GB2160265A

GB2160265A - Turbofan exhaust mixers

Info

Publication number: GB2160265A
Application number: GB08414899A
Authority: GB
Inventors: Sandra Ann Hiles; Malchom Roy Pike
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1984-06-12
Filing date: 1984-06-12
Publication date: 1985-12-18
Also published as: GB8414899D0; FR2565631A1; JPS614849A; DE3520726A1

Abstract

A multi-lobed mixer 11 for mixing the turbine exhaust stream 19 with the bypass air stream 15 comprises non-divergent outlets at the tops of the lobes 37 to give minimum radial outflow of the exhaust gases leaving them, at least some of the troughs defined between the lobes having trailing edges 47,49 which are rearwardly raked with a rake angle changing from zero at the radially outer tops of the lobes 37 to a maximum value beta ' at distance R' radially inwards thereof. The change in angle may be smooth, as shown, or abrupt. The trailing edges of alternate troughs are raked forwardly at a maximum rake angle of beta . <IMAGE>

Description

SPECIFICATION Exhaust mixers for gas turbine aeroengines The present invention relates to exhaust mixers for bypass gas turbine aeroengines (known as turbofans), whereby the turbine exhaust gas stream and the bypass air stream are combined with each other within the engine before exit from a final propulsion nozzle.

A well known type of exhaust mixer nozzle proposed for use in such engines is the socalled "multi-lobed" mixer, which projects interdigitated portions of the turbine exhaust stream and the bypass stream into each other and which also increases the area of contact between the two streams, thereby improving the propulsive efficiency of the turbofan by improving the efficiency of the mixing process.

Unfortunately, such multi-lobed mixer nozzles contribute extra length and weight to the engine, so that there is a special need to minimise length and weight if such a mixer is fitted. One way of minimising engine length is to ensure that the mixing process between the turbine exhaust stream and the bypass stream is made as efficient as possible, since this reduces the length of exhaust duct required to allow the mixing process to procede towards completion before the combined stream exits from the final propulsion nozzle.

In some types of known multi-lobed mixer, the mixing process is optimised by forwardly and/or rearwardly "scarfing" or "ranking" the exit planes of the troughs or channels between the lobes so as to obtain initial and progressive mixing between the two streams before they actually leave the most downstream portions of the mixer. In this way, the streams are already partially combined before they leave the mixer and consequently the exhaust duct length can be reduced. Scarfing also results in a lighter construction for the mixer through elimination of material.

Examples of this type of lobed mixer are that shown in British Patent Application Number 21 19859A (the large outer nozzle) and that shown in United States Patent Number 4117671, to which documents the reader is referred.

It will be noted that the latter document differs from the former in disclosing a mixer in which the trailing edges of all the troughs are forwardly raked instead of the troughs being raked alternately forward and rearward in peripheral sequence, and also in that the tops of the lobes have an outlet angle relative to the horizontal centreline which causes their downstream ends to be inclined gently towards, i.e.

convergent on, the axial centreline of the nozzle, whereas the former document shows lobes having an outlet angle causing divergence from the centreline.

The object of providing a multi-lobed exhaust mixer with this type of lobe is to minimise the radial outflow of the hot turbine exhaust gases from the tops of the lobes, thereby avoiding impingement of the hot gases on the outer walls of the exhaust duct and minimising boundary layer interactions.

Such impingement can cause overheating of the duct wall and the boundary layer interactions can also cause extra noise through turbulence. For the purposes of this specification, lobes whose tops have an outlet which extends parallel to the centreline of the mixer, or which converges on it, will be called "minimum outflow lobes".

However, if it is desired to utilise a multilobed exhaust mixer having troughs with rearwardly-raked trailing edges but also having minimum outflow lobes, a problem arises in that the shape of the lobe top necessitates an increase in the overall length of the mixer, and hence of its weight.

The present invention alleviates this problem by altering the geometry of the mixer body so that both minimum outflow lobes and troughs with rearwardly-raked trailing edges can be utilised without an increase in length of the mixer body.

According to the present invention, an exhaust mixer for a bypass turbine aeroengine is adapted to aid combination of the turbine exhaust gas stream with the bypass air stream before exit of a resultant combined stream from a final propulsion nozzle of the aeroengine, the exhaust mixer having a plurality of minimum outflow lobes, as hereinbefore defined, with a trough defined between each adjacent pair of lobes and at least some of the troughs having rearwardly-raked trailing edges, the rake angle of the rearwardly-raked trailing edges changing from zero at the radially outer tops of the lobes to a maximum value at a first predetermined radial distance inward from the tops of the lobes.In a preferred embodiment, alternate troughs in peripheral sequence around the exhaust mixer have forwardly-raked trailing edges and the other troughs have the rearwardly-raked trailing edges, the rake angle of the forwardlyraked trailing edges changing from zero at the radially outer tops of the lobes to a maximum at a second predetermined radial distance inward from the tops of the lobes.

