WO2021032962A1

WO2021032962A1 - Rapid humidification device and process

Info

Publication number: WO2021032962A1
Application number: PCT/GB2020/051957
Authority: WO
Inventors: Antonis SERGIS; Yannis Hardalupas
Original assignee: Imperial College Innovations Limited
Priority date: 2019-08-16
Filing date: 2020-08-14
Publication date: 2021-02-25
Also published as: GB201913407D0

Abstract

System comprising: an inlet (16) connectable to a source of fluid; an outlet (20); a fluid flow path (22) defined from the inlet to the outlet; an annular diffuser (14) for diverging and slowing flow within the fluid flow path; a contraction section (18) situated downstream of the diffuser for contracting and accelerating flow within the fluid flow path; and an atomiser nozzle (24) arranged to introduce liquid droplets into the fluid flow path between the diffuser and the contraction section.

Description

RAPID HUMIDIFICATION DEVICE AND PROCESS

This invention relates to a system that may be used as a flow seeder or a humidifier. The invention also relates to a method of seeding or humidifying a fluid flow.

Background

Flow seeders are devices employed in high fidelity flow visualisation methods (e.g. Particle Image Velocimetry - PIV). An example may be a water droplet seeder. Seeders are devices designed to create uniform suspensions of micron sized tracing particles in transparent flows of gases and liquids. The particles are illuminated by strong coherent light sources (i.e. lasers) and tracked using specialised cameras as they are carried by the flow containing them. This helps create high fidelity, high spatial resolution maps of the motion of the flow by means of the inferred motion of the illuminated tracing particles. The types of tracing particles used can be anything that can reflect and scatter light. The seeders are categorised into liquid and solid particle seeders. The former are usually used to seed very large volumetric flows and the latter for smaller flows.

Commercial liquid seeders used to seed large airflows typically employ smoke machines (e.g. modified theatrical smoke machines). Smoke is essentially micron sized particles ordroplets suspended in air. Glycol is usually burnt to create the smoke. Alternatively, silicon oil may be burnt, which occurs at higher temperatures. A problem with these methods is that the smoke will usually condense when it comes in contact with colder surfaces. In the case of silicon oil, there is no effective and practical way to completely remove deposited silicon condensate from the surfaces. This can create blockages or damage on instruments with fine features (e.g. small holes to measure pressure) or make the optical windows and lenses of measurement equipment dirty to the point of no longer being able to optically analyse the flows.

There is therefore a need for a flow seeder that causes reduced condensation or deposition on surfaces.

Industrial humidifiers generate water droplets and expel a supersaturated mixture of water droplets and air at a certain cone angle. When the supersaturated mixture meets a surface the water droplets condensate. As such, this type of humidifier has not yet been used inside ducted heating, ventilation and air conditioning (HVAC) systems, but is instead placed inside large cavernous spaces which have a high tolerance to the emerging condensation that could occur on neighbouring surfaces before the air and water droplet mixture de-saturates.

There is a need fora humidifier that causes reduced condensation before de-saturation of the air and water droplet mixture. Summary

Aspects and features of the present invention are defined in the accompanying claims.

According to a first aspect, there is provided a system as defined in claim 1. The system could be used as either a flow seeder or a humidifier.

According to a second aspect, there is provided a method as claimed in claim 13.

Brief description of the drawings

Embodiments will now be described, by way of example only, and with reference to the drawings in which: Figure 1 illustrates a section view of an example of a flow seeder.

Figure 2 illustrates a section view of the flow seeder of Figure 1.

Figure 3 illustrates a section view of the flow seeder of Figure 1 with an airflow.

Figure 4 illustrates a section view of the contraction section of the flow seeder of Figure 1 with an airflow. Figure 5 illustrates an atomiser nozzle for use in the flow seeder of Figure 1 .

Figure 6 illustrates the velocity profile of the flow produced using the flow seeder of Figure 1 .

Figures 7 and 8 illustrate the dispersion of droplets produced using the flow seeder of Figure 1.

In the Figures, like elements are indicated by like reference numerals throughout.

Detailed description

As discussed in the background section, flow seeders used to seed large fluid flows typically achieve this by burning glycol or silicon oil. However, this results in condensation and deposition on surfaces which may be difficult to remove and create blockages or damage on instruments. Industrial humidifiers eject a supersaturated air/water mixture at a cone angle, which results in undesirable condensation unless the humidifier is situated in a large cavernous space to allow the mixture to de-saturate before coming into contact with surfaces.

