GB2094892A - Mixed exhaust and bypass turbofan aeroengine - Google Patents

Abstract

A turbofan aeroengine 1 is of the type in which the turbine exhaust stream 23 and the bypass exhaust stream 17 are mixed together within the engine before issuing from the propulsion nozzle 15 as a propulsive jet 25. Streams 17 and 23 are brought together for mixing at the downstream end of a mixer ring 13, stream 17 having a swirl velocity which is opposed to that of stream 23 and an axial velocity which differs in magnitude from that of stream 23. This produces circumferential and axial shear between streams 17 and 23 resulting in the formation of stream-mixing vortices which originate at or near the trailing edge of the mixer ring 13. The mixer ring can be modified in various ways to facilitate control of the vorticity. <IMAGE>

Description

SPECIFICATION Bypass gas turbine engines The present invention relates to turbofan or bypass gas turbine aeroengines of the type in which the turbine exhaust stream is made confluent with the bypass air stream in a manner conducive to mixing before the combined exhaust stream passes through the engine's final propulsion nozzle.

In the past, mixing of bypass and turbine exhaust flows within bypass gas turbine aeroengines has been accomplished using the well-known multi-lobed and multi-chuted types of exhaust mixers - see for example U.S. Patent Number 3,655,009 and p. 52 of our publication "The Jet Engine", 3rd Edition. Such mixers are used in order to produce a more uniform velocity and temperature across the diameter of the combined jet efflux. Efficient mixing of the bypass and turbine exhaust flows can lead to an increase in thrust and hence a decrease in specific fuel consumption of the engine. It is also desirable for aerodynamic and noise control reasons. Clearly, it is desirable to obtain a high degree of mixing efficiency if possible.

Unfortunately, incorporation of prior art mixers in bypass gas turbine aeroengines, whilst increasing efficiency, has also increased weight and manufacturing cost, and it is therefore desirable to find an efficient means of mixing the exhaust flows which also enables weight and cost to be reduced. This is accomplished in the invention by utilising the properties of swirling fluid flow.

Accordingly, in a turbofan aeroengine in which the turbine exhaust stream and the bypass exhaust stream are mixed together within the engine before issuing therefrom as a propulsive jet, the invention in essence comprises the concept of bringing the two streams together for mixing as coaxial streams both of which have swirl and axial velocity vectors, the swirl velocity vectors of the two streams being opposed to each other and the axial velocity vectors of the two streams being different in magnitude, whereby circumferential and axial shear occurs between the two streams and said shear results in the formation of stream-mixing vortices. Gases from both streams are thereby entrained and mixed together without the need for any flow-deflecting hardware other than guide vanes or blades configured to put the desired degree of swirl into the two streams.

The invention may be described in terms of a turbofan aeroengine having a turbine exhaust duct, a bypass duct which surrounds the turbine exhaust duct and is coaxial therewith, an exhaust stream mixing duct for receiving from the turbine exhaust duct and the bypass duct the turbine exhaust stream and the bypass exhaust stream respectively, a propulsion nozzle which defines the downstream end of the mixing duct, a mixer ring which defines the downstream end of the turbine exhaust duct and the downstream end of the radially inner wall of the bypass duct, and means for imparting differing swirl and axial velocity vectors to the two streams before they pass the mixer ring, the swirl velocity vectors of the two streams being opposed to each other and the axial velocity vectors of the two streams being different in magnitude, whereby when the two streams contact each other at the downstream edge of the mixer ring, their differing velocity vectors give rise to circumferential and axial shear between the two streams and said shear results in the formation of stream-mixing vortices extending downstream of the mixer ring.

In one embodiment of the invention, the mixer ring has a plain trailing edge, the abovementioned shear between the two streams thereby resulting in the formation of funnelshaped trailing vortices which depend from said trailing edge. In order to stabilise the vortices and prevent possible migration of the vortices around the trailing edge, further embodiments of the invention provide the trailing edge with a plurality of discontinuities equi-angularly spaced around the circumference of the trailing edge, which discontinuities act to initiate vortex formation in one or both of the streams so that the shear between the two streams results in the formation of funnel-shaped trailing vortices which depend from said discontinuities instead of other portions of the trailing edge.

The discontinuities may, for example, comprise fins which project radially from the trailing edge of the mixer ring into at least one of the streams and which are aligned in the axial direction.

Alternatively, the discontinuities may comprise tooth-like features which project axially from the trailing edge of the mixer ring. As a further alternative, the discontinuities may comprise streamlined excrescences which project axially from the trailing edge of the mixer ring.

