Atomiser assembly

Abstract

A compact apparatus for atomisation of fluid samples comprises a sonotrode (11), placed so that an ultrasonic wave emitted by the sonotrode is directed through a channel (25) in a separate channel device (21) and reflected by from the interface (26) in a high-low impedance transition zone (Tz), so that a standing wave is formed within the channel. A positive air flow through the channel, driven by a pressure differential at each end of the channel, interacts with the working fluid or slurry being delivered by a fluid delivery device (30) to atomise it. The speed of the air flow and the dispersal, homogeneity, and size of particles in the slurry sample can be controlled by varying the shape of the channel outlet.

Claims

1. A method of spray drying a particulate substance from a slurry of the particulate substance suspended in a fluid, the method comprising generating a dispersion of particles from the slurry using an atomiser device, the atomiser device comprising an energy generator having an active face, a channel device having a channel comprising a bore extending parallel to an axis of the channel, with a channel inlet and a channel outlet, wherein the channel device comprises a plate having opposite inlet and outlet surfaces on which the channel inlet and channel outlet are respectively disposed, and wherein the channel is filled by a gas, and a fluid delivery device having a fluid outlet, the method comprising: separating the inlet surface of the plate from the active face of the energy generator by a gap filled with a gas; generating an energy wave from the energy generator; passing the energy wave generated by the energy generator into the channel inlet and through the bore of the channel and emitting the energy wave from the channel outlet, wherein the energy wave has a frequency selected from the range of frequencies consisting of 20 kHz to 70 kHz; establishing a standing wave in the energy wave within the channel; axially separating the channel outlet from the energy generator by a distance; flowing the fluid through the fluid delivery device, and discharging the fluid from the fluid outlet into the energy wave emitted from the channel outlet; the method including flowing the gas from the channel inlet to the channel outlet, and establishing the standing wave in the gas, and drying the dispersion of particles.

2. The method of claim 1, including axially separating the channel inlet from the active face of the energy generator by a distance ranging from 0.1 mm to 0.35 mm.

3. The method of claim 1, including discharging the fluid from the fluid outlet at an axial location with respect to the axis of the channel corresponding to a pressure node on the energy wave.

4. The method of claim 1, including discharging the fluid from the fluid outlet within a transition zone formed outside the channel outlet, the transition zone having an acoustic impedance gradient at the interface between the interior of the channel and the exterior of the channel, and wherein the method includes reflecting the incident energy wave from the acoustic impedance gradient within the channel and towards the energy generator.

5. The method of claim 1, including flowing the fluid into a torus-shaped region of low pressure outside the channel outlet.

6. The method of claim 1, including discharging the fluid from the fluid outlet of the fluid delivery device into an annular chamber surrounding the channel outlet, and flowing the fluid from the annular chamber past the channel outlet.

7. The method of claim 6, wherein the annular chamber extends beyond the channel outlet in an axial direction with respect to the channel, and wherein the outlet of the fluid delivery device is disposed within the annular chamber surrounding the channel outlet.

8. The method of claim 7, wherein the annular chamber comprises a wall, and wherein the wall of the annular chamber extends beyond the channel outlet in an axial direction with respect to the channel by a distance in the range of 0.1-0.3 mm.

9. The method of claim 8, wherein the wall of the annular chamber tapers towards the channel outlet such that the radius of the annular chamber decreases along the axis of the channel in a direction towards the outlet surface of the channel device.

10. The method of claim 1, including propagating the energy wave within the channel in a direction aligned with the axis of the channel.

