HK1104341B - Improved particle interactions in a fluid flow - Google Patents

Description

Improved particle interaction in fluid streams

Technical Field

The present invention relates generally to methods and apparatus for promoting or enhancing interactions between different types of particles in a fluid stream. The present invention provides a method of designing a vortex generator to generate particle-scale turbulence and induce interactions between specific types of particles in a fluid stream in an efficient manner.

The present invention has particular application in air pollution control by promoting agglomeration of fine contaminant particles in an air stream into larger particles and thereby facilitating their subsequent filtration or removal from the air stream, although the invention is not limited to this application.

Background

Many industrial processes result in the emission of small hazardous particles into the atmosphere. These particles often include very fine submicron particles of toxic compounds that are easily inhaled. The combination of toxicity and ease of inhalation has prompted governments around the world to make legislation to tightly control the emission of particles less than ten microns in diameter (PM10), particularly less than 2.5 microns in diameter (PM 2.5).

Smaller particles in atmospheric emissions are also primarily responsible for the adverse visual effects of air pollution. The opacity is largely determined by the fine emission particle composition, since the absorption coefficient is highest around the wavelength of light between 0.1 and 1 micron.

Various methods have been used to remove dust and other contaminating particles from gas streams. While these methods are generally suitable for removing larger particles from gas streams, they are often ineffective at filtering out smaller particles, particularly PM2.5 particles.

By means of collision and adhesion, fine particles in the gas flow can be agglomerated into larger particles, thereby facilitating subsequent removal of the particles by filtration. Our international patent application numbers PCT/NZ00/00223 and PCT/AU2004/000546 disclose active and passive devices for agglomerating particles. The agglomeration efficiency depends on the incidence or frequency of collisions and similar interactions between particles.

Many pollution control strategies also rely on contact between individual elements of a particular species in order to promote reactions or interactions that are beneficial for subsequent removal of the contaminants of interest. For example, an adsorbent such as activated carbon may be injected into the contaminated gas stream to remove mercury (adsorption), or calcium may be injected to remove sulfur dioxide (chemisorption).

In order for these interactions to occur, the two substances of interest must remain in contact. For many industrial pollutants in standard flues, this is difficult for several reasons. For example, the period for reaction/interaction is short (on the order of 0.5-1 seconds), the species of interest is distributed very little in the exhaust gas (relative to the bulk fluid), and the flue is larger in scale than the pollutant particles.

Typically, the exhaust gases from the production process are fed into large conduits, carrying them as uniformly as possible to some downstream collection device (e.g. an electrostatic precipitator, a filter bag or a cyclone collector), with little turbulence/energy loss. The turbulence created en route is typically mass-diverted gas along guide vanes, along internal duct support/stiffening ribs, through a diffuser screen, and the like. This turbulence is of the scale of the duct and requires that there should be minimal disturbance and pressure drop so that the correct flow required can be achieved.

Similarly, when mixing devices are used for specific purposes, such as the absorption of specific pollutants, they are generally devices that generate large-scale turbulent fields (on the order of the width and height of the tubes) and are arranged as a short series of curtains through which the gas must pass.

The most known mixing devices aim to obtain a mixing of two or more substances. The device is not specifically designed to promote interaction between the fine particles in the mixture. In most industrial scale devices involving particle transport, the turbulence created by mixing is large scale relative to the particles. Under which the particles tend to move in the same path rather than in the course of collisions.

It is also known to use vortex generators in the mixing chamber to promote mixing of the fluids. However, the device is not normally used in a flow filled with particles to create collisions between the particles.

Whether they are particles (e.g. silver ash), gases (e.g. SO)₂) Mist (e.g. NO)_x) Or elements (e.g., mercury), the contaminant species that are more difficult to collect in an industrial exhaust stack are species on the order of microns (i.e., 10 a)^-6Rice). Because of their small size, they occupy a small proportion of the volume in the total fluid flow. For example, if uniformly distributed, one million particles of 1 micron diameter will occupy less than 1cm³0.00005% of the gas volume (assuming the particles are spherical). Even at 10 micron diameter, the ratio only increases to 0.05%. When it is considered that pollutants such as mercury constitute only parts per million of the total species present, it is clear that at this particle scale there is a large amount of space/distance between the species transported by the industrial flue gas. Where particles are already "well mixed" in a flow, such as being somewhat freely distributed throughout a duct (e.g., in an exhaust flue), turbulence of any scale may not mix them more thoroughly.

