Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
  • B01F25/00—Flow mixers; Mixers for falling materials, e.g. solid particles
  • B01F25/10—Mixing by creating a vortex flow, e.g. by tangential introduction of flow components
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
  • B01F25/00—Flow mixers; Mixers for falling materials, e.g. solid particles
  • B01F25/30—Injector mixers
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23D—BURNERS
  • F23D14/00—Burners for combustion of a gas, e.g. of a gas stored under pressure as a liquid
  • F23D14/46—Details, e.g. noise reduction means
  • F23D14/62—Mixing devices; Mixing tubes
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23D—BURNERS
  • F23D2900/00—Special features of, or arrangements for burners using fluid fuels or solid fuels suspended in a carrier gas
  • F23D2900/14—Special features of gas burners
  • F23D2900/14021—Premixing burners with swirling or vortices creating means for fuel or air
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23D—BURNERS
  • F23D2900/00—Special features of, or arrangements for burners using fluid fuels or solid fuels suspended in a carrier gas
  • F23D2900/14—Special features of gas burners
  • F23D2900/14701—Swirling means inside the mixing tube or chamber to improve premixing

Definitions

• 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.
• 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.
• US 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
• step (d) causing the stream to diverge from its axis, the method including introducing a second fluid into the stream before step (d).
• Figure 1 is a schematic axial section through one embodiment of a mixer
• Figure 2 is a graph of angular velocity, ⁇ , in the circumferential direction against radial distance, r, from the axis of a swirling stream created in a preferred embodiment of the mixer;
• Figure 3 shows the angular velocity field produced by the swirling stream having the radial distribution of angular velocity shown in Figure 2;
• Figure 4a is a cross-section through the convergent section of a mixer, showing one possible arrangement of injectors
• Figure 4b in a view similar to Figure 4a, showing another possible arrangement of injectors
• Figure 5 is a schematic axial section through a mixer combined with a burner sector
• Figure 6 is an inlet end view of the mixer in Figure 5;
• Figure 7 is an outlet end view of the burner sector.
• the mixer illustrated in Figure 1 comprises a passageway 1 having an axis 2 along which a stream of air (the carrier fluid or first fluid) flows in tlie 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 ⁇ with respect to the axis 2, a divergent section 7 which is conical and diverges at an angle ⁇ 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.
• FIG. 1 is a graph of the angular velocity, ⁇ , 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 Figure 2 is depicted in Figure 3.
• a swirl generator 13 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.
• vortex breakdown Whilst strong levels of swirl are beneficial to mixing, vortex breakdown has to be avoided if flashback is to be prevented in combustion applications.
• 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.
• 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 0 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.
• ⁇ should lie between 15° and 25°, whereas in a configuration where the swirl angle changes from 15° to 30°, ⁇ 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 tlie 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.
• ⁇ 5°
• 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.
• Figures 4a and 4b show possible orientations of the injection ports 11. In Figure 4a the ports 11 are symmetrically arranged with respect to planes containing the axis of the passageway. In Figure 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.
• the swirl generator 13 may circumferentially surround a central fuel injection lance, which could additionally inject air, in order to further enhance mixing.
• FIG. 5 illustrates such an embodiment.
• the circular cross-section of the divergent section 7 gradually changes into a sector of an annulus ( Figure 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 5mm) act as flame arresters. 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 15mm.
• Tlie embodiment of Figure 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 s at 3 bar.
• the mixer could be used for mixing any two (or more) different fluids.
• the throat 9 may be of substantial length and the second fluid may be introduced into the throat.
• the second fluid may be introduced into the inlet section 4 (upstream or, preferably, downstream of the swirl generator 13).
• 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.

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• 2003
  • 2003-02-14 GB GB0303495A patent/GB2398375A/en not_active Withdrawn
• 2004
  • 2004-02-03 AT AT04707535T patent/ATE357965T1/de not_active IP Right Cessation
  • 2004-02-03 WO PCT/EP2004/050074 patent/WO2004071637A1/en active IP Right Grant
  • 2004-02-03 EP EP04707535A patent/EP1592495B1/de not_active Expired - Lifetime
  • 2004-02-03 DE DE602004005572T patent/DE602004005572T2/de not_active Expired - Lifetime
• 2005
  • 2005-08-12 US US11/202,208 patent/US20060035183A1/en not_active Abandoned

Non-Patent Citations (1)

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