U.S. patent application number 10/588535 was filed with the patent office on 2008-04-17 for particle interactions in a fluid flow.
This patent application is currently assigned to INDIGO TECHNOLOGIES GROUP PTY. Invention is credited to Peter Anthony Markus Kalt, Richard Malcolm Kelso, Graham Jerrold Nathan, Rodney John Truce, John Walter Wilkins.
Application Number | 20080087347 10/588535 |
Document ID | / |
Family ID | 34831686 |
Filed Date | 2008-04-17 |
United States Patent
Application |
20080087347 |
Kind Code |
A1 |
Truce; Rodney John ; et
al. |
April 17, 2008 |
Particle Interactions in a Fluid Flow
Abstract
Interaction between two different species of particle(s) in a
fluid stream is promoted by generating turbulent eddies in a fluid
stream. The turbulent eddies are designed to be of such size and/or
intensity that the different sized particle(s) are entrained into
the eddies to significantly different extents and forced to follow
different trajectories, increasing the likelihood of collisions and
interactions. Optimum collision rates will occur for a system which
maintains a Stokes Number much less than 1 for one sized particle,
and or order 1 or greater for the other sized particle. The
invention has particular application in air pollution control,
whereby agglomeration of fine particles into larger particles is
promoted, subsequent to their removal.
Inventors: |
Truce; Rodney John;
(Queensland, AU) ; Wilkins; John Walter;
(Queensland, AU) ; Nathan; Graham Jerrold; (South
Australia, AU) ; Kelso; Richard Malcolm; (South
Autralia, AU) ; Kalt; Peter Anthony Markus; (South
Autralia, AU) |
Correspondence
Address: |
THE WEBB LAW FIRM, P.C.
700 KOPPERS BUILDING, 436 SEVENTH AVENUE
PITTSBURGH
PA
15219
US
|
Assignee: |
INDIGO TECHNOLOGIES GROUP
PTY
Milton
AU
|
Family ID: |
34831686 |
Appl. No.: |
10/588535 |
Filed: |
February 9, 2005 |
PCT Filed: |
February 9, 2005 |
PCT NO: |
PCT/AU05/00160 |
371 Date: |
August 7, 2006 |
Current U.S.
Class: |
137/896 ;
137/1 |
Current CPC
Class: |
B01J 2/16 20130101; Y10T
137/0318 20150401; B01F 5/0618 20130101; B01D 51/02 20130101; Y10T
137/87652 20150401; B01F 5/0643 20130101; B01F 2215/0431
20130101 |
Class at
Publication: |
137/896 ;
137/1 |
International
Class: |
F15D 1/10 20060101
F15D001/10; B01F 3/06 20060101 B01F003/06; B01F 5/00 20060101
B01F005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 9, 2004 |
AU |
2004900593 |
Claims
1-32. (canceled)
33. A method of designing a formation of vortex generators for
generating turbulent eddies in a fluid stream to promote
interaction between at least two types of particles in the
turbulent eddies, comprising the steps of: (i) identifying relevant
characteristics of the two types of particles, (ii) performing a
Stokes Number analysis to determine the optimal characteristic eddy
size to cause one type of particle to have a significantly higher
slip velocity than the other type of particle, and (iii) designing
a formation to generate eddies in the fluid stream having the
optimal size determined in step (ii) above.
34. The method as claimed in claim 33, wherein the relevant
characteristics of the two types of particles include the size and
density of the particles.
35. The method as claimed in claim 33, wherein the determination of
the optimal characteristic eddy size involves an iteration
process.
36. The method as claimed in claim 33, wherein the Stokes Number
for one type of particle is at least an order of magnitude greater
than that of the other type of particle.
37. The method as claimed in claim 36, wherein at least one of the
particles has a Stokes Number in the range 10.sup.-2 to
10.sup.2.
38. The method as claimed in claim 33, wherein the optimal
characteristic eddy size is one at which the difference in the
Stokes Numbers of the two types of particles is maximized.
39. The method as claimed in claim 33, wherein the formation is
designed to comprise a plurality of vanes.
40. The method as claimed in claim 33, wherein one type of particle
is solid, liquid or gaseous, and the other type of particle is
solid, liquid or gaseous.
41. A method of promoting interaction between at least two types of
particles in a fluid stream, comprising generating turbulent eddies
in the fluid stream to cause interactions between the two types of
particles in the turbulent eddies, wherein the eddies are of such a
size and/or intensity that the two types of particles are entrained
into the eddies to significantly different extents.
