GB2429937A - Apparatus for mixing gas streams - Google Patents

Abstract

An apparatus for mixing gas streams includes a duct 46 for carrying the first gas stream in a certain direction; at least one conduit 32 for carrying the second gas stream so that it interacts with the first gas stream, and a number of first holes 40a,b arranged along the conduit for allowing the second gas stream to leave the conduit and enter the first gas stream for mixing purposes. Each first hole is angled against the certain direction by an angle a , where 0{ < a < 45{ and 45{ < a < 90{ and where 0{ corresponds to a direction of flow of the second gas stream that is directly opposite to the certain direction. Preferably each hole is angled against the certain direction by an angle of 15{ or 75{, with a tolerance of 12{, 8{ or 5{. The conduits may be located either a short distance inside the duct at an upstream end in terms of first gas stream flow, or a short distance outside the duct likewise at its upstream end.

Description

APPARATUS FOR MIXING GAS STREAMS

The invention relates to an apparatus for mixing gas streams and to a gas turbine comprising an apparatus for mixing gas streams.

Gas turbines are known which employ a gas feedback arrangement for feeding some of the compressed air produced by the compressor back to the inlet of the compressor. This scheme is shown in simplified form in Fig. I a. In Fig. I a a conduit 10 is shown connecting the outlet of the compressor stage 12 with the air inlet 14 of the compressor. The conduit 10 is positioned in a duct 16, which supplies the majority air to the compressor. The air fed back from the compressor outlet is at high pressure and temperature and acts as a source of minority air which is mixed with the majority air in the duct 16. In order to achieve an effective degree of mixing, the portion of the conduit inside the duct has a series of holes 18, which are oriented with respect to the direction of majority airflow at an angle of 45 (see Fig. lb. which shows the conduit 10 in pian view inside the duct).

Such a mixing arrangement is employed in a variety of applications of the air feedback, including the reduction of harmfiul emissions or the heating of the inlet or de- icing of the compressor. While the described air-feedback arrangement is reasonably efficient, it is not ideal by any means for any of the applications for which it is employed.

There is therefore an incentive to provide better mixing, since this can minimise the stratification of flow and thereby reduce the potential for reduced compressor and combustor perfonnance, especially in view of the elevated temperatures (e.g., in excess of 400 C) of the fed-back air.

The present invention is based on a recognition by the present inventors that the described prior-art system is a compromise solution to the pmblem of providing effective mixing of two gases and that better mixing can be achieved by tailoring the mixing characteristics to the individual applications. Such better mixing is achieved in the present invention by varying the angular orientation of the holes, through which the minority air passes, with respect to the direction of the majority air flow.

Accordingly there is provided under a first aspect of the present invention an apparatus for mixing first and second gas streams, comprising: a duct for carrying the first gas stream in a predetermined direction; at least one conduit for carrying the second gas stream, the at least one conduit being disposed so that, in use, it is impacted by the first gas stream; and a plurality of first holes disposed along the conduit and providing a route for the second gas stream to leave the conduit and enter the first gas stream to mix therewith; wherein each first hole is angled against said predetermined direction by an angle which is greater than 00 and less than 45 , or by an angle which is greater than 45 and less than 90 , the O angle corresponding to a direction of flow of said second gas stream which is directly opposite to said predetermined direction.

Preferably each first hole is angled against said predetermined direction by an angle of 15 plus or minus 12 , or by an angle of 75 plus or minus 12 . More preferably the tolerance on these angles is 8 , and even more preferably 5 .

A plurality of conduits may be arranged spaced apart over a cross-section of said duct. The conduits are preferably arranged approximately equidistantly to each other over the cross-section of said duct.

Each conduit advantageously has first and second sets of angled holes at substantially equal, but opposite, angles to said predetermined direction. Of the mutually opposite-facing sets of angled holes of two adjacent conduits, the angled holes of one of the conduits may be staggered with respect to the angled holes of the other conduit.