The above-mentioned predetermined radial distances inward from the tops of the lobes are preferably substantially less than the total radial distance from the tops of the lobes to the bottoms of the troughs, though this is not essential. The changes in rake angle for the forwardly and/or rearwardly raked trailing edges may comprise a single increment at the relevant predetermined radial distance inward from the tops of the lobes, but preferably the changes in rake angle are distributed over the relevant predetermined radial distance inward from the tops of the lobes so as to produce a gradual increase in rake angle. Preferably, the rake angle of the forwardly and/or rearwardly raked trailing edges is constant at its maximum value over those portions of the trailing edges which are radially inward of the positions where the maximum rake angle is attained.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows a partly "cut-away" side elevation in diagrammatic form of a turbofan aeroengine fitted with a multi-lobed exhaust mixer in accordance with the invention; Figure 2 shows a perspective view of the multilobed exhaust mixer of Fig. 1, showing the overall lobe and trough configuration; Figure 3 is a composite view of axial sections taken through the centrelines of a lobe and its adjacent troughs to show the major features of the embodiment in diagrammatic form; Figure 4 is a further view similar to Fig. 3 but illustrating an alternative embodiment of the invention.

Referring first to Fig. 1, a medium bypass ratio gas turbine aeroengine or turbofan 1 includes: an engine core 3; a bypass duct 5 defined between the engine core 3 and an outer engine casing/nacelle 7; a short turbine exhaust duct 9; an exhaust bullet 10; a multilobed exhaust mixer nozzle 11; an exhaust mixing duct 12; and a final propulsion nozzle 1 3. The bypass duct 5 is supplied with bypass air 1 5 from low pressure compressor or fan 17, which also supplies engine core 3.

Mixing of the bypass air stream 1 5 with the turbine exhaust stream 1 9 is facilitated by the mixer nozzle 11, which is attached to the rear of the engine core 3 and defines the ends of the bypass duct 5 and the turbine exhaust duct 9. Mixing of the two streams 1 5 and 1 9 continues in mixing duct 1 2 before exit of the combined stream to atmosphere through propulsion nozzle 1 3.

In addition to the above-mentioned features, it will be noted that engine core 3 is suspended within the outer engine casing/nacelle 7 by means of struts 21 at a forward location on the core 3, and suspension linkages 23, 25 at a rear location thereon. Fan outlet guide vanes 27 straighten the by-pass flow 1 5 as it enters the bypass duct 5, and acoustic linings 29 and 31 are provided in the intake 33 of the engine 1 and in the bypass duct 5 in order to attenuate noise from fan 17. Turbine noise, and noise due to the mixing process in mixing duct 12, is attenuated by an acoustic lining 35 in the mixing duct.

Referring now to Figs. 1 to 3 inclusive, the multi-lobed exhaust mixer nozzle 11 has twelve convex outward bulges or lobes 37 angularly spaced apart from each other in an annular array around the nozzle, troughs 39 and 39' being defined between adjacent lobes. Lobed mixer nozzles are well known as a general type and the structure and function of nozzle 11 will not be described in detail except where germane to the invention.

It will be noticed that each trough 39 and 39' can be considered as being defined by a pair of mutually confronting side walls 41, 43 and 41', 43' respectively and the side-walls 41, 43 of alternate troughs 39 in peripheral sequence around the mixer 11 have forwardly-raked trailing edges, the side-walls 41', 43' of the other troughs 39' having rearwardly-raked trailing edges. The purpose of the forward and rearward raking of the trailing edges of the troughs 39 and 39' respectively is to initiate mixing of the two streams 1 5 and 1 9 before they reach the downstream rearmost end of mixer 11.

Another characteristic of the mixer 11 seen in Figs. 1 to 3 is that is possesses "minimum outflow lobes". As already explained, this terminology means that radial outflow of the hot turbine exhaust gases is minimised by ensuring that the tops of the lobes 37 have an outlet angle relative to the axial direction which causes their downstream ends to extend parallel to the axial centreline 45 of mixer 11, or which, as in the present case, causes them to be convergent on the centreline.

As indicated by the chain-link lines in Fig.

3, the provision of minimum outflow lobes in a mixer with troughs whose side-walls have rearwardly raked trailing edges would formerly have dictated that if it were desired to maintain the same angle of rake as in a mixer with lobes having a divergent outlet, the length of the mixer would have to be increased by dimension L to accommodate the minimum outflow lobes.

In the present invention this disadvantage associated with minimum outflow lobes is avoided by changing the geometry of the mixer body so that the rake angle of the rearwardly-raked trailing edges of side walls 41', 43', is no longer constant but changes from zero at the radially outer tops of the lobes 37 to a maximum value ' at the radially inner bottoms of the troughs 39'.

