TECHNICAL FIELD

The present invention relates to microbubble generators and methods of microbubble generation.

BACKGROUND

Microbubble generators are used for aeration of liquids, in particular being used in water treatment, where the microbubbles are used to form free radicals (OH ions) for waste-water disinfection, and in the dissolved air flotation of particulate. Microbubble aeration is also used in bacteria digestion and fermentation processes, and to oxygenate water for aquaculture.

For aeration applications, the size of bubbles has an important affect upon aeration performance. Large bubbles (e.g. >50 pm diameter) rise rapidly, spending less time within water, and so provide lower aeration performance. Nanobubbles (<1 pm diameter) are extremely stable due to the absorption of ions on their surface, which prevents the gas inside the nanobubble from being absorbed into the bulk liquid, allowing the nanobubble to last much longer, reducing aeration performance. Microbubbles (1 pm < diameter < 50 pm) have a higher internal pressure, shrink and dissolve in water, providing good aeration performance.

There remains a need to improve the generation of microbubbles by microbubble generators and their aeration performance.

SUMMARY OF THE DISCLOSURE

According to the present disclosure, there is provided a microbubble generator, an aeration system, an aquaculture facility, and a boat, as set forth in the appended claims.

According to a first aspect, there is provided a microbubble generator comprising: a first Venturi section; a second Venturi section; and a mixing section for coupling fluid flow between the first Venturi section and the second Venturi section, wherein the first Venturi section has a first convergent section coupled to a first divergent section, providing a first constriction for a flow of liquid through the microbubble generator, and having a gas inlet aperture at or close to the narrowest part of the constriction for drawing gas into a liquid flowing through the first constriction, the second Venturi section has a second convergent section coupled to a second divergent section, and providing a second constriction for a flow of liquid through the microbubble generator, and wherein the mixing section couples between the first divergent section and the second convergent section.

According to a second aspect, there is provided an aeration system comprising: a plurality of microbubble generators according to the first aspect; and a liquid pump for pumping liquid through each of the microbubble generators.

According to a third aspect, there is provided an aquaculture facility comprising the aeration system of the second aspect installed within a body of water.

According to a fourth aspect, there is provided a boat comprising the aeration system of the second aspect, wherein the microbubble generators are arranged for the supply of aerated liquid adjacent an exterior surface of a hull of the boat.

According to a fifth aspect, there is provided a cell culture system comprising the aeration system of the second aspect, wherein the microbubble generators are arranged for the supply of aerated liquid within a cell culture tank.

The first Venturi section may comprise a pipe of substantially constant cross-section coupled between the first convergent section and the first divergent section.

The second Venturi section may be configured to provide a lower hydrodynamic resistance to liquid flowing through the microbubble generator than the first Venturi section.

The hydrodynamic resistance of the second Venturi section may be less than 50% of the hydrodynamic resistance of the first Venturi section. The hydrodynamic resistance of the second Venturi section may be less than 40% of the hydrodynamic resistance of the first Venturi section. The hydrodynamic resistance of the second Venturi section may be less than 30% of the hydrodynamic resistance of the first Venturi section. The hydrodynamic resistance of the second Venturi section may be less than 20% of the hydrodynamic resistance of the first Venturi section.

The narrowest part of the second constriction may have a larger cross-sectional area than the cross-sectional area of the narrowest part of the first constriction.

The second constriction may have a cross-sectional area that is at least 50% larger than the first constriction.

The narrowest part of the second constriction may have a larger internal diameter than the diameter of the narrowest part of the first constriction.

The second Venturi section may comprise a plurality of second Venturi sub-sections arranged in parallel and each providing a second constriction for a subsidiary flow of liquid through the microbubble generator, and the microbubble generator may comprise a flow path splitting section coupling between the first Venturi section and the plurality of second Venturi sub-sections.

The plurality of second Venturi sub-sections may be configured to provide a lower hydrodynamic resistance to liquid flowing through the second Venturi section than the first Venturi section.

The narrowest parts of each of the plurality of second constrictions and the first constriction may have substantially equal cross-sectional areas.

A gas supply tube may be provided, extending from the gas inlet aperture.

A helical thread may be provided within the mixing section.