Furthermore the first set of angled holes of a conduit may be staggered with respect to the second set of angled holes of the same conduit.

It is preferable if, in each of those conduits which are disposed nearest the ends of a width dimension of said duct, the set of angled holes facing toward said ends are greater in number than the set of angled holes facing away from said ends, and are of correspondingly smaller size.

The conduits may be disposed outside of said duct and upstream thereof with respect to the flow direction of said first gas stream. Alternatively, the conduits may be disposed inside said duct, and those conduits which are disposed adjacent the side-walls of said duct may be spaced apart from said side-walls by a distance equal to 0.75 0.25 times the distance between adjacent conduits. An alternative - and preferred distance is 0.75 0.10 times the distance between adjacent conduits. l0

The angled holes at the ends of the conduits may be spaced apart from the top or bottom walls of the duct by a distance equal to 0.75 0.25 times the distance between adjacent angled holes in the same set of angled holes. An alternative - and preferred - distance is 0.75 0.10 times the distance between adjacent angled holes in the same set of angled holes.

The at least one conduit may be a conduit in spiral form, in which the angled holes are provided along the length of the spiral.

It is expedient if the apparatus further comprises a plurality of second holes disposed along each of the at least one conduit, each second hole being angled against said predetermined direction by an angle of substantially 00. The relative sizes of said first and second holes may be such as to provide a greater flow of the second gas through the first holes than through the second holes. More precisely, these relative sizes may be such as to provide between inclusively four times and 20 times as much flow of the second gas through the first holes as through the second holes.

The second holes may be disposed between adjacent said first holes. There may be at least two second holes between adjacent said first holes.

The at least one conduit may have a circular or aerofoil cross-section.

A plurality of condu its may be provided, said conduits being connected together by way of a manifold for supply of said second gas stream to all of said conduits from a common source. A plurality of sets of conduits may be provided, the conduits of each set being connected together by way of a manifold for supply of said second gas stream to all of the conduits in that set.

The conduits in one set may alternate with the conduits in another set, in which case the inner ends of the conduits of the one set may or may not extend beyond the inner ends of the conduits in the other set. Alternatively, the conduits in each set may be adjacent to each other.

There may be two sets of conduits located at opposite sides of the duct and the conduits may extend to less than one half of the distance between said sides, the inner ends of the two sets of conduits being blind and the outer ends of the two sets of conduits being fed from respective manifolds.

In a second aspect of the invention a gas turbine is pmvided, comprising: a compressor, and an apparatus for mixing first and second gas streams as claimed in any one of the preceding claims, wherein said duct is located at an inlet of said compressor, said first gas stream corresponds to an air stream provided from outside the compressor and said second gas stream corresponds to air at an elevated temperature and pressure fed back from an outlet of said compressor to said at least one conduit for mixing with said air stream.

The outlet may be a final outlet of the compressor or an intermediate bleed point of the compressor.

An embodiment of the invention will now be described, by way of nonlimiting example only, with the aid of the drawings, of which: Figs I a and lb are schematic diagrams of a typical gas turbine incorporating a known mixing arrangement for majority and minority gas streams; Fig. 2 is a perspective view of a duct suitable for use as part of a mixing apparatus in accordance with the present invention; Fig 3 is a side-view of a conduit arrangement employed as part of a first embodiment of a mixing apparatus according to the invention; Fig. 4 is an end-view of a conduit employed in the conduit arrangement under the first embodiment; Fig. 5 is a side-view of a conduit arrangement employed as part of a second embodiment of a mixing apparatus according to the invention; Fig. 6 is an end-view of a conduit employed in the conduit arrangement under the second embodiment; Figs 7a and 7b are conduit end-views correspond to Figs 4 and 6, respectively, and showing two components of the minority flow in the conduits; Figs 8a and 8b are side views of two adjacent conduits in a mixing apparatus according to the invention and illustrating two different strengths of minority flow injection; Figs 9a and 9b show two different exemplaiy hole geometries in a conduit associated with a mixing apparatus according to the invention; Figs l0a- lOd are four variant conduit configurations that may be employed in a mixing apparatus according to the present invention; and Figs ha, 1 lb and lie show three alternative forms of conduit that may be employed in a mixing apparatus according to the present invention.

