GB2566459A

GB2566459A - An acoustic apparatus

Info

Publication number: GB2566459A
Application number: GB1714700.0A
Authority: GB
Inventors: Stevenson Adrian; Nock William; Phillips Mark; Li Dr Nan; Bhatta Devaki
Original assignee: Labxero Ltd; Cambridge Enterprise Ltd
Current assignee: Labxero Ltd; Cambridge Enterprise Ltd
Priority date: 2017-09-13
Filing date: 2017-09-13
Publication date: 2019-03-20
Also published as: GB201714700D0

Abstract

A method and an acoustic apparatus for inhibiting a flow of particles or cells in a fluid. The apparatus comprises an acoustic source transducer such as an open-ended piezoelectric tube 2 and a tubular cavity in which fluid is located. The source transducer is configured to form resonant nodal planes (3, Fig 5) perpendicular to a direction of the flow within the tubular cavity, and the nodal planes impede the flow of particles or cells such that the particles or cells from the fluid are separated near the resonant nodal planes and a remainder of the fluid is flown through the tubular cavity. Preferably a flexible de-coupler 4 such as an o-ring links a tube 5 of differing acoustic impedance to the acoustic source transducer. The acoustic apparatus can be used for particle or cell separations from a fluid, such as dewatering applications, including harvesting microalgae from water.

Description

AN ACOUSTIC APPARATUS

FIELD OF THE INVENTION

The present invention relates to an acoustic apparatus, particularly but not exclusively, for acoustic separation of particles or cells in a fluid.

BACKGROUND OF THE INVENTION

Microalgae are microscopic, photosynthetic organisms, which can be found across the globe, and in freshwater and marine environments. They are unicellular organisms, and as a result, this makes them more efficient in the conversion of solar energy into biomass during photosynthesis. These diverse micro-organisms are able to biosynthesise a wide variety of natural compounds. These include vitamins and oils that are essential for human nutrition and have potential benefits in pharmaceutical and medical applications.

There is commercial interest to cultivate micro-algal biomass as a source of oils and vitamins for human consumption. Harvesting of micro-algal cells is recognized as a bottleneck for the commercial manufacture of high value micro-algal products. Harvesting technologies are desired to be able to process high flow rates, at a high concentration factor, with high recovery efficiency and low operating and capital costs.

Centrifugation is an effective means to concentrate micro-algal cells, this can process large flow rates, however high capital costs and energy consumption have restricted its use for micro-algal harvesting.

Membrane filtration units are also able to dewater micro-algal biomass, this technology can be scaled up and is used for the treatment of wastewater. Issues with membrane clogging and fouling are a drawback, and a pressure difference is required across the membrane, which increases energy requirements.

Sedimentation is an option for many algal species, which will naturally settle due to their higher density than water. This generally involves a long duration, which can be problematic in a commercial process and the effectiveness of sedimentation is dependent on the micro-algal species.

Flocculation of micro-algal cells can be promoted by changing the pH of the algal media, this can require large quantities of chemicals. This could lead to issues with chemical contamination and is not a cost effective means to harvest large volumes of micro-algal biomass.

Flotation can also be used to separate micro-algal cells by using gas bubbles, which will rise up through the culture, resulting in microalgae cells attaching to the bubble surface, as a result of the electrostatic charge. This method often requires a flocculant to promote the attraction between the cells and the bubble surface, however, leading to the same drawbacks as chemical flocculation.

Conventional acoustic separation mechanisms have used standing waves to form nodal planes which act as zones to concentrate micro-algal biomass. These approaches have only exploited the radial modes which form across the inner diameter of a tube. This does not allow for a continuous process as the direction of flow is parallel to the acoustic nodal planes.

SUMMARY OF THE INVENTION

The proposed invention employs an ultrasonic transducer and resonant tube to act as a trap to the flow of particles or cells within a fluid allowing for separation of the concentrated cells/particles. This consists of a cylindrical piezoelectric transducer connected to a tube, which allows for acoustic energy transfer to the tube. The piezoelectric transducer causes the tube to resonate, which imparts a force on the particles within the fluid retaining them within the tube while allowing fluid to flow through the tube concentrating the particles from the fluid.