The microbubble generator may be integrally formed (e.g. formed in a single manufacturing step). The microbubble generator may be integrally formed by injection moulding. The microbubble generators and liquid pump may be configured to provide a pressure drop between an inlet to the first convergent section and an outlet from the second divergent section of each microbubble generator of at least 3.5bar.

The aeration system of the second aspect may comprise a gas compressor coupled for the supply of compressed gas to the gas inlet aperture of each microbubble generator.

DESCRIPTION OF THE DRAWINGS

Examples are further described hereinafter with reference to the accompanying drawings, in which:

• Figure 1 A shows a side view along the length of a microbubble generator;

• Figure 1B shows the microbubble generator of Figure 1A in use;

• Figures 2A & 2B show side views along the length of further microbubble generators provided with helical screw threads;

• Figure 3 shows a side view along the length of a further microbubble generator;

• Figure 4 shows a cell culture tank provided with an aeration system; and

• Figure 5 shows a boat provided with an aeration system.

DETAILED DESCRIPTION

In the described examples, like features have been identified with like numerals, albeit in some cases having typographical marks (primes) or increments of 100. For example, in different figures, 100, 100’, 100” and 200 have been used to indicate a microbubble generator.

Figure 1A shows a microbubble generator 100, and Figure 1B shows the microbubble generator in use, submerged in a body of liquid L (e.g. water). The microbubble generator 100 is generally a tube for high speed liquid to flow F through, the interior of which has two Venturi sections 110, 130, coupled together by an intervening mixing section 120.

The first Venturi section 110 has a first convergent section 112 coupled to a first divergent section 116, providing a first throat (constriction) for a flow F of liquid through the microbubble generator 100. As illustrated in Figure 1A, the first throat of the first Venturi section 110 may be provided with a first pipe 114 of substantially constant cross-section, coupled between the first convergent section 112 and the first divergent section 116. A gas inlet aperture 118 is provided at, or close to the narrowest part of the first throat, through which a flow of gas (e.g. air) is drawn into the liquid flowing through the first throat, by the reduced pressure within the first throat, produced by the Venturi effect. The flow of gas may be drawn in directly from the atmosphere. Alternatively, the flow of gas may be provided from a pressurised gas supply.

The internal walls of the first convergent section 112 may have a convergence angle of 15 - 25° (e.g. 16.05°). The internal walls of the first divergent section 116 may have a divergence angle of 2.5 - 7.5° (e.g. 3.75°). (All angles of convergence and divergence are specified with respect to the central axis of the microbubble generator.) The inlet diameter of the first convergent section 112 may be 8 - 15mm. The length of the first throat may be 5 - 15mm. The internal diameter of the first throat may be 2.5 - 3.5mm (e.g. 2.8mm). The outlet diameter of the first divergent section 116 may be 8 - 15mm. The diameter of the gas inlet aperture 118 at the first throat may be 1 - 2mm.

The gas supply G (e.g. air) to the microbubble generator 100 may be supplied through a tube T coupled to a connection port 119 coupled to the gas inlet aperture 118, as shown in Figure 1B. For example, such a tube T may be used to supply gas G to the gas inlet aperture 118 when the microbubble generator 100 is immersed in liquid L. For example, the microbubble generator 100 may be installed beneath the surface of the liquid L, with the tube T extending above the surface of the liquid, to enable gas G (e.g. ambient air) to be drawn in through the gas inlet aperture 118 in use. The microbubble generator 100 may be connected to a liquid supply pump (not shown) through pipework P, for the supply of the flow F of liquid through the microbubble generator.

The second Venturi section 130 has a second convergent section 132 coupled to a second divergent section 136, providing a second throat (constriction) for a flow of liquid through the microbubble generator 100. The second throat of the second Venturi section 130 may be provided with a second pipe 134 of substantially constant cross-section, coupled between the first convergent section 132 and the first divergent section 136, as illustrated in Figure 1A. Alternatively, the throat of the second Venturi section 130 may be provided with a smoothly curved transition between the second convergent section 132 and the second divergent section 136.

