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Back calculating surface roughness

Back calculating surface roughness

Back calculating surface roughness

(OP)
thread378-220785: PVC pipe absolute roughness

I have read a number of posts on this site which mention "back calculating" to find the roughness of a pipe/hose (see link). I have a fabric duct used for HVAC which was sent to us for R&D purposes from the manufacturer. I am having trouble back calculating my test results to find the duct roughness. It's a simple fix surely -- can someone outline in detail their process for calculating epsilon?

Thanks in advance   

RE: Back calculating surface roughness

Probably easier to assume an initial value for ε and iterate with it until you get the observed pressure drop.

Solving a Colebrook or similar fluid head loss equation for ε isn't on my priority list.

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RE: Back calculating surface roughness

(OP)
I will try iterating ... I solved the Haaland equation for ε and, as I mentioned earlier, saw results which were not realistic.  

RE: Back calculating surface roughness

kapo84,

For flow in a duct the flow is probably fully turbulent and you can use the rough pipe law:

1/√f = 2Log (3.7D/ke) or
(1/2)1/√f = Log (3.7D/ke)

Take the anti log of both sides and get:

10(1/2)1/√f=(3.7D/ke)

Solve for ke:

ke=(3.7D/10(1/2)1/√f)

f is the Darcy friction factor
D is the pipe diameter
ke is the effective roughness

You can do the same for the Colebrook equation.
 

RE: Back calculating surface roughness

(OP)
vzeos:

Thanks for the help! I iterated per the instructions of BigInch and received the results I needed.

The equation you provided gave me near exact results to those which I iterated. However, it is handy to have an equation which Excel can equate for large amounts of data.

In hind sight I should have tried to solve for the Colebrook equation rather than the Haaland equation since the Haaland equation is derived from the Colebrook equation.

Thanks again BigInch and vzeos.   

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