In order to minimise thrust losses at the propulsion nozzle, it is preferred that immediately before mixing of the streams, the swirl momentum of the bypass exhaust stream and the swirl momentum of the turbine exhaust stream are substantially equal in magnitude but opposed in direction under predetermined operating conditions of the engine, whereby the gases at the downstream end of the mixing duct have no substantial net swirl momentum.

In order to produce the mutually opposed swirl velocity vectors in the turbine exhaust stream and the bypass exhaust stream, the turbofan aeroengine can be provided with suitably configured turbine outlet guide vanes and fan outlet guide vanes. However, it may be more convenient to omit turbine outlet guide vanes and produce the desired swirl velocity vectors by suitably configured fan outlet guide vanes and last stage turbine blades. In order to reduce aerodynamic losses in the bypass duct, it may be advantageous to provide the engine with fan outlet guide vanes configured to substantially remove swirl from the bypass stream, and bypass outlet guide vanes at the downstream end of the bypass duct, these latter vanes being configured to re-impart swirl to the bypass exhaust stream before the commencement of the mixing process.

Specific embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which Figure 1 is a diagrammatic part-sectional side elevation of a high bypass ratio turbofan aeroengine with part of the nacelle and the engine core casing cut away to reveal internal details, the turbine exhaust and bypass air streams being mixed within the engine in accordance with the present invention; Figure 2 is a prior art diagrammatic representation of the development of vorticity in the shear layer of a circular air jet exhausting into ambient air; Figures 3a to 3d are diagrammatic representations of the mixing process within the engine of Figure 1; and Figures 4a to 4dare perspective views of various alternative configurations for a mixer ring according to the present invention.

The drawings are not to scale.

Referring first to Figure 1, a high bypass ratio gas turbine engine 1, otherwise known as a turbofan, has an engine core 3 carrying front fan 19, a bypass duct 5 defined between the engine core 3 and the surrounding bypass duct casing/nacelle 7, and an exhaust system 9 including an exhaust bullet 11, an exhaust flow mixer ring 1 3 (which forms the rear end of the engine core 3), a mixing duct 14 and a final propulsion nozzle 1 5. The engine core 3 is supported within the bypass duct 5 by fan outlet guide vanes 20 and streamlined struts 6. The bypass air stream 1 7 is supplied from front fan 19, which also supplies air to the compressor (not shown) of the engine core 3, the fan 19 being driven from turbine 21 in the engine core. Turbine 21 has turbine outlet guide vanes 8.Downstream of the mixer ring 13 the bypass air stream 17 is made confluent with the turbine exhaust stream 23 in the mixing duct 14 to produce at least a partially mixed exhaust flow 25, which passes to atmosphere through propulsion nozzle 1 5.

In the engine 1, the bypass stream 17 is a low temperature, low velocity flow, whilst the turbine exhaust stream 23 is a high temperature, high velocity flow.

By mixing streams 17 and 23 together, a significant benefit is realised in terms of an increase in thrust at the propulsion nozzle relative to an unmixed jet. It can be thermodynamically proved that the sum of the thrusts available from a hot high velocity turbine exhaust stream surrounded by a cool low velocity bypass stream is less than the thrust available from a homogeneous jet resulting from thorough mixing of the two streams before exit from the propulsion nozzle.

Since greater thrust is being produced per unit weight of fuel burnt, efficient mixing in this way increases the fuel economy of the engine. In practice a number of adjustments, familiar to those skilled in the art, would be made to match the engine cycle to the change in the arrangement of the propelling jets. However, the most thorough mixing possible, consistent with low pressure loss and the least possible increase in the engine's size and weight, is still fundamental to this engine arrangement.

Were the two streams 1 7 and 23 to be allowed to issue from propulsion nozzle 1 5 without first being forcibly mixed internally of the engine, mixing would proceed naturally for a considerable number of nozzle diameters downstream of the engine, the velocity and temperature disparity between the turbine exhaust stream 23 and the surrounding bypass stream 1 7 causing a significant amount of noise throughout the mixing zone. It is anticipated that thorough mixing of the two streams within the engine 1 will ensure that by the time the combined exhaust flow exits from propulsion nozzle 15, the noisiest part of the mixing process will have been accomplished and the efflux will be approaching homogeneity.Note that internal mixing allows absorption of mixing noise as it arises by means of sound absorbing linings (not shown) e.g. on the duct wall 22.