Also, sufficiently small particles delivered into the flowing fluid will follow the streamlines in the fluid flow. This occurs when the viscosity of the fluid exceeds the inertial force of the particles. Known turbulent mixing conditions of the duct scale are many orders of magnitude larger than particles. They are far from chaotic when viewed from the perspective of the particles, but rather smooth. Although particles undergo many changes in direction as they pass through turbulence in the conduit or through a standard mixing zone, they are in a relatively long range compared to the size and scale of the particles. Therefore, particles in the gas stream under typical industrial dust-laden flow conditions follow more or less the same path as their neighboring particles, which results in little interaction with surrounding particles. On a particle scale there are relatively few interaction by turbulence and therefore the known mixing industry has a very low efficiency in terms of agglomeration.

Systems that tend to maximize the collision rate of very small pollutants, which occupy a very small proportion of the total fluid flow volume, must move them as often as possible along different trajectories and/or at different velocities to each other. In addition, these differences in trajectory and/or velocity must be tolerated for the size of the particles to have the best effect. Unfortunately, current design concepts do not adequately account for these criteria.

It is an object of the present invention to provide a method and apparatus for obtaining improved interaction in a fluid stream.

It is another object of the present invention to provide a method of custom designing structures to create particle-scale turbulence to induce interactions between particular types of particles in a fluid stream in an efficient manner.

Disclosure of Invention

The invention is based on the recognition that: two types of particles of different substances and/or aerodynamic properties in a flowing fluid will respond differently to turbulent eddies of predetermined size in the fluid flow. In particular, if the vortex is of a particular size, different particles will be entrained into the vortex to different extents and will therefore follow different trajectories. Therefore, the possibility of collision or interaction between particles is increased.

Particles of similar material and/or aerodynamic properties that are captured and entrained by the turbulent eddies will generally follow the same path and therefore do not impinge upon each other to any significant extent. Particles of larger material and/or different aerodynamic properties will not be entrained into the vortex, or will be entrained to a substantially lesser extent, and will therefore move through the vortex in a different trajectory and be impacted by many other particles entrained in the same vortex.

To increase the likelihood of collisions between two particles in a fluid stream, for example to promote agglomeration or adsorption of smaller particles by larger particles, a structure is designed to create turbulence of such a magnitude that different particles are entrained to significantly different extents.

In one broad form, the present invention provides such a method of promoting interaction between at least two types of particles in a fluid stream, comprising the steps of: generating turbulent eddies in a fluid stream to cause two particles in the turbulent eddies to interact, characterised in that the eddies are of such size and/or intensity that the two particles are entrained into the eddies to a significantly different extent.

In another form, the invention provides an apparatus for promoting interaction between at least two particles in a fluid stream, comprising means for generating a turbulent vortex in the fluid stream so as to cause interaction between the two particles in the turbulent vortex, characterised in that the vortex is of such size and/or strength that the two particles are entrained into the vortex to a significantly different extent.

Preferably, the turbulent eddies are of such size and/or intensity that one particle is substantially completely entrained while the other particle is not substantially entrained, thereby maximizing the likelihood of relative slipstreaming and interaction between the two particles.

In another broad form, the present invention provides a method of custom designing a structure 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) the relevant features of the two types of particles are identified,

(ii) stokes Number (Stokes Number) analysis was performed to determine the best characterized vortex size, with one type of particle having a significantly higher slipstream velocity than the other type of particle, an

(iii) (iii) designing a structure to create vortices in the fluid flow having the optimum dimensions determined in step (ii) above.

The relevant characteristics of both particles generally include the size and density of the particles.

The determination of the optimal characteristic vortex size may include an iterative process.

Since the standard equation for the stokes number assumes that the particles are spherical, an empirical "shape factor" can be used to account for the shape of the particles.

