Patent Description:
It is generally known in the art to power aircraft gas turbine engines with gases expelled from combustion chambers. In the gas turbine engine, a fuel is combusted in an oxygen rich environment. The fuel may be any appropriate fuel such as a liquid or gas. Exemplary fuels include hydrocarbons (for example methane or kerosene) or hydrogen. These combustion systems may emit undesirable compounds such as water vapor, nitrous oxide compounds (NOx), carbon containing compounds. In some cases, it may be desirable to decrease the emission of various compounds as much as possible so that the selected compounds may not enter the atmosphere. There is a need in the art, therefore, for improved systems and methods for treating combustion gas emissions from aircraft gas turbine engines.

<CIT> discloses a prior art diffuser for an axial flow machine. The diffuser has a transition from a ring channel having a first cross sectional area into an outlet space with a second, larger cross sectional area along a machine axis of the axial flow machine. The transition includes a plurality of steps.

According to an aspect of the present disclosure, there is provided a diffuser nozzle for a gas turbine engine as set forth in claim <NUM>.

In any of the aspects or embodiments described above and herein, within the first axially-extending duct segment, the duct cross-sectional area of the first duct section may be greater than a first combined duct cross-sectional area of all of the other duct section of the plurality of duct sections and within the second axially-extending duct segment, the duct cross-sectional area of the second duct section may be greater than a second combined duct cross-sectional area of all of the other duct sections of the plurality of duct sections.

In any of the aspects or embodiments described above and herein, a first number of duct sections of the plurality of duct sections may be equal to a second number of axially-extending duct segments of the plurality of axially-extending duct segments.

In any of the aspects or embodiments described above and herein, the first duct section may include a first exhaust treatment system in only the first axially-extending duct segment and the second duct section may include a second exhaust treatment system in only the second axially-extending duct segment.

In any of the aspects or embodiments described above and herein, wherein each of the first exhaust treatment system and the second exhaust treatment system is configured to remove one or more of water vapor, carbon compounds, or nitrogen oxides (NOx) from an exhaust gas stream passing through the diffuser nozzle.

In any of the aspects or embodiments described above and herein, each axially-extending duct segment of the plurality of axially-extending duct segments may include a total cross-sectional area of the nozzle duct.

In any of the aspects or embodiments described above and herein, each axially-extending duct segment of the plurality of axially-extending duct segments may be located in a first axial portion of the housing and wherein a first diameter of the housing in the first axial portion may be greater than a second diameter of the nozzle inlet and a third diameter of the nozzle outlet.

In any of the aspects or embodiments described above and herein, the housing may further include a second axial portion of the housing positioned between the nozzle inlet and the first axial portion. The second axial portion may have a diffusion diameter which transitions from the second diameter to the first diameter in an axial direction.

In any of the aspects or embodiments described above and herein, the housing may further include a third axial portion of the housing positioned between the nozzle outlet and the first axial portion. The third axial portion may have a diffusion diameter which transitions from first diameter to the third diameter in an axial direction.

In any of the aspects or embodiments described above and herein, each duct section of the plurality of duct sections may be fluidly independent of each other duct section of the plurality of duct sections from the nozzle inlet to the nozzle outlet.

According to another aspect of the present disclosure, there is provided a gas turbine engine for an aircraft as set forth in claim <NUM>.

In any of the aspects or embodiments described above and herein, the gas turbine engine may be a turboprop or a turboshaft gas turbine engine.

In any of the aspects or embodiments described above and herein, the diffuser nozzle may have a circular cross-sectional shape.

In any of the aspects or embodiments described above and herein, each axially-extending duct segment of the plurality of axially-extending duct segments may include an exhaust treatment system in only one duct section of the plurality of duct sections.

In any of the aspects or embodiments described above and herein, the exhaust treatment system is configured to remove one or more of water vapor, carbon compounds, or nitrogen oxides (NOx) from the exhaust gas stream passing through the diffuser nozzle.

According to another aspect of the present disclosure, there is provided a method for treating exhaust gases from a gas turbine engine for an aircraft as set forth in claim <NUM>.

In any of the aspects or embodiments described above and herein, the method may further include directing each exhaust gas sub-stream of the plurality of exhaust gas sub-streams through a respective exhaust treatment system within the diffuser nozzle to remove one or more of water vapor, carbon compounds, or nitrogen oxides (NOx) from each exhaust gas sub-stream.

