GB2316162A - Device for imparting swirl to a fluid flow - Google Patents

Abstract

A device (10) for generating enhanced streamwise vorticity in fluid flows comprises a plurality of concave flow surfaces (12, 13) arranged such that adjacent flow surfaces: (i) extend parallel to each other in a desired streamwise direction (M); (ii) are joined to each other along their lengths (L); and (iii) are concave transverse of the streamwise direction such that their joined edges form cusps (14) (Fig.1B) dividing the adjacent flow surfaces from each other. Streams of fluid (17) are directed onto the cusps (14) in an initial direction such that each stream (17) flowing in the initial direction is divided into two resulting streams (18, 19), each of which follows its respective concave flow surface (12, 13) such that a swirling fluid flow is established with respect to the concave flow surface, vortices (18, 19) on opposed sides of the cusps (14) thereby rotating in opposite senses. A flow momentum (M) of the resulting streams along their respective flow surfaces is established in the desired streamwise direction, so that the fluid flow in the resulting streams is helical. The device has application in mixing fluids such as fuel and oxidants in a combustion chamber, or in enhancement of heat transfer across a wall such as in cooling a wall of a combustion chamber.

Description

DEVICES FOR IMPARTING SWIRL TO FLUID FLOW This invention relates to devices which can be utilised to intercept a stream of fluid flowing in an initial direction in a predominantly linear manner, change the stream's direction of flow and simultaneously impart swirling or vortex motion to it, so that in its new direction the fluid stream has rotational components to its streamwise motion.

One of the requirements for state-of-the-art gas turbine combustors, particularly of the lean-burn type, is to obtain thorough mixing of the air and fuel reactants prior to their ignition at the flame front in the combustor. If the mixing process is insufficiently thorough, the combustion process will be spatially and temporally non-uniform, leading to pronounced temperature gradients in the combustion gases, with consequent heat stresses in combustor and turbine materials and production of pollutants such as oxides of nitrogen, carbon monoxide, and unburnt hydrocarbons.

A known device for promoting mixing of fuel with a supply of combustion air in gas turbine engines is a so called "swirler" or "swirl vane ring", used to impart a swirling motion to air or an air/fuel mixture prior to combustion in a combustion chamber.

Such a swirler commonly comprises a set of vanes, i.e. blade-like members, arranged in a ring to extend across a fluid stream at right angles to the direction of fluid flow while presenting their span-wise edges to it. The blade-like members may, e.g., be of thin sheet material, or may be bodies which have an aerofoil type of cross-section in the streamwise direction, or may take the form of wedges which present their thin edges to the oncoming stream. In any case, the vanes are disposed so that their chordwise sides define passages therebetween which deflect the fluid flow and impart swirling or vortical motion to it. A known way of utilising swirlers to obtain good mixing of streams of reactants before combustion is to pass the streams through respective swirlers arranged adjacently in the head of the combustor. As the swirling streams expand into the combustor, they interfere with each other in a viscous shear layer mixing interaction before they encounter the flame front. However, such an mixing interaction, besides incurring significant viscous losses in the fluid flow, requires a significant length of combustor in which to occur, and more compact arrangements are therefore required.

A requirement for walls of combustion chambers is adequate cooling to prevent material temperatures rising too far towards those of the combustion gases. Maintaining relatively low combustor wall temperatures prevents rapid degradation of the wall materials and maintains their strength. A well known means of cooling a combustor wall utilises so-called "impingement cooling", in which small jets of air are directed against the outside of the wall. The air jets have a scrubbing action on the wall surface, thus promoting good heat transfer from the wall to the cooling air. Subsequent to such impingement, the air is allowed to flow along the outside of the combustor wall in response to a pressure gradient. This air may still have appreciable heat capacity, but has only a low velocity along the wall, due to its impingement on the wall.

Hence, its ability to pick up further heat from the wall surface is limited because it can exert no further scrubbing action. A means of increasing the cooling duty obtainable from impinged air is therefore desirable.

The present invention is intended to provide devices helpful in improving efficiency in mixing of different fluids and in heat exchange between surfaces and fluids, the above prior art examples being only two out of many possible ones.

According to the present invention, a device for generating streamwise vorticity in fluid flows comprises: (a) a plurality of concave flow surfaces arranged such that adjacent flow surfaces (i) extend parallel to each other in a desired streamwise direction, (ii) are contiguous (i.e., are joined) along their adjacent streamwise extending edges, and (iii) are concave transverse of the streamwise direction such that their contiguous edges form a cusp dividing the adjacent flow surfaces from each other; (b) flow directing means for directing a stream of fluid in an initial direction onto a corresponding cusp such that the stream flowing in the initial direction is divided into two resulting streams, each of which follows its respective concave flow surface such that a swirling fluid flow is established with respect to the concave flow surface, vortices on opposed sides of the cusps thereby rotating in opposite senses; and (c) means for establishing flow momentum of the resulting streams along their respective flow surfaces in the desired streamwise direction, whereby the fluid flow in the resulting streams is helical.

It is believed that embodiments of the invention impart to fluid impinging on them in the above manner vortices analogous to those produced by a sea-gull's wings in the air in which it flies. A swirling effect is introduced into the impinging fluid and streamwise vorticity is shed into the field downstream of the device. Since vortices on opposed sides of the cusps rotate in opposite senses, the most prominent structures in the downstream flow field are counter-rotating pairs of vortices.

