6 replies to this topic

[axl456]

Hello..

I know this is silly, but its intriguing me..

What is the actual Poiseuille equation, because i've seen two slightly different version of the same equation:

is it this:

f=64/Re

or this one:

f=16/Re

I know they are very similar, but i would like to know why are there two of them?

Edited by axl456, 25 January 2010 - 11:21 PM.

[katmar]

Forgive me for being lazy and not typing the whole thing out again, but I have just answered this question on another forum.

Please see http://www.eng-tips....d=263226&page=1

[axl456]

Forgive me for being lazy and not typing the whole thing out again, but I have just answered this question on another forum.

Please see http://www.eng-tips....d=263226&page=1

dont worry, thanks for the reply

Actually I have just find that answer and I was going to edit to say it, but you already reply

I find it, when i came across another similar problem with to different, colebrook equation for turbulent flow in smooth pipes:
f=0.079xRe^-0.25 (fanning factor)
f=0.3164xRe^-0.25 (darcy factor)

now, in another topic, you where talking about the Stanton friction factor, and the churchill equation in the other forum.

Just to be totally secure, that churchill equation in the link posted by bigInch, was for the darcy factor or the stanton factor?

I am right now studying about piping for my thesis and am having a lot of trouble partially because is really hard for me to find documentation here in venezuela and specially in my university.

and one last question (sorry for bothering to much), is the churchill equation, the only equation that can give me the friction factor for transient flow? or at least the best one (in terms of precision)

I'm planing to use the hagen-poiseuille one for the laminar flow, the colebrook for turbulent flow (in smooth and in rough pipes in his two variation) and the churchill one for transient flow, in my thesis..

[katmar]

I am 99% certain that the reference by BigInch uses the Stanton form of the friction factor. The best way to check is to calculate a friction factor for laminar flow and compare it with the Hagen-Poiseuille equation. The results you get will be

Moody (Darcy) f=64/Re
Fanning f=16/Re
Stanton f=8/Re

I'm going to be lazy again - for my opinion on the accuracy/relevance of Churchill for the transition zone between laminar and turbulent see my post of 17 Sept 2009 in
http://www.cheresour...-pipe-fittings/

Perhaps in Spanish it is less ambiguous, but many English texts use the expression "transition zone" for conflicting parts of the Colebrook diagram. The sub-zone, in the turbulent zone, between the smooth flow line and the area of complete turbulence is called the transition zone by many authors. Other authors have called the area between Re=2,000 and Re=3,000 the transition zone. I prefer (but I am not always consistent in this) to call this second zone the "critical" zone. To me, the word "critical" conveys the need to stay out of this zone better than "transition" does.

[axl456]

I am 99% certain that the reference by BigInch uses the Stanton form of the friction factor. The best way to check is to calculate a friction factor for laminar flow and compare it with the Hagen-Poiseuille equation. The results you get will be

Moody (Darcy) f=64/Re
Fanning f=16/Re
Stanton f=8/Re

Thanks again for answer me

I calculate the friction factor for a pipeline with this characteristic:
D= 5000 mm
e= 0.2 mm

And a Reynolds number of 707.4, and the result for the Hagen-Poiseuille is 0.09047.
With the following Churchill equation the result is exactly the same:


so, if am not doing anything wrong that specific version of the churchill equation is calculating the darcy factor

I'm going to be lazy again - for my opinion on the accuracy/relevance of Churchill for the transition zone between laminar and turbulent see my post of 17 Sept 2009 in
http://www.cheresour...-pipe-fittings/

Perhaps in Spanish it is less ambiguous, but many English texts use the expression "transition zone" for conflicting parts of the Colebrook diagram. The sub-zone, in the turbulent zone, between the smooth flow line and the area of complete turbulence is called the transition zone by many authors. Other authors have called the area between Re=2,000 and Re=3,000 the transition zone. I prefer (but I am not always consistent in this) to call this second zone the "critical" zone. To me, the word "critical" conveys the need to stay out of this zone better than "transition" does.

I understand, thanks for the clarify..

actually am using the word "transient flow" because is the exact translation of "flujo de transicion", the spanish denomination for a flow with a reynolds number between 2100 and 4000. But now that you mention it, here we also use "flujo critico" (critical flow) as well as transient flow..

as I say, am new in all this "piping design" and, you mention a conflicting part, in the colebrook diagram, can you point me to some literature to read about it..

right now am working in a pipe sizing open source software proposal for my university, and I need as you may see for my noobs question to study a lot more about pipe sizing..

[katmar]

axl456, I was wrong about the form of the friction factor used in the Churchill equation in the article on Eng-Tips referenced by BigInch. It is indeed the Darcy form. I did not pick up that an "8" had been inserted in the f=8((... equation. In the original article by Churchill there is no "8".

The best example I have come across of the Colebrook or Moody diagram is in the Crane 410 manual. If you are serious about doing pipe calculations it would be well worth your while to purchase a copy. If there is any "standard reference text" in the pipe hydraulics calculation world it is this manual. It is not perfect, but very very useful. You will probably find there is a copy in your department or library. The diagram is on page A-24.

[axl456]

axl456, I was wrong about the form of the friction factor used in the Churchill equation in the article on Eng-Tips referenced by BigInch. It is indeed the Darcy form. I did not pick up that an "8" had been inserted in the f=8((... equation. In the original article by Churchill there is no "8".

The best example I have come across of the Colebrook or Moody diagram is in the Crane 410 manual. If you are serious about doing pipe calculations it would be well worth your while to purchase a copy. If there is any "standard reference text" in the pipe hydraulics calculation world it is this manual. It is not perfect, but very very useful. You will probably find there is a copy in your department or library. The diagram is on page A-24.

thanks for the reference, am reading now the crane 410 manual


I check the diagram, and i can now perfectly see what you told about the differences between "critical" and "transient" flow..

for what I have read about in other books and what I have seen so far in the crane manual, there are 4 well specified regimens in a fluid inside a pipe, those are:

Laminar flow (Re< 2000)
Critical flow(2000< Re < 4000 according to the a-24 diagram in the crane manual)
Turbulent flow - transient regimen (4000 < Re < 10e4 according to the a-24 diagram in the crane manual)
Turbulent flow - fully turbulent regimen (Re > 10e4 according to the a-24 diagram in the crane manual)

The problem is special with the "critical" flow, because of what i have read (if am not interpreting this wrong), this is a zone where NO constant value of f can be calculated, because the flow is too unstable, behaving like laminar or turbulent depending on several factors (changes in direction, obstruction etc)

In the turbulent flow with transient regimen, the f factor depends on both Reynolds number and relative roughness, and can be calculated with the colebrook equation..

In the fully turbulent flow, it appears as if the flow stop depending on the Reynolds number and depends only on the relative roughness,because the relative roughness lines are almost totally horizontal. This I havent read in any books, is just an observation of the diagram, and a conclusion I made because I have read that the colebrook equation works only on Reynolds number between the lines of smooth pipes and totally turbulent flow on the diagram (McGraw-Hill Piping Handbook 7th edition page B.372)

This of course is contradictory with other observation I have read claiming that the colebrook equation can be used for all Reynolds number above 4000 (I think this is because the % of error is to small)

Well I think that I have stole to much time from you, thanks for all the answer, they have been very very helpful

I actually have many things i would like to ask, sadly I haven't find any teacher in my university that cant help me with all this, but i feel that am being rude asking to much

If you are willing to help me with others question it will be very helpful if not i will understand and move my noobness to another thread or site

Again, thank you very much for helping..

Edited by axl456, 28 January 2010 - 09:57 PM.