In any of the aspects or embodiments described above and herein, the respective exhaust treatment system for each exhaust gas sub-stream of the plurality of exhaust gas sub-streams may have a different axial catalyst location than the respective exhaust treatment system for each other exhaust gas sub-stream of the plurality of exhaust gas sub-streams.

The present disclosure, and all its aspects, embodiments and advantages associated therewith will become more readily apparent in view of the detailed description provided below, including the accompanying drawings.

<FIG> illustrates a gas turbine engine <NUM> of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication an air inlet <NUM>, a compressor section <NUM> for pressurizing the air from the air inlet <NUM>, a combustor <NUM> in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, a turbine section <NUM> for extracting energy from the combustion gases, and an exhaust section <NUM> through which the combustion exhaust gases exit the gas turbine engine <NUM>.

The gas turbine engine <NUM> of <FIG> generally includes a high-pressure spool <NUM>, a low-pressure spool <NUM>, and a power spool <NUM> mounted for rotation about an axial centerline <NUM> (e.g., a rotational axis) of the gas turbine engine <NUM>. The high-pressure spool <NUM> generally includes a high-pressure shaft <NUM> that interconnects a high-pressure compressor <NUM> and a high-pressure turbine <NUM>. The low-pressure spool <NUM> generally includes a low-pressure shaft <NUM> that interconnects a low-pressure compressor <NUM> and a low-pressure turbine <NUM>. The power spool <NUM> generally includes a drive output shaft <NUM> in rotational communication with a power turbine <NUM> having a forward end configured to drive a rotatable load <NUM>. The rotatable load <NUM> can, for instance, take the form of a propeller. In alternative embodiments, the gas turbine engine <NUM> may be configured such that the rotatable load <NUM> may include a rotor, such as a helicopter main rotor, driven by the drive output shaft <NUM>. The drive output shaft <NUM> may be connected to the rotatable load <NUM> through a gear assembly <NUM> to drive the rotatable load <NUM> at a lower speed than the power spool <NUM>. It should be understood that "low pressure" and "high pressure," or variations thereof, as used herein, are relative terms indicating that the high pressure is greater than the low pressure. The high-pressure shaft <NUM>, the low-pressure shaft <NUM>, and the drive output shaft <NUM> may be concentric about the axial centerline <NUM>. The gas turbine engine <NUM> of <FIG> further includes a nacelle <NUM> defining an exterior housing of the gas turbine engine <NUM>. The gas turbine engine <NUM> of <FIG> further includes an aircraft wing <NUM> mounted to and extending outward from the nacelle <NUM>.

The gas turbine engine <NUM> of <FIG> may be configured, for example, as a turboprop or a turboshaft gas turbine engine. It should be understood that the concepts described herein are not limited to use with turboprops as the teachings may be applied to other types of gas turbine engines such as turbofan gas turbine engines as well as those gas turbine engines including single-spool or two-spool architectures.

The present disclosure gas turbine engine <NUM> further includes a diffuser nozzle <NUM> in the exhaust section <NUM> of the gas turbine engine <NUM>. The diffuser nozzle <NUM> is configured for post-combustion treatment of combustion exhaust gases, for example, to reduce or otherwise mitigate the emission of undesirable compounds from the gas turbine engine <NUM>. In particular, the diffuser nozzle <NUM> is configured to reduce the velocity of combustion exhaust gases passing therethrough in order to increase the effectiveness of an exhaust treatment process, as will be discussed in further detail. The gas turbine engine <NUM> may include a fixed structure <NUM> such as a casing or cowl surrounding at least a portion of the turbine section <NUM>. The diffuser nozzle <NUM> may be mounted to the fixed structure <NUM> axially downstream of the turbine section <NUM>. As shown in <FIG>, the diffuser nozzle <NUM> may be located within the nacelle <NUM> surrounding the gas turbine engine <NUM>. Aspects of the present disclosure diffuser nozzle <NUM> maybe particularly relevant for the treatment of combustion exhaust gases from turboprop or turboshaft gas turbine engines, as the combustion exhaust gases in these gas turbine engines may not be used to generate a substantial amount of thrust for an associated aircraft. Accordingly, treatment of the combustion exhaust gases according to the present disclosure may provide a valuable means of controlling gas turbine engine emissions without restricting the operational capacity of the associated gas turbine engine. However, it should be understood that aspects of the present disclosure may also be relevant to other types of aircraft gas turbine engines such as turbofan and turbojet gas turbine engines.

