laminar flow in an annulus 4

[proinwv2]

I have need to determine laminar flow in an annulus, and I am not sure what to use for the diameter (hydraulic diameter).

My references tell me that hydraulic radius or diameter is not applicable to laminar flow. So my question is what "diameter" should be used to calculate Re as well as flow in Darcy's equation?

(If important, these are going to be conduits less than about 0.2" diameter with an annulus space of about 0.02" and a length of about 0.2". Of course these are all guesses until I can make a reasonable calculation. Calcs will be proven in lab tests.)

Thanks very much.



Paul

http://www.ostand.com

 

[vpl]

Do you have Crane's Technical Paper 210 "Flow of Fluids"?

It has an equation for determing the hydraulic diameter for flow in an annulus.

There was also a previous thread that discussed what happened when the annulus was very small and the Crane formula no longer appeared to apply.

Patricia Lougheed

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[Latexman]

Paul,

In case this reference is convenient - the original 1960 Transport Phenomenon by Bird, Stewart and Lightfoot has an analytical solution to laminar flow in an annulus. It doesn't use hydraulic radius.

Good luck,
Latexman

 

[ione]

Take a glance at the attached paper. Surprisingly (not too much) the reference suggested by Latexman is quoted there.

 

[zdas04]

I've found that the "hydraulic area" or "wetted perimiter" methods understate observed pressure drop in a full pipe (they seem to work well in horizontal flow of a liquid if the annulus is not full). For mostly vertical flow or flow with the annulus full (either turbulent or laminar), I use the equation in faq378-1142

David

 

[SNORGY]

I believe zdas04's post to be correct. I quasi-derived similar expressions based on theory in a textbook "FLUID MECHANICS" - Frank M. White. I have since adopted them for use in my annular flow problems, such as glycol jacketed lines, etc., with good results.

Regards,

SNORGY.

 

[SNORGY]

vpl:

I *know* you *meant* to type "410"...




Regards,

SNORGY.

 

[vpl]

Yes I did, thanks Snorgy

Patricia Lougheed

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[ione]

David,

I have missed your FAQ on this topic: really interesting.

 

[proinwv2]

I want to thank all for your answers, and I do have TP-410 but have been suspicious of their equation. I don't have access to the reference mentioned by Latexman.

My annulus is quite small, a fraction of an inch in diameter and using the method described in the faq I get an effective diameter 2.7 times larger than that calculated by the conventional method (4*A)/wetted perimeter.

This along with not having assurance, at this time, that we will be able to maintain concentricity, will lead to some serious lab work to evaluate our device.

Thanks again!



Paul

http://www.ostand.com

 

[zdas04]

I've used the Petroleum technique on 1.5 inch OD coiled tubing in 2-3/8 tubing (ID 1.996) which works out to 1.08 inch equivilant diameter and it represented measured data very well. The wetted perimiter method for full pipe converges on the difference in ID vs. OD and would be 0.496 inches or half the result from the Petroleum technique.

David