Non-Newtonian fluid ratholing 1

[Mechanical Farmer]

Hello,

Long time listener, first time caller.

I have been diving into the behavior of non-Newtonian (shear thinning, specifically) and believe that I am experiencing rat-holing in a Pseudoplastic flow.

My question is this: Provided this is a correct assumption, is there a way to determine the thickness of the "stagnant" layer on the pipe walls. Is there a way to determine a radial pressure gradient from the centerline of the pipe to the pipe walls?

 

[georgeverghese]

See if the narrative presented on pages 6-13 and 6-14 in Perry Chem Engg Handbook 7th edn on laminar and turbulent flow regimes in non Newtonian fluids helps - there is some material here on derivation of this radial velocity profile for laminar flow.

 

[Latexman]

I’m familiar with rat-holing in solids/powder flow, but not in non-Newtonian fluid flow. Can you provide a reference?

 

[Mechanical Farmer]

See if the narrative presented on pages 6-13 and 6-14 in Perry Chem Engg Handbook 7th edn on laminar and turbulent flow regimes in non Newtonian fluids helps - there is some material here on derivation of this radial velocity profile for laminar flow.

Do you know how I can access this?

 

[Mechanical Farmer]

I’m familiar with rat-holing in solids/powder flow, but not in non-Newtonian fluid flow. Can you provide a reference?

I've seen your comments a lot. I was hoping you would reply. You've reference some Chemical Engineering Magazine articles from '68. I can't find anywhere to view or purchase these. I've ordered Darby's text (second edition) as some have suggested. Hopefully that is insightful.

My background is mechanical engineering and I am new to Non-Newtonian fluid flow, so I'm somewhere to the left of the Dunning Kruger curve (Little knowledge with a high degree of confidence).

This is in regard to polymer flow through relatively long pipelines and at low velocity, i.e. low shear rate. (~3 s-1)
Our extruder vendor along with a board member of the Society of Plastics Engineers have suggested that we are experiencing ratholing.
It was explained that the shear rate needs to be in the hundreds, at a minimum to induce surface renewal. I understand that not having surface renewal doesn't necessarily mean ratholing.
Intuitively (take that as you will) I believe the velocity profile we are experiencing in our pipeline is probably more of a bell curve than a parabola.
Unfortunately, I can't really provide any more information than this.

 

[Latexman]

Is there a basic problem? High dP, need more flow, need more heat transfer, burnt product at the wall, or, as stated, you are just coming up to speed on your process? Those articles are great. I got copies (poor ones) long ago from my megaCorp's library. I recommend calling ChE magazine and ask how to get a copy. Surely, they know how. Does your company have an "Information Department"? We had a guy that specialized in bird dogging info we needed. If you strike out, let me know after April 1. I'm at my FL condo now. *I think*, the articles are at my home in NC. I retired in July and I'm pretty sure I brought those home.

Have you characterized the rheology of your shear thinning flow?

 

[LittleInch]

Can you at least sketch up or describe a bit better what YOU mean by rat holing as the term is a little bit vague.

I've had experience of very viscous waxy crude oils being transported by pipeline so not as viscousa s the goo latexman is familiar with, but close.

What sort of viscosities are we talking here at your low shear rate?
Velocity? Size of pipe?

Don't think that's sensitive information...

 

[Mechanical Farmer]

Is there a basic problem? High dP, need more flow, need more heat transfer, burnt product at the wall, or, as stated, you are just coming up to speed on your process? Those articles are great. I got copies (poor ones) long ago from my megaCorp's library. I recommend calling ChE magazine and ask how to get a copy. Surely, they know how. Does your company have an "Information Department"? We had a guy that specialized in bird dogging info we needed. If you strike out, let me know after April 1. I'm at my FL condo now. *I think*, the articles are at my home in NC. I retired in July and I'm pretty sure I brought those home.

Have you characterized the rheology of your shear thinning flow?

I am just trying to wrap my head around the pseudoplastic fluid dynamics. We are seeing things that we don't really know how to explain at the moment, like flowing through the pipeline without losses through an open vent [in vertical piping], which triggered my question about ratholing. I am assuming there was a sufficient layer of stagnant fluid along the pipe wall to act as a liner. By my calculations and based on my limited understanding, I believe we are more than a few NPSs too large. Again, purely speculative, but I think we've got more of the cross-sectional area or the pipe "stagnant" than flowing.