42. The method as claimed in claim 41, wherein the eddies are of
such a size and/or intensity that one type of particle is
substantially fully entrained while the other type of particle is
not substantially entrained, to thereby maximize relative slip and
the likelihood of interactions between the two types of particles
in the eddies.
43. The method as claimed in claim 41, wherein the Stokes Number
for one type of particle is at least an order of magnitude greater
than that of the other type of particle.
44. The method as claimed in claim 43, wherein the Stokes Number
for at least one of the particles is in the range 10.sup.-2 to
10.sup.2.
45. The method as claimed in claim 41, wherein one type of particle
is solid, liquid or gaseous, and the other type of particle is
solid, liquid or gaseous.
46. The method as claimed in claim 41, wherein the fluid stream is
in a duct and the step of generating turbulent eddies comprises
placing a plurality of vane members in spaced relationship across
the duct to generate a multiplicity of eddies.
47. The method as claimed in claim 46, wherein the spacing between
the vane members is on the order of the width of the vane
members.
48. The method as claimed in claim 46, further comprising the step
of placing additional rows of spaced vane members across the duct
to form an array of vane members, the additional rows being spaced
longitudinally along the duct.
49. The method as claimed in claim 48, wherein the longitudinal
spacing between the additional rows is on the order of 1 to 3 times
the width of the vane members.
50. The method as claimed in claim 46, wherein there are sufficient
additional rows of spaced vane members spaced longitudinally along
the duct such that the time taken for the fluid stream to pass the
array is at least 0.1 seconds.
51. An apparatus for promoting interaction between at least two
types of particles in a fluid stream, comprising means for
generating turbulent eddies in the fluid stream to cause
interactions between the two types of particles in the turbulent
eddies, wherein the eddies are of such a size and/or intensity that
the two types of particles are entrained into the eddies to
significantly different extents.
52. The apparatus as claimed in claim 51, wherein the eddies are of
such a size and/or intensity that one type of particle is
substantially fully entrained while the other type of particle is
not substantially entrained, to thereby maximize relative slip and
the likelihood of interactions between the two types of particles
in the eddies.
53. The apparatus as claimed in claim 51, wherein the Stokes Number
for one type of particle is at least an order of magnitude greater
than that of the other type of particle.
54. The apparatus as claimed in claim 53, wherein the Stokes Number
for at least one of the particles is in the range 10.sup.-2 to
10.sup.2.
55. The apparatus as claimed in claim 51, wherein one type of
particle is solid, liquid or gaseous, and the other type of
particle is solid, liquid or gaseous.
56. The apparatus as claimed in claim 51, wherein the fluid stream
is in a duct and the means for generating turbulent eddies
comprises a plurality of vane members in spaced relationship across
the duct to generate a multiplicity of eddies.
57. The apparatus as claimed in claim 56, wherein the spacing
between the vane members is on the order of the width of the vane
members.
58. The apparatus as claimed in claim 56, further comprising
additional rows of spaced vane members across the duct to form an
array of vane members, the additional rows being spaced
longitudinally along the duct.
59. The apparatus as claimed in claim 58, wherein the longitudinal
spacing between the additional rows is on the order of 1 to 3 times
the width of the vane members.
60. The apparatus as claimed in claim 56, wherein each vane member
is of Z-shaped cross-section.
61. The apparatus as claimed in claim 60, wherein each vane member
has spaced tooth portions along its longitudinal edges.
62. An apparatus for causing interaction between large particles
and fine particles in a fluid stream, comprising an array of
micro-vortex generating formations for generating a multiplicity of
micro-vortices across the fluid stream, the array including a
plurality of longitudinally spaced rows of micro-vortex generating
formations, each row having a plurality of transversely spaced
micro-vortex generating formations, and wherein the fine particles
are substantially entrained in the micro-vortices while the large
particles are not substantially entrained, to thereby maximize
relative slip and the likelihood of interactions between the two
types of particles.
63. The apparatus as claimed in claim 62, each micro-vortex
generating formation is a vane member of Z-shaped cross-section
with scalloped longitudinal edges.
Description
[0001] This invention relates generally to a method and apparatus
for promoting or increasing interactions between different types of
particles in a fluid flow. The invention provides a method of
designing a formation of vortex generators to generate particle
scale turbulence to cause interactions between particular types of
particles in a fluid flow in a highly efficient manner.
[0002] The invention has particular application in air pollution
control, by promoting agglomeration of fine pollutant particles in
air streams into larger particles to thereby facilitate their
subsequent filtration or other removal from the air streams,
although the invention is not limited to that application.
