Mixer

Abstract

A mixer for mixing first and second fluids has a passageway along which a stream comprising the first fluid flows along an axis of the passageway. The passageway has, in sequence in the downstream direction, a convergent section, a throat, and a divergent section. An injector introduces the second fluid into the stream in the passageway upstream of the divergent section. A swirl generator in the passageway is upstream of the convergent section.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of the U.S. National Stage designation of co-pending International Patent Application PCT/EP2004/050074 filed Feb. 3, 2004, which claims priority to Great Britain patent application no. 0303495.6 filed Feb. 14, 2003, and the entire contents of these applications are expressly incorporated herein by reference thereto.

FIELD OF THE INVENTION

This invention relates to a mixer and a method for mixing first and second fluids. The two fluids may be gases, e.g. air and a combustible gas, or a gas and a liquid, e.g. air and a liquid fuel, or liquids. The mixer may, in particular, form part of a combustion device.

BACKGROUND OF THE INVENTION

A number of applications require that separate fluid streams be thoroughly mixed. One such application is catalytic combustion, where the fuel and air must be very well mixed prior to entry in the catalyst. This requirement also holds true for conventional lean-premix burners. However, current mixing techniques generally do not achieve homogeneous mixtures, and therefore the resulting combustion process is non-uniform; large temperature variations are observed, and significant NOx emissions associated with high-temperature areas are recorded.

Attaining high levels of mixedness between fluids is normally accompanied by large, undesirable pressure losses. The most promising option to date is that involving counter-swirling flows. However, complex aerodynamic designs (i.e. aerofoil sections) are essential components of such units, because the structures generating swirl must not form wakes including recirculation zones (which can lead to flashback and severe damage); furthermore, there is scope for significant improvement in mixing quality. For certain applications, e.g. those involving catalytic units, the catalyst inlet velocity distribution must be as uniform as possible; this is a difficult objective for conventional mixers which employ swirling flows. A further problem with many units is that of flashback, i.e. the phenomenon which entails a homogeneous flame moving upstream and into the mixer, often resulting in damage.

Venturi injectors are relatively simple devices for attaining reasonable mixing; however, the quality falls short of that achieved by the swirl-based concepts. Venturi units rely upon low local pressures to draw additive fluid into a carrier fluid; mixing is attained by virtue of the shear layer across the longitudinal jet of fluid, whose principal velocity component is axial. U.S. Pat. No. 4,123,800 describes a mixer in which a certain degree of twisting motion is imparted to the flow downstream of the Venturi constriction, to further aid in mixing, by means of skewed grooves machined into the walls of the divergent section downstream of a throat section into which the additive fluid is injected. However, this twisting motion is only imparted near the walls, without significantly affecting the bulk of the flow, and does not meaningfully assist the mixing process.

SUMMARY OF THE INVENTION

The present invention provides a passageway along which a stream comprising the first fluid flows along an axis of the passageway, the passageway having, in sequence in the downstream direction, a convergent section, a throat, and a divergent section; an injector for introducing the second fluid into the stream in the passageway upstream of the divergent section; and a swirl generator in the passageway upstream of the convergent section.

The invention also provides a method of mixing fluids, comprising the sequential steps of:

• • (a) providing a stream comprising a first fluid and having an axis along which the stream flows,
  • (b) inducing swirl in the stream about its axis,
  • (c) causing the stream to converge towards its axis, and
  • (d) causing the stream to diverge from its axis,
    The method including introducing a second fluid into the stream before step (d).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described further, by way of example only, with reference to the accompanying drawing, in which:

FIG. 1 is a schematic axial section through one embodiment of a mixer;

FIG. 2 is a graph of angular velocity, ω, in the circumferential direction against radial distance, r, from the axis of a swirling stream created in a preferred embodiment of the mixer;

FIG. 3 shows the angular velocity field produced by the swirling stream having the radial distribution of angular velocity shown in FIG. 2;

FIG. 4a is a cross-section through the convergent section of a mixer, showing one possible arrangement of injectors;

FIG. 4b in a view similar to FIG. 4a, showing another possible arrangement of injectors;

FIG. 5 is a schematic axial section through a mixer combined with a burner sector;