This proposed invention can be used for particle or cell separations from a fluid, such as dewatering applications, including harvesting microalgae from water. This invention provides an approach to acoustic cell separations, with current approaches using standing waves to form nodal planes which act as zones to concentrate micro-algal biomass. The advantages of this approach lie in the ability to employ this to a continuous flow with easier extraction of cells and particles from the volume in the resonant tube. This will enable easier scale-up of this technology for biotechnology applications.

According to one aspect of the present invention, there is provided an acoustic apparatus for inhibiting a flow of particles or cells in a fluid, the apparatus comprising: an acoustic source transducer;

a tubular cavity in which the fluid is located;

wherein the source transducer is configured to form resonant nodal planes perpendicular to a direction of the flow within the tubular cavity, and wherein the nodal planes impede the flow of particles or cells such that the particles or cells from the fluid are separated near the resonant nodal planes and a remainder of the fluid is flown through the tubular cavity.

In one embodiment, the resonant nodal planes may be nodal standing wave planes.

The acoustic source transducer may be a piezoelectric source transducer. In this example, the piezoelectric source transducer is a discrete transducer.

The tubular cavity may be an open ended auxiliary tubular cavity and a face of the acoustic source transducer may be perpendicular with a central axis of the auxiliary tubular cavity. In this example, the transducer source is separate from the open ended tube.

The tubular cavity may be an open ended piezo-electric tube. The open ended piezoelectric tube may serve as both the tubular cavity and the acoustic source transducer. Advantageously, the open-ended acoustic apparatus is a virtual acoustic filter as it does not have any filtering component within the tubular cavity.

The perpendicular nodal plane is created within the tubular cavity (or open-ended tube or piezo-tube) when an electrical energy (or an AC electric field) is applied at a predetermined resonant frequency.

Generally speaking, the capacity of the piezoelectric tube to immobilise particles or cells in the nodal planes may be dependent on acoustic energy density and its net volume.

The piezoelectric tube may be configured such that a radial breathing mode of the tube is coupled with a vibrational mode. This facilitates energy transfer from the piezoelectric tube to the fluid within the tube, and also allows energy transfer from the radial mode to the longitudinal mode.

The perpendicular nodal planes may be generated when a tube length and the fluid in the tube are resonated at a same resonant frequency as a wall of the piezoelectric tube.

The resonant frequency of the wall of the piezoelectric tube may be defined by:

N*piezo tube velocity/2*piezo wall thickness, wherein N is an integer corresponding to a resonant harmonic.

The resonant frequency of the tube length and fluid in the tube may be defined by: N(fluid velocity)/2*tube length, wherein N is an integer corresponding to a resonant harmonic.

An inner diameter of the piezo-electric tube may be chosen so that a radial standing wave cannot form, which is achieved by choosing a matched anti-resonant frequency. This increases coupling to the longitudinal mode, which will increase the energy efficiency of the device.

The acoustic apparatus may further comprise:

a flexible de-coupler at an upper end of the piezoelectric tube, and a further flexible de-coupler at a lower end of the piezoelectric tube.

The flexible de-couplers reduce the amount the further tubes interfere with the acoustic filter. The de-couplers reduce the amount of vibrational energy transferred to the further tubes, leading to better piezoelectric tube resonances.

At least one of the flexible de-couplers may be an O-ring de-coupler. Any flexible elastomeric type gaskets could be used instead of the O-ring de-coupler.

The piezoelectric tube may be coupled with further tubes of differing acoustic impedance through the flexible de-couplers. The use of further tubes of differing impedance increases the Q factor of the longitudinal standing waves, increasing the strength of the filtering action.

The further tubes may have different elasticity, different thickness and stiffness of wall compared to those of the piezoelectric tube. This increases the impedance difference between tubes, increasing Q factor.

The further tubes may have substantially (almost) the same inner diameter as the piezoelectric tube.

At least one of the further tubes may have a larger inner diameter compared to that of the piezoelectric tube so that an increased impedance difference is achieved between the piezoelectric tube and the further tubes. This increases the Q factor of the tube which influences how strongly the nodal planes impede the flow of particles.

At least one of the further tubes may have a tapered shape so that a circumference of the further tubes gradually mates with that of the piezoelectric tube. This reduces turbulence in the system, which helps preserve the standing wave formed within the tubular cavity.

The acoustic apparatus may further comprise an auxiliary tube coupled with the piezoelectric tube through the de-couplers. A central axis of the auxiliary tube may be aligned with a central axis of the piezoelectric tube. This allows for the rigid tube to be excited by the piezoelectric tube.