BACKGROUND ART
[0003] Many industrial processes result in the emission of small
hazardous particles into the atmosphere. These particles often
include very fine sub-micron particles of toxic compounds which are
easily inhaled. Their combination of toxicity and ease of
respiration has prompted governments around the world to enact
legislation for more stringent control of emission of particles
less than ten microns in diameter (PM10), and particularly
particles less than 2.5 microns (PM2.5).
[0004] Smaller particles in atmospheric emissions are also
predominantly responsible for the adverse visual effects of air
pollution. Opacity is largely determined by the fine particulate
fraction of the emission since the light extinction coefficient
peaks near the wavelength of light which is between 0.1 and 1
microns.
[0005] Various methods have been used to remove dust and other
pollutant particles from air streams. Although these methods are
generally suitable for removing larger particles from air streams,
they are usually much less effective in filtering out smaller
particles, particularly PM2.5 particles.
[0006] Fine particles in air streams can be made to agglomerate
into larger particles by collision/adhesion, thereby facilitating
subsequent removal of the particles by filtration. Our
international patent applications nos. PCT/NZ00/00223 and
PCT/AU2004/000546 disclose energized and passive devices for
agglomerating particles. The agglomeration efficiency is dependent
upon the incidence or frequency of collisions and similar
interactions between the particles.
[0007] Many pollution control strategies also rely on contact
between individual elements of specific species to promote a
reaction or interaction beneficial to the subsequent removal of the
pollutant concerned. For example, sorbents such as activated carbon
can be injected into the polluted air stream to remove mercury
(adsorption), or calcium can be injected to remove sulfur dioxide
(chemisorption).
[0008] In order for these interactions to take place, the two
species of interest must be brought into contact. For many
industrial pollutants in standard flue ducts, this is difficult for
several reasons. For example, the time frames for
reaction/interaction are short (of the order of 0.5-1 second), the
species of interest are spread very sparsely (relative to the bulk
fluid) through the exhaust gases, and the scale of the flue ducting
is large compared to the scale of the pollutant particles.
[0009] Normally, exhaust gases from the outlet of an industrial
process are fed into a large duct which transports them to some
downstream collection device (e.g. an electrostatic precipitator,
bag filter, or cyclone collector) as uniformly and with as little
turbulence/energy loss as possible. Such turbulence as is generated
en route is normally a large scale diversion of gases around
turning vanes, around internal duct supports/stiffeners, through
diffusion screens and the like. This turbulence is of the scale of
the duct and should desirably be the minimum disturbance, and hence
pressure drop, possible to achieve the desired flow correction.
[0010] Similarly, when mixing devices are employed for a specific
application, eg. sorption of a particular pollutant, they are
usually devices that generate a large-scale turbulence field (of
the order of the duct width or height) and are arranged as a short
series of curtains that the gases must pass through.
[0011] The aim of most known mixing devices is to achieve a
homogeneous mixture of two or more substances. Such devices are not
specifically designed to promote interactions between fine
particles in the mixture. In most industrial-scale devices
involving the transport of particles, the turbulence generated by
the mixing is of a large scale relative to the particles. Under
such conditions the particles tend to travel in similar paths
rather than in collision courses.
[0012] It is also known that vortex generators can be used in
mixing chambers to promote mixing of fluids. However such devices
are not generally used in particle laden flows to create collisions
between particles.
[0013] Whether they be particulate (e.g. flyash), gaseous (e.g.
SO.sub.2), mist (e.g. NO.sub.x), or elemental (eg. Mercury), the
pollution species which are the more difficult to collect within
industrial exhaust flues are those of the order of micrometers in
diameter (i.e. 10.sup.-6 metres). Due to their small size, they
occupy a very small volumetric proportion of the total fluid flow.
For example, if uniformly distributed, one million 1 .mu.m diameter
particles would occupy less than 0.00005% of the volume of 1
cm.sup.3 of gas (assuming that the particles are spherical). Even
at 10 .mu.m diameter, this proportion only increases to 0.05%. When
it is considered that a pollutant such as Mercury may only account
for a few parts per million (ppm) of the total species present, it
is apparent that at particle scale, there is a significant amount
of space/distance between the species being transported by an
industrial flue gas. Where particles are already "well-mixed" in a
flow, e.g. disbursed more-or-less randomly throughout a duct (as in
an exhaust flue), turbulence of any scale will not be able to mix
them more thoroughly.
[0014] Furthermore, sufficiently small particles that are entrained
in a flowing fluid will follow the streamlines in the fluid flow.