FIG. 6 is an inlet end view of the mixer in FIG. 5; and

FIG. 7 is an outlet end view of the burner sector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The mixer illustrated in FIG. 1 comprises a passageway 1 having an axis 2 along which a stream of air (the carrier fluid or first fluid) flows in the direction of the arrow 3. The passageway 1 has an upstream end portion or inlet section 4 which is cylindrical, a convergent section 6 which is conical and which converges at an angle θ with respect to the axis 2, a divergent section 7 which is conical and diverges at an angle a with respect to the axis 2, and a downstream end portion or outlet section 8 which is cylindrical. The passageway has a throat 9 between the convergent and divergent sections 6, 7; in the embodiment illustrated, the throat 9 is of negligible axial length. The convergent section 6, throat 9, and divergent section 7 together constitute a Venturi section.

An injector comprising a plurality of injection ports 11 in the peripheral wall 12 of the passageway 1 introduces fuel (the additive fluid or second fluid) into the stream in the convergent section 6 at multiple locations along and around the axis 2. If the additive fluid is a liquid, it can be injected as sprays, and droplet atomisation and penetration can be enhanced by using high-pressure injectors.

A swirl generator 13 is provided in the inlet section 4 of the passageway 1. This imparts swirl to the bulk flow of the carrier fluid prior to the convergent section 6 and prior to the injection of the fuel. Conservation of angular momentum results in increased angular velocities of the swirling stream at the throat 9. Such a configuration enhances mixing between the carrier fluid and additive fluid by virtue of the circumferential shear layers which are formed. These shear layers promote cross-stream diffusion. Mixing begins earlier than in a conventional Venturi injector and results in a longer time being available for mixing and a more uniform concentration profile.

Mixing can be significantly improved by generating angular velocity profiles which form a number of intense circumferential shear layers. The intensity of these shear layers is directly proportional to dω/dr (the radial gradient of the angular velocity). Thus, abrupt changes in the circumferential component of the angular velocity are desirable. FIG. 2 is a graph of the angular velocity, ω, in the circumferential direction against radial distance, r, from the axis 2, illustrating a radical form of such an angular velocity profile. A swirling velocity field resulting from the application of inlet angular velocities similar to those of FIG. 2 is depicted in FIG. 3. Whilst distinct swirling annular bodies are clearly visible at the inlet of the convergent section 6, these are seen to merge with each other as the longitudinal distance from the inlet increases. Such “blurring” of the annular layers is indicative of cross-stream interactions, which result in mixing of the first and second fluids.

In order to generate such an angular velocity profile, a swirl generator 13 is used in which the swirl angle varies in the radial direction, typically increasing with distance from the axis.

Whilst strong levels of swirl are beneficial to mixing, vortex breakdown has to be avoided if flashback is to be prevented in combustion applications. Although studies have demonstrated that vortex breakdown is promoted by expansion downstream of a swirl generator, we have found that vortex breakdown does not occur so readily if a convergent section is placed between the swirl generator and the divergent section. Also, studies have shown that abrupt changes in tangential velocity profiles tend to reduce the tendency of vortex breakdown. Instead, flashback is hindered by the strongly swirling axial jet which is formed. The value of θ required to avoid vortex breakdown is a function of the angular velocity profile produced by the swirl generator 13. For example, we have found that (for a given operating condition, i.e. velocity, pressure, temperature) if the swirl angle varies (radially) between 15° and 45°, θ should lie between 15° and 25°, whereas in a configuration where the swirl angle changes from 15° to 30°, θ may be reduced to less than 15°.

The nature of the divergence downstream of the throat 9 can be selected for various needs. If recirculation zones are not desired, expansion must not be sudden, so a more gradual increase in the cross-section of the divergent section 7 is needed. Such a configuration may be applicable to cases where no negative axial velocities are desired, for example in catalytic combustion. On the other hand, the mixer may be used for premixed combustion, in which case sudden expansion serves to aerodynamically anchor the homogeneous flame.

The mixer does not require the large inlet to throat diameter ratio (typically 2) normally necessary for strongly accelerating a carrier fluid, because of the high degree of mixing resulting from tangential shear in the carrier fluid, for which the axial velocities need no longer be so high. Conventional Venturi injectors require small angles of divergent (diffuser angles), typically α=5°, if flow separation is to be avoided, but the resulting long diffuser lengths result in significant pressure losses such that, in a typical conventional Venturi injector, 95% of the loss is incurred during diffusion. In the present mixer, if flow separation is to be avoided, small angles of divergence are still necessary, but the relatively large throat diameter results in shorter diffusers and hence smaller pressure losses. Such a saving of space may be highly advantageous in catalytic combustion applications.