The auxiliary tube may be configured to resonate at a predetermined frequency.

Both the auxiliary tube and the piezoelectric tube may be configured to resonate at a same frequency. This allows a larger net trapped volume of particles.

One of the further tubes may comprise a valve which may be configured to obtain the inhibited particles or cells and allow de-watering of the remainder of the fluid through the further tube.

The acoustic apparatus may further comprise a controller which may be configured to apply an electrical energy to the tubular cavity to create a standing wave barrier perpendicular to the flow in the tubular cavity. Any types of controllers or drivers could be used.

The acoustic apparatus may further comprise a plurality of tubular cavities connected in parallel, wherein each tubular cavity comprises a relatively smaller inner diameter. Advantageously, the Q factor for each tube may be raised. The energy may be distributed across a set of smaller tubes, resulting in a higher impedance filter than consumes less power. Tubes connected in parallel can increase the volume filtered by the apparatus (or a relevant system), increasing the output from the system.

The acoustic apparatus may further comprise a plurality of tubular cavities connected in series. This could amplify the concentration of the cells or particles collected as the output undergoes more than one dewatering process.

The tubular cavity may comprise a material comprising any one of metal, ceramic, glass, polymer, and a composite. This list of materials is not limiting. Other suitable materials could be used.

The apparatus may be configured such that the particles or cells from the fluid are concentrated near the resonant nodal planes. This technique is generally used for micro-algae processing.

The apparatus may be configured such that the particles or cells from the fluid are filtered by the resonant nodal planes. This technique could be used in applications like bioprocessing, industrial biotechnology, biological separation and other suitable applications.

The apparatus may be configured such that a continuous flow is processed by the apparatus. A continuous flow may provide easier scale-up of the technology.

The apparatus may be configured to apply a tapping technique to process the flow on a batch by batch basis. Tapping can avoid the accumulation of cells or particles in the filter which can interfere with the standing wave formed.

The apparatus may be configured such that the fluid flows through from an upper end to a lower end of the tubular cavity. In this configuration, gravity is the same direction as the flow velocity and so a pressure pump may not be needed to pass the fluid through the device.

The apparatus may be configured such that the fluid flows through from a lower end to an upper end of the tubular cavity. In this configuration, gravity is in the opposite direction to the flow velocity and so particles will need a larger flow velocity to break through the filter. This increases the efficiency of the filter.

The apparatus may be configured such the nodal planes filter particles or cells having a size ranging from micrometre to macro metre.

In one embodiment, we disclose a biological separation system comprising the acoustic apparatus as described above. The biological separation system is generally used in bioprocessing, industrial biotechnology and other related applications.

In an alternative embodiment, we disclose a non-biological separation system comprising the acoustic apparatus as described above. This system is generally used in non-biological separation applications.

According to a further aspect of the present invention, there is provided a method of inhibiting a flow of particles or cells in a fluid, the method comprising:

providing an acoustic source transducer;

providing a tubular cavity in which the fluid is located; and forming resonant nodal planes perpendicular to a direction of the flow within the tubular cavity, impeding the flow of particles or cells by the resonant nodal planes such that the particles or cells from the fluid are separated near the resonant nodal planes and a remainder of the fluid is flown through the tubular cavity.

The method may further comprise forming a filter plane from the acoustic source transducer.

The method may further comprise producing one quasi two-dimensional cavitation plane for impeding the flow of particles.

The method may further comprise producing two quasi two-dimensional cavitation planes with different radiation directions for impeding the flow of particles.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present disclosure will be understood more fully from the detailed description that follows and from the accompanying drawings, which however, should not be taken to limit the invention to the specific embodiments shown, but are for explanation and understanding only.