This occurs where the viscous forces of the fluid dominate the
inertial forces of the particle. Known turbulent mixing regimes of
the scale of the duct are many orders of magnitude larger than the
particle. When viewed from the perspective of the particle, they
are far from being chaotic but rather, are relatively smooth.
Whilst there may be many changes of direction for a particle in its
passage through a turbulent flow in a duct or through a standard
mixing region, they are all relatively long range compared with the
size or scale of the particle. Consequently, particles in a stream
under conditions typical of industrial dust-laden flows follow more
or less the same paths as their neighbouring particles, resulting
in few interactions with the surrounding particles. At particle
scale therefore, there are relatively few turbulence-generated
interactions, and consequently, the known mixing processes achieve
poor efficiency in agglomeration.
[0015] Systems intended to maximise the collision rate of very
small pollution species which occupy a tiny proportion of the
volume of the total fluid flow must cause them to move along
different trajectories, and/or at different speeds, to each other,
as often as possible. Additionally such differences in trajectory
and/or speed must be brought to bear at the scale of the particle
to have the most effect.
[0016] Unfortunately, current design philosophies do not adequately
address these criteria.
[0017] It is an aim of the present invention to provide method and
apparatus for achieving improved interaction of particles in fluid
flows.
[0018] It is another aim of this invention to provide a method of
custom designing a formation to generate particle scale turbulence
to cause interactions between particular types of particles in a
fluid flow in a highly efficient manner.
SUMMARY OF THE INVENTION
[0019] This invention is based on the recognition that two
particles of different mass and/or aerodynamic properties in a
flowing fluid will respond differently to a turbulence eddy of a
predetermined size in the fluid flow. More specifically, if the
eddy is of a particular scale, the different particles will be
entrained in the eddy to different extents, and will therefore
follow different trajectories. Consequently, the likelihood of
collision or interaction between the particles is increased.
[0020] Particles of similar mass and/or aerodynamic properties
which are captured by, and entrained in, a turbulent eddy will
follow roughly the same path and consequently do not impact with
each other to any significant extent. A particle of larger mass
and/or different aerodynamic property will not be entrained into
the eddy, or will be entrained to a substantially lesser extent,
and will therefore travel through the eddy on a different
trajectory and be impacted by many more other particles entrained
into the same eddy.
[0021] To improve the likelihood of collisions between two types of
particles in a fluid flow, e.g. to promote their agglomeration or
the adsorption of the smaller particle by the larger particle, a
formation is designed to generate turbulence of such scale that
different particles are entrained to significantly different
extents.
[0022] In one broad form, the present invention provides a method
of promoting interaction between at least two types of particles in
a fluid stream by generating turbulent eddies in the fluid stream,
characterised in that the eddies are of such size and/or intensity
that the two types of particles are entrained in the eddies to
significantly different extents.
[0023] In another form, the invention provides apparatus for
promoting interaction between at least two types of particles in a
fluid stream, comprising means for generating turbulent eddies in
the fluid stream, characterised in that the eddies are of such size
and/or intensity that the two types of particles are entrained in
the eddies to significantly different extents.
[0024] Preferably, the turbulent eddies are of such size and/or
intensity that one type of particle is substantially fully
entrained while the other type of particle is not substantially
entrained, to thereby maximize relative slip and the likelihood of
interactions between the two type of particles.
[0025] In yet another broad form, the invention provides a method
of custom designing a formation for generating turbulence in a
fluid stream to promote interaction between at least two types of
particles in the fluid stream, comprising the steps of:
[0026] (i) identifying relevant characteristics of the two types of
particles,
[0027] (ii) performing a Stokes Number analysis to determine the
optimal characteristic eddy size to cause one type of particle to
have a significantly higher slip velocity than the other type of
particle, and
[0028] (iii) designing a formation to generate eddies in the fluid
stream having the optimal size determined in step (ii) above.
[0029] The relevant characteristics of the two types of particles
normally include the size and density of the particles.
[0030] The determination of the optimal characteristic eddy size
may involve an iteration process.
[0031] As the standard equation for Stokes Number assumes that
particles are spherical, an empirical "shape factor" may be applied
to account for the shape of the particles.
[0032] For two given types of particles, e.g. a collector particle
and a collected particle, the invention provides a method of custom
designing a formation to generate turbulent eddies of such size and
scale as to maximise the differential slip velocities of the two
particles and thereby maximise the likelihood of interactions
between the particles. Preferably, the eddies in the generated
turbulence will be of such size that the slip velocity of the
collector particle is maximised, while the slip velocity of the
collected particle is minimised.