The expected advantages of the invention, when used to enhance mixing between different fluids, such as fuel and air, include enhancement of the rapidity and spatial uniformity of the mixing process compared to known swirler devices, while minimising stagnation pressure loss. When the invention is used to enhance heat transfer, it is believed that the counter-rotating pairs of vortices are more effective in "scrubbing" the surfaces over which they pass, and hence bringing cooling or heating fluid for heat transfer into intimate contact with the surfaces, than ordinary linear or turbulent flow.

In preferred embodiments of the invention, the concave flow surfaces are arranged equiangularly around a common centre such that they define a flow swirling body having a longitudinal centreline extending in a predetermined direction. In one embodiment, the concave flow surfaces extend in the same direction as the longitudinal centreline. In another embodiment, they extend helically around it. In the latter case, the fluid flow along the flow surfaces with respect to the longitudinal centreline will be a compound helical motion.

A circumscribing envelope drawn to touch the tips of the cusps of the concave flow surfaces along their lengths may be parallel to the longitudinal centreline of the flow swirling body, or may be tapered with respect thereto. In the former case, such a circumscribing envelope will be cylindrical and in the latter case, will be conical if the taper is constant. The tapering shape may be either convergent or divergent in the downstream direction.

Various types of flow directing means may be contemplated for use in the invention.

For example, the device may be provided with nozzles for directing fluid streams onto the cusps, or apertures or other passage means may be provided in wall means spaced from the flow surfaces, the passage means being located in registration with the cusps such that fluid streams issuing from the passage means are directed onto the cusps.

Flow momentum of the helically flowing streams along their respective flow surfaces in the streamwise direction may be established by orienting the flow directing means with respect to the cusps such that the stream of fluid flowing in the initial direction has a component of velocity in the desired streamwise direction. Alternatively, or in addition, the device may include means for establishing a pressure gradient in the streamwise direction such that the fluid pressure is greater at the upstream parts of the flow surfaces than at their downstream parts. Such means for establishing a pressure gradient may conveniently comprise wall means at upstream ends of the flow surfaces, the wall means being effective to prevent or hinder flow of fluid therepast.

In some exemplary embodiments of the invention, the concave flow surfaces comprise an external surface of the flow-swirling body. In this case the streams of fluid directed onto the cusps have at least a component of velocity towards the longitudinal centreline of the flow-swirling body. Nevertheless, arrangements may be conceived in which the concave flow surfaces comprise an internal surface of the flow-swirling body, and in this case the streams of fluid directed onto the cusps have at least a component of velocity away from the longitudinal centreline of the flow-swirling body.

Assuming the concave flow surfaces comprise an external surface of the flow-swirling body, the device may conveniently be constructed in the form of a housing containing a centrebody, the centrebody comprising the flow-swirling body, the housing having (a) peripheral wall means including passages or apertures for directing fluid streams onto the cusps and (b) end wall means at an upstream end of the flow-swirling body, the housing being open-ended at an extremity thereof opposite the end wall means, whereby the resulting streams flow off the downstream ends of the flow surfaces on the flow-swirling body and commingle as they exit the open end of the housing.

Alternatively, if the concave flow surfaces define an internal surface of the flowswirling body, the device may again be constructed in the form of a housing containing a centrebody, but in this case the housing's internal wall means comprises the flow-swirling body, while the centrebody comprises wall means containing passages or apertures for directing fluid streams onto the cusps, the housing again having end wall means at an upstream end of the flow-swirling body and being open-ended at an extremity thereof opposite the end wall means, whereby the resulting streams flow off the downstream ends of the flow surfaces and commingle as they exit the open end of the housing.

It may be advantageous if both the centrebody and the internal wall means of the housing are provided with confronting concave flow surfaces, the concave flow surfaces on one component being shaped and disposed in a manner which is complementary to the shape and disposition of the concave flow surfaces on the other component, whereby the confronting flow surfaces at least partially define therebetween channels for constraining the helically flowing streams to flow in the streamwise direction.

Confronting concave flow surfaces are preferably disposed so that their respective cusps are in angular registration with each other.

As mentioned above, a first possible use of the invention is in enhanced mixing of fluids having different characteristics, such as fuels and oxidants, prior to use of the fluid mixture in a further process, such as combustion in a combustion chamber. A second possible use of the invention is in enhancement of heat transfer rates across a boundary for cooling or heating purposes.

In an example of the first-mentioned use, the concave flow surfaces of the flowswirling body are provided with spray holes for spraying of a fuel in fluid form therefrom for mixing with an oxidant comprising the helically flowing streams flowing along their respective concave flow surfaces, the spray holes being connected with fuel supply passage means in the flow-swirling body. Preferably, the spray holes are arranged in rows along the streamwise extent of the concave flow surfaces.

