Viscosity - an important property in fluid mechanics and atomization technology

The viscosity is one of the most important material properties in fluid mechanics.

And of course also in the technology of nozzles and atomizers. Every real fluid, including liquids and gases, has a certain viscosity value. This depends on the temperature and, in particular with gases, additionally on the pressure. The value of the viscosity is decisive for the pressure losses occurring during flow processes.


The influence of viscosity starts already before the nozzle!

This also has to be taken into account in the field of nozzle technology and is unfortunately sometimes overlooked!

Liquids or suspensions to be atomized must be fed to the atomizers via pipes or hoses. In particular, pressure nozzles must be operated with a certain pressure differential in order to supply both the desired spray result and the specified volume flow.

Liquids often have to be pumped through long pipelines

In the chemical industry and in production plants, there are often long pipe or hose sections. In addition, there are fittings that are also flowed through. Normally, pumps are used to provide the required pressure for fluid transport.

When passing through the pipes, however, there is a pressure drop that should not be underestimated, which depends on the viscosity, among other parameters!

Example:

If a volume flow of 10 l/min of oil with a dynamic viscosity of 50 mPas is pumped through a steel pipe with an inner diameter of 10 mm over a length of 20 m, a pressure drop of almost 2 bar occurs! If there are additional installations, the pressure drop will be even higher.

When designing a nozzle technology, it must therefore be taken into account whether the required pressures are actually available for the operation of a nozzle. Naturally, our engineers pay attention to this important aspect!


Viscosity is very important for nozzle technology!

Viscosity is often responsible for the failure of nozzle technology!

The dynamic viscosity η or kinematic viscosity ν of a fluid is therefore the rheological property with the greatest influence on atomization processes. This applies not only to the supply of the nozzles but also to spray formation!

If nozzles fail, this can often be traced back to the influence of viscosity!

The disintegration of liquid lamellae or liquid jets into individual drops is influenced by the viscosity. This dampens the desired breakup. As a result, pressure nozzles in particular then quickly fail and instead of a spray, they only deliver isolated and very coarse droplets.

If the flow rates to be atomized are still small, this undesirable effect occurs very quickly. In this case, preheating of the liquid may be a possible solution.

In any case, it is therefore important to know not only the viscosity values but also the volume flow rate to be atomized! If at least these two data are not known, a nozzle cannot be reliably selected. This is particularly important when using pressure nozzles!


The viscosity of Newtonian and non-Newtonian liquids.

Many organic liquids have a Newtonian flow behaviour. This means that the viscosity does not change with the shear rate; this is to be understood as the velocitiy gradient. Time-related influences also play no role.

Typical representatives of this class are water and most oils. In this case, it is therefore sufficient to specify the value at a defined temperature.

The viscosity of Newtonian liquids can be easily measured. For example, with capillary viscometers.

Ketchup, for example, has a non-Newtonian flow behaviour.

However, it is much more problematic if non-Newtonian liquids need to be atomized. The viscosity of these liquids changes with the shear rate and thus with the flow rate. These changes can occur not only in the nozzle but also in the supply lines.

There are also certain liquids which also show a time-dependent change in viscosity.

Non-Newtonian liquids, which usually also include suspensions, often cause major problems in the entire nozzle technology. Also, a lot of paints, varnishes, plastic melts and foodstuffs such as ketchup belong to the group of non-Newtonian liquids.

Therefore, this issue is absolutely necessary and important! Exact information on the flow behaviour must already be available in the run-up to the selection of a nozzle or atomizer.

In case of doubt, do not assume a Newtonian flow!


Measuring the viscosity of non-Newtonian liquids

The viscosity of non-Newtonian liquids cannot be determined with capillary viscometers. Simply due to the fact that the values change at different shearing speeds (gradients). In some cases, a time-dependent effect must also be taken into account.

Therefore, other measurement methods are necessary. With modern rotational rheometers we measure the flow curves of non-Newtonian liquids on demand. Of course also as a function of the liquid temperature.


Let our rheology experts advise you now!

For you, we measure the viscosity of your fluids as a separate service or as part of a project execution!

Both, Newtonian and non-Newtonian fluids. Of course also as a function of the desired liquid temperature!

Call our hotline now under the phone number

+49 251 2 87 99 53-0

and let us advise you comprehensively!