11. The method of claim 1, wherein the diameter to length ratio of the channel is selected from a range of 0.5 to 0.8.

12. The method of claim 1, wherein the energy wave is an ultrasound wave.

13. A method of spray drying a particulate substance from a slurry of the particulate substance suspended in a fluid, the method comprising generating a dispersion of particles from the slurry using an atomiser device, the atomiser device comprising an energy generator having an active face, a channel device having a channel comprising a bore extending parallel to an axis of the channel, with a channel inlet and a channel outlet, wherein the channel device comprises a plate having opposite inlet and outlet surfaces on which the channel inlet and channel outlet are respectively disposed, and wherein the channel is filled by a gas, and a fluid delivery device having a fluid outlet, the method comprising: separating the inlet surface of the plate from the active face of the energy generator by a gap of 0.1 mm to 0.35 mm, the gap being filled with a gas; generating an energy wave from the energy generator; passing the energy wave generated by the energy generator into the channel inlet and through the bore of the channel and emitting the energy wave from the channel outlet, wherein the energy wave has a frequency selected from the range of frequencies consisting of 20 kHz to 70 kHz; establishing a standing wave in the energy wave within the channel; axially separating the channel outlet from the energy generator by a distance; flowing the fluid through the fluid delivery device, and discharging the fluid from the fluid outlet into a transition zone of the energy wave formed outside the channel outlet, the transition zone having an acoustic impedance gradient at the interface between the interior of the channel and the exterior of the channel; reflecting the incident energy wave back into the channel towards the energy generator from the acoustic impedance gradient in the transition zone; and drying the dispersion of particles.

14. The method of claim 13, including discharging fluid from the fluid outlet at an axial location with respect to the axis of the channel corresponding to a pressure node on the energy wave.

15. The method of claim 13, wherein the transition zone comprises a torus-shaped region of low pressure outside the channel outlet.

16. The method of claim 13, including discharging the fluid from the fluid outlet of the fluid delivery device into an annular chamber surrounding the channel outlet, and flowing the fluid from the annular chamber past the channel outlet.

17. The method of claim 16, wherein the annular chamber extends beyond the channel outlet in an axial direction with respect to the channel, and wherein the outlet of the fluid delivery device is disposed within the annular chamber surrounding the channel outlet.

18. The method of claim 17, wherein the annular chamber comprises a wall, and wherein the wall of the annular chamber extends beyond the channel outlet in an axial direction with respect to the channel by a distance in the range of 0.1-0.3 mm.

19. The method of claim 18, wherein the wall of the annular chamber tapers towards the channel outlet such that the radius of the annular chamber decreases along the axis of the channel in a direction towards the outlet surface of the channel device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the accompanying drawings:

(2) FIG. 1 shows a schematic side view of an atomiser assembly;

(3) FIG. 2 shows an end view of the atomiser assembly of FIG. 1;

(4) FIG. 3 shows an enlarged side view of the atomiser assembly of FIG. 1, showing a schematic transition zone and impedance gradient between low and high acoustic impedance outside the channel outlet;

(5) FIG. 4 shows a graph plotting variations of the length of the channel (providing different displacements between the sonotrode active face and the outlet end of the channel) (x-axis) in the atomiser assembly of FIG. 1 against static air pressure obtained at the channel inlet (y-axis) with the different variations;

(6) FIG. 5 shows a graph plotting variations of the diameter of the channel (x-axis) in the atomiser assembly of FIG. 1 against static air pressure obtained at the channel inlet (y-axis) with the different variations;

(7) FIG. 6 shows a graph plotting variations of the separation (x-axis) in the atomiser assembly of FIG. 1 between the channel device and the energy generator against airflow obtained at the channel inlet (y-axis) with the different variations;

(8) FIG. 7 shows a schematic view of second example of a an atomiser assembly with a channel outlet and the annular space formed around it by the edge of an annular channel, with other parts removed for clarity;

(9) FIG. 8 shows a schematic side view of the FIG. 7 assembly including the sonotrode, channel, annular chamber and fluid delivery device;

(10) FIG. 9 shows a schematic detail view of the FIG. 8 channel with the low pressure torus at the channel outlet illustrated and other parts removed for clarity;

(11) FIG. 10 shows a schematic side view of a third example of an atomiser assembly with the sonotrode end face housed within a recess in the plate, and an annular chamber around the channel; and

(12) FIG. 11 shows a schematic side view of the assembly of FIG. 10 with the fluid delivery device shown, and the plate comprising different segments connected together.