Referring to <FIG>, in a first embodiment of the present disclosure, the diffuser nozzle <NUM> includes a housing <NUM> disposed about a nozzle axis <NUM> and extending between a first nozzle end <NUM> and a second nozzle end <NUM>. The nozzle axis <NUM> may or may not be colinear with the axial centerline <NUM> of the gas turbine engine <NUM>. The housing <NUM> includes a nozzle inlet <NUM> located at the first nozzle end <NUM> and a nozzle outlet <NUM> located at the second nozzle end <NUM>. Combustion exhaust gases (schematically illustrated in <FIG> and <FIG> as exhaust gas stream <NUM>) are directed from the turbine section <NUM> to the nozzle inlet <NUM> and then through the diffuser nozzle <NUM> in a direction from the nozzle inlet <NUM> to the nozzle outlet <NUM>. The housing <NUM> radially surrounds and defines a nozzle duct <NUM> of the diffuser nozzle <NUM> extending from the nozzle inlet <NUM> to the nozzle outlet <NUM> and including the nozzle inlet <NUM> and the nozzle outlet <NUM>. While diffuser nozzle <NUM> is shown in <FIG> as having a circular cross-sectional shape, the present disclosure is not limited to this particular cross-sectional shape and other shapes (e.g., polygonal cross-sectional shapes) for the diffuser nozzle <NUM> may also be used.

The diffuser nozzle <NUM> may include a plurality of walls <NUM> disposed within the nozzle duct <NUM> and extending along at least a portion of the axial span of the diffuser nozzle <NUM>. The plurality of walls <NUM> may subdivide the nozzle duct <NUM> into a plurality of duct sections <NUM> with each duct section <NUM> extending from the nozzle inlet <NUM> to the nozzle outlet <NUM>. Each duct section <NUM> may be fluidly independent of each other duct section <NUM> from the nozzle inlet <NUM> to the nozzle outlet <NUM>. In other words, the plurality of walls <NUM> may isolate each duct section <NUM> from each other duct section <NUM>, with respect to the combustion exhaust gases passing therethrough, from the nozzle inlet <NUM> to the nozzle outlet <NUM>. Accordingly, the diffuser nozzle <NUM> may separate the exhaust gas stream <NUM> into a plurality of exhaust gas sub-streams with each exhaust gas sub-stream flowing through a respective duct section of the plurality of duct sections <NUM>. As shown in <FIG>, the nozzle duct <NUM> includes a first duct section 74A, a second duct section 74B, a third duct section 74C, and a fourth duct section 74D. However, the present disclosure is not limited to any particular number of duct sections of the plurality of duct sections <NUM>.

Each duct section <NUM> has a duct cross-sectional area which may vary through an axial span of each duct section <NUM> from the nozzle inlet <NUM> to the nozzle outlet <NUM>. <FIG> illustrate duct cross-sectional areas of the plurality of ducts <NUM> at various axial positions of the diffuser nozzle <NUM>. In an upstream-to-downstream direction as shown in <FIG>, the diffuser nozzle <NUM> may include the nozzle inlet <NUM> (see <FIG>), a diffusing axial portion <NUM> (see <FIG>), a treatment axial portion <NUM> (see <FIG>), a concentrating axial portion <NUM> (see <FIG>), and the nozzle outlet <NUM> (see <FIG>). The treatment axial portion <NUM> includes a maximum cross-sectional area of the nozzle duct <NUM>. A diameter D1 of the housing <NUM> along the treatment axial portion <NUM> is greater than a diameter D2 of the housing <NUM> at the nozzle inlet <NUM> and a diameter D3 of the housing <NUM> at the nozzle outlet <NUM>. Within the diffusing axial portion <NUM>, the duct cross-sectional area of each duct section <NUM> gradually increases until reaching a maximum duct cross-sectional area within the treatment axial portion <NUM>. Within the concentrating axial portion <NUM>, the duct cross-sectional area of each duct section <NUM> gradually decreases from the maximum duct cross-sectional area of the treatment axial portion <NUM> until reaching the nozzle outlet <NUM>.