Hence, it is desirable to provide a flow seeder or humidifier that does not cause deposition of glycol or silicon oil and does not need to be situated in a large cavernous space. Turning to Figure 1 , an example of a flow seeder 10 is disclosed. Although referred to as flow seeder 10 throughout, it will be understood that the device could be used as a humidifier.

A central axis of the flow seeder 10 is indicated by dashed line 12. The flow seeder 10 has a proximal end (bottom of page as viewed in Figure 1) and a distal end (top of page as viewed in Figure 1). The flow seeder 10 has continuous rotational symmetry about the central axis 12. This allows smooth and continuous fluid flow through the flow seeder 10.

The flow seeder 10 includes an annular diffuser 14. The annular diffuser 14 slows fluid flow passing through the annular diffuser. Annular diffuser 14 includes an inlet 16 for receiving incoming fluid flow. Inlet 16 is provided at a proximal end of the flow seeder 10. In use, the inlet 16 may be coupled to a fluid delivery system (not shown), for example via a flexible pipe. For example, in use as a humidifier, relatively dry air may be directed into the inlet 16.

The diffuser defines a portion of a fluid flow path which is discussed in more detail below. The portion of the fluid flow path in the diffuser diverges. When fluid flows along the fluid flow path, the same volumetric flow rate is maintained but the cross-sectional area of the flow increases, such that the velocity of the flow decreases. Therefore the fluid flow slows as it moves through the annular diffuser 14.

The flow seeder 10 further includes an atomiser nozzle 24 in communication with the fluid flow path downstream of the annular diffuser, which is described in further detail below. An atomiser nozzle is a device suitable for ejecting airborne liquid droplets, for example droplets of water. Typically, the atomiser nozzle is supplied with pressurised air and water and ejects a saturated air/water mixture. However other fluids may be used with appropriate modification. The liquid droplets are introduced to the flow path at a point downstream of the diffuser.

The flow seeder 10 further includes a contraction section 18. Contraction section 18 accelerates the fluid flow before leaving the flow seeder 10. Contraction section 18 includes an outlet 20. Fluid exits the flow seeder 10 via the outlet 20. Outlet 20 is provided at a distal end of the flow seeder 10.

The contraction section defines a further portion of the fluid flow path which is discussed in more detail below. The portion of the fluid flow path in the contraction section converges. When fluid flows along the fluid flow path, the same volumetric flow rate is maintained but the cross-sectional area of the flow decreases, such that the velocity of the flow increases. Therefore the airflow accelerates as it moves through the contraction section.

Annular diffuser 14 and contraction section 18 together define a fluid flow path 22 from inlet 16 to outlet 20. Fluid entering inlet 16 follows fluid flow path 22 to the outlet 20 and exits the flow seeder 10. The fluid is decelerated through the annular diffuser, is then mixed with liquid droplets from the atomiser nozzle and finally accelerated through the contraction section. This ensures that large volumes of fluid mix adequately with the mixture of fluid and liquid droplets emitted from the atomiser nozzle.

It will therefore be appreciated that in the flow seeder 10, the majority of the fluid flowing through the device enters through a dedicated duct: inlet 16 and fluid flow path 22. A small amount of fluid may enter through the atomiser nozzle 24. The flow seeder 10 can therefore handle a larger volume of flow, compared to a device in which all or the majority of fluid must pass through an atomiser nozzle. Passing fluid through an atomiser nozzle would limit the fluid flow volume. Further, this avoids producing a dense spray at the device exit, which would later need to be combined with a bulk fluid flow in a large space to avoid condensation. Finally, adequate mixing of the saturated mixture from the atomiser nozzle with the fliud entering the inlet 16 is ensured, as the fluid is slowed by the diffuser for mixing.

In other embodiments, additional components may be provided between the annular diffuser 14 and the contraction section 18 to define the fluid flow path 22. In other words, the fluid flow path 22 may be defined by more than just the annular diffuser 14 and contraction section 18.

Annular diffuser 14 and contraction section 18 are each bodies with a substantially cylindrical outer profile, joined end to end. Annular diffuser 14 and contraction section 18 may be portions of a single integral body or may be separate parts joined together. For example, they may be joined by a screw fit. A seal may be provided between annular diffuser 14 and contraction section 18.