DETAILED DESCRIPTION OF AT LEAST ONE EXAMPLE OF THE INVENTION

(13) Referring now to the drawings, an atomiser assembly 1 has an energy generator 10 which in this example comprises an ultrasonic transducer (in this case, a Model CL334 by Qsonica of Newtown Conn., USA driven by a model Q700 generator also by Qsonica, although other examples can use the AFG-2105 function generator, by GW Instek of Taipei, Taiwan, and a P200 linear amplifier, by FLC Electronics of Partille, Sweden) fitted with a sonotrode 11 designed to amplify the ultrasonic energy wave emitted by the ultrasonic transducer. The nominal sonotrode energy wave has an amplitude of 120 m at a frequency of 20 kHz. Operating in air, under these parameters, the wavelength of the wave emitted from the sonotrode is provided by the equation =v.sub.c/f, where v.sub.c is the speed of sound in air (340 m/s) and f is the frequency of oscillation (20 kHz in the case of this transducer) hence in this example=17 mm. The sonotrode 11 is generally cylindrical with a long axis x-x, and has a flat active face 12 at one end, with a flared edge. The energy generator comprising the ultrasonic transducer and sonotrode 11 is optionally mounted on a frame (not shown) adjacent to a channel device which in this example comprises a metal plate. In this example, the channel device comprises an aluminium plate 20 arranged parallel to the sonotrode's flat active face, but spaced therefrom and held in the spaced relationship by the frame. An optional linear bearing assembly on the frame (not shown) allows m adjustment of the air gap separating the sonotrode 11 and the plate 20.

(14) The plate 20 has a channel 25 extending from a channel inlet located on an inlet face 21 of the plate 20, to a channel outlet located on an outlet face 22 of the plate 20. The channel 25 extending between the channel inlet and the channel outlet is typically straight, and is aligned with the axis x-x of the sonotrode 11, as the inlet face 21 of the plate 20 is parallel to the active face of the sonotrode 11. In this example, the channel 25 is generally cylindrical, as best seen in FIG. 2, which shows an end view of the outlet face 22 of the plate 20, such that the sides of the channel 25 are straight and mutually parallel, and such that the axis of the channel is coaxial with the axis x-x of the sonotrode 11.

(15) The inlet face 21 of the plate 20 is axially spaced from the active face of the sonotrode 11 by a gap approaching 0.35 mm filled with air at atmospheric pressure and at room temperature. The static air pressure produced at the end of the channel 25 nearest to the sonotrode 11 was measured with different lengths of channel 25, while maintaining a consistent axial separation of 0.35 mm from the sonotrode 11. Measurements were taken by inserting a hypodermic needle (0.5 mm diameter) through the channel device (for example into the channel 25) into close proximity to the face of the sonotrode 11, connected by vinyl tubing (which optionally passed through the channel device, for example through the channel 25) to a manometer, always ensuring that the needle was mounted on a linear ball-bearing slide which was fitted with a location stop, so that it could be ensured that successive readings were taken with the needle point in the same position.

(16) It was found that a peak static air pressure within the channel occurred when the displacement between the active face of the sonotrode 11 and the outlet face of the channel device was approximately 3.8-4.5 mm, typically approaching 4 mm in channel length. Summing this channel length with the typical separation between the channel inlet and the active face of the sonotrode, this overall value of channel length+separation correlates well with a 1(/4) in this example=4.25 mm (since for 20 kHz in air in this example=17 mm based on the above frequency characteristics of the transducer and the medium of air) as shown in FIG. 4. Accordingly a higher static pressure at the channel inlet was obtained when the displacement approached a value of n(/4) where n=1. Peaks could also be obtained in this example when n=other odd numbers, e.g. where n=3, 5 etc. Hence, the outlet of the channel device was axially displaced along the axis of the wave at a node on the wave which substantially coincided with the outlet of the channel.

