Atomization technology

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The sputtering technique is a discipline of mechanical process technology and deals with the atomization , that is the division of liquids, suspensions or dispersions in fine droplets.

The aim here is often a large increase in the free surface in order to promote mass or heat exchange processes. An ideal spray consists only of droplets with the same diameter; this is called a monodisperse spray. A collective of drops with individual drops of the same size can be easily calculated with regard to the total surface, whereas collective drops with a broader drop size distribution can at best be calculated approximately. Laser-optical methods are used in practice to measure real droplet size distributions on nozzles and atomizers. These methods work without contact or interference.

However, a purely monodisperse spray is very rarely achieved. In contrast, sprays with a narrow droplet size distribution are realistic. The applications in the technical and domestic sector are very diverse and range from spray cans, ultrasonic nebulizers to large reactors in the spray drying of food and chemical products. The nozzles and atomizers used for atomization are expediently divided into groups according to the type of energy supply.

Single-substance pressure nozzles

This type of nozzle exclusively uses the kinetic energy of a liquid jet emerging from a nozzle orifice or a liquid lamella for atomization. For this purpose, a pressure difference p is applied to the liquid . This results in a specific flow velocity v of the fluid. The exiting jet of liquid or the lamella disintegrates into individual drops due to the turbulent flow and forms a spray. In addition, aerodynamic effects due to the interaction with the surrounding atmosphere must be taken into account in the formation of drops.

The exit speed of the liquid from the nozzle mouth, its contour and the droplet size spectrum generated depend on a large number of influencing variables. The pressure difference, the rheological properties of the liquid and the geometric design of the nozzle itself play an important role. An exit speed of v max cannot be exceeded.

Atomization with single-fluid pressure nozzles is fundamentally problematic when small volume flows of highly viscous liquids are to be atomized into fine droplets. Relatively high pressure differentials are required for this. At the same time, the smallest flow cross-section within the nozzle, which is usually the nozzle outlet, must be relatively small. This quickly leads to the degree of turbulence of the flow, characterized by the Reynolds number Re , assuming an amount of Re <2300.

In this case there is a so-called laminar nozzle flow . In most cases it is no longer possible to create a spray with fine droplets. Another dimensionless number describes the break-up of liquid jets or lamellae into drops. This is the no- worries number Oh ,

A laminar liquid jet emerging from the nozzle orifice breaks down into almost monodisperse droplets under certain conditions. This disintegration mechanism is known as Rayleigh's or laminar jet disintegration. It is particularly interesting that liquids with a high viscosity disintegrate into particularly fine drops. The reason for this is that, due to the effective acceleration due to gravity, the liquid jet flows faster and faster with increasing distance from the nozzle mouth. According to the rules of the continuity equation , this is accompanied by a decrease in the beam diameter. If this thin liquid jet disintegrates, correspondingly small droplet diameters result.

The droplet diameter x for low-viscosity liquids can be calculated as a good approximation,

,

where d in this case describes the diameter of the nozzle mouth. When higher viscosity liquids break down, the relevant rheological properties must also be taken into account.

denotes the beam diameter at the point of decay.

The principle of Rayleigh's jet disintegration is used, for example, in garden showers. Perforated plates with defined bore diameters provide an almost monodisperse range of droplet sizes, which roughly corresponds to natural rain events.

Turbulence and jet-forming nozzles

A compact jet of liquid emerges from the nozzle mouth. This type of nozzle is primarily suitable for generating a high-impulse liquid jet. The targeted cleaning of surfaces or the high-pressure cutting of metals are typical applications. The rapid break-up of the liquid jet and thus the formation of finer droplets can be achieved if the liquid is often deflected within the nozzle. Changes in cross-section in the flow channels or so-called Borda mouths also support the jet disintegration. With moderate pressure differences p, Coanda orifices are also used.

Lamellar nozzles

Disintegration of a liquid lamella due to edge bead contraction and hole formation.

These include, for example, the flat jet and hollow cone pressure nozzles as well as tongue and cone nozzles. A liquid lamella with the lamella thickness is formed at the nozzle mouth . This breaks down into a spray through various disintegration mechanisms. These can roughly as the amount of Weber number We with

can be divided into four areas:

  • We <2: Forming a lamella is not possible.
  • We <1640: disintegration due to edge bulge contraction and possible hole formation. Relatively large droplets are produced.
  • We> 1640: Aerodynamic curling. Strong interaction with the surrounding gas atmosphere. Relatively fine drops are formed.
  • We >> 1640: The decay is increasingly determined by turbulent effects. Fine drops are created.

