Exhaust nozzle

There is disclosed an exhaust nozzle for a gas turbine engine, the exhaust nozzle comprising a frame extending along a longitudinal axis. The exhaust nozzle comprises a convergent petal pivotably attached at a convergent pivot point to the frame and extending axially downstream and radially inward from the frame, a follower roller fixed to the convergent petal on a radially outer side of the convergent petal, and a cam defining a working surface configured to engage the follower roller to react a force from the convergent petal. The cam is movable along a travel in an axial direction to actuate radial movement of the follower roller to pivot the convergent petal. The cam defines a concave working surface such that a contact angle between the follower roller and the cam varies along the travel to thereby vary a radial component of the force reacted by the cam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This disclosure is based upon and claims the benefit of UK Patent Application No. GB 1915793.2, filed on 31 Oct. 2019, the entire contents which are hereby incorporated herein by reference.

BACKGROUND

Technical Field

The present disclosure relates to an exhaust nozzle for a gas turbine engine, and a gas turbine engine comprising the exhaust nozzle.

Description of the Related Art

Gas turbine engines may use a variable geometry convergent-divergent (con-di) exhaust nozzle to maximise the production of thrust. A typical exhaust nozzle comprises a plurality of convergent petals which can be pivoted to converge, to reduce the size of an area for air flow exhausting from the engine.

SUMMARY

According to a first aspect of the disclosure, there is provided an exhaust nozzle for a gas turbine engine, the exhaust nozzle comprising a frame extending along a longitudinal axis and the exhaust nozzle comprising: a convergent petal pivotably attached at a convergent pivot point to the frame and extending axially downstream and radially inward from the frame; a follower roller fixed to the convergent petal on a radially outer side of the convergent petal; and a cam defining a working surface configured to engage the follower roller to react a force from the petal; wherein the cam is movable along a travel in an axial direction to actuate radial movement of the follower roller to pivot the convergent petal, whereby the cam defines a concave working surface such that a contact angle between the follower roller and the cam varies along the travel to thereby vary a radial component of the force reacted by the cam.

A concave cam is intended to mean a cam having a curved profile, where the centre of curvature is radially inward in the nozzle from the cam.

The exhaust nozzle may comprise a divergent petal pivotably attached at a divergent pivot point to a downstream end of the convergent petal, the divergent petal extending axially downstream and radially outward from the divergent pivot point.

The divergent petal may be connected to the frame by a linkage such that the frame, convergent petal, divergent petal and linkage form a four-bar linkage.

The linkage may be a thrust linkage which is actuatable to change length.

The convergent petal may define a chord length from the convergent pivot point to the divergent pivot point. The roller may be fixed between 40-80% along the chord length of the convergent petal from the convergent pivot point.

The cam may be moveable between a contracted position in which the cam is in a furthest upstream position and an expanded position in which the cam is in a furthest downstream position. The cam may be configured so that a contact angle between the cam and the follower roller in the expanded position is between 80-100 degrees from the longitudinal axis, preferably between 85-95 degrees from the longitudinal axis.

A ratio of a radius of the follower roller to an average radius of curvature of the cam may be 0.05 or above. A ratio of a radius of the follower roller to a maximum radius of curvature of the cam may be 0.2 or below.

The exhaust nozzle may comprise a plurality of convergent petals angularly distributed around the exhaust nozzle, each comprising respective rollers, and the exhaust nozzle comprising a corresponding plurality of cams circumferentially spaced around the exhaust nozzle and configured to maintain engagement with a respective roller.

The cam may be fixed to a unison ring. Axial movement of the cam may be actuated by axial movement of the unison ring.

There may be a plurality of divergent petals corresponding to the plurality of convergent petals.

According to a second aspect of the disclosure, there is provided a gas turbine engine comprising an exhaust nozzle in accordance with the first aspect.

DETAILED DESCRIPTION

With reference toFIG. 1, a gas turbine engine is generally indicated at10, having a principal and rotational axis11. The engine10comprises, in axial flow series, an air intake12, a propulsive fan13, an intermediate pressure compressor14, a high-pressure compressor15, combustion equipment16, a high-pressure turbine17, an intermediate pressure turbine18, a low-pressure turbine19and an exhaust nozzle20. A nacelle21generally surrounds the engine10and defines both the intake12and the exhaust nozzle20.

