GB2149456A

GB2149456A - Exhaust mixing in turbofan aeroengines

Info

Publication number: GB2149456A
Application number: GB08329733A
Authority: GB
Inventors: John Mathieson Robertson
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1983-11-08
Filing date: 1983-11-08
Publication date: 1985-06-12
Also published as: GB2149456B; US4577462A

Description

1 E GB 2 149 456 A 1

SPECIFICATION

Exhaust mixing in turbofan aeroengines The present invention relatesto bypass gasturbine aeroengines, otherwise known as "turbofans", wherein theturbine exhaust gas stream and the bypass airstream: are combined with each other before exitfrom a final propulsion nozzle, combination of the two streams being facilitated by a "mixer" structure which divides the turbine exhaust stream into a plurality of discretejets which penetrate and mixwith the bypass airstream.

It is known that mixing of the bypass anciturbine exhaust streams improvesthe propulsive efficiency of turbofans by transferring thermal energyfrom the hot turbine gases to the cooler bypass air. Even small improvements in the efficiency of the mixing process will significantly improve the propulsive efficiency of the turbofan, thereby allowing lower specificfuel consumption. So called "multi-lobed" and "multitubed- mixer nozzles, which project portions of the two streams into each other and increase the area of contact between them, have been usedto improve the propulsive efficiency by improving the efficiency of the mixing process. During design of such mixer nozzles,the object is to maximisethe contribution of the mixerto mixing efficiency whilst minimising mixer weight and the thrust losses inherent in the mixing process itself.

According to the present invention there is provided a bypass gas turbine aeroengine wherein, during operation of the aeroengine, a turbine exhaust stream, issuing from the engine core, and a bypass airstream issuing from the bypass duct surrounding the engine core, are combined with each other before exit of the combined stream from a final propulsion nozzle of the aeroengine, mixing of the two streams being facilitated by means wherebythe turbine exhaust stream is divided into an annular array of discrete turbine exhaustjets, each suchjet having: a rearward component of velocity; a radially outward component of velocity, whereby thejets penetrate into the surrounding bypass airstream and the bypass air stream flows in sheets between the jets; and a tangential component of velocity.

Preferably, the tu rbine exhaust jets are given cross-sections of elongate shape and are oriented so thatthe longitudinal centrelines of such cross-sections are substantially aligned with the direction of flow of the bypass stream therepast.

In spite of the tangential velocity components of the tu rbine exhaustjets, substantially zero net swirl velocity in the combined stream can be ensured by arranging thatthe bypass stream has a swirl compo- nent of velocity sufficieritto balance the tangential momentum of theturbine exhaustjets. Note that a distinction between tangential and swirl components of velocity is maintained in orderto distinguish respectively between the exhaustjets on the one hand andtheturbine exhaust and bypass streams onthe other.

In orderto attain the desired components of velocity of theturbine exhaustjets, the invention proposes that the engine core is arranged to give the turbine exhaust stream within the engine core a resultant velocity comprising swirl and rearward components, a plurality of turbine exhaust ducts being provided to convey respective portions of the turbine exhaust stream from the engine corefor production of the turbine exhaustiets,the turbine exhaust ducts being oriented and configured such thatthey acceptand conveythe turbine exhauststrearn portions without substantial modification of the directions of flowthereof immediately before entryto said ducts, the swirl and rearward components of velocity of the turbine exhaust stream thereby becoming the tangential, radially outward and rearward components of velocity of the tu rbine exhaust jets.

Preferably, the tu rbine exhaust ducts are conf igured such that the tu rbine exhaust stream portions follow substantially straight paths between the inlets and outlets of the turbine exhaust ducts, the ducts preferably changing in cross-sectionai shape between their inlets and outlets in orderto produce turbine exhaustjets having cross-sections of a desired shape. The ducts extend through a rearwardly convergent (substantially conical) afterbody attached to the engine core and are preferably configured to act as diffusers forthe turbine exhaust stream portions.

Nozzles forthe turbine exhaust jets are provided at locations where the turbine exhaust ducts intersect the surface of the afterbody.

The invention also includes an afterbodyforthe engine core of a bypass gasturbine aeroengine as described above.