The internal walls of the second convergent section 132 may have a convergence angle of 15 - 25° (e.g. 17.73°). The internal walls of the second divergent section 136 may have a divergence angle of 2.5 - 7.5° (e.g. 3.12°). The inlet diameter of the second convergent section 132 may be 8 - 15mm. The length of the second throat may be 5 - 10mm. The internal diameter of the second throat may be 2.5 - 4.5mm (e.g. 4.0mm). The outlet diameter of the second divergent section 136 may be 8 - 15mm. The second Venturi section 130 has a lower hydrodynamic resistance to the flow F of liquid through the microbubble generator than the first Venturi section 110, so that a substantial proportion of the pressure differential across the microbubble generator 100 is maintained across the first Venturi section 110, in use. The hydrodynamic resistance of the second Venturi section 130 may be less than 50% of the hydrodynamic resistance of the first Venturi section 110 (e.g. less than 20%). The second throat may have a larger cross-sectional area than the first throat. The second throat may have a cross-sectional area that is at least 50% larger than the first throat (e.g. approximately double the area of the first throat). The first and second throats may have a circular cross-section, and the second throat may have a larger diameter than the first throat. Additionally, or alternatively, the second throat may be shorter than the first throat, along the direction of flow F of the liquid passing through the microbubble generator, in use.

The hydrodynamic resistance of each Venturi section 110, 130 is dependent upon the pressure drop arising in liquid flow F through the respective throat, in use. Table 1 below shows exemplary pressure drops for different liquid flows F through a microbubble generator 100 according to Figure 1A, in which first throat diameter is 2.8mm, the second throat diameter is 4.0mm, both throats have constant diameter lengths of 10mm, and the first and second Venturi sections 110, 130 otherwise have substantially the same design. TABLE 1 For the exemplary microbubble generator, the pressure drop in the second throat is less than 20% (18.3%) of the pressure drop in the first throat, providing a second Venturi section 130 with a hydrodynamic resistance that is significantly less than the hydrodynamic resistance in the first Venturi section 110.

The mixing section 120 couples between the first divergent section 116 and the second convergent section 132, and has a larger internal cross-sectional area than both of the first and second throats (constrictions).

In use, a flow F of liquid (e.g. water) passes into the inlet 102 of the microbubble generator 100, and flows through to the outlet 104. Within the first throat (constriction) of the first Venturi section 110, the hydrostatic pressure of the high-speed liquid is reduced, in accordance with the Venturi effect. The lower hydrostatic pressure adjacent the gas inlet aperture 118 draws gas through the gas inlet aperture, which becomes entrained as bubbles in the liquid flow.

Bubbles may be entrained from the gas inlet aperture 118 with a range of different volumes (i.e. the bubbles would have different volumes, if measured at the same hydrostatic pressure, e.g. atmospheric pressure at sea level). The first divergent section 116 provides the first stage of the process of breaking up the bubbles entrained from the gas inlet aperture 118.

Along the flow direction, the first divergent section 116 diverges gradually, causing deceleration of the flow of liquid and entrained bubbles, and causing an increase in pressure along the length of the first divergent section, and instability in the flow, creating turbulent shear forces in the liquid flow, e.g. in a central core of the liquid flow. The flow of liquid also creates viscous shear forces near the internal wall of the first divergent section 116. The shear forces and turbulent fluctuation in the liquid flow deform the bubbles within the first divergent section 116, leading larger bubbles to split into smaller bubbles.

The shear forces in the liquid vary across the width of the first divergent section 116 (e.g. the shear forces may vary in an approximately radial manner, from the central axis of the first divergent section) causing an inhomogeneity in bubble splitting performance.

As the liquid flows along the mixing section 120, between the first and second Venturi sections 110, 130, turbulence mixes the liquid, reducing the variation in the bubble size distributions across the width (i.e. transverse to the flow direction) of the microbubble generator 100 in the liquid that flows into the second Venturi section.

The second convergent section 132 provides a gradual change in velocity, which reduces energy loss from the flow and reduces (e.g. prevents) flow separation and reattachment within the second Venturi section 130.