The peripheral wall 12 of the passageway, particularly the Venturi section constituted by the convergent and divergent sections 6, 7, may be coated with a catalytic material for the purpose of quenching radicals, which are precursors of homogeneous ignition and combustion. This assists in preventing flashback and flame anchoring, these two phenomena being encouraged by the lower velocities encountered in the boundary layer near the peripheral wall.

The injection ports 11 may simply be holes which each face the axis 2. However, introducing the additive fluid in a direction which is skewed to the axis 2 results in increased turbulence and better mixing of the additive fluid with the carrier fluid. FIGS. 4a and 4b show possible orientations of the injection ports 11. In FIG. 4a the ports 11 are symmetrically arranged with respect to planes containing the axis of the passageway. In FIG. 4b the ports 11 are angled so as to assist the swirling motion of the carrier fluid. However, the injection ports may instead be angled in the opposite sense with respect to the swirl direction of the carrier fluid.

Injection ports 11 of different sizes may be provided in order to achieve different depths of penetration of the additive fluid into the stream. Fuels which are particularly prone to causing flashback due to their high flames speeds and diffusivity, for example hydrogen-containing gases such as synthesis gas, can be used in the mixer because of the very high velocities achievable and the possibility of avoiding recirculation zones. The swirl generator 13 may surround a central member or mandrel, which may be in the form of a central injection tube for providing a central air jet hindering the formation of recirculating regions at the exit. Whilst the recirculation zones which tend to form behind a solid mandrel would not normally cause flame attachment (because the axial velocities at the throat 9 are very high, typically tens of times the homogeneous flame speed of natural gas), extra caution has to be exercised when hydrogen-containing fuels are used because of the very high flame speed and diffusivity of hydrogen.

The swirl generator 13 may circumferentially surround a central fuel injection lance, which could additionally inject air, in order to further enhance mixing.

Increasing the size of the mixer makes it possible to generate a larger number of coaxial swirling layers. It may therefore be advantageous to use the mixer in a combustion device having multiple burners, by using one mixer for a burner sector (comprising a number of burners). FIG. 5 illustrates such an embodiment. The circular cross-section of the divergent section 7 gradually changes into a sector of an annulus (FIG. 7) in which a number of burners 14 are located (three burners being shown by way of example). The burners 14 may be very simple (e.g. utilising sudden expansion without swirl) because complete fuel/air mixing has already been achieved prior to entry into the burners.

A flow straightener 16, which has also has the function of flashback prevention is placed near the exit of the divergent section 7, upstream of the burners 14. The flow straightener 16 has a similar construction to the swirl generator 13, except that it has straight channels. Flow straightening ensures that the flow distribution into each burner is identical. Small channels (hydraulic diameter typically less than 5 mm) act as flame arrestors. The channels may be coated with a catalyst for quenching radicals, further hindering flashback. In order to minimise pressures losses, the flow straightener has a very small axial length, typically less than 15 mm. The fact that the mixing process is decoupled from the burners results in more uniform burner entry conditions (flow distribution, pre-mixedness, temperature) and hence more uniform combustion among the burners and fewer instabilities (which may arise if there are differences between burners). This embodiment is thus especially applicable to catalytic combustion.

The embodiment of FIG. 5 can be used for liquid fuels if the geometry ensures very high velocities such that the mixer residence time (i.e. the time taken for the fuel to move from the injection point to the burners) is very short, typically less than 3 ms at 3 bar.

Various modifications may be made within the scope of the invention. In particular, although the mixer has been particularly described in the context of mixing air and fuel, the mixer could be used for mixing any two (or more) different fluids. It is possible to introduce the second fluid into the passageway at any convenient location upstream of the divergent section 7. In particular, the throat 9 may be of substantial length and the second fluid may be introduced into the throat. Alternatively, or additionally, the second fluid may be introduced into the inlet section 4 (upstream or, preferably, downstream of the swirl generator 13). Alternatively, or additionally, the second fluid may be introduced through a tube extending along the axis 2. It is also possible to introduce at least one further fluid into the passageway upstream of the divergent section 7.