In some embodiments, the diffuser nozzle <NUM> may include an exhaust treatment system <NUM> in the treatment axial portion <NUM> of the diffuser nozzle <NUM>. The exhaust treatment system <NUM> may be configured to treat combustion exhaust gases from the gas turbine engine <NUM> so as to eliminate or reduce the quantity of one or more compounds within the combustion exhaust gases. Additionally or alternatively, the exhaust treatment system <NUM> may be configured to alter the physical properties (e.g., pressure, temperature, velocity, etc.) of the exhaust gas stream <NUM> passing therethrough. In some embodiments, the exhaust treatment system <NUM> may include a heat exchanger or condenser configured to reduce an amount of water or other fluid vapors in the combustion exhaust gases, for example, minimize or eliminate the formation of condensation trails (i.e., contrails) formed from operation of the gas turbine engine <NUM>. In some embodiments, the exhaust treatment system <NUM> may additionally or alternatively be configured to absorb or capture carbon containing compounds (e.g., carbon dioxide (CO<NUM>)) from the combustion exhaust gases. In some embodiments, the exhaust treatment system <NUM> may additionally or alternatively be configured to reduce the concentration of air pollutants such as, but not limited to, nitrogen oxides (NOx) from the combustion exhaust gases. For example, the exhaust treatment system <NUM> may include a monolithic catalyst structure configured for the treatment of NOx within the combustion exhaust gases. The present disclosure, however, is not limited to any particular form or configuration of exhaust treatment system <NUM> for the diffuser nozzle <NUM>.

Combustion exhaust gases of the exhaust gas stream <NUM> passing through the diffuser nozzle <NUM> are directed through the exhaust treatment system <NUM> where the exhaust gas stream <NUM> is treated. Diffusion of the exhaust gas stream <NUM> within the diffusing axial portion <NUM> of the diffuser nozzle <NUM> from the nozzle inlet <NUM> to the maximum cross-sectional area provided by the treatment axial portion <NUM> provides for an increase in the static pressure of the exhaust gas stream <NUM> and a reduction in velocity of the exhaust gas stream <NUM>, within the treatment axial portion <NUM> of the diffuser nozzle <NUM>. By reducing the velocity of the exhaust gas stream <NUM> within the treatment axial portion <NUM>, the length of time for interaction of the exhaust gas stream <NUM> with the exhaust treatment system <NUM> is increased, thereby improving post-combustion treatment of the exhaust gas stream <NUM>. Moreover, pressure losses of the exhaust gas stream <NUM> passing through the diffuser nozzle <NUM> may be reduced in comparison to at least some conventional exhaust systems. Concentration of the exhaust gas stream <NUM> within the concentrating axial portion <NUM> of the diffuser nozzle <NUM> from the treatment axial portion <NUM> to the nozzle outlet <NUM> provides for a decrease in the static pressure of the exhaust gas stream <NUM> and an increase in velocity of the exhaust gas stream <NUM> which exits the nozzle outlet <NUM> of the diffuser nozzle <NUM>, thereby providing some amount of usable thrust. Accordingly, the configuration of the diffuser nozzle <NUM> may provide a tradeoff whereby an axial length of the diffuser nozzle <NUM> may be decreased and a diameter of the diffuser nozzle <NUM> (e.g., the diameter D1 of the housing <NUM> along the treatment axial portion <NUM>) may be increased, while maintaining the post-combustion treatment capability of the diffuser nozzle <NUM> with respect to the exhaust gas stream <NUM>. The diffuser nozzle <NUM> may, therefore, provide a form factor which can more readily be incorporated into gas turbine engines such as the gas turbine engine <NUM> and, for example, be retained within a nacelle for the respective gas turbine engine.

Referring to <FIG>, in a second embodiment of the present disclosure, the diffuser nozzle <NUM> includes the plurality of duct sections <NUM> and the cross-sectional area of each duct section <NUM> of the plurality of duct sections <NUM> may vary relative to one or more other duct sections <NUM> of the plurality of duct sections <NUM> at one or more axial positions within the diffuser nozzle <NUM>. The diffuser nozzle <NUM> illustrated in <FIG> includes components which are similar to components of the diffuser nozzle <NUM> illustrated in <FIG>. For the sake of brevity, descriptions of these similar components will not be repeated with respect to the diffuser nozzle <NUM> illustrated in <FIG>.