The compressed air exhausted from the high-pressure compressor15is directed into the combustion equipment16where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines17,18,19before being exhausted through the nozzle20to provide additional propulsive thrust. The high17, intermediate18and low19pressure turbines drive respectively the high-pressure compressor15, intermediate pressure compressor14and fan13, each by suitable interconnecting shaft.

FIG. 2shows an example exhaust nozzle20having a variable converging-diverging (con-di) exhaust in a contracted configuration. In other examples, the exhaust nozzle may only have a variable converging exhaust, without a divergent section.

The nozzle20comprises a radially outer support frame30extending from an upstream end (left side in the Figures) to a downstream end (right side in the Figures) along a longitudinal axis50, which is coaxial with the rotational axis11of the gas turbine engine10described with reference toFIG. 1, when the nozzle20is mounted in the gas turbine engine10. The frame30comprises an annular portion30awhich is coaxial with the longitudinal axis50and which converges from the upstream end to the downstream end (i.e. the radius of the annular portion30areduces in a downstream direction along the longitudinal axis50).

The nozzle20comprises a plurality of convergent petals32which are angularly distributed about the longitudinal axis50within the nozzle20at an upstream end of the nozzle20, and a corresponding plurality of divergent petals34which are angularly distributed about the longitudinal axis50within the nozzle20downstream of (and connected to) the plurality of convergent petals32. The plurality of convergent petals32and divergent petals34are configured to provide a converging and then diverging cross-sectional area for air flow exhausting from the gas turbine engine10, for example to choke the flow and achieve supersonic exit velocities in the divergent section.

The extent of convergence of the convergent petal32and the extent of divergence of the divergent petal34is variable, as will be explained in detail below. In the contracted configuration, the convergent petal32is in a contracted position in which it is at a maximum convergence (i.e. it reduces the air flow area to a minimum along the longitudinal axis50). In this example, the convergent petal32in the contracted position is angled radially inwardly at an angle of approximately 40 degrees with respect to the longitudinal axis50(with the frame of reference being such that 0 degrees would correspond to the convergent petal32being parallel with the longitudinal axis50). In other examples, the angle of the convergent petal with respect to the longitudinal axis in the contracted position may be less than 40 degrees, such as 35 degrees or 30 degrees.

The configuration of each convergent petal32and respective divergent petal34is identical, and as such it will be described below with respect to a single convergent petal32and respective divergent petal34.

The frame30comprises a first extension30bextending radially inwards from the annular portion30aof the frame30at an upstream end of the frame30. The convergent petal32is pivotably attached to the first extension30bat a convergent pivot point36. The convergent petal32extends axially downstream and radially inwardly from the frame30and from the convergent pivot point36, in the contracted position.

The divergent petal34is pivotably attached to a downstream end of the convergent petal32, at a divergent pivot point38. The divergent petal34is pivotably attached to a linkage40at a point on the divergent petal34downstream of the divergent pivot point38. The linkage40is pivotably attached to a second extension30cwhich extends radially inwardly from the annular portion30aof the frame30, at a location on the annular portion30adownstream of the first extension30b. The divergent petal34is connected to the linkage40and convergent petal32such that it extends axially downstream and radially outwardly form the divergent pivot point38.

The frame30, convergent petal32, divergent petal34and linkage40therefore form a four-bar linkage, such that pivoting movement of the convergent petal32induces predictable pivoting movement of the divergent petal34. Whilst all members of the four-bar linkage are of constant length, the four-bar linkage is said to have one degree of freedom (i.e. such that for each angular position of the convergent petal32there is a single corresponding angular position of the divergent petal34).

The linkage40in this example is a thrust linkage comprising a telescopic extension. The thrust linkage40is actuatable to change in length, so that the pivoting movement of the divergent petal34in response to pivoting movement of the convergent petal32can be adjusted, thereby providing a second degree of freedom in the four-bar linkage.