Within the second throat (constriction) of the second Venturi section 130, the hydrostatic pressure of the liquid is reduced, in accordance with the Venturi effect, and the liquid and entrained bubbles are subjected to shear forces (the flow of liquid creates viscous shear forces near the internal wall of the throat of the second Venturi section 130). The reduced hydrostatic pressure causes the entrained bubbles to expand within the throat of the second Venturi section 130, and the shear forces deform the expanded bubbles, causing some larger bubbles to split into smaller bubbles.

Along the flow direction, the second divergent section 136 diverges gradually, causing deceleration of the flow of liquid and entrained bubbles and an increase in pressure along the length of the second divergent section, and instability in the flow, creating turbulent shear forces in the liquid flow, e.g. in a central core of the liquid flow. The flow of liquid creates viscous shear forces near the internal wall of the second divergent section 136. The shear forces and turbulent fluctuation in the liquid flow deform the bubbles within the second divergent section 136, leading larger bubbles to split into smaller bubbles.

In particular, where larger bubbles flow through regions of higher shear force, they may be divided, which increases the level of uniformity of size of the microbubbles (i.e. uniformity of volume under the same hydrostatic conditions) produced by the microbubble generator 100. Improved uniformity of microbubble size increases the efficiency of absorption of oxygen into the bulk liquid, so increasing the aeration performance of the microbubble generator 100.

The second Venturi section 130 has a lower hydrodynamic resistance to liquid flowing through the microbubble generator than the first Venturi section 110. In the illustrated microbubble generator 100, the cross-sectional area of the second throat of the second Venturi section 130 is larger than the cross-sectional area of the first throat of the first Venturi section 110, reducing the hydrodynamic resistance of the second Venturi section 130 relative to the hydrodynamic resistance of the first Venturi section 110. The cross- sectional shape of the first and second Venturi sections 110, 130 may be circular, perpendicular to the fluid flow direction, and the diameter D2 of the second throat of the second Venturi section 130 may be larger than the diameter D1 of the first throat of the first Venturi section 110. In the illustrated microbubble generator 100, the length of the throat in the second Venturi section 130 is shorter than the throat of the first Venturi section 110, reducing the hydrodynamic resistance of the second Venturi section 130 relative to the hydrodynamic resistance of the first Venturi section 110.

The provision of a lower hydrodynamic resistance in the second Venturi section 130 than in the first Venturi section 110 maintains a significant pressure drop across the first Venturi section 110 and reduces the resistance to flow through the microbubble generator 100.

The illustrated microbubble generator 100 is substantially rotationally symmetric about a central axis C. However, the microbubble generator is not limited to this shape, and may have a non-circular cross-sectional shape, and/or may have a shape that is not straight.

The microbubble generator 100 comprising two Venturi sections coupled in series with an intervening mixing section enables the microbubbles to be generated in the flow through the outlet 104 with a reduced distribution of volumes (under hydrostatic conditions). In particular, the provision of the mixing section 120 and the second Venturi section 130 provide enhanced bubble generation compared with a single Venturi section device. Further, for corresponding aeration performance, the microbubble generator 100 comprising two Venturi sections coupled in series with an intervening mixing section enables microbubbles to be generated with a lower liquid flow rate than in a single Venturi microbubble generator, reducing operational power consumption.

The illustrated microbubble generators 100, 100’, 100” have two Venturi sections that are coupled in series with an intervening mixing section. However, one or more further Venturi sections (e.g. a third Venturi section) may additionally be coupled in series with the second Venturi section, with an intervening mixing section coupling between the successive Venturi sections.

Entraining gas (e.g. air) at atmospheric pressure, through the gas inlet aperture 118 and into the liquid flow reduces the cost of the aeration system and its running costs, and increases reliability in use, compared with the use of compressed gas. Alternatively, compressed gas may be provided to the gas inlet aperture 118 for entraining into the liquid flowing through the microbubble generator 100. Figures 2A and 2B show further microbubble generators 100’ and 100”. The microbubble generator 100’ differs from the microbubble generator 100 of Figures 1A and 1B by the additional provision of a first helical screw thread 122’ (having three turns) within the mixing section 120’. The microbubble generator 100” of Figure 2B differs from the microbubble generator 100’ of Figures 2A by the provision of a longer, second helical screw thread 122” (having seven turns) within the mixing section 120”. The helical screw threads 122’ and 122” may induce additional shearing forces within the bubbles travelling through the mixing sections 120’, 120”, causing additional elongation and splitting of bubbles within the mixing section, particularly for larger bubbles.