Fig. 1 shows two open-ended tubes having an impedance difference between them;

Fig. 2 shows wider pipe sections with a reduced impedance level;

Fig 3 shows an illustration of different forces acting on a particle that has caught in a nodal plane;

Fig. 4 shows an illustration of radial modes of a piezoelectric tube, which are mode converted into longitudinal modes;

Fig. 5 shows a schematic representation of an open ended piezoelectric tube, in line with tubes at the upper and lower ends, according to one embodiment of the invention;

Fig. 6 shows a schematic representation of an open-ended piezoelectric tube, connected to wider tubes at either end, according to a further embodiment of the invention;

Fig. 7 shows an alternative schematic representation of the embodiment of Fig. 6;

Fig. 8 shows an alternative schematic representation of the embodiment of Fig. 6 or 7;

Fig. 9 shows a schematic representation of a further embodiment of the invention, in which the piezoelectric tube is replaced with a rigid tube;

Fig. 10 shows a transducer introduced at a frequency such that a vibrational energy of the transducer is transferred to an open-ended tube;

Fig. 11 shows a schematic representation of a further embodiment of the invention, in which an auxiliary tube is in line with the piezoelectric tube; and

Fig. 12 shows a schematic representation of a further embodiment of the invention in which a piezoelectric tube is used as an exit point from a tank.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

We will describe the physical phenomenon of the devices of the present invention in general terms. Specific embodiments will follow after the general description of the physics of the devices.

Broadly speaking, a piezoelectric tube once excited by an electrical source can be made to create nodal standing wave planes that impede the flow of particles when arranged perpendicular to flow.

A standing wave can be formed when a wave is reflected onto itself, allowing the reflected wave to superimpose on the incidence wave. This interference results in a wave with an increased amplitude resulting from the combination of both waves. This combined wave, known as a standing wave, has nodes at the point of minimum amplitude, which remain stationary and anti-nodes at the point of maximum amplitude, which is the sum of the amplitudes of the incidence and the reflected wave. In a resonating system standing waves occur at the resonance frequency.

Within the proposed invention an effect known as mode conversion is exploited. This allows the radial breathing mode of the tube, which launches compressive energy perpendicular to the walls and to its centre, to couple to other vibrational modes.

Mechanical constitutive equations are used for determining the natural mechanical resonant frequencies of the system, and depend on Newton’s law and Hooke’s law.

Oij.i = pQrf °ij,i ^{= c}ijkf kl + hijklS kl

Ski = 0.5(u_k[ + U_[k) (1)

Piezoelectric behaviour is added via Gauss’ law, a modification to Hooke’s law and the dielectric and field equations. This amounts to the driving forces that lead to stored energy in a structure (although there are other transduction mechanisms that can be used to add acoustic energy, e.g. a magnetostrictive or moving coil based approach).

^Dt.j = ⁰ ^σί7,ί ^{= c}ijklSkl T ^eijkE_k T PijklS kl

Bi ^{= e}ikl$kl + ^ikEk

Ek = ~<P,k (2)

The solutions that satisfy these equations can be reduced to simple standing wave components in the fluid or solid along paths of length d as shown in equation 3.

In order to couple energy from an electrically excited piezoelectric tube on to the fluid, there is a resonant angular frequency associated with the piezoelectric wall of the tube, which is matched to the angular frequency associated with radial acoustic modes of the tube shown in equation 4, where c_piezo and c_water refer to their respective acoustic velocity, n in both cases to the harmonic number of the standing wave and t to the piezotube wall thickness and D to its inner diameter.

p(x,t) = Aej^^t+k(-^d~^x^ +	Q_ej[a>t-k{d-x)}	(3)
Tic ^iiUpiezo	fir ^llcwater
d^radtal ~ ⁿp ~	D ^Uw	(4)

In the acoustic filter the compressive energy moves parallel to the walls. This uses mode conversion from the radial mode to the longitudinal mode. Therefore if the tube length and the fluid within it are arranged to resonate at the same frequency as the wall of the piezotube, then there will be a transfer of energy to the open ended tube mode, and perpendicular nodal planes will be created.

Therefore, the resonant frequency of the piezotube wall and the resonant frequency of the fluid within the tube is (or need to be) matched. The following frequency or wavelength conditions, shown in equations 5 and 6, should be satisfied.

d>long ~ d)_radi_ai (5) k-iong ~ k_radiai (6)

The resonant frequency of the piezoelectric tube wall and the resonant frequency of the fluid within the tube are given in equations (7) and (8) respectfully.

_ ^Cpi^ezo „ d/radial ~ ⁿp (7) _ ^Cwater ~ d>long ~ n_w (8)

To further optimise the coupling to the desired mode, the inner diameter of the piezotube should be chosen so that a radial standing wave cannot form, which can be achieved by choosing an anti-resonant frequency to match, given in equation (9).

