Dynamic viscosity @ kinematic!

[mrswax]

hi all,

I am working on rotational viscometer to measure dynamic viscosity and I have to use just one value to calculate the kinematic viscosity, but in this case I've already got many values of dynamic viscosity at shear rate range from 0 -1000 s-1.
my question now is can I use the only first value of dynamic. because i need only one value?

if you have any idea please help me.

thank a lot

 

[25362]

Evidently the fluid is non-Newtonian. As such, I think one is obliged to report all the results.

 

[mrswax]

thanks 25362 for ur reply, however as you know there are different values of dynamic( minimum and maximum ). so what do you think the perfect value as represent the all.

 

[rbcoulter]

It will depend on your application or end point use. There is no perfect value. High shear rate applications will require that the viscosity be measured a similar shear rates. For example, pouring is generally a low shear rate application but high speed coating through a narrow feed bar could be a high shear rate application. Must pumping of higher viscosity materials tend to be low shear rates must this could vary depending on your material. Chances are your fluid is thixotropic also meaning its viscosity changes with shear history.

 

[jmw]

If you have different viscosities at different shear rates it is because your fluid is non-Newtonian.
This means that all the data is valid and all could be converted.
It helps to report what type of fluid you have:
is it dialatant? (shear thickening)
pseudoplastic? (shear thinning)
Bingham palstic or whatever.
Does it show time or work dependency?
Is the behaviour reversable?

Now the key question is:
Why are you measuring the viscosity?
If you know the context for the measurement then, perhaps, you can narrow it down as to if there is a value that is appropriate.
For example, if it is pseudoplastic i.e. has a high viscosity at low shear rates then if you are trying to predict the behaviour when pumped you may need to extend your shear rate to higher values for a gear pump or to low values for a piston pump or other low shear pump if theproduct is permanently deformed (damaged) by high shear.




JMW

http://www.ViscoAnalyser.com

 

[TrevorP]

It is often possible to provide a viscosity/shear rate correlation. I believe that there are various correlation models, depending on the type of behaviour observed. This correlation is important as the shear in a pump will be different to the shear in a pipeline, and will be different again through a restriction such as a control valve or orifice plate. The viscosities at all points are required for a suitable system design.

 

[25362]

Suggest reading an article titled Program helps friction factor for non-Newtonian fluid flow by H.A. Ohen, that appeared in the OGJ issue of Jan 2, 1989.