Embodiments of the invention will now be described by way of example onlywith referenceto the accompanying drawings in which -- Figure I shows a partly "cutaway" side elevation in diagrammatic form of a turbofan aeroengine according to the invention.

Figu re 2 is a "ghosted" perspective view showing detail in the rear of the engine core and its afterbody which is hidden in Figures 1 and 3, the rear of the engine core and its afterbody being shown in isolation from otherstructure; and Figure 3 is a view on arrowA in Figure 1, showing the afterbody of the turbofan's engine core in isolation from other sturctures.

The drawings are not to scale.

Referring primarily to Figure 1, a bypass gas turbine aeroengine orturbofan 1 is of high bypass ratio and has: an engine core 3; a bypass duct 5 defined between the engine core 3 and the outer engine cowl 7; a conical afterbody 9 on engine core 3; an exhaust mixing duct 11, and afinal propulsion nozzle 13. The bypass duct 5 is supplied with bypass airfrorn low pressure compressor orfan 15, which also supplies engine core 3, the fan 15 being driven from the low pressure turbine 44 (Figure 2) in the rear of core 3. Combination of the bypass air stream 17with the hot exhauststream4l (Figure 2) from the turbine is facilitated by the afterbody 9; this divides the turbine exhaust stream into an annular array of (e.g,)ten discrete jets 19 which diverge from each other as they The drawing(s) originally filed was (were) informal and the print here reproduced is taken from a later filed formal copy.

2 GB 2 149 456 A 2 penetratethe bypass stream 17 and mixwith itthe bypass stream flowing in sheets between thejets.

Mixing of theturbine exhaust gaseswith the bypass stream continues in the mixing duct 11 before exitof the combined stream 20 to atmosphere through propulsion nozzle 13. In orderto absorb mixing noise as it arises, mixing duct 11 and afterbody 9 may be provided with acousticfacing panels as known.

Turbofan 1 is supported from the underside of a wing 21 of an aircraft (notshown) via a pylon 23, the engine core 3 being attached to structure (not shown) inside the external skin of pylon 23 via a suspension linkage 25 atthe front and further suspension linkages 27 at the rear. Front suspension linkage 25 extends across bypass duct 5 within one of the fan outlet guide vanes 29, which remove some swirl velocity from the bypass stream 17 after it has been energised by the fan 15. Rear suspension linkages 27 extend across bypass duct 5 within a streamlined fairing 31, which occupies a sector of the bypass duct at its midlength, but which tapers to thin leading and trailing edges.

By the invention, the penetration of the turbine exhaust jets 19 into the bypass stream 17, and their combination by mixing, is facilitated by arranging that the jets 19, as they issue from slot-shaped nozzles 33 in 90 conical afterbody 9, have both tangential and radial components of velocity in addition to their axial rearward velocity component, the directions of these velocities being relative to the centreline of the afterbody 9 and the propulsion nozzle 13. The radially 95 outward components of velocity can be achieved without activelyturning theturbine exhauststream, even though it has only axial and swirl components of velocity within core 3. Howthis is achieved will be explained in relation to Figure 2. The radially outward 100 components and the tangential components of flow in jets 19 are both importantto the efficiency of the mixing process, since the former ensure good penetration of the bypass stream bythe turbine exhaust gases, and the latter ensure good vortical 105 interactions between the flows.

As will be seen in Figure 1, tu rbine exhaustjets 19 are shown as having a left hand tangential component of velocity (described in the conventional mannerfor screwthreads) but in orderto maximise the thrust of 110 theturbofan tthe combined stream 20 should flow through propulsion nozzle 13 in a substantially axial manner. Consequently, in orderto balance outthe tangential momentum of theturbine exhaust gases in jets 19, it is necessaryto ensure that bypass stream 17, 115 with which the turbine exhaust gases mix in mixing duct 11, has a right hand swirl component of velocity giving it a swirl momentum of substantiallythe same magnitude as the tangential momentum of the turbine exhaust gases, but of opposite effect. To achieve this 120 state of affairs, the fan outlet guide vanes 29, which in a conventionally arranged turbofan actto straighten outthe bypass stream as much as possible after it has been energised bythe fan 15, are of such camberas to leave an appropriate residual swirl compnentof 125 velocity in the bypass stream 17 as indicated by the arrows. For example, if the bypass ratio of turbofan 1 is 5: 1, the swirl angle of the bypass stream must be approximately 115 of that of the turbine exhaust gases.