Note that in the engine shown in Figure 1, mixer ring 13 takes the form of a circular nozzle on the rear of engine core 3, having a plain trailing edge which does not significantly disturb streams 1 7 and 23 as they flow over it. This gives minimum aerodynamic drag losses. It is apparent that substantial mixing of the two streams 1 7 and 23 can only proceed if turbulence occurs at the interface between the streams downstream of the mixer ring 13; such turbulence is promoted by velocity differentials between the streams, which produce a shearing action at their interface. The turbulence produced by shearing action is vortical.

Note that the bypass stream has a lower axial velocity than the turbine exhaust stream, axial shear between the streams arising as a result of this. In itself this axial shear is inadequate to produce thorough mixing in a short length of mixing duct, and in the prior art this fact led to the use of flow deflecting devices such as multi-lobed or multi-chuted mixers, to cause the streams to interpenetrate each other more vigorously.

However, in Figure 1, efficient mixing -- with its attendant benefits as mentioned above - is achieved not by physically intercepting and deflecting the streams, but by ensuring that considerable circumferential shear is present at the interface between the streams, in addition to axial shear. This gives a desirable type of streammixing vortical interaction between the two streams, as explained later.

The circumferential shear is produced and maximised by ensuring that the turbine exhaust stream 23 and the bypass air stream 1 7 each have a significant swirl component of velocity as they enter the mixing region downstream of mixer ring 13, the swirl in the bypass air stream being of opposite hand (i.e. opposite sense of rotation) to the swirl in the turbine exhaust stream.

The development of vorticity due to shear in the shear layer at the interface between two coflowing streams moving at different velocities has been, and continues to be, the subject of considerable research. Lord Rayleigh investigated instability at the interface (otherwise called the surface of discontinuity) and noted how small wave-like disturbances increase in amplitude and become vortices which disarrange streamline patterns -- see, for example, "Fundamentals of Hydro- and Aerodynamics" by 0. G. Tietjens published in 1934 by McGraw-Hill, pp. 222, 223.

More recent examples of research are contained in the following two papers: C. D. Winant and F. K. Browsand "Vortex pairing: the mechanism of turbulent mixing-layer growth at moderate Reynolds number", J. Fluid Mech (1974), vol. 63, part 2, pp. 237-255.

C. J. Moore "The role of shear-layer instability waves in jet exhaust noise", J. Fluid Mech (1977), vol. 80, part 2, pp. 321-367.

The former reference describes vorticity phenomena seen in an initially plane shear layer, and the latter reference describes vorticity phenomena in the initially cylindrical shear layer between a circular air jet and ambient air into which the jet exhausts. Figures 10 and 11 of the latter reference describe how the jet shear layer develops, Figure 10 being reproduced as Figure 2 of the present specification. In Figure 2, the air jet exhausts from a plain circular nozzle 29 and immediately starts to mix with ambient air. A divergent shear layer 31 forms, which comprises a mixture of jet air and ambient air and is the transition between the unmixed core 33 of the jet and ambient air. The vortices formed during the mixing process are due to the shear between the jet and the still air.Because the only substantial shear present is in the direction of the major axis 30 of the jet, the vortices are toroidal about the major axis. The stages of vortex development in Figure 2 are indicated as: (a) shear layer oscillates; (b) ambient air becomes entrained; (c) vortices form; (d) vortices form pairs and so increase axial spacing. Although toroidal, the vortices are indicated schematically on one side of the jet only as sections through the toroids.

In the present invention, both circumferential and axial shear are present at the transition or interface between the two streams 1 7 and 23, due to the presence of both swirl and axial components of velocity in both streams as already mentioned. In Figure 1 this leads to the vortices produced in the shear layer between the streams being funnel-shaped instead of toroidal, the streamlines within the funnels being helical in the manner of a corkscrew.

The process of vortex formation in mixing duct 14 of Figure 1 will now be described with reference to diagrammatic Figures 3a to 3d.