In this embodiment of the invention, the fuel/oxidant mixing process is enhanced in comparison with the prior art in three ways. Firstly, as the oxidant fluid streams are split into two by impingement on the cusps, there is a resulting multiplication of shear contact area between the oxidant and the fuel. Secondly, while flowing along the length of the concave flow surfaces of the device, there is a further continuing increase of shear contact area between the fuel being injected and the oxidant, as new regions of the oxidant streams are continually presented to the spray holes by the rotating vortices. Thirdly, after the flows exit the device, there is increased interaction between the counter-rotating vortices to yet further increase the shear contact area.

There may also be some interaction between adjacent oppositely rotating vortical streams before they exit the housing.

When used as a fuel/air mixing device to feed the mixture to a downstream combustion chamber, a further advantage of the invention becomes apparent. When compared with arrangements utilising conventional swirlers, the advantage is a reduction of tangential flow momentum in the combustion chamber. This is achieved because not only can it be ensured that each region of swirl exiting the device is relatively small compared to the cross-sectional area of the chamber, but also the net swirl of the mixture exiting the device is substantially zero, because the device produces counterrotating vortex pairs. Such a reduction in tangential flow momentum reduces combustion instability and dynamic pressure fluctuations in the combustion chamber.

In an example of the use of the invention for heat exchange purposes, the concave flow surfaces comprise a surface of a first wall which serves as the flow-swirling body, heat being transferred between the first wall and a heat transfer fluid flowing therepast, a second wall being spaced from the first wall but extending substantially parallel thereto to define a gap therebetween, the second wall having holes in registration with the cusps of the flow surfaces to direct the heat transfer fluid at the cusps to produce resulting streams of heat transfer fluid with counter-rotating helical flow in the desired streamwise direction along the first wall.

Exemplary embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 A is a diagrammatic perspective view to illustrate aspects of the basic geometry of a device utilising the principles of the present invention and Figure 1 B is a sectional end view of the same device illustrating how it can produce vortical flow from an impinging linear flow; Figure 2 is a view similar to Figure 1A, illustrating further basic aspects of the invention; Figures 3A and 3B are sectional end and side elevations respectively showing diagrammatically the construction of a practical device according to the present invention; Figure 4A is a view similar to Figure 3A illustrating how such device can be utilised as a fuel/air swirler and mixer and Figure 4B is an enlargement of the area within the dotted circle of Figure 4A; Figures 5, 6 and 7 are diagrammatic representations of further embodiments of the invention as a fuel/air swirler and mixer; Figure 8 is a partial side elevation of a combustor showing how an embodiment similar to that of Figure 4 may be installed in a combustion chamber; Figure 9 is a partial side elevation of a combustor showing how an embodiment similar to that of Figure 7 may be installed in a combustion chamber; Figure 10 is an enlargement of the area within the dotted rectangle in Figure 8, showing how the invention may be used to increase the effectiveness of impingement cooling of the wall; and Figure 11 illustrates an alternative to the embodiment of Figure 10.

Referring to Figure 1A, a device 10 for generating streamwise vorticity in fluid flows comprises two adjacent concave flow surfaces 12 and 13 of length L, which have been formed out of an article 11. This comprises a piece of metal, plastic, or other material, and the forming process may be machining, moulding, casting, etching, or any other suitable technique which results in a good surface finish. The two adjacent concave flow surfaces 12 and 13 extend parallel to each other in a desired streamwise direction, their concavity being transverse of the streamwise direction. The flow surfaces 12 and 13 are contiguous along their adjacent streamwise extending edges, their joined edges thereby forming a cusp 14 which divides the adjacent flow surfaces from each other.

As illustrated in Figure 1B, the flow surfaces 12 and 13 may be considered to be parts of the surfaces of two imaginary parallel cylinders of equal radius R which are touching or intersecting each other along their lengths. In this case, intersecting cylinders are indicated by the dotted circles. Cusps 14 may be considered to mark the line along which the cylinders intersect or touch. However, the surfaces 12 and 13 need not be part-cylindrical. As shown by the dashed lines 15 and 16 in Figure 1B, their cross-sections may depart from being parts of true circles and in this example they are parts of respective ellipses.

In accordance with the invention, as illustrated in Figure 1B, if the cusp edge 14 ofthe concave flow surfaces 12 and 13 is positioned as shown to divide an initial uniform linear stream 17 of air or other fluid into two separate resulting streams 18 and 19, each resulting stream 18 and 19 then follows its own concave surface 12 and 13. In so doing, each stream 18 and 19 establishes a vortex motion, such that vortices on opposed sides of the cusp edge 14 rotate in opposite senses, as indicated by the arrows.

Assuming establishment of a streamwise flow momentum in the direction of arrows M along the length L of the concave flow surfaces 12, 13, the vortices will roll longitudinally of the concave flow surfaces in the direction of arrow M, thus establishing twin resulting streams 18 and 19 in which stream elements therein have a helical flow motion, as indicated for stream 18 in Figure 1A. Stream 19 has been omitted from Figure 1 A for reasons of clarity. The scale of the vortices is of course controlled by the radius of curvature R of the concave flow surfaces 12 and 13.