7r _ ^ώ Nvater ^anti ~ 3D n_w (9)

Trapping acoustic energy in this longitudinal mode depends on the end reflection coefficient of the tube, which is given by equation (10), and largely depends on piezoelectric tube area (A_o) relative to the adjoining tube (4!) and the acoustic velocity c. A difference in the cross-section of the tubes fitting onto the piezoelectric tube, will create a reflection that can support the desired longitudinal standing wave mode.

4p _ Αγ n ₌ _C____C_ do.dl c c (10)

The stored energy in open ended tube, the radiation of longitudinal energy and the Q factor of open ended tube, and other material and geometry variables represent other aspects that can be adjusted to tune the ratio between the impeding force of the acoustic filter, and the amount of electrical power applied.

When two materials of different elasticity are joined together, then a reflection from that interface occurs that is proportional to the difference in impedance. However if that scenario is an open-ended pipe, the impedance difference is defined as the pressure divided by the velocity, in the two different regions. For the open-ended pipe this impedance difference is generally tuned in order to get the maximum Q factor. Q factor is a measure of how damped a resonator is. A higher Q factor means that oscillations die out slower, and there is a lower rate of energy loss relative to the stored energy of the resonator. Therefore a device with a high Q factor will be more efficient and use less power.

Fig. 1 shows two open-ended tubes having an impedance difference between them. It is shown that for maximum reflection at the boundary between tube A and tube B, there is an impedance difference between them such that Z_A#Z_B.

Wider pipe sections 200 as in Fig 2 will have reduced impedance, and therefore a greater impedance difference between A and B, resulting in greater Q factor and reflection at the boundary.

The maximum impeding force of the acoustic filter is given by equation (11) and depends on the particle size (a), the acoustic contrast factor of the particle (K_p) and principally on the energy density (E_s) of the longitudinal acoustic field.

Ffiiter 4tccl kE_sK_p (11)

The filter operation zone defining an operating state whereby particles are held back by Ffuter . and cannot pass through the filter is given by equation (12). The competing drag force is the Stoke’s force from particle flow (F_draq), and the buoyancy force (F_buoy), which is related to any density difference between the fluid and particle, and can be positive or negative.

Ff ilter > F_drag + F_buOy (12)

So generally speaking, the acoustic filter given sufficient electrical energy, will create an optimally oriented standing wave barrier (i.e. perpendicular to flow) that stops particles in their track.

Fig 3 shows an illustration of the forces acting on a particle that has caught in a nodal plane 11. Here the movement in the radial direction 12 (in this case piezoelectrically driven) is shown to mode convert, so the radial mode transfers energy to the longitudinal mode. The net result is that fluid movement (i.e. energy) in the radial direction 12 becomes fluid movement in the longitudinal direction 13. Once polarised in this way, it can create standing waves perpendicular to the moving faces. These provide ‘periodic energy hills’ that impede particles that try to flow through them. To ‘break’ the filter, flow related stokes forces should increase beyond a threshold determined by the acoustic energy density, as shown in equation (12).

Fig. 4 shows an illustration of the radial modes of the piezoelectric tube, these are mode converted into longitudinal modes.

Fig. 5 shows a schematic representation of an open ended piezoelectric tube, in line with tubes at the upper and lower ends, according to one embodiment of the invention. An open-ended piezoelectric tube 2 is terminated by a flexible de-coupler 4 at its upper and lower ends. In one embodiment, O-rings may be used as the flexible de-couplers. The O-rings 4 are linked to two tubes 5 of differing acoustic impedance. The open ended piezoelectric tube 2 is used to create an acoustic standing wave 1 with nodal planes 3 that are perpendicular to its central axis.

Variables affecting the impedance include the tubes wall thickness, the stiffness of the wall and the tubes inner diameter. The structure of the two end termination tubes 5 alters the Q factor of the piezoelectric tube which influences how strongly the nodal planes impede the flow of particles. In operation, the capacity of the piezotube to immobilise particles in the nodal planes is related to acoustic energy density and its net volume.

In use, a radio frequency signal source is connected to the piezoelectric tube and its frequency is set to drive the piezoelectric tube and sample to resonance. Ideally this frequency is set to the series resonance frequency of the piezotube, although in some instances we may operate at the parallel resonance frequency, as this is more easily driven by the amplifier and least removed from its standard operating impedance.