A duct suitable for use in the present invention, and which corresponds to the duct 16 shown in Fig. I a, is shown as item 20 in Fig. 2. The duct is of rectangular cross-section and has openings at the front and the back, as shown. The majority flow of air from the outside is in the direction of the arrow 22, while the minority flow of fed-back air from the outlet of the compressor enters a manifold 24 as shown by the arrow 26. The manifold 24 is blind at its distal end 30 and therefore the flow entering the proximal end of the manifold must exit at the tap-off points 28 to flow along the conduits 32. The conduits 32 penetrate the longer sides of the duct 20 close to the front opening thereof and are secured at the penetration points so that they cannot move. The conduits are substantially equidistantly spaced apart from each other by a distance s and are oriented substantially parallel to the short sides of the duct 20.

Each conduit has several sets of equidistantly spaced holes along its length, as shown in Figs 3 and 4. The inner four conduits 32 are provided with a first set of larger holes 40a and a second set of holes 40b of the same size running longitudinally and spaced apart by a distance b. In a first embodiment of the invention these holes subtend an angle of approximately 75 ( 5 ) with a line 42 extending through the centre of the conduit 32 and parallel to the direction of the majority flow of the air into the duct (i.e. into the page in Fig. 3). Hence they will henceforth be termed "angled holes". The first set of angled holes 40a are staggered longitudinally with respect to the second set of angled holes 40b.

Furthermore the second set of angled holes in one conduit is staggered longitudinally with respect to the first set of angled holes in the next conduit, as is clearly seen in Fig. 3. In addition a third set of equidistantly spaced holes 44 is provided. These holes 44 are substantially smaller than the holes 40 (they are between 5% and 25% of the size of holes 40, typically being 10%) and are located where the line 42 meets the periphery of the conduit 32 at its upstream point. Hence these holes form an angle of 0 with the direction of the majority flow into the duct. The holes 44 are spaced apart from each other by half the distance between adjacent angled holes 40a or 40b and by the same distance as the spacing between an angled hole 40a and an adjacent angled hole 40b.

The end-conduits 32' are of the same design as the inner conduits 32, except that the first set of angled holes 40a in the right-hand conduit 32' are approximately twice as numerous as the second set of angled holes 40b in the same conduit, while the second set of angled holes 40b in the left-hand conduit 32' are approximately twice as numerous as the first set of angled holes 40a in the same conduit. This is in order to create a "blanket" of minority flow in the wall area due to the lack of another adjacent conduit to provide half of the flow in this area. At the same time these more numerous holes are approximately half the size of the other angled holes 40a, 40b. Furthermore, the distance between the endconduits 32' from the side-walls 46 is approximately equal to 0.75 ( 0.25, preferably 0.10) x s, while the distance between the outermost angled hole at the ends of each conduit and the top and bottom walls 48 is approximately 0. 75 ( 0.25, preferably 0.10) x b. The cross-sectional dimensions of the duct are shown as width x and height z in Fig. 3.

In a second embodiment of the invention the angle subtended by the angled holes with respect to the line 42 in Fig. 4 is appmximately 15 ( 5 ) instead of approximately 75 . The other details are the same. This situation is shown in Figs 5 and 6.

The effect of both these embodiments is illustrated in Figs 7a and 7b.

Fig. 7a shows the situation in the first embodiment, in which the angle of the holes is appmximately 75 ( 5 ). The minority flow is split into two parts: a first part 50 through the angled holes 40a, 40b and a second part 52 through the non-angled holes 44.