Claims

1. A mixer for mixing first and second fluids, comprising:

a passageway along which a stream flows along an axis thereof, the stream comprising the first fluid, and the passageway comprising, in sequence in a downstream direction, a convergent section, a throat, and a divergent section;

an injector for introducing the second fluid into the stream in the passageway upstream of the divergent section; and

a swirl generator in the passageway upstream of the convergent section.

2. The mixer of claim 1, wherein the injector introduces the second fluid into the stream in the convergent section of the passageway.

3. The mixer of claim 1, wherein the injector comprises at least one injection port in a peripheral wall of the passageway.

4. The mixer of claim 3, wherein the injector comprises at least two injection ports of different sizes.

5. The mixer of claim 1, wherein the injector introduces the second fluid at a plurality of locations along the passageway.

6. The mixer of claim 1, wherein the injector introduces the second fluid in at least one direction which is transverse to the axis of the passageway.

7. The mixer of claim 1, wherein the swirl generator has a swirl angle that varies as a function of distance from the axis.

8. The mixer of claim 7, wherein the swirl angle varies such that there is at least one abrupt change in circumferential velocity of the stream about the axis, between one radial position and another.

9. The mixer of claim 1, further comprising a central injection tube opening in the passageway upstream of the divergent section.

10. The mixer of claim 9, wherein the swirl generator circumferentially surrounds the central injection tube.

11. The mixer of claim 1, wherein upstream and downstream ends of the convergent section have a diameter ratio of less than 2.

12. The mixer of claim 1, wherein the convergent section converges at an angle of at most 25° with respect to the axis of the passageway.

13. The mixer of claim 12, wherein the angle is at least 10°.

14. The mixer of claim 12, wherein the angle is at least 15°.

15. The mixer of claim 1, wherein a peripheral wall of the passageway comprises a coating of a catalyst for quenching radicals that are precursors to ignition of a mixture of the first and second fluids.

16. The mixer of claim 1, further comprising a flow straightener in the passageway downstream of the throat.

17. The mixer of claim 16, wherein the flow straightener comprises channels with a hydraulic diameter of at most 5 mm.

18. The mixer of claim 16, wherein the flow straightener carries a catalyst for quenching radicals.

19. The mixer of claim 16, wherein the flow straightener has a length in the axial direction of at most 15 mm.

20. The mixer of claim 1, wherein the passageway forms part of a combustion device.

21. The mixer of claim 20, wherein the combustion device comprises a plurality of burners supplied with a mixture of the fluids by the passageway.

22. The mixer of claim 21, wherein the burners form a burner sector, a downstream end portion of the passageway gradually changing cross-section into a sector of an annulus.

23. A method of mixing fluids, comprising the sequential steps of:

(a) providing a stream comprising a first fluid and having an axis along which the stream flows;

(b) inducing swirl in the stream about an axis thereof;

(c) causing the stream to converge towards the axis; and

(d) causing the stream to diverge from the axis;

wherein a second fluid is introduced into the stream before step (d).

24. The method of claim 23, wherein the second fluid is introduced into the stream in a direction that is transverse to the axis of the stream.

25. The method of claim 23, wherein the second fluid is introduced into the stream during step (c).

26. The method of claim 25, wherein the second fluid is introduced into the stream in a direction that is transverse to the axis of the stream.

27. The method of claim 23, wherein a swirl angle of the swirl induced in step (b) varies as a function of distance from the axis.

28. The method of claims 23, wherein the swirl induced in step (b) is such that there is at least one abrupt change in circumferential velocity of the stream about the axis, between one radial position and another.

29. The method of claim 23, further comprising introducing a fluid into a central region of the stream after step (b) and before step (d).

30. The method of claim 29, wherein the fluid is the first fluid.

31. The method of claim 29, wherein the fluid is the second fluid.

32. The method of claim 23, wherein the first and second fluids are both introduced into a central region of the stream.

33. The method of claim 23, further comprising straightening the flow of the stream after step (d).

34. The method of claim 23, wherein at least one of the first and second fluids is a gas.

35. The method of claim 34, wherein the first fluid is air and the second fluid is a combustible fluid.

36. The method of claim 34, wherein the first fluid is a gas and the second fluid is a liquid.

37. The method of claim 36, in which the first fluid is air and the second fluid is a combustible fluid.