An atomiser nozzle 24 is disposed within the annular diffuser 14. The atomiser nozzle 24 is configured to introduce liquid droplets into the fluid flow path 22 when in use. In use as a humidifier, relatively dry airflow entering inlet 16 is thus humidified, and an air/water mixture exits the humidifier. In use as a seeder, a tracer substance may also be additionally introduced into the fluid entering inlet 16, such that a tracer/fluid mixture exits the seeder. The atomiser nozzle 24 is preferably a shear induced humidifying device. The atomiser nozzle 24 may be connected to sources of air and water (not shown).

The shear induced humidifying device is a passive device, that is, it does not require electrical power to operate. In alternative embodiments, a different atomiser nozzle may be used. An atomiser nozzle that requires electrical power may be used.

The annular diffuser 14 comprises an outer wall section 26 and an inner wall section 28. The outer wall section 26 and inner wall section 28 define a first portion of the fluid flow path 22 therebetween. Outer wall section 26 has a substantially cylindrical outer surface 30. Outer wall section 26 has a hollow interior defined by an inner surface 32.

Inner wall section 28 is disposed within the hollow interior of outer wall section 26. Inner wall section 28 is a tapered body having a tapered outer surface 34. Inner wall section 28 also has a front surface 36 at its distal end. A cross section of the inner wall section 28 in a plane perpendicular to central axis 12 increases in a direction along central axis 12 from the proximal end to the distal end. The cross section is circular. In other words, inner wall section 28 is narrower at the proximal end.

A first part of the fluid flow path 22 is defined between the outer surface 34 of the inner wall section 28 and the inner surface 32 of the outer wall section 26. In use, fluid flow entering inlet 16 flows along the outer surface 34 of the inner wall section 28.

Because the outer surface 34 is tapered, a cross section of the first portion of the fluid flow path 22 expands in a direction from the proximal end to the distal end. The first portion of the fluid flow path 22 therefore diverges. In use, this slows the airflow as it moves through the annular diffuser 14. This is because the same volumetric flow rate is maintained, but the cross-sectional area of the flow increases, so the velocity of the flow decreases.

It will be appreciated that a cross section of the first portion of the fluid flow path 22 is annular in shape.

Inner wall section 28 has a bevelled front edge 38. The bevelled front edge 38 is between the outer surface 34 and the front surface 36. Bevelled front edge 38 and inner surface 32 of outer wall section 26 define an annular space therebetween. A directional vane ring 56 is disposed in this space. A compact arrangement of the flow seeder 10 is thus achieved. This is particularly advantageous in wind tunnel applications, in which a compact and aerodynamic arrangement is preferred in order to minimise turbulence in the wind tunnel. Directional vane ring 56 is disposed in the first part of the fluid flow path 22.

Directional vane ring 56 is a circular structure having a substantially triangular cross section (in a plane normal to the plane of the circle). That is, directional vane ring 56 tapers towards the proximal end of the flow seeder 10. Directional vane ring 56 has an angled inner surface 56a (angled relative to central axis 12) and an outer surface 56b. Outer surface 56b is substantially cylindrical about central axis 12. Directional vane ring 56 is preferably manufactured using high precision CNC lathing and milling as well as rapid prototyping (3D printing) methods.

The vane ring ensures stable deceleration of the flow in a short distance. This allows the diffuser to be shorter and more compact, which is advantageous in wind tunnel applications as explained above.

Fluid flow incident on the directional vane ring 56 is separated into an inner portion and an outer portion by the directional vane ring 56. The inner portion follows an inner flow path 50 (shown in Figure 3). In this embodiment, the inner portion is deflected inwards towards central axis 12 by angled inner surface 56a to follow the inner flow path 50. The outer portion follows an outer flow path 52 (shown in Figure 3). In this embodiment, the outer portion continues substantially parallel to the central axis 12 to follow outer flow path 52. It will therefore be understood that the inner flow path 50 converges towards the central axis 12. Contraction section 18 has an inner wall 48 which defines a mixing region 54. Mixing region 54 is a chamber within the flow seeder 10 which is significantly larger in cross sectional area than the first part of the fluid flow path. Outlet 20 communicates with mixing region 54. The inner flow path 50 is within an interior section of the mixing region 54. The outer flow path 52 is along the inner wall 48 of the contraction section 18. The outer flow path 52 is therefore in an outer section of the mixing region 54. It will be understood that the inner and outer flow paths referred to are not separated, but form a single continuous volume. In other embodiments they may be separated.