In practice the fed-back air from the compressor outlet is injected at a predetermined mach number, into the manifold 24 (see Fig. 2) and, since the angled holes 40a, b are typically ten times the size of the nonangled holes 44, approximately 91% of the minority flow passes thmugh the angled holes, whereas only 9% passes thmugh the non-angled holes. Consequently the flow through the angled holes is stronger and is able to reach into the intervening space between the conduits and the flow through the non-angled holes mixes in the wake behind the conduit. The flow 50 is impacted upon by the majority flow entering the duct and, since the minority flow is injected at the angle shown, mixing occurs through the shear interaction of the minority flow with the majority flow. Indeed, the inventors have found that most of the mixing occurs due to this very interaction in the area of the conduits themselves and only a little flirther mixing occurs further downstream in the rest of the duct. The flow lines 50 and 52 in Fig. 7a clearly show how the momentum of the jets exiting the angled holes collides with the mainstream (majority) flow, causing the jets to disperse, turn and mix with the mainstream. The same applies to the smaller jets exiting the non-angled holes 44, but at a lower momentum.

Fig. 7b illustrates the same situation for the second embodiment, in which the angle of the holes 40 is set to 150 (+50) instead of 750 The arrangement shown in Fig 7b achieves comparable mixing performance to that of Fig 7a at the same minority flow rate and injection mach numbers. The smaller injection angle achieves higher penetration against the majority flow so that the flow has a longer path in which to mix. This compensates for the lower lateral interaction between minority and majority flows. However the arrangement of Fig 7b gives higher injection momentum against the majority flow, thus providing an increased majority flow pressure loss which is beneficial to Gas Turbine part-load emission control through a reduced mass flow and hence power for a given firing temperature. On the other hand, the arrangement of Fig 7a has a lower axial component of injection momentum against the flow, and consequently it produces a lower majority flow pressure loss. This is advantageous for applications such as heating the majority flow at low ambient temperatures, and anti-icing, where it is preferable to maximise the engine power at a given firing temperature.

In order to achieve effective mixing, it is preferable if the penetration distance of the angled jets is at least one-half of the inter-conduit spacing s. This is shown in Fig. 8a, in which the jets 50 from the angled holes of one conduit reach, at the predetermined lull mach number, Mj, more than halfway across to the next conduit. Also visible is the effect of the interleaving between the jets of the two adjacent conduits. Due to this configuration it is ensured that stratification of the mixing process (i.e. uneven mixing over the cross- section of the duct) is reduced to a low level. Assisting in this is the use of the smaller "0 " holes 44. These provide a flow having only a small reach, but which effectively helps to fill in the gaps 52 near and behind the conduits 32 and on both sides thereof, which the main minority flow coming fhm the angled holes 40 cannot reach. Hence these smaller holes may be called "filler" holes, in contrast to the "angled" holes 40.

Fig. Sb shows the situation where the minority flow is injected at less than the tuii Mach number, Penetration into the gap between the conduits can be seen to be less, due to the decrease in the momentum of the jets, but even then the gap between the bars is adequately covered.

The procedure for designing the size and number of the holes and the number of the conduits will now be described. It is assumed in what follows that the angles of the angled holes will be as already stipulated, namely either approximately 150 ( 5 ) or approximately 75 ( 5 ). These values have been found by the inventors to provide the best performance for the various applications mentioned earlier.

Firstly it is decided how many conduits will be employed. This is to some extent an arbitrary decision, though it is assumed that a larger number of conduits will result in better mixing over the cross-section of the duct due to the better spread of the minority flow over that cross- section. However, for a given duct size, too many conduits could be counter-productive because the majority flow pressure loss would be increased at all conditions, which is potentially detrimental when the minority flow is set to zero. In addition too many conduits would add to the materials and manufacturing cost of the gas turbine. The number also depends on the relative sizes of the conduit diameter and the duct crosssection. Fig. 2 shows a typical application employing a total of six conduits spaced over the width of the duct.