(17) The high static pressure produced by the peak length of channel 25 (resulting in a total displacement between the active face of the sonotrode and the channel outlet of (/4)) is evidence of the formation of a standing wave within the channel 25. Acoustic energy from the sonotrode 11 travels through the channel, and the expansion of the incident wave front from the channel outlet creates a transient interface 26 between low and high impedance established within a transition zone T.sub.z just outside the channel outlet. The incident wave travelling through the channel 25 from the sonotrode 11 therefore reflects back into the channel 25 from the interface 26 as a reflected wave. Choosing a channel length that provides a total displacement of n(/4) where n is odd (in this example n=1), creates a particularly beneficial reinforcing reflection, hence supporting a standing wave within the channel 25, with a node at or adjacent to the channel outlet. This value of n (n=1) reduces the attenuation effect of the energy wave reducing in intensity as it approaches the channel outlet. Clearly, useful examples of the invention can be reproduced with variations departing from this displacement value, but better results can be obtained closer to the stated value.

(18) The suspension of fluid to be atomised is discharged from the fluid outlet of a fluid delivery device 30 at or near the channel outlet, within the transition zone 26, which is particularly beneficial because at the node formed at the channel outlet, steep pressure gradients exist so that suspension discharged from the fluid delivery device 30 into this part of the wave absorbs large amounts of energy from the incident and reflected waves and is atomised with high efficacy and efficiency. Optionally, the fluid is discharged from the fluid outlet at an axial location with respect to the axis of the channel 25 between the outlet face 22 of the plate 20, and the boundary of the transition zone 26. Optionally the tip of the fluid delivery device 30 can be disposed anywhere in the area 31 adjacent to the boundary of the channel 25. Optionally, a node is formed at the outlet of the channel 25, and the fluid is discharged at or near to the node.

(19) Adjusting the diameter of the channel 25 also has a beneficial effect on the formation of the standing wave inside the channel, as the diameter affects the relative acoustic impedance between the inside of the channel 25 and the outside. A smaller channel gives a greater difference from the absolute acoustic impedance of the unconstrained air outside the channel, but limits the amount of energy that can be transmitted by the energy wave through the channel 25. Further, a small diameter channel causes a more sharply-defined reflection. Larger diameters in the channel reduce the definition of the reflection, but allow more energy transfer by the wave. Hence for suitable examples of atomiser assemblies, a balance needs to be struck between sufficient energy transfer through a large enough diameter of channel, and a sufficiently small diameter of channel in order to create an acoustic impedance gradient to provide a sufficiently definite reflection from the boundary of the transition zone 26. We conducted experiments to estimate the amplitude of the standing wave produced in the channel by measuring the static pressure at the channel inlet with different diameters of channel 25. Our results suggested that there is a practical limit to the ratio of diameter to length of the channel, w=d/l, and that as the reflection forms progressively and from a range of different axial locations, the effective mean point of reflection lies outside of the channel outlet. We found from these results that a reasonably well-defined standing wave can be formed within the channel 25 with good nebulisation effects with the ratio w approaching 0.7. Clearly, a range of values of ratio w on either side of this ratio will also achieve good results, and the experiments showed that good reflection can be achieved in the standing wave in values of ratio w ranging from 0.5-1, particularly 0.6-0.8, as shown in FIG. 5. Hence, with a channel length of 4 mm in this example, the most suitable diameter of channel 25 of the examples studied was obtained when the diameter approached 2.8 mm.