The inner wall 48 of the contraction section 18 is substantially dome shaped. The apex of the dome is towards the distal end of the flow seeder 10. The outlet 20 is situated at the apex of the dome. The contraction section 18 thus reduces the cross sectional area of the fluid flow path 22 in the distal direction. This accelerates fluid flow before leaving the device.

Inner wall section 28 has a hollow interior defining an inner volute 40. Inner volute 40 communicates with mixing region 54 via an opening 44 provided in the front surface 36 of the inner wall section 28. The atomiser nozzle 24 is disposed in the opening 44. The atomiser nozzle 24 is configured to introduce liquid droplets into the mixing region 50. Inner volute 40 communicates with the first part of the fluid flow path 22 via inner volute vacating hole 42. Inner volute vacating hole 42 is situated at a proximal end of the inner volute 40. Inner volute vacating hole 42 is a bore extending through the inner wall section 28.

Liquid droplets from the atomiser nozzle 24 mix with fluid following the inner flow path 50 within the inner section of the mixing region 54 to produce a mixture. Fluid that has not has liquid droplets introduced flows along the outer flow path 52, following the inner wall 48 of the contraction section 18. Thus the mixture is prevented from contacting the inner wall 48 of the contraction section 18 by the unmixed fluid of the outer flow path 52.

Advantageously, impingement of liquid droplets on interior surfaces of flow seeder 10 may thus be avoided. This prevents or minimises condensation inside the device. This is advantageous as it prevents the accumulation of fluid, which may require drainage or become a health hazard in the case of stagnant water. Further, this reduces the requirement for regular cleaning and maintenance of the flow seeder 10.

The inner flow path 50 and outer flow path 52 meet at the apex of the dome shaped inner wall 48 of the contraction section 18.

It will therefore be understood that fluid entering the flow seeder 10 is separated into an inner flow path 50 for mixing with liquid droplets from the atomiser nozzle 24, and an outer flow path 52 for following an innerwall of the flow seederto prevent condensation within the flow seeder 10. The unmixed fluid of the outer flow path 52 and the mixture of the inner flow path 50 are then recombined before exiting the flow seeder 10 to form a de-saturated mixture. The de-saturated mixture exits the flow seeder 10 via the outlet 20. Condensation outside the flow seeder 10 is therefore also reduced or avoided, because the mixture exits the humidifier 10 in a de-saturated state. The de-saturated mixture exits the flow seeder 10 with velocity substantially parallel to the central axis 12.

It will therefore be understood that the flow seeder 10 splits fluid flow into an inner flow path 50 and an outer flow path 52. Atomiser nozzle 24 is arranged to introduce liquid vapor into the inner flow path 50. The outer flow path 52 bypasses the atomiser nozzle. This has the effect that the flow seeder emits a desaturated mixture, rather than a super-saturated mixture, reducing or avoiding condensation on nearby surfaces. The flow seeder therefore does not need to be situated in a large volume to operate with acceptable levels of condensation.

Airflow and water droplets have been used as examples above. However, other fluid flows may also be used with appropriate modification to the seeder design. For example, a flow of water could be seeded. In this case, the contours of the diffuser may need to be adjusted. For example, given the higher viscosity of water as opposed to air, the diffuser section could be shortened as the liquid would be inherently more stable than air. The atomising nozzle would also need to change to accommodate the change to accommodate the change of liquid droplet generation. Such modifications will be apparent to the skilled person.

As mentioned above, the device can also be used as a humidifier. In use as a humidifier, relatively dry airflow enters inlet 16. The relatively dry airflow is split by the humidifier into an inner flow path 50 and an outer flow path 52. Atomiser nozzle 24 is arranged to introduce water vapor into the inner flow path 50. The relatively dry air entering inlet 16 is thus humidified, and an air/water mixture exits the humidifier. The outer flow path 52 bypasses the atomiser nozzle. This has the effect that the humidifier emits a desaturated mixture, rather than a super-saturated mixture, reducing or avoiding condensation on nearby surfaces. The humidifier therefore does not need to be situated in a large volume to operate with acceptable levels of condensation.