The nozzle20further comprises a unison ring44disposed radially outwardly of the convergent pivot point36. The unison ring44is annular and extends around the longitudinal axis50within the annular portion30aof the frame30. A cam46is fixedly attached to the unison ring44and extends axially downstream and radially inwards from the unison ring44, such that it is disposed radially outwardly from the convergent petal32. Since the cam46is fixedly attached to the unison ring44, it cannot translate or rotate with respect to the unison ring44. The cam46defines a curved, concave working surface48on a radially inner side of the cam46(i.e. the radius of curvature of the cam46is located radially inward of the cam46within the nozzle20, with respect to the longitudinal axis50).

A follower roller42is fixed to the convergent petal32on a radially outer side of the convergent petal32(i.e. with respect to the longitudinal axis50). The working surface48of the cam46is configured to engage the follower roller42, such that contact is maintained between the cam46and the follower roller42. It will be appreciated that in use, aerodynamic forces act on the convergent petal32so that the follower roller42is urged against the cam46, as explained in further detail below.

The convergent petal32defines a chord length along the convergent petal32from the convergent pivot point36to the divergent pivot point38. The follower roller42in this example is fixed at 50% of the chord length from the convergent pivot point36. In other examples, the follower roller may be fixed to the convergent petal at any suitable location, for example between 40-80% of the chord length from the convergent pivot point.

During use, air flow through the exhaust nozzle is directed radially inward by the convergent petal32, and permitted to flow radially outward past the divergent petal34. Therefore, the air flow exerts a force on the convergent petal32as it is directed radially inward. This force is transferred through the follower roller42to the cam46which reacts the force from the convergent petal32.

The cam46is moveable along a travel in an axial direction. Movement of the cam46in the axial direction actuates radial movement of the follower roller42, because the cam46and follower roller42are engaged. Radial movement of the follower roller42pivots the convergent petal32about the convergent pivot point36. Therefore, the pivoting angle of the convergent petal32with respect to the longitudinal axis50can be controlled by axial movement of the cam46.

As explained above, the pivoting of the convergent petal32induces pivoting of the divergent petal34about the divergent pivot point38due to the four-bar linkage arrangement.

In this example, there are a plurality of cams46such that there is a cam46for every convergent petal32around the nozzle20. In other examples, there may be fewer cams, such as one cam for every two convergent petals. The convergent petals for which there is no corresponding cam may then be coupled to an adjacent convergent petal which does have a corresponding cam, such that pivoting movement of the adjacent convergent petal induces identical movement of the convergent petal without a corresponding cam.

The unison ring44, which is fixedly attached to the cam46, is moveable in an axial direction. Around the circumference of the nozzle20, the plurality of cams46are fixed to the unison ring44such that axial movement of the unison ring actuates corresponding axial movement of each of the plurality of cams46. Therefore, each of the plurality of convergent petals32can be actuated to pivot by a corresponding amount by axially moving the unison ring44, and the amounts may be substantially identical with sufficient control of manufacturing tolerances.

Axial movement of the unison ring44is controlled by a plurality of actuators52. In this example, there are four actuators distributed around and within the nozzle20(only two are shown). In other examples, there may be any suitable number of actuators distributed around the nozzle to move the unison ring axially. The actuators52in this example are in the form of telescopic cylinders, which can move axially between a fully retracted position, and a fully extended position.

As explained above,FIG. 2shows the nozzle20in a contracted configuration. In the contracted configuration, the actuator52is in a fully retracted position, such that the unison ring44and cam46are in a furthest upstream position. A downstream part of the cam46is therefore engaged, and in contact with the follower roller42, such that the convergent petal32is in the contracted position. It will be appreciated that the total force exerted on the convergent petal32will tend to be largest in use (for given turbine exit flow conditions) when the convergent petal32is in the contracted position, because it is at a maximum convergence corresponding to a maximum change in direction of the air flow through the nozzle20.

FIG. 3shows the example exhaust nozzle20ofFIG. 2in an expanded configuration. In the expanded configuration, the convergent petal32is in an expanded position, in which it is pivoted furthest radially outward. The convergent petal32in the expanded position is angled radially inwardly at an angle of approximately 10 degrees with respect to the longitudinal axis50. In other examples, the angle of the convergent petal with respect to the longitudinal axis in the expanded position may be more than 10 degrees, such as at least 15 degrees or at least 20 degrees.