Having decided on the number of conduits n to be used, a value of conduit spacing s is determined according to the following formula based on the cross-sectional dimension x of the duct across which the conduits are to be arranged. x s=

(n+0.5) Thus, for the Fig. 2 example just mentioned, in which the duct has six conduits, whereas it might normally be considered appropriate to make the spacing between the conduits equal to one-sixth of the duct width, which would leave a spacing between the outer conduits and the side-walls of the duct equal to one-half of the inter-conduit spacing, in practice this end-spacing to the duct wall is increased to approximately three-quarters of the inter- conduit spacing in order to reduce the risk of hot air clinging to the wall. This means that the spacing between the conduits is equal to the duct width divided by 6.5 instead of 6.

A decision then has to be made on the size of the holes ("angled" and "filler") and their spacing and angle with respect to the majority flow. The first consideration, however, is to establish a maximum value of injection Mach number, This should be subsonic at all times, in order to prevent choking of the flow across the holes and the subsequent shockwaves and flow disturbance that would result from this and could damage the conduits and/or the duct and also give rise to poor mixing of the flow and possible noise issues. It is also assumed that, on any particular application, the size and angle of the spray holes will remain unchanged, consequently the Mach number will be reduced from its maximum value when reduced flow is required.

Having established the number of conduits required, their spacing and a maximum value of Mach number, it is then possible to determine the number and size of the holes and their spacing. Using the formula: flaX I 25x lflg < (pU,2) xsin(9 30)) Ifl +fllg pgU where Ymax = penetration distance required from each jet (m) d1 effective jet diameter required (m) mg = mass flow rate of majority flow (kg/s) m mass flow rate ofjets (kg/s) Ug = velocity of majority flow (mis) U velocity ofjets (mis) Pg density of majority flow (kg/m3) p3 density ofjet stream (kg/m3), 0 angle of exiting jet relative to majority-flow direction (zero-degrees angle is directly against the majority flow) and given a desired value of maximum inter-conduit penetration, which as mentioned earlier will be at least equal to one-half of the inter-conduit spacing s, a value of effective jet diameter d can be determined. From this effective jet diameter and the hole discharge coefficient (typically 0.75), the required hole diameter is calculated. The number of conduits (e.g. six) arrived at earlier, in conjunction with the required maximum Mach number, likewise decided on earlier, allows the number of holes per conduit] to be determined. In calculating the number of holes per conduit, allowance must be made for the flow through the filler holes 44. This is assumed to be a maximum of 25% of the total minority flow, and is preferably not lower than 5%, for example.

The value of the spacing (3s/4) between the end-conduits and the sidewalls of the duct and of the spacing (3b/4) between the end-holes on any one conduit and the top and bottom walls of the duct have already been specified earlier. These spacing values are to ensure that hot air from the minority flow does not cling to the cooler walls of the duct.

The resulting hole geometry is defined in Figs 9a and 9b, for odd and even values of j respectively.

In addition, again as already mentioned, the outer-facing angled holes of the end- conduits will be larger in number (i.e. approximately twice as many) than the rest of the sets of angled holes. This reduces the risk of mixing stratification, which is particularly acute near to the walls of the duct. As already mentioned, the size of these outer-facing angled holes in the end-conduits should be proportionately smaller. This ensures that there is the same effective total hole-injection area in each conduit.

What has been described is a preferred realisation of the invention involving two particular angles of the angled holes. Also given is a figure for the tolerance on that angle, namely 5 . However, other angles and tolerances are possible. Indeed, the invention envisages the use of any angle except for 0 , 45 and 90 , since the invention is based on the recognition that the angle of the main jets can be varied from the known 45 in order to suit various applications of the feedback arrangement. However, experiment has shown that an angle of around 150 is best suited when the feedback mechanism is being used to reduce harmful emissions, as this means increased majority flow pressure loss due to injection jet momentum during operation. On the other hand, 75 is best when feedback is being used for heating and anti-icing purposes. The tolerances given on this angle may be anything between approximately 5 and 150, with 50, 8 and 12 being preferred values.