Exact matching of swirlltangential momenta between 130 bypass stream 17 and turbine exhaustjets 19 will of course only be achieved at a certain engine running condition forwhich the entire mixing arrangement is designed, this usually beingthe "cruise" condition.

Note that in orderto accommodatethe swirling component of velocity in the bypassstrearn 17, the suspension fairing 31, as well, as otherstructures extending acrossthe bypass duct 5, such as strut35 or fairing 37 for a shaft driving accessory components 39, must be appropriately configured and aligned withintheduct.

Turning nowto a detailed consideration of Figures 2 and 3, we can consider the manner of production of the turbine exhaustjets by conical afterbody 9.

It is common in turbofan aeroenginesforthe gas stream leaving the last set of rotating turbine blades to have a swirl component of velocity aboutthe engine centreline. This has advantages in the design of the turbine but in order notto lose propulsive thrust it is usual to remove this swirl by passing the gas through a set of stationary outlet guide vanes. Thus, referring to Figure 2, the swirling turbine exhaust gas stream 41 which flowsthrough the annular outlet duct43 from turbine 44would, in a conventional arrangement, be intercepted by such a set of outlet guide vaneswhose leading edges would be correctly aligned to acceptthe swirling flow4l with minimum disturbanceto it, the vanes being cambered so asto removethe swirl component of velocity and leave onlythe axial component. It can be considered thatsuch a set of outlet guidevaneswould divide the turbine outlet duct 43 into a number of passages defined between the vanes, these separate passages terminating atthe downstream edges of the vanes, and the separate exhaust gas flows through them being thus reunited.

In the present embodiment of the invention the passages between guide vanes are replaced by turbine exhaust ducts 45 which do not merge with each otherto reunite the turbine gases into a further annularflow, but instead continue separately from each otherth rough afterbody 9 until they intersect its conical surface to produce slot-shaped "nozzles" 33 from which turbine exhaustjets 19 issue. For convenience and simplicity of presentation only one of these ducts 45 is shown in Figure 2, though in factthere are ten of them arrayed around the afterbody 9 in this particular embodiment of the invention. The entrance and exit areas of the duct 45 are shown as crosshatched.

Ducts 45 areformed from suitable heat resistant panels or plates. In figure 2, two of the most upstream plates are referenced as 49 and 51; the leading edges of these two plates extend radially between the inner wall 53 of turbine outlet duct43 and its outerwall 55. In overall orientation, plates 49 and 51 are alignedto acceptthe swirling flow 41 with minimum disturbance to it in the same way as would the leading edges of the guide vanes which they replace. Notethat although for reasons of clarity in Figure 2, a considerable axial distance is shown between theturbine 44 and the downstream end of the turbine outlet duct43, the swirling flow4l downstream of theturbine would in fact pass into the ducts 45 within a very short axial distanceto minimise aerodynamic losses and engine weight.

3 GB 2 149 456 A 3 Once the swirling turbine exhaust stream 41 has been intercepted atthe downstream end of turbine outlet duct43 and separated into a numberof discrete sub-streams by ducts 45, each such sub-stream is permitted to flow along a straight path so that at some 70 distance along each duct, each sub-stream has components of velocity in the axial, tangential and radial directions, this being achieved without deflect ing the sub-streams from their initial instantaneous overall directions of flow at the entrances to the passages. The flows in the passages 45 continue sensibly straight until the passages terminate at nozzles 33 in the surface of afterbody 9, the jets 19 issuing from the nozzles having the required compo nents of velocity in the axial, tangential and radial directions.