As shown in <FIG>, the treatment axial portion <NUM> of the diffuser nozzle <NUM> may include a plurality of axially-extending duct segments <NUM> of the nozzle duct <NUM>. A number of duct sections in the plurality of duct sections <NUM> is equal to a number of axially-extending duct segments of the plurality of duct segments <NUM>. As shown in <FIG>, the nozzle duct <NUM> includes the first duct section 74A, the second duct section 74B, the third duct section 74C, and the fourth duct section 74D. As shown in <FIG>, the treatment axial portion <NUM> of the diffuser nozzle <NUM> includes a first axially-extending duct segment 88A, a second axially-extending duct segment 88B, a third axially-extending duct segment 88C, and a fourth axially-extending duct segment 88D. However, the present disclosure is not limited to any particular number of duct sections <NUM> of the plurality of duct sections <NUM> or axially-extending duct segments <NUM> of the plurality of axially-extending duct segments <NUM>. Each axially-extending duct segment of the plurality of axially-extending duct segments <NUM> includes the duct cross-sectional area of each duct section <NUM> of the plurality of duct sections <NUM> located axially therein. In other words, each axially-extending duct segment <NUM> of the plurality of axially-extending duct segments <NUM> includes a total cross-sectional area of the nozzle duct <NUM> within each respective axially-extending duct segment <NUM> of the plurality of axially-extending duct segments <NUM>.

Within the nozzle duct <NUM>, the plurality of walls <NUM> may be configured to provide for axially staggered diffusion of each of the duct sections <NUM> of the plurality of duct sections <NUM>. In each axially-extending duct segment <NUM> of the plurality of axially-extending duct segments <NUM>, one duct section <NUM> of the plurality of duct sections <NUM> may have a duct cross-sectional area which is greater than the duct cross-sectional area of each other duct section <NUM> of the plurality of duct sections <NUM>. Each other duct section <NUM> of the plurality of duct sections <NUM> may have duct cross-sectional areas which are substantially equal to one another. In each axially-extending duct segment <NUM> of the plurality of axially-extending duct segments <NUM>, the duct section <NUM> having the greater duct cross-sectional area may be different than the duct section <NUM> having the greater duct cross-sectional area in each other axially-extending duct segment <NUM> of the plurality of axially-extending duct segments <NUM>. In some embodiments, in each axially-extending duct segment <NUM> of the plurality of axially-extending duct segments <NUM>, one duct section <NUM> of the plurality of duct sections <NUM> may have a duct cross-sectional area which is greater than all of the duct cross-sectional areas of each other duct section <NUM> (i.e., the combined duct cross-sectional area of each other duct section <NUM>) of the plurality of duct sections <NUM>. The staggered diffusion configuration of the present disclosure diffuser nozzle of <FIG> may provide a tradeoff whereby a diameter of the diffuser nozzle <NUM> is decreased and an axial length of the diffuser nozzle <NUM> is increased, with respect to the diffuser nozzle <NUM> of <FIG>, while maintaining the post-combustion treatment capability of the diffuser nozzle <NUM> with respect to the exhaust gas stream <NUM>. The diffuser nozzle <NUM> may, therefore, provide a form factor which can more readily be incorporated into gas turbine engines such as the gas turbine engine <NUM> and, for example, be retained within a nacelle for the respective gas turbine engine.

While each of the axially-extending duct segments <NUM> of the plurality of axially-extending duct segments <NUM> are illustrated in <FIG> has having a substantially equal axial length, it should be understood that the present disclosure is not limited to any particular axial lengths for the plurality of axially-extending duct segments <NUM>. In some embodiments, the axial lengths of the plurality of axially-extending duct segments <NUM> may be varied relative to one another, for example, to account for changes in pressure and/or velocity of the exhaust gas stream <NUM> as the exhaust gas stream <NUM> passes through the nozzle duct <NUM> from the nozzle inlet <NUM> to the nozzle outlet <NUM>.