For two given particle types, e.g., collector particles and collected particles, the present invention provides a method of custom designing a configuration that generates vortex vortices of such size and scale as to maximize the different slip velocities of the two particles, thereby maximizing the likelihood of particle-particle interaction. The vortices in the generated turbulence are preferably of such size that the slip velocity of the collector particles is maximized while the slip velocity of the collected particles is minimized.

Throughout the context of the allowed description, the term "particle" refers to a component of a flowing fluid that can be manipulated to collide with another particle in the same fluid stream. The "particles" may be solid (e.g., fly ash particles), liquid (e.g., hanging water droplets), or gaseous (e.g., SO)₃Hg or NO_xA molecule). The invention can be used for interaction of gas and solid particles, interaction of gas and liquid droplets, interaction of liquid and solid particles, interaction of liquid droplets of different sizes, and interaction of particles of different sizes. Particles of different sizes may be suspended in a gas or liquid and droplets of different sizes may be suspended in a gas.

The term "collector particle" refers to a larger and/or heavier particle that is intended to collide with and/or react with a "collected" or "collecting" particle.

The term "interaction" refers to particles colliding or contacting or sufficiently close to cause agglomeration, adsorption, agglomeration, catalysis, or chemical reaction.

In addition, the terms "slip stream" and "slip stream velocity" are used to describe the relative velocity between a particle and the fluid surrounding it. So, if a particle is fully entrained in a turbulent flow, its slip velocity is zero. The more the path of a particle deviates from the fluid surrounding it, the greater its slip velocity. So, in this context, if small particles follow the fluid flow more closely than large particles, their slip velocity will be less, they are said to have less "slip".

Typically, the fluid stream is a gas or gas stream and the at least one particle is a micron or sub-micron sized contaminant particle. However, the invention is not limited to the use of pollution control, but may be applied more broadly to other applications in which it is sought to obtain interaction between particles in a fluid stream in an efficient manner.

Turbulent eddies typically include a swirling motion having a plurality of different sizes and shapes.

In one embodiment, a plurality of small, low intensity vortices are used to entrain fine (contaminant) particles and subject them to turbulence. One or more larger "collector" particles are introduced into the gas stream for removal of contaminant particles. Larger collector particles are not entrained into the vortex or are entrained to a very small extent, whereby they follow different trajectories with the fine contaminant particles, resulting in a higher probability of contact and/or interaction between the contaminant particles and the larger matter.

As the contaminant particles contact the larger material, they tend to adhere to and react with them. The removal substance may be chemical, such as calcium, which chemically reacts with the contaminant particles (e.g., sulfur dioxide) to form a third compound (e.g., calcium sulfate). Alternatively, the particulate removal material may remove the contaminant particles by absorption, or by adsorption (carbon particles adsorbing contaminant mercury particles), or the particulate removal material may simply remove the fine contaminants by agglomerating with the contaminants by impact adhesion. Larger or agglomerated particles are then more easily removed from the gas stream using known methods.

Typically, a stokes number very less than 1 will ensure the inhalation of contaminant particles. The larger removed species of the particles should have a stokes number much greater than 1 so that they are not inhaled. In fact, the eddies or vortices generated by the gas flow are small, unlike the large-scale turbulence of known mixers. Therefore, the structure generally includes a plurality of components that generate a plurality of small vortices or vortices.

The plurality of small vortices or vortices entrain the (small) particles of interest and subject them to turbulence. Larger particles do not have to be entrained by these small vortices, or are entrained to a small extent. Relative movement between small and large particles results in a higher frequency of collisions between them, and the fine (contaminant) particles are removed more efficiently by the larger (collector) particles.

It is counterintuitive to use a plurality of small vortices in the flow where the particles are already well distributed in the duct. Generally, it is desirable that the pressure drop in the gas stream be as low as possible. For this reason, the vanes are generally only used to keep the particles as evenly distributed as possible in the duct. Such blades are therefore generally of relatively large scale-only slightly smaller than the scale of the duct. For example, large scale "turning vanes" may be used in a curve to prevent all particles from going outside the curve and creating an uneven distribution after the curve.