Specifically, in the embodiment of Figs. 1 to 3, the rake angle changes gradually from zero at the radially outer tops of the lobes to the maximum value ss' at a predetermined radial distance R' inward from the tops of the lobes 37. This gives a curved appearance in the side view of the radially outer part 47 of the trailing edge in Fig 3, and a straight appearance to the radially inner part 49.

As will be seen particularly in Fig. 3, a similar (though not identical) gradual change in forward rake angle from zero to a maximum of ss is also evident for side walls 41, 43 of mixer 11, the maximum rake angle being achieved at a radial distance R from the tops of lobes 37. In this way the transition from rearward rake in troughs 39' to forward rake in troughs 39 is accomplished in an aerodynamically smooth way.

Though radial dimensions R and R' are shown as different in this embodiment of the invention, they need not be in all embodiments of the invention.

Although Figs. 1 to 3 show a gradual change in rake angles of the trailing edges for the purpose of avoiding corners which might be noise generators, it would also be possible as shown in Fig. 4 to shape the trailing edges so as to exhibit an incremental (i.e. step-) change from the zero rake angle at the tops of the lobes to the maximum rake angles ss2 and ss3, the change for forward and rearward rakes occurring in this instance at radial distances R2 and R3 respectively from the tops of the lobes.

It should be noted that the mixer shown in Figs. 1 to 3 is designed for a turbofan aeroengine having a bypass ratio of about 3.0, i.e.

in the medium range of bypass ratios, and, relative to large high bypass turbofan aeroengines, is adapted for use in the context of small diameter gas ducts with a small diameter exhaust bullet. In this context, model tests of a mixer and exhaust system similar to that shown in Figs. 1 to 3 attained a high mixing efficiency at the end of the exhaust mixing duct 1 2 of 75%, with a low pressure loss of less than 3% of exhaust stream dynamic head, whilst avoiding any separation of turbine exhaust flow or bypass flow from the surfaces of the mixer body. When a temperature survey of the combined stream was performed at the propulsion nozzle exit, it was confirmed that the mixer model was performing with a high mixing efficiency by promoting large radial flow components in the two streams. At the same time the minimum outflow lobes were shielding the exhaust mixing duct walls from contact with outwardlydirected portions of the hot turbine exhaust stream by allowing a layer of cool bypass air to flow next to the mixing duct wall.

Claims

1. An exhaust mixer for a bypass gas turbine aeroengine, the exhaust mixer being adapted to aid combination of the turbine exhaust gas stream with the bypass air stream before exit of a resultant combined stream from a final propulsion nozzle of the aeroengine, the exhaust mixer having a plurality of minimum outflow lobes, as hereinbefore defined, with a trough defined between each adjacent pair of lobes and at least some of the troughs having rearwardly-raked trailing edges, the rake angle of the rearwardly-raked trailing edges changing from zero at the radially outer tops of the lobes to a maximum value at a first predetermined radial distance inward from the tops of the lobes.

2. An exhaust mixer according to claim 1 in which alternate troughs in peripheral sequence around the exhaust mixer have forwardly-raked trailing edges, the other troughs having the rearwardly-raked trailing edges, the rake angle of the forwardly-raked trailing edges changing from zero at the radially outer tops of the lobes to a maximum at a second predetermined radial distance inward from the tops of the lobes.

3. An exhaust mixer according to claim 1 or claim 2 in which at least the first predetermined radial distance is substantially less than the total radial extent of the lobes.

4. An exhaust mixer according to claim 1 in which the change in rake angle of the rearwardly-raked trailing edges is a single increment at the first predetermined radial distance inward from the tops of the lobes.

5. An exhaust mixer according to claim 2 or claim 4 in which the change in rake angle of the forwardly-raked trailing edges is a single increment at the second predetermined radial distance inward from the tops of the lobes.

6. An exhaust mixer according to claim 1 in which the change in rake angle of the rearwardly-raked trailing edges is distributed over the first predetermined radial distance inward from the tops of the lobes so as to produce a gradual increase in rake angle.

7. An exhaust mixer according to claim 2 or claim 6 in which the change in rake angle of the forwardly-raked trailing edges is distributed over the second predetermined radial distance inward from the tops of the lobes so as to produce a gradual increase in rake angle.

8. An exhaust mixer according to any one of claims 1 to 7 in which the rake angle is constant at its maximum value over those portions of the trailing edges which are radially inward of positions where the maximum rake angle is attained.

9. An exhaust mixer substantially as described in this specification with reference to and as illustrated by Figs. 1 to 3 of the accompanying drawings.

10. An exhaust mixer substantially as described in this specification with reference to and as illustrated by Fig. 4 of the accompanying drawings.