In operation the pressure fluctuations in the fluid are the driving force behind the particle motions observed. Increasing the level of these pressure fluctuations, improves the strength of the filtering action, thus restricting particle flow.

To create the invisible acoustic filter, that impedes particles flowing along the long axis of the piezotube, it is important the axial mode of the tube is excited. This creates nodal planes perpendicular to particle flow.

The piezoelectric tube may be formed by bonding piezoelectric crystals to a tube with epoxy resin to excite the wanted nodal modes.

The proposed device can operate in both an upwards flow configuration and downwards flow configuration. In the downwards flow configuration, gravity is acting in the same direction as the flow. This increases the likelihood of particles passing through the filter. In the upwards flow configuration, a pressure pump is used to move the fluid through the system and the particles will need a higher flow velocity in order to break through the acoustic filter. The direction of gravity influences the efficiency of the acoustic filter as particles are more likely to pass through the filter in the downwards flow configuration.

The power of the piezoelectric tube may be adjusted so that the system is operated at a point that avoids cavitation. Bubbles caused by cavitation may interfere with the operation of the acoustic filter.

A tapping point may be used to collect particles. When particles fall into the standing wave formed, they change the character of the standing wave and so are tapped. Tapping may be used so that the particle concentration does not exceed a critical threshold. This could be done in batch or flow separation scenarios. For example, in a batch separation scenario, a process is started on a batch and then the process is stopped so that the processed batch is separated. After this, the process starts again for the next batch. For the flow separation scenario, there is generally no start or stop process. It operates on the entire flow.

Fig. 6 shows a schematic representation of an open-ended piezoelectric tube, connected to wider tubes at either end, according to a further embodiment of the invention. An open-ended piezoelectric tube 2 is connected to two O-rings 4 at its upper and lower ends. The O-rings are linked to a sheet 6. The sheet 6 represents a part of a much larger volume than the piezoelectric tube volume, for example a wider tube. Many features are the same as those shown in Fig. 5 and therefore carry the same reference numerals.

This allows the volume to increase significantly at either end of the piezoelectric tube, approximating an open-ended pipe. This embodiment has a greater Q factor than the first embodiment, which increases the impedance to particle flow.

Fig. 7 shows an alternative representation of the embodiment of Fig. 6. The sheet 6 can be seen as two tubes with larger width than the piezoelectric tube 2. The impedance is inversely proportional to the end area increase. A desired impedance difference may be achieved by increasing the diameter by a factor of 10, further increase in diameter would make little difference to the energy confinement in the piezoelectric tube. The dimensions shown in these figures are not limiting.

Fig. 8 shows an alternative representation of the embodiment of Fig. 6 or 7. In this embodiment, the tubes 6 are tapered at the ends adjacent to the piezoelectric tube 2. The tubes 6 connected to the upper and lower ends of the piezoelectric tube 2 are tapered so that the circumference of the external tubes 6 gradually mates with the piezoelectric tube. This will reduce turbulence in the system, which will in turn preserve the standing wave within the piezoelectric tube.

Fig. 9 shows a schematic representation of a further embodiment of the invention, in which the piezoelectric tube is replaced with a rigid tube 8. Many features are the same as those shown in Fig. 5 and therefore carry the same reference numerals. A separate transducer 7 is used instead of the piezoelectric tube. The transducer 7 could be stationed either within the fluid itself, in contact with the side wall of the rigid tube, or at any other convenient location.

Generally speaking, the preferred location is for the transducer centre to be aligned along the central axis of the rigid tube 8, with the transducer face perpendicular to the rigid tube axis, and the separation distance between the rigid tube 8 and the transducer 7 to be a multiple of half a wavelength of the standing wave within the rigid tube 8. It will be appreciated that the same termination conditions described earlier may apply and influence the Q factor of the rigid tube and therefore the nodal planes within it. The transducer generates pressure waves which become resonantly trapped in the cavity part of the rigid tube 8.

As shown in Fig. 10 the transducer 7 is introduced at a frequency such that the vibrational energy of the transducer is transferred to the open-ended tube 8.

Fig. 11 shows a schematic representation of a further embodiment of the invention, in which an auxiliary tube is in line with the piezoelectric tube. Many features are the same as those shown in Fig. 5 and therefore carry the same reference numerals. A rigid tube 8 is connected to the end of the piezoelectric tube 2. The piezoelectric tube 2 is used as a source of longitudinal wave energy launched from the ends of the piezoelectric tube 2. The central axis of the piezoelectric tube 2 and the rigid tube 8 may be aligned, and the frequency of the piezoelectric tube 2 may be adjusted so that the rigid tube 8 is excited. The auxiliary tube 8 acts only as an open-ended cavity, and not a source of acoustic energy.

