaust flow can be modulated to control exhaust velocities depending on engine operational conditions.

The recuperator may be configured to accommodate between <NUM> and <NUM>% of core engine flow, with the remainder being bypassed around the recuperator heat exchanger.

A hydrogen-fuelled airliner is illustrated in <FIG>. In this example, the airliner <NUM> is of substantially conventional tube-and-wing twinjet configuration with a central fuselage <NUM> and substantially identical underwing-mounted turbofan engines <NUM>. In the present embodiment, the turbofan engines <NUM> are geared turbofan engines.

A hydrogen storage tank <NUM> located in the fuselage <NUM>. In the present embodiment, the hydrogen storage tank <NUM> is a cryogenic hydrogen storage tank and thus stores the hydrogen fuel in a liquid state, in a specific example at <NUM> kelvin. In this example, the hydrogen fuel is pressurised to a pressure from around <NUM> bar to around <NUM> bar, in a specific example <NUM> bar. In other cases, the hydrogen could be stored as a cryogenically cooled, compressed gas or supercritical fluid.

A block diagram of one of the turbofan engines <NUM> is shown in <FIG>.

The turbofan engine <NUM> comprises a core gas turbine <NUM>.

The core gas turbine <NUM> comprises, in fluid flow series, a low-pressure compressor <NUM>, a high-pressure compressor <NUM>, a fuel injection system <NUM>, a combustor <NUM>, a high-pressure turbine <NUM>, a low-pressure turbine <NUM>, and a core nozzle <NUM>. The high-pressure compressor <NUM> is driven by the high-pressure turbine <NUM> via a first shaft <NUM>, and the low-pressure compressor <NUM> is driven by the low-pressure turbine <NUM> via a second shaft <NUM>. It will be appreciated that in alternative embodiments, the core gas turbine could be of three-shaft configuration.

The turbofan <NUM> also defines a fan <NUM>, which is driven by the low-pressure turbine <NUM>. The fan provides airflow to the core gas turbine <NUM>, and to a bypass duct <NUM>. As such, distinct bypass and core flows are provided through the bypass passage <NUM> and gas turbine engine core <NUM> respectively.

In operation, hydrogen fuel is pumped from the hydrogen storage tank <NUM> by a pump <NUM> and into a main fuel conduit <NUM> which ultimately delivers fuel to the fuel injection system <NUM>.

As will be appreciated, it is desirable to increase the temperature of the fuel from cryogenic storage condition to a temperature much closer to the firing temperature of the core gas turbine <NUM>; of course this is subject to the constraint of not exceeding the autoignition temperature of the hydrogen fuel prior to admission into the combustor <NUM>. In an example, the injection temperature is from <NUM> to <NUM> kelvin, for example <NUM> kelvin. In some cases, it may be desirable to increase the fuel temperature to above an icing temperature, such as <NUM> kelvin.

A preheater <NUM> is therefore provided for heating of the hydrogen fuel. This takes place between the pump <NUM> and the fuel injection system <NUM>. In an embodiment, the preheater <NUM> is configured to raise the temperature of the hydrogen fuel to the required injection temperature. The heating may provide a phase change (for example from liquid to supercritical or to gas), or the fluid may remain in a supercritical state after heating by the preheater.

In another embodiment, the preheater <NUM> is configured to raise the temperature of the hydrogen fuel to an intermediate temperature less than the injection temperature. This could for example be from <NUM> to <NUM> kelvin, for example <NUM> kelvin.

The pre-heater <NUM> comprises a recuperator heat exchanger configured to exchange heat from the gas turbine engine core exhaust to the hydrogen fuel in the main fuel conduit <NUM>, prior to delivery to the fuel injector <NUM>. In some cases, at certain points in the operational envelope there will be insufficient heat output from the engine to raise the fuel temperature to the injection temperature using the recuperator alone. Such occasions may include, for example, ground start, in-flight relight, end of cruise idle, etc. An such cases, an additional auxiliary preheater (not shown) may be provided.