As mentioned above, to establish a desired upstream/downstream direction of flow for the vortical streams 18, 19 with respect to the length L of the flow surfaces, it is necessary to establish a flow momentum in the desired streamwise direction. This may be done as shown in Figure 2 by directing an initial stream, shown as arrow 17', so that it impinges on the cusp edge 14 at an angle A to the perpendicular. Note that arrow 17 represents impingement of the stream 17 on the cusp edge 14 in a direction perpendicular or normal thereto, as in Figure 1. The plane C of the cusp edge 14 is indicated by dashed lines, and arrows 17 and 17' both lie in this plane. Impingement of stream 17' at the angle A gives resulting vortical streams (not shown, but similar to streams 18 and 19 in Figure 1B), having a component of velocity in direction 23 along the flow surfaces 12 and 13, thereby establishing direction 23 as the downstream direction. Of course, it may be arranged that the initial stream impinges at any angle A to the edge 14, but in general as the angle A veers further from the perpendicular, the vortical component of flow in the streams 18 and 19 reduces as the downstream component of flow increases. Furthermore, although it is not essential for the streams 17 or 17' to lie exactly in the plane C of the cusp 14, substantial deviation from such an orientation would cause the streams on opposed sides of the cusp to be unequal in flow and in their degree of swirl and would introduce cross-stream turbulence into the streams, which would interfere with their streamwise motion 23.

Another way of establishing a flow momentum in a desired direction - which is not exclusive of the way described immediately above - is to establish a pressure gradient along the length L between opposed ends of the device 10. Even with perpendicular impingement of initial stream 17 on cusp edge 14 along line 21, such a pressure gradient can be simply achieved by abutting a wall 25, indicated by broken lines, against a chosen end of the device 10. With the flow path of the streams 18 and 19 along the surfaces 12 and 13 in that direction blocked by the wall 25, a slight pressure gradient is established along length L due to the conversion of flow momentum towards the wall 25 to static pressure as the flow is halted, the flow in direction 23 being unrestricted.

The pressure gradient facilitated by a such a blocking wall can be substantially increased if the article 11 is encompassed by a duct having an aperture therein for the entry of stream 17, the duct being closed at one end by wall 25 and open at the opposite end of the article. The encompassing duct inhibits the helically flowing streams from dissipating in directions which are perpendicular to the surfaces 12 and 13.

A device 30 having such an arrangement is illustrated diagrammatically in Figures 3A and 3B, the device being constructed in the form of a cylindrical housing 31 containing a cylindrical centrebody 32 acting as the flow swirling body. The external surface of the centrebody 32 comprises a plurality of concave flow surfaces 33 arranged equiangularly around a longitudinal centreline CL of the centrebody, which extends in the desired streamwise direction. The centrebody 32 may be considered to be a cylinder of basic radius R1 on which the concave flow surfaces 33 are superimposed. Cusps 34 project from the cylinder radius R1 to a radius R2 of an imaginary cylinder circumscribing the cusps.

Housing 31 comprises a wall of inner radius R3 and outer radius R4 which surrounds the flow swirling body 32, but is spaced radially outwardly of it by a distance R3 - R2 and is concentric therewith. The housing 31 includes apertures or passages 35 for directing initial fluid streams 36 inwardly onto the cusps 34. Housing 31 also has a circular end wall 37 to which both the flow swirling body 32 and the housing 31 are concentrically fixed The end wall 37 forms the upstream end of the housing, the housing being open at the opposing downstream end.

Apertures 35 comprise first and second upstream and downstream rings of apertures 35A and 35B, the two rings being closely spaced in tandem in the streamwise direction, giving corresponding initial fluid streams 36A, 36B. In each ring of apertures 35A, 35B, the apertures are equiangularly spaced from each other around the housing 31 so as to be located in registration with alternate ones of the cusps. In this particular embodiment, this geometry provides an arrangement in which the centrelines of the apertures 35 have an angular separation of 90" around the device 30. Each stream 36 is thereby divided equally into two swirling streams 38A, 38B, indicated in unbroken arrow lines in Figure 3B and in broken arrow lines in Figure 3B.

It will be realised that for initial fluid streams 36 to exist, there must be a pressure difference between the inside of the housing 31 and the outside. As seen in the radial plane of Figure 3A, the apertures 35A, 35B are in the shape of convergent nozzles to increase the velocity of the fluid streams 36 before they impinge on the cusps 34.

Furthermore, the two rings of apertures 35A and 35B are oriented with their centrelines at an angle B to the centreline CL of the device 30, thereby giving the initial streams of fluid 36A, 36B and their resulting streams 38A, 38B, a component of velocity in the desired streamwise direction. Flow momentum in the downstream direction is enhanced by a pressure gradient created by the presence of the end-wall 37.

Note that the distance R3 - R1 should be sufficient to allow development and rotation of the vortices without substantial interference by the wall of the housing 31. A suggested value is about twice the radius of curvature of the flow surfaces 33.

The vortically flowing streams 38A, 38B roll down the length of the flow surfaces 33 on centrebody 32. Neighbouring streams commingle as they exit the open end of the housing.

Figure 4 illustrates how a device similar to that shown in Figure 3 can be utilised to enhance mixing of fluids having different characteristics, such as reactants for a chemical process. This may be prior to use of the fluid mixture or reaction products in a further process. In this case, an oxidant and a fuel are to be mixed together prior to combustion in a combustion chamber. Features in Figure 4 which are the same as in Figure 3 are given the same numbers and only the differences will be described.