In the expanded configuration, the actuator52is fully extended so that the unison ring44and cam46are in a furthest downstream position. An upstream part of the cam46is therefore engaged, and in contact with the follower roller42, such that the convergent petal32is in the expanded position.

Axial movement of the cam46is therefore controlled by extension and retraction of the actuators52, and the cam46is moveable along a travel between the contracted configuration (FIG. 2) and the expanded configuration (FIG. 3), in which the cam46is in a respective contracted position and expanded position.

The concave working surface48of the cam46ensures that a contact angle between the follower roller42and the cam46varies along the travel to thereby vary a radial component of the force from the convergent petal32reacted by the cam46.

Referring back toFIG. 2, a contact angle of a contact line54between the cam46and the follower roller42in the contracted configuration is approximately 45 degrees with respect to the longitudinal axis50. This is lower than in previously considered systems due to the concave profile of the working surface48of the cam46. This reduces the radial load transferred to the cam46and the unison ring44in the contracted configuration compared to previously considered systems.

Since the convergent petal32experiences the highest forces in the contracted position, the lower contact angle between the cam46and follower roller42in the contracted configuration reduces the maximum radial load transferred to the cam46, and to the unison ring44in use. This enables a weight saving, as the cams46and the unison ring44can be made more lightweight. This is particularly advantageous in examples in which there is no unison ring, for example for a 2D non-axisymmetric nozzle, as the radial load cannot be constrained in a hoop continuous component, such that the radial loads must be reduced for the same weight nozzle.

As can be seen inFIG. 3, the concave working surface48of the cam46is configured so that a contact angle of a contact line56between the cam46and the follower roller42in the expanded position is approximately 90 degrees with respect to the longitudinal axis50. In other examples, the contact angle in the expanded position may be between 80 to 100 degrees. This ensures that the axial load which is transferred to the cam46, and therefore to the unison ring44from the convergent petal32, is low or tends to zero in the expanded configuration.

Further, fixing the follower roller42to the convergent petal32between 40-80% along the chord length of the convergent petal32reduces the radial load on the unison ring44in the convergent position because a moment arm between the convergent pivot point36and the contact point between the cam46and the follower roller42is increased compared with previously considered arrangements. A moment about the convergent pivot point36from the pressure of the gas on the convergent petal32must be reacted by the cam46at the contact point between the cam46and the follower roller42. The reacting moment of the cam46is therefore achieved with a lower reaction force if the moment arm is increased, and the reacting moment of the cam46is achieved with a higher reaction force if the moment arm is reduced.

The applicant has found that mounting the follower roller42at a position less than 40% along the chord length of the convergent petal32from the convergent pivot point36results in a rapid increase in radial load on the unison ring44and actuator52in the contracted configuration due to the decreasing moment arm. In contrast, although the radial load reduces as the distance of mounting the follower roller42from the convergent pivot point36increases, the applicant has found that mounting it further than 80% along the chord length from the convergent pivot point results in clashes between the follower roller and surrounding components in use.

In this example, a radial ratio of a radius of the follower roller to the radius of curvature of the cam is approximately 0.1. In other examples, the ratio may be between 0.05 and 0.2. The applicant has found that a ratio over 0.2 results in a rapidly decreasing contact angle between the cam46and the follower roller42with respect to the longitudinal axis, such that the axial load transferred to the unison ring44and actuators52rapidly increases towards the contracted configuration. The applicant has also found that a ratio of below 0.05 results in the cam46being too long in a typical nozzle such that it would collide with the divergent petal34when moving towards the expanded configuration. The radius of curvature in this example is the radius of curvature in a plane intersecting the longitudinal axis50, which may be the engine centreline axis11.

In some examples, the radius of curvature of the cam may not be constant. The radius of curvature used in such examples to calculate the comparable radial ratio is the average radius of curvature along curved portions of the cam for lower limit of 0.05 and the maximum radius of curvature along curved portions of the cam for the upper limit of 0.2.

Although it has been described that the frame comprises discrete radially inward extending extensions to which the convergent petal and linkage are coupled, the extensions may be in the form of annular extensions, which are continuous around the circumference of the nozzle and frame, or any suitable support structure.