An alternative of this embodiment would be that both tubes could resonant at the same frequency. This would contribute to a larger net trapped volume. The energy density would be reduced as the same energy would be distributed over the larger volume.

Fig. 12 shows a schematic representation of a further embodiment of the invention in which a piezoelectric tube is used as an exit point from a tank. Many features are the same as those shown in Fig. 5 and therefore carry the same reference numerals. The piezoelectric tube 2 can be longer and narrower then the piezoelectric tube used in other embodiments. In use a large water volume will flow under gravity from the tank through the piezoelectric tube. The lower end of the piezoelectric tube 2 is connected to an O-ring 4. The O-ring 4 is connected to an adjoining fluidic (rigid) tube 5. There can be an exit hole 17 on the side of the rigid tube 5 beneath the O-ring 4. In one example, the exit hole 17 is sealed with a rubber seal 18 and a valve 19. This is generally used as a dewatering configuration.

Due to the raised aspect ratio of the piezoelectric tube and the narrow aperture of the piezoelectric tube, this embodiment is closer to a one-dimensional resonator and has a high Q factor. Generally speaking, an exit point of the piezoelectric tube of this embodiment is not bounded by a container, so the open-ended boundary condition is achieved. Water flows continuously under gravity and so there is no pressure noise from the pump action.

A further embodiment of the invention would be a filter comprising of many long and narrow tube elements, each of high aspect ratio. The tube elements should not be too narrow such that they collect particles in their own right. Each tube element would have an individual high Q factor. In use, the energy is distributed across the set of smaller tube elements. This can result in a higher impedance acoustic filter that consumes less power.

This can be achieved by using a set of smaller piezoelectric tubes, running in parallel. Alternatively, this can also be achieved by using one large piezoelectric tube with a set of auxiliary tubes. Energy is transferred from the larger piezoelectric tube to the smaller piezoelectric tubes through the fluid or through a mounting structure.

Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

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Claims

1. An acoustic apparatus for inhibiting a flow of particles or cells in a fluid, the apparatus comprising:

an acoustic source transducer;

a tubular cavity in which the fluid is located;

wherein multiple source transducers are configured to form resonant nodal standing wave planes perpendicular to a direction of the flow within the tubular cavity, and wherein the nodal planes impede the flow of particles or cells such that the particles or cells from the fluid are separated near the resonant nodal planes and a remainder of the fluid is flown through the tubular cavity.

2. An acoustic apparatus according to claim 1, wherein the acoustic source transducer is a piezoelectric source transducer.

3. An acoustic apparatus according to claim 1, wherein the tubular cavity is an open ended auxiliary tubular cavity, and wherein a face of the acoustic source transducer is aligned with the central axis of the auxiliary tubular cavity.

4. An acoustic apparatus according to claim 1, wherein the tubular cavity is an open ended piezo-electric tube, and wherein the open ended piezo-electric tube serves as both the tubular cavity and the acoustic source transducer.

5. An acoustic apparatus according to claim 4, wherein the capacity of the piezoelectric tube to immobilise particles or cells in any of the nodal planes is dependent on acoustic energy density and its net volume.

6. An acoustic apparatus according to claim 4 or 5, wherein the piezoelectric tube is configured such that a radial breathing mode of the tube is coupled with a vibrational mode.

7. An acoustic apparatus according to claim 6, wherein the perpendicular nodal planes are generated when a tube length and the fluid in the tube are resonated at a same resonant frequency as a wall of the piezoelectric tube.

8. An acoustic apparatus according to claim 7, wherein the resonant frequency of the wall of the piezoelectric tube is defined by:

9. An acoustic apparatus according to claim 7 or 8, wherein the resonant frequency of the tube length and fluid in the tube is defined by:

N(fluid velocity)/2*tube length, wherein N is an integer corresponding to a resonant harmonic.

10. An acoustic apparatus according to any one of claims 6 to 9, wherein an inner diameter of the piezo-electric tube is chosen so that a radial standing wave cannot form, which is achieved by choosing a matched anti-resonant frequency.