[0033] Throughout this specification where the context permits, the
term "particle" is intended to mean a constituent of a flowing
fluid that can be manipulated to effect its collision with another
particle in the same fluid flow. The "particle" can be solid (e.g.
a fly ash particle), liquid (e.g. a suspended water droplet) or
gaseous (e.g. SO.sub.3, Hg or NO.sub.x molecules). This invention
can be applied to gas and solid particle interactions, gas and
liquid droplet interactions, liquid and solid particle
interactions, interactions between different sized droplets, and
interactions between different sized particles. The different sized
particles may be suspended in a gas or in a liquid, and the
different sized droplets may be suspended in a gas.
[0034] The term "collector particle" is intended to mean the larger
and/or heavier particle used to collide and/or interact with the
"collected" or "collection" particle.
[0035] The term "interaction" is intended to mean that the
particles collide or contact or come into sufficiently close
proximity so as to result in their agglomeration, sorption,
coagulation, catalysation or chemical reaction.
[0036] Additionally, the terms "slip" and "slip velocity" are used
to describe the relative velocity between a particle and its
surrounding fluid. Hence, if a particle is fully entrained in a
turbulent flow, its slip velocity is zero. The more a particle's
path diverges from that of its surrounding fluid, the greater will
be its slip velocity. Therefore, in this context, if small
particles follow the flow more closely than large ones, their slip
velocity will be smaller and they will be said to have less
"slip".
[0037] Typically, the fluid stream is a gas or air stream, and the
particles of at least one type are pollutant particles of micron or
sub-micron size. However, this invention is not limited to
pollution control uses, and has wider application to other uses in
which interaction between particles in a fluid stream is sought to
be achieved in a highly efficient manner.
[0038] Turbulent eddies typically comprise vortex motions with a
plurality of different sizes and shapes.
[0039] In one embodiment, a multiplicity of small, low intensity
vortices are used to entrain fine (pollutant) particles and subject
them to turbulent flow. One or more species of larger "collector"
particles are introduced into the gas stream for removal of the
pollutant particles The larger collector particles are either not
entrained into the vortices, or are entrained to a much smaller
extent, so that they follow different trajectories to the fine
pollutant particles, resulting in a higher likelihood of contact
and/or interaction between the pollutant particles and the larger
species.
[0040] When the pollutant particles contact the larger species,
they tend to adhere thereto or react therewith. The removal species
may be a chemical, such as calcium, which reacts chemically with
pollutant particles, (such as sulphur dioxide) to form a third
compound (e.g. gypsum). Alternatively, the removal species of
particles may remove the pollutant particles by absorption, or by
adsorption (carbon particles adsorbing pollutant mercury
particles), or the removal species of particles may simply remove
the fine pollutants by agglomerating with the pollutants through
impact adhesion. The larger or agglomerated particles are
subsequently easier to remove from the gas stream using known
methods.
[0041] Typically, a Stokes Number much less than 1 will ensure
entrainment of the fine pollutant particles. The larger removal
species of particles should have a Stokes Number much greater than
1 so that they are not entrained. In practice, the eddies or
vortices generated in the gas stream are small, unlike the large
scale turbulence of known mixers. Consequently, the formation
typically comprises a multitude of components generating a
multiplicity of small eddies or vortices.
[0042] The multiplicity of small eddies or vortices entrain the
(small) particles of interest and subject them to turbulent flow.
Larger particles are not necessarily entrained by these small
vortices, or are entrained to a much lesser extent. Relative
movement between the small and large particles results in higher
frequency of collisions between them, and more efficient removal of
the fine (pollutant) particles by the larger (collector)
particles.
[0043] The use of a multiplicity of small vortices is
counterintuitive in flows where the particles are already well
disbursed in a duct. Normally, it is desirable that the pressure
drop in the gas stream be as low as possible. For this reason vanes
are typically only used to maintain as uniform a distribution of
particles as possible in a duct. Such vanes are therefore typically
of a relatively large scale--only slightly smaller than the scale
of the duct. For example, large-scale "turning vanes" may be used
in a bend to prevent all of the particles from going to the outside
of the bend and creating a non-uniform distribution after the
bend.
[0044] Alternatively, known mixers are used when two different
substances are not initially well distributed in a vessel or duct,
to generate a homogenous mixture. Again, large-scale devices are
typically used. They are not generally used to promote collisions
between substances that are already well distributed in a duct. The
present invention, on the other hand, uses many vortex generators
which create small scale vortices to increase interactions between
the fine (pollutant) particles and collector particles that are
already sufficiently well distributed throughout the flow.