Alternatively, known mixers are used to produce uniform mixing when two different substances are not initially well distributed in a vessel or pipe. Further, a large-scale apparatus is generally used. They are not generally used to promote collisions between substances that are already well distributed in the pipe. On the other hand, the present invention uses a number of vortex generators which generate small scale vortices to increase the interaction between fine (contaminant) particles and collector particles which are already sufficiently well distributed throughout the fluid flow.

In order that the invention may be more fully understood and put into practical effect, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings.

Drawings

FIG. 1 is a perspective view of a blade according to one embodiment of the present invention.

FIG. 2 is a cross-sectional view of the blade array of FIG. 1.

FIG. 3 is a cross-sectional view of a blade according to another embodiment of the invention.

FIG. 4 is a partial perspective view of a blade according to another embodiment of the present invention.

Fig. 5 is a partial perspective view of a blade according to yet another embodiment of the invention.

Figure 6 shows the turbulent eddies formed by the array of figure 2.

FIG. 7 is a cross-sectional view of a modification of the vane array of FIG. 2.

Detailed Description

In a preferred embodiment, the present invention involves the use of turbulent eddies to manipulate the relative trajectories of very small contaminant particles and larger collector particles carried by a flowing fluid, typically the effluent gas stream from a production process, in order to increase the likelihood that the particles will collide or interact to agglomerate or otherwise react with each other to more readily form removable particles. A structure is designed to provide turbulence of a desired size and scale to cause particles of different materials to have substantially different slip velocities.

The turbulence should be such that the Stokes (St) of small contaminant particles is very small than 1(St < 1), while the Stokes (St) of larger collector particles is very large than 1(St > 1).

The stokes number (St) is a theoretical measure of the ability of a particle to follow a streamline. The stokes number is defined as the ratio of the particle response time to the fluid flow time and is characterized by:

St＝τ_p/τ_f＝ρ_pUd_p ²/18μL，(1)

wherein: tau is_pIs the particle response time, τ_fIs specialFlow time, p_pIs the particle density, U is the fluid velocity, d_pIs the particle diameter, μ is the fluid viscosity, and L is the vortex size. In general, for St < 1, the particles are able to respond adequately to turbulent eddies of scale L and follow closely. At the other extreme, where St > 1, the particles do not respond at all to turbulent motion of this scale, their trajectories being substantially unaffected. In the mid-range, for St ≈ 1, the particles respond partially to the fluid motion, but still have particle trajectories that are significantly separated from the fluid motion.

When Stokes analysis is performed on common pollutants in, for example, the flue of an industrial coal-fired boiler, it is found that the Stokes analysis is performed on a common pipe (say 4 m)²) And the velocity of the gas (8-16m/sec), all particles of a commonly found size will respond fully to turbulent eddies, i.e. St < 1 for all particles. Even for turbulent flow on a scale responsive to the dimensions of the conduit height/width mixer, rotating vanes, reinforcing ribs, etc. (say 400mm), most particles below 100 microns will respond fully to turbulent vortex flow. Until the turbulence scale drops significantly below the following dimensions: for sizes ranging from 0.1 micron to 100 microns, the particles have a response range from St < 1 to St > 1. Under such conditions, the trajectories of the large and small particles deviate to different extents from those of the flow, so that the probability of collision is increased.

From the foregoing, it is apparent that if the turbulent eddies are correctly sized, the number of collisions between components of different sizes within the same fluid stream can be increased based on their different interaction with and path through the turbulent eddies of fixed size. Moreover, the major dimensions of the turbulent eddies can be tailored to maximize the interaction between specific components based on their relative inertia, and thus respond to the turbulent eddies. Suitable configurations can then be designed to provide the desired combination of vortex sizes.

Vortex generators may be used to generate vortices. Vortex generators are well known in the art and need not be described in detail in this application. A common vortex generator is a blade. A structure comprising a plurality of vanes may be used to create a plurality of vortices in the fluid flow.