Figure 3a shows in simplified diagrammatic form the exhaust system 9 of Figure 1. Stream velocity vectors 7 and23 indicate the flow of the bypass airstream and the turbine exhaust stream respectively as they pass over the rear edge of mixer ring 13, these vectors being the resultants of the axial and swirl components of velocity of the two streams. Figure 3b is a view on section line B-B in Figure 3a, illustrating in two dimensions the development of vortices at the notional interface 35, i.e. in the shear layer, between streams 1 7 and 23 which have opposed swirl velocities.Figure 3c is a vector diagram showing the relationship, at any point on interface 35 near mixer ring 13, between: the axial and swirl components of velocity of each of the streams 17 and 23, namely 7,VA23 and V,17, V523 respectively: the resultant stream velocities V1 7 and V23; and the axis of vorticity, which is the line along which the vortex is centred. The axis of vorticity is at right angles to the vector V23-V1 7. Finally, Figure 3d shows a threedimentional sketch view of the vorticity illustrated two-dimensionally in Figure 3b.It will be seen that the vortices are funnel-shaped, with their apices on the downstream edge of mixer ring 13, i.e. they are trailing vortices, starting where the two streams 1 7 and 23 first have an interface with each other. The angle of the axis of vorticity relative to the axial direction defines a helix angle for the vortices, which are helical about the centre line 16 of the mixer ring 13.

Assuming circumferentially uniform conditions of flow within streams 1 7 and 23 before they pass mixer ring 13, the vortices will be approximately equi-angularly spaced around the downstream edge of the mixer ring. As they move downstream the vortices entrain more of the streams 1 7 and 23 and expand so that further down the mixing duct 14 (Figure 1) they become sufficiently large in diameter to interfere with each other, thereby tending to cancel out each other's angular momentum and substantially entirely fill the rear portion of the mixing duct with eddies which complete the mixing process.If the angular momentums of the streams 17 and 23 immediately before mixing are the same in magnitude but opposed in direction, the eddies cancel each other out, leaving no net swirl in the final mixed flow 25 as it passes through propulsion nozzle 1 5.

In Figure 3d, for the sake of clarity in the illustration, the vortices are shown as funnels with clearly defined boundaries and a fairly small angle of divergence. In practice the boundaries of the vortices will be shear layers and their angle of divergence will be greater. Interference between the vortices and consequent mixing across the width of mixing duct 14 will thus actually begin only a short distance downstream of the mixer ring 13.

Counterswirling of the two streams 1 7 and 23 as described therefore not only ensures that large amounts of shear can be generated at their interface, but also that the opposing swirl velocities can be used to cancel or partly cancel each other out, thus avoiding thrust losses due to large swirl velocities in the mixed exhaust flow through propulsion nozzle 1 5. Counterswirling of both streams is thus better than swirling only one of the streams.

To illustrate this point consider the following two cases:- (i) both streams have swirl, but in opposite senses to each other (as above); (ii) only the turbine exhaust stream 23 has swirl, the bypass airstream having no substantial swirl component of velocity.

Plainly, for (ii) to achieve the same amount of shear as (i) at the interface between the streams, the swirl component of velocity of the turbine exhaust stream in (ii) would have to be substantially higher than it would need to be in (i).

Furthermore, in (ii) the swirl component of the turbine exhaust stream would not be completely or partially cancelled by a swirl component of the bypass air stream, so that all the swirl would remain throughout the mixing process and result in a larger thrust loss than in case (i).

In respect of the turbine exhaust stream only, a further benefit of ensuring that it has a swirl component of velocity is seen in the fact that the swirl increases the rate of radial spread of the stream relative to an unswirlej state, thereby minimising the axial length of the exhaust system 9 required for complete mixing.

Since it is desirable to minimise thrust losses by avoiding swirling flow in propulsion nozzle 15, it is preferred that at the important engine operating conditions, such as cruise, it is arranged that the swirl velocities of the two streams are such as to ensure that the mixed exhaust flow has substantially no net swirl. In the high bypass ratio turbofan sketched in Figure 1 , the mass flow of the bypass stream 1 7 will be greater than that of the turbine exhaust stream 23. Hence, to achieve no substantial net swirl in propulsion nozzle 15, the swirl component of velocity of stream 1 7 will be sufficiently less than that of stream 23 to ensure substantially complete swirl momentum cancellation between the streams.

Dealing now in more detail with the design changes in existing types of turbofan engines necessary to put the invention into practice, it is first noted that it is common practice to leave some swirl in the turbine exhaust stream in order to optimise blade velocity triangles, which swirl is normally removed by appropriately configured turbine outlet guide vanes to minimise thrust losses, Similarly, the fan puts swirl into the bypass air stream, and this swirl is normally removed by fan outlet guide vanes to minimise thrust loss and to avoid swirl-generated losses in the bypass duct downstream of the fan outlet guide vanes.Under normal running conditions of such engines, assuming that the fan rotates in the same direction as the last turbine stage, the swirl in the bypass stream is of opposite hand to the swirl in the turbine exhaust stream, but the utility of this fact for purposes of mixing the streams has not hitherto been appreciated. However, in engine 1 of Figure 1, these opposing swirls are not eliminated before mixing occurs. Instead, the correct swirl characteristics for efficient mixing in exhaust system 9 are achieved by specially configuring the turbine outlet guide vanes 22 and the fan outlet guide vanes 20 to leave some swirl in both streams; i.e. their pressure surfaces are less concave, and their suction surfaces less convex, than required for complete removal of the swirl components of the respective streams.