In this example of an advantageous use of the invention, as better seen in Figure 4B, the concave flow surfaces 33 of the flow-swirling body 32 are provided with spray holes 40, which spray a fuel in gaseous, liquid, or finely comminuted solid form into the air or other reactant comprising the helically flowing streams 38A, 38B, as they flow along their respective concave flow surfaces. The spray holes 40 are connected with fuel supply passages 42 in the flow-swirling centrebody 32 and holes 40 may be arranged in rows along the streamwise extent of the concave flow surfaces33.

As previously discussed, a device like that in Figure 4 enhances the fuel/oxidant mixing process by radically increasing shear contact area between the fuel and oxidant relative to the prior art. Such a device could theoretically produce a uniform mixture at a ratio of mixing length L to diameter D in the range 0.5 to 1.5. Typical L/D ratios for prior art devices relying on shear layer mixing are in the range 5 to 7.

As shown by dashed arcuate lines in Figure 4A, it may be advantageous if the inner peripheral wall of the housing 31 is also provided with concave flow surfaces 44 intersecting each other to define cusps 46. Concave flow surfaces 44 and cusps 46 confront flow surfaces 33 and cusps 34 provided on the centrebody 32, the better to constrain the helically flowing streams 38A, 38B to flow in the streamwise direction..

In Figure 4A, the concave flow surfaces 44 on the inner wall of housing 31 extend only over about half of the arc length of flow surfaces 33 on centrebody 32, though their arcs could be extended somewhat further towards adjacent apertures 35 without interfering with the inflowing streams 36 of reactant. In any case, the flow surfaces 44 are shaped and disposed in a manner which is complementary to the shape and disposition of the concave flow surfaces 33 on the centrebody 32. Hence, confronting flow surfaces are disposed so that their respective cusps 34, 46 are in angular registration with each other, so more effectively defining channels 48 which help to prevent cross-flows between adjacent streams. Provision of confronting flow surfaces 44 will be desirable only if, in their absence, cross-flow between adjacent streams creates uneven mixing between the reactants or other fluids being mixed by the device.

Figure 5 is a partial end sectional view of a further exemplary device showing two possible variations on the arrangement of Figure 4A. In Figure 5, twice as many apertures 35' are provided in housing 31', giving twice as many initial streams 36' directed radially inwards. As in Figure 4A, the apertures are equally spaced around the housing 31', but now each aperture 35' is in registration with a cusp 34 on the centrebody 32. As shown by arrows, the streams 38A', 38B', which result from impingement of the streams 36' on the cusps 34, flow towards each other on the concave flow surfaces 33 and meet in a region 50 of chaotic turbulence at the longitudinal centrelines of the flow surfaces 33, the turbulence being carried downstream by the established flow momentum. However, the rotational velocity imposed on the streams 38A', 38B' by surfaces 33 is not immediately dissipated in the turbulence where the streams meet and some residual opposing swirl components 52, 54 survive to roll downstream in a helical flow mode for at least a short distance before being overcome by turbulent motions. This configuration may be of use in situations where only a short streamwise distance is available for thorough mixing of the reactants, e.g., fuel exiting from spray holes 40 and air comprising streams 38A', 38B'.

If desired, the severity of the turbulence in region 50 can be reduced - and more pronounced swirl components 52, 54 can be preserved to roll downstream - by providing auxiliary concave flow surfaces 56 which are superimposed on flow surfaces 33 to define minor cusps 58 between major cusps 34. Minor cusps 58 are in registration with the longitudinal centrelines of the surfaces 33. It should be recognised that if cusps 58 were to be extended to the same radial position as the cusps 34, a configuration would be reached in which the centrebody 32 would have twice as many flow surfaces and cusps as in Figure 4A, but which would be similar to Figure 4A in that only alternate cusps would be impinged by an initial stream 36'.

Figure 6 shows a further alternative embodiment of the invention, in which concave flow surfaces define an internal surface of a flow-swirling body. The device is again constructed in the form of a cylindrical housing 60 containing a cylindrical centrebody 61, but in this case the internal surface of the housing's wall comprises the concave flow surfaces 62, while the centrebody is hollow, acting as a duct 63 for air or other fluid. Centrebody 61 comprises a cylindrical wall penetrated by passages or apertures 64 for directing fluid streams 65 onto the cusps 66 of the flow surfaces 62. Depending on the radial and axial orientation of the apertures 64, it is plain that fluid streams 65 will have at least a radial component of velocity away from the longitudinal centreline of the flow-swirling bod

In a manner analogous to that explained in connection with flow surfaces 44 in Figure 4A, it may be advantageous if both the centrebody 61 and the internal wall of the housing 60 are provided with confronting concave flow surfaces and cusps to define channels for constraining the helically flowing streams to flow in the streamwise direction.

In Figures 3 to 6, a circumscribing envelope drawn to touch the tips of the cusps of the concave flow surfaces along their lengths would be parallel to the longitudinal centreline of the flow swirling body, since the flow surfaces in these examples are superimposed on a base cylinder. Figure 7 illustrates an alternative arrangement in which such a circumscribing envelope of cusps 70 is tapered with respect to the longitudinal centreline of a centrebody 71, the basic shape of the centrebody being a conical shape which is convergent in the downstream direction.