As will be appreciated, the provision of a recuperator heat exchanger increases the thermal efficiency of the engine, since waste heat that would normally be expelled from the exhaust is reintroduced into the engine cycle via the fuel. However, in studies conducted by the inventors, the advantages of a recuperated cycle are greatly (and in some cases, entirely) offset by reduced propulsive efficiency in view of the flow restriction provided by the recuperator. Additionally, the blockage of the recuperator may increase turbine back pressure, thereby reducing available pressure drop across the turbine, and so reducing turbine work and engine power density. A further disadvantage of recuperated designs is the presence of a relatively delicate heat exchanger in the core engine gas path. Foreign or domestic objects present in the exhaust flow may impinge on the heat exchanger, thereby damaging it. The present invention may solve some or all of these problems.

A first embodiment of the preheater <NUM> is shown in further detail in <FIG> which shows an aft part of the engine <NUM>.

The heat exchanger <NUM> comprises a heat exchange matrix <NUM> configured to flow hydrogen fuel through a first set of channels, and hot exhaust air through a second set of channels, to allow for heat exchange therebetween. The heat exchange matrix <NUM> is provided within a recuperator channel <NUM> arranged to guide a portion (i.e. less than the whole) of core gas turbine engine exhaust gas flow A. Typically, the recuperator channel is configured to accommodate between <NUM>% and <NUM>% of core mass flow, with the remainder of core mass flow being bypassed. In a particular example, the inventors have found that a preferred range of recuperator mass flow is between <NUM>% and <NUM>%. In engine modelling experiments, the inventors have found that <NUM>% recuperator mass flow provides optimum heat exchange to the fuel without providing excessive blockage of the core exhaust.

The recuperator channel <NUM> is provided adjacent a radially inner side of the gas turbine engine core, and is mounted to a radially inner side wall <NUM> aft of the low-pressure turbine <NUM>. The radially inner side wall <NUM> extends annularly around the engine core to form a core centre body <NUM>, which projects from a rear of the engine <NUM>. The radially inner side wall <NUM> defines an inner extent of the recuperator channel <NUM>, while a radially outer recuperator duct wall <NUM> defines a radially outer wall of the recuperator channel. As such, a generally annular duct <NUM> is defined, which guides core flow generally axially through the duct <NUM> from an inlet, through the heat exchange matrix, and to an exhaust.

Radially outward of the duct <NUM> is a core bypass passage <NUM> through which the remainder of the core flow passes, without extending through the recuperator matrix. This core bypass passage <NUM> is distinct from the turbofan bypass duct <NUM>, and flows only core flow. The core bypass passage <NUM> is defined by an annulus between the recuperator channel outer wall <NUM> and a core outer wall <NUM>. The two flows mix aft of the core bypass passage, and are expelled through the exhaust nozzle <NUM>. As such, a portion of core bypass flow extends out of the exhaust without being restricted by the recuperator heat exchanger <NUM>. This may increase core outlet velocity relative to an engine in which all core flow extends through a recuperator, and may reduce backpressure, thereby increasing turbine effectiveness. These disadvantages may be reduced by employing heat exchanger designs with low pressure drop. However, such designs are typically large and bulky, and may require large diffusion ducts upstream of the heat exchanger. These design compromises may lead to increased overall weight, volume and cost of the propulsion system.

<FIG> shows a first alternative arrangement of a recuperator heat exchanger <NUM> in a gas turbine engine core. The arrangement is similar to that shown in <FIG>, but the recuperator heat exchanger <NUM> is provided radially relative to the arrangement shown in <FIG>. The recuperator heat exchanger <NUM> is provided within the core centre body <NUM>. A scoop <NUM> is provided, which extends into the core flow, to ingest a portion of the core flow aft of the turbine <NUM>, which is then redirected to the recuperator heat exchanger <NUM> through a recuperator channel <NUM>. The remainder of the core flow continues unabated out of the exhaust nozzle <NUM>.

The recuperator channel is divergent from an inlet to the front face of the heat exchanger <NUM>. Consequently, flow velocity is reduced, and pressure is increased. Consequently, heat exchanger effectiveness is increased. In addition, the probability of recuperator damage is reduced in view of the lower velocity, and the presence of the inlet at a low radial position, since particles in the exhaust are likely to be concentrated at the radially outer wall of the turbine. In some cases, a diverter (not shown) may be provided in the recuperator channel to further reduce the probability of debris damage.

Consequently, the recuperator is provided within a space that is normally empty, thereby improving engine packaging. Furthermore, engine core flows relatively uninterrupted through the gas turbine engine exhaust, thereby improving propulsive efficiency. Additionally, the recuperator is protected from damage.