11. An acoustic apparatus according to any one of claims 4 to 10, further comprising:

12. An acoustic apparatus according to claim 11, wherein at least one of the flexible de-couplers is an O-ring de-coupler.

13. An acoustic apparatus according to claim 11 or 12, wherein the piezoelectric tube is coupled to closed tubes containing axial reflectors of differing acoustic impedance through the flexible de-couplers.

14. An acoustic apparatus according to claim 13, wherein the further tubes have different elasticity, different thickness and stiffness of wall compared to those of the piezoelectric tube.

15. An acoustic apparatus according to claim 13 or 14, wherein the further tubes have substantially the same inner diameter as the piezoelectric tube.

16. An acoustic apparatus according to claim 13 or 14, wherein at least one of the further tubes has a larger inner diameter compared to that of the piezoelectric tube so that an increased impedance difference is achieved between the piezoelectric tube and the further tubes.

17. An acoustic apparatus according to claim 16, wherein at least one of the further tubes has a tapered shape so that a circumference of the further tubes gradually mates with that of the piezoelectric tube.

18. An acoustic apparatus according to claim 13 or 14, further comprising an auxiliary tube coupled with the piezoelectric tube through the de-couplers, wherein a central axis of the auxiliary tube is aligned with a central axis of the piezoelectric tube.

19. An acoustic apparatus according to claim 18, wherein the auxiliary tube is configured to resonate at a predetermined frequency.

20. An acoustic apparatus according to claim 18, wherein both the auxiliary tube and the piezoelectric tube are configured to resonate at a same frequency.

21. An acoustic apparatus according to claim 13 or 14, wherein one of the further tubes comprises a valve which is configured to obtain the inhibited particles or cells and allow de-watering of the remainder of the fluid through the further tube.

22. An acoustic apparatus according to any preceding claim, further comprising a controller which is configured to apply an electrical energy to the tubular cavity to create a standing wave barrier perpendicular to the flow in the tubular cavity.

23. An acoustic apparatus according to any preceding claim, further comprising a plurality of tubular cavities connected in parallel to increase scale and throughput, wherein each tubular cavity comprises a relatively smaller inner diameter.

24. An acoustic apparatus according to any one of claims 1 to 22, further comprising a plurality of tubular cavities connected in series.

25. An acoustic apparatus according to any preceding claim, wherein the tubular cavity comprises a material comprising any one of metal, ceramic, glass, polymer, and a composite.

26. An acoustic apparatus according to any preceding claim, wherein the apparatus is configured such that the particles or cells from the fluid are concentrated near the resonant nodal planes within the tubular cavity.

27. An acoustic apparatus according to any one of claims 1 to 25, wherein the apparatus is configured such that the particles or cells from the fluid are filtered by the resonant nodal planes.

28. An acoustic apparatus according to any preceding claim, wherein the apparatus is configured such that a continuous flow is processed by the apparatus.

29. An acoustic apparatus according to any one of claims 1 to 27, wherein the apparatus is configured to apply a tapping technique to process the flow on a batch by batch basis.

30. An acoustic apparatus according to any preceding claim, wherein the apparatus is configured such that the fluid flows through from an upper end to a lower end of the tubular cavity aided by gravity.

31. An acoustic apparatus according to any one of claims 1 to 29, wherein the apparatus is configured such that the fluid flows through from a lower end to an upper end of the tubular cavity against gravity.

32. An acoustic apparatus according to any one of claims 1 to 29, wherein apparatus is configured such that the fluid flows through from one side to the other of a horizontal or inclined tubular cavity.

33. An acoustic apparatus according to any preceding claim, wherein the apparatus is configured such that the nodal planes filter particles or cells having a size ranging from sub-micrometre to macrometre.

34. A biological separation system comprising the acoustic apparatus according to any preceding claim.

35. A non-biological separation system comprising the acoustic apparatus according to any one of claims 1 to 33.

36. A method of inhibiting a flow of particles or cells in a fluid, the method comprising:

providing an acoustic source transducer;

37. A method according to claim 36, further comprising forming a filter plane from the acoustic source transducer.

38. A method according to claim 36, further comprising producing one quasi twodimensional cavitation plane for impeding the flow of particles.

39. A method according to claim 36, further comprising producing two quasi twodimensional cavitation planes with different radiation directions for impeding the flow of particles.