It should also be noted in connection with Figure 7 that the concave flow surfaces 72 extend in helically convergent manner around the longitudinal centreline of the centrebody, whereas in previously described embodiments of the invention, they extend in the same direction as the longitudinal centreline. However, such helically formed flow surfaces are not necessarily a preferred configuration for either a cylindrical flow swirling body or a tapered flow swirling body. In either case, the result aimed for is a fluid flow along the flow surfaces which is a compound helical motion with respect to the longitudinal centreline, so providing a greater length of swirling flow over which to accomplish the mixing task.

In Figure 7, a sheet metal housing 73 has an upstream end wall 74 and a convergent frustoconical wall portion 75 which is coextensive with, but spaced radially outwards from the conical centrebody 71. The angle of convergence, X, of the frustoconical wall 75 and the centrebody 71 may be in the range 30 - 45". At its downstream end the housing 73 is joined to a divergent duct 76 in the present example. However, the angle of divergence, Y, defining the abruptness of the transition from the exit of the mixing device to the entry to the duct 76, could be anywhere in the range zero to 90 , preferably 450 to 900.

An axial gap exists between the end wall 74 of the housing 73 and the convergent wall 75. A ring of equiangularly spaced vanes or wedges 77 bridges the gap to connect the two parts of the housing and define passages between the vanes or wedges for directing streams of air 78 or other reactant radially inwards onto the helically extending cusps 70 of the centrebody. Initial streams 78 impinge the cusps at multiple points around the centrebody 71. In order that each cusp should intercept a stream 78 and that the concave flow surfaces 72 should carry their resulting streams along the length of the centrebody in a continuous helical path around its centreline, the cusps and flow surfaces are configured with respect to the centrebody in the manner of a multi-start screw thread.

Entry of streams 78 need not be in an exactly radial orientation and Figure 7 shows in dashed lines an alternative arrangement to provide initial streams 78' with an initial axial (downstream) component of velocity before they impinge on the cusps 70. Angle Z, defining the angle of the streams 78' relative to a purely radial direction, may vary between zero and 45" and should preferably match the helix angle of the cusps 70 relative to the centrebody 71.

As in Figures 4 to 6, the concave flow surfaces 72 of the flow-swirling centrebody 71 are provided with spray holes 79 for spraying of a fuel or other fluid into the helically flowing streams of air or other fluid which are flowing along their respective concave flow surfaces 72. As before, the spray holes 79 are arranged in rows along the con cave flow surfaces and connected with supply passages (not shown) in the flowswirling centrebody.

Referring now to Figure 8, a combustor 80 comprises a combustor can 81, being one of several such cans arranged around the circumference of a gas turbine engine. The can 81 is seen as a cross-section along its longitudinal centreline CL. Each can 81 has a relatively large internal diameter D over most of its axial length, but at its head or upstream end narrows quite abruptly to a smaller internal diameter d. Although described with reference to this generally cylindrical type of combustor can, the invention should not be restricted to such, and is also applicable to the annular type of combustor.

Each can 81 is provided with a so-called "burner" or fuel injector assembly 82 at its head end. In the assembly 82, a liquid or gaseous fuel feed 83 connects to a central fuel injector body 82A on the centreline CL of the can 81. In turn, the central injector body 82A is located in a central bore of an outer injector body 82B.

In injector body 82A, the fuel feed 83 is connected to a central bore 82C, which conducts the fuel to a flow swirling centrebody 82D fixed to an end plate 82E and projecting into the small diameter head end of the combustor can 81. A flanged cylindrical member 82G of internal diameter d forms the combustor can's circumferential wall at its head end and provides the housing required to enable a flow-swirling arrangement similar to that shown in Figures 4 or 5.

In common with central injector body 82A, outer injector body 82B is connected to end plate 82E. An annular array of vanes or wedges 82F are secured to the rear face of end plate 82E so that flow passages are defined between adjacent vanes. These vanes or wedges 82F are disposed radially outwards of the flow swirling centrebody 82D and concentric with it. In operation, the main fuel inlet passage 82C feeds a gallery (not shown) within centrebody 82D. As described in connection with Figure 4, the gallery in turn is connected through drillings (not shown) to spray holes 84 near the cusps of the flow surfaces 85. Compressed air 86 flows inwards towards the centrebody 82D between the adjacent vanes 82F, impinges on the cusps and forms resulting vortical streams on flow surfaces 85, thereby mixing with fuel from spray holes 84. Air 86 is taken from the volume surrounding the combustor can 81 and is supplied in the usual way from the outlet of the gas turbine engine's compressor (not shown).

When the fuel and air leaves the downstream end of the centrebody 82D, it is substantially fully mixed and is ready for burning in a combustion zone 81 A, the zone being substantially symmetrical about the can's centreline CL. The overall combustion process in combustor can 81 may be initiated and sustained at start-up and low engine loadings by combustion of a fuel-rich fuel/air mixture in region 81 A, the mixture being rich at these engine loadings to ensure stable combustion. If desired, such a fuel-rich mixture could be supplied to region 81A through a separate fuel feed (not shown) in the centrebody 82D to a small injector nozzle (e.g., an airspray burner, not shown) or spray bar (not shown) located in the centrebody's downstream end. This would provide the combustor can 81 with a suitable pilot burner for supporting a diffusion pilot flame at low engine powers. At higher part-load and full load engine conditions the overall combustion process in region 81A is preferably lean-burn and any pilot fuel supply would be turned down or off.

