I am testing a hydrocarbon sludge that will be subjected to chemical treatment to reduce its viscosity for pumping purposes. How do i determine the absolute viscosity of this sludge using a rotary viscometer when i get differing results across different speeds with the same spindle?
- 6,200 mPa @ 31% torque, 30 RPM
- 3,400 mPa @ 34.8% torque, 60 RPM
- 5,300 mPa @ 10.5% torque, 12 RPM
How do i know which one is correct?
Dear Lee,
It is well known, that If the viscosity does not depend on the shear rate, this means that the liquid under study is Newtonian, if the viscosity decreases with the shear rate, it means that the fluid is pseudo-plastic, and if the viscosity increases with the shear rate, it means that the liquid is dilatant.
HI Hakim Fikret Abbasov , thanks for the explanation but can you help clarify which of the viscosity readings would be correct, or how it would be reported for this material?
Thanks
The hydrocarbon sludge that you are measuring is non-Newtonian, i.e. its viscosity changes under different RPM. You can plot a graph showing the viscosity-RPM relationship to observe the trend. From the results you provided, I can see that there is a shear-thinning behaviour (reduction of viscosity at a higher RPM) but I also aware that the viscosity at 12RPM is lower than expected. Wall slip tends to take place at a lower RPM and this gives wrong rheological information. Rough surfaces can minimize the wall slip error.
You must again carefully measure the viscosity of the test fluid on a rotational viscometer.
Hi Hakim Fikret Abbasov the original measurements were conducted using a rotary viscometer, so you are suggesting just to perform the test again?
Hi Ernest Ong if i plot a graph showing viscosity-RPM trend what am i looking at specifically to help determine the Absolute Viscosity since the curve will be quite smooth i.e. no peak. What RPM should i be referencing to determine the correct viscosity?
Is there any indication that defines what the Absolute Viscosity is since repeated tests provide different answers? Or should i be using a rotational rheometer or something else instead?
Thanks Gents
@Lee Hunter The absolute viscosity or dynamic viscosity of your hydrocarbon sludge cannot be represented in a single value. As I mentioned earlier, yout hydrocarbon sludge is non-Newtonian and its dynamic viscosity changes under different RPM.
@Lee Hunter You mentioned that the viscosity changes under different tests. It shows that your material is thixotropic in nature. It means the material has time-dependent rheological properties. To get a more consistent result, you need to use fresh sample (fresh hydrocarbon sludge) in each of your tests. You need to read some books on rheology. As a beginner, you can read the introduction to rheology by Howard Barnes.
http://www.rheosense.com/applications/viscosity/newtonian-non-newtonian
What steps have you taken to ensure that sample properties remain the same/
Have you maintained the sample temperature constant?
What about cleanliness of the instrument?
What is the expected range of viscosity?
In any case, the second reading looks to be a wild point.
Hi Lee, you have to be careful. There are rotational viscometers which will give you an absolute viscosity value but there are also models available, which only give relative values.
Since you find different viscosities at different rotational speeds (and I assume lower viscosity values at higher speeds) you know already that you materila is shear thinning aka pseudoplastic. If your sample also shows a viscosity decrease over time under constant shear (rotational speed) then you know that it is thixotropic as well.
To characterize this material you can either give one or more viscosity values with their corresponding shear rates or measure the viscosity curve, fit a suitable rheological model and give the model parameters.
Hi Lee, the problem is complex and you may look at the attached paper to see that an understanding of turbulence is here essential. So we possibly need to talk about a full spectrum of turbulence under non-Newtonian conds and then about an additional excitation at a specific "singular" frequency controlled by your pump. It'll need longer talk and more detailed knowedge of the hydromech/hydrodyn framework.
If I were you I would run a flow sweep test on my samples. A flow sweep is a test at constant temperature in which you shear the samples from very low speeds (essentially zero) to high speeds at steady state conditions. At very low shear rates (say 0.001 s-1) the sample starts with a plateau which is called zero shear viscosity (viscosity at static condition). As the shear rate increases, for your sample you might see a decrease in viscosity which might be modeled with the Power Law equation. At higher shear rates (lets say 1000 s-1) you might again see a plateau which is called infinite shear viscosity.
Keep in mind that the number and trends I mentioned are not exact numbers for any sample. Depending on the sample, these numbers and trends might change.
If your sample is more like a very viscous fluid (Paste), I would recommend running an oscillatory test instead of a shear test.
For oscillatory tests, take a look at this paper Article Diffusion of Nanoparticles in Entangled Poly(vinyl alcohol) ...
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