The flow seeder 10 uniformly suspends large amounts of micron sized liquid droplets inside large airflows. Up to 12 litres per hour (3.33x10⁶ m³/s) or the equivalent of 6.4 trillion droplets per second may be suspended.

The liquid droplets are preferably around 1pm in diameter.

The fluid flow is preferably from 500 litres per minute (0.0083 m³/s) to 4000 litres per minute (0.067 m³/s). The fluid flow is most preferably 3000 litres per minute (0.05 m³/s).

Flow seeder 10 employs a high-performance industrial shear-induced humidifying device to suspend micron-sized droplets of water inside a conditioned airflow void. Industrial shear humidifiers, unlike ultrasonic humidifiers, suffer less from bacterial contaminations (e.g. legionella disease, bacterial large- scale growths, mould, etc.) as they use a continuous fresh stream of pressurised water and air to generate the water droplets. There is no need for electricity at the installation point to operate them; the device operates instead via the actuation of valves through pressurised water and air supplies. There is low energy consumption in operating the device.

Flow seeder 10 uses aerodynamic components to ensure that large volumes of airflow mix adequately with the saturated mixture of air and micron-sized water droplets emitted from the shear humidifier. This ensures fast desaturation of the mixture coming out of the humidifier and manipulation of the emerging desaturated mixture. This avoids the condensation issues mentioned above.

As a seeding device this is a very useful property when used in large-scale wind tunnel experiments where laser diagnostic methods are to be used to analyse full sized models (such as cars, airplanes etc.).

As a humidifying device, the shear humidification device may be used in large-scale HVAC systems as well as clean rooms to rapidly control static charges (especially useful for microelectronic assembly rooms etc.) developing. The device can rapidly humidify large spaces in seconds. The device may be used in places where industrial shear humidifiers have been used in the past such as:

• Process and materials humidification

• Binding of dust

• Humidification in electrostatic painting

• Paint booths

• Refrigerated rooms

• Plastics industry

• Greenhouses and refrigerated rooms

• Evaporative cooling of warm premises

All airflow paths and wall contours are preferably designed based on wind tunnel design theory. A computational analysis (Computational Fluid Dynamics - CFD) is preferably performed on the geometries to ensure nominal operation.

The flow seeder 10 is preferably 0.5m in length and 0.2m in diameter. The flow seeder 10 is preferably placed inside an industrial flanged cast iron pressure sleeve to fulfil the health and safety criteria for large components under pressure as well as allowing customisation.

The atomiser nozzle 24 is preferably a SIBE SWED-FOG® Nozzle system. The atomiser nozzle 24 preferably has a variable seeding cone angle at its exit. The atomiser nozzle 24 is preferably able to produce up to 12kg/hr (0.0033 kg/s) of water droplets around 1 pm in size. Service hole fins, entrance redirection vanes as well as the supports of the different components have been omitted from the Figures but may be provided in embodiments.

A flow seeder housing may also be provided. The flow seeder housing is identical to the flow seeder save that the atomiser nozzle is omitted, leaving the annular diffuser and contraction section. An atomiser nozzle may be interchangeably inserted into the flow seeder housing to provide the above-described advantages.

The above description refers to a flow seeder. It will be understood that the flow seeder, when used to seed a flow of air with water, may be referred to as a humidifier and is suitable for use as a humidifier, such as in large-scale HVAC systems as well as clean rooms to rapidly control static charge development. The claims refer to a system, and it will be understood that the system may be referred to as a flow seeder or may be referred to as a humidifier.

Performance simulations

3D RANS simulations were performed via the SolidWorks Flow Simulation package. No-slip and adiabatic conditions were applied for the wall boundaries. At the inlet, a 2% turbulence intensity and a fully developed flow profile were set. Velocity and pressure boundary conditions are imposed at the inlet and outlet to simulate realistic conditions expected during the operation. Adaptive cubic meshing to both the geometry and calculation was used on an exact CAD model of the diffuser. Due to the asymmetry of the supports as well as the asymmetry of the service ports of the diffuser, the entire diffuser has been processed via the CFD tool. A total of 3.4 million mesh cells were used to capture the flow. The smallest mesh cell has a cube side length of 118pm. A single-phase simulation took place (air) with the seeding droplets omitted from the calculation.

