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A.H.
science forum beginner

Joined: 03 May 2005
Posts: 17

Posted: Fri Apr 28, 2006 8:52 pm    Post subject: Re: theory: viscosity of mixtures?

 Quote: Say for instance I have two liquids: a vegetable oil with viscosity of 70 cp, and diesel fuel of viscosity 3 cp. If I mix them 50:50 by mole fraction, somehow I get a feeling that the viscosity will not be just the arithmetic average of the two (36.5 cp). So, is there a theory which predicts the viscosities of fluid mixtures?

As others have mentioned, there is no good theory for this, and mixing
models are largely semiempirical. A review of viscosity estimation
techniques, including for mixtures, is given in the valuable book "The
Properties of Gases and Liquids." Something called the Grunberg-Nissan
equation seems to be the most popular way to interpolate between pure
values to estimate a mixture viscosity.

Dr. Allan H. Harvey, Boulder, Colorado
(Usual disclaimers here)
rekuci@gmail.com

Joined: 22 Sep 2005
Posts: 98

 Posted: Tue Apr 18, 2006 11:39 am    Post subject: Re: theory: viscosity of mixtures? I could imagine that 50/50 might significantly decrease the viscosity from that of 100% vegetable oil. Take glycerol for example, it's so thick and syrupy you can't even pipette it, but add 50% water and the viscosity goes way down. In some battery chemistries, a small amount of cosolvent is added, which can greatly reduce viscosity at low temperatures.
science forum Guru Wannabe

Joined: 07 Oct 2005
Posts: 107

Posted: Mon Apr 17, 2006 10:49 pm    Post subject: Re: theory: viscosity of mixtures?

rekuci@gmail.com wrote:
 Quote: onehappymadman@yahoo.com wrote: Say for instance I have two liquids: a vegetable oil with viscosity of 70 cp, and diesel fuel of viscosity 3 cp. If I mix them 50:50 by mole fraction, somehow I get a feeling that the viscosity will not be just the arithmetic average of the two (36.5 cp). You are correct - if you think about what exactly makes viscosity happen at the molecular level, you wouldn't expect it to be any sort of arithmetic average. So, is there a theory which predicts the viscosities of fluid mixtures? Here's one, if you can access it - http://ej.iop.org/links/q24/Ia1YGlrSkNlAu46bDxZgLw/jcv8i15p2376.pdf You won't be able to get around the nasty coagulated parameters that are specific to each component, but they can be determined empirically. And vegetable oil and diesel fuel wouldn't even be a binary mixture, since each is composed of multiple components. The more components, the more pairwise interactions that need to be considered. This is a case of measurement being far easier than calculation.

Ooh, thanks all.

Yes, I figured measurement would be easier than calculation.
(Especially since "diesel fuel" isn't a pure component.)

The motivation for asking: one of my co-workers mentioned that he can
run diesel fuel / used vegetable oil mixtures 50:50 by volume in his
diesel tractor, with no modifications (no fuel preheater, etc.) I was
curious what the viscosity would be, since vegetable oil is supposed to
be too viscous for diesel engines... I thought that adding 10% veg oil
to 90% diesel would be about the upper limit of what's safe, but
apparently some people have done just fine with > 50% veg oil...
rekuci@gmail.com

Joined: 22 Sep 2005
Posts: 98

Posted: Mon Apr 17, 2006 10:38 pm    Post subject: Re: theory: viscosity of mixtures?

 Quote: Say for instance I have two liquids: a vegetable oil with viscosity of 70 cp, and diesel fuel of viscosity 3 cp. If I mix them 50:50 by mole fraction, somehow I get a feeling that the viscosity will not be just the arithmetic average of the two (36.5 cp).

You are correct - if you think about what exactly makes viscosity
happen at the molecular level, you wouldn't expect it to be any sort of
arithmetic average.

 Quote: So, is there a theory which predicts the viscosities of fluid mixtures?

Here's one, if you can access it -

You won't be able to get around the nasty coagulated parameters that
are specific to each component, but they can be determined empirically.
And vegetable oil and diesel fuel wouldn't even be a binary mixture,
since each is composed of multiple components. The more components,
the more pairwise interactions that need to be considered. This is a
case of measurement being far easier than calculation.
Thiophilus
science forum beginner

Joined: 11 Nov 2005
Posts: 3

Posted: Mon Apr 17, 2006 10:27 pm    Post subject: Re: theory: viscosity of mixtures?

On 17 Apr 2006 15:02:32 -0700, onehappymadman@yahoo.com wrote:

 Quote: Say for instance I have two liquids: a vegetable oil with viscosity of 70 cp, and diesel fuel of viscosity 3 cp. If I mix them 50:50 by mole fraction, somehow I get a feeling that the viscosity will not be just the arithmetic average of the two (36.5 cp). So, is there a theory which predicts the viscosities of fluid mixtures?

There are many ways to blend viscosities mathematically. I seem to
recall that API procedure 11A4.3 was one that was used in oil
refining. Look under Transport Property Models in
http://tinyurl.com/jktjb

Check Google using terms like viscosity /blend /blending /calculator
/calculation and so on.
science forum Guru Wannabe

Joined: 07 Oct 2005
Posts: 107

 Posted: Mon Apr 17, 2006 10:02 pm    Post subject: theory: viscosity of mixtures? Say for instance I have two liquids: a vegetable oil with viscosity of 70 cp, and diesel fuel of viscosity 3 cp. If I mix them 50:50 by mole fraction, somehow I get a feeling that the viscosity will not be just the arithmetic average of the two (36.5 cp). So, is there a theory which predicts the viscosities of fluid mixtures?

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