Different configurations of the conduits are possible within the scope of the invention. For example, although it has been assumed that the conduits will be situated inside the duct, they may alternatively be situated a short way outside the duct and upstream of it with respect to the majority flow. Also, instead of the conduits being fed from a single manifold, as shown in Fig. 2, they may instead be fed from two or more.

Two examples of this are illustrated in Figs l0a and lOb. In Fig. lOa the six conduits present in the preferred embodiment described above are divided into two sets of three, alternate conduits 32a and 32b being associated with different manifolds 24a and 24b, respectively. An advantage of this configuration is that, since the manifolds have to serve fewer conduits, they can be made of smaller diameter, which may be better suited to some environments. A similar arrangement is shown in Fig. lOb, in which instead of the two manifolds 24a and 24b serving alternative conduits, they serve different adjacent sets of conduits, 32a and 32b.

Fig. lOc shows a further possible arrangement. Here each conduit 32 is divided into two, each half-conduit thereby produced is blocked off at its inner end and corresponding half-conduits are disposed end-to-end with each other. The upper half- conduits 32a are serviced by the manifold 24a, while the lower half- conduits 32b are serviced by manifold 24b.

In Fig. lOd a further variant is shown, in which, as in Fig. lOa, alternate conduits are staggered, but only extend halfway along the height of the duct. The upper conduits 32a are fed from a first manifold 24a, while the lower conduits 32b are fed from a second manifold 24b.

Instead of using a number of discrete conduits, it is also possible to use a single conduit member, which runs continuously in a spiral shape across the cross-sectional area of the duct. Two examples of this are shown in Figs lEa and II b. In Fig. 11 a, for a cireular duct 56, a circular or aerofoil cross-section conduit 58 is arranged in a spiral over the duct area and the spiral is supported periodically along its length by struts 60 in some suitable manner. The filler holes are not shown in this example, but are assumed to be present in a manner similar to that shown in the earlier-described preferred embodiments.

The spiral is blind at its non-fed end, which may be either of the two ends shown.

However, it will normally be more expedient to supply the spiral from its outer end, since that is nearer the duct wall and will therefore presumably penetrate it, as shown, for feeding (and fixing) purposes.

Fig. 1 lb shows an alternative arrangement for rectangular ducts, in which the spiral conduit is composed of discrete pipe sections, which are welded together. Alternatively, a continuous pipe may be employed, which is bent at a tight radius at the corners.

It is also possible to employ a wave-shaped conduit instead of a spiral one. This is shown in Fig. 11 c (the basic shape only is shown, the holes and the supporting struts being omitted for convenience). The conduit cross-section may be any convenient shape, e.g. rectangular or circular, etc. As with Fig. I Ib, the Fig. lie configuration may be all in one piece, the corners then having a minimum required radius of curvature, or in discrete welded sections, in which case the corners may have a negligible radius of curvature.

In all of the Fig. II configurations, but in particular Figs lib and lic, the considerations previously set forth as regards hole and conduit spacing apply here also.

This includes the preferred staggered arrangement of the two sets of angled holes in each section of the spiral or wave and the preferred staggered arrangement of the mutually facing sets of angled holes in adjacent parallel sections of the spiral or wave, also the preferred spacing between the end-sections and the nearby walls of the duct and the preferred spacing between the angled holes at the longitudinal ends of the relevant sections and the nearby walls. The same applies as far as possible to Fig. I la, though it is understood that it may not be necessary to provide smaller and more numerous outer- facing angled holes over much of the length of the spiral pmximate the duct, since this length may already be sufficiently far from the duct.

Whereas the invention has been described thus far with only one filler hole 44 located between adjacent angled holes on different sides of the 00 line of each conduit, it is within the scope of the invention to employ two or more. Allowance for this must, of course, be made in the calculation stage when arriving at the sizes of the various holes.

The use of more filler holes in this way has the advantage of providing more even mixing coverage from top to bottom of the duct cross-section.