[0045] In order that the invention may be more fully understood and
put into practice, an embodiment thereof will now be described, by
way of example only, with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] FIG. 1 is a perspective view of a vane according to one
embodiment of the invention.
[0047] FIG. 2 is a section plan view of an array of vanes of FIG.
1.
[0048] FIG. 3 is a section plan view of an array of vanes according
to another embodiment of the invention.
[0049] FIG. 4 is a partial perspective view of an array of vanes
according to another embodiment of the invention.
[0050] FIG. 5 is a partial perspective view of an array of vanes
according to yet another embodiment of the invention.
[0051] FIG. 6 illustrates turbulent eddies formed by the array of
FIG. 2.
[0052] FIG. 7 is a section plan view of a modified version of the
array of vanes of FIG. 2.
DESCRIPTION OF PREFERRED EMBODIMENT
[0053] In a preferred embodiment, this invention involves the use
of turbulent eddies to manipulate the relative trajectories of very
small pollutant particles and larger collector particles carried by
a flowing fluid, which is typically an exhaust gas stream from an
industrial process, to increase the probability of the particles
colliding or interacting to agglomerate, or otherwise react with
each other, to form more easily removable particles. A formation is
designed to provide turbulence of the required size and scale to
cause the different species of particles to have substantially
differential slip velocities.
[0054] The turbulence should be such that the Stokes Number (St) of
the small pollutant particles is much less than 1 (St<<1),
while the Stokes Number (St) of the larger collector particles is
much greater than 1 (St>>1).
[0055] The Stokes number (St) is a theoretical measure of the
ability of a particle to follow a turbulence streamline. The Stokes
number is defined as the ratio of the particle response time to a
fluid flow time and is characterised by:
St=.tau..sub.p/.tau..sub.f.rho..sub.p U d.sub.p.sup.2/18 .mu.L,
(1)
where; .tau..sub.p is the particle response time, .tau..sub.f is
the characteristic flow time, .rho..sub.p is the particle density,
U is the fluid velocity, d.sub.p is the particle diameter, .mu. is
the fluid viscosity and L is the eddy dimension. Typically, for
St<<1 a particle is able to respond fully to a turbulent eddy
of scale L, and follows it closely. At the other extreme, where
St>>1, a particle does not respond to turbulent motions of
that scale at all and its trajectory is largely unaffected. In the
intermediate range, for St.apprxeq.1, particles respond partially
to the fluid motions, but there is still a significant departure of
the particle trajectory from the fluid motions.
[0056] When a Stokes analysis is performed for the common pollution
species in the flue of, for example, an industrial coal fired
boiler, it is found that for turbulence eddies at the scale of a
typical duct (say 4 m.sup.2) and velocity of the gas (8-16 m/sec),
all particles of all commonly found sizes will respond fully to the
turbulence eddies, i.e. St<<1 for all particles. Even for
turbulence of a scale corresponding to the dimensions of duct
height/width mixers, turning vanes, stiffeners etc (say 400 mm),
the majority of particles below 100 .mu.m will respond fully to the
turbulence eddies. It is not until the turbulence scales are
reduced significantly below this size that the particles exhibit a
range of responses from St<<1 to St>>1 for sizes
ranging from 0.1 .mu.m up to 100 .mu.m. Under such conditions, the
trajectories of the large and small particles diverge from those of
the flow to different extents, causing increased probability of
collisions.
[0057] From the foregoing, it is evident that, if turbulent eddies
are sized correctly, it is possible to increase the number of
collisions between different sized constituents within the same
fluid flow on the basis of their differing interaction with, and
hence path through, a turbulent eddy of fixed size. Further, it is
possible to tailor the dominant size of a turbulent eddy to
maximise the interaction between specific constituents on the basis
of their relative inertia and hence responses to the turbulent
eddy. A suitable formation can then be designed to provide the
desired combination of eddy sizes.
[0058] Vortex generators can be used to create the eddies. Vortex
generators are generally known in the art, and need not be
described in detail in this application. A common vortex generator
is a vane. A formation comprising a plurality of vanes can be used
to generate a multitude of eddies in the fluid stream.
[0059] In one embodiment, illustrated in FIGS. 1 and 2, an array of
angle section vanes 10 is used to generate the vortices. A vane 10
is shown in FIG. 1 and comprises a strip of Z-shaped metal having
protrusions or "teeth" 12 spaced along its length. The teeth 12 may
be formed by spaced cut-outs 11 along the edges of the strip 10.
