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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG. 2 is a section plan view of an array of vanes of FIG. 1.

FIG. 3 is a section plan view of an array of vanes according to another embodiment of the invention.

FIG. 4 is a partial perspective view of an array of vanes according to another embodiment of the invention.

FIG. 5 is a partial perspective view of an array of vanes according to yet another embodiment of the invention.

FIG. 6 illustrates turbulent eddies formed by the array of FIG. 2.

FIG. 7 is a section plan view of a modified version of the array of vanes of FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENT

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.

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).

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=τ _p/τ_fρ_p U d_p²/18 μL,  (1)

where; τ_p is the particle response time, τ_f is the characteristic flow time, ρ_p is the particle density, U is the fluid velocity, d_p is the particle diameter, μ 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≈1, particles respond partially to the fluid motions, but there is still a significant departure of the particle trajectory from the fluid motions.

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 m²) 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 μm 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 μm up to 100 μm. 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.

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.

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.

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_d and the tooth pitch T_p.

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 V₁ in the direction of fluid flow, and are spaced apart by a distance V_s. The body portions of the vanes 10 have a width V_w in the direction transverse to the flow.

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_w, the length of the vane V_l, the tooth depth T_d and the tooth pitch T_p. The separation distance between successive vanes V_s is selected so that the eddies may form fully in the inter vane region.

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.

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.

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.

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.

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:

• 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.
• 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).
• 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.
• 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.
• 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 10⁻²<St<10².
• 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.
• An iteration process may be used to determine the optimal characteristic eddy size (L).

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.

• 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.
• 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.
• 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.
• If necessary, an empirical “shape factor” could be applied to account for the shape of non-spherical particles.

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_w, V_l, V_s, T_p and T_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.

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.

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.

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_s equivalent to the vane width V_w.

Alignment of the vanes is not critical and may be horizontal, vertical or at some angle between these two directions.

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.

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≧1 for the other species. 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 its application in pollution control, it can be used to design high efficiency mixers for other applications.

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.

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.

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.

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 10−2 to 102.

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 10−2 to 102.

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 10−2 to 102.

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.