The invention has so far been described in connection with its use in association with the compressor stages of gas turbines, in which the fed-back minority gas and the majority gas is air. However, the mixing arrangement applies to any pair of gases, which may be required to mix with each other. Furthermore, these two gases may be of different compositions and may also be at different temperatures, as in the described preferred embodiment, or at the same temperature.

Although the duct has been assumed to be rectangular in cross-section, it may have other configurations, e.g. circular, as mentioned above in connection with one of the spiral arrangements. Also, the conduits may, in a rectangular cross-section duct, be disposed between the shorter sides, i.e. parallel to the longer sides. However, the conduits do not have to be parallel to any of the duct sides, but may instead be at an angle thereto.

Nevertheless, the preferred orientation is parallel, since that simplifies both the mechanical mounting arrangements and the calculations of conduit and hole spacing, sizes, etc. Fig. Ia showed the minority flow being sourced from the outlet of the compressor stage 12. This may be one of a number of such stages arranged in series, in known manner. However, the minority flow may also be derived from a bleed point between the inlet and outlet of the compressor stage. The invention also envisages a situation in which the second gas stream takes the form of a gaseous by- product of a remote industrial process which is fed into the gas turbine in order to destroy impurities contained therein.

An example is volatile organic compounds (VOCs), which may be found in paints, varnishes and wax, all of which contain organic solvents, as do many cleaning, disinfecting, cosmetic, degreasing and hobby products. Fuels are also made up of organic chemicals. Hence, for example, a situation is envisaged in which it may be desired to extract air from a paint box containing VOCs. Using the invention this air may, following compression, be injected into the duct as the second gas stream.

A still further possibility is for the second gas stream to be provided in the form of JO injected steam. The location of the duct could then be downstream of the compressor, but preferably before the combustion chamber. The duct in this case would preferably have an annular crosssection having an inner radius and an outer radius. An alternative location for the duct in such an injected-steam arrangement would be in an intermediate section before the power turbine. This could provide additional boost, which was either continuous or intermittent, depending on the availability of steam.

It is not strictly necessary for the angled and filler holes to be equidistantly spaced along the conduits, though this will generally provide a more even mixing action over the cross-sectional area of the duct.

Claims (31)

1. An apparatus for mixing first and second gas streams comprising: a duct for carrying the first gas stream in a predetermined direction; at least one conduit for carrying the second gas stream, the at least one conduit being disposed so that, in use, it is impacted by the first gas stream; and a plurality of first holes disposed along the conduit and providing a route for the second gas stream to leave the conduit and enter the first gas stream to mix therewith; wherein each first hole is angled against said predetermined direction by an angle which is greater than 0 and less than 45 , or by an angle which is greater than 45 and less than 900, the 0 angle corresponding to a direction of flow of said second gas stream which is directly opposite to said predetermined direction.

2. Apparatus according to claim 1, wherein each first hole is angled against said predetermined direction by an angle of 15 plus or minus 12 , or by an angle of 75 plus or minus 12 .

3. Apparatus according to claim 1, wherein each first hole is angled against said predetermined direction by an angle of 150 plus or minus 8 , or by an angle of 750 plus or minus 8 .

4. Apparatus according to claim 1, wherein each first hole is angled against said predetermined direction by an angle of 150 plus or minus 5 , or by an angle of 75 plus or minus 5 .

5. Apparatus according to any one of the preceding claims, wherein a plurality of conduits are arranged spaced apart over a cross-section of said duct.

6. Apparatus according to claim 5, wherein said conduits are arranged appmximately equidistantly to each other over the cmss-section of said duct.

7. Apparatus according to claim 6, wherein each conduit has first and second sets of angled holes at substantially equal, but opposite, angles to said predetermined direction.

8. Apparatus according to claim 7, wherein, of the mutually oppositefacing sets of angled holes of two adjacent conduits, the angled holes of one of the conduits are staggered with respect to the angled holes of the other conduit.

9. Apparatus according to claim 7 or claim 8, wherein the first set of angled holes of a conduit are staggered with respect to the second set of angled holes of the same conduit.