The teeth 12 have a depth T.sub.d and the tooth pitch T.sub.p.
[0060] The vanes 10 are arranged in an array comprising a plurality
of parallel rows each extending in the direction of flow, each row
containing a plurality of spaced vanes, orientated transversely to
the fluid flow, as shown in the section view of FIG. 2. (The rows
of vanes are normally mounted in planar frames which have been
omitted for clarity). The body portions of the vanes 10 extend
V.sub.1 in the direction of fluid flow, and are spaced apart by a
distance V.sub.s. The body portions of the vanes 10 have a width
V.sub.w in the direction transverse to the flow.
[0061] Turbulent eddies are formed in the wake of the folds and
protrusions 12 of the vanes 10. The dominant sizes of eddies
created by this design approximate the significant dimensions of
the generator, and include the width of the vane V.sub.w, the
length of the vane V.sub.l, the tooth depth T.sub.d and the tooth
pitch T.sub.p. The separation distance between successive vanes
V.sub.s is selected so that the eddies may form fully in the inter
vane region.
[0062] The combination of dimensions determines the combination of
eddy sizes that are formed. The optimal range of eddy sizes is
selected, and the vane design is optimized to achieve this within
other constraints, such as pressure drop.
[0063] Although teeth are used on the illustrated vane 10 and the
vanes are angled to the direction of fluid flow, other variations
are possible because eddies will form in the wake of any planar
cylindrical or other shaped body placed in the path of the fluid
flow and the eddies formed will be approximately the same size as
the obstructing vane.
[0064] For example, as shown in FIG. 3, an array of flat strips
mounted transversely to the fluid flow may be used. Alternatively,
an array of flat strips with scalloped edges as shown in FIG. 4, or
an array of round posts as shown in FIG. 5, may be used. An single
transverse row of spaced wires or rods, orientated across the flow,
may also be used.
[0065] The multiple small scale vortices or eddies generated by the
array of vanes extend across the entire duct as it is preferable
for the turbulence field to encompass the entire flow path.
However, although the vanes may be mounted in a duct in which the
subject air stream flows, it is to be noted that the invention does
not require that vanes to be mounted in a duct or other
conduit.
[0066] Thus, in one embodiment, a formation for causing turbulent
flow of the desired size and scale in a fluid flow can be designed
and constructed as follows: [0067] 1. Determine the size
distribution and density of the particles to be agglomerated (both
collector and collected particles), including the relative
quantities of particles of each size. [0068] 2. Identify the
distribution of size, density and shape and the number density of
the particles to act as the "collector particle" (i.e. the particle
that will have the greatest slip). These particles may be naturally
present in the system (e.g. in the upper size fraction of particles
in a pulverised fuel ash stream) or may be introduced (e.g. sorbent
particles for mercury collection). [0069] In certain systems, it is
possible for the collector and collection particles to have
significantly different densities and shapes. Variation in the slip
characteristics of the collector and collected particles may be
achieved by differences in density or shape, as well as by
differences in size. [0070] The collector particles will also be
selected to ensure that there are sufficient numbers of them
present to produce a significant number of collisions between
collector and collection particles. [0071] 3. Perform a Stokes
Number analysis of the system as defined in 2 (above) using
equation (1) to determine the optimal characteristic eddy size (L)
to cause the collector particles to have a significantly higher
slip velocity than the collected particles. This would typically
require the Stokes number for the collector particle to be at least
an order of magnitude greater than that of the collected particle.
In a preferred methodology, the Stokes number of spherical
collector particles would be in the range
10.sup.-2<St<10.sup.2. [0072] Note that once the critical
particle sizes are determined, the Stokes Number analysis can be
performed because St can be set (as St>>1 for high slip
particles) and all other variables in the Stokes equation with the
exception of L (the eddy size) are (or can be assumed to be)
constant. [0073] An iteration process may be used to determine the
optimal characteristic eddy size (L).