In one embodiment, as shown in fig. 1 and 2, angular cross-section blades are used to generate the vortex. FIG. 1 shows a blade 10 comprising a Z-shaped metal strip having projections or "teeth" 12 along its length. The teeth 12 may be formed by notches 11 spaced along the edge of the belt 10. The teeth 12 have a depth T_dAnd a tooth spacing T_p。

The vanes 10 are arranged in an array comprising a plurality of parallel rows, each row extending in the direction of flow and containing a plurality of spaced vanes oriented transversely to the fluid flow, as shown in the cross-sectional view of fig. 2. (typically rows of blades are mounted in a planar frame, which is omitted for clarity). The main part of the blade 10 extends in the direction of the fluid flow V₁And are separated by a distance V_s. The main portion of the blade 10 has a width V in the direction across the flow_w。

A vortex is formed after the folding of the blade 10 and the protrusion 12. The major dimensions of the vortex created by this design approximate the important dimensions of the generator, including the blade width V_wLength of blade V₁Tooth depth T_dAnd a tooth spacing T_p. Selecting the spacing distance V between successive vanes_sSo that a vortex can be sufficiently formed in the inner blade region.

The combination of sizes determines the combination of sizes of vortices formed. An optimal range of vortex sizes is selected and the vane design is optimized to achieve it within other limits, such as pressure drop.

Although teeth are used on the illustrated vane 10 and the vane is angled with respect to the direction of fluid flow, other variations are possible because any planar cylinder or other shaped body disposed in the path of fluid flow will then form a vortex and the vortex will form almost the same size as the blocking vane.

For example, as shown in FIG. 3, an array of flat bands mounted across the fluid stream may be used. Alternatively, a flat band array with scalloped edges as shown in FIG. 4 and a cylindrical array as shown in FIG. 5 may be used. A single transverse row of spaced wires or rods across the flow direction may also be used.

The plurality of small scale vortices or vortices generated by the array of vanes extend across the entire duct, preferably surrounding the entire flow path for the turbulent flow field. However, although the vanes may be mounted in a duct in which the main gas stream flows, it should be noted that the present invention does not require the vanes to be mounted in a duct or other conduit.

Thus, in one embodiment, the structure for creating turbulence in a fluid stream of a desired size and scale may be designed and configured as follows:

1. the size distribution and density of the particles to be agglomerated (collector and collected particles) are determined, including the relative amount of particles of each size.

2. The size distribution, density and shape and number density of the particles used as "collector particles" (i.e. the particles with the largest slip) are identified. These particles may be naturally present in the system (e.g., higher size particle size of particles in a loose fuel ash stream) or may be introduced (e.g., sorbent particles for mercury collection).

In a certain system, the collector and the collecting particles may have significantly different densities and shapes. Variations in the slip stream characteristics of the collector and collected particles can be obtained by differences in density and shape as well as differences in size.

The collector particles will also be chosen to ensure that they are in sufficient number to produce a significant number of collisions between the collector and the collecting particles.

3. A stokes number analysis as defined in 2 (above) was performed using equation (1) to determine the optimal characteristic vortex size (L) for collector particles to have a significantly higher slip velocity than the collected particles. This typically requires that the stokes number of the collector particles be at least an order of magnitude greater than the collected particles. In a preferred embodimentIn the method, the Stokes number of the spherical collector particles is 10^-2＜St＜10²Within the range of (1).

Note that the Stokes number analysis can be performed once the adjacent particle size is determined, since St can be set (for high slip particles, St > 1), and all other variables in the Stokes equation except L (vortex size) are (or can be assumed to be) constant.

An iterative process is used to determine the optimal characteristic vortex size (L). St for the desired "collected particles" size (St < 1 for low slip particles) is examined, using the vortex size (L) determined in step 3 (above). Using the vortex size (L) determined in step 3 (above), St ═ 1, the median particle response was examined. These steps are iterated, adjusting the vortex size (L) to obtain the desired particle response.

The preferred size of the vortex is generally small, for example much less than 400mm, typically on the order of 10mm, but depends on the type of particles and their associated characteristics.

4. The required dimensions of the main dimensions of the vortex generator blades W are determined to generate vortices of the size (L) as determined in step 3 (above). In one approach, W is estimated to be equal to L. In another preferred method, the size of the blades is determined by stokes number simulation. This requires dimensioning the blades to match as closely as possible the stokes numbers of the collector and the collected particles, which have been found to perform well under different sets of conditions, i.e. with different particle size distributions, densities, shapes, flow rates and/or dynamic viscosities.