Optionally, additional gaseous or liquid fuel may be fed through outer injector body 82B to spray bars or the like (not shown) situated to spray fuel into the flow passages between vanes 82F. In this case, mixing of the initial streams of fuel and air with further gaseous or liquid fuel from spray holes 84 would be completed by the vortical mixing action on concave flow surfaces 85.

It is important to ensure that the combustion process does not propagate or "flash back" from the combustion zone 81A up the flow of fuel/air mixture along concave flow surfaces 85, since the heat of combustion would damage the centrebody 82D and the vane ring 82F. Hence, the downstream velocity of the fuel/air mixture through the circumferential gap 81B, between the centrebody 82D and the housing 82G, should be at least equal to the upstream propagation velocity of the combustion process at the same axial station.

Support for the combustor can 81 in its location within the engine is conveniently provided at its rear, i.e. downstream, end (not shown) by attachment in known ways through a combustor exit nozzle to suitable static structure of the engine, such as nozzle guide vanes at the entry to the high pressure turbine. Support at the can's front, i.e. upstream, end is conveniently provided by securing the injector assembly 82 to a portion of combustor casing 88. The latter has an aperture within which is secured the outer injector body 82B. Connection of the can 81 to the casing 88 is completed by fixing an upstream flange 82H of the can to the rear face of a ring 82I to which the vanes 82F are fixed. Fixing of the various parts of the injector assembly 82 to each other and to the casing 88 is conventionally achieved by setscrews, bolts, or the like.

Figure 9 shows a combustor with an overall structure and function substantially the same as Figure 8, so will not be described in detail. It differs from Figure 8 in that its flow swirling centrebody 92D and its surrounding housing 92G are conical or frustoconical in shape, giving a fuel/air mixing and swirling arrangement similar to that described in relation to Figure 7. However, although the conical centrebody 92D is illustrated with spirally arranged concave flow surfaces and cusps, as in Figure 7, it could alternatively be provided with concave flow surfaces which extend linearly along it, as in Figure 8, but in a mutually convergent arrangement. Hence, the flow surfaces comprising each pair of flow surfaces would extend parallel to each other, but adjacent pairs of flow surfaces with their dividing cusps would be widely circumferentially spaced apart at the upstream end of the centrebody 92D, but closely spaced or contiguous at its downstream end.

Figure 10 shows the portion of the combustor wall 81 within the dotted rectangle in Figure 8. It will be seen that the wall 81 comprises an inner wall 101 and an outer wall 102 which is spaced from wall 101 but extends substantially parallel to it, so define a gap between the walls. Rows of small holes 103 extend longitudinally along the outer wall 102. Due to the higher air pressure in the volume surrounding the combustor can 81, the holes 103 cause jets of air 104 to impinge on the radially outer surface of the inner wall 101 to cool it, because it is exposed to heat from the combustion process it contains.

This provides an example of the use of the invention for heat exchange purposes, in that the outer surface of wall 101 comprises concave flow surfaces 105, so that wall 101 essentially serves as a flow-swirling body similar to that in Figure 3. Heat is transferred from the inner wall 101 to the counter-rotating swirling streams 108 of cooling air flowing past it along the concave flow surfaces 105, the rows of holes 103 being in registration with the cusps 106 to direct the fluid at the cusps as described previously. Flow surfaces 105 may be formed integrally with the inner wall 101, or alternatively as part of a thin sheet metal layer 107 as shown. Such a sheet metal layer may be brazed or diffusion bonded to the wall surface.

Figure 11 shows an alternative arrangement in which the combustor can 81 is doublewalled as in Figure 10, but the outer wall 102 is provided with a single circumferentially extending band of elongate holes or slits 110 and the inner wall 101 is provided with a circumferentially extending band of concave flow surfaces 111 whose cusps 112 are in one-to-one registration with the slits 110. As in Figure 10, the flow surfaces 111 are formed as part of a thin sheet metal layer 113 bonded to the wall surface, but in this case their axial extent is essentially restricted to that of the slits 110.

Nevertheless, after initial impingement of air or other coolant 114 (e.g., steam) on the cusps 112, swirling streams 115 are produced which flow downstream over the outer surface of the inner wall 101 to remove heat therefrom by a continuing scrubbing action.

Claims (23)

1. A device for generating streamwise vorticity in fluid flows, comprising: (a) a plurality of concave flow surfaces arranged such that adjacent flow surfaces (i) extend parallel to each other in a desired streamwise direction, (ii) have contiguous streamwise extending edges, and (iii) are concave transverse of the streamwise direction such that the contigu ous edges of adjacent concave flow surfaces form a cusp dividing the adjacent concave flow surfaces from each other; (b) flow directing means for directing a stream of fluid in an initial direction onto a corresponding cusp such that the stream flowing in the initial direction is di vided into two resulting streams, each of which follows its respective concave flow surface such that a swirling fluid flow is established with respect to the con cave flow surface, vortices on opposed sides of the cusps thereby rotating in op posite senses; and (c) means for establishing flow momentum of the resulting streams along their respective flow surfaces in the desired streamwise direction, whereby the fluid flow in the resulting streams is helical.