<FIG> shows a second alternative arrangement of a recuperator heat exchanger <NUM>. In this arrangement, the recuperator heat exchanger <NUM> is again provided within a duct <NUM> within the centre body <NUM>. However, in this arrangement, the heat exchanger <NUM> and duct <NUM> are configured such that core flow flows generally axially from an inlet <NUM> toward the recuperator <NUM> before being turned generally radially inwardly and flowing through the recuperator <NUM> heat exchange matrix in a generally radially inward direction. Flow downstream of the recuperator <NUM> is again turned to a generally axial direction, where it flows out of the engine through a recuperator exhaust <NUM> the centre of the centre body <NUM>.

The recuperator <NUM> has a generally annular profile, and is arranged to flow core flow radially inward. Alternatively, the recuperator <NUM> may be part annular. As such, a relatively large area can be provided, since inlet flow area can be increased by increasing the axial extent of the recuperator <NUM>. The combination of large area and low velocity results in high heat exchange effectiveness for a given mass flow. Consequently, a relatively small quantity of air A can be drawn from the core flow, and decelerated to relatively slow velocities, before flowing through the recuperator heat exchanger <NUM>. As such, the impact on turbine backpressure and recuperator bypass flow B is still further reduced, while a high rate of heat transfer is maintained.

<FIG> shows a third alternative arrangement of the recuperator heat exchanger <NUM>. The arrangement is similar to that shown in <FIG>, but with the addition of a translating centre-body configured to control the outlet area of the recuperator channel <NUM>.

The upper half of <FIG> shows the centre body translated aft, such that the outlet area of the recuperator channel <NUM> is reduced, whereas the lower half of <FIG> shows the centre body translated forward such that the outlet area of the recuperator channel <NUM> is increased. By changing the outlet area, the mass flow and / or velocity through the channel <NUM> can be controlled, thereby controlling heat input to the fuel within the recuperator <NUM>. Consequently, fuel temperature can be controlled independently of engine core flow and temperature. For example, during operation at low power, fuel flow will typically be low, while core flow velocity will also be low, while core temperature may remain high. As such, the temperature rise of the fuel may be excessive under these conditions. Consequently, mass flow can be decreased by closing the nozzle, thereby reducing heat input. In some cases, the nozzle may be controlled completely, thereby halting heat input to the fuel. As will be appreciated, other flow control means could be employed, such as valves of various types.

<FIG> show a fourth alternative arrangement of the recuperator heat exchanger <NUM>.

In this arrangement, the recuperator channel <NUM> is provided at a radially outer portion of the engine core, adjacent a core outer wall <NUM> of the engine, with the recuperator heat exchanger <NUM> being provided within the recuperator channel <NUM>, and a core bypass being provided radially inwards. This arrangement provides the cooler, lower velocity air from the recuperator heat exchanger at the outer radius of the core exhaust, providing intermediate temperature and velocity airflow between the core and bypass flows, which may result in lower noise output from the engine.

As shown in <FIG>, the recuperator channel comprises a valve arrangement comprising a plurality of rotatable covers, which can rotate in the directions shown by the arrows to cover or uncover the heat exchange matrix <NUM>. As such, the mass flow and / or velocity of air through the heat exchangers can be managed.

Claim 1:
A propulsive aircraft gas turbine engine (<NUM>) comprising:
a turbine (<NUM>, <NUM>) disposed in a gas turbine engine core (<NUM>) flow;
a recuperator heat exchanger (<NUM>) disposed downstream of the turbine (<NUM>, <NUM>) in gas turbine engine core flow and installed adjacent a radially inner wall (<NUM>) of a gas turbine core exhaust nozzle (<NUM>), the recuperator heat exchanger (<NUM>) being configured to transfer heat from the gas turbine engine core flow to gas turbine engine fuel; wherein
the recuperator heat exchanger (<NUM>) is configured to accommodate a portion of the gas turbine engine core flow therethrough, the remainder being bypassed around the recuperator heat exchanger;
and wherein the engine exhaust nozzle comprises a generally radially inner recuperator flow and a generally radially outer core bypass flow, the recuperator being provided within a core centre body (<NUM>) of the gas turbine engine, aft of the turbine (<NUM>, <NUM>);
characterised in that:
the gas turbine engine comprises an axially translating turbine centre body (606a, 606b) configured to vary an outlet area of the recuperator (<NUM>).