The hollow cone pressure nozzle (HKD) is often found in technical applications . Either through special swirl bodies inside the nozzle or through tangential entries into the so-called swirl chamber, it is achieved that the liquid does not fill the entire nozzle outlet diameter. A relatively thin liquid lamella is thus formed, which disintegrates into fine droplets. Relatively large flow cross-sections can therefore be used with the tangential HKD. This minimizes the tendency of the nozzle to clog when using contaminated liquids. With the HKD, calculating the volume flow as a function of the pressure difference p as well as the density and viscosity of the liquid is complex. The HKD paradox should also be noted . This means that, in contrast to nozzles with a completely filled nozzle outlet, the volume flow initially increases with increasing liquid viscosity. On the other hand, when the viscosity decreases, it decreases. This leads, for example, to the fact that when oil is preheated at a defined pressure difference, the volume flow decreases.

Most lamella-forming nozzles deliver significantly finer droplets than jet and turbulence nozzles under identical operating conditions and the same rheology of the liquid.

Dual-substance or pneumatic atomizers

With these nozzle types, a gas or steam mass flow flowing at high speed serves as the energy supplier for the atomization process. This offers the advantage that, in contrast to single-substance pressure nozzles, smaller volume flows of liquids with a higher viscosity can also be atomized into a fine range of droplet sizes. The mass flow ratio between the gas and the liquid plays an important role here .

This mass flow ratio is also referred to as loading. The droplets produced tend to become finer with increasing loading. The larger the loading number, the more leeway there is with regard to the mass flows for a constant characteristic droplet diameter.

Two-substance nozzles with external mixture

The pefilming area on the two-substance nozzle of the outer mixture is decisive for the droplet size spectrum generated
Pre-filming surface on a two-component nozzle with an external mixture.

The liquid to be atomized and the gas only interact with one another outside the nozzle. The prefilming nozzle is often found here. The liquid emerges almost without pressure in the center of the nozzle. The gas flows out of a surrounding annular channel at high speed. This results in a negative pressure in the vicinity of the nozzle orifice, which spreads the liquid as a film on the pre-filming surface. This thin film hits the gas flowing at high speed and is broken up into fine droplets. Under certain conditions this type of nozzle works self-priming.

Two-substance nozzles with internal mixture

With these types of nozzles, a two-phase mixture is already created inside the nozzle. This has a low speed of sound. This results in a so-called pressure jump in the nozzle outlet plane. Drops with a critical diameter experience a further breakdown and contribute to a high proportion of fine droplets in the spray. In contrast to the two-substance nozzles with an external mixture, the gas and liquid pressure must be coordinated with one another. In this respect, a higher technical control effort is required.

Rotary atomizer

Rotary atomizers belong to the group of mechanical atomizers. A rotating disk or cup is exposed to liquid almost without pressure. The liquid is accelerated towards the edge due to the adhesion conditions. Depending on the operating conditions, it forms individual liquid threads or a lamella. These disintegrate into drops at a certain distance from the edge of the atomizer.

Rotary atomizers are considered to be almost clog-free, as no critical cross-sections are required. In addition, as a result of centrifugal acceleration, they clean themselves automatically when the fluid supply is interrupted. For this reason, they are often used to atomize suspensions. What is particularly interesting is that under certain circumstances they are able to deliver an almost monodisperse spray.

Depending on the operating conditions, the following droplet formation mechanisms occur on a rotary atomizer:

Dripping processes: bimodal droplet size distribution
Thread disintegration: Almost monodisperse drops are formed
Lamellar formation: droplet size distribution similar to that of single-fluid pressure nozzles that form lamellae

Rotary atomizers are often used in painting technology in the operational area of ​​thread disintegration. The additional electrostatic guiding of the drops minimizes the undesirable overspray effect.

literature

  • Thomas Richter: Atomization of Liquids - Nozzles in Theory and Practice . expert-Verlag, Renningen 2016, ISBN 978-3-8169-3359-5 .
  • Günter Wozniak: Atomization technology: principles, processes, devices . Verlag Springer, Berlin 2002, ISBN 3-540-41170-4 .
  • Ghasem G. Nasr, Andrew J. Yule, Lothar Bendig: Industrial Sprays and Atomization: Design, Analysis and Applications. Springer-Verlag, Berlin 2002, ISBN 1-85233-460-6 .
  • Gerhard Kifferle, Walter Stahli: Spraying and spraying methods in crop protection and liquid fertilization in area crops. Books on Demand, Norderstedt 2001, ISBN 3-8311-2538-4 .