Rheology for nozzle technology and fluid mechanics

Rheology deals with the deformation and flow behaviour of materials. It includes liquids and gases.

Nozzles and atomizers can only be selected, designed or optimized if the relevant rheological data for atomizing liquids are known in detail. In addition to the rheological data, the flow rate must also be taken into account in nozzle technology.

Within the frame of fluid mechanics - atomization technology can be regarded as a particular sub-area of fluid mechanics - the rheology of fluids is therefore of great importance.

Liquids can be very viscous. And besides, they can show a non-Newtonian flow behavior.

Experience has already shown that it makes a big difference whether water or highly viscous oil is pumped through a pipeline.

The required performance of a pump will be much higher incase of oil depending on the length and diameter of the pipe. Also, there is an  flow rate influence.

The rheological properties also play an enormously important role in atomization technology. For example, if a liquid is too viscous, it can no longer be atomized with standard nozzles.

This prevents the formation of a classic spray with fine drops. Rather, only liquid jets and so-called fragments occur.

Why rheology must be known in atomization technology!

The rheology of liquids or suspensions to be atomized in conjunction with the volume flow rate provides information in advance as to whether certain nozzle types are suitable for solving the task at all. Expensive and time-consuming wrong decisions can thus be avoided.

Generally, three different rheological properties of the liquid to be atomized play an important role.

  • First of all the viscosity η resp. ν must be mentioned. This is a value for the internal friction of a flowing liquid or gas. It is thus significantly influencing a "pressure loss" or "speed loss". Viscosity plays an important role, especially with pressure nozzles. In addition, it is imperative to consider whether the fluid to be atomized has a Newtonian or non-Newtonian flow behaviour.
  • The density ρ of a liquid should also be known. Its value also influences the atomisation behaviour. For example, in the self-priming of liquids in two-substance nozzles with an external mixture. In case of pressure nozzles, the density is important for the maximum flow velocity as a function of the pressure difference. More detailed information on density can be found here.
  • The surface tension σ is important for drop formation. The disintegration of a liquid jet or liquid lamella is influenced by it. Drop sizes tend to become slightly finer when the surface tension is lower. In ultrasonic nebulisers, on the other hand, a low surface tension can be problematic. Read more about surface tension here.

If the rheology of liquids to be atomized differs from water

Many liquids to be atomized have a rheology that differs from water. It is very dangerous to rely on results from water atomisation tests if the original fluid has different properties.

If, for example, the fluid to be atomized is viscous, i. e. more viscous than water, the spray properties are considerably different. In extreme cases, even the nozzle or atomizer can fail completely!

The influence of the surface tension σ and the density ρ of liquids is not quite as important for the atomization process as the viscosity. However, these rheological properties should also be known. The knowledge of  all relevant rheological data is a major advantage.

Tip on rheology for nozzle and flow optimization

If the rheology of the original fluid is known, this results in a further and very practical advantage!

Occasionally, liquids may be toxic, explosive or particularly costly. In these cases, it makes little sense to carry out optimizations on nozzles in the laboratory or in test benches with the original fluid. Model fluids can be used for measuring drop size distributions as well as for the development and optimization of nozzle systems.

These then have largely the same rheology as the original fluid. However, the difference is that they are easy to handle and inexpensive. 

Our rheology laboratory quickly provides the desired model fluids in required quantities.

Of course, this is also part of our full service for you!

Measuring rheological data

For a large number of liquids, the rheological data are given as a function of temperature in tables from the open literature. However, it is occasionally the case that the data of special liquids are not known.

If required, we measure the rheological properties of your liquids for you!

Of course, this is no problem!

If required and on your behalf, we can determine the necessary data for you in our fully equipped rheology laboratory.

Of course also as a function of the temperature!

We can also create viscosity curves of so-called non-Newtonian liquids. Modern shear rheometers are available for this purpose.

We can also offer you the measurement of rheological properties as a separate and independent service.

Not only for solving atomization problems. But also when it comes to process optimization in the fields of fluid mechanics, process or production engineering.

We will be happy to answer your questions about rheology!

Do you have any questions about rheology?

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