(20) Adjusting the separation between the channel inlet face 21 and the sonotrode 11 also affected the characteristics of the dispersion formed at the outlet face 22, and particularly could be adjusted in order to affect the direction of travel of the dispersion, and the density of the spray; for example, with a suitable separation between the channel inlet face 21 and the active face of the sonotrode 11, the dispersion could be formed as a relatively tight cone with a relatively defined vector away from the outlet face 22, rather than a diffuse dispersion with little or no definition to any particular vector of movement. We postulate that the proximity of the inlet face 21 to the sonotrode active face gives rise to the formation of a small, positive air pressure because of acoustic radiation pressure effects see references: Non-contact transportation using near-field acoustic levitation (Sadayuki Ueha, Yoshiki Hashimoto, Yoshikazu Koike. Ultrasonics 38 (2000) 26-32) and Acoustic radiation pressure produced by a beam of sound (Boa-Teh Chu, Robert Apfel. J. Acoust Soc Am. 72(6) (1982) 1673-1687), which are incorporated herein by reference. This pressure gives rise to a flow of air through the channel 25 which causes the dispersion formed at the channel outlet to be discharged in a direction away from the outlet face 22, hence reducing contamination of the face of the sonotrode 11 and the plate 20. The radiation pressure appeared to be independent of the frequency of radiation but shows correlation between the distance between the active face of the sonotrode 11 and the inlet face 21 of the plate 20, and our experiments suggest that a separation approaching 0.35 mm is effective. Other separation values could be useful for gasses of different density as a medium, and references 9 and 10 provide sufficient formulae to enable the determination of other values for other gasses. Clearly, useful examples of the invention can be reproduced with variations departing from this separation value, but our results shown in FIG. 6 plotting the air flow obtained through the channel 25 against separation between the channel inlet face 21 and the active face of the sonotrode 11 indicate that a range of separation values between 0.25 and 0.4 mm in air is capable of achieving a suitable effect directing the spray of the dispersion formed at the channel outlet in a more precise conical configuration, away from the atomiser assembly, and towards any target being coated.

(21) In certain examples of the invention, the assembly produces a more directed spray, forming a cone with a lower angle of divergence from the axis of the channel 25, and a consequentially narrower surface area of coverage. This leads to less waste of sprayed material, and more accurate spraying of the fluid onto the target. Certain examples of the invention may also exhibit reduced susceptibility to clogging, and may more easily spray very viscous liquids. Certain examples of the invention may also be particularly useful for spraying of hazardous or toxic materials, for example asbestos, for laboratory and/or industrial purposes.

(22) Another example of the invention is shown in FIGS. 7-9. For conciseness, features that remain the same as described above will not be described in detail again. Similar features to those of the example shown in FIGS. 1-3 will be given the same reference number, increased by 100. Hence, the atomiser assembly 101 of FIGS. 7-9 has an energy generator 110 comprising a sonotrode 111 with an active face 112, and a plate 120 with a channel 125 as described for the previous example.

(23) The active face 112 of the sonotrode 111 is parallel with the channel outlet surface 121 of the plate 120, with a gap approaching 0.35 mm filled with air at atmospheric pressure and at room temperature as described in the first example. The sonotrode 111 emits an ultrasonic energy wave as described before, which sets up a standing wave within the channel 125. As before, acoustic energy emitted by the sonotrode 111 travels through the channel 125. The incident wave front expands from the channel outlet 127o and establishes a transient interface between low and high impedance within a transition zone just outside the channel outlet 127o. When the displacement of the outlet 127o of the channel 125 relative to the active face 112 of the sonotrode 111 approaches an axial length of n(/4) (where n is an odd number), a particularly beneficial reinforcing reflection of the incident wave back into the channel 125 is created, supporting the standing wave within the channel 125, with a node at or adjacent to the channel outlet 127o as before. In this example, the peak static air pressure adjacent to the channel inlet 127i was measured (using a manometer provided with a narrow gauge syringe tip as previously described) to be 30 mbar, and the low pressure region adjacent to the channel outlet 127o was measured to be 30 mbar. This is sufficient to set up a pressure differential through the channel 125, resulting in the air moving from the high pressure area at the channel inlet 127i to the low pressure area at the channel outlet 127o, producing a steady positive flow of air through the channel 125.

(24) FIG. 7 shows an end view of the channel 125, with an annular chamber 150 formed in the outlet face of the plate 120, the annular chamber 150 having a tapered wall 151, the inner edge of which defines the chamber 150. There is a small gap 150a (see FIG. 7) between the exterior of the channel 125 and the inner edge of the wall 151 of the annular chamber 150, forming an annular outlet of the chamber through which fluid exits the chamber into the energy wave produced by the sonotrode 111.

(25) FIG. 8 shows a schematic side view of the energy generator 110 (not to scale). The sonotrode 111 comprises a flat active face 112 in close proximity to the channel inlet face 121 of the plate 120, as before. The annular chamber 150 extends around the channel 125 and protrudes slightly further than the channel outlet in the axial direction of the channel 125.