Nowfrom theoretical considerations it is believed thatthe most effective mixing between a bypass air stream and a turbine exhaust stream results from bringing the streamstogether in such a waythatthe hydraulic mean depth of the streams is small and the inter-face area between the streams is large-see, for exampie,the manner in which multilobed mixers attemptto bring the streams together in alternating "iaminates". It is forthis reason thatthe nozzles 33 are 90 in the form of slots whose longitudinal dimensions are aligned so as to be substantially parallel with the direction of flow of the bypass stream 17 as it swirls overthe outside of the conical afterbody in the direction shown bythe arrows. Thejets 19 from 95 nozzles 33 arethus flattened and aligned with the direction of flow of the bypass stream 17, the bypass stream thereby also being divided into a number of substreams, or sheets, of shallow depth in the circumferential direction, which flow between the jets. 100 It should be understood that because of the Cooanda effect and the diffuser effect of the divergent annular duct formed between afterbody 9 and the outer cowling of the engine, the bypass stream tends to followthe convergent surface of the afterbody 9 105 instead of detaching itself therefrom, the convergence of the afterbody being sufficiently gradual to prevent detachment.An appreciation of the arrangementwill be gained byviewing Figure 2 in conjunction with Figure 3, which shows all ten nozzle openings 33.

In orderto producethe correct shape for nozzles 33, the turbine exhaust ducts 45 gradually change in shape between their upstream and downstream ends as indicated in Figure 2, the ducts 45 being gradually flattened in transitional sections-57. In the present embodimentthe ducts 45transition from a height/ width ratio of the order of one to a flattened section with a height/width ratio of the order of four.

Optimum performance of the mixing function in the above-described turbofan aeroengine will in practice depend upon detailed consideration of a number of thermodynamic and mechanicallstructural factors in the design. For example, the operating pressure ratio between the bypass stream 17 and theturbine exhaust jets 19 will affectthe degree of radial penetration through the bypass stream achieved by the hotjets 19; too little penetration will leave the outer portion of the bypass stream unmixed as itflows through the mixing duct 11, whilsttoo much penetration could cause unacceptably high temperature---hot-spots- on 130 the wall of the mixing duct 11, with no additional mixing benefit compared with that achievable at acceptable wall temperatures. Note that adequate penetration produces adequate mixing within a shorter axial length of mixing duct 11, thereby allowing the length of mixing duct 11 to be minimised.

Afurtherfactor needing consideration will be the exact shape of nozzles 33 and their orientation with respectto the direction of flow of the bypass stream 17. For example, it could be advantageous to make the nozzles 33 (and hence the jets 19) somewhat wedgeshaped, with the thin ends of the wedges being their leading edges with respectto the direction of bypass flow, so thatthe "collision" between the bypass stream and thejets is of more gradual onset; or slot-shaped nozzles could be oriented somewhat out of alignmentwith the bypassflowto encourage extra turbulence if this were desireablefor enhancing mixing.

Again, although the turbine exhaust ducts 45 are said above to be substantially straight, it would be possible to introduce a small curvature in the centreline of the ducts to produce an enhanced radially outward velocity component in the jets 19, thereby possibly reducing the length required for mixing in mixing duct 11 and allowing the engineto be shortened. This would haveto be a balance between internal drag losses introduced by curving the ducts 45, and weight saving with reduction of internal and external drag losses brought about by having a shorter mixing duct 11.

Aerodynamic tests showthatthe lower are the Mach numbers of the bypass and turbine exhaust streams asthey startto mix, the lower are the aerodynamic losses due to mixing. Hence it is desirableto havethe maximum possible amountof diffusion in the ducts 45 and in the pre-mixing portion of the mixing duct 11 through which the bypass stream is diffused after it leaves bypass duct 5. This also reduces the amount of diffusion required in the mixing duct 11 downstream of nozzles 33, and thereby allows its length to be minimised.

It should be noted that conical afterbody 9 will not experience direct impingement of any of the hot turbine exhaust gases and it can therefore be constructed of such materials as will stand the temperature of the combined stream in mixing duct 11, plus radiated and conducted heat from turbine exhaust ducts 45. Light-weight materials such as aluminium alloys orfibre-reinforced composites may therefore be adequate forthis duty. Ducts 45 will of course experience the direct heating effects of the turbine exhaust gases and will need to be constructed of highly heatresistant materials as already known to gasturbine engineers. However, hot ducts are inherently good structures from the point of view of vibration and stress, and it isfurther pointed out that ducts 45 offer an excellent opportunityfor attenuating noisefrom theturbine by applying suitable sound attenuating acoustic linings to them.