5. Vanes of appropriate shape and size are designed to produce a vortex of the size determined in 4 above, the preferred shape of the vanes being shown in figure 1.

If necessary, an empirical "shape factor" is used to account for the shape of the non-spherical particles.

For the system by the physical properties of the system, the manufacturing size requirements and the device engineering limitationsMay have a range of sizes. In general, however, the variable V_w、V₁、V_s、T_pAnd T_dThe size, shape, intensity and frequency of the turbulence generated will be determined, which in turn will control the degree to which individual particles slip and collide into the turbulence behind the blades. Important design criteria are the size and spacing of the blades.

Furthermore, the purpose is to cause collisions of suspended particles for useful purposes, such as agglomeration, adsorption, catalysis, agglomeration, and the like. Thus, sufficient particle interaction should occur that substantially all particles undergo at least one (preferably multiple) collision phenomenon while traversing the device. In a practical sense, this requires multiple vanes in the direction of and across the flow. The multiple vanes across the flow ensure that there is no flow path through the device without an appropriately sized vortex, while the multiple vanes in the direction of flow ensure that the flow remains in the device for a sufficient time for a useful number of particle collisions to occur.

In a preferred embodiment, the device is sufficiently long in the direction of flow that it treats the flowing fluid for at least 0.1 seconds. For a typical industrial flow of, say, 10 m/s, it is required that the device be at least 1m deep in the direction of flow.

The spacing between adjacent vanes in the direction of flow should be such that the swirl generated by the vanes is enhanced by the swirl generating action of the vanes immediately downstream, as shown in figure 6, where the swirl 1 generated by the vanes is enhanced by the following successive vanes at 2. Fig. 6 also shows different trajectories of low slipstream particles 3 and high slipstream particles 4. In a preferred embodiment, the blades are separated by a distance V_sAnd width V of blade_wAre equal.

The arrangement of the vanes is not critical and may be horizontal, vertical or at some angle in both directions.

The present invention has the advantage of designing the mixing device to suit a particular application. In particular, a desired scale of turbulence may be achieved, whereby small contaminant particles are entrained into turbulent eddies or vortices, while larger collector particles are entrained to a lesser or negligible extent). The resulting different slip velocities and trajectories of small contaminant particles and larger dislodged particles result in more collisions between the two particles. Therefore, there is greater interaction (e.g., impact adhesion, adsorption, or chemical reaction) between the particles, increasing the efficiency of contaminant removal.

The present invention therefore relates to the generation of turbulence of such a scale that the two substances of interest are entrained to significantly different extents and is not limited to any particular apparatus or process. An optimum collision rate will occur for a system that holds one substance St < 1 and another substance St.gtoreq.1. The turbulence itself may be generated in any suitable manner and is not limited to known vortex generators.

Although the invention has been described with particular reference to pollution control applications, it may be used to design high efficiency mixers for other applications.

Furthermore, although the invention has been described with particular reference to mixing particles in a gas stream, it may also be used for mixing in other fluid streams, such as liquids.

The blades need not be mounted in a linear array. As shown in fig. 7, the vanes may be mounted in successive rows across the flow direction, the vanes in each row intersecting the flow path with respect to the vanes of an adjacent row.

In another embodiment of the invention, two or more vortex generators are continuously spaced along the flow path, creating progressively increasing vortex vortices to promote progressively increasing particle impingement. This configuration is suitable for agglomeration that gradually increases 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 catalytic processes where the purpose of the successively larger components is to improve process efficiency.

Claims (30)

1. A method of designing a structure of a vortex generator 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) the relevant features of the two types of particles are identified,

(ii) stokes number analysis was performed to determine the optimum characteristic vortex size so that one particle had a significantly higher slip velocity than the other, an

(iii) (iii) designing a structure to create vortices in the fluid flow having the optimum dimensions determined in step (ii) above.

2. The method of claim 1, wherein the relevant characteristics of both particles include size and density of the particles.