In the first axially-extending duct segment 88A, the first duct section 74A has a duct cross-sectional area that gradually expands, relative to the duct sections 74B, 74C, 74D, to a maximum duct cross-sectional area (see, e.g., <FIG>) which is greater than each of the duct cross-sectional areas of the duct sections 74B, 74C, 74D. The duct cross-sectional area of the first duct section 74A subsequently gradually contracts relative to the duct sections 74B, 74C, 74D within the first axially-extending duct segment 88A. In the second axially-extending duct segment 88B, the second duct section 74B has a duct cross-sectional area that gradually expands, relative to the duct sections 74A, 74C, 74D, to a maximum duct cross-sectional area (see, e.g., <FIG>) which is greater than each of the duct cross-sectional areas of the duct sections 74A, 74C, 74D. The duct cross-sectional area of the second duct section 74B subsequently gradually contracts relative to the duct sections 74A, 74C, 74D within the second axially-extending duct segment 88B. In the third axially-extending duct segment 88C, the third duct section 74C has a duct cross-sectional area that gradually expands, relative to the duct sections 74A, 74B, 74D, to a maximum duct cross-sectional area (see, e.g., <FIG>) which is greater than each of the duct cross-sectional areas of the duct sections 74A, 74B, 74D. The duct cross-sectional area of the third duct section 74C subsequently gradually contracts relative to the duct sections 74A, 74B, 74D within the third axially-extending duct segment 88C. In the fourth axially-extending duct segment 88D, the fourth duct section 74D has a duct cross-sectional area that gradually expands, relative to the duct sections 74A, 74B, 74C, to a maximum duct cross-sectional area (see, e.g., <FIG>) which is greater than each of the duct cross-sectional areas of the duct sections 74A, 74B, 74C. The duct cross-sectional area of the fourth duct section 74D subsequently gradually contracts relative to the duct sections 74A, 74B, 74C within the fourth axially-extending duct segment 88D. The shapes of the duct cross-sectional areas shown in <FIG> are exemplary and the present disclosure is not limited to any particular cross-sectional shape of the plurality of duct sections <NUM> or the nozzle duct <NUM>. Accordingly, exhaust gas sub-streams of the exhaust gas stream <NUM> passing through the duct sections 74A-D are sequentially diffused by diffusing and subsequently concentrating each exhaust gas sub-stream associated with each respective duct section 74A-D.

In some embodiments, each duct section <NUM> of the plurality of duct sections <NUM> may include the exhaust treatment system <NUM> at an axial location which is different than an axial location of the exhaust treatment system <NUM> located in each other duct section <NUM> of the plurality of duct sections <NUM>. Referring again to <FIG>, for example, the first duct section 74A may include the exhaust treatment system <NUM> only within the first axially-extending duct segment 88A (e.g., at a location of the maximum duct cross-sectional area of the first duct section 74A). Similarly, duct sections 74B-D may include the exhaust treatment system <NUM> only within the respective axially-extending duct segments 88B-D. Accordingly, the cross-sectional area of the exhaust treatment system <NUM> may be optimized for each duct section <NUM> of the plurality of duct sections <NUM>. It should be understood, however, that the present disclosure is not limited to any particular axial location for the exhaust treatment system <NUM> within the nozzle duct <NUM> or within the various duct sections <NUM> of the plurality of duct sections <NUM>.

Claim 1:
A diffuser nozzle (<NUM>) for a gas turbine engine (<NUM>), the diffuser nozzle (<NUM>) comprising:
a housing (<NUM>) disposed about a nozzle axis (<NUM>) and extending between a first nozzle end (<NUM>) and a second nozzle end (<NUM>), the housing (<NUM>) comprising a nozzle inlet (<NUM>) located at the first nozzle end (<NUM>) and a nozzle outlet (<NUM>) located at the second nozzle end (<NUM>), the housing (<NUM>) defining a nozzle duct (<NUM>) extending from the nozzle inlet (<NUM>) to the nozzle outlet (<NUM>); and
a plurality of walls (<NUM>) disposed within the nozzle duct (<NUM>), the plurality of walls (<NUM>) subdividing the nozzle duct (<NUM>) into a plurality of duct sections (74A-D) with each duct section of the plurality of duct sections (74A-D) extending from the nozzle inlet (<NUM>) to the nozzle outlet (<NUM>) and having a duct cross-sectional area, the plurality of walls (<NUM>) further defining a plurality of axially-extending duct segments (88A-D) of the nozzle duct (<NUM>),
the diffuser nozzle being characterised in that within a first axially-extending duct segment (88A) of the plurality of axially-extending duct segments (88A-D), the duct cross-sectional area of a first duct section (74A) of the plurality of duct sections (74A-D) is greater than the duct cross-sectional area of each other duct section of the plurality of duct sections (74A-D), and
within a second axially-extending duct segment (88B) of the plurality of axially-extending duct segments (88A-D), the duct cross-sectional area of a second duct section (74B) of the plurality of duct sections (74A-D) is greater than the duct cross-sectional area of each other duct section of the plurality of duct sections (74A-D).