Although in the above description itwas stated that the bypass stream 17 partakes of a swirl component of velocity throughoutthe length of bypass duct 5, itwill be appreciated thatfan outlet guide vanes 29 could remove substantially all swirl from the bypass stream,

4 GB 2 149 456 A 4

Claims

it being reintroduced as necessary by a ring of guide vanes atthe downstream end of bypass duct 5. This would eliminate the requirement for fairing structures in duct 5to be skewed out of axial alignment to 5 accommodate swirling flow. CLAIMS

1. A bypass gas turbine aeroengine wherein, during operation of the aeroengine, a turbine exhaust stream, issuing from the engine core, and a bypass airstream, issuing from the bypass cluctsurrounding the engine core, are combined with each other before exit of the combined stream from a final propulsion nozzle of the aeroengine, mixing of the two streams being facilitated by means for dividing theturbine exhaust stream into an annular array of discrete turbine exhaustjets, each turbine exhaustjet having: a rearward component of velocity; a radially outward component of velocity, wherebythe turbine exhaust jets penetrate into the surrounding bypass air stream and the bypass air stream flows in sheets between the 85 turbine exhaustjets; and a tangential component of velocity.

2. A bypass gas turbine aeroengine according to claim 1 incorporating means wherebythe turbine exhaustjets are given cross-sections of elongate shape and are oriented such the longitudinal centrelines of said cross-sections are substantially aligned with the direction of flow of the bypass stream therepast.

3. A bypass gas turbine aeroengine according to claim 1 or claim 2 in which substantially zero net swirl velocity in the combined stream is ensured by arranging thatthe bypass stream has a swirl component of velocity sufficientto balance thetangential momentum of theturbine exhaustjets.

4. A bypass gas turbine aeroengine according to any one of claims 1 to 3 in which the engine core is arranged to give the turbine exhaust stream within the engine core a resultant velocity comprising swirl and rearward components, turbine exhaust ducts being provided to convey respective portions of theturbine exhaust stream from the engine corefor production of theturbine exhaustjets, the turbine exhaust ducts being oriented and configured such thatthey accept and conveythe turbine exhaust stream portions without substantial modification of the directions of flowthereof immediately before entryto said ducts, the swirl and rearward components of velocity of the turbine exhauststrearn thereby becoming the tangential, radially outward and rearward components of velocity of the turbine exhaustjets.

5. A bypass gas turbine aeroengine according to claim 4 in which the turbine exhaust ducts are configured such that the turbine exhaust stream portions follow substantially straight paths between the inlets and outlets of the turbine exhaust ducts.

6. A bypass gas turbine aeroengine according to claim 4 or claim 5 in which the turbine exhaust ducts change in cross-sectional shape between their inlets and outlets in orderto produce turbine exhaustjets having cross-sections of a desired shape.

7. A bypass gas turbine aeroengine according to anyone of claims 4to 6 in which the turbine exhaust ducts are configured to act as diffusers for theturbine exhauststrearn portions.

8. A bypass gas turbine aeroengine according to anyone of claims 4to 7 in which the turbine exhaust ducts extend through a rearwardly convergent afterbody attached to the engine core.

9. A bypass gas turbine aeroengine according to claim 8 in which nozzles for the turbine exhaustjets are provided at locations where the turbine exhaust ducts intersectthe surface of the afterbody.

10. A bypass gas turbine aeroengine according to claim 8 or claim 9 in which the rearwardly convergent afterbody is substantially conical in shape.

11. An afterbody for the engine core of a bypass gas turbine aeroengine according to anyone of claims 8 to 10.

12. A bypass gas turbine aeroengine substantially as described in this specification with reference to the accompanying drawings.

13. An afterbodyforthe engine core of a bypass gas turbine aeroengine substantially as described in this specification with reference to the accompanying drawings.

Printed in the United Kingdom for Her Majesty's Stationery Office, 8818935, 6185, 18996. Published at the Patent Office, 25 Southampton Buildings, London WC2A lAY, from which copies may be obtained.