Patent Application
20060035183
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.
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.
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:
The invention will be described further, by way of example only, with reference to the accompanying drawing, in which:
a is a cross-section through the convergent section of a mixer, showing one possible arrangement of injectors;
b in a view similar to
The mixer illustrated in
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.
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.
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).
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
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0303495.6 | Feb 2003 | GB | national |
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.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP04/50074 | Feb 2004 | US |
| Child | 11202208 | Aug 2005 | US |