We have R&D but not an information department per-se. I suppose that would be handled by IT. I will keep digging to find those documents and try giving ChemE a call.
We just recently received some lab results of our fluid providing shear sweeps from 10^-1 to 10^3 (1/s) at a number of different temperatures. I attempted to derive K and n from those charts and am skeptical of the results. Plugging two sets of values into τ = Kγ ^n and solving for K and n I got
K=~19.5 and
n=~0.0354851.
Which return:
Re = 0.000155466 from (https://cheguide.com/power_law_fluid.html) and a friction factor of
f = 102,916, and a pressure drop of
delP = 435,381,518 psi.

The building is still standing so I think its safe to say these numbers are incorrect.

 

[Compositepro]

Rat-holing is something that many of us are familiar with in silos and hoppers storing granular solids. It is not a term used to describe fluids, but it is understandable as an analogy. The proper term for this effect is Bingham plastic. A certain shear force is required to before a material starts to flow and behave like a fluid. With Bingham plastics there is always a stagnant layer of material at any wall; it is only a question of how thick it may be. This layer tends to grow with time due to cross-linking. This is why polymer process equipment often uses wall scrapers and is designed to be frequently disassembled for cleaning with brushes and other tools.

 

[Mechanical Farmer]

Rat-holing is something that many of us are familiar with in silos and hoppers storing granular solids. It is not a term used to describe fluids, but it is understandable as an analogy. The proper term for this effect is Bingham plastic. A certain shear force is required to before a material starts to flow and behave like a fluid. With Bingham plastics there is always a stagnant layer of material at any wall; it is only a question of how thick it may be. This layer tends to grow with time due to cross-linking. This is why polymer process equipment often uses wall scrapers and is designed to be frequently disassembled for cleaning with brushes and other tools.

Thanks for the reply.

My previous job was on a chicken farm. ratholing, bridging, and channeling were common with feed systems there. I think you are right that the term was meant to be analogous when presented to us.

Does this mean that the material is a Bingham plastic, rather than pseudoplastic or is this just, as you stated a dynamic effect of the pseudoplastic? The lab result of viscosity testing have shown that we are experiencing shear-thinning.

I assume your suggestion would also mean that the thickness of the stagnant layer is constantly growing and therefore cannot be calculated?
I am trying to build the argument for decreasing pipe size to encourage surface renewal and hopefully prevent (or at least mitigate) this stagnation.

 

[Mechanical Farmer]

Can you at least sketch up or describe a bit better what YOU mean by rat holing as the term is a little bit vague.

I've had experience of very viscous waxy crude oils being transported by pipeline so not as viscousa s the goo latexman is familiar with, but close.

What sort of viscosities are we talking here at your low shear rate?
Velocity? Size of pipe?

Don't think that's sensitive information...

You'd be surprised what our legal department deems sensitive. Pipe size + velocity = feed rate
Its difficult trying to vet vendors when you cant tell them anything about what you need without an NDA.

I'm envisioning a pipe with scale as a good visual aid.
Imagine the scale as a stagnant layer of the fluid with flowing fluid in the open space.



The below rheology is our "worst case." For the sake of being conservative and for ease of measurement I am considering a shear rate of 1 which gives a viscosity of only 11Pa-s. which is much lower than we originally assumed.

I am now assuming that the fluid not only has a velocity gradient along the radius of the pipe but probably exhibits a nonlinear shear rate and thus a viscosity gradient.

 

[Mechanical Farmer]

See if the narrative presented on pages 6-13 and 6-14 in Perry Chem Engg Handbook 7th edn on laminar and turbulent flow regimes in non Newtonian fluids helps - there is some material here on derivation of this radial velocity profile for laminar flow.

https://students.aiu.edu/submissions/profiles/resources/onlineBook/z5y2E6_Perry-s_Chemical_Engineers-_Handbook.pdf#page=634.53&gsr=0

Attempting to read the whole of chapter 6. It may take me a while.