Minimum L/D for DarcyûWeisbach equation

[Wilbur55]

Is there some value of L/D for which the Darcy–Weisbach equation is applicable?

At what point does 'relative roughness' become meaningless?

Is there any expectation of accuracy in calculating the pressure drop through a 10-foot steel duct that is 5 feet long?

Seemingly not because the flow is not close to "fully developed" in such a short length.

For such a short duct, how meaningful are the friction factor based on e/D (in this example = 0.15/10 = 0.015) and the pressure drop based on L/D (in this example = 0.5)?

If using an orifice equation, what is an appropriate value for C_d, coefficient of discharge for the incompressible flow equation in Wikipedia:

http://en.wikipedia.org/wiki/Orifice_plate

Or is some other calculation more appropriate for this situation?

 

[ProcessHVAC]

What is you velocity and reynolds number inside the duct? If not very high, I wouldn't worry too much about the accuracy of Darcy Weisbach. If it's high enough I would treat the entry as an exit "minor" loss with the appropriate k factor. Depending on the velocity, the "minor" pressure drop could end up being significant.

 

[KiwiMace]

I agree, and would treat it as a fitting taking into account what is happening upstream and downstream.

 

[Wilbur55]

So dynamic losses in this situation are far more significant than whatever friction loss might be calculated with the Darcy–Weisbach equation.

For a conduit between reservoirs I don't imagine the flow is fully developed if L/D < 10. There may be a better limit, but I suppose L/D = 0.5 for a conduit between reservoirs isn't even close to an appropriate application of the Darcy–Weisbach equation.

Thanks.