3. The method of claim 1, wherein the determination of the optimal characteristic vortex size comprises an iterative process.

4. The method of claim 1, wherein the stokes number of one particle is at least an order of magnitude greater than the stokes number of another particle.

5. The method of claim 4, wherein at least one particle has a particle size of 10^-2To 10²The stokes number of the range.

6. The method of claim 1, wherein the optimal characteristic vortex size is the size: the difference in stokes numbers of the two particles is greatest at this size.

7. The method of claim 1, wherein the structure is designed to include a plurality of blades.

8. A method of promoting interaction between at least two types of particles in a fluid stream, comprising the steps of: generating turbulent eddies in a fluid stream to cause interaction between two types of particles in the turbulent eddies, characterized by: the vortices are of such size and/or strength that the two particles are entrained into the vortex to a significantly different extent.

9. A method as claimed in claim 8, wherein the vortices are of such size and/or strength that one particle is substantially fully entrained and the other particle is substantially unabsorbed, thereby maximising the likelihood of relative slipstreaming and interaction between the two particles in the vortices.

10. The method of claim 8, wherein the stokes number of one particle is at least an order of magnitude greater than the stokes number of another particle.

11. The method of claim 10, wherein at least one particle has a stokes number of 10^-2To 10²Within the range.

12. An apparatus for promoting interaction between at least two particles in a fluid stream, comprising means for generating turbulent eddies in the fluid stream to cause interaction between the two particles in the turbulent eddies, characterized by: the vortices are of such size and/or strength that the two particles are entrained into the vortex to a significantly different extent.

13. The apparatus of claim 12, wherein the vortices are of such size and/or strength that one particle is substantially fully entrained while the other particle is substantially not entrained, thereby maximizing the likelihood of relative slipstreaming and interaction between the two particles in the vortices.

14. The device of claim 12, wherein the stokes number of one particle is at least an order of magnitude greater than the stokes number of another particle.

15. The device of claim 14, wherein at least one particle has a stokes number of 10^-2To 10²Within the range.

16. A structure for generating vortices in a fluid flow, the structure being designed by the method of claim 1.

17. The method of claim 1, wherein one particle is solid, liquid or gaseous and the other particle is solid, liquid or gaseous.

18. The method of claim 8, wherein one particle is solid, liquid or gaseous and the other particle is solid, liquid or gaseous.

19. The method of claim 8, wherein the fluid flow is in a conduit and the step of generating turbulent eddies includes disposing a plurality of blade members in spaced relation across the conduit to generate a plurality of eddies.

20. The method of claim 19, wherein the spacing between the blade members is about the width of the blade members.

21. The method of claim 19, further comprising the step of arranging the blade members to form an array of blade members.

22. The method of claim 21, wherein the array of blade members extends longitudinally along the pipe, such that it takes at least 0.1 seconds for the fluid flow to pass through the array.

23. The apparatus of claim 12, wherein one particle is solid, liquid or gaseous and the other particle is solid, liquid or gaseous.

24. The apparatus of claim 12, wherein the fluid flow is in a conduit and the means for generating turbulent eddies comprises a plurality of blade members disposed in spaced relation across the conduit to generate the plurality of eddies.

25. The apparatus of claim 24, wherein the spacing between the blade members is about the width of the blade members.

26. The apparatus of claim 24 wherein the blade members form an array of blade members.

27. The apparatus of claim 24 wherein each blade member has a Z-shaped cross-section.

28. The apparatus of claim 27 wherein each blade member has spaced apart tooth portions along its longitudinal edges.

29. Apparatus for creating an interaction between large and fine particles in a fluid stream, comprising an array of micro-vortex generating structures for generating a plurality of micro-vortices across the fluid stream, the array comprising a plurality of longitudinally spaced rows of micro-vortex generating structures, each row having a plurality of laterally spaced micro-vortex generating structures, wherein fine particles are substantially entrained into the micro-vortices and large particles are substantially not entrained, thereby maximising the likelihood of relative slippage and interaction between the two particles.

30. The apparatus of claim 29, each minute vortex generating structure being a blade member having a Z-shaped cross section with a fan-shaped longitudinal edge.