[0074] Namely, using the eddy size (L) as determined in step 3
(above), check St for the desired "collected particle" size (for
low slip particles, St<<1). Using eddy size (L) as determined
in step 3 (above) and St=1, check the intermediate particle
response. Iterate these steps, adjusting the eddy size (L) to
obtain the desired particle response. [0075] The optimum eddy size
will normally be small, e.g. much less than 400 mm, and typically
of the order of 10 mm, but will depend on the species of particles
and their relevant characteristics. [0076] 4. Determine the
required size of the dominant dimension of the vane(s), W, of the
vortex generator to create an eddy of size (L), as determined in
step 3 (above). In one methodology, W would be estimated to equal
L. In another preferred methodology, the size of the vane would be
determined by Stokes number similarity. This requires scaling the
size of the vane to match as closely as possible the Stokes numbers
of the collector and collected particles found to perform well in a
different set of conditions, i.e. with different distribution(s) of
particle size, density, shape and flow velocity and/or dynamic
viscosity. [0077] 5. Design a vane with the appropriate shape and
dimensions to generate eddies of the size determined in 4 above. A
preferred shape of vane is shown in FIG. 1. [0078] If necessary, an
empirical "shape factor" could be applied to account for the shape
of non-spherical particles.
[0079] There may be a range of sizes for each of the critical vane
dimensions as dictated by the physical properties of the system,
the dimensional requirements of manufacturing, and the engineering
constraints of the apparatus. However, in general, the variables
V.sub.w, V.sub.l, V.sub.s, T.sub.p and T.sub.d will determine the
size, shape, intensity and frequency of the turbulence created,
which in turn will control the degree to which individual particles
will slip and collide in the turbulence behind the vanes. The
important design criteria are the size and spacing of the
vanes.
[0080] In addition, the objective is to cause the collision of
suspended particles for a useful purpose e.g. agglomeration,
sorption, catalisation, condensation etc. Hence, sufficient
particle interactions should occur that substantially all particles
experience at least one (and preferably multiple) collision event/s
while traversing the device. In a practical sense, this requires a
multiplicity of vanes in the direction of flow as well as across
the flow. A multiplicity of vanes across the flow ensures that
there is no flow path through the device that is free of
appropriately sized eddies, while a multiplicity of vanes in the
direction of flow ensures the flow remains in the device for a
sufficient time for a useful number of particle collisions to
occur.
[0081] In a preferred embodiment, the device is long enough in the
direction of flow that the flowing fluid is treated by it for at
least 0.1 second. For a typical industrial flow of (say) 10 m/sec,
this would require a device at least 1 m deep in the direction of
flow.
[0082] Separation between subsequent vanes in the direction of flow
should be such that the eddies created by a vane are reinforced by
the eddy creating action of the vane immediately downstream, as
illustrated in FIG. 6 in which vortices 1 created by a vane are
reinforced at 2 by the next successive vane. FIG. 6 also
illustrates the different trajectories of a low slip particle 3 and
a high slip particle 4. In a preferred embodiment, the vanes are
separated by a distance V.sub.s equivalent to the vane width
V.sub.w.
[0083] Alignment of the vanes is not critical and may be
horizontal, vertical or at some angle between these two
directions.
[0084] The present invention has the advantage that mixing devices
can be designed to suit particular applications. More specifically,
turbulence of a desired scale can be achieved, so that small
pollutant particles are entrained into the turbulent eddies and
vortices, whereas larger collector particles are entrained to a
smaller or negligible degree). The resultant differential slip
velocities and trajectories of the small pollutant particles and
the larger removal particles result in more collisions between the
two types of particles. Consequently, there is greater interaction
between the particles (e.g impact adhesion, absorption, adsorption
or chemical reaction), improving the efficiency of pollutant
removal.
[0085] Conceptually, the invention involves generating turbulence
of such a scale that the two species of interest are entrained to
significantly differing extents, and is not limited to any
particular apparatus and process. Optimum collision rates will
occur for a system which maintains St<<1 for one species and
St.gtoreq.1 for the other species. The turbulence itself may be
generated in any suitable manner, and is not limited to known
vortex generators.
[0086] Although the invention has been described with particular
reference to its application in pollution control, it can be used
to design high efficiency mixers for other applications.
[0087] Further, although the invention has been described with
particular reference to the mixing of particles in a gas stream, it
also has application to mixing in other fluid flows, e.g.
liquids.
[0088] The vanes need not be mounted in a rectilinear array. As
shown in FIG. 7, the vanes may be mounted in successive rows
transverse to the direction of flow, with the vanes in each row
being staggered across the flow path relative to vanes in the
adjacent rows.
[0089] In a further embodiment of the invention, two or more
turbulence generators are spaced successively along the flow path,
generating progressively larger turbulence eddies to promote the
impact of progressively larger particles. Such an arrangement
accommodates agglomerates which are progressively increased in size
along the flow path. This embodiment has potential application in
mist eliminators and fine particle agglomerators, as well as in
chemical interaction or catalisation processes in which
successively larger constituents are targeted to enhance the
process efficiency.
* * * * *