(26) FIG. 9 shows a detailed, close-up view of the channel outlet 127o, illustrating the toroidal region of low pressure 140 around the outer surface of the wall of the channel outlet 127o (not to scale). For clarity, the other parts of the assembly 101 are not illustrated in FIG. 9. The region of low pressure 140 is well-defined, but extremely small relative to the rest of the apparatus.

(27) In FIG. 8, the protruding edges of annular chamber 150 extend beyond the location of this low-pressure torus 140. Having an area of lower pressure 140 within the boundaries of the annular chamber may be advantageous to fluid uptake, as it can encourage the fluid within the chamber 150 to flow towards the low pressure areas 140 at the chamber outlet and into the energy wave at the channel outlet 127o. The energy wave then atomises or aerosolises the fluid, and disperses it.

(28) To a greater or lesser extent, the region of low pressure 140 illustrated in the example of FIG. 9 is present in all examples of the invention. The energy wave produced by the sonotrode 11, 111, 211 is reflected back into the channel 25, 125, 225 and as the low-high impedance transition boundary is most abrupt at the outer edges of the channel outlet, the reflection of the wave at this location may contribute to the formation of the low pressure region 140. The low pressure region 140 is well-defined when the displacement between the active face 112 of the sonotrode 111 and the channel outlet 127 approaches n(/4) where n=1, more so than when n=3, and is not seen where the displacement approaches a multiple of n(/4) where n is an even number. The low pressure region 140 was easily mapped in various examples by taking pressure readings using a manometer equipped with a syringe tip, and placing the syringe tip at different locations around the outlet of the channel to map the boundaries of the low pressure area.

(29) The fluid delivery device 130 in the form of an injection line for the fluid slurry being atomised is also schematically illustrated in FIG. 8. The fluid enters annular chamber 150 at the injection point 131. The rate of fluid delivery varies according to the viscosity of the fluid being passed through the delivery device 130. Some fluids may be discharged in a slow but steady stream, while others may be dripped into the chamber 150. The angle of the wall 151 of the annular chamber 150, combined with the tendency for the fluid to flow towards the low pressure torus, can act to focus the stream of fluid into the path of the energy wave emitting from the channel 125. The fluid then absorbs large quantities of energy from the wave and is atomised as described above. The fluid may also be drawn through the chamber outlet by capillary action in some examples.

(30) A third example of the invention is shown in FIGS. 10 and 11. For conciseness, features that remain the same as described in the previous two examples above will not be described in detail again. Similar features to those of the example shown in FIGS. 1-3 and 7-9 will be given the same reference number, increased by 200. Hence, the atomiser assembly 201 of FIGS. 10-11 has an energy generator 210 comprising a sonotrode 211, and a plate 220 with a channel 225 as described for the previous example.

(31) FIG. 10 shows a schematic side view of an example of the invention where the active face 212 of the sonotrode 211 is disposed within a recessed area 213 of the plate 220, with the active face 212 of the sonotrode 211 being parallel to the channel inlet face 221 of the recess 213, and the channel inlet 227i. The gap between the active face 212 of the sonotrode 211 approaches 0.35 mm, and is filled with air at atmospheric pressure and at room temperature as described in the first two examples. As before, the sonotrode 211 emits an ultrasonic wave, the wave front creating a transient interface between low and high impedance, where the interface acts to reflect the incident wave back into the channel 225. As before, the reflection is reinforced when the displacement of the outlet of the channel 225 relative to the active face 212 of the sonotrode 211 approaches an axial length of n(/4) (where n is an odd number), supporting the standing wave within the channel 225, with a node at or adjacent to the channel outlet 227o as before.

(32) When the tapered section 255 is fitted over the channel 225, it forms the annular chamber 250. A region of low pressure is set up at the channel outlet 227o, as illustrated generally in FIG. 9. As before, as the air surrounding the apparatus is at a higher pressure than the region of low pressure, it flows towards the low-pressure torus and is carried forwards by its own inertia into the stream of air moving from the high pressure region adjacent to the channel inlet 227i towards the low pressure torus around the exterior of the channel outlet 227o.