A section view of the CFD results in the mid-plane of the seeder can be found in Figure 3. PIV analysis at the outlet of the diffuser was performed and confirmed a close to top hat average airflow exit profile from the diffuser at the nominal condition, airflow of 3000 litres per minute (0.05 m³/s).

Figure 6 shows the velocity profile of the flow produced using the flow seeder. The velocity profile shown in Figure 6 is an average of a plurality of images (2000 images) taken at the exit of the flow seeder. The velocity profiles are measured using a Particle Image Velocimetry (PIV) technique, which relies on a uniform dispersion of droplets within the flow, which are used as flow tracers to describe the flow. The measured velocity profiles shown in Figure 6 confirm that a top hat velocity profile is established at the exit of the flow seeder, thereby confirming the functionality of the flow seeder.

Figure 7 is a raw image showing laser light being scattered by the droplets produced by the flow seeder. The droplets scatter the light, making them appear white against a black background. Figure 7 shows that the flow seeder is able to uniformly suspend a cloud of uniformly sized droplets in high volumetric flow rates. Figure 7 also shows that the droplets do not merge to create visibly large conglomerates. Figure 8 is a binarized version of the image shown in Figure 7. Figure 8 is produced by correcting and normalising the illumination across the image to better indicate the number and location of the droplets within the image. It should be understood by those skilled in the art that while the present invention has been described with reference to exemplary embodiments, it is not limited to the disclosed exemplary embodiments. Various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof. Features from any example or embodiment of the present disclosure can be combined with features from any other example or embodiment of the present disclosure.

Claims

1 . A system comprising: an inlet connectable to a source of fluid; an outlet; a fluid flow path defined from the inlet to the outlet; an annular diffuser for diverging and slowing flow within the fluid flow path; a contraction section situated downstream of the diffuser for contracting and accelerating flow within the fluid flow path; and an atomiser nozzle arranged to introduce liquid droplets into the fluid flow path between the diffuser and the contraction section.

2. A system as claimed in claim 1 , in which the diffuser comprises an outer wall section for containing the fluid flow path and an inner wall section disposed within the outer wall section for diverging the fluid flow path, part of the fluid flow path being defined between the outer wall section and the inner wall section.

3. A system as claimed in claim 2, in which the inner wall section has a tapered profile for diverging airflow.

4. A system as claimed in any preceding claim, in which the contraction section includes a dome shaped inner wall for contracting the fluid flow path to accelerate the flow within the fluid flow path.

5. A system as claimed in any preceding claim, in which the atomiser nozzle is a shear-induced humidifying device.

6. A system as claimed in any preceding claim, further comprising a vane ring for dividing the fluid flow path into an inner flow path and an outer flow path.

7. A system as claimed in claim 6 when dependent on claim 4, in which the vane ring is arranged such that the outer flow path follows the dome shaped inner wall.

8. A system as claimed in claim 6 or claim 7 in which the vane ring comprises an outer cylindrical surface parallel to an axis of the system, and an angled inner surface for deflecting a portion of the flow within the fluid flow path towards the central axis of the system.

9. A system as claimed in any of claims 6 to 8, in which the atomiser nozzle is situated on a central axis of the vane ring, such that the liquid droplets are introduced into the inner flow path and the outer flow path bypasses the atomiser nozzle.

10. A system as claimed in any of claims 6 to 9 when dependent on claim 2, in which the inner wall section comprises a bevelled front edge for accommodating the vane ring.

11. A system as claimed in any of claims 6 to 10, in which the inner flow path is arranged to mix with the water droplets from the atomiser nozzle.

12. A system as claimed in any preceding claim, further comprising a pressure sleeve surrounding an exterior of the flow seeder.

13. A method of seeding or humidifying a fluid flow, comprising: first, diverging and decelerating the fluid flow by passing it through an annular diffuser second, introducing liquid droplets into the fluid flow from an atomiser nozzle third, contracting and accelerating the fluid flow by passing it through a contraction.

14. A method as claimed in claim 13, further comprising: dividing the fluid flow into an inner flow path and an outer flow path, in which the outer flow path follows an inner wall of the contraction and in which the liquid droplets are introduced into the inner flow path; and recombining the outer flow path and the inner flow path.

15. A method as claimed in claim 13 or claim 14, comprising passing the fluid flow through the system of any of claims 1 to 12.