2. A device according to claim 1, in which the concave flow surfaces are arranged equiangularly around a common centre such that they define a flow swirling body having a longitudinal centreline extending in a predetermined direction.

3. A device according to claim 1 or claim 2, in which the concave flow surfaces ex tend in the same direction as the longitudinal centreline.

4. A device according to claim 1 or claim 2, in which the concave flow surfaces ex tend helically around the longitudinal centreline, such that the fluid flow along the flow surfaces with respect to the longitudinal centreline is a compound helical motion.

5. A device according to any preceding claim, in which a circumscribing envelope drawn to touch the tips of the cusps of the concave flow surfaces along their lengths is parallel to the longitudinal centreline of the flow swirling body, being of cylindrical form.

6. A device according to any one of claims 1 to 4, in which a circumscribing enve lope drawn to touch the tips of the cusps of the concave flow surfaces along their lengths is tapered with respect to the longitudinal centreline of the flow swirling body.

7. A device according to any preceding claim, in which the flow directing means for directing a fluid stream onto a corresponding cusp comprises nozzle means.

8. A device according to any preceding claim, in which the flow directing means for directing a fluid stream onto a corresponding cusp comprises passage means provided in wall means spaced from the flow surfaces, the passage means being located in registration with the cusps.

9. A device according to any preceding claim, in which flow momentum of the heli cally flowing streams along their respective flow surfaces in the streamwise di rection is established by orienting the flow directing means with respect to the cusps such that each stream of fluid flowing in the initial direction has a compo nent of velocity in the streamwise direction.

10. A device according to any preceding claim, in which the device includes means for establishing a pressure gradient in the streamwise direction such that the fluid pressure is greater at the upstream parts of the flow surfaces than at their down stream parts.

11. A device according to claim 10, in which means for establishing a pressure gradi ent comprises wall means at upstream ends of the flow surfaces, the wall means being effective to impede flow of fluid therepast.

12. A device according to any preceding claim, in which the concave flow surfaces comprise an external surface of the flow-swirling body, such that each stream of fluid directed onto a corresponding cusp has at least a component of velocity to wards the longitudinal centreline of the flow-swirling body.

13. A device according to any one of claims 1 to 11, in which the concave flow sur faces comprise an internal surface of the flow-swirling body, such that each stream of fluid directed onto a corresponding cusp has at least a component of velocity away from the longitudinal centreline of the flow-swirling body.

14. A device according to claim 12, in which the device is constructed in the form of a housing containing a centrebody, the centrebody comprising the flow-swirling body, the housing having (a) peripheral wall means including passage means for directing fluid streams onto corresponding cusps and (b) end wall means at an upstream end of the flow-swirling body, the housing being open-ended at an extremity thereof opposite the end wall means, whereby the resulting streams flow off the downstream ends of the flow surfaces on the flow-swirling body and commingle as they exit the open end of the housing.

15. A device according to claim 13, in which the device is constructed in the form of a housing containing a centrebody, the housing's internal wall means comprising the flow-swirling body, and the centrebody comprising wall means containing passage means for directing fluid streams onto corresponding cusps, the housing having end wall means at an upstream end of the flow-swirling body and being open-ended at an extremity thereof opposite the end wall means, whereby the re sulting streams flow off the downstream ends of the flow surfaces and commingle as they exit the open end of the housing.

16. A device according to claim 14 or claim 15, in which both the centrebody com ponent and the internal wall means component of the housing are provided with confronting concave flow surfaces, the concave flow surfaces on one component being shaped and disposed in a manner which is complementary to the shape and disposition of the concave flow surfaces on the other component, whereby the confronting flow surfaces define therebetween channels for constraining the heli cally flowing streams to flow in the streamwise direction.

17. A device according to any preceding claim, in which the concave flow surfaces of the flow-swirling body are provided with holes for spraying of a fuel in fluid form therefrom to mix with a fluid oxidant comprising the helically flowing streams flowing along their respective concave flow surfaces, the spray holes be ing connected with fuel supply passage means in the flow-swirling body.

18. A device according to claim 17, in which the spray holes are arranged in rows along the streamwise extent of the concave flow surfaces.

19. A device according to any one of claims 1 to 16, in which the concave flow sur faces comprise a surface of a first wall which serves as the flow-swirling body, heat being transferred between the first wall and a heat transfer fluid flowing therepast, a second wall being spaced from the first wall but extending substan tially parallel thereto to define a gap therebetween, the second wall having holes in registration with the cusps of the flow surfaces to direct the heat transfer fluid at the cusps to produce resulting streams of heat transfer fluid with counter rotating helical flow in the desired streamwise direction along the first wall.

20. A combustor provided with a device according to claim 17 or claim 18, in which the device feeds a mixture of fuel and oxidant to a combustion chamber.

21. A combustor provided with a device according to claim 19, in which the device serves to cool a wall of a combustion chamber.

22. A device for generating streamwise vorticity in fluid flows, substantially as de scribed herein with reference to any one of the accompanying drawings.

23. A combustion chamber, substantially as described herein with reference to any one of Figures 8 to 11 of the accompanying drawings.