The exact shape of the respective guide vanes will depend upon the characteristics of the particular turbofan engine being considered, such as bypass ratio, stream velicities, mass flow and the dimensions of the exhaust system, since all these will have an effect on the swirl velocities required for efficient mixing before exit of the combined exhaust through propulsion nozzle 1 5.

These considerations are within the scope of the competent designer.

For some turbofans, it might be possible to omit the fan and turbine outlet guide vanes altogether, leaving the swirl velocities in the bypass and turbine exhaust streams unchanged from their values on exit from the fan or turbine blades, apart from losses due to aerodynamic drag in the engine ducts or other causes. Omission of these guide vanes as such would still leave a requirement to carry support structure and such things as accessory drives across the bypass duct or the turbine gas passage, a requiremeni which has hitherto been partly met by using guide vanes as support struts orfairings.However, this requirement can easily be met without guide vanes by employing purpose-designed struts or fairings instead, which can be much more widely spaced than is necessary for guide vanes and need only a streamlined shape suitably aligned with the swirling flows, not a flow turning shape; this leads to weight and cost savings.

It is preferable for the swirl in the bypass duct to be of a low value in order to minimise swirlgenerated losses in the bypass duct and to avoid the need for the bypass duct to be radially deepened to cope with greater streamline Mach No. caused by greater swirl velocities. For high bypass ratio turbofans, this preference is in accord with the earlier stated preference for zero net swirl in the propulsion nozzle, achieved by low swirl velocities in the high mass flow bypass stream, and higher swirl velocity in the lower mass flow turbine exhaust stream.

The available evidence shows that mixing of two coaxial streams will be enhanced if the outer stream has less angular momentum than the inner stream, or if the outer stream is less dense than the inner stream. The latter condition cannot be met by turbofan arrangements in which the outer stream comprises cool bypess air and the inner stream comprises hot turbine gases, but the former condition is met.

One conclusion from these considerations is that an optimum design solution for a high bypass turbofan could feature fan outlet guide vanes with reduced flow turning ability relative to current designs, and the complete omission of turbine outlet guide vanes, i.e. the turbine exhaust passage is unobstructed by a flow-turning hardware downstream of the last turbine blade stage.

Alternatively, the swirl in the bypass stream could be completely removed by fan outlet guide vanes to avoid swirl-generated losses in the bypass duct, swirl being put back into the bypass stream at the outlet from the bypass duct by an array of bypass outlet guide vanes replacing struts 6 in Figure 1.

Referring now to Figure 4a, plain circular mixer ring 13 (see also Figure 1) is .lso illustrated as a separate component. A possible problem resulting from the use of such a mixer ring would be migration of the trailing vortices around the circumference of the trailing edge, causing vibration or unsteady flow problems. Such migration of the vortices would occur, for example, if the flow conditions in the mixing duct were theoretically insufficient to support an integer number of vortices. In practice, an integer number of vortices would be formed, but the vortices would circulate clockwise or anticlockwise around the duct under the influence of pressure gradients resulting from waves in the shear layer corresponding to the notional "fractional" vortex.

In order to overcome the problem of vortex migration, the trailing edge portion of the mixer ring may be provided with equi-angularly spaced discontinuities around its circumference which disturb the flow over them and in conjunction with the shearing action between streams 1 7 and 23 initiate vortex formation so that each discontinuity propagates a trailing vortex. Each discontinuity "captures" the trailing vortex it produces and prevents it migrating circumferentially, so that the number and spacing of the vortices is the same as the number and spacing of the discontinuities.

For example, the discontinuities may take the form of radially projecting fins 41 as Figure 4b, or of axially projecting teeth 43 as in Figure 4c, or of axially projecting streamlined excrescences, such as the "tear-drop" or "carrot"-shaped bodies 45 as in Figure 4d.

The large stream-mixing trailing vortices attached to the mixer rings of Figures 4a to 4d are maintained as long as there is sufficient swirl velocity in the streams 17 and 23 to energise them.