10. Apparatus according to claim 8 or claim 9, wherein, in each of those conduits which are disposed nearest the ends of a width dimension of said duct, the set of angled holes facing toward said ends are greater in number than the set of angled holes facing away from said ends, and are of correspondingly smaller size.

11. Apparatus according to any one of the preceding claims, wherein said conduits are disposed outside of said duct and upstream thereof with respect to the flow direction of said first gas stream.

12. Apparatus according to any one of claims I to 10, wherein said conduits are disposed inside said duct, and those conduits which are disposed adjacent the side-walls of said duct are spaced apart from said side-walls by a distance equal to 0.75 0.25 times the distance between adjacent conduits.

13. Apparatus according to any one of claims 1 to 10, wherein said conduits are disposed inside said duct, and those conduits which are disposed adjacent the side-walls of said duct are spaced apart from said side-walls by a distance equal to 0.75 0.10 times the distance between adjacent conduits.

14. Apparatus according to claim 12 or claim 13, wherein the angled holes at the ends of the conduits are spaced apart from the top or bottom walls of the duct by a distance equal to 0.75 0.25 times the distance between adjacent angled holes in the same set of angled holes.

15. Apparatus according to claim 12 or claim 13, wherein the angled holes at the ends of the conduits are spaced apart from the top or bottom walls of the duct by a distance equal to 0.75 0.10 times the distance between adjacent angled holes in the same set of angled holes.

16. Apparatus according to any one of claims I to 4, wherein said at least one conduit is a conduit in spiral form, in which the angled holes are provided along the length of the spiral.

17. Apparatus according to any one of the preceding claims, further comprising a plurality of second holes disposed along each of the at least one conduit, each second hole being angled against said predetermined direction by an angle of substantially 0

18. Apparatus according to claim 17, wherein the relative sizes of said first and second holes is such as to provide a greater flow of the second gas through the first holes than through the second holes.

19. Apparatus according to claim 18, wherein the relative sizes of said first and second holes is such as to provide between inclusively four times and twenty times as much flow of the second gas through the first holes as thmugh the second holes.

20. Apparatus according to any one of claims 17 to 19, wherein said second holes are disposed between adjacent said first holes.

21. Apparatus according to claim 20, wherein at least two second holes are disposed between adjacent said first holes.

22. Apparatus according to any one of the preceding claims, wherein said at least one conduit has a circular or aerofoil cross-section.

23. Apparatus according to claim I, wherein a plurality of conduits are provided, said conduits being connected together by way of a manifold for supply of said second gas stream to all of said conduits from a common source.

24. Apparatus according to claim 1, wherein a plurality of sets of conduits is provided, the conduits of each set being connected together by way of a manifold for supply of said second gas stream to all of the conduits in that set.

25. Apparatus according to claim 24, wherein the conduits in one set alternate with the conduits in another set.

26. Apparatus according to claim 25, wherein the inner ends of the conduits of the one set do not extend beyond the inner ends of the conduits in the other set.

27. Apparatus according to claim 24, wherein the conduits in each set are adjacent to each other.

28. Apparatus according to claim 27, wherein there are two sets of conduits located at opposite sides of the duct and the conduits extend to less than one half of the distance between said sides, the inner ends of the two sets of condu its being blind and the outer ends of the two sets of conduits being fed from respective manifolds.

29. A gas turbine comprising: a compressor, and an apparatus for mixing first and second gas streams as claimed in any one of the preceding claims, wherein said duct is located at an inlet of said compressor, said first gas stream corresponds to an air stream provided from outside the compressor and said second gas stream corresponds to air at an elevated temperature and pressure fed back from an outlet of said compressor to said at least one conduit for mixing with said air stream.

30. A gas turbine according to claim 29, wherein said outlet is a final outlet of the compressor.

31. A gas turbine according to claim 29, wherein said outlet is an intermediate bleed point of the compressor.