(33) The external profile 256 of the tapered section 255 has a tapered external diameter that decreases in an axial direction, away from the sonotrode 111. Experimental measurements of the velocity of air flowing towards the region of low pressure show the velocity is increased when the channel outlet has a tapered external profile. The air flows over (and is focussed by) the tapered profile 256, towards the low pressure region, and is, we believe, carried through the low pressure region by inertial forces. As the air passes through the torus, it is entrained in the stream of air flowing from the channel outlet 227o as a result of the pressure differential between the channel inlet and outlet, and forms a powerful jet.

(34) By changing the angle of the external profile 256, it is possible to alter the power of the air jet. For example, removing the taper altogether so that the outer face of the plate 220 is flush with the channel outlet 227 substantially reduces the air jet effect.

(35) In some examples, the powerful jet effect can be minimised by using a non-tapered external profile with the surface of the plate at the channel outlet being perpendicular to and flush with the channel outlet (for example, as schematically drawn in FIG. 3), and optionally in or near the same plane as the channel outlet. Provided that the spacing between the active face of the sonotrode and the channel outlet is very close or equal to n(/4) (where n is an odd number), the torus of low pressure continues to be formed around the exterior of the channel outlet. The jet effect is then reduced, we believe, by the flow of air being drawn to the torus from all directions. The air is not focussed in any given direction, and may therefore meet and combine to reduce or cancel out the velocity of air being drawn in from opposing directions.

(36) The plate 220 is formed from segments of the same or optionally different materials. The outer segment 228 is in this example a single annular-shaped piece, which fits over the inner section 229 of the plate 220. In this example, the fitting is a bayonet-style fitting. Inner segment 228 comprises an L-slot (but a J-slot or similar would also be suitable). A further annular-shaped segment (not shown), comprising protrusions adapted to fit into the corresponding slot in inner segment 228, then fits over the outwardly-facing end of inner segment 228 to hold the segmented plate 220 together.

(37) FIG. 11 shows the apparatus of FIG. 10, with the fluid delivery device 230 illustrated, and threaded fixings in the form of bolts 220f shown as one example of a means of fixing the tapered section 255 to the inner plate section 229.

(38) The following disclosures are incorporated herein by reference: 1. The mechanisms of the formation of fogs by ultrasonic waves. Sollner K. Trans Faraday Soc 32 (1936) 1537-1538 2. Ultrasonic atomization of liquids. Peskin R L, Raco R J. J. Acoust Soc Am 35 (1963) 1378-1381 3. Radiation Pressurethe history of a mislabelled tensor. Beyer, R T. J. Acoust Soc Am 63(4) (1978) 1025-1030. 4. Ultrasonic separation of suspended particles. Part 1Fundamentals. Groschl M. Acustica Acta Acustica 84 (1998) 432-447 5. US Patent 20070017441 A1 (2007) 6. Inverter topologies for ultrasonic piezoelectric transducers with high mechanical Q-factor. Kauczor C, Frohleke N. IEEE Power Electronics Specialists Conference. IEEE 35.sup.th Annual (2004) 4, 2736-2741 7. Production of fine particles from melts of metals or highly viscous fluids by Ultrasonic Standing Wave Atomisation. Anderson O, Hansmann S, Bauckhage K. Particle and Particle Systems Characterisation 13 (1996) 217-223 8. Modelling and simulation of the disintegration process in Ultrasonic Standing Wave Atomisation. Reipschlager O, Bothe H-J, Warnecke B, Monien B, Pruss J, Weigand B. University of Paderborn. ILASS-Europe 2002. 9. Non-Contact transportation using near-field acoustic levitation. Sadayuki Ueha, Yoshiki Hashimoto, Yoshikazu Koike. Ultrasonics 38 (2000) 26-32 10. Acoustic radiation pressure produced by a beam of sound. Boa-Teh Chu, Robert Apfel. J. Acoust Soc Am. 72(6) (1982) 1673-1687