U.S. patent application number 11/202208 was filed with the patent office on 2006-02-16 for mixer.
Invention is credited to Richard Carroni, Timothy Griffin.
Application Number | 20060035183 11/202208 |
Document ID | / |
Family ID | 9953073 |
Filed Date | 2006-02-16 |
United States Patent
Application |
20060035183 |
Kind Code |
A1 |
Carroni; Richard ; et
al. |
February 16, 2006 |
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.
Inventors: |
Carroni; Richard;
(Niederrohrdorf, CH) ; Griffin; Timothy;
(Ennetbaden, CH) |
Correspondence
Address: |
STEPTOE & JOHNSON LLP
1330 CONNECTICUT AVENUE, N.W.
WASHINGTON
DC
20036
US
|
Family ID: |
9953073 |
Appl. No.: |
11/202208 |
Filed: |
August 12, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP04/50074 |
Feb 3, 2004 |
|
|
|
11202208 |
Aug 12, 2005 |
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Current U.S.
Class: |
431/7 ;
431/284 |
Current CPC
Class: |
B01F 5/0057 20130101;
F23D 2900/14701 20130101; B01F 5/04 20130101; F23D 2900/14021
20130101; F23D 14/62 20130101 |
Class at
Publication: |
431/007 ;
431/284 |
International
Class: |
F23D 3/40 20060101
F23D003/40; F23Q 9/00 20060101 F23Q009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 14, 2003 |
GB |
0303495.6 |
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.degree. with respect to the axis of the
passageway.
13. The mixer of claim 12, wherein the angle is at least
10.degree..
14. The mixer of claim 12, wherein the angle is at least
15.degree..
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.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] 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
[0002] 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
[0003] 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.
[0004] 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.
[0005] 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
[0006] 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.
[0007] The invention also provides a method of mixing fluids,
comprising the sequential steps of: [0008] (a) providing a stream
comprising a first fluid and having an axis along which the stream
flows, [0009] (b) inducing swirl in the stream about its axis,
[0010] (c) causing the stream to converge towards its axis, and
[0011] (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
[0012] The invention will be described further, by way of example
only, with reference to the accompanying drawing, in which:
[0013] FIG. 1 is a schematic axial section through one embodiment
of a mixer;
[0014] FIG. 2 is a graph of angular velocity, .omega., in the
circumferential direction against radial distance, r, from the axis
of a swirling stream created in a preferred embodiment of the
mixer;
[0015] FIG. 3 shows the angular velocity field produced by the
swirling stream having the radial distribution of angular velocity
shown in FIG. 2;
[0016] FIG. 4a is a cross-section through the convergent section of
a mixer, showing one possible arrangement of injectors;
[0017] FIG. 4b in a view similar to FIG. 4a, showing another
possible arrangement of injectors;
[0018] FIG. 5 is a schematic axial section through a mixer combined
with a burner sector;
[0019] FIG. 6 is an inlet end view of the mixer in FIG. 5; and
[0020] FIG. 7 is an outlet end view of the burner sector.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] 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 .theta. 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.
[0022] 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.
[0023] 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.
[0024] 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 d.omega./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, .omega., 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.
[0025] 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.
[0026] 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 .theta.
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.degree. and 45.degree., .theta. should lie between 15.degree.
and 25.degree., whereas in a configuration where the swirl angle
changes from 15.degree. to 30.degree., .theta. may be reduced to
less than 15.degree..
[0027] 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.
[0028] 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 .alpha.=5.degree., 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] The swirl generator 13 may circumferentially surround a
central fuel injection lance, which could additionally inject air,
in order to further enhance mixing.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
* * * * *