Although in the above description it has been assumed that the guide vanes associated with the turbine and bypass exhaust streams are fixed in position, it would clearly be possible to provide the streams with guide vanes which are pivotable about radially extending axes so as to control the amount of swirl which they impart to the streams, the better to take account of varying engine conditions.

Claims (11)

1. A turbofan aeroengine in which the turbine exhaust stream and the bypass exhaust stream are mixed together within the engine before issuing therefrom as a propulsive jet, the engine having means for bringing said two exhaust streams together for mixing as coaxial streams both of which have swirl and axial velocity vectors, the swirl velocity vectors of said two streams being opposed to each other and the axial velocity vectors of the two streams being different in magnitude, whereby circumferential and axial shear occurs between said two exhaust streams and said shear results in the formation of streammixing vortices.

2. A turbofan aeroengine according to claim 1 in which the two exhaust streams are brought together for mixing at the trailing edge of a mixer ring at which the stream-mixing vortices originate.

3. A turbofan aeroengine having a turbine exhaust duct, a bypass duct which surrounds the turbine exhaust duct and is coaxial therewith, an exhaust stream mixing duct for receiving from the turbine exhaust duct and the bypass duct the turbine exhaust stream and the bypass exhaust stream respectively, a propulsion nozzle which defines the downstream end of the mixing duct, a mixer ring which defines the downstream end of the turbine exhaust duct and means for imparting differing swirl and axial velocity vectors to said two exhaust streams before they pass the mixer ring, the swirl velocity vectors of said two exhaust streams being opposed to each other and the axial velocity vectors of said two exhaust streams being different in magnitude, whereby when said two exhaust streams contact each other at the downstream edge of the mixer ring, their differing velocity vectors give rise to circumferential and axial shear between said two exhaust streams and said shear results in the formation of streammixing vortices extending downstream of the mixer ring.

4. A turbofan aeroengine according to claim 2 or claim 3 in which the mixer ring has a plain trailing edge which does not substantially disturb the flow of the two exhaust streams thereover, the shear between said two exhaust streams thereby resulting in the formation of funnel-shaped trailing vortices which depend from said trailing edge.

5. A turbofan aeroengine according to claim 2 or claim 3 in which the trailing edge of the mixer ring is provided with a plurality of equi-angularly spaced discontinuities which act to initiate vortex formation in one or both of the two exhaust streams so that the shear between said two exhaust streams results in the formation of funnelshaped trailing vortices which depend from said discontinuities.

6. A turbofan aeroengine according to claim 5 in which the discontinuities at the trailing edge of the mixer ring comprise fins which project radially into at least one of the two exhaust streams and which are aligned with the axial direction.

7. A turbofan aeroengine according to claim 5 in which the discontinuities at the trailing edge of the mixer ring comprise tooth-like features which project in the axial direction from the trailing edge of the mixer ring.

8. A turbofan aeroengine according to claim 5 in which the discontinuities at the trailing edge of the mixer ring comprise streamlined excrescences which project in the axial direction from the trailing edge of the mixer ring.

9. A turbofan aeroengine according to any one of claims 1 to 8 provided with means for ensuring that at predetermined operating conditions of the engine, the swirl momentum of the bypass exhaust stream and the swirl momentum of the turbine exhaust stream are substantially equal in magnitude but opposed in direction, whereby the gases in the propulsive jet have no substantial net swirl momentum.

10. A turbofan aeroengine according to any one of claims 1 to 9, provided with turbine outlet guide vanes and fan outlet guide vanes configured to produce the mutually opposed swirl velocity vectors in the turbine exhaust stream and the bypass exhaust stream.

11. A turbofan aeroengine according to any one of claims 1 to 9, provided with fan outlet guide vanes and a final stage of turbine blades, said vanes and blades being configured to produce the mutually opposed swirl velocity vectors in the bypass exhaust stream and the turbine exhaust stream, the turbine exhaust stream being unobstructed by any flow-turning means between the final stage of turbine blading and the commencement of the mixing process.

1 2. A turbofan aeroengine according to any one of claims 1 to 9, provided with fan outlet guide vanes a short distance downstream of the fan and bypass outlet guide vanes at the downstream end of the bypass duct, the fan outlet guide vanes being configured substantially to remove swirl from the bypass stream, and the bypass outlet guide vanes being configured to reimpart swirl to the bypass exhaust stream.

1 3. A turbofan aeroengine substantially as described in this specification with reference to and as illustrated by the accompanying drawings.