In the microbubble generators may be integrally formed (formed in a single manufacturing step), for example by an injection moulding process. Integral formation may enable the provision of an uninterrupted internal wall surface, which will not interfere with the adjacent liquid flow.

In the microbubble generators 100, 100’, 100” illustrated in Figures 1A, 2A and 2B, the same flow F of liquid passing through the first Venturi section 110 passes, in series, through the second Venturi section 130, and the second Venturi section 130 has a lower hydrodynamic resistance to the flow F of liquid through the microbubble generator than the first Venturi section 110, so that a substantial proportion of the pressure differential across the microbubble generator 100 is maintained across the first Venturi section 110, in use.

Alternatively, or additionally, after the first Venturi section 210, the flow path of the liquid passing through the microbubble generator 200 may split into two (or more) parallel, subsidiary flow paths, each leading to a respective second Venturi section 230, 230’ and outlet 204’, 204”, as shown in Figure 3. In the illustrated microbubble generator 200, the flow path splitting section 240 is provided between the first Venturi section 210 and respective mixing sections 220’, 220” as shown in Figure 3. The internal liquid flow path of the microbubble generator 200 is branched at the flow path splitting section 240, providing a common flow path section 200A, in which the first Venturi section 210 is provided, and two (or more) subsidiary flow path sections 200B’, 200B”, in which the second Venturi sections 230’, 230” are provided.

In the microbubble generator of Figure 3, the flow path splitting section 240 is provided between the first Venturi section 210 and respective mixing sections 220’, 220”. However, alternatively, the mixing section may be provided between the first Venturi section and a common mixing section, and the flow path splitting section may be provided between the common mixing section and the respective second Venturi sections.

In microbubble generators with a flow path splitting section, the cross-sectional area of the second throats of the second Venturi sections may be comparable, or equal, to the cross- sectional area of the first throat of the first Venturi section. Further, the internal shapes of the first Venturi section and the two (or more) second Venturi sections may be the same. The internal diameters of the first throat and plurality of second throats may each be 2.5 - 3.5mm (e.g. 2.8mm). In the case that the microbubble generator with a flow splitting section is formed by modular construction, use of first and second Venturi sections having the same internal shapes may reduce manufacturing cost.

An aeration system may be formed by coupling one or a plurality (two or more) of microbubble generators 100, 100’ and 100” to a pump 184 for pumping liquid (e.g. water) through each of the microbubble generators, and enabling or supplying gas G (e.g. ambient air or compressed air) to flow to each gas inlet aperture 118 (e.g. with each gas inlet aperture being provided with an air supply tube extending from the water surface). The microbubble generators in the aeration system may be of the same or different designs (e.g. providing the same or different distributions of bubble sizes).

For example, an aquaculture facility may comprise the aeration system. A network of pipework may connect underwater microbubble generators, which may be distributed within a body of water used for aquaculture.

Figure 4 illustrates a cell culture system 180 having a cell culture tank 182 provided with an aeration system, e.g. for the growth of mammalian cells or microorganisms, such as animal or bacterial cells. The microbubble generators submerged in a liquid growth medium L within the tank 182 are coupled to a , to provide aeration of the growth medium.

Where the microbubble generators are submerged, in use, the microbubble generators and liquid pump may be configured to provide a pressure drop between in the inlet 102 and outlet 104 of each microbubble generator of at least 3.5bar.

A boat B may be provided with an aeration system, in which a water pump provides air to a plurality of microbubble generators arranged to emit aerated water through outlets 190, into the water adjacent the hull, as shown in Figure 5. For example, the plurality of microbubble generators may supply aerated water to an array of outlets 190 on each side of the hull of the boat B, so that the aerated water is swept across the surface of the hull, towards the stern, as the boat travels forwards through the water. The aerated water adjacent the hull may reduce the hydrodynamic drag of the hull, through the water L. A boat provided with an aeration system may be driven around an